Chapter 4: Cell: Basic Unit of Life
In 1884, a young Danish physician named Hans Christian Gram was working in a laboratory in Berlin, studying lung tissue taken from patients who had died of pneumonia. His problem was simple to state and hard to solve: under the microscope, bacteria and human tissue looked almost the same — pale, crowded, indistinguishable. He needed a dye that would pick out the bacteria and leave everything else alone. So he tried a violet dye, fixed it with an iodine solution, and then washed the slide with alcohol. Some bacteria held on to the violet colour stubbornly. Others let go of it completely and turned almost invisible. Gram was disappointed. He had wanted one clean method for seeing all bacteria, and instead he had a method that worked on some and failed on others. When he published, he wrote modestly that the technique was imperfect and would need improvement by others. What he had actually found was not a failure at all. The bacteria that kept the dye and the bacteria that lost it were built differently — their outer walls were not the same. A stain that had been designed to make bacteria visible had accidentally started sorting them into two great groups, and it is still doing so in hospital laboratories today.
Diagram
Fig. 4.1 – Opening illustration: a split-panel microscope view showing a field of deep violet Gram-positive cocci on one side and pink Gram-negative rods on the other, with a magnified inset of each cell wall — thick peptidoglycan layer versus thin peptidoglycan with an outer membrane
This chapter will help you answer each of these questions.
4.1 Cell Theory and the Cell as the Basic Unit of Life
Every living thing you can name — the bacterium in a spoonful of curd, the mango tree outside the window, the person reading this page — is built out of cells. A cell is the smallest unit that can carry out all the activities we call “life”: it takes in materials, releases energy, responds to its surroundings, grows and makes copies of itself. Nothing smaller than a cell can do all of this on its own.
This idea sits underneath the whole of biotechnology. When a laboratory produces a medicine using bacteria, or regenerates a whole plant from a piece of leaf, it is relying on the fact that a single cell already contains the complete machinery and the complete instructions needed for life. The biotechnologist does not build that machinery — the cell supplies it. Understanding what a cell is, and why it counts as the basic unit of life, is therefore the natural starting point of this course.
How the Cell Was Discovered
The story begins with a piece of cork. In 1665, the English scientist Robert Hooke cut a very thin slice of cork and examined it under a simple microscope he had built himself. He saw a honeycomb of tiny empty compartments and, because they reminded him of the small rooms in a monastery, he called them “cells” — a name that has stayed with biology ever since.
Nearly two centuries passed before anyone drew the general conclusion. In 1838–39, two German biologists working separately arrived at the same answer — M.J. Schleiden, studying plants, and Theodor Schwann, studying animals — and proposed that all plants and animals are made of cells. Their statement had one serious gap: it did not explain where new cells come from. That gap was closed in 1855 by Rudolf Virchow, who argued that every cell arises from a cell already present, summarising it in the Latin phrase Omnis cellula e cellula. The steady growth of the idea, from Hooke’s cork to Virchow’s statement, is traced in Fig. 4.2.
Diagram
Fig. 4.2 – Timeline of the development of cell theory: Hooke (1665, cork) → Leeuwenhoek (living cells) → Schleiden and Schwann (1838–39, all plants and animals made of cells) → Virchow (1855, Omnis cellula e cellula), each stage with a small sketch of what was observed
The Postulates of Cell Theory
Taken together, these contributions give the cell theory its three classical postulates:
- All living organisms are composed of one or more cells.
- The cell is the basic structural and functional unit of life.
- All cells arise from pre-existing cells (Omnis cellula e cellula).
Why the Cell Qualifies as the Basic Unit of Life
Calling the cell the “basic unit of life” is not a slogan; it rests on five separate observations, each of which can be checked.
All organisms are made of cells. From bacteria to human beings, no exception has been found. Some organisms are unicellular, meaning the entire organism is a single cell — Amoeba and Paramecium are familiar examples. Others are multicellular, built from very large numbers of cells working together, as in Homo sapiens or the mango tree, Mangifera indica.
The cell is the structural unit. In a multicellular body, similar cells group into tissues, tissues combine into organs, organs work together as organ systems, and the organ systems together make the organism. Muscle cells, for instance, form muscle tissue, which forms the heart, which is part of the circulatory system. Follow that sequence in FC 4.1 and you will see that the cell is the lowest floor of the building — every higher level is assembled from it.
Flowchart
FC 4.1 – Levels of biological organisation: cell → tissue → organ → organ system → organism, with muscle cell → muscle tissue → heart → circulatory system → human body shown alongside as the worked example
The cell is the functional unit. Every process we call living — respiration, nutrition, excretion, growth, reproduction — happens inside cells. It is not the leaf that respires but the cells of the leaf; not the kidney that excretes but its cells. In a unicellular organism this is especially clear, because one single cell performs every one of these functions by itself.
New cells come only from existing cells. Cell division is what allows an organism to grow, to repair damaged tissue and to reproduce. Because each new cell is produced by an older one, there is an unbroken chain of cells running back through every generation — this is what continuity of life means at the cellular level.
Cells carry the genetic material. Each cell contains DNA, the hereditary material that directs inheritance, controls the cell’s day-to-day activities and specifies which proteins are made. The cell is therefore also the unit of heredity, and this is precisely why a cell — rather than a tissue or an organ — is the object a genetic engineer works on.
Exceptions to the Cell Theory
Every good scientific generalisation is worth testing against the cases that resist it. Viruses are the clearest exception. A virus is made of nucleic acid enclosed in a protein coat; it has no plasma membrane of its own, no protoplasm, no organelles, and it cannot generate energy or reproduce by itself. It multiplies only after entering a living host cell and using that cell’s machinery. Since a virus is neither made of cells nor able to arise from a pre-existing cell independently, it falls outside the theory — which is one reason biologists argue about whether viruses are truly alive.
A second, milder exception comes from organisms whose body is not divided into separate cells at all. In some fungi and algae, the growing filament contains many nuclei sharing one continuous mass of cytoplasm, with no cross walls separating them into individual cells. Such a structure is called coenocytic, and it does not fit neatly into the “one nucleus, one cell” picture.
Size and Shape of Cells
Most cells are far too small to see without help. A typical cell measures roughly 10 µm to 100 µm across, where one micrometre (µm) is one-thousandth of a millimetre. The smallest known living cells are those of PPLO (pleuropneumonia-like organisms), which measure only about 0.1–0.4 µm — small enough to pass through filters that hold back ordinary bacteria.
At the other extreme, size is limited less strictly than you might expect. The largest single cell is the egg of the ostrich, about 15 cm in outer diameter, most of it stored food. The longest animal cell is the nerve cell, which in a large animal may run close to a metre from one end to the other, while the longest plant cell is the sclerenchymatous fibre of the ramie plant, Boehmeria nivea, at about 55 cm.
Shape varies just as widely, and it follows function: a nerve cell is drawn out into a long thread because its job is to carry a signal over a distance, a red blood cell is a flattened disc because that shape gives it a large surface for exchanging gases, and a bacterial cell may be spherical, rod-shaped or spiral. Whenever you meet an unfamiliar cell, asking what its shape is for is usually more informative than memorising the shape itself.
Because almost all cells lie below the limit of unaided vision, everything in this chapter depends on instruments — the microscope above all, and the centrifuge and staining methods that go with it. Those techniques are taken up at the end of the chapter, in Section 4.11.
4.2 Prokaryotic and Eukaryotic Cells
Although all cells share the same basic job, they are not all built to the same plan. Biologists divide all cells into two great grades of organisation, and the dividing line is drawn on one feature above all others: whether the genetic material is shut away inside a membrane-bound nucleus, or lies free in the cytoplasm.
The Two Grades of Cellular Organisation
In a prokaryotic cell, there is no nuclear envelope, so the DNA is not enclosed in a true nucleus. Membrane-bound organelles such as mitochondria, chloroplasts, lysosomes and Golgi bodies are also absent. The word itself records this — pro meaning “before” and karyon meaning “nucleus”. Bacteria, blue-green algae (cyanobacteria) and mycoplasma are all prokaryotic.
In a eukaryotic cell, the DNA is enclosed within a double-membrane nuclear envelope to form a distinct nucleus, and the cytoplasm contains a range of membrane-bound organelles, each specialising in one kind of work. All algae, protists, fungi, plants and animals are made of eukaryotic cells.
This one structural difference has a consequence worth pausing on. In a prokaryote, the DNA sits directly in the cytoplasm, so a messenger RNA molecule can begin to be translated into protein while it is still being copied from the DNA. In a eukaryote, the nuclear envelope separates the two events in space: the RNA is made inside the nucleus and must be exported before a ribosome can use it. Compartmentalisation costs the cell a step, but it buys control — an idea that will return when you study gene expression in Class XII.
Flowchart
FC 4.2 – Classifying an unknown cell: does it have a nuclear envelope? → No → check for 70S ribosomes, circular DNA, absence of membrane-bound organelles → prokaryotic cell; → Yes → check for 80S ribosomes, linear DNA with histones, membrane-bound organelles → eukaryotic cell → then check for cell wall, plastids and a large central vacuole → plant cell if present, animal cell if absent
Differences Between Prokaryotic and Eukaryotic Cells
The full contrast runs well beyond the nucleus and is set out below. Read it as a set of linked consequences rather than a list to be memorised: a cell without a nuclear envelope also turns out to have a smaller, circular chromosome, a simpler ribosome and no internal membrane system.
An Overview of the Eukaryotic Cell
A typical plant cell or animal cell is eukaryotic, and both are organised on the same general plan: an outer plasma membrane enclosing the cytoplasm, a nucleus holding the genetic material, and a set of organelles distributed through the cytoplasm. The structures of an animal cell are shown in Fig. 4.3.
Diagram
Fig. 4.3 – A typical animal cell in section, labelled: cell membrane, cytoplasm, rough and smooth endoplasmic reticulum, ribosomes, Golgi bodies, lysosome, mitochondrion, centrosome, vacuole, nuclear membrane, nucleoplasm, nucleolus and chromatin
The plant cell shares all of these, but adds three features of its own — a rigid cell wall outside the plasma membrane, plastids including chloroplasts, and a single large central vacuole that pushes the cytoplasm and nucleus towards the edge of the cell. Compare Fig. 4.4 with Fig. 4.3 and notice how much of the plant cell’s interior volume the vacuole occupies.
Diagram
Fig. 4.4 – A typical plant cell in section, labelled: cell wall, plasma membrane, plasmodesmata, chloroplast, mitochondrion, rough and smooth endoplasmic reticulum, free ribosomes, Golgi complex, large central vacuole, cytoplasmic matrix, nucleus with nucleolus and nucleopore
Each of the components named in these two figures is examined in the sections that follow — the cell wall and plasma membrane in Section 4.4, the endomembrane system in Section 4.5, and the energy-related and other organelles in Section 4.6. Before that, Section 4.3 looks closely at the prokaryotic cell, using the bacterium as its example, because bacteria are the workhorses of biotechnology and much of what you will do later in this course begins with a bacterial cell.
4.3 Structure of Bacteria, Plasmids and Gram Staining
Bacteria are prokaryotic, unicellular organisms. They have no organised nucleus, but they do have a rigid cell wall built mainly of peptidoglycan (also called murein), a mesh-like polymer in which chains of sugar units are cross-linked by short chains of amino acids. It is this cross-linked mesh that gives the bacterial cell its shape and lets it survive without bursting when it takes in water.
Bacteria are classified by shape into four common forms: spherical (coccus), rod-shaped (bacillus), comma-shaped (vibrio) and spirally coiled (spirillum). Some species can occur in more than one of these forms. The general structure of a bacterial cell is shown in Fig. 4.5.
Diagram
Fig. 4.5 – A typical prokaryotic (bacterial) cell in section, labelled: cell wall, cytoplasmic membrane, glycocalyx, cytoplasm, nucleoid (DNA), ribosomes, plasmid, lipid body, volutin granule, gas vesicles, fimbriae, pilus, and flagellum showing filament, hook region and basal body
Diagram
Fig. 4.6 – The four common bacterial shapes drawn side by side and named: coccus, bacillus, vibrio, spirillum
Plasmids
Besides its single main chromosome, a bacterium very often carries one or more plasmids — small, circular, double-stranded DNA molecules that lie separately in the cytoplasm.
Two properties of plasmids matter for everything that follows. First, a plasmid replicates on its own, independently of the bacterial chromosome, so a cell can hold several copies of it. Second, the genes it carries are not the ones needed for basic survival but ones that give the cell an extra ability in a particular situation — most famously, resistance to an antibiotic.
The same two properties make the plasmid the biotechnologist’s favourite tool. A gene of interest can be inserted into a plasmid, the plasmid put back into a bacterium, and the bacterium then grown in bulk — every daughter cell carrying a copy of the foreign gene. A DNA molecule used to carry a foreign gene into a host cell in this way is called a vector, and you will meet named plasmid vectors such as pBR322 and pUC in Class XII.
Gram Positive and Gram Negative Bacteria
Bacteria are sorted into two large groups by the way they respond to a staining procedure: Gram positive (Gram +ve) bacteria keep the violet stain, while Gram negative (Gram −ve) bacteria lose it and take up the counterstain instead. The difference is not a matter of chemistry inside the cell but of the architecture of the wall around it.
The two groups differ in several linked ways, summarised below.
Notice how the last four rows follow from the first two. A thick, heavily cross-linked wall makes a Gram positive cell physically tough but leaves its peptidoglycan exposed to drugs and dyes; the outer membrane of a Gram negative cell is a weaker mechanical structure but acts as a barrier that many antibiotics, detergents and basic dyes cannot cross. This is why the Gram reaction reported by a laboratory is a genuinely useful piece of clinical information and not merely a label.
4.4 The Cell Wall and the Plasma Membrane
Every cell has to draw a line between itself and the outside world. In animal cells that line is a single structure, the plasma membrane. In plant cells, and in bacteria and fungi, there is a second, tougher layer outside it — the cell wall. The two are easy to confuse but do quite different jobs, and the difference is worth fixing firmly before going further.
The Cell Wall
The presence of a cell wall is a characteristic feature of plant cells. It is not laid down from outside; it is always formed by the activity of the protoplast, the living contents of the cell itself. Where two plant cells lie side by side, their walls are cemented together by a layer called the middle lamella, made of calcium or magnesium pectate. The wall of a mature plant cell is therefore described in three layers — the middle lamella between neighbouring cells, and then the primary and secondary walls belonging to each cell, as shown in Fig. 4.7.
Diagram
Fig. 4.7 – Layers of the plant cell wall between two adjacent cells, labelled from outside inwards: middle lamella, primary cell wall, secondary cell wall, plasma membrane and protoplast, with a plasmodesma passing through
Primary cell wall. This is the outermost layer of the cell’s own wall, present on both sides of the middle lamella. It is usually thin (1–3 µm) and elastic, so it can stretch as the cell grows. Its chief constituents are cellulose and hemicellulose, with some pectin often present as well. In thin-walled cells — meristematic cells, parenchyma, collenchyma and mesophyll cells — the primary wall remains the only wall the cell ever has, which is precisely what allows these cells to keep enlarging or dividing.
Secondary cell wall. This is much thicker (5–10 µm), rigid and inelastic. It is formed only after the primary wall has stopped increasing in surface area — that is, once the cell has finished growing — and it lies between the primary wall and the protoplast. It may be further thickened by the deposition of substances such as cutin, suberin, lignin and pectin, each of which hardens or waterproofs the wall in a different way.
The functions of the cell wall are largely mechanical. It acts as a skeleton for the plant, providing rigidity, strength and a degree of flexibility; it maintains the shape of individual cells and of the tissues they build; and it protects the protoplasm against mechanical injury. It is also freely permeable — water and dissolved substances pass through it without being selected — so it is a support structure, not a gatekeeper. The selecting is done by the membrane just inside it.
Protoplasm and Cytoplasm
The living material inside the cell is called protoplasm. The term was coined independently by Johannes E. Purkinje (1839) and Hugo von Mohl (1846), and T.H. Huxley described protoplasm as the physical basis of life, since life cannot exist apart from it. Protoplasm is colloidal in nature — a fluid in which large molecules and particles remain dispersed rather than dissolved or settled.
The part of the protoplasm lying between the plasma membrane and the nucleus is the cytoplasm. It is composed of two distinct kinds of material: a continuous fluid-like substance called the cytosol, and a number of organelles suspended in it, each with a definite function. Everything described in Sections 4.5 and 4.6 sits within this cytoplasm.
The Plasma Membrane
The protoplasm of every living cell is enclosed by a living membrane called the cell membrane, plasma membrane or plasmalemma. Several models have been proposed to explain its structure, and the one that is accepted today is the fluid mosaic model.
The fluid mosaic model was proposed by Singer and Nicolson in 1972. According to this model, the membrane consists of a continuous bilayer of phospholipid molecules with globular proteins embedded in it. The membrane is therefore regarded as a semifluid structure in which both the lipids and the intrinsic proteins can move sideways within the bilayer — the “fluid” part of the name — while the scattered, irregular arrangement of proteins across the lipid sheet gives the “mosaic”. Study Fig. 4.8 and note that the proteins are not arranged in any regular pattern and do not all sit at the same depth.
Diagram
Fig. 4.8 – Fluid mosaic model of the cell membrane: bimolecular phospholipid layer with hydrophilic heads outward and hydrophobic tails inward, labelled with peripheral (extrinsic) protein, integral (intrinsic) protein, a pore formed by an integral protein, network proteins and sugar chains of a glycoprotein
The fluidity is not a detail — it is what makes the membrane work. A rigid sheet could not fold inwards to engulf a food particle, could not fuse with a vesicle arriving from the Golgi apparatus, and could not reseal itself after a puncture. A semifluid sheet can do all three.
Proteins and Carbohydrates of the Membrane
The proteins of the membrane are of two types, distinguished by how deeply they sit in the bilayer.
Some carbohydrates are covalently linked to the polar heads of phospholipids or to proteins, forming glycolipids and glycoproteins. On the animal-cell surface these carbohydrate chains together form the glycocalyx, which is helpful in cell adhesion and cell recognition — it is, in effect, the chemical identity card a cell shows to its neighbours.
Functions of the Plasma Membrane
- It forms the limiting boundary of the cell, separating the protoplasm from the surroundings.
- Being selectively permeable, it allows only useful substances to enter the cell and thus maintains the homeostasis of the cell — its steady internal condition.
- Through its receptors it helps in binding hormones, drugs, neurotransmitters, growth factors and similar signalling molecules.
- Its glycocalyx helps in cell recognition, cell adhesion and the exchange of materials or information between cells.
- It helps in bulk transport by phagocytosis, pinocytosis and exocytosis — processes in which the membrane itself folds inwards or outwards to move material in bulk across the boundary.
Selective permeability is the property to hold on to. Because an intact membrane admits only what the cell allows, a cell that has died and lost membrane integrity will let stray dyes flood in, while a living one will keep them out. That simple difference is the basis of the cell viability tests used in tissue culture laboratories, which you will meet later in the course.
4.5 The Endomembrane System
Some of the organelles inside a eukaryotic cell are not really separate departments at all. The endoplasmic reticulum, the Golgi complex, lysosomes and vacuoles are built of membrane, exchange material with one another through small membrane sacs called vesicles, and work as stages of a single coordinated pathway. Because of this, they are grouped together as the endomembrane system.
The best way to hold the system in mind is to follow one protein through it, from the place where it is made to the place where it is finally used. That is the sequence set out in FC 4.3, and the rest of this section explains each stage of it.
Flowchart
FC 4.3 – Path of a secretory protein through the endomembrane system: ribosome on RER synthesises protein → protein enters the lumen of the RER → transport vesicle buds off → vesicle fuses with the cis (forming) face of the Golgi apparatus → protein is modified in the cisternae → sorted at the trans (maturing) face → branch 1: secretory vesicle fuses with plasma membrane and releases the protein by exocytosis; branch 2: vesicle retained as a primary lysosome inside the cell
Endoplasmic Reticulum (ER)
The endoplasmic reticulum is a network of tiny tubular structures scattered through the cytoplasm. Its membranes divide the intracellular space into two compartments: the luminal space inside the ER, and the extra luminal space, which is the cytoplasm outside it. Keeping these two compartments apart is what allows a protein to be handled inside the ER without interfering with the reactions going on in the cytoplasm.
Endoplasmic reticulum is of two types, shown together in Fig. 4.9.
Diagram
Fig. 4.9 – Endoplasmic reticulum: granular (rough) endoplasmic reticulum with ribosomes on its surface and agranular (smooth) endoplasmic reticulum without them, labelled with tubules, cisternae and vesicles
Rough endoplasmic reticulum (RER) bears ribosomes on its surface, which is what makes it look rough under the electron microscope. RER is frequently observed in cells that are actively involved in protein synthesis and secretion, and its membranes extend up to and join the outer membrane of the nucleus — a direct physical link between the place where the genetic instructions are stored and the place where they are carried out.
Smooth endoplasmic reticulum (SER) has no ribosomes. It is the major site for the synthesis of lipids, and in animal cells the lipid-like steroidal hormones are synthesised in the SER.
Functions of the Endoplasmic Reticulum
- It provides mechanical support for the colloidal structure of the cytoplasm.
- It helps in the exchange of materials between the nucleus and the cytoplasm.
- It may help in the intracellular circulation of various substances.
- The RER provides a site for protein synthesis by attaching ribosomes to itself.
- The SER helps in the synthesis and storage of lipids, cholesterol and glycogen.
Golgi Apparatus
The Golgi apparatus consists of densely stained reticular structures lying near the nucleus. They are made of flat, disc-shaped sacs called cisternae, each 0.5–1.0 µm in diameter, stacked parallel to one another and arranged concentrically. The stack has two ends: a convex cis or forming face, which receives vesicles arriving from the ER, and a concave trans or maturing face, from which finished vesicles depart. The two faces are totally different in composition, yet the stack between them is interconnected — material entering at one end works its way to the other. In plants, the Golgi apparatus is known by a separate name, the dictyosome.
Diagram
Fig. 4.10 – Golgi apparatus, labelled: stacked cisternae, membrane of the Golgi complex, tubule, convex cis (forming) face receiving vesicles from the ER, concave trans (maturing) face, and secretory vesicles budding off
Functions of the Golgi Apparatus
- It secretes materials to intracellular targets or to the outside of the cell.
- Proteins synthesised by ribosomes on the ER are modified in the cisternae of the Golgi apparatus before they are released from its trans face.
- It forms glycoproteins and glycolipids.
- It forms the cell plate during cytokinesis in plant cells.
- Primary lysosomes are formed by the Golgi complex.
The word modified in the second function is doing a great deal of work, and it is worth unpacking. A protein leaving the ER is functional in outline but not finished; in the Golgi cisternae, sugar chains are attached or trimmed and the molecule is sorted according to where it must go. The Golgi apparatus is therefore best understood as the cell’s despatch department — it does the final packaging, labelling and routing.
Lysosomes
Lysosomes are membrane-bound vesicular structures formed by the process of packaging in the Golgi apparatus. Lysosomal vesicles contain almost all types of hydrolytic enzymes — hydrolases such as lipases, proteases and carbohydrases — which are active at acidic pH. They digest carbohydrates, proteins, lipids and nucleic acids, and are thus involved in intracellular digestion.
Two design features of the lysosome deserve attention. First, its enzymes work best in acid conditions, while the surrounding cytoplasm is close to neutral; so even if a few enzymes leak out, they are largely inactive there. Second, those enzymes are kept behind a membrane. If that membrane ruptures, the enzymes are released and digest the cell’s own contents — which is why lysosomes are sometimes called the suicidal bags of the cell. Controlled self-digestion of this kind is not always a disaster; it is how a cell disposes of worn-out organelles and how unwanted tissue is removed during development.
Vacuoles
Vacuoles are membrane-bound spaces found in the cytoplasm. A vacuole contains water, sap, excretory products and other materials that are not useful for the cell, and it is bounded by a single membrane called the tonoplast.
In plant cells, vacuoles can occupy up to 90% of the volume of the cell. The tonoplast facilitates the transport of ions and other materials into the vacuole against concentration gradients, so their concentration inside the vacuole becomes higher than in the cytoplasm. This is what allows a plant cell to accumulate solutes, draw in water by osmosis and stay turgid — the pressure discussed in Section 4.4 comes ultimately from what the tonoplast pumps in.
4.6 Mitochondria, Plastids, Ribosomes and Other Cell Inclusions
The organelles of the endomembrane system handle the making and moving of materials. The organelles in this section do something different — they supply the cell with energy, and they build its proteins. Between them they explain how a cell pays for everything else it does.
Mitochondria
Mitochondria are clearly visible only when stained, which is why they were among the later organelles to be recognised. Their number, shape and size per cell are variable and depend on the physiological activity of the cell. A mitochondrion is usually cylindrical, with a diameter of 0.2–1.0 µm (average 0.5 µm) and a length of 1.0–4.1 µm.
A mitochondrion is a double membrane-bound structure, with an outer membrane and an inner membrane. The inner membrane forms a number of infoldings called cristae projecting towards the matrix, and these infoldings increase the surface area available inside the organelle. The matrix — the fluid enclosed by the inner membrane — possesses a circular DNA molecule, a few RNA molecules, ribosomes of the 70S type and the components needed for protein synthesis. In other words, a mitochondrion carries a small amount of its own genetic information and can make some of its own proteins.
Diagram
Fig. 4.11 – A mitochondrion in section, labelled: outer membrane, inner membrane, cristae, inner membrane spheres (oxysomes), matrix space, circular DNA, mitoribosomes, granule, perimitochondrial space, and the C face and M face of the cristae membrane
Function: Mitochondria are the sites of aerobic respiration. They produce energy in the form of ATP, and for this reason they are also known as the power houses of the cell.
Plastids
Plastids are found in all plant cells and in other eukaryotic autotrophs. They are large organelles containing pigments, and on the basis of the type of pigment they are of three kinds.
1. Chloroplasts. These are double membrane-bound organelles found mainly in the mesophyll cells of leaves. They vary in length from 5–10 µm and in width from 2–4 µm. The inner membrane of the chloroplast is less permeable than the outer one, and the space it encloses is called the stroma. Within the stroma are many organised membranous sacs called thylakoids, which contain the pigment chlorophyll. Thylakoids are arranged in stacks called grana, and the grana are connected to one another by membranous bridges called fret. The stroma itself contains small double-stranded circular DNA molecules, ribosomes and the enzymes for the synthesis of carbohydrates and proteins. The ribosomes of the chloroplast are smaller (70S) than the cytoplasmic ribosomes (80S).
Diagram
Fig. 4.12 – A chloroplast in section, labelled: outer membrane, inner membrane, stroma, thylakoid, granum (stack of thylakoids), fret connecting the grana, circular DNA and ribosomes
2. Chromoplasts. These contain fat-soluble carotenoid pigments such as carotene and xanthophylls, which give a yellow, orange or red colour — the colours of ripe fruits and many flowers.
3. Leucoplasts. These are colourless plastids of varied shapes and sizes that store nutrients. They include:
- Amyloplasts — store starch, e.g., in potato.
- Elaioplasts — store oils and fats.
- Aleuroplasts — store proteins.
Ribosomes
Ribosomes are non-membranous granular structures composed of ribonucleic acid (RNA) and proteins. They were first observed by George Palade in the mid-1950s. A eukaryotic ribosome has two subunits — the large 60S subunit and the small 40S subunit — which together form the 80S ribosome. The two subunits stay apart until protein synthesis begins and come together on the messenger RNA, so a ribosome is best pictured as a machine that assembles itself around its work.
Ribosomes synthesise proteins for the cell. During protein synthesis, many ribosomes become aggregated linearly along a common mRNA to form a polyribosome, also called a polysome or ergosome. Because several ribosomes read the same mRNA one after another, many copies of the protein can be produced from a single message. Polysome formation also protects the mRNA from ribonucleases, the enzymes that would otherwise break it down.
Diagram
Fig. 4.13 – (a) A eukaryotic ribosome showing the 40S small subunit and the 60S large subunit combining to form the 80S ribosome; (b) a polyribosome — several ribosomes attached along one mRNA strand, each carrying a growing polypeptide chain
Centrosome and Centrioles
The centrosome is an organelle usually containing two non-membrane-bound cylindrical structures called centrioles, which lie perpendicular to each other. Each centriole is made up of nine evenly spaced peripheral triplets of microtubules, composed of the protein tubulin.
Centrioles help in initiating cell division in animal cells, and the asters formed by the centrosomes give rise to the spindle fibres that draw chromosomes apart. Their full importance therefore appears in the next chapter, where cell division is taken up in detail.
Other Cell Inclusions
Besides the organelles described so far, the cytoplasm holds several structures grouped loosely as cell inclusions.
The cytoskeleton is a network of protein filaments running through the cytoplasm — chiefly microtubules made of tubulin, along with finer microfilaments. It maintains the shape of the cell, holds organelles in position and provides the tracks along which vesicles are moved from one part of the cell to another. A cell whose cytoskeleton is disrupted loses both its shape and its ability to transport material internally.
Cilia and flagella are hair-like projections from the cell surface that bring about movement — either of the cell itself, as in a sperm cell, or of fluid across the cell surface, as in the lining of the respiratory tract. In eukaryotic cells they are built on the 9 + 2 arrangement of microtubules described in Section 4.2.
Finally, cells contain non-living inclusions stored in the cytoplasm — starch grains, glycogen granules, oil droplets, protein granules and crystals. In bacterial cells these include the volutin granules, lipid bodies and gas vesicles labelled earlier in Fig. 4.5. Such materials are reserves or waste rather than machinery: they take no part in the cell’s activities, but their presence tells you a good deal about what the cell has been doing.
4.7 The Nucleus
Of all the structures in a eukaryotic cell, the nucleus is the one the syllabus asks you to know best, and for good reason: it holds the genetic material. Every technique you will study later — cloning a gene, sequencing DNA, transforming a plant — is ultimately an attempt to reach, read or rewrite what is kept here.
The nucleus was discovered by Robert Brown in 1833, and it regulates the overall activities of an individual cell. Although a nucleus is present in almost all kinds of cells, a well-organised nucleus is generally absent in bacteria and blue-green algae, which are prokaryotic. It may also be absent from certain eukaryotic cells that have given it up during maturation — mature red blood cells and the sieve tube cells of phloem are the standard examples. In both, the space and simplicity gained are worth the loss: an RBC packs in more haemoglobin, and a sieve tube offers less obstruction to the flow of sap.
Structure of the Nucleus
The nucleus is made up of four structures, all visible in Fig. 4.13.
Diagram
Fig. 4.13 – A nucleus in section, labelled: outer membrane, inner membrane, perinuclear space, nuclear pore, nuclear envelope, nucleoplasm, nucleolus, euchromatin, heterochromatin, perinucleolar chromatin, intranucleolar chromatin, and the endoplasmic reticulum with ribosomes continuous with the outer membrane
1. Nuclear envelope. The nucleus is externally surrounded by a nuclear envelope, also called the nuclear membrane. It is composed of an outer and an inner membrane, separated by a perinuclear space. The envelope is perforated by many circular or octagonal openings called nuclear pores. These pores are the only route in and out: materials such as RNA and proteins pass through them, and everything else is held back. Because the outer membrane is continuous with the endoplasmic reticulum, the nucleus is physically joined to the transport system of the cytoplasm described in Section 4.5.
2. Nucleoplasm. The nucleus is filled with a transparent, semi-solid, jelly-like granular and acidophilic ground substance or matrix known as nucleoplasm, also called karyoplasm or nuclear sap. The chromatin fibres and the nucleolus are suspended in it.
3. Nucleolus. The nucleolus is attached to a specific region of a particular chromosome, and this region is called the nucleolar organiser or secondary constriction. The nucleolus is not membrane-bound; it is a dense working area in which ribosomal RNA is made and ribosome subunits are assembled before being exported to the cytoplasm. This explains an observation you will meet in questions: a cell with several prominent nucleoli is almost always a cell that is manufacturing protein heavily, because it must first manufacture the ribosomes to do it with.
4. Chromatin fibres. These thread-like structures can be seen only during interphase, the period between two divisions. During cell division these threads become thick and condensed, and are then called chromosomes. On the basis of their staining properties, two types of chromatin material have been identified — heterochromatin and euchromatin.
Heterochromatin consists of the dark-staining, tightly condensed regions of chromatin. Because the DNA in these regions is packed so densely, the machinery that reads genes cannot easily reach it, and heterochromatin is therefore described as genetically and metabolically inactive.
Euchromatin is the light-staining, diffused region of chromatin. Here the DNA is loosely packed and accessible, so euchromatin contains most of the genes that are actually being expressed and is genetically as well as metabolically active.
Functions of the Nucleus
The nucleus acts as the brain of the cell. It performs the following important functions:
- It co-ordinates the activities of different parts of the cell.
- The DNA of the nucleus regulates protein synthesis by transcription.
- It transfers parental characters to the offspring through chromosomes.
- It determines the sex of the offspring.
- It regulates growth and reproduction.
The chromatin fibres introduced here are the raw material of the chromosome. What that material is made of, and how a length of DNA nearly two metres long is folded to fit inside a nucleus a few micrometres wide, is the subject of the next section.
4.8 Chromosome Structure and Chromatin Organisation
All living organisms have specific characteristics that they transmit to their offspring through successive generations. These characteristics are identified as hereditary traits, and they are controlled by special units called genes, which are borne on the chromosomes. Chromosomes are therefore defined as self-duplicating nuclear filaments having a specific organisation and individuality.
The name arrived after the object. Walther Flemming, working in the early 1880s, saw deeply stainable thread-like material in the nucleus and called it chromatin, following its behaviour through cell division. In 1888 Waldeyer gave these threads the name chromosomes. A little later, Sutton and Boveri suggested — and it was subsequently proved experimentally — that chromosomes are the physical carriers of hereditary characters. The reason their proposal was convincing is worth noting: chromosomes were observed to pair, separate and be distributed to daughter cells in exactly the pattern that inherited characters were known to follow.
Morphology of Chromosomes
The size of chromosomes varies from species to species but is generally specific for a particular species. Chromosome size is normally measured at mitotic prophase, when the threads have condensed enough to be seen clearly but have not yet been pulled apart. Plant chromosomes are usually larger than animal chromosomes, and likewise the chromosomes of monocots are larger than those of dicots.
Chromosomal Organisation in Prokaryotes
In prokaryotes such as bacteria, the genetic material is not enclosed within a true nucleus. Instead, the chromosome lies in a region of the cytoplasm called the nucleoid. The prokaryotic chromosome is typically a single, circular, double-stranded DNA molecule. It is not associated with histone proteins as in eukaryotes (except in some archaea), but it is compacted with the help of nucleoid-associated proteins, and the DNA is highly folded and supercoiled so that it fits inside the small bacterial cell.
In addition to the main chromosome, many bacteria possess the plasmids described in Section 4.3. Since prokaryotes lack a nuclear membrane, their chromosome lies freely in the cytoplasm, and replication and transcription occur directly in this region.
Chromosomal Organisation in Eukaryotes
The major constituents of eukaryotic chromosomes are DNA, RNA, histone and non-histone proteins, and metal ions. The chromosome carries the genetic information from one generation to the next. RNA is transcribed from DNA and most of it is transported to the cytoplasm. The histone proteins present in chromosomes are basic proteins, composed largely of basic amino acids such as lysine and arginine. Being basic, they carry a positive charge, and the DNA they bind carries a negative charge along its sugar–phosphate backbone — so the two attract each other strongly, and that attraction is what holds the packing system together.
The Nucleosome
The basic unit of a eukaryotic chromosome, made of DNA and histones, is called the nucleosome. It consists of a core made of eight histone proteins, called a histone octamer, which includes two molecules each of H2A, H2B, H3 and H4. Around this histone core, about 146 base pairs of DNA are tightly wrapped in approximately 1.65 turns. The nucleosomes are connected to one another by a short stretch of linker DNA, and another histone protein called H1, the linker histone, helps to hold the DNA in place and assists in further compaction of the chromatin. Under the electron microscope, chromatin appears like beads on a string, where each bead represents a nucleosome.
Diagram
Fig. 4.14 – Levels of DNA packing: DNA double helix → DNA wound around a histone octamer to form a nucleosome, with linker DNA and H1 marked → the “beads on a string” chromatin fibre → the coiled and looped chromatin fibre → the fully condensed metaphase chromosome, drawn to show the increase in thickness at each stage
Flowchart
FC 4.4 – Levels of chromatin organisation: DNA double helix → wrapped around histone octamer (146 bp, ~1.65 turns) → nucleosome → nucleosomes joined by linker DNA → “beads on a string” chromatin fibre → compacted further with H1 histone → coiled and looped chromatin → condensed chromosome visible at metaphase
Structure of a Condensed Chromosome
During interphase, the stained chromosome appears as a thin and coiled filament composed of chromatin. These fibres get condensed during cell division, and only then can the following parts be distinguished, as labelled in Fig. 4.15.
Diagram
Fig. 4.15 – A typical condensed chromosome, labelled: chromatid, primary constriction, centromere, kinetochore, secondary constriction I, secondary constriction II, satellite, telomere and nucleolus; alongside, a coiled-thread view of the same chromosome
1. Primary constriction. Each chromosome has a non-stainable region at a specific point along its length, and this region is called the primary constriction.
2. Centromere. Within the primary constriction there is a clear central zone called the centromere. This is the point of attachment of the sister chromatids, and also the site of attachment of the mitotic spindle fibre through a protein structure called the kinetochore. The portion of the chromosome on either side of the centromere is called an arm. Functionally, the centromere is related to the movement of chromosomes at anaphase, and during this movement the chromosome takes on the shape of I, J, L or V depending on the relative ratio of its two arms.
3. Secondary constriction. This is also called the nucleolar organiser — the region to which the nucleolus described in Section 4.7 remains attached. The part of the chromosome lying beyond the secondary constriction is called the satellite, and chromosomes having a satellite are called SAT chromosomes.
4. Telomere. The tips of the chromosomes, containing heterochromatic material or repetitive DNA sequences, are called telomeres. Each telomere has a definite polarity. It does not allow other chromosomes to stick to it, nor does it allow union with broken ends — so a telomere is what tells the cell that a chromosome end is a proper end and not a break needing repair.
Sister chromatids, mentioned above, are the two identical copies of a chromosome produced by replication and held together at the centromere; they separate at anaphase. Chromatids belonging to two different members of a homologous pair are called non-sister chromatids, and the distinction matters because it is non-sister chromatids that exchange segments during crossing over. Homologous chromosomes themselves are taken up in the next section.
4.9 Types, Number and Functions of Chromosomes
Chromosomes within a single nucleus are not a random collection. They occur in matched pairs, they can be sorted into definite types by the position of the centromere, and their total number is fixed for a species. Each of these three facts is used later in the course — in inheritance, in karyotyping, and in the diagnosis of chromosomal disorders.
Homologous and Non-Homologous Chromosomes
Homologous chromosomes are chromosomes that have the same size, the same shape, the same position of the centromere, and genes for the same characters arranged at the same positions, or loci, along their length. They occur as pairs, and the two members of a pair come from different parents: for the first pair of chromosomes in an individual, one member has been passed on by the mother — the maternal chromosome — while the other comes from the father, the paternal chromosome.
The two members of a homologous pair carry genes for the same characters, but not necessarily the same versions of those genes. One member may carry the form of a gene for a rounded seed and the other the form for a wrinkled seed at exactly the same locus. This is the whole basis of inheritance as Mendel described it, and it is why the word homologous means “corresponding”, not “identical”.
Non-homologous chromosomes belong to different pairs. They differ in size, in centromere position, or in both, and they carry genes for entirely different characters.
Classification of Chromosomes According to the Position of the Centromere
According to the position of the centromere, the following types of chromosomes are identified. Their shapes at anaphase, when the chromosome is being pulled by its centromere, are drawn in Fig. 4.16.
- Telocentric: the centromere is situated at one end. At anaphase such a chromosome looks like the letter I.
- Acrocentric: the centromere lies almost near the tip of the chromosome, so that one arm is exceptionally short and the other is long. At anaphase these chromosomes look like the letter J.
- Submetacentric: the centromere lies a little away from the centre, dividing the chromosome into two unequal arms. Such a chromosome looks like the letter L at anaphase.
- Metacentric: the centromere lies in the middle of the chromosome, dividing it into two equal arms. A metacentric chromosome becomes V-shaped during anaphase.
Diagram
Fig. 4.16 – The four chromosome types drawn side by side at anaphase and named: metacentric, submetacentric, acrocentric, telocentric, with the centromere position and the resulting V, L, J and I shapes marked on each
An acentric chromosome — one without a centromere — does not exist as a stable structure. If such a fragment is formed by an error during cell division, it has no point of attachment for the spindle fibre, cannot be pulled to either pole, and disintegrates almost immediately.
Chromosome Number in Different Species
The number of chromosomes is constant for a species and is one of its defining features. Body cells are usually diploid, written 2n, meaning they carry two complete sets — one from each parent — while gametes are haploid, written n, carrying a single set. The number itself has nothing to do with how complex an organism is, as the table makes plain.
| Species | Chromosome number |
|---|---|
| Man (Homo sapiens) | 2n = 23 pairs (46) |
| Mouse (Mus musculus) | 2n = 20 pairs (40) |
| Fruit fly (Drosophila melanogaster) | 2n = 4 pairs (8) |
| Pea (Pisum sativum) | 2n = 7 pairs (14) |
In human beings, the 23 pairs are made up of 22 pairs of autosomes, which are alike in both sexes, and one pair of sex chromosomes, which differ — XX in the female and XY in the male. It is this last pair that answers the question of how the nucleus determines the sex of the offspring, listed among the functions of the nucleus in Section 4.7.
Functions of Chromosomes
Chromosomes are the most vital component of the cell. They control almost all cellular activities at the physiological, molecular and morphological levels. Besides this, they perform the following main functions:
- They maintain the identity of the species, since chromosome number and form are characteristic of each species.
- They determine the sex of animals and plants.
- They act as the vehicle carrying hereditary characters from one generation to the next.
- With the help of their chemical constituents, DNA and RNA, they direct the synthesis of proteins and enzymes.
Being able to count chromosomes and sort them by type is not merely descriptive work. Arranged systematically, they form a karyotype — a picture in which an extra chromosome, a missing one or a structurally altered one becomes immediately visible. Before that arrangement can be made, however, the individual chromosomes have to be told apart reliably, and the technique that makes this possible is the subject of the next section.
4.10 Chromosome Banding Patterns and Their Applications
Under an ordinary stain, a metaphase chromosome is a uniformly dark rod. Two chromosomes of similar size and similar centromere position then look alike, and there is no reliable way to say which is which. Banding techniques solve exactly this problem: they treat the chromosome so that it takes up stain unevenly along its length, producing a pattern of dark and light cross-bands.
That pattern is the point. It is constant for a given chromosome and reproducible from cell to cell, so it works as a barcode — every chromosome in a set can be identified individually by its own banding pattern, as shown in Fig. 4.17.
Diagram
Fig. 4.17 – A banded metaphase chromosome pair: one chromosome shown plain and the same chromosome shown after banding, with alternating dark and light cross-bands, the centromere marked, and the short arm and long arm labelled
How Banding Works
During cell division the heterochromatin condenses, and bands are formed that contain a complex of protein and DNA. The regions lying between the dark bands contain the active genes. Different pre-treatments and different stains bring out different parts of this pattern, and the four types used are named after the stain or the treatment involved.
The Four Types of Banding
Q-bands are produced by the quinacrine mustard stain, which is a fluorescent dye. The bands appear as bright and dull regions under a fluorescence microscope, and the bright regions correspond to A–T rich stretches of DNA.
G-bands are produced by Giemsa stain. The pattern obtained closely matches the Q-band pattern, and G-banding is the method most often used in routine work because it needs only an ordinary light microscope rather than a fluorescence one. G-bands are not obtained satisfactorily in plant chromosomes.
R-bands are produced by a reverse Giemsa method and give a pattern that is the reverse of the G-band pattern: the regions that stay pale after G-banding stain dark here. R-banding is therefore useful for examining the ends of chromosomes, which are often faint in a G-banded preparation.
C-bands are produced by staining with Giemsa after incubating the chromosomes in strong alkali. This treatment removes most of the chromosomal material but leaves the tightly packed heterochromatin near the centromere, so C-banding stains the centromeric regions specifically.
Significance of Chromosomal Banding
- It is useful for assigning linkage groups to specific chromosomes.
- It is useful in chromosome mapping.
- Individual autosomes can be identified by this technique, which is otherwise difficult when several of them are of similar size.
- Any kind of chromosomal aberration — a missing segment, a duplicated one or a segment moved to another chromosome — can be easily identified and studied, because the aberration shows up as a break in the expected band pattern.
- Evolutionary relationships can be identified by chromosomal banding; for example, the banding patterns of human chromosomes and those of apes show marked similarities.
- The technique helps in distinguishing different genera or species even when they show a similar karyotype structure. If fluorochrome dyes are used, the resolution of banding can be enhanced further.
- The bands in animal chromosomes show better resolution than those in plant chromosomes.
- A consistent banding pattern of heterochromatin can be observed even where there is variability in intraspecific karyotyping.
Banding is what turns a photograph of scattered chromosomes into a readable document. In practice it is used together with karyotyping, which arranges the banded chromosomes in a standard order so that the whole set can be examined at a glance — and that, along with the other instruments of the cytology laboratory, is taken up in the next section.
4.11 Techniques in Cytology
Everything described in this chapter — the cristae of a mitochondrion, the beads-on-a-string appearance of chromatin, the banding pattern of a chromosome — is invisible to the unaided eye. Cytology, the study of cells, has therefore always advanced with its instruments. Three of these instruments matter most at this stage: the microscope, which lets us see the cell; karyotyping, which lets us read its chromosomes; and the centrifuge, which lets us take the cell apart and study its components separately.
Microscopy
A microscope does two distinct things, and it is important not to confuse them. Magnification is how much larger the image appears than the object. Resolution is the smallest distance between two points at which they can still be seen as two points rather than one blur. Magnification without resolution is useless — enlarging a blurred image only produces a larger blur — so it is resolution that sets the real limit of any microscope.
The Light Microscope
Principle. A light microscope uses a beam of visible light passed through a thin specimen and focused by a series of glass lenses. The objective lens forms a magnified image, which the eyepiece magnifies further. The total magnification is simply the product of the two:
\[\text{Total magnification} = \text{magnification of objective} \times \text{magnification of eyepiece}\]
Because the resolving power of any lens system depends on the wavelength of the radiation used, and visible light has a wavelength of a few hundred nanometres, a light microscope cannot resolve two points closer together than about 0.2 µm. In practice this allows useful magnification up to roughly 1000–1500 times.
Applications. The light microscope is used to examine whole cells and tissues, to observe living material, and to study stained preparations. Nearly all the practical work of this course depends on it — identifying the stages of mitosis in an onion root tip, distinguishing Gram positive from Gram negative bacteria, examining a blood smear stained with Leishmann’s stain, and counting cells on a haemocytometer.
The Electron Microscope
Principle. An electron microscope uses a beam of electrons instead of light, and electromagnetic lenses instead of glass ones. Electrons behave as waves of far shorter wavelength than visible light, and since resolution improves as wavelength decreases, the electron microscope resolves detail hundreds of times finer — down to a fraction of a nanometre — with useful magnifications of several hundred thousand times. Because electrons are absorbed by air, the whole instrument must be kept under vacuum, and the specimen must be dried, fixed and made electron-dense with heavy metal stains. This is the instrument’s one great limitation: living material cannot be examined.
Two forms are in general use. The transmission electron microscope passes electrons through an ultra-thin section and shows internal structure. The scanning electron microscope sweeps the beam across a surface and gives a three-dimensional view of the exterior.
Applications. The electron microscope revealed the ultrastructure of the cell — the double membrane and cristae of mitochondria, the stacked thylakoids of the chloroplast, the cisternae of the Golgi apparatus, the nucleosome, and the structure of viruses, which lie far below the limit of light microscopy.
Karyotyping
Principle. Chromosomes can be identified individually by their size, the position of their centromere and their banding pattern. If a cell is arrested at metaphase, when its chromosomes are most condensed, the chromosomes can be spread out, stained, photographed, and then arranged in a standard order — in decreasing order of size, with homologous chromosomes placed together as pairs. The systematic arrangement so obtained is the karyotype.
Diagram
Fig. 4.18 – A human karyotype: banded metaphase chromosomes arranged in 22 numbered pairs of autosomes in decreasing order of size, followed by the sex chromosome pair, each pair aligned at the centromere
Applications. Karyotyping establishes the normal chromosome number and form for a species and makes any departure from it visible at once — an extra chromosome, a missing one, or a segment that has been lost, duplicated or moved. It is used in clinical and prenatal diagnosis of chromosomal disorders, in determining sex from a chromosome preparation, and in comparing the chromosome sets of related species to study their evolutionary relationships. In the ISC practical course you will prepare a karyotype from a metaphase plate of mitosis, which is the same procedure carried out on a smaller scale.
Centrifugation
Principle. When a suspension is spun rapidly, the particles in it experience a centrifugal force directed outwards, many times stronger than gravity. Particles sediment towards the bottom of the tube at a rate that depends on their size, their shape and how much denser they are than the surrounding fluid. Larger and denser particles therefore settle first, and if the speed is increased in stages, the components of a cell homogenate can be brought down one group at a time. Separating cell components in this way is called differential centrifugation, and the sediment collected at each stage is the pellet, while the liquid above it is the supernatant.
Flowchart
FC 4.5 – Differential centrifugation of a cell homogenate: homogenise cells in buffer → spin at low speed → pellet of nuclei and unbroken cells, keep supernatant → spin at medium speed → pellet of mitochondria and chloroplasts, keep supernatant → spin at high speed → pellet of membrane fragments → spin at very high speed → pellet of ribosomes, leaving soluble cytosol in the supernatant
Diagram
Fig. 4.19 – A tube before and after centrifugation, showing the uniform suspension on the left and, on the right, the pellet at the bottom with the supernatant above it, with the direction of the centrifugal force marked
Applications. Centrifugation is used to separate cell organelles for study — a technique known as cell fractionation, which is how the enzymes of respiration were first shown to belong to the mitochondrion. It is also used to sediment cells from a culture, to separate the components of blood, to recover DNA after precipitation with ethanol, and to purify proteins. You will use a table-top centrifuge repeatedly in the practical course; when you do, remember that its tubes must always be balanced in opposite pairs, since an unbalanced rotor spinning at high speed is dangerous.
4.12 Summary
The cell is the smallest unit that can carry out all the activities of life. Robert Hooke (1665) first saw and named cells in cork, Anton van Leeuwenhoek first saw living cells, Schleiden and Schwann (1838–39) proposed that all plants and animals are made of cells, and Rudolf Virchow (1855) added that every cell arises from a pre-existing cell — Omnis cellula e cellula.
Cell theory has three postulates: all living organisms are composed of one or more cells; the cell is the basic structural and functional unit of life; and all cells arise from pre-existing cells.
The cell qualifies as the basic unit of life on five grounds — all organisms are made of cells, the cell is the structural unit, the cell is the functional unit, new cells arise only from existing cells, and cells carry the genetic material.
Viruses are the clearest exception to cell theory, since they have no protoplasm or organelles and multiply only inside a host cell. Coenocytic fungi and algae, whose many nuclei share one continuous cytoplasm, are a milder exception.
Most cells measure about 10–100 µm. PPLO are the smallest cells (0.1–0.4 µm), the ostrich egg is the largest cell, the nerve cell is the longest animal cell, and the fibre of Boehmeria nivea is the longest plant cell.
Cells are of two grades. Prokaryotic cells have no nuclear envelope and no membrane-bound organelles, have circular DNA without histones and 70S ribosomes, and may carry plasmids and pili. Eukaryotic cells have a true nucleus, membrane-bound organelles, linear DNA with histones and 80S ribosomes.
Bacteria are prokaryotic and unicellular, with a rigid peptidoglycan cell wall. They occur as coccus, bacillus, vibrio and spirillum forms.
A plasmid is a small circular extra-chromosomal DNA molecule that replicates independently and carries genes for special functions such as antibiotic resistance. The same properties make plasmids useful as vectors in gene cloning.
Gram positive bacteria have a thick, single-layered peptidoglycan wall with teichoic acid and retain crystal violet; Gram negative bacteria have a thin peptidoglycan layer covered by a lipid-rich outer membrane, lose the violet stain during the alcohol wash and take up the safranin counterstain.
The plant cell wall is formed by the protoplast and consists of the middle lamella, the thin elastic primary wall of cellulose and hemicellulose, and the thicker rigid secondary wall. It is freely permeable and gives mechanical strength and shape.
Protoplasm is the living material of the cell and was described by Huxley as the physical basis of life. The cytoplasm lies between the plasma membrane and the nucleus and consists of the fluid cytosol together with the organelles.
The plasma membrane is described by the fluid mosaic model of Singer and Nicolson (1972): a semifluid phospholipid bilayer with globular proteins embedded in it and free to move laterally. Its fluidity is what allows engulfing, vesicle fusion and self-sealing.
Membrane proteins are extrinsic (peripheral), attached at the surfaces, or intrinsic (integral), penetrating the bilayer. Carbohydrates linked to lipids and proteins form the glycocalyx, which helps in cell adhesion and cell recognition.
The plasma membrane forms the cell boundary, is selectively permeable and maintains homeostasis, carries receptors, and performs bulk transport by phagocytosis, pinocytosis and exocytosis.
The endomembrane system — endoplasmic reticulum, Golgi complex, lysosomes and vacuoles — works as one coordinated pathway. RER bears ribosomes and handles protein synthesis and transport; SER lacks them and synthesises lipids and steroidal hormones.
The Golgi apparatus consists of stacked cisternae with a cis (forming) face and a trans (maturing) face. It modifies, packages and despatches proteins, forms glycoproteins and glycolipids, forms the cell plate, and produces primary lysosomes. In plants it is called the dictyosome.
Lysosomes contain hydrolytic enzymes active at acidic pH and carry out intracellular digestion. Because rupture of their membrane releases these enzymes into the cell, they are called suicidal bags.
Vacuoles are bounded by the tonoplast and may occupy up to 90% of a plant cell’s volume. The tonoplast pumps ions in against a concentration gradient, which draws water in and keeps the cell turgid.
Mitochondria are double membrane-bound organelles whose inner membrane is folded into cristae, increasing surface area. Their matrix contains circular DNA and 70S ribosomes. They are the sites of aerobic respiration and produce ATP, and so are called the power houses of the cell.
Plastids are of three types: chloroplasts, with stroma, thylakoids stacked into grana connected by fret, and chlorophyll; chromoplasts, with carotenoid pigments; and colourless leucoplasts, which store starch, oils or proteins.
Ribosomes are non-membranous structures of RNA and protein. The eukaryotic 80S ribosome has 60S and 40S subunits. Many ribosomes working on one mRNA form a polyribosome, which also protects the mRNA from ribonucleases.
The centrosome contains two perpendicular centrioles made of nine peripheral fibrils of tubulin; they initiate cell division in animal cells and their asters form spindle fibres. The cytoskeleton of microtubules and microfilaments maintains shape, anchors organelles and forms cilia and flagella.
The nucleus, discovered by Robert Brown (1833), consists of a double nuclear envelope with nuclear pores, the nucleoplasm, the nucleolus attached at the nucleolar organiser, and chromatin fibres. The nucleolus makes ribosomal RNA and assembles ribosome subunits.
Heterochromatin is condensed, dark-staining, late-replicating and transcriptionally inactive; euchromatin is loosely packed, light-staining, early-replicating and active. Which regions are held condensed differs between cell types and helps determine what each cell does.
In prokaryotes the single circular chromosome lies in the nucleoid, compacted by nucleoid-associated proteins and supercoiling. In eukaryotes, about 146 base pairs of DNA are wrapped in roughly 1.65 turns around a histone octamer of two molecules each of H2A, H2B, H3 and H4 to form a nucleosome; nucleosomes joined by linker DNA give the beads-on-a-string appearance, and H1 assists further compaction.
A condensed chromosome shows a primary constriction containing the centromere, to which sister chromatids and the spindle fibre attach; a secondary constriction or nucleolar organiser, beyond which lies the satellite of a SAT chromosome; and telomeres, the repetitive tips that prevent fusion and degradation of chromosome ends.
Homologous chromosomes are alike in size, shape and centromere position and carry genes for the same characters at the same loci, one member being maternal and the other paternal. Non-homologous chromosomes belong to different pairs.
By centromere position, chromosomes are metacentric (V-shaped at anaphase), submetacentric (L), acrocentric (J) or telocentric (I). An acentric fragment has no spindle attachment and disintegrates.
Chromosome number is constant for a species: man 2n = 46, mouse 2n = 40, Drosophila melanogaster 2n = 8, pea 2n = 14. In human beings these are 22 pairs of autosomes and one pair of sex chromosomes. Chromosomes maintain species identity, determine sex, carry hereditary characters and direct protein synthesis.
Banding techniques make each chromosome individually identifiable. Q-bands use quinacrine mustard and mark A–T rich regions, G-bands use Giemsa, R-bands give the reverse of the G-band pattern, and C-bands stain centromeric heterochromatin. Banding is used in chromosome mapping, detecting aberrations, and comparing species.
The light microscope uses visible light and glass lenses, resolves to about 0.2 µm and can examine living material; the electron microscope uses electrons and electromagnetic lenses, resolves a fraction of a nanometre and revealed cell ultrastructure, but the specimen must be dead and held in vacuum.
A karyotype is the systematic arrangement of an individual’s metaphase chromosomes by size, centromere position and banding pattern; it is used to detect extra, missing or altered chromosomes in clinical and prenatal diagnosis.
Centrifugation separates particles by spinning, with larger and denser particles sedimenting first. Differential centrifugation at increasing speeds separates cell organelles one group at a time, and the method is also used to recover DNA, separate blood components and purify proteins.
Flowchart
FC 4.6 – The route of this chapter: cell as the basic unit of life → two grades of organisation → prokaryotic cell (bacterium, nucleoid, plasmid, Gram reaction) and eukaryotic cell → boundary structures (cell wall, plasma membrane) → cytoplasmic organelles (endomembrane system, mitochondria, plastids, ribosomes, cytoskeleton) → nucleus → chromatin → nucleosome → chromosome (types, number, banding) → the techniques that make all of it visible (microscopy, karyotyping, centrifugation)
4.13 Key Terms
Cell the smallest independently functioning unit of a living organism, bounded by a membrane and containing genetic material.
Cell theory the theory that all organisms are made of one or more cells, that the cell is the basic structural and functional unit of life, and that all cells arise from pre-existing cells.
Unicellular organism an organism whose entire body consists of a single cell, such as Amoeba.
Multicellular organism an organism built of many cells organised into tissues, organs and organ systems.
PPLO (pleuropneumonia-like organisms) the smallest known living cells, measuring only about 0.1–0.4 µm.
Prokaryotic cell a cell in which the genetic material is not enclosed by a nuclear envelope and which lacks membrane-bound organelles.
Eukaryotic cell a cell with a true nucleus enclosed by a nuclear envelope and with membrane-bound organelles in its cytoplasm.
Peptidoglycan (murein) the polymer of sugar chains cross-linked by short amino acid chains that forms the rigid bacterial cell wall.
Coccus a spherical bacterium.
Bacillus a rod-shaped bacterium.
Vibrio a comma-shaped bacterium.
Spirillum a spirally coiled bacterium.
Plasmid a small circular extra-chromosomal DNA molecule in bacteria that replicates independently and carries genes for special functions such as antibiotic resistance.
Vector a DNA molecule used to carry a foreign gene into a host cell.
Gram positive bacteria bacteria with a thick, single-layered peptidoglycan wall that retain the crystal violet stain and appear violet.
Gram negative bacteria bacteria with a thin peptidoglycan layer covered by a lipid-rich outer membrane, which lose the crystal violet stain and take up the pink counterstain.
Teichoic acid a wall component present in Gram positive but absent in Gram negative bacteria.
Cell wall the rigid, freely permeable, non-living layer secreted outside the plasma membrane by the protoplast, made chiefly of cellulose in plant cells.
Middle lamella the layer of calcium or magnesium pectate that cements the walls of adjacent plant cells together.
Primary cell wall the thin, elastic outer wall layer of cellulose and hemicellulose that allows a growing cell to enlarge.
Secondary cell wall the thicker, rigid, inelastic wall layer laid down between the primary wall and the protoplast after growth has ceased.
Protoplasm the living, colloidal material of a cell, described by Huxley as the physical basis of life.
Cytoplasm the part of the protoplasm lying between the plasma membrane and the nucleus, consisting of the cytosol and the organelles suspended in it.
Cytosol the continuous fluid-like substance of the cytoplasm in which the organelles are suspended.
Plasma membrane (plasmalemma) the living, selectively permeable membrane that encloses the protoplasm of every cell.
Fluid mosaic model the model of Singer and Nicolson describing the plasma membrane as a semifluid phospholipid bilayer with globular proteins embedded in it and free to move laterally.
Extrinsic (peripheral) proteins membrane proteins loosely attached to the surfaces of the lipid bilayer, which can readily dissociate from it.
Intrinsic (integral) proteins amphipathic membrane proteins that penetrate the lipid bilayer partially or completely.
Glycocalyx the layer of carbohydrate chains linked to lipids and proteins on the animal cell surface, involved in cell adhesion and recognition.
Selective permeability the property of the plasma membrane by which it admits only certain substances and thereby maintains the cell’s internal steady state.
Endomembrane system the group of membranous organelles — endoplasmic reticulum, Golgi complex, lysosomes and vacuoles — whose functions are coordinated with one another.
Endoplasmic reticulum (ER) the network of tubular membranous structures that divides the intracellular space into luminal and extra luminal compartments.
Rough endoplasmic reticulum (RER) endoplasmic reticulum bearing ribosomes on its surface, associated with protein synthesis and secretion.
Smooth endoplasmic reticulum (SER) endoplasmic reticulum without ribosomes, which synthesises lipids and steroidal hormones.
Golgi apparatus the stack of flat cisternae near the nucleus that modifies, packages and despatches materials made in the cell.
Cisternae the flat, disc-shaped membranous sacs, 0.5–1.0 µm across, that make up the Golgi stack.
Cis (forming) face the convex face of the Golgi stack that receives vesicles arriving from the endoplasmic reticulum.
Trans (maturing) face the concave face of the Golgi stack from which finished vesicles are released.
Dictyosome the name given to the Golgi apparatus in plant cells.
Lysosome a membrane-bound vesicle containing hydrolytic enzymes active at acidic pH, responsible for intracellular digestion.
Vacuole a membrane-bound space in the cytoplasm containing water, sap, excretory products and other materials not useful to the cell.
Tonoplast the single membrane bounding the vacuole, which transports ions into it against a concentration gradient.
Mitochondrion the double membrane-bound organelle that is the site of aerobic respiration and produces ATP, known as the power house of the cell.
Cristae the infoldings of the inner mitochondrial membrane that increase its surface area.
Matrix the inner compartment of the mitochondrion, containing circular DNA, RNA and 70S ribosomes.
ATP the molecule in which the energy released by respiration is made available to the cell.
Plastid a large pigment-containing organelle found in plant cells and other eukaryotic autotrophs.
Chloroplast the double membrane-bound plastid containing chlorophyll, in which light energy is used to build carbohydrates.
Stroma the space lined by the inner membrane of the chloroplast, containing circular DNA, ribosomes and enzymes.
Thylakoid the membranous sac within the chloroplast stroma that contains chlorophyll.
Granum a stack of thylakoids within the chloroplast.
Fret the membranous connection linking adjacent grana.
Chromoplast a plastid containing fat-soluble carotenoid pigments that give yellow, orange or red colour.
Leucoplast a colourless plastid that stores nutrients, including amyloplasts for starch, elaioplasts for oils and fats, and aleuroplasts for proteins.
Ribosome the non-membranous granular structure of RNA and protein on which proteins are synthesised.
Polyribosome (polysome) a group of ribosomes translating a single mRNA molecule simultaneously.
Svedberg unit (S) the unit describing how fast a particle sediments in a centrifuge, which depends on shape as well as mass and therefore is not additive.
Centrosome the organelle containing two perpendicular centrioles, which initiates cell division in animal cells.
Centriole a cylindrical structure of nine peripheral fibrils of tubulin, whose asters form the spindle fibres.
Cytoskeleton the network of microtubules and microfilaments that maintains cell shape, anchors organelles and provides tracks for movement.
Nucleus the membrane-bound organelle that holds the genetic material and regulates the overall activities of the cell.
Nuclear envelope the double membrane surrounding the nucleus, separated by a perinuclear space and perforated by nuclear pores.
Nuclear pore an opening in the nuclear envelope through which materials pass between the nucleus and the cytoplasm.
Nucleoplasm (karyoplasm) the semi-solid ground substance filling the nucleus, in which chromatin and the nucleolus are suspended.
Nucleolus the dense non-membranous region of the nucleus where ribosomal RNA is made and ribosome subunits are assembled.
Nucleolar organiser the chromosomal region, at the secondary constriction, to which the nucleolus is attached.
Chromatin the thread-like complex of DNA and proteins seen in the nucleus during interphase, which condenses into chromosomes at cell division.
Heterochromatin the condensed, dark-staining, late-replicating and transcriptionally inactive region of chromatin.
Euchromatin the loosely packed, light-staining, early-replicating and transcriptionally active region of chromatin.
Chromosome a self-duplicating nuclear filament of DNA and protein that carries the genes.
Gene the unit borne on a chromosome that controls a hereditary trait.
Nucleoid the region of the prokaryotic cytoplasm, not bounded by a membrane, in which the circular chromosome lies.
Histone a basic, positively charged protein rich in lysine and arginine, around which DNA is wound in eukaryotic chromosomes.
Nucleosome the basic unit of eukaryotic chromatin, about 146 base pairs of DNA wrapped in roughly 1.65 turns around a histone octamer.
Histone octamer the core of eight histone proteins, two each of H2A, H2B, H3 and H4, at the centre of a nucleosome.
Linker DNA the short stretch of DNA joining one nucleosome to the next, held in place by the H1 histone.
Primary constriction the non-stainable region of a chromosome within which the centromere lies.
Centromere the region of a chromosome at which sister chromatids are joined and to which the spindle fibre attaches.
Kinetochore the protein structure at the centromere through which the spindle fibre attaches to the chromosome.
Secondary constriction the constriction, also called the nucleolar organiser, beyond which the satellite lies.
Satellite the portion of a chromosome lying beyond the secondary constriction.
SAT chromosome a chromosome bearing a satellite.
Telomere the repetitive DNA sequence at the tip of a eukaryotic chromosome that prevents fusion with other chromosomes and degradation of the end.
Sister chromatids the two identical copies of a replicated chromosome, joined at the centromere.
Non-sister chromatids chromatids belonging to the two different members of a homologous pair.
Homologous chromosomes a maternal and a paternal chromosome alike in size, shape and centromere position and carrying genes for the same characters at the same loci.
Non-homologous chromosomes chromosomes belonging to different pairs, differing in size or centromere position and carrying genes for different characters.
Metacentric chromosome a chromosome with a central centromere and two equal arms, V-shaped at anaphase.
Submetacentric chromosome a chromosome whose centromere lies slightly off-centre, giving unequal arms and an L shape at anaphase.
Acrocentric chromosome a chromosome whose centromere lies near one tip, giving one very short arm and a J shape at anaphase.
Telocentric chromosome a chromosome with the centromere at one end, appearing I-shaped at anaphase.
Acentric chromosome a chromosome fragment lacking a centromere, which cannot attach to the spindle and disintegrates.
Diploid (2n) a cell carrying two complete sets of chromosomes, one from each parent.
Haploid (n) a cell carrying a single set of chromosomes, as in a gamete.
Autosomes the chromosome pairs that are alike in both sexes.
Sex chromosomes the pair of chromosomes that differ between the sexes and determine the sex of the offspring.
Chromosome banding the treatment and staining of chromosomes to produce a constant pattern of dark and light cross-bands by which each chromosome can be identified.
Q-bands bands produced by the fluorescent quinacrine mustard stain, marking A–T rich regions.
G-bands bands produced by Giemsa stain, giving a pattern corresponding to the Q-bands, but poorly obtained in plant chromosomes.
R-bands bands produced by a reverse Giemsa method, giving the reverse of the G-band pattern.
C-bands bands produced by Giemsa staining after incubation in strong alkali, marking centromeric heterochromatin.
Magnification the number of times larger an image appears than the object itself.
Resolution the smallest distance between two points at which they can still be distinguished as two.
Light microscope an instrument using visible light and glass lenses, resolving to about 0.2 µm and able to examine living material.
Electron microscope an instrument using a beam of electrons and electromagnetic lenses, resolving a fraction of a nanometre but requiring a dead specimen held in vacuum.
Karyotype the complete set of an individual’s chromosomes arranged in a standard order by size, centromere position and banding pattern.
Idiogram a diagrammatic representation of a karyotype.
Centrifugation the separation of particles in a suspension by spinning, so that larger and denser particles sediment first.
Differential centrifugation the separation of cell organelles one group at a time by spinning a homogenate at successively higher speeds.
Pellet the sedimented material collected at the bottom of the tube after centrifugation.
Supernatant the liquid remaining above the pellet after centrifugation.
4.14 Points to Ponder
A virus has genetic material and can multiply, yet it has no protoplasm and cannot reproduce outside a host cell. Should the definition of “living” be widened to include it, or is it more honest to keep cell theory as it stands and accept that viruses sit outside it? What does your answer suggest about how scientific definitions are made?
Every cell of your body carries the same complete set of genes, yet a nerve cell and a liver cell behave nothing alike. If the instructions are identical, where does the difference between cell types actually reside — and what does that imply about how much of an organism’s character is written in its DNA sequence alone?
Antibiotic resistance genes are often carried on plasmids, which can move between bacteria and even between species. Given this, is the decision to prescribe an antibiotic ever purely a matter between one doctor and one patient? Consider who else has a stake in that decision.
Penicillin works because human cells have no peptidoglycan wall to attack. How many other drugs might be possible if we systematically listed every structure a pathogen has and we do not? What limits this approach when the pathogen is a eukaryote, such as a fungus?
Our most detailed pictures of cell ultrastructure come from electron microscopy, which requires the specimen to be dead, dried and fixed. How confident should we be that what we see is the cell as it was when alive, and how might a scientist guard against being misled by an artefact of the technique?
Karyotyping can reveal a great deal about a foetus, including its sex. In India, using prenatal diagnostic techniques to determine sex is prohibited by law. Consider why a technique that is valuable for detecting chromosomal disorders needed to be restricted in this way, and what is lost and gained by such a restriction.
Cells taken from a patient during treatment can be grown in culture and used in research for decades afterwards. Who should be considered the owner of such a cell line — the patient it came from, the hospital, or the laboratory that maintained it? What would a fair arrangement look like?
Lysosomes can digest the cell that contains them, and this self-destruction is a normal part of development rather than an accident. Why might an organism have evolved to destroy its own cells deliberately, and what might go wrong if that ability were lost?
Telomere length is sometimes described as a measure of “biological age”, and commercial tests offering to report it are now available. What would you want to know about the evidence before deciding whether such a test tells a person anything useful about their health?
Prokaryotic cells multiply far faster than eukaryotic ones, while eukaryotic cells achieve far greater structural complexity. Is one of these organisational plans better than the other, or does the question itself assume something it should not?
4.15 Objective Questions
- The longest plant cell is the sclerenchymatous fibre of:
- Pisum sativum
- Boehmeria nivea
- Allium cepa
- Mangifera indica
- The number of base pairs of DNA wrapped around one histone octamer is approximately:
- 46
- 100
- 146
- 200
- The histone octamer of a nucleosome consists of two molecules each of:
- H1, H2A, H2B and H3
- H2A, H2B, H3 and H4
- H1, H2, H3 and H4
- H2B, H3, H4 and H5
- The histone that lies on the linker DNA and assists in further compaction of chromatin is:
- H1
- H2A
- H3
- H4
- A chromosome whose centromere lies exactly in the middle appears at anaphase in the shape of the letter:
- I
- J
- L
- V
- Q-bands on a chromosome are produced by staining with:
- Giemsa stain
- Quinacrine mustard
- Acetocarmine
- Safranin
- Giemsa staining carried out after incubating chromosomes in strong alkali specifically reveals:
- A–T rich regions along the whole chromosome
- The reverse of the G-band pattern
- Heterochromatin near the centromere
- The satellite of a SAT chromosome
- The diploid chromosome number of Drosophila melanogaster* is:*
- 8
- 14
- 16
- 40
- The resolution of a light microscope is limited to about:
- 0.2 nm
- 2 nm
- 0.2 µm
- 2 mm
- Living material cannot be examined under an electron microscope mainly because:
- Electrons cannot pass through water
- The specimen must be kept under vacuum
- Electromagnetic lenses cannot focus on moving objects
- The magnification is too high for living cells
- In differential centrifugation of a cell homogenate, the fraction that sediments first at the lowest speed is:
- Ribosomes
- Mitochondria
- Nuclei
- Soluble cytosol
- The part of a chromosome lying beyond the secondary constriction is called the:
- Telomere
- Kinetochore
- Satellite
- Chromatid
- Teichoic acid is a component of the cell wall of:
- Gram positive bacteria only
- Gram negative bacteria only
- Both Gram positive and Gram negative bacteria
- Plant cells
- The single membrane that bounds a vacuole is called the:
- Plasmalemma
- Tonoplast
- Perinuclear membrane
- Cristae
- In a chloroplast, adjacent grana are connected to one another by:
- Fret
- Stroma
- Cristae
- Linker DNA
- The Golgi apparatus of a plant cell is known as the:
- Dictyosome
- Oxysome
- Amyloplast
- Polysome
- Which of the following is NOT a part of the endomembrane system?
- Golgi complex
- Lysosome
- Mitochondrion
- Endoplasmic reticulum
- The middle lamella that cements adjacent plant cell walls together is composed of:
- Cellulose and hemicellulose
- Calcium or magnesium pectate
- Lignin and suberin
- Peptidoglycan
- Euchromatin differs from heterochromatin in that it:
- Stains darker and appears condensed
- Replicates at the end of the S-phase
- Replicates during the early stage of the S-phase
- Is genetically inactive
- Spectrin of the red blood cell membrane is an example of:
- An intrinsic protein
- An extrinsic protein
- A glycolipid
- A carbohydrate of the glycocalyx
- For the following questions, choose from the following options: (a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion. (b) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion. (c) Assertion is true, but Reason is false. (d) Both Assertion and Reason are false.
- Assertion (A): Streptomycin blocks protein synthesis in bacteria without stopping protein synthesis in human cells. Reason (R): Bacterial ribosomes are of the 70S type while human cytoplasmic ribosomes are of the 80S type.
- Assertion (A): Chromatin is not maintained in its fully condensed form throughout the life of a cell. Reason (R): Tightly packed DNA is inaccessible to the machinery that transcribes genes.
- Assertion (A): A plant cell placed in pure water becomes turgid but does not burst. Reason (R): The plant cell wall is freely permeable to water and dissolved substances.
- Assertion (A): An acentric chromosome fragment is lost during cell division. Reason (R): It has no centromere and therefore no point of attachment for the spindle fibre.
- Assertion (A): G-banding is preferred over Q-banding for routine chromosome analysis. Reason (R): G-banded preparations can be examined with an ordinary light microscope, whereas Q-banding requires a fluorescence microscope.
- Assertion (A): Mitochondria are able to synthesise some of their own proteins. Reason (R): The mitochondrial matrix contains 80S ribosomes and linear DNA.
- Assertion (A): Onion root tip is the material of choice for observing the stages of mitosis. Reason (R): Onion has few, large chromosomes and its root tips divide rapidly.
- Assertion (A): Centrifuge tubes must always be loaded in balanced pairs placed opposite each other. Reason (R): An unbalanced rotor spinning at high speed is dangerous.
- A student observes a long, thread-like cell extending up to a metre in length. Which cell type is this most likely to be?
- Muscle cell
- Nerve cell
- Bacterial cell
- Epithelial cell
- During a lecture, a teacher mentions the “physical basis of life”. This term refers to:
- DNA
- Cytoplasm
- Protoplasm
- Nucleus
- A researcher identifies the smallest known living cells (PPLO). These belong to:
- Fungi
- Bacteria-like organisms
- Protists
- Viruses
- A cell shows DNA associated with histone proteins. This feature is characteristic of:
- Prokaryotes
- Eukaryotes
- Viruses
- Bacteria only
- A student studies chromosome ends that prevent fusion with other chromosomes. These structures are called:
- Centromeres
- Telomeres
- Chromatids
- Satellites
- A plant cell remains firm even after absorbing excess water. This is mainly due to:
- Plasma membrane
- Cell wall resistance
- Cytoplasm density
- Nucleolus
- A bacterial cell fails to retain the Gram stain and appears pink. This suggests:
- Thick peptidoglycan
- Presence of outer membrane
- Absence of lipids
- High protein content
- A cell membrane shows embedded proteins that can move laterally. This supports which concept?
- Unit membrane model
- Fluid mosaic model
- Sandwich model
- Micelle model
- A cell is unable to recognise neighbouring cells due to the absence of surface carbohydrates. Which structure is affected?
- Cytoskeleton
- Glycocalyx
- Nucleoplasm
- Centrosome
- A plant cell stops expanding after formation of the secondary wall. This is because of:
- Increased permeability
- Loss of elasticity
- Increased cytoplasm
- Decreased DNA activity
- A scientist observes a cell with no membrane-bound organelles but active metabolism. Which feature allows metabolic control?
- Nucleoid DNA
- Lysosomes
- Golgi bodies
- Vacuoles
- A plant cell placed in saline water shrinks. This phenomenon is best explained by:
- Endocytosis
- Plasmolysis
- Diffusion
- Active transport
- A cell shows rapid secretion of enzymes. Which organelle combination will be most active?
- SER and lysosome
- RER and Golgi
- Nucleus and vacuole
- Ribosome and centriole
- A student detects circular DNA inside an organelle of a eukaryotic cell. This organelle is most likely:
- Nucleus
- Golgi apparatus
- Mitochondrion
- Lysosome
- A cell shows high detoxification activity. Which organelle is primarily responsible?
- Rough ER
- Smooth ER
- Ribosome
- Centrosome
- A cell shows reduced ATP production but normal protein synthesis. Which organelle is likely defective?
- Ribosome
- Mitochondria
- Golgi
- Nucleus
- Two cells differ in ribosome size (70S and 80S). This suggests a difference in:
- Shape
- Genetic material
- Cellular organisation
- Cell size only
- A bacterium with a capsule survives immune attack better. This indicates that the capsule helps in:
- Energy production
- Protection from host defences
- Protein synthesis
- DNA replication
- A cell fails to transport proteins outside despite normal synthesis. Which organelle is malfunctioning?
- Ribosome
- Golgi apparatus
- Nucleus
- Mitochondria
- A mutation reduces membrane fluidity. Which component is most affected?
- Carbohydrates
- Lipid bilayer
- Ribosomes
- DNA
- A cell loses its ability to maintain internal balance. Which structure’s failure is most responsible?
- Nucleus
- Plasma membrane
- Ribosome
- Centriole
- A mutation causes chromatin to remain highly condensed permanently. This will most likely:
- Increase gene expression
- Reduce gene expression
- Increase replication
- Enhance transcription
- A bacterium loses its plasmids. This may lead to:
- Increased resistance
- Loss of special survival traits
- Faster growth
- Increased mutation
- A cell loses centromere function. This will affect:
- DNA synthesis
- Chromosome segregation
- Protein synthesis
- Cell respiration
- A defect in histone proteins occurs. This will impact:
- Cell wall formation
- DNA packaging
- ATP synthesis
- Ribosome structure
- For the following questions, choose from the following options: (a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion. (b) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion. (c) Assertion is true, but Reason is false. (d) Both Assertion and Reason are false.
- Assertion (A): A plant cell placed in a hypotonic solution becomes turgid. Reason (R): Water enters the cell due to an osmotic gradient across the plasma membrane.
- Assertion (A): Gram positive bacteria retain the crystal violet stain. Reason (R): They possess a thick peptidoglycan layer in their cell wall.
- Assertion (A): Cells rich in mitochondria show high metabolic activity. Reason (R): Mitochondria generate the ATP required for cellular processes.
- Assertion (A): Rough endoplasmic reticulum is involved in protein synthesis. Reason (R): Ribosomes are attached to its surface.
- Assertion (A): A mutation affecting spindle fibres leads to abnormal cell division. Reason (R): Spindle fibres help in chromosome movement during cell division.
- Assertion (A): A cell lacking functional nuclear pores cannot maintain normal cellular activity. Reason (R): Nuclear pores regulate the exchange of materials between nucleus and cytoplasm.
Fill in the blank: The scientist who coined the term “cell” after observing cork was __________.
Fill in the blank: The statement “Omnis cellula e cellula” was proposed by __________.
Fill in the blank: The genetic material in prokaryotic cells is present in a region called the __________.
Fill in the blank: The plasma membrane is described by the __________ model.
Fill in the blank: The rigid outer layer of plant cells is mainly composed of __________.
Fill in the blank: Ribosomes found in prokaryotic cells are of the __________ type.
Fill in the blank: The organelle known as the powerhouse of the cell is the __________.
Fill in the blank: The chromosomal region to which the nucleolus remains attached is called the __________.
Fill in the blank: The protective ends of chromosomes are known as __________.
4.16 Check Your Concepts
Hooke observed cork and named what he saw “cells”, yet he never saw a living cell. Explain what he actually observed and why the compartments appeared empty.
Cell theory states that all cells arise from pre-existing cells. Explain why viruses are regarded as an exception to this statement, and state one other kind of exception.
An organism is described as unicellular. Explain why this does not mean that the organism is simpler than a single specialised cell of a multicellular body.
Arrange the following in the correct sequence, from the smallest to the largest level of organisation, and give one example of each: organ, cell, organism, tissue, organ system.
In a prokaryote, translation of an mRNA molecule can begin before its synthesis from DNA is complete, while in a eukaryote it cannot. Explain the structural reason for this difference and state one advantage of the eukaryotic arrangement.
A student writes that a prokaryotic cell is one in which “mitochondria and chloroplasts are absent”. Explain why this definition is unsatisfactory, and give a better one.
The steps of Gram staining are listed below in the wrong order. Rearrange them correctly, and state what would be observed if the fourth step were carried out for far too long.
- Counterstain with safranin
- Flood the smear with crystal violet
- Prepare a thin bacterial smear on a slide
- Decolourise with alcohol
- Add iodine solution
Penicillin damages bacterial cells but not the cells of the patient taking it. Explain the structural basis of this selectivity, and predict whether the same drug would be expected to harm a plant cell.
Two bacterial cultures are treated with the same antibiotic. Culture A is wiped out; in culture B, a few cells survive and multiply. When plasmids are extracted from the survivors, resistance genes are found on them. Explain how the surviving cells could have acquired these genes, and why the situation is of concern beyond this one laboratory.
Compare the plant cell wall and the plasma membrane with respect to composition, permeability and function. Explain why the wall alone cannot control what enters the cell.
A cell that has just finished growing begins to deposit lignin in its wall. State which wall layer is being formed and explain what this tells you about whether the cell will enlarge further.
Why is the plasma membrane described as a “fluid mosaic” rather than a fixed sheet? Give two cellular processes that would be impossible if the membrane were rigid.
Match the membrane component in Column I with its correct description in Column II. | Column A | Column B | |—|—| | a. Spectrin | i. Amphipathic protein penetrating the membrane | | b. ATPase | ii. Carbohydrate layer aiding cell recognition | | c. Glycocalyx | iii. Peripheral protein of the RBC membrane | | d. Phospholipid bilayer | iv. Semifluid framework of the membrane |
Arrange the following events in the correct order to trace a secretory protein from its synthesis to its release, and name the organelle acting at each stage: modification in cisternae; release outside the cell; synthesis on ribosomes; transport in a vesicle to the cis* face; departure from the trans face in a secretory vesicle.*
Lysosomal enzymes work best at acidic pH while the cytoplasm around them is nearly neutral. Explain why this arrangement is an advantage to the cell.
A drug blocks the fusion of vesicles with the plasma membrane but leaves protein synthesis untouched. Predict what would accumulate inside the cell and where, and explain your reasoning.
Explain why the tonoplast pumping ions into the vacuole results in the plant cell becoming turgid. Link your answer to why an unwatered plant wilts.
Heart muscle mitochondria have far more densely packed cristae than the mitochondria of a less active cell. Explain the connection between cristae and the cell’s energy output.
Match the plastid in Column I with what it contains or stores in Column II. | Column A | Column B | |—|—| | a. Chloroplast | i. Carotenoid pigments | | b. Chromoplast | ii. Proteins | | c. Amyloplast | iii. Chlorophyll in thylakoids | | d. Aleuroplast | iv. Starch |
A eukaryotic cell is found to contain both 70S and 80S ribosomes. Explain how this is possible, and name the two organelles responsible.
Explain why many ribosomes attach to a single mRNA molecule during protein synthesis, and state one additional benefit of this arrangement besides speed.
The nuclear envelope has pores, whereas most other organelle membranes do not. Explain why the nucleus requires them, and predict the consequence of blocking them.
A cell is observed to have several large, prominent nucleoli. What can be inferred about its activity, and why? Explain the link between the nucleolus and protein synthesis.
Every cell of an individual carries the same genes, yet different cell types express different ones. Explain how the distribution of heterochromatin and euchromatin accounts for this.
Arrange the following levels of DNA organisation in order of increasing compaction: chromatin fibre compacted with H1; condensed metaphase chromosome; DNA double helix; nucleosome; “beads on a string” fibre. Then explain why the packing must be orderly rather than random.
Distinguish between sister chromatids and non-sister chromatids, and between homologous and non-homologous chromosomes. State which pair of structures exchanges segments during crossing over.
Explain why an acentric chromosome fragment cannot survive a cell division, and identify which part of the normal chromosome it is missing.
Draw and label a diagram of a condensed chromosome showing the chromatid, primary constriction, centromere, secondary constriction, satellite and telomere. Beside the diagram, state in one line what each labelled part does.
Two chromosomes in a set are of very similar size and have the centromere in a similar position. Explain how banding allows them to be told apart, and name the band type you would use to examine faint regions near the chromosome tips.
Explain why increasing the magnification of a light microscope beyond a certain point does not reveal any further detail. In your answer, distinguish clearly between magnification and resolution.
A student wishes to observe the movement of cytoplasm in a living cell and proposes to use an electron microscope for better detail. Explain why this plan cannot work, and suggest the correct instrument.
A student preparing a karyotype stains and photographs a dividing cell but obtains chromosomes that are long, thin and overlapping, and cannot be identified. Suggest what went wrong in the procedure and how it should be corrected.
A laboratory assistant places a single tube of sample in a centrifuge rotor and switches it on at high speed. State what is wrong with this and explain the reason.
A cell homogenate is spun at low speed and the pellet is discarded, after which the supernatant is spun at a higher speed. Name the organelles expected in the second pellet, and explain what would have been lost by discarding the first one.
Compare the light microscope and the electron microscope under the headings: source used, resolution, nature of the specimen, and one structure best studied by each.
A photograph shows a bacterial cell labelled with a nucleoid, ribosomes, a plasmid, a cell wall and a flagellum. Explain how you would use these labels alone to argue that the cell is prokaryotic rather than eukaryotic.
What is a nucleoid?
What type of ribosomes are found in prokaryotes?
Name the basic unit of heredity present in cells.
What is the main component of the bacterial cell wall?
Name the stain used in Gram staining.
Explain the role of plasmids in bacteria.
Differentiate between RER and SER (one point).
Explain why lysosomes are called suicidal bags.
Describe the function of the Golgi apparatus.
What is the significance of the glycocalyx?
Which cells contain chloroplasts and a large vacuole?
Classify cells that contain 70S ribosomes.
Classify an organism whose cells have a nucleus and membrane-bound organelles.
What does the presence of many mitochondria in a cell indicate?
Why can some bacteria survive extreme conditions?
Why does a cell rich in RER indicate high protein synthesis?
Why are plasmids important in antibiotic resistance?
Predict the effect of the absence of telomeres.
Predict the effect of excessive endocytosis.
Suggest how a cell might compensate for the absence of mitochondria.
A teacher explains that new cells arise from existing ones. State the scientist and the statement associated with this idea.
A bacterial cell is observed with DNA not enclosed by a nucleus. Name this region and the cell type.
During lab observation, ribosomes of size 70S are detected. Identify the type of cell showing these ribosomes.
A plant tissue sample shows rigid outer boundaries. Name the structure responsible and its main component.
A stained bacterial sample appears purple after Gram staining. Explain why this occurs.
A plant cell shows a thick cellulose wall. Explain how this structure supports the plant.
A cell contains many lysosomes. Explain why such cells require numerous lysosomes.
A cell has chloroplasts and a large vacuole. Identify the cell type and justify your answer.
A cell placed in a hypotonic solution becomes turgid. Explain the reason for this change.
A microorganism has circular DNA and no membrane-bound organelles. Classify it and justify your answer.
A cell has numerous mitochondria. Analyse what this suggests about its function.
Two bacteria show different Gram staining results. Analyse the structural differences responsible.
A cell with abundant rough ER is observed. Analyse its likely role in the body.
A mutation disrupts spindle formation. Analyse its effect on chromosome distribution.
A cell shows many cilia. Analyse how their structure relates to their function.
A cell lacks lysosomes. Evaluate how this affects cellular waste management.
Telomeres are absent in a cell. Evaluate the effect on chromosome stability.
A bacterial cell gains antibiotic resistance genes. Evaluate the impact on treatment strategies.
How can a cell survive without mitochondria?
Explain how a cell could regulate excessive lysosomal activity.
A student traces the discovery of cells from the observation of cork to modern cell theory. Describe the contributions of Robert Hooke, Anton van Leeuwenhoek, Matthias Schleiden, Theodor Schwann and Rudolf Virchow to the development of cell theory.
During a lab session, a teacher asks students to explain the postulates of cell theory using examples from unicellular and multicellular organisms. Describe the postulates and justify them with examples.
A student prepares a chart on differences between prokaryotic and eukaryotic cells. Describe at least five differences with suitable examples.
A biology student is asked to explain the structure of the plasma membrane using the fluid mosaic model. Describe the model and its components.
While studying chromosomes, a student is asked to explain their structure and components. Describe the structure of chromosomes and the role of DNA and histone proteins.
A bacterium is observed to contain plasmids and a peptidoglycan cell wall. Explain how these features help in survival and adaptation.
A plant cell shows a thick cellulose wall and a large vacuole. Explain how these structures contribute to plant rigidity and function.
A cell shows both rough and smooth endoplasmic reticulum. Explain their structural differences and functional roles in the cell.
A cell shows multiple nucleoli and active chromatin regions. Explain how these features relate to protein synthesis and gene expression.
During Gram staining, two bacterial samples show different colours. Explain the structural basis of this difference and its biological significance.
A cell observed under the microscope has chloroplasts, a large vacuole and a cellulose wall. Identify the cell type and explain how each structure supports its function.
A plant cell is placed in hypotonic and hypertonic solutions separately. Explain the changes observed in both conditions, with reasons.
A cell is found to contain 70S ribosomes and circular DNA. Classify the cell and explain how these features influence its functioning.
A researcher isolates a unicellular organism with membrane-bound organelles and a nucleus. Classify the organism and explain its cellular organisation.
A cell contains numerous mitochondria and extensive rough ER. Analyse the functional specialisation of this cell and suggest its possible role in the body.
Compare Gram positive and Gram negative bacteria in terms of structure, staining behaviour and antibiotic susceptibility.
A toxin disrupts cytoskeleton organisation in a cell. Analyse its effects on cell shape, intracellular transport and division.
A mutation results in the absence of lysosomes in a cell. Evaluate the consequences for intracellular digestion and cellular health.
A defective Golgi apparatus is observed in a cell. Evaluate its impact on protein processing, packaging and secretion.
Telomeres are absent in a eukaryotic cell. Evaluate the impact on chromosomal stability, ageing and cell division.
A bacterial cell acquires plasmids carrying antibiotic resistance genes. Evaluate the implications for medical treatment and public health.
Propose how a cell lacking mitochondria can adapt to meet its energy requirements.
Design a mechanism by which chromosomes could segregate in the absence of spindle fibres.
Suggest an experimental setup to differentiate prokaryotic and eukaryotic cells based on ribosome types and organelles.
Propose regulatory mechanisms that a cell can use to control excessive lysosomal activity and prevent self-digestion.
A newly discovered organism lacks a defined centromere. Propose possible mechanisms that could ensure proper chromosome segregation during cell division.
A group of students studies prokaryotic and eukaryotic cells. They find that some cells lack a well-defined nucleus and membrane-bound organelles, while others have complex internal structures. Explain the differences between prokaryotic and eukaryotic cells with suitable examples.
A healthcare trainee studies muscle cells and notices that they contain a large number of mitochondria. She relates this to the energy requirements of muscle contraction. Explain why cells with a high energy demand contain more mitochondria.
A cell studied by a research team shows extensive rough endoplasmic reticulum (RER). The researchers conclude that the cell is actively involved in protein synthesis. Analyse why the presence of RER indicates high protein production and how it supports cellular function.
During a genetic study, scientists observe that the absence of telomeres leads to chromosomal instability, with chromosomes fusing and degrading over time. Analyse the role of telomeres and explain the consequences of their absence.
4.17 Case-Based Questions
- A patient is admitted with a wound infection that is not responding to the antibiotic first prescribed. A swab is sent to the microbiology laboratory, where a smear is prepared and stained. The report returns within the hour: rod-shaped organisms, staining pink. The physician changes the prescription on the basis of this single line of information.
- To which group do these bacteria belong, and what is the shape term for a rod-shaped bacterium?
- Explain, in terms of cell wall structure, why these organisms stained pink rather than violet.
- Suggest why organisms of this group are often harder to treat with antibiotics than those of the other group.
- A junior technician suggests that the pink result may have been caused by an error at the decolourisation step. Explain what kind of error would produce a false pink result, and in which group of bacteria.
- In a cytogenetics laboratory, cells obtained from a sample are cultured and then arrested while dividing. The chromosomes are spread on a slide, treated with Giemsa stain, photographed, and finally cut out and arranged in pairs in decreasing order of size. On examining the arrangement, the cytogeneticist notes that one of the smaller pairs appears to have three members rather than two.
- Name the arrangement that has been prepared, and define it.
- Explain why the cells had to be arrested at metaphase before the chromosomes could be spread.
- Name the banding technique used here, and explain why banding is necessary before individual chromosomes can be identified with confidence.
- Such preparations can also reveal the sex chromosomes of a foetus. In India, the use of prenatal diagnostic techniques to determine sex is prohibited by law. Explain briefly why a technique of genuine diagnostic value required this restriction.
- A research student grinds liver tissue in a cold buffer to produce a homogenate and spins it in a centrifuge at a low speed. She keeps the liquid above the sediment and spins it again at a higher speed, obtaining a second sediment. When she tests the two sediments, she finds that the second one carries out reactions that consume oxygen and release ATP.
- Name the technique she is using and state the principle on which it works.
- Name the components expected in the first sediment and in the second.
- Identify the organelle responsible for the reactions detected in the second sediment, and name two of its structural features that suit it to this role.
- State the terms for the sediment and for the liquid above it, and explain why the student kept the liquid rather than discarding it after the first spin.
- A farmer irrigates a field with water drawn from a well that has become saline. Within two days the young plants droop and their leaves lose firmness, although the soil around them is visibly wet. Cells taken from a leaf and examined under the microscope show the protoplast shrunken away from the cell wall.
- Name the phenomenon observed in these cells and explain why it has occurred despite the soil being wet.
- Explain how a plant cell placed in pure water behaves differently, and why it does not burst.
- Explain the role of the tonoplast and the vacuole in keeping a plant cell firm under normal conditions.
- State which structure of the plant cell is responsible for the fact that the wall itself remains unchanged while the protoplast shrinks, and give the reason.
- A team examining a newly isolated microorganism finds it too small to be resolved clearly under a light microscope. They prepare a fixed, dried specimen and examine it with a beam of electrons under vacuum. The images show a cell with no nuclear envelope, a single circular DNA molecule lying free in the cytoplasm, ribosomes of the 70S type, and a wall outside the membrane.
- Classify the organism as prokaryotic or eukaryotic, giving three features from the passage that support your answer.
- Explain why the light microscope failed and why the electron microscope succeeded, referring to resolution in your answer.
- State one important limitation of the method used here, and explain what it prevents the team from observing.
- Name the region in which the circular DNA lies, and explain how such a long DNA molecule is compacted in the absence of histone proteins.
- In a biotechnology laboratory, a gene of interest is inserted into a small circular DNA molecule, which is then introduced into bacterial cells. The bacteria are grown overnight, and by morning the culture contains many millions of cells, each carrying a copy of the inserted gene. The circular DNA used also carries a gene for resistance to an antibiotic, which allows the researchers to select the bacteria that took it up.
- Name the circular DNA molecule used, and define the term applied to any DNA molecule used to carry a foreign gene into a host cell.
- State the two properties of this molecule that make it suitable for the purpose described.
- Explain how the antibiotic resistance gene allows the researchers to pick out the bacteria that took up the construct.
- Some critics object to the routine use of antibiotic resistance markers in laboratory work. Explain the concern, referring to how such genes can spread between bacteria.
- During a biology workshop, students observe different types of cells under a microscope. They notice that all living organisms, whether simple or complex, are made up of cells, and that cells arise from pre-existing cells.
- State any two postulates of cell theory.
- Who proposed the idea “Omnis cellula e cellula”?
- Why is the cell called the structural unit of life?
- Why is the cell considered the functional unit of life?
- In a school laboratory, students are examining a cell sample. They observe that the cell contains chloroplasts, a large vacuole and a rigid cell wall. Based on these features, they attempt to classify the cell type.
- Identify the type of cell observed.
- Which organelle is responsible for photosynthesis?
- What is the role of the vacuole in this cell?
- How does the cell wall help the cell in maintaining its structure?
- Given below is the diagram of a structure found in a typical cell observed under the microscope, with parts labelled A to E.
- What is the role of the part labelled “A”?
- What is the composition of the part labelled “B”? Also mention its role in the cell.
- Draw a well-labelled diagram of the structure of the part labelled “B”.
- Observe the diagram given below, in which parts are labelled P, Q, R, S and T, and answer the questions that follow.
- Identify the structure shown in the figure.
- Label all the parts.
- What is the role of the part labelled “Q”?
- What is the significance of the part labelled “R”?
- The figure shown below is a cell organelle, with parts labelled 1 to 4. Answer the questions that follow.
- What role does this structure play in the cell?
- What is the role of the part labelled “3”?
- What is the significance of the part labelled “2”?
- Observe the figure given below, in which parts are labelled 1, 2 and 3 along with the membrane, and answer the questions that follow.
- What role does this structure play in the cell?
- Label the parts 1, 2 and 3.
- What role do the parts labelled 1 and 2 play in the metabolic reaction it performs?
4.18 Practice With Data
A microscope is fitted with a 10× eyepiece and three objectives of 10×, 45× and 100×. (a) Calculate the total magnification obtained with each objective. (b) Working at the highest magnification, a student measures the image of a cell as 5 mm across. Calculate the actual diameter of the cell in micrometres. (c) The same student wishes to examine a structure 0.05 µm across. State, with a reason, whether this microscope can resolve it.
The table gives approximate sizes of four cells or cell types.
| Cell | Approximate size |
|---|---|
| PPLO | 0.2 µm |
| Typical human epithelial cell | 60 µm |
| Ostrich egg | 15 cm |
| Ramie fibre | 55 cm |
(a) Express the size of the epithelial cell and the ostrich egg in millimetres. (b) How many times larger than a PPLO cell is the epithelial cell? (c) Arrange the four in increasing order of size after converting all values to a common unit.
In a eukaryotic nucleus, about 146 base pairs of DNA are wrapped around each histone octamer, and successive nucleosomes are joined by a linker stretch of about 54 base pairs. (a) Calculate the number of base pairs accounted for by one nucleosome together with its linker. (b) Calculate how many nucleosomes would be formed along a stretch of 20,000 base pairs. (c) State how many histone molecules of the octamer type would be required for the stretch in part (b).
The table gives the diploid chromosome number of four species.
| Species | 2n |
|---|---|
| Man (Homo sapiens) | 46 |
| Mouse (Mus musculus) | 40 |
| Fruit fly (Drosophila melanogaster) | 8 |
| Pea (Pisum sativum) | 14 |
(a) State the haploid number for each species. (b) State the number of homologous pairs in a somatic cell of each. (c) For man, state how many of the pairs are autosomes and how many are sex chromosomes. (d) A student concludes from this table that chromosome number increases with the complexity of an organism. Comment on this conclusion using the data given.
A somatic cell of a certain plant contains 24 chromosomes. (a) How many chromatids will the cell contain immediately after DNA replication in the S-phase, before division begins? (b) How many chromosomes will each gamete of this plant contain? (c) A fragment of one chromosome breaks off without a centromere. Predict its fate and state the resulting chromosome number of the daughter cells if the fragment is lost.
Two bacterial isolates were analysed for cell wall composition, with the results shown below.
| Isolate | Murein content of the wall | Lipid content of the wall | Teichoic acid |
|---|---|---|---|
| P | 75% | Low | Present |
| Q | 15% | 25% | Absent |
(a) Identify each isolate as Gram positive or Gram negative, giving reasons drawn from the data. (b) Predict the colour each isolate would show after Gram staining. (c) Predict which isolate is likely to be more resistant to antibiotics, and explain why using the data.
- A liver homogenate was subjected to centrifugation at increasing speeds, and each pellet was analysed. The results are given below.
| Spin | Relative speed | Main content of the pellet |
|---|---|---|
| 1 | Low | Nuclei and unbroken cells |
| 2 | Medium | Mitochondria |
| 3 | High | Membrane fragments |
| 4 | Very high | Ribosomes |
(a) Name the technique used and state the property of the particles on which the separation depends. (b) Explain why the nuclei sediment before the mitochondria. (c) In which pellet would you expect to find the enzymes of aerobic respiration, and why? (d) Which fraction would contain the soluble cytosol at the end of the procedure?
A student examines diluted blood on a haemocytometer under the microscope and counts the cells in 16 small squares of the grid, obtaining the following counts: 12, 15, 11, 14, 13, 16, 12, 14, 15, 13, 11, 14, 12, 15, 13 and 14. (a) Calculate the total number of cells counted. (b) Calculate the average number of cells per small square. (c) State one precaution the student must take while filling the counting chamber, and explain how ignoring it would affect the count.
The central vacuole of a mature plant cell occupies 88% of the cell’s total volume, and the cell’s total volume is 5000 µm³. (a) Calculate the volume occupied by the vacuole. (b) Calculate the volume left for the cytoplasm, nucleus and organelles together. (c) Explain what advantage a plant cell gains by filling most of its volume in this way.
Chromosomes from a cell were banded and measured, giving the data below. All lengths are in arbitrary units.
| Chromosome | Length of short arm | Length of long arm |
|---|---|---|
| A | 4.0 | 4.0 |
| B | 1.5 | 6.5 |
| C | 0.3 | 7.7 |
| D | 0.0 | 8.0 |
(a) Classify each chromosome as metacentric, submetacentric, acrocentric or telocentric. (b) State the shape each would take at anaphase. (c) Explain why two chromosomes of the same total length may still be distinguished from each other in a banded preparation.