Chapter 5: Cell Structure And Organization
In the 1670s, a cloth merchant in the Dutch town of Delft began doing something no cloth merchant was expected to do. Antonie van Leeuwenhoek used small glass lenses to check the quality of thread — and became fascinated by what lenses could reveal. He taught himself to grind lenses far better than anyone else of his time, setting each tiny bead of glass into a small brass plate. The result was a simple microscope you held up to the eye like a peephole, yet it magnified far beyond the instruments of the day.
He pointed it at everything within reach. A drop of pond water. Rainwater standing in a gutter. Scrapings from his own teeth. And in each, he found the same startling thing: tiny bodies moving, spinning, swimming. He called them “animalcules” and wrote long letters describing them to the Royal Society in London, which at first found the reports hard to believe.
Leeuwenhoek had no idea that he was looking at bacteria — at whole living organisms made of a single cell. But he had opened a door. Once people accepted that life continues below the limit of the naked eye, the obvious question followed: what is every living body actually built out of?
Diagram
Fig. 5.0 – A hand holding Leeuwenhoek’s simple brass single-lens microscope up to the eye, with a magnified circular view beside it showing tiny moving single-celled organisms in a drop of pond water.
This chapter will help you answer each of these questions.
5.1 Cell
Every living thing you can name — a mango tree, a earthworm, a Paramoecium in pond water, or you — is built out of cells. A cell is not simply a building block like a brick in a wall. It is a working unit: it takes in materials, releases energy, responds to its surroundings, grows, and makes more of itself.
That is why the cell is described as both a structural and a functional unit of life. “Structural” means the body is made of cells. “Functional” means every activity of the body is really the sum of activities happening inside cells.
The phrase “capable of independent existence” is worth pausing on. A single cell of Amoeba is a complete organism on its own. In a multicellular body, cells cooperate instead, but each one still runs the same basic machinery of life inside itself.
Seeing the Cell
Most cells are far too small for the unaided eye, so studying them depends completely on the microscope. A simple microscope uses a single lens system and can magnify an image about 50 to 100 times, which is enough for larger cells.
To see smaller cells clearly we need a compound microscope, which uses a combination of lenses and can magnify 1000 times or more. The microscope used in your school laboratory is of this kind, and it is shown in Fig. 5.1.
Diagram
Fig. 5.1 – A simple single-lens microscope of the kind used in the late 1600s, with the lens holder, specimen pin and focusing screw labelled.
In the laboratory microscope, a beam of light is passed through the specimen to make it visible, which is why it is called a light microscope. Light, however, sets a limit: very fine details inside the cell simply cannot be separated by it.
For that we use the electron microscope, which can magnify the image up to 500000 times. It is this instrument that allowed biologists to see the interior of the cell — the membranes, tubules and particles that fill it.
Shape and Size of Cells
There is no typical shape of a cell. Cells may be spherical, rectangular, flattened, polygonal, oval, triangular, conical or columnar.
This variety is not random. Shape follows the job the cell has to do — a flattened cell suits a lining that substances must cross quickly, while a long, drawn-out cell suits carrying a signal over a distance.
Cell size also varies greatly in different plants and animals. Some cells are not visible to the naked eye, some are barely visible, and some are clearly macroscopic. The range across cells and their parts is shown on a logarithmic scale in Fig. 5.2.
Diagram
Fig. 5.2 – Logarithmic size scale from 0.1 nm to 1 mm showing, in order, atom, lipids, C60, protein, flu virus, bacteria, mitochondria, red blood cell, animal cell, plant cell, pollen, human egg and frog egg.
The smallest cell size is seen in mycoplasma, about 0.3 µm in length. A bacterial cell is 3 to 5 µm, while the largest cell is the ostrich egg, nearly 15 cm across. The longest cells in the body are the nerve cells.
How Cell Theory Took Shape
You already know that the cell theory was proposed by Schwann and Schleiden. Their work established that plants and animals alike are built of cells.
There was, however, a gap in this theory. It described what organisms are made of, but it said nothing about where new cells come from. If cells build every body, then each new cell must come from somewhere.
In later years, advanced research in cytology led to modification of the cell theory. The improved version is now known as the Modern Cell Theory, and its postulates are given below.
The growth of this idea, from the first cell theory to its modern form, is summarised in FC 5.1.
Flowchart
FC 5.1 – Development of cell theory: Schleiden (1838, plant tissues are made of cells) → Schwann (1839, cells are bound by a thin membrane; plants and animals are made of cells and cell products) → cell theory proposed, but origin of new cells unexplained → Virchow (1855, Omnis cellula-e-cellulla) → advances in cytology → Modern Cell Theory (six postulates)
Totipotency and Stem Cells
A skin cell and a muscle cell look nothing alike, yet both came from the same fertilised egg. This raises a striking possibility: does a specialised cell still carry the instructions for building any other cell type?
A cell is totipotent because it has the entire genetic information of the organism in its nucleus. Nothing has been thrown away during specialisation; different parts of the same information are simply put to use in different cells. Note the word “nucleated” in the definition — a cell that has lost its nucleus, and with it the full set of instructions, cannot be totipotent.
Embryonic animal cells are totipotent and are termed stem cells. Because they can still become many different cell types, stem cells have great medical applications, including the cure of diseases.
5.2 Kinds of Cells
All the cells of the living world can be sorted into two great groups: the Prokaryotes and the Eukaryotes. The dividing line is simple to state — whether or not the genetic material is enclosed in a proper nucleus.
The prokaryotes have a simple cellular organisation. The eukaryotes exhibit a high degree of organisation, with their work divided among many separate compartments. This basic split is shown in FC 5.2.
Flowchart
FC 5.2 – Kinds of cells: Living organisms → Prokaryotes (no true nucleus, no membrane-bound organelles; e.g. bacteria, cyanobacteria) and Eukaryotes (true nucleus with nuclear membrane, membrane-bound organelles; e.g. protists, fungi, plants, animals)
Prokaryotic Cells
The cells of prokaryotes show certain main features. Such a cell has a chemically complex, protective cell envelope on the outside. On the inside, however, it does not have a well-defined nucleus, nor does it have other membrane-bound cell organelles.
This does not mean a prokaryote does less work. It carries out respiration, protein synthesis and DNA replication just as we do — but it must do all of it in one shared space, using the cell membrane itself wherever a membrane is needed.
The Cell Envelope
The cell envelope is a three-layered structure. Starting from the outside and moving inwards, it consists of the glycocalyx, then the cell wall, and then the plasma membrane, as summarised in FC 5.3.
Flowchart
FC 5.3 – Layers of the bacterial cell envelope, outermost to innermost: Glycocalyx (slime layer, loose sheath / capsule, tough) → Cell wall (peptidoglycan or murein) → Plasma membrane (phospholipid bilayer)
The glycocalyx is the outermost sugary coat. It is present either as a slime layer, which is a loose sheath, or as a capsule, which is tough and firmly attached.
Because bacteria are almost transparent, they are better observed when stained. Staining also does more than make the cell visible — it separates bacteria into groups that behave differently.
The most followed staining method is ‘Gram staining’, developed by the Danish bacteriologist Hans Christian Gram.
The cell wall lies beneath the glycocalyx. It is made up of peptidoglycan in Gram positive bacteria and murein in Gram negative bacteria. Its main contribution is mechanical strength, which is what stops the cell from bursting.
Inside the wall lies the cell membrane, which is a phospholipid bilayer. Taken together, all these structures give protection to the cell and also help in inter-cellular transport.
Surface Structures and Movement
In motile bacteria, either cilia or flagella are found. Both are driven by a rotatory movement produced by the basal body, which works as a motor. The other parts of the structure are the filament and the hook.
This is a genuine rotating machine, not a lashing whip: the basal body turns, the hook acts as a flexible joint, and the long filament spins like a propeller to push the cell forward.
Some other surface projections are the tubular pili, which help in inter-cellular communication, and the fimbriae, which help the cell in clinging to a support.
Notice that the whole outer surface of a prokaryote is doing several jobs at once — protection by the envelope, movement by flagella, attachment by fimbriae and contact by pili. With no internal compartments available, the surface has to serve as the cell’s main working boundary.
Membrane Infoldings and Cytoplasm
The cell membrane shows infoldings called mesosomes. These help in cell wall formation, in cellular respiration and in DNA replication.
Mesosomes solve a problem created by the absence of organelles. A eukaryotic cell has mitochondria for respiration; a bacterium has no such organelle, so it folds its own membrane inward to gain extra surface for these activities.
Some bacteria, especially the photosynthetic cyanobacteria, show longer extensions called chromatophores. These carry the photosynthetic pigments, so they are the sites where light energy is captured in such cells.
The cytoplasm contains dense particles called ribosomes, which help in protein synthesis. Ribosomes are described by their sedimentation rate in Svedberg units, written with the letter S — a measure of how a particle settles when spun at high speed.
Bacterial ribosomes are 70S, composed of a larger subunit 50S and a smaller subunit 30S. All these features can be located in the prokaryotic cell shown in Fig. 5.3.
Diagram
Fig. 5.3 – A prokaryotic (bacterial) cell in section, labelled: capsule or slime layer, cell wall, plasma membrane, cytosol, chromosome (nucleoid region), plasmid (DNA), ribosomes, food granule, pili and flagellum.
Eukaryotic Cells
Cells in which the nucleus has a definite nuclear membrane are known as eukaryotic cells. These cells also exhibit the presence of membrane-bound cell organelles — for example, the cells of protists, plants, animals and fungi.
The value of these membranes is that they create compartments. Reactions that would interfere with one another can be kept apart, each with its own set of enzymes and its own internal conditions — an arrangement a prokaryote cannot achieve.
Eukaryotic cells differ widely in shape, size and physiology. Yet all of them are typically composed of the plasma membrane, the cytoplasm and its organelles — mitochondria, endoplasmic reticulum, ribosomes, Golgi complex and others — together with a true nucleus.
5.3 Components of Eukaryotic Cell
A eukaryotic cell is best understood as a workshop divided into rooms. Each organelle is a room with its own tools, its own conditions and its own job, and the whole cell works because these rooms cooperate. We will now go through them one by one, as they appear in the plant cell of Fig. 5.4 and the animal cell of Fig. 5.5.
Diagram
Fig. 5.4 – A plant cell in section, labelled: cell wall, cell membrane, Golgi apparatus, Golgi vesicles, chloroplast, vacuole membrane, large central vacuole, raphide crystal, druse crystal, mitochondrion, amyloplast (starch grain), rough ER, smooth ER, ribosome, nucleus, nucleolus and cytoplasm.
Diagram
Fig. 5.5 – An animal cell in section, labelled: cell (plasma) membrane, pinocytotic vesicle, lysosome, Golgi apparatus, Golgi vesicles, rough ER, smooth ER, mitochondrion, nucleus, nucleolus, centrioles, microtubules, ribosome and cytoplasm.
1. Cell Wall
The cell wall is the rigid, supportive and protective outer covering that lies outside the plasma membrane. It is found in plant cells, in fungi and in some protists, but not in animal cells.
Its chemical make-up varies with the group. Algae show the presence of cellulose, galactans, mannans and minerals such as calcium carbonate in the cell wall. In other plants it is made up of hemicelluloses, pectin, lipids and protein.
The rigidity itself comes from one component in particular. The microfibrils of the plant cell wall contain cellulose, and it is this cellulose that is responsible for the wall’s strength.
Further materials may be deposited in the wall to suit special needs. Such depositions include silica in the grass stem, cutin in the epidermal walls of land plants, suberin in the endodermal cells of the root, and also wax and lignin.
Because of all this, the wall gives shape to the cell and protects it from mechanical injury and infection. A wall that resists a fungal hypha trying to push its way in is doing defensive work quite as important as its mechanical work.
In plants, the cell wall shows three parts — the middle lamella, the primary wall and the secondary wall — arranged from the outside inwards as shown in FC 5.4 and in Fig. 5.6.
Flowchart
FC 5.4 – Layers of the plant cell wall, outermost to innermost: Middle lamella (pectin, calcium and magnesium pectate; shared between two adjacent cells) → Primary wall (capable of growth; the only wall in meristematic tissue, mesophyll, pith) → Secondary wall (laid after growth of the primary wall stops; thickening absent at some places → pits)
Middle lamella: It is thin and lies between two adjacent cells. It is the first structure formed from the cell plate during cytokinesis. It is mainly made up of pectin, calcium and magnesium pectate. The softening of ripe fruit is due to the solubilization of pectin — which is why a ripe papaya presses apart so easily between the fingers.
Primary wall: In a young plant cell this wall is capable of growth. It is laid down inner to the middle lamella. It is the only wall seen in the cells of meristematic tissue, mesophyll, pith and similar tissues, all of which are still growing or remain thin-walled.
Secondary wall: It is present inner to the primary wall. Once the growth of the primary wall stops, the secondary wall is laid down. At some places this thickening is absent, and these gaps lead to the formation of pits.
Diagram
Fig. 5.6 – Cell wall of a plant cell in section, labelled: middle lamella, pectin, primary cell wall, cellulose microfibril, hemicellulose, protein in plasma membrane and lipid bilayer of plasma membrane.
Plasmodesmata are cytoplasmic bridges between neighbouring cells. They communicate through pores between the primary cell wall and the middle lamella, so that the cytoplasm of one cell is continuous with the cytoplasm of the next.
2. Cell Membrane / Plasma Membrane / Biomembrane
The cell membrane is a thin, quasifluid structure present both extracellularly and intracellularly. Extracellularly it is present around the protoplast; intracellularly it is present around most of the cell organelles in a eukaryotic cell, where it separates those organelles from the cytosol.
The word “biomembrane” reminds us that the same basic membrane design is used everywhere in the cell — around the whole cell, around the nucleus, around a mitochondrion and around a vacuole.
The thickness of a biomembrane is about \(75\ \mathring{A}\). Under the electron microscope the cell membrane appears trilamellate, that is, made up of three layers.
It shows the presence of lipids, mostly phospholipids, arranged in a bilayer. Lipids possess one hydrophilic polar head and two hydrophobic non-polar tails, and so phospholipids are said to be amphipathic — a single molecule that is water-loving at one end and water-fearing at the other.
The lipid molecules are arranged in two layers in such a way that the tails are sandwiched in between the heads. Due to this arrangement, the tails never come in direct contact with the aqueous surroundings.
The cell membrane also shows the presence of proteins and carbohydrates. The ratio of proteins to lipids varies in different cells; for example, in human beings, RBCs show approximately 52% protein and 40% lipids.
Fluid Mosaic Model
The fluid mosaic model is the most accepted model of the cell membrane. It was proposed by Singer and Nicholson in 1972, and it is illustrated in Fig. 5.7.
According to this model, the membrane is made up of a phospholipid bilayer together with proteins. The proteins are like icebergs floating in a sea of lipids, and they can change their position.
Some proteins are intrinsic, that is, they occur at different depths of the bilayer. They span the entire thickness of the membrane, and so they are called transmembrane proteins. They form channels for the passage of water.
Extrinsic or peripheral proteins are found on the two surfaces of the membrane, rather than buried within it.
Diagram
Fig. 5.7 – Fluid mosaic model of the plasma membrane, labelled: phospholipid bilayer, integral membrane proteins, peripheral membrane protein, protein channel, glycoprotein, glycolipid, cholesterol and cytoskeletal filament.
The quasifluid nature of the lipid enables lateral movement of the proteins. This ability of molecules to move within the membrane is measured as fluidity.
Fluidity is not a minor detail. A membrane that can flow is a membrane that can be pinched off into a vesicle, fused with another membrane, and repaired after damage — all of which the cell does constantly.
Transport Across the Membrane
The main function of the plasma membrane is the transport of molecules across it. This membrane is selectively permeable, which means it allows some substances to pass while holding others back.
During passive transport, many molecules move across the membrane without spending energy. Some solutes move by simple diffusion along the concentration gradient, that is, from a higher to a lower concentration. Neutral solutes may cross the membrane by this process of simple diffusion, and this is called passive transport. Water may also move by osmosis.
During active transport, a few ions or molecules are transported against the concentration gradient, that is, from a lower to a higher concentration. This cannot happen on its own, so it requires energy and ATP is utilised. Because such transport is an energy-dependent process in which ATP is used, it is called active transport — for example, the \(\text{Na}^+/\text{K}^+\) pump.
Polar molecules cannot pass through the non-polar lipid bilayer. Such molecules therefore require carrier proteins to take them across.
3. Cytoplasm
The cell contains a ground substance called the cytoplasmic matrix or cytosol. This colloidal, jelly-like material is not static — it shows streaming movements called cyclosis.
The cytoplasm contains water as its major component, along with organic and inorganic molecules such as sugars, amino acids, vitamins, enzymes, nucleotides, minerals and waste products.
It also contains the various cell organelles — endoplasmic reticulum, Golgi complex, mitochondria, plastids, nucleus, microbodies — and cytoskeletal elements such as microtubules.
The cytoplasm thus acts as a source of raw materials as well as the seat of the various metabolic activities taking place in the cell. It also helps in the distribution and exchange of materials between the various cell organelles, and the streaming movement of cyclosis assists in this circulation.
Cell organelles are nothing but compartments in the cell that carry out specific functions. Some of them coordinate with each other and complete specific tasks for the cell.
The nuclear membrane, endoplasmic reticulum, Golgi complex, lysosomes and various types of vesicles and vacuoles form such a group, and together they are considered the endomembrane system of the cell. Their membranes are in constant physical exchange with one another through vesicles.
Organelles having distinct functions are not included in the endomembrane system. For example, mitochondria and chloroplasts carry out specific types of energy conversion in the cell and stand apart from this group.
Flowchart
FC 5.5 – Endomembrane system: Nuclear membrane → endoplasmic reticulum → Golgi complex → lysosomes and vesicles/vacuoles (membranes exchanged through transport vesicles). Not included: mitochondria and chloroplasts (independent energy-converting organelles)
4. Endoplasmic Reticulum (ER)
The endoplasmic reticulum is a little network within the cytosol. It is present in all eukaryotic cells except the ova and the mature red blood corpuscles.
Under the electron microscope it appears as a network of membranous tubules and sacs called cisternae, as seen in Fig. 5.8. It forms more than 50% of the total membrane of a eukaryotic cell — a striking figure that shows how much of the cell’s surface is internal rather than external.
Diagram
Fig. 5.8 – Endoplasmic reticulum continuous with the nuclear envelope, labelled: nucleus, nuclear envelope, rough endoplasmic reticulum with ribosomes, and smooth endoplasmic reticulum.
The ER divides the cytoplasm into two parts: the cytoplasm within the lumen of the ER, called luminal cytoplasm, and the non-luminal cytoplasm that lies outside the ER. This separation lets the cell keep newly made products apart from the general cytoplasm.
The membrane of the endoplasmic reticulum is continuous with the nuclear envelope at one end and extends up to the cell membrane. It thus acts as an intracellular supporting framework and helps in maintaining the position of the various cell organelles in the cytoplasm.
The outer surface of the endoplasmic reticular membrane may or may not be studded with ribosomes. Accordingly it is called rough ER or smooth ER respectively, and the two differ in their functions.
Smooth ER is involved in various processes in different cells. Depending on the cell type, it helps in the synthesis of lipids — for example in the steroid-secreting cells of the cortical region of the adrenal gland, the testes and the ovaries — in the detoxification of drugs and poisons in liver cells, and in the storage of calcium ions in muscle cells.
Rough ER is primarily involved in protein synthesis, for example in the pancreatic cells that secrete insulin. These proteins are made by the ribosomes attached to the rough ER and are called secretory proteins.
These proteins get wrapped in membrane that buds off from the transitional region of the ER. Such membrane-bound proteins depart from the ER as transport vesicles.
Rough ER is also involved in the formation of membrane for the cell. The ER membrane grows in place by the addition of membrane proteins and phospholipids to its own membrane, and portions of this expanded membrane are then transferred to other components of the endomembrane system.
5. Golgi Complex
The Golgi complex — also called the Golgi apparatus or Golgi body — is the manufacturing, packaging and transport unit of the cell. Different names are used for the same assembly.
The Golgi complex essentially consists of stacks of hollow membranous sacs called cisternae, shown in Fig. 5.9. The diameter of the cisternae varies from 0.5 to 1 µm, and a cell may have a few to several cisternae depending on its function — a cell that secretes heavily needs more of them.
Diagram
Fig. 5.9 – Golgi complex as a stack of curved cisternae, labelled: vesicle from ER, “receiving” side of Golgi apparatus (cis face), trans face and new vesicle forming.
The thickness and molecular composition of the two membranes of a Golgi sac differ from each other. The Golgi sacs also show a specific orientation in the cell: each cisterna has a forming or ‘cis’ face (cis: on the same side) and a maturing or ‘trans’ face (trans: the opposite side).
This orientation gives the organelle a direction of flow. Material enters at one face, is worked upon as it moves across the stack, and leaves from the other face.
Transport vesicles that pinch off from the transitional ER merge with the cis face of the Golgi cisterna and add their contents into the lumen. This explains why Golgi bodies are usually located near the ER.
Modified and condensed secretions leave the Golgi through the trans face, again as membrane-bound vesicles.
The Golgi body carries out two types of function: modification of the secretions of the ER, and production of its own secretions.
The cisternae contain specific enzymes for specific functions, so the refining of a product takes place in an orderly manner, step by step. For example, the glycolipids and glycoproteins brought from the ER lose certain sugars and gain others, and in this way a variety of products is formed.
Golgi bodies also manufacture their own products. In many plant cells they produce non-cellulose polysaccharides such as pectin — the very material that goes into the middle lamella. Whether manufactured or merely modified, all products of the Golgi complex leave the cisternae from the trans face as transport vesicles.
While these vesicles are leaving the Golgi, certain markers may get impregnated on their membrane so that they can identify their specific target cell or cell organelle. Without such an address label, a vesicle would have no way of knowing where to deliver its contents.
Flowchart
FC 5.6 – Path of a secretory protein: ribosomes on rough ER → protein enters ER lumen → membrane buds from transitional ER → transport vesicle → merges with cis face of Golgi → stepwise modification across cisternae → leaves from trans face as a transport vesicle with markers → target organelle or plasma membrane → secretion out of the cell
6. Lysosomes
Lysosomes can be considered the dismantling and restructuring units of a cell. They are membrane-bound vesicles containing hydrolytic enzymes, and their arrangement is shown in Fig. 5.10.
The enzymes in lysosomes are used by most eukaryotic cells to digest, that is, to hydrolyse, macromolecules. These lysosomal enzymes show optimal activity at acidic pH, so the inside of a lysosome is kept more acidic than the cytosol.
Lysosomes arise from the Golgi-associated endoplasmic reticulum, which places them squarely within the endomembrane system.
The list of lysosomal enzymes includes all types of hydrolases, namely amylases, proteases and lipases — enzymes for carbohydrates, proteins and fats respectively.
These enzymes are in an inactive state and are activated only when a lysosome comes in contact with another particular organelle to form a hybrid structure. This is an important safety arrangement: the enzymes wake up only when there is something to digest.
After the action of the enzymes is over, the lysosome is reformed and re-used. Lysosomes are thus found in various structural forms and carry out various functions for the cell.
Diagram
Fig. 5.10 – Lysosome structure showing single-wall membrane and enzyme complexes, together with the pathway: rough ER → transport vesicle → Golgi apparatus → lysosomes → fusion with food vacuole formed by phagocytosis, and autophagy of an old organelle.
Lysosomes are polymorphic in nature, that is, they exist in several forms. We can classify them as follows.
Primary lysosomes are nothing but membrane-bound vesicles in which the enzymes are in an inactive state — freshly formed and not yet put to work.
Secondary lysosomes, also called hybrid lysosomes, are formed by the fusion of a lysosome with an endocytic vesicle containing material to be digested. Such a structure is represented as a heterophagic vesicle, and it is larger in size than a primary lysosome.
Residual body is the vesicle containing the undigested remains left over in the heterophagic vesicle, after the products of digestion have been released into the cytosol.
Autophagic vesicles are lysosomes that bring about the digestion of the cell’s own organic material, such as a damaged cell organelle. They are also called suicide bags. An autophagic vesicle essentially consists of a lysosome fused with a membrane-bound old cell organelle or with organic molecules to be recycled.
The scale of this recycling is remarkable: every week, a human liver cell recycles half of its macromolecules. Worn-out parts are broken down and their building blocks used again.
Flowchart
FC 5.7 – Forms of the lysosome: Golgi-associated ER → primary lysosome (enzymes inactive) → branch one: fuses with endocytic vesicle → secondary (heterophagic) vesicle → digestion → products to cytosol → residual body with undigested remains; branch two: fuses with old or damaged organelle → autophagic vesicle (suicide bag) → contents recycled
Lysosomes bring about both intracellular and extracellular digestion. The intracellular digestion is brought about by autophagic vesicles or by secondary lysosomes, which contain foreign material brought in by processes such as phagocytosis. Examples are the food vacuole in Amoeba, and the macrophages in human blood that engulf and destroy harmful microbes entering the body.
Extracellular digestion is brought about by the release of lysosomal enzymes outside the cell. For example, the acrosome, a cap-like structure in the human sperm, is a modified lysosome containing various enzymes such as hyaluronidase. These enzymes bring about fertilization by dissolving the protective layers of the ovum.
During the process of metamorphosis found in many organisms, lysosomal enzymes help in reusing the tissues of redundant organs — the tadpole’s tail, for instance, is not thrown away but dismantled and its material reused. Lysosomes also help in the destruction of malignant cells, for example by T-lymphocytes.
7. Vacuoles
Vacuoles are membrane-bound sacs, prominently found in plant cells. In animal cells, whenever they are present, they are few in number and smaller in size. Generally there are two or three permanent vacuoles in a plant cell.
In some large plant cells a single large vacuole occupies the central part of the cell, and it is called the central vacuole. In such cells the vacuole can occupy as much as 90% of the total volume of the cell — the cytoplasm is pressed into a thin layer against the wall.
The vacuoles are bound by a semipermeable membrane called the tonoplast membrane. This membrane helps in maintaining the composition of the vacuolar fluid, the cell sap, different from that of the cytosol.
The composition of the cell sap differs in different types of cells. The cell sap of the central vacuole is a storehouse of various ions and is thus hypertonic to the cytosol. Because it is hypertonic, water tends to move into the vacuole, and this is what presses the cytoplasm outwards against the wall.
Small vacuoles in the seeds of certain plants store organic materials such as proteins. In vacuoles, along with excretory products, other compounds are stored that are harmful or unpalatable to herbivores, thereby protecting the plant — a plant that cannot run away defends itself chemically instead.
The attractive colours of petals are due to the storage of such pigments in vacuoles.
The intake of food or of a foreign particle by phagocytosis involves the formation of a food vacuole. In fresh water unicellular forms like Paramoecium, excretion and osmoregulation take place through contractile vacuoles. Vacuoles also maintain the turgidity of the cell.
8. Glyoxysomes
Glyoxysomes are membrane-bound organelles that contain enzymes which convert fatty acids to sugar. Their structure is shown in Fig. 5.11.
Diagram
Fig. 5.11 – A glyoxysome in section, labelled: membrane, matrix and core.
They can be observed in the cells of germinating seeds. Here the cells utilise stored fats as a source of sugar by reverse glycolysis, and they continue to do so until the young plant starts photosynthesising on its own.
This makes biological sense: a seed underground has no light and cannot yet make sugar, but it does carry a rich store of fat, and glyoxysomes are the organelles that turn that store into usable sugar.
9. Mitochondria (Singular: Mitochondrion)
Mitochondria are important cell organelles involved in aerobic respiration. They are absent in prokaryotic cells and in red blood corpuscles (RBCs). Their shape may be oval, spherical, or like a spiral strip.
The mitochondrion is a double membrane-bound organelle, as seen in Fig. 5.12. The outer membrane is permeable to various metabolites due to the presence of a protein — porin, also known as Parson’s particles. The inner membrane is selectively permeable to only a few substances. Both membranes are separated by a space, the outer chamber.
The inner membrane shows several finger-like or plate-like folds called cristae. These folds greatly increase the inner membrane’s area, and since this membrane carries the machinery of energy release, more folds mean more capacity.
The inner membrane bears numerous particles — oxysomes — and also cytochromes, which are electron carriers.
The inner membrane encloses a cavity, the inner chamber, containing a fluid called the matrix. The matrix contains a few coils of circular DNA, RNA, 70S types of ribosomes, lipids and various enzymes of the Krebs cycle and other pathways.
Diagram
Fig. 5.12 – Mitochondrion cut open, labelled: outer membrane, inner membrane, intermembrane space, cristae, matrix, DNA, ribosomes, \(F_0\) portion and \(F_1\) portion; with an enlarged oxysome showing head (\(F_1\)), pedicel (\(F_0\)) and base.
Oxysomes
The inner membrane of the mitochondrion bears numerous particles called oxysomes. They are also known as F1–F0 particles, Fernandez–Moran particles, elementary particles or mitochondrial particles.
Each particle consists of a head and a stalk or foot. The head (\(F_1\)), described as a lollipop head, faces towards the matrix, and the foot (\(F_0\)) is embedded in the inner membrane.
The head acts as the enzyme ATP synthase and the foot acts as a proton channel. Oxysomes are therefore involved in proton pumping and in ATP synthesis — protons pass through the foot and the head uses their movement to make ATP.
10. Plastids
Like mitochondria, plastids too are double-membraned organelles containing DNA, RNA and 70S ribosomes. But they are larger in size and can be observed under the light microscope.
Plastids are classified according to the pigments present in them as leucoplasts, chromoplasts and chloroplasts, as shown in FC 5.8.
Flowchart
FC 5.8 – Classification of plastids: Plastids → Leucoplasts (no pigment; storage) → amyloplasts (starch), elaioplasts (oils), aleuroplasts (proteins); Plastids → Chromoplasts (carotene, xanthophyll; red, yellow, orange colour of flowers and fruits); Plastids → Chloroplasts (chlorophyll; photosynthesis)
Leucoplasts do not contain any pigments and are of various shapes and sizes. These are meant for the storage of nutrients — for example, amyloplasts that store starch, elaioplasts that store oils and aleuroplasts that store proteins.
Chromoplasts contain pigments such as carotene and xanthophyll. They impart red, yellow or orange colour to flowers and fruits.
Now you must have understood why the potato looks white in colour, while the petals of the shoe flower are red — the first stores its material in colourless leucoplasts, the second carries chromoplasts. Both leucoplasts and amyloplasts do not contain the photosynthetic apparatus that we find in chloroplasts. Let us now study the chloroplast in detail.
Plant cells, the cells of algae and a few protists such as Euglena contain chloroplasts. You have observed the ribbon-shaped chloroplast in Spirogyra.
The chloroplast differs in size, number and shape in the various cells in which it is found. In plants it is found in green regions, mainly in the mesophyll of the leaf, where it is lens shaped. But it can also be oval, spherical, discoid or ribbon-like.
A cell may contain a single large chloroplast, as in Chlamydomonas, or there may be 20 to 40 chloroplasts per cell, as seen in mesophyll cells. Chloroplasts contain the green pigment chlorophyll, along with other enzymes that help in the production of sugar by photosynthesis.
The inner membrane of the double-walled chloroplast is comparatively less permeable. Inside the cavity of the inner membrane there is another set of membranous sacs called thylakoids, shown in Fig. 5.13.
Diagram
Fig. 5.13 – Chloroplast in section, labelled: external membrane, internal membrane, intermembrane space, stroma, granum, thylakoid, thylakoid membrane, lumen, lamella, chloroplast DNA, ribosome and plastoglobule.
Thylakoids are arranged in the form of stacks called grana (singular: granum). The grana are connected to each other by means of membranous tubules called stromal lamellae.
The space outside the thylakoids is filled with stroma. The stroma, and the space inside the thylakoids, contain various enzymes essential for photosynthesis. Like other plastids, the stroma of the chloroplast also contains DNA and ribosomes.
11. Ribosomes
You are aware that ribosomes are the protein factories of the cell. They use the genetic information to synthesise proteins.
Ribosomes were first observed as dense particles in an electron micrograph of a cell by the scientist Pallade in 1953. They are made up of ribosomal RNA and proteins, and they do not have any membranous covering around them. This absence of a membrane is why they are not part of the endomembrane system.
The composition of ribosomes in prokaryotes and eukaryotes is compared in Fig. 5.14.
Diagram
Fig. 5.14 – Ribosome composition compared: prokaryotes — small subunit 30S (16S rRNA + 21 proteins), large subunit 50S (23S rRNA + 5S rRNA + 34 proteins), complete ribosome 70S; eukaryotes — small subunit 40S (18S rRNA + 33 proteins), large subunit 60S (28S rRNA + 5.8S rRNA + 5S rRNA + 50 proteins), complete ribosome 80S.
In a eukaryotic cell, ribosomes are present in mitochondria, in plastids and in the cytosol. The ribosomes in the cytoplasm are either found attached to the outer surface of the rough endoplasmic reticulum and the nuclear membrane, or freely suspended in the cytoplasm.
Both kinds are similar in structure and are of the 80S type. Each ribosome is made up of two subunits, a large one and a small one.
Bound ribosomes generally produce proteins that are transported outside the cell after processing in the ER and the Golgi body. For example, the bound ribosomes of the acinar cells of the pancreas produce pancreatic digestive enzymes.
Free ribosomes come together and form chains called polyribosomes for protein synthesis. They generally produce enzymatic proteins that are used up within the cytoplasm, such as the enzymes required for the breakdown of sugar.
Both types of ribosomes can interchange position and function — the difference between them lies in where the protein is destined to go, not in the ribosome itself. The number of ribosomes is high in cells actively engaged in protein synthesis.
12. Nucleus
The structure of the nucleus of a eukaryotic cell becomes distinct in a non-dividing cell, that is, during interphase. Such an interphase nucleus is made up of the nuclear envelope, the nucleoplasm, the nucleolus and the chromatin network, as shown in Fig. 5.15 and in more detail in Fig. 5.16.
Diagram
Fig. 5.15 – Nucleus in section, labelled: nuclear membrane, nuclear pore, nucleoplasm and nucleolus.
The nuclear envelope is the double-layered delimiting membrane of the nucleus. The two membranes are separated from each other by the perinuclear space, which is 10 to 50 nm wide.
The outer membrane is connected with the endoplasmic reticulum at places, and it also harbours ribosomes on it. This connection is what makes the nuclear membrane a part of the endomembrane system.
The inner membrane is lined on its inner side by the nuclear lamina, a network of protein fibres that helps in maintaining the shape of the nucleus.
The two membranes, along with the perinuclear space, help in separating the nucleoplasm from the cytoplasm. However, the nuclear membrane is not continuous. At places there are small openings called nucleopores.
The nucleopores are guarded by pore complexes, which regulate the flow of substances from the nucleus to the cytoplasm and in the reverse direction. This is essential, because instructions must leave the nucleus and materials must enter it, while the genetic material itself stays inside.
The nucleoplasm or karyolymph contains various substances such as nucleic acids, protein molecules, minerals and salts. It contains the chromatin network and the nucleolus.
The nucleolus is another component which is not bound by a cell membrane — it is a dense body lying naked within the nucleoplasm.
The nucleolus is made up of rRNA and ribosomal proteins, and it is best known as the site of ribosome biogenesis.
Depending on the synthetic activity of a cell, there are one or more nucleoli present in the nucleoplasm. For example, the cells of an oocyte contain a large nucleolus, whereas sperm cells contain a small, inconspicuous one — an oocyte must stock up on ribosomes for the early embryo, while a sperm need not.
Nucleoli appear as dense spherical bodies present near the chromatin network. They produce rRNA and ribosomal proteins, which are then transported to the cytoplasm and assembled together to form ribosomes.
Chromatin Material
The nucleus contains genetic information in the form of chromosomes, which are nothing but DNA molecules associated with proteins.
In a non-dividing cell, the chromosomes appear as a thread-like network and cannot be identified individually. This network is called chromatin material. The chromatin material contains DNA, histone and non-histone proteins, and RNA.
In some regions of chromatin, the DNA is in a more extended state and is genetically active; this is called euchromatin. Some regions contain more protein, and here the DNA is in a highly condensed state and is genetically inert; these are called heterochromatin.
The logic is straightforward: DNA that is loosely packed can be read, while DNA that is tightly packed cannot. A cell controls which of its genes are in use partly by controlling how tightly they are wound.
Diagram
Fig. 5.16 – Ultrastructure of the nucleus, labelled: outer nuclear membrane, inner nuclear membrane, perinuclear space, nuclear pore, nucleolus, karyolymph, chromatin threads, heterochromatin, chromocentre and karyosome.
When the cell prepares to divide, the chromosomes coil and get condensed. At the metaphase stage they become distinct and can be clearly identified. You will study this process, as well as the structure and types of chromosome, in other chapters.
Every species of living organism has a specific number of chromosomes in its body cells; for example, a normal human cell has 46 chromosomes.
The nucleus contains the entire genetic information, and hence it plays an important role in heredity and variation. It is the site for the synthesis of DNA, RNA and ribosomes, and it plays an important role in protein synthesis.
Since the chromosome number is constant for a species, it is also important in phylogenetic studies, that is, in working out relationships between organisms. The nucleus is thus rightly called the master cell organelle.
13. Cytoskeleton
With advancement in light and electron microscopy, scientists revealed the presence of a network of fibrils throughout the cytoplasm. This network is called the cytoskeleton.
The cytoskeleton consists of microtubules, microfilaments and intermediate filaments. Microtubules are made up of the protein tubulin, microfilaments are made up of actin, and intermediate filaments are composed of fibrous proteins.
The cytoskeleton helps in the maintenance of the shape of the cell, in contraction of the cell, in the mobility of the cell and of cell organelles, in changes in the shape of cells, and in cell division.
Note that it is called a skeleton but does far more than a skeleton does. It holds the cell in shape, and it also moves things about within the cell.
Flowchart
FC 5.9 – Cytoskeleton: Cytoskeleton → microtubules (protein tubulin) + microfilaments (actin) + intermediate filaments (fibrous proteins) → functions: cell shape, contraction, mobility of cell and organelles, change of shape, cell division
Cilia and Flagella
Cilia and flagella are fine, hair-like, contractile, membrane-bound protoplasmic outgrowths that occur on the free surface of the cell. They generate a current in the fluid medium for the passage of material, and for locomotion.
Cilia are small in size and many in number. Cilia act as oars, causing coordinated movement of the cell — they beat in sequence rather than at random, which is why the current they set up flows in one direction.
Flagella are longer and few in number. Flagella present in prokaryotic bacteria are structurally different from those of eukaryotic flagella, even though both are used for movement.
A cilium or flagellum consists of a basal body, a basal plate and a shaft.
The basal body is placed in the outer part of the cytoplasm. It is derived from the centriole and has nine peripheral triplets of fibrils.
The shaft is the exposed part of the cilium or flagellum. It consists of two parts — the sheath and the axoneme. The sheath is the covering membrane of the cilium or flagellum.
The core, called the axoneme, possesses 11 fibrils running parallel to the long axis. It shows 9 peripheral doublets and two single central fibrils, an arrangement written as (9+2) and seen in Fig. 5.17. The central tubules are enclosed by a central sheath.
Diagram
Fig. 5.17 – Transverse section of a cilium showing the 9+2 arrangement, labelled: plasma membrane, central microtubules, inner sheath, radial spokes, nexin, dynein arms, doublet microtubule with A tubule and B tubule.
This sheath is connected to one of the tubules of the peripheral doublets by a radial spoke. The central tubules are connected to each other by bridges. The peripheral doublets are connected to each other through linkers, also called the interdoublet bridge.
All these connections matter for movement. Because the doublets are tied to one another and to the centre, they cannot simply slide apart; the sliding is converted into a bending of the whole cilium.
Centrioles and Centrosomes
The centrosome is usually found near the nucleus of an animal cell. It contains a pair of cylindrical structures called centrioles, shown in Fig. 5.18.
The cylinders are placed perpendicular to each other and are surrounded by an amorphous substance called the pericentriolar material.
Each cylinder of the centriole is made up of nine sets of triplet microtubules, which are made up of tubulin. The evenly spaced triplets are connected to each other by means of non-tubulin proteins.
Diagram
Fig. 5.18 – Structure of the centriole, shown lengthwise and in transverse section, labelled: microtubule triplet, connecting fibres, A–C linker, foot and radial fibre (spoke).
At the proximal end of the centriole there is a set of tubules called the hub. The peripheral triplets are connected to the hub by means of radial spokes, and because of this the proximal end of the centriole looks like a cartwheel.
The centrosomes help in the assembly of the spindle apparatus during cell division. The centriole also forms the basal body of cilia and flagella — the same structure serving two different purposes in the life of the cell.
5.4 Summary
The cell is the structural and functional unit of life in all living organisms. It is capable of independent existence and can carry out all the functions of life.
Cells are studied with microscopes. A simple microscope magnifies 50 to 100 times, a compound microscope 1000 times or more, and an electron microscope up to 500000 times. A light microscope uses a beam of light; the interior of the cell is seen with the electron microscope.
There is no typical shape of a cell — cells may be spherical, rectangular, flattened, polygonal, oval, triangular, conical or columnar.
Cell size varies greatly. The smallest cell is that of mycoplasma (0.3 µm), a bacterial cell is 3 to 5 µm, and the largest cell is the ostrich egg (nearly 15 cm). The longest cells are the nerve cells.
Cell theory was proposed by Schleiden and Schwann, but it did not explain how new cells are formed. Rudolf Virchow (1855) showed that new cells arise by division of pre-existing cells (Omnis cellula-e-cellulla). Later work in cytology gave the Modern Cell Theory.
The postulates of modern cell theory are: all living organisms are made of cells; living cells arise from pre-existing cells; a cell is the structural and functional unit of life; the total activities of cells are responsible for the activity of an organism; cells show transformation of energy; and cells contain nucleic acids — DNA and RNA in the nucleus and cytoplasm.
Totipotency is the capacity of a living nucleated cell to differentiate and divide to form any other type of cell, and thereby a complete new organism. Embryonic animal cells are totipotent and are called stem cells, which have great medical applications.
Cells are of two kinds. Prokaryotic cells lack a well-defined nucleus and membrane-bound organelles; eukaryotic cells have a nucleus with a definite nuclear membrane and membrane-bound organelles.
The prokaryotic cell envelope has three layers — glycocalyx (slime layer or capsule), cell wall (peptidoglycan in Gram positive, murein in Gram negative bacteria) and plasma membrane. Gram staining was developed by Hans Christian Gram.
Prokaryotes may bear flagella or cilia driven by a rotatory basal body, along with pili (inter-cellular communication) and fimbriae (clinging to support). Mesosomes are membrane infoldings that help in cell wall formation, cellular respiration and DNA replication; chromatophores of cyanobacteria carry photosynthetic pigments. Bacterial ribosomes are 70S (50S + 30S).
Bacterial genetic material is a single circular, coiled DNA ring attached to the mesosome, replicated by the theta model and not associated with histones. Plasmids are extrachromosomal, self-replicating DNA molecules — F-plasmid for reproduction and R-plasmid for antibiotic resistance.
The cell wall of plants is rigid and protective, made rigid by cellulose microfibrils, with depositions such as silica, cutin, suberin, wax and lignin. It has three parts — middle lamella, primary wall and secondary wall. Plasmodesmata are cytoplasmic bridges connecting neighbouring cells.
The plasma membrane is about \(75\ \mathring{A}\) thick and appears trilamellate. It is a bilayer of amphipathic phospholipids with proteins and carbohydrates. The fluid mosaic model (Singer and Nicholson, 1972) describes proteins as icebergs in a sea of lipids; intrinsic proteins span the membrane, extrinsic proteins lie on its surfaces.
The membrane is selectively permeable. Passive transport (simple diffusion, osmosis) needs no energy and follows the concentration gradient; active transport moves substances against the gradient using ATP, as in the \(\text{Na}^+/\text{K}^+\) pump. Polar molecules need carrier proteins.
The cytoplasm is a colloidal matrix showing streaming movement called cyclosis. It holds raw materials, enzymes and organelles, and is the seat of metabolic activity. The nuclear membrane, ER, Golgi complex, lysosomes, vesicles and vacuoles together form the endomembrane system; mitochondria and chloroplasts are not part of it.
The endoplasmic reticulum is a network of cisternae forming more than 50% of the cell’s membrane. Smooth ER carries out lipid synthesis, detoxification and calcium storage; rough ER, studded with ribosomes, makes secretory proteins and new membrane.
The Golgi complex is the manufacturing, packaging and transport unit. Its cisternae (0.5 to 1 µm) receive vesicles at the cis face, modify the contents in an orderly manner, and release them from the trans face as transport vesicles carrying markers.
Lysosomes are membrane-bound vesicles of hydrolytic enzymes that act at acidic pH and arise from Golgi-associated ER. They exist as primary lysosomes, secondary (heterophagic) lysosomes, residual bodies and autophagic vesicles, and carry out intracellular and extracellular digestion.
Vacuoles are bound by the tonoplast and hold cell sap. A central vacuole may occupy 90% of a plant cell’s volume, storing ions, proteins, pigments and defensive compounds, and maintaining turgidity. Contractile vacuoles of Paramoecium carry out excretion and osmoregulation.
Microbodies include sphaerosomes, which store fats and have a half unit membrane, and peroxisomes, which produce \(H_2O_2\) and also convert it to water. Glyoxysomes convert fatty acids to sugar in germinating seeds by reverse glycolysis.
Mitochondria are double-membraned organelles of aerobic respiration, absent in prokaryotes and RBCs. The inner membrane forms cristae bearing oxysomes and cytochromes; the matrix holds circular DNA, RNA, 70S ribosomes and Krebs cycle enzymes. In an oxysome, the head (\(F_1\)) is ATP synthase and the foot (\(F_0\)) is a proton channel.
Plastids are double-membraned organelles with DNA, RNA and 70S ribosomes. Leucoplasts store nutrients (amyloplast, elaioplast, aleuroplast), chromoplasts give red, yellow and orange colours, and chloroplasts contain chlorophyll. In a chloroplast, thylakoids are stacked into grana joined by stromal lamellae, surrounded by stroma.
Ribosomes are non-membranous particles of rRNA and protein, first observed by Pallade (1953). Eukaryotic cytoplasmic ribosomes are 80S; bound ribosomes make proteins for export, free ribosomes form polyribosomes and make proteins used within the cytoplasm.
The interphase nucleus consists of the nuclear envelope with perinuclear space (10 to 50 nm) and nucleopores guarded by pore complexes, the nucleoplasm, the nucleolus (site of ribosome biogenesis) and the chromatin network. Extended, active chromatin is euchromatin; condensed, inert chromatin is heterochromatin. A normal human cell has 46 chromosomes.
The cytoskeleton of microtubules (tubulin), microfilaments (actin) and intermediate filaments maintains cell shape and helps in contraction, mobility and cell division.
Cilia and flagella are protoplasmic outgrowths with a basal body, basal plate and shaft; the axoneme shows the 9+2 arrangement of 11 fibrils. The centrosome, found near the nucleus of an animal cell, contains two perpendicular centrioles of nine triplet microtubules; it assembles the spindle apparatus and forms the basal body of cilia and flagella.
The overall plan of the cell, as covered in this chapter, is condensed in FC 5.10.
Flowchart
FC 5.10 – Chapter overview: Cell → Prokaryotic cell (envelope: glycocalyx → cell wall → plasma membrane; nucleoid, plasmids, mesosomes, 70S ribosomes, flagella/pili/fimbriae) and Eukaryotic cell → boundary (cell wall, plasma membrane) → cytoplasm (cyclosis) → endomembrane system (ER → Golgi → lysosomes, vacuoles) + independent organelles (mitochondria, plastids) + non-membranous parts (ribosomes, cytoskeleton, cilia and flagella, centrioles) → nucleus (nuclear envelope, nucleoplasm, nucleolus, chromatin)
5.5 Points to Ponder
A cell is called the unit of life because it can exist independently. Then in what sense is a single cell in your liver “independent”, when it would die within minutes if removed from the body?
Virchow’s statement that every cell comes from a pre-existing cell explains the cells alive today. Where, then, could the very first cell have come from?
Cell size ranges from mycoplasma at 0.3 µm to an ostrich egg nearly 15 cm across. Why do most working cells of a large animal stay microscopic instead of simply growing bigger as the animal grows?
A eukaryotic cell divides its work among membrane-bound compartments, while a prokaryotic cell manages the same essential processes in one open space. What does a cell gain by making compartments, and what must it spend to maintain them?
The bacterial cell has survived for billions of years without a nucleus. Does “simple organisation” mean “less successful”? What would you use as a measure of success in living things?
Plasmids carry only a few genes, yet they can pass between bacteria. What does this suggest about how quickly antibiotic resistance can spread through a bacterial population, and what should that mean for how we use antibiotics?
The plasma membrane is described as fluid, yet it is also a boundary that holds the cell together. How can the same structure be both flowing and reliably intact?
Lysosomal enzymes work best at acidic pH while the cytosol is near neutral. Is this difference an accident of chemistry, or can you argue that it is a safety arrangement the cell depends on?
A liver cell recycles half of its macromolecules every week. Why would a cell dismantle structures that are still functioning, instead of simply keeping them?
The endosymbiont theory proposes that mitochondria and chloroplasts were once free-living prokaryotes. What kinds of evidence would strengthen this idea, and what kind of observation would count against it?
A plant cell is enclosed in a rigid wall, and a large central vacuole may fill 90% of its volume. In what ways does this arrangement suit an organism that cannot move, and in what ways does it limit one?
The nucleus is called the master organelle, yet mature human RBCs and mature xylem elements function without one. What does this tell you about the difference between being alive and being able to continue living?
The same structure — nine triplet microtubules — serves as a centriole during cell division and as a basal body for cilia and flagella. Why might living things reuse one design for very different purposes?
5.6 Check Your Concepts
The cell theory of Schleiden and Schwann was considered incomplete until 1855. What was missing from it, and how did Virchow complete it?
Arrange the following in order of increasing size and justify your sequence: bacterial cell, mycoplasma, ostrich egg, mitochondrion.
A student says, “Our laboratory microscope is a compound microscope, so it cannot be a light microscope.” Explain what is wrong with this statement.
Why is a light microscope of no use for studying the internal structure of a mitochondrion, however much its magnification is increased?
Name the three layers of the bacterial cell envelope from outside inwards, and state one function of each.
What happens to a bacterium if its cell wall is destroyed while the surrounding medium remains dilute? Explain your answer using the function of the wall.
A bacterium has no mitochondria, yet it carries out cellular respiration. Which structure makes this possible, and how is it suited to the job?
Distinguish between pili and fimbriae, and between a slime layer and a capsule.
Bacterial DNA is described as “naked”. What does this mean, and why is bacterial genetic material therefore not called chromatin?
A cell in a meristematic tissue has no secondary wall. Why would a secondary wall be a disadvantage in such a cell?
Ripe fruits become soft while unripe ones stay firm. Which part of the cell wall is responsible, and what change takes place in it?
Phospholipids arrange themselves into a bilayer as soon as they are placed in water, with no help from the cell. Explain why, using the term amphipathic.
Why can a small non-polar molecule such as oxygen cross the plasma membrane freely, while a similar-sized polar molecule needs a carrier protein?
What would happen to the \(\text{Na}^+/\text{K}^+\) pump of a cell whose ATP supply is suddenly cut off? Explain the effect on the movement of ions.
Mitochondria and chloroplasts are not included in the endomembrane system, even though both are membrane-bound. Give reasons.
Arrange the following in the correct sequence followed by a secretory protein, starting from its synthesis: transport vesicle from transitional ER, cis face of Golgi, ribosome on rough ER, trans face of Golgi, secretion outside the cell.
Cells of the adrenal cortex are rich in smooth ER, while pancreatic cells that secrete insulin are rich in rough ER. Explain this difference in terms of the work each cell does.
Golgi bodies are usually found close to the endoplasmic reticulum. What feature of their working explains this position?
Vesicles leaving the trans face of the Golgi complex carry markers on their membranes. What would go wrong in the cell if these markers were absent?
Lysosomes are called suicide bags, yet they do not normally destroy the cell that contains them. Give two reasons why the cell stays safe.
Distinguish between a primary lysosome, a secondary lysosome and a residual body.
What is the role of lysosomal enzymes in fertilization, and which structure of the sperm supplies them?
An insufficiency of a single lysosomal enzyme can lead to a disorder such as Tey Sach’s disease. Explain how the lack of one enzyme causes damage in an organ.
The cell sap of a central vacuole is hypertonic to the cytosol. What does this mean for the movement of water, and how does it help a soft-stemmed plant stay upright?
What happens to a plant cell, and to the plant as a whole, when the central vacuoles lose water on a hot afternoon? Is the change reversible? Explain.
Peroxisomes produce a toxic substance and also destroy it. Name the substance, write the conversion involved, and explain why both sets of enzymes are present in the same organelle.
Why are glyoxysomes especially important in a germinating oil seed, and why do they matter less once the seedling turns green?
Describe the structure of an oxysome and state the function of its head and its foot.
Mature human RBCs have no mitochondria. Suggest a reason why this is an advantage rather than a defect.
A potato tuber is white while a carrot root is orange. Which plastids are responsible in each case, and how do they differ?
List the features shared by mitochondria and chloroplasts that support the endosymbiont theory. Which of these features is also found in a bacterial cell?
Draw a labelled diagram of a chloroplast and mark the stroma, granum, thylakoid and stromal lamella. State where the enzymes of photosynthesis are found.
A 50S subunit and a 30S subunit together form a 70S ribosome and not an 80S one. Explain why Svedberg values cannot simply be added.
Distinguish between bound and free ribosomes with respect to their position and the fate of the proteins they make.
Why is the number of ribosomes very high in the acinar cells of the pancreas?
Ribosomes are found in the cytosol, in mitochondria and in plastids of a eukaryotic cell. What is different about the ribosomes present in the last two, and what does this difference suggest?
The nuclear membrane is not continuous. Describe the openings present in it and explain why the cell needs them.
An oocyte has a large nucleolus while a sperm cell has a small, inconspicuous one. Explain this difference in terms of the function of the nucleolus.
Distinguish between euchromatin and heterochromatin, and state which of the two is genetically active.
Why can individual chromosomes not be identified in an interphase nucleus, while they can be identified clearly at metaphase?
The nucleus is called the master cell organelle. Give three reasons that justify this description.
Name the three components of the cytoskeleton along with the protein each is made of, and state two functions of the cytoskeleton as a whole.
Draw a labelled transverse section of a cilium and explain what is meant by the 9+2 arrangement. How many fibrils does the axoneme contain in all?
Bacterial flagella and eukaryotic flagella both bring about movement, yet they are said to be structurally different. Describe the difference.
The proximal end of a centriole is described as looking like a cartwheel. Which structures give it this appearance?
State two roles of the centrosome in an animal cell, and explain how a centriole is related to a cilium.
Plants have no circulatory system. Then how do cells manage intercellular transport?
Is the nucleolus covered by a membrane?
The fluid mosaic model proposed by Singer and Nicholson replaced the sandwich model proposed by Danielli and Davson. Why?
The RBC surface normally shows glycoprotein molecules. Do they play any role while determining the blood group?
How does cytoplasm differ from nucleoplasm in chemical composition?
Distinguish between smooth and rough endoplasmic reticulum.
Mitochondria are the power house of the cell. Give reasons.
What are the types of plastids?
Label the diagrams and write down the details of the concept in your own words.
Diagram
Fig. 5.19 – Four unlabelled organelles for the student to label, with blank label lines: A – mitochondrion cut open; B – chloroplast cut open; C – Golgi complex; D – endoplasmic reticulum with nuclear envelope.
- Complete the flow chart.
Flowchart
FC 5.11 – Incomplete flow chart for the student to fill: blank box → Transport vesicle → Golgi apparatus → blank box → Autophagy
- Label A, B, C and D in the diagram given below, and write the functions of organelles A and B.
Diagram
Fig. 5.20 – A plant cell in section with four unlabelled leader lines A, B, C and D pointing to different organelles.
- Identify each cell structure or organelle from its description below.
- Manufactures / generates ribosomes
- Carries out photosynthesis
- Manufactures ATP in animal and plant cells
- Selectively permeable
- Onion cells have no chloroplast. How can we tell that they are plant cells?
5.7 Objective Questions
Growth of the cell wall during cell elongation takes place by ………….
- Apposition b. Intussusception c. Both a & b d. Super position
Cell membrane is composed of
- Proteins and cellulose b. Proteins and phospholipid c. Proteins and carbohydrates d. Proteins, phospholipid and some carbohydrates
Plasma membrane is a fluid structure due to the presence of –
- Carbohydrates b. Lipid c. Glycoprotein d. Polysaccharide
Cell wall is present in –
- Plant cell b. Prokaryotic cell c. Algal cell d. All of the above
Plasma membrane is –
- Selectively permeable b. Permeable c. Impermeable d. Semipermeable
Mitochondrial DNA is –
- Naked b. Circular c. Double stranded d. Each one of the above
Which of the following sets of organelles contain DNA?
- Mitochondria, Peroxisome b. Plasma membrane, Ribosome c. Mitochondria, Chloroplast d. Chloroplast, Golgi body
Assertion: A mature human red blood corpuscle cannot be called totipotent. Reason: Totipotency requires the presence of the entire genetic information in the nucleus of a living cell. Study the assertion and the reason, and state whether each is true, and whether the reason correctly explains the assertion.
Assertion: An R-plasmid can make a bacterial population resistant to an antibiotic much faster than ordinary inheritance would allow. Reason: Plasmids are extrachromosomal, self-replicating DNA molecules that can be passed between bacteria. Comment on both statements.
Assertion: The fluidity of the plasma membrane is essential for the formation of transport vesicles. Reason: The quasifluid nature of lipids allows lateral movement within the membrane. Comment on both statements.
Assertion: A cell that needs a great deal of energy usually has mitochondria with numerous cristae. Reason: Cristae increase the surface area of the inner membrane, which bears the oxysomes. Comment on both statements.
Match the following.
| Column A | Column B |
|---|---|
| a. Amyloplast | i. stores proteins |
| b. Elaioplast | ii. stores starch |
| c. Aleuroplast | iii. contains chlorophyll |
| d. Chloroplast | iv. stores oils |
- Match the following.
| Column A | Column B |
|---|---|
| a. Mesosome | i. rotatory movement |
| b. Chromatophore | ii. cell wall formation and DNA replication |
| c. Basal body | iii. photosynthetic pigments in cyanobacteria |
| d. Fimbriae | iv. clinging to a support |