Chapter 6: Photosynthesis
In the summer of 1771, in a laboratory in Leeds, Joseph Priestley lit a candle and sealed it inside a glass jar. The flame burned for a while and then went out. He tried a fresh candle in the same jar, and it would not stay alight. Something in the air had been used up, and nothing he did could bring the flame back.
Then Priestley tried something odd. He put a sprig of mint into a jar of this spent air and left it standing for several days. When he came back and lowered a lighted candle in, the flame held. A mouse, which could not have stayed alive in that jar earlier, could now live in it. The plant had repaired the air.
Priestley could not say what the mint had put back. Oxygen had not yet been named. He knew only that a green plant somehow undoes the damage that burning and breathing do to air.
Eight years later a Dutch physician, Jan Ingenhousz, repeated the work with more care and found the missing condition. The mint mended the air only in sunlight. Left in the dark, it did the opposite. And only the green parts of the plant could do it at all.
Green, light, air. Three clues, gathered two and a half centuries ago. This chapter is about what those two men were watching.
Diagram
Fig. 6.0-1 — Priestley’s Sealed Jar Experiment Line drawing of two sealed bell jars standing side by side on a bench; jar A holds only a burning candle with its flame guttering out, jar B holds a burning candle together with a sprig of mint in a small water dish and the flame burning steadily; label sealed glass jar, candle, sprig of mint, flame goes out (A), flame keeps burning (B).
6.1 Plants As Their Own Food Producers
Every living thing needs food. Food does two jobs at once: it releases the energy needed for growth, movement and repair, and it supplies the material out of which new living matter is built.
Animals cannot manufacture this food. A cow crops grass, and you eat rice that a plant made. Every animal, without exception, takes food that some other organism has already prepared.
Green plants do what no animal can. Using the energy of sunlight, they build food out of two very simple substances — carbon dioxide from the air and water from the soil. This building up of food in green cells, using light energy, is called photosynthesis.
Two Ways Of Getting Food
Organisms that prepare their own food from simple, non-living raw materials are called autotrophs, which means self-feeders. Green plants are autotrophs, and so are algae, which are green and carry out the same process in water.
Organisms that cannot prepare their own food, and must take it ready-made from other organisms, are called heterotrophs, which means other-feeders. All animals are heterotrophs. So are non-green plants such as fungi, which have no chlorophyll and draw their nourishment from other organic matter.
The difference between the two ways of feeding is set out in Table 6.1.
Because heterotrophs feed on autotrophs, or on other heterotrophs that fed on autotrophs, every meal eaten anywhere on the earth can be traced back to a green cell that once trapped sunlight. That single process is the subject of this chapter.
6.2 What Photosynthesis Is
Photosynthesis is the making of food inside a green cell. The plant takes two very ordinary substances and builds a sugar out of them.
The first is carbon dioxide, a gas present in the air in small amounts. The second is water, which the roots absorb from the soil and the stem carries up to the leaves.
Two other things must be present, though neither is used up. One is light. The other is chlorophyll, the green pigment held inside the cells of the leaf, which absorbs light energy and makes it available for the reaction.
The food formed is glucose, a simple sugar that dissolves readily in water. A leaf turns most of this glucose almost at once into starch, an insoluble substance that can be stored where it is made.
There is one more product. As the food is built, oxygen is set free and passes out of the leaf into the air. It is a by-product — a substance produced alongside the main product, and not the purpose of the process.
The Four Essentials
Four things, then, are essential: carbon dioxide, water, chlorophyll and light. Remove any one of them and no food is made. FC 6.1 sets out what goes in and what comes out.
Flowchart
FC 6.1 — What Goes Into Photosynthesis And What Comes Out Carbon dioxide (from the air) + water (from the soil) → enter a cell containing chlorophyll → light energy absorbed → glucose formed (soon stored as starch) + oxygen released into the air
These chemical steps are the same in every green plant. A patch of moss on a damp wall and a banyan tree spreading over a field are running the same process, and differ only in scale.
6.3 Chlorophyll — The Light-Trapping Pigment
The name of the green pigment says where it is found. It comes from two Greek words, chloros meaning green and phyllon meaning leaf.
A pigment is a substance that gives colour by absorbing some colours of light and turning back the rest. Chlorophyll is the pigment that makes leaves green, and it is also the only substance in a plant that can capture light energy for food-making.
Chlorophyll is not spread loosely through the cell. It is packed inside tiny green bodies, and everything about the shape of those bodies is arranged around the job of catching light.
6.3.1 The Chloroplast And Its Internal Structure
A chloroplast is the small green organelle inside a plant cell in which photosynthesis takes place. Chloroplasts are minute oval bodies, and an ordinary leaf cell holds about 40 to 50 of them. There may be more than 500,000 chloroplasts under every square millimetre of leaf surface.
Each chloroplast is bounded by an envelope of two membranes, an outer and an inner one. This double envelope keeps the contents of the chloroplast separate from the rest of the cell, so the reactions inside are not disturbed.
Inside the envelope lie closely packed flattened sacs called thylakoids. The chlorophyll is held in the membranes of these sacs, which is why the thylakoids and not the rest of the chloroplast are where light is trapped.
The thylakoids are not scattered. They are stacked into neat piles, and each pile is called a granum (plural, grana). Slender interconnecting membranes called frets run between one granum and the next, linking them into a single system. The internal structure is shown in Fig. 6.3-1.
Surrounding the grana is a colourless ground fluid called stroma. It fills the rest of the chloroplast and holds the enzymes needed for the later stages of food-making.
Diagram
Fig. 6.3-1 — Internal Structure Of A Chloroplast Cut-away diagram of a single oval chloroplast with part of the envelope removed to show the interior; label outer membrane, inner membrane, thylakoid (flattened sac), granum (pile of thylakoids), fret (interconnecting membrane), stroma (ground fluid); one granum shown enlarged in vertical section alongside.
Keep these two regions clearly apart in your mind — the stacked sacs, and the fluid around them. Photosynthesis runs in two stages, and each stage takes place in one of these two regions.
Chlorophyll itself is a complex substance built from carbon, hydrogen, oxygen, nitrogen and magnesium. The magnesium is worth noting, because a plant short of magnesium in the soil cannot build enough chlorophyll, however bright the sunlight is.
Chloroplasts are found chiefly in the green tissue lying between the upper and lower skin of the leaf. They also occur in the guard cells that border the tiny pores of the leaf, and in the outer layers of young green stems.
6.3.2 Types Of Chlorophyll And The Light They Absorb
Chlorophyll is not a single substance. Several types are known, and of these, chlorophyll-a and chlorophyll-b are the best known and the most abundant in green plants.
To see what these pigments do with light, first recall what sunlight is. White light is a mixture of colours, and a glass prism or a rainbow separates it into violet, indigo, blue, green, yellow, orange and red.
Chlorophyll does not absorb these colours equally. It absorbs strongly at both ends of the visible spectrum — the blue light at one end and the red light at the other.
Green light, lying in the middle, is largely not absorbed. It is reflected away from the leaf and reaches your eye, and that is exactly why chlorophyll, and every leaf containing it, looks green to you. Fig. 6.3-2 shows this.
Diagram
Fig. 6.3-2 — Light Absorbed And Light Reflected By Chlorophyll Horizontal band of the visible spectrum from violet to red falling on a leaf; arrows pointing into the leaf at the blue and red ends marked absorbed, and an arrow bouncing away from the middle of the band marked green light reflected; caption labels blue region, green region, red region.
Since blue and red light are the colours actually taken in, they are the most effective for photosynthesis. Green light, being turned away, does the plant very little good.
6.3.3 Light And The Formation Of Chlorophyll
Light and chlorophyll have a double relationship. A plant needs light to make chlorophyll, and yet very strong light can destroy the chlorophyll it has already made.
Chlorophyll is highly sensitive to light. Under excessive light it breaks down faster than the plant can replace it.
At the same time, the formation of fresh chlorophyll depends on the plant being exposed to light. A plant kept in the dark makes no new chlorophyll at all, while the chlorophyll it already holds slowly disintegrates.
You can see the result on any lawn. Lift a stone that has been lying on grass for a week, and the grass beneath it is pale yellow. No new chlorophyll was formed in the dark, and the old chlorophyll broke down with nothing to replace it.
6.4 Stomata And The Entry Of Carbon Dioxide
Carbon dioxide has to reach the green cells inside a leaf. But a leaf is covered on both faces by a skin coated with wax, and that coat is more or less airtight.
The gas gets in through stomata (singular, stoma) — minute pores that open through this skin. They occur in very large numbers, and in most leaves they lie chiefly on the lower surface.
An open pore, however, does not admit gas in one direction only. Water vapour escapes outwards through the same opening, and this loss of water from the plant surface is called transpiration.
So the plant faces a real cost. Every time it opens its stomata to take in carbon dioxide, it loses water that had to be raised all the way from the roots.
It settles the matter by opening the pores only when they are needed. After sunrise, when light is available and food-making begins, the stomata open. In darkness, when no photosynthesis can occur, they close and water loss is reduced.
6.4.1 Structure Of The Stomatal Apparatus
A stoma is not simply a hole in a wall. It is the gap between two specialised cells, called guard cells, which surround the pore and control its width.
Guard cells differ from the ordinary epidermal cells around them in one important respect: their cytoplasm contains chloroplasts. Each guard cell also has a nucleus, and the pair together makes up the stomatal apparatus.
The two walls of a guard cell are not alike. The inner wall, the one facing the pore, is thick and stiff. The outer wall, on the far side, is thin and easily stretched. Fig. 6.4-1 shows the pore open and closed.
This unevenness is the whole mechanism. When a guard cell fills with water and swells, only the thin outer wall can stretch, so the cell curves outwards and the pore between the pair is pulled open.
Diagram
Fig. 6.4-1 — The Stomatal Apparatus, Open And Closed Surface view of a pair of guard cells shown twice side by side; A with the guard cells swollen and curved apart so the pore is wide open, B with them limp and straight so the pore is closed; label guard cell, thick inner wall, thin outer wall, chloroplasts, nucleus, stomatal pore, epidermal cells, and mark A open, B closed.
6.4.2 Opening And Closing Of Stomata
Opening and closing therefore come down to one thing: the movement of water into and out of the guard cells. A cell swollen and firm with water is said to be turgid; a cell that has lost water and gone limp is said to be flaccid.
Turgid guard cells hold the pore open. Flaccid guard cells let it close.
What remains to be explained is why the water moves in and out at the right times of day. Two theories have been offered — the potassium ion concentration theory, which is the recent one, and the sugar concentration theory, which is the older.
6.4.2.1 Potassium Ion Concentration Theory
On this theory, opening and closing depend on a gradient of potassium ions (\(K^+\)) built up between the guard cells and their neighbours.
During the daytime the chloroplasts inside the guard cells photosynthesise, and this yields ATP (adenosine triphosphate), the compound in which a cell holds energy in a form it can spend at once.
That ATP is spent on actively pumping potassium ions out of the neighbouring epidermal cells and into the guard cells. Active pumping means the ions are driven in even though they are already more concentrated inside, and that is precisely why energy has to be supplied.
The guard cells now hold far more dissolved particles than their neighbours do — they have become hypertonic to them. Water therefore passes into them from the surrounding cells by endosmosis, the entry of water into a cell across its membrane.
The guard cells swell, their thin outer walls bulge outwards, and the stomatal pore opens. FC 6.2 sets out the whole cycle.
At night the sequence runs backwards. Photosynthesis stops, the potassium ions leak out of the guard cells, water follows them, the guard cells lose their turgor and the pore closes.
Flowchart
FC 6.2 — Opening And Closing Of Stomata By Potassium Ion Exchange Daytime: photosynthesis in guard cell chloroplasts → ATP produced → \(K^+\) pumped in from epidermal cells → guard cells become hypertonic → endosmosis, water enters → guard cells turgid and bulge → stoma opens Night: photosynthesis stops → ATP no longer supplied → \(K^+\) leaks out of guard cells → guard cells become hypotonic → exosmosis, water leaves → guard cells flaccid → stoma closes → (at sunrise the cycle returns to the daytime sequence)
6.4.2.2 Sugar Concentration Theory
The older theory explains the same swelling in a different way, naming sugar rather than potassium as the substance that concentrates the cell sap.
By this account the guard cells photosynthesise during the daytime, and the glucose formed accumulates in them. The sugar raises the osmotic pressure of the guard cells, so water is drawn in from the adjoining cells by endosmosis.
The guard cells become turgid and bulge outwards at their thin outer walls, widening the pore between them. Once the stoma is open, gases diffuse freely in both directions — carbon dioxide inwards for photosynthesis, oxygen and water vapour outwards.
Closing can also happen for a quite different reason, and both theories agree on it. If the water content of the leaf falls short, water is drawn out of the guard cells by exosmosis, its exit from a cell, whatever the hour. They go flaccid, their thick inner walls straighten, and the pore shuts.
The two accounts are set side by side in Table 6.2. They agree entirely on what happens to the guard cells, and differ only on what makes the water move.
6.5 The Process Of Photosynthesis
You now have the pigment, the container that holds it and the pore that lets the gas in. What remains is to see where in the leaf the work is actually done, how the raw materials reach that place, and what the whole reaction adds up to.
6.5.1 Mesophyll Cells As The Site Of The Process
Between the upper skin of a leaf and the lower skin lies a layer of green tissue called the mesophyll. Its cells are the principal centres of photosynthesis, and they are of two kinds.
Just under the upper surface stand the palisade cells, long and column-shaped, packed side by side like a row of pillars. They hold the largest number of chloroplasts in the leaf.
Below them lie the spongy cells, rounded and loosely arranged, with wide air spaces between them. These air spaces connect with the stomata, so gases can reach every cell.
Sunlight falls first on the upper surface, so it is the chlorophyll of the palisade cells that traps most of the light energy. This is also why the upper surface of a leaf usually looks a deeper green than the lower one. Fig. 6.5-1 shows the arrangement.
Diagram
Fig. 6.5-1 — Transverse Section Of A Leaf Showing The Mesophyll Transverse section through a dorsiventral leaf, drawn as a line diagram; label upper epidermis, cuticle, palisade cells, spongy cells, mesophyll, air space, lower epidermis, stoma, guard cell, vein; show arrows for sunlight falling on the upper surface, carbon dioxide entering through the stoma and water vapour leaving through it.
6.5.2 Arrival Of The Raw Materials
The two raw materials reach the mesophyll by completely different routes.
Carbon dioxide comes from the atmosphere. Its concentration is higher in the air outside the leaf than inside, because the mesophyll cells are steadily consuming it. The gas therefore moves in by diffusion — the spreading of a substance from where it is more concentrated to where it is less concentrated — passing through the stomata into the air spaces, and from there into the moist walls of the mesophyll cells.
Water comes from the soil. It is absorbed by the roots, carried up the stem through the xylem, the water-conducting tissue of the plant, and delivered through the veins of the leaf into the mesophyll tissue. The two paths are set out in FC 6.3.
Flowchart
FC 6.3 — How The Raw Materials Reach A Mesophyll Cell Carbon dioxide: atmosphere → stoma → air spaces of the spongy mesophyll → moist wall of a mesophyll cell → chloroplast Water: soil → root → stem xylem → veins of the leaf → mesophyll tissue → chloroplast
6.5.3 The Overall Balanced Equation
The whole process can be written in one line. In words:
carbon dioxide + water, in the presence of light energy and chlorophyll, gives glucose + water + oxygen.
Written as a balanced chemical equation:
\[6CO_2 + 12H_2O \xrightarrow{\text{light energy, chlorophyll}} C_6H_{12}O_6 + 6H_2O + 6O_2 \uparrow\]
The glucose molecule, \(C_6H_{12}O_6\), is a simple sugar that dissolves readily in water. The upward arrow shows that the oxygen is given off as a gas.
One thing in this equation puzzles most students. Twelve molecules of water go in, but six come out again — so why not simply write six on the left and none on the right?
The answer is that the six molecules on the right are not left-over water. Every one of the twelve original molecules is broken up early in the process. The six that appear at the end are built afresh during the chain of reactions that follows. Writing the equation in this fuller form keeps that fact visible.
6.6 The Two Phases Of Photosynthesis
Photosynthesis is not a single reaction. It is two sets of reactions, run one after the other, and they differ in what they need and where they happen.
The first set cannot begin without light, and is called the light-dependent phase. The second set does not use light at all, and is called the light-independent phase.
Both go on inside the same chloroplast, but in different parts of it. The light-dependent reactions take place in the thylakoids, where the chlorophyll is held. The light-independent reactions take place in the stroma around them. Fig. 6.6-1 marks the two sites.
Diagram
Fig. 6.6-1 — Sites Of The Two Phases Inside A Chloroplast Simplified outline of a chloroplast in section, showing one granum of stacked thylakoids surrounded by stroma; label thylakoid — site of the light-dependent phase, stroma — site of the light-independent phase; show an arrow from the granum to the stroma marked hydrogen and energy passed on, an arrow marked light entering the granum, and an arrow marked carbon dioxide entering the stroma.
6.6.1 The Light-Dependent Or Photochemical Phase
In this phase light plays the leading part. A series of chemical reactions follows one another in very quick succession, and every one of them is set off by light.
Because light starts these chemical changes, the phase is also called the photochemical phase. It takes place in the thylakoids, which are the only part of the chloroplast that contains chlorophyll.
The phase runs in two steps.
6.6.1.1 Activation Of Chlorophyll
Light travels in tiny packets of energy. A single such packet, the smallest unit of light energy, is called a photon.
When light falls on a leaf, the chlorophyll in the thylakoids absorbs these photons. The pigment is then said to be activated, meaning it now holds more energy than it did before and can pass that energy on.
6.6.1.2 Photolysis Of Water
The energy taken up by the activated chlorophyll is used to break apart water molecules that lie in the thylakoids.
Each pair of water molecules is split into hydrogen ions, electrons and oxygen:
\[2H_2O \xrightarrow{\text{energy of 4 photons}} 4H^+ + 4e^- + O_2\]
This splitting is called photolysis, from photo meaning light and lysis meaning breaking. It is the one step in the whole of photosynthesis at which oxygen is produced.
Notice what this settles. The oxygen a plant gives out is torn from water, not from carbon dioxide. The carbon dioxide has not even entered the reaction yet.
6.6.1.3 Fate Of The Products Of Photolysis
Photolysis leaves three things behind, and each is dealt with separately.
The hydrogen ions are picked up by a compound called NADP, or nicotinamide adenine dinucleotide phosphate. Along with electrons it becomes NADPH, or reduced NADP, and this compound now carries the hydrogen to wherever it is needed:
\[NADP^+ + 2e^- + H^+ \rightarrow NADPH\]
The oxygen is not needed by the plant at this point. It is released as molecular oxygen, \(O_2\), and diffuses out of the leaf through the stomata.
The electrons carry energy, and as they are passed along inside the thylakoid they give that energy up. It is used to join a molecule of ADP, adenosine diphosphate, to a molecule of inorganic phosphate, making ATP, or adenosine triphosphate:
\[ADP + P_i \rightarrow ATP\]
Adding a phosphate group in this way is called phosphorylation. Here the energy for it has come from light, so the process is called photophosphorylation. The whole light-dependent phase is summarised in FC 6.4.
Flowchart
FC 6.4 — Events Of The Light-Dependent Phase Light falls on thylakoid → chlorophyll absorbs photons and is activated → energy splits water (photolysis) → three products formed, which branch as follows: hydrogen ions and electrons → taken up by NADP → NADPH formed, carrying hydrogen to the stroma; electrons → energy released as they are passed along → ADP + inorganic phosphate → ATP formed (photophosphorylation); oxygen → released through the stomata into the air
6.6.2 The Light-Independent Or Biosynthetic Phase
This phase was for many years called the dark phase, and that name has misled generations of students. It never meant that these reactions happen at night.
It meant only that they are not dependent on light. In fact they run at the same time as the light reactions, taking the ATP and NADPH the moment those are made. Calling the phase light-independent states this correctly.
It is also called the biosynthetic phase, because this is where a food substance is actually built. The reactions are driven by enzymes and take place in the stroma of the chloroplast.
6.6.2.1 Formation Of Glucose From Carbon Dioxide
Carbon dioxide has by now diffused into the stroma. NADPH arrives from the thylakoids carrying hydrogen, and ATP arrives carrying energy.
The hydrogen released by NADPH is combined with the carbon dioxide, using the energy supplied by ATP, and glucose is formed. This is the first food substance made in photosynthesis, and it is the reason the whole process exists.
6.6.2.2 Polymerisation Of Glucose Into Starch
Glucose dissolves readily in water, and a leaf making it all day would soon have cell sap far too concentrated. Most green plants therefore convert glucose into starch almost as soon as it is formed.
Several glucose molecules are joined together to build one large molecule of starch. Joining many small molecules into one big one in this way is called polymerisation.
Not every plant stores its food as starch. Some convert glucose into sucrose, the cane sugar of everyday use, which is a double sugar or disaccharide. Others convert it into oils.
The two phases are set side by side in Table 6.2.
6.7 Adaptations In Leaves For Photosynthesis
A leaf is not simply a green flap on a stem. Almost every feature of it can be read as an answer to one of three demands: catch as much light as possible, exchange gases quickly, and move water in and food out.
Seven such features are worth knowing.
Large surface area. A leaf is broad and flat, so a great deal of chlorophyll is spread out facing the light at one time and very little sunlight falls on bare stem instead. A few plants go the other way. The leaves of oleander (Nerium oleander) and of pines (Pinus species) are narrow, which costs them light but greatly reduces the water they lose, and in dry places that trade is worth making.
Leaf arrangement. Leaves are held at right angles to the light coming towards them, which is the position that catches the most of it. Their stalks also differ in length, so that the leaves of a shoot fill the gaps between one another rather than lying one above the other in shade.
Cuticle and upper epidermis. The cuticle is the waxy layer covering the outer skin of the leaf. Both it and the epidermis beneath are transparent, so light passes straight through to the mesophyll, and both are waterproof, so the leaf does not dry out while the light gets in.
Numerous stomata. A single leaf carries thousands of these pores. Carbon dioxide can therefore enter, and oxygen leave, fast enough to keep pace with the reactions inside.
Thinness of the leaf. No mesophyll cell is far from a surface or from an air space. Gases, water and food all have a short distance to travel, so nothing is held up on the way.
Position of the chloroplasts. They are concentrated in the upper layers of the leaf, in the palisade cells, which is where the light arrives first and is strongest.
Extensive vein system. Veins branch through the whole blade of the leaf, so water reaches every mesophyll cell quickly and the food made there can be carried away just as quickly. Fig. 6.7-1 shows how the branching and the arrangement work together.
Diagram
Fig. 6.7-1 — Two Adaptations Seen From Above Two-part line drawing; part A shows a shoot viewed from directly above, with leaves of differing stalk lengths filling the gaps between one another so that none is shaded, labelled leaf arrangement, long stalk, short stalk, no overlap; part B shows a single leaf blade with its branching vein system drawn in, labelled midrib, lateral vein, fine vein network reaching the whole blade.
Taken together, these features explain why the leaf, and not the stem or the root, is the organ in which almost all of a plant’s food is made.
6.8 The End Products And Their Fate
Photosynthesis leaves three end products behind: glucose, water and oxygen. Not one of them is thrown away.
6.8.1 Fate Of Glucose
Glucose is the product the whole process exists to make, and the plant puts it to four different uses.
- Some is consumed immediately by the plant cells themselves, which break it down to release the energy they need.
- Some is stored in the form of insoluble starch.
- Some is converted into sucrose, in which form the food can travel in solution.
- Some is used as raw material for building other substances, such as fats and proteins.
That last use is worth pausing on. A plant does not live on sugar alone, and every fat and protein in its body is built, in the end, from the glucose a leaf made out of air and water.
6.8.2 Fate Of Water
Six molecules of water appear among the products of the reaction. This water is not lost to the plant.
It is simply released into the cell, where it becomes part of the plant’s own water again. It may then be drawn back into photosynthesis, so the process continues using water it has itself produced.
6.8.3 Fate Of Oxygen
A small part of the oxygen is taken up by the leaf cells themselves, since they respire like any other living cell. Uptake of oxygen by cells while they are in the light is given the name photorespiration.
The greater part of the oxygen is not needed by the plant at all. It diffuses out through the stomata into the atmosphere.
Even so, calling it a waste product would be wrong. That released oxygen is what every respiring organism on the earth uses, including the plant itself at night, when photosynthesis has stopped and only respiration continues.
6.8.4 Storage And Translocation Of The Food Made
Food is made in the leaf, but it is needed everywhere — in the root tip that is growing, in the flower that is opening, in the stem that is thickening. It has to be moved.
The difficulty is one of speed. In bright sunlight a leaf makes glucose far faster than it can send it away, and glucose dissolves easily, so it would make the cell sap dangerously concentrated. The leaf therefore converts it into insoluble starch, which stays quietly where it is put. This is temporary storage, and the leaf is only a warehouse for the day.
At night the starch is changed back into soluble sugar. In this dissolved form it can travel, and it passes into the veins of the leaf and down the stem through the phloem, the food-conducting tissue of the plant.
The movement of prepared food from the leaf to the other parts of the plant is called translocation. Where the food arrives, it may be used at once for energy and growth, or converted into starch again and stored — as it is in a potato. FC 6.5 follows the whole route.
Flowchart
FC 6.5 — From Glucose In The Leaf To Starch In A Storage Organ Glucose made in mesophyll cell → converted to insoluble starch → stored in the leaf through the day → at night changed back to soluble sugar → veins of the leaf → phloem of the stem → carried to roots, stems, buds and flowers → used for energy and growth, or reconverted to starch and stored (as in a potato)
6.9 Factors Affecting Photosynthesis
The rate of photosynthesis is not fixed. It rises and falls through the day and from place to place, and seven factors account for the change — four outside the plant and three within it.
6.9.1 External Factors
The four external factors are light intensity, carbon dioxide concentration, temperature and water content.
6.9.1.1 Light Intensity And Carbon Dioxide Concentration
These two are best studied together, because what one of them does depends on the other.
It seems obvious that brighter light should mean faster photosynthesis, and up to a point it does. Beyond that point, however, the rate stops climbing and levels off, however bright the light becomes. Fig. 6.9-1 shows what is happening.
Diagram
Fig. 6.9-1 — Photosynthesis At Increasing Light Intensities And Two Carbon Dioxide Concentrations Two line graphs drawn side by side, each with increasing light intensity on the horizontal axis and rate of photosynthesis on the vertical axis; graph A shows a curve rising steeply and then flattening into a horizontal line at the point marked S′, with the whole graph labelled carbon dioxide concentration 0.02 per cent; graph B repeats the same rising curve and flattening at S′, then shows the curve rising again from S′ and flattening a second time at the higher point marked S″, with the second stretch labelled carbon dioxide concentration increased to 0.05 per cent.
6.9.1.2 Temperature
As the temperature rises, the rate of photosynthesis rises with it. A rise of 10°C roughly doubles the rate, so a plant at 30°C works about twice as fast as the same plant at 20°C.
This cannot continue indefinitely. The rate is highest at about 35°C, the optimum temperature — the temperature at which the process runs best.
Above the optimum the rate falls, and above about 40°C photosynthesis stops altogether. The reactions are controlled by enzymes, and enzymes are proteins that are destroyed by heat. Once they are destroyed the process cannot run at any speed.
6.9.1.3 Water Content
Water is a raw material of photosynthesis, but shortage of it acts mainly in an indirect way.
When the soil is dry, or when transpiration is losing water faster than the roots absorb it, the guard cells lose their turgidity and the stomata close. With the pores shut, carbon dioxide can no longer diffuse in, and the rate falls for want of that gas.
It is worth knowing how little water the process itself consumes. Only about 1 per cent of all the water a root absorbs is actually used in photosynthesis. Nearly all the rest is lost in transpiration.
6.9.2 Internal Factors
Three factors belong to the plant itself.
The first is chlorophyll. A plant short of the minerals it needs, magnesium among them, cannot build enough chlorophyll, and with less pigment it traps less sunlight.
The second is the protoplasm of the cell. If it becomes dehydrated the rate falls. The rate also falls when carbohydrates — sugar and starch — accumulate in the leaf faster than they are carried away, since the products of a process, left to pile up, slow that process down.
The third is the structure of the leaf. The thickness of the cuticle, the number and distribution of the stomata and the size of the leaf together decide how much light and how much carbon dioxide actually reach the mesophyll.
6.9.3 Changes From Dawn To Dusk
Light intensity does not merely differ between one place and another. It changes through every twenty-four hours, and the rate of photosynthesis follows it — rising from dawn to a peak around midday, then declining as dusk comes on.
A second process is going on the whole time. Plants respire like every other organism, taking in oxygen and giving out carbon dioxide, and they do so by day and by night alike.
In the light the two run together, and photosynthesis is much the faster of the two. Its effects mask those of respiration, and the leaf shows a net output of oxygen.
In darkness photosynthesis stops and respiration alone continues. It is then the only process changing the gases around the plant, so the leaf takes in oxygen and gives out carbon dioxide. Table 6.3 sets the two processes against each other.
6.10 Experiments On Photosynthesis
Everything said so far can be tested. Each of the essentials — chlorophyll, light and carbon dioxide — can be taken away from a leaf one at a time, and the leaf then asked whether it has made any food. The release of oxygen can be caught and identified.
All of these experiments rest on one idea. Food made in a leaf is stored there as starch, so a leaf that has made food will contain starch and a leaf that has not will contain none. Two preparatory procedures therefore come first.
6.10.1 Destarching A Plant
A leaf picked from a plant in a garden is already full of starch made on earlier days. Test it and it will give a positive result whatever you do to it afterwards, so the experiment would prove nothing.
The plant must therefore be emptied of starch before the experiment begins. It is kept in complete darkness for 24 to 48 hours. With no light there is no photosynthesis, while respiration and translocation continue, so the starch already in the leaves is changed to sugar, part of it used up in respiration and the rest carried away to the storage organs.
A plant whose leaves are free from starch in this way is said to be destarched. Only now will a starch test mean something, because any starch found afterwards must have been made during the experiment itself.
6.10.2 Testing A Leaf For Starch
Starch gives a blue-black colour with iodine solution. A green leaf, however, cannot simply be dipped in iodine, because its own colour would hide any colour change. The test is done in four steps, and each step has a reason.
- Dip the leaf in boiling water for about a minute. This kills the cells and stops all further activity in the leaf.
- Boil the leaf in methylated spirit until it turns pale white. The spirit dissolves the chlorophyll out, so the leaf’s green colour will not mask the result. Methylated spirit catches fire very easily, so the tube holding it is heated in a water bath and never directly in a flame. Fig. 6.10-1 shows the arrangement.
- Put the pale leaf back into hot water for a moment. After the spirit treatment it is hard and brittle, and this softens it so it can be spread out without breaking.
- Spread the leaf in a dish and pour iodine solution over it. Parts containing starch turn blue-black. Parts without starch stay brown, the colour of the iodine itself.
The iodine solution used is made by dissolving 0.3 g of iodine and 1.5 g of potassium iodide in 100 ml of water. FC 6.6 sets out the sequence.
Diagram
Fig. 6.10-1 — Removing Chlorophyll From A Leaf Line diagram of the apparatus: a beaker of boiling water heated by a burner on a tripod stand, with a test tube standing in the water and a leaf inside the test tube in methylated spirit; label leaf, methylated spirit, test tube, boiling water, beaker, tripod stand, burner; show a note line to the burner reading flame heats the water, not the spirit.
Flowchart
FC 6.6 — Steps Of The Starch Test Leaf dipped in boiling water (cells killed) → boiled in methylated spirit in a water bath (chlorophyll removed, leaf turns pale) → dipped in hot water (leaf softened) → spread in a dish and covered with iodine solution → blue-black colour where starch is present, brown where it is absent
6.10.3 Showing That Chlorophyll Is Necessary
This experiment uses a variegated leaf — one with green patches and pale, non-green patches on the same blade. Coleus, geranium and croton all have such leaves. Since both kinds of tissue are on one leaf, they receive the same light, the same air and the same water, and chlorophyll is the only difference between them.
- Destarch the plant by keeping it in a dark room for a few days.
- Place the plant in sunlight.
- After a few hours, pluck one variegated leaf.
- Draw the outline of the leaf on paper and mark on the drawing which areas were green and which were not.
- Test the leaf for starch.
Only the areas that were green turn blue-black. The parts that had no chlorophyll stay brown, as Fig. 6.10-2 shows. Since nothing else about them differed, chlorophyll must be necessary for photosynthesis.
Diagram
Fig. 6.10-2 — A Variegated Leaf Before And After The Starch Test Two drawings of the same variegated leaf side by side with an arrow between them; the left leaf is labelled before treatment with its green areas and white non-green areas marked, the right leaf is labelled after treatment with iodine solution with the formerly green areas shaded blue-black and the formerly white areas shaded brown; label green area, non-green area, blue-black, brown.
6.10.4 Showing That Light Is Necessary
Here the leaf is uniformly green, and light is what differs from one part of it to another.
- Take a destarched potted plant.
- Cover one of its leaves with black paper in which a design has been cut out.
- Place the plant in sunlight for a few hours.
- Remove the paper and test that leaf for starch.
Only the parts of the leaf that received light through the cut-out design, and any part left uncovered, turn blue-black. The covered parts remain brown, as in Fig. 6.10-3. The covered and uncovered parts of one leaf differed in nothing but light, so light is necessary for photosynthesis.
Diagram
Fig. 6.10-3 — The Design Experiment Two drawings side by side; the left shows a leaf on the plant covered with black paper in which a star shape has been cut out, labelled black paper, cut-out design; the right shows the same leaf after the starch test, with a blue-black star on a brown background, labelled blue-black where light fell, brown where light was cut off.
6.10.5 Showing That Carbon Dioxide Is Necessary
Carbon dioxide cannot be cut out with paper, so it is removed chemically. Potassium hydroxide absorbs carbon dioxide from the air around it.
- Take a destarched potted plant.
- Insert half of one leaf, still attached to the plant, through a split cork into a conical flask containing potassium hydroxide.
- Leave the plant in sunlight for a few hours.
- Test the whole leaf for starch.
The half that stayed outside in ordinary air turns blue-black. The half enclosed with the potassium hydroxide stays brown. Both halves had the same chlorophyll, the same light and the same water supply, and only the enclosed half was deprived of carbon dioxide, so that gas is necessary for photosynthesis. Fig. 6.10-4 shows the set-up.
Diagram
Fig. 6.10-4 — Testing The Need For Carbon Dioxide Line diagram of a potted plant with one twig bent down so that half of a single leaf passes through a split cork into the mouth of a conical flask; the flask holds a shallow layer of liquid at the bottom; label leaf half inside the flask, leaf half outside in air, split cork, conical flask, potassium hydroxide, sunlight; mark the outside half as the control.
The half of the leaf left outside the flask is the control. A control is the part of an experiment in which nothing has been altered, and it is what makes the result meaningful — without it, a brown half-leaf might have been blamed on the cork, the flask or the shade, rather than on the missing gas.
6.10.6 Showing That Oxygen Is Released
The other three experiments looked for starch. This one collects the gas the plant gives off, and it is done with a water plant so that the gas can be seen leaving as bubbles.
- Place a few twigs of a water plant such as Hydrilla or Elodea in a beaker of pond water, and cover them with a short-stemmed funnel.
- Fill a test tube with water and invert it over the stem of the funnel, keeping the water level in the beaker above the top of the stem so that no air can get in.
- Place the apparatus in sunlight for a few hours.
- Bubbles rise from the plant, pass up the funnel and collect in the test tube, driving the water down.
- Test the collected gas by bringing a glowing splinter to the mouth of the tube.
The splinter bursts into flame, which is the test for oxygen. Since the gas was given off only in sunlight and only by the green plant, the oxygen must have come from photosynthesis. Fig. 6.10-5 shows the apparatus.
If the bubbling is slow, a pinch of sodium bicarbonate stirred into the water speeds it up. Sodium bicarbonate supplies extra carbon dioxide to the water, and with more raw material available the plant photosynthesises faster.
Diagram
Fig. 6.10-5 — Collecting The Gas Given Off By A Water Plant Line diagram of a beaker of water on a bench in sunlight, containing twigs of a water plant covered by an inverted short-stemmed funnel resting on small supports, with a water-filled test tube inverted over the funnel stem and gas collected at its closed end; label pond water, water plant twigs, funnel, support to keep the funnel off the bottom, test tube filled with water, gas bubbles, collected oxygen, sunlight.
6.11 Importance Of Photosynthesis
Photosynthesis supports all life on the earth. Remove the green plants and life would not merely become harder; it would come to an end.
Two reasons account for that, and they are worth taking one at a time. Green plants supply the food, and they supply the oxygen.
6.11.1 Food For All Living Things
Every animal, including you, depends in the end on plants. A herbivore, or plant-eater, takes plant material directly. A carnivore, or flesh-eater, takes the flesh of an animal that was itself a herbivore, or that fed on one.
This chain of feeding, in which each organism is eaten by the next, is called a food chain. It may have several stages, but its starting point is always a green plant.
The plant at the start is called a producer, because it produces its own food. Every organism that follows it is a consumer, because it consumes food that something else has made.
That first link cannot be dispensed with. Energy has to enter a living system from outside, and photosynthesis is the only doorway by which it does so. A chain of consumers with no producer at its head would have nothing to live on.
Here are some food chains you can trace for yourself:
- corn → rat → snake → hawk
- grass → rabbit → tiger
- grass → grasshopper → lizard → kite
- grass → insects → frog → snake → peacock
- wheat → human → mosquito
Consumers are grouped by their place in the chain. The herbivore that eats the producer is a primary consumer; the carnivore that eats the herbivore is a secondary consumer; and a carnivore that feeds on that one is a tertiary consumer. Fig. 6.11-1 shows several chains together.
Diagram
Fig. 6.11-1 — Producers And The Consumers That Follow Them Chart in four labelled columns — producers, primary consumers (herbivores), secondary consumers (carnivores), tertiary consumers (carnivores); column one shows a tree and grass labelled vegetation; column two shows a grasshopper, a rabbit and a rat; column three shows a lizard and a snake; column four shows a hawk; draw arrows from vegetation to each of the three herbivores, from grasshopper to lizard, from rat to snake, and from lizard, snake and rabbit to the hawk, each arrow marked is eaten by.
Non-green organisms such as fungi and bacteria seem to stand outside all this, since they eat no plants. They feed on decaying organic matter instead. But that matter is the remains of dead plants and animals, and those animals fed on plants, so their food also traces back to photosynthesis.
6.11.2 Oxygen In The Atmosphere
The air you breathe is about one-fifth oxygen. That free oxygen is there because of photosynthesis, which is the only biological process that releases oxygen into the atmosphere.
It was not always there. For a long stretch of the earth’s early history the atmosphere held almost no free oxygen at all, although simple living things already existed in the seas.
Oxygen began to collect in the air roughly 2,400 million years ago, and it collected because photosynthesising organisms were releasing it faster than it could be used up. Once oxygen was plentiful, life that depends on it became possible.
Almost every organism now alive needs oxygen for respiration, plants as much as animals. A leaf that gives out oxygen all day takes it in again at night, when photosynthesis has stopped and respiration continues alone.
6.12 The Carbon Cycle
Carbon is in every carbohydrate, every fat and every protein in every living body. All of it was taken from the air, as carbon dioxide, by photosynthesis.
Yet the air holds very little carbon dioxide — about 0.04 per cent of it by volume, or a little over 400 parts in a million (2024). At the rate the world’s plants remove it, that small supply would have been used up long ago.
It has not been used up, because the carbon comes back. The carbon cycle is the round trip carbon makes between the air and living things.
The Six Steps Of The Cycle
- Photosynthesis. Green plants, the producers, take carbon dioxide from the air and build it into carbohydrates such as sugar and starch. This is the only step that removes carbon from the air.
- Food chains. The food passes from plants to animals, the consumers, and with it goes the carbon it contains. Carbon is an essential element of the body tissues of plants and animals alike.
- Respiration. Every plant and animal oxidises carbohydrates inside its cells to release energy, and gives out carbon dioxide into the atmosphere. Carbon returns to the air here in small amounts, day and night.
- Decay. The dead remains of plants and animals are consumed by bacteria and fungi, the decomposers. As they break the organic matter down, carbon dioxide is released back into the air.
- Combustion. Some remains are buried before they can decay and are slowly changed into fossil fuels — coal, petroleum and natural gas. When wood or any of these fuels is burnt, the carbon in it is oxidised to carbon dioxide and returned to the air. Every one of these fuels was once part of a living organism.
- Heating limestone. Limestone also holds carbon, laid down long ago. When it is heated in lime kilns, a certain amount of carbon dioxide is released from it.
Fig. 6.12-1 puts all six steps into one picture.
Diagram
Fig. 6.12-1 — The Carbon Cycle Cycle diagram with a box at the top labelled carbon dioxide in the air; an arrow marked photosynthesis leading from it down to a box labelled producers (green plants); an arrow marked food chain from producers to a box labelled consumers (animals), and an arrow from consumers to a box labelled decomposers (bacteria and fungi); arrows marked respiration, decay and combustion leading from all three boxes back up to the carbon dioxide box; an arrow marked fossilisation leading down from the boxes to a box labelled fossil fuels — coal, oil and gas, and limestone; and arrows marked combustion and heating in lime kilns leading from that box back up to the carbon dioxide box.
Notice how the steps divide. One step takes carbon out of the air, and five put it back. The cycle turns because of that balance.
6.13 Summary
- Organisms that build their own food from simple raw materials are autotrophs; those that take food ready-made from other organisms are heterotrophs.
- Photosynthesis is the process by which plant cells containing chlorophyll build glucose and starch from carbon dioxide and water, using light energy, and release oxygen as a by-product.
- Four things are essential: carbon dioxide, water, chlorophyll and light. Carbon dioxide and water are raw materials and are used up; light and chlorophyll are conditions and are not.
- Chlorophyll is held inside chloroplasts, which are bounded by a double membrane and contain flattened sacs called thylakoids, stacked into piles called grana and linked by frets, lying in a colourless fluid called stroma.
- Chlorophyll absorbs light at both ends of the visible spectrum, the blue and the red, and reflects green — which is why leaves look green and why blue and red light are the most effective for photosynthesis.
- Light is needed for chlorophyll to be formed, and excess light destroys it; grass kept in darkness under a stone turns yellow.
- Carbon dioxide enters the leaf through stomata, minute pores bounded by two guard cells. Guard cells have a thick inner wall, a thin outer wall and chloroplasts, which ordinary epidermal cells lack.
- By the potassium ion concentration theory, ATP from photosynthesis in the guard cells pumps potassium ions in from neighbouring cells, water follows by endosmosis, the guard cells become turgid and the pore opens; at night the ions leak out and the pore closes.
- By the older sugar concentration theory, glucose made in the guard cells raises the concentration of their sap and draws water in by endosmosis, with the same result.
- Water is lost as vapour through open stomata, so transpiration is the price the plant pays for photosynthesis.
- Photosynthesis takes place chiefly in the mesophyll — the palisade cells near the upper surface, which hold the most chloroplasts, and the spongy cells with their air spaces below.
- Carbon dioxide reaches the mesophyll by diffusion through the stomata; water reaches it from the soil by way of the roots, the stem and the veins of the leaf.
- The overall reaction is \(6CO_2 + 12H_2O \rightarrow C_6H_{12}O_6 + 6H_2O + 6O_2\) in the presence of light energy and chlorophyll; the six water molecules on the right are formed afresh during the reaction.
- Photosynthesis runs in two phases. The light-dependent or photochemical phase occurs in the thylakoids; the light-independent or biosynthetic phase occurs in the stroma.
- In the light-dependent phase, chlorophyll absorbs photons and is activated, and that energy splits water into hydrogen ions, electrons and oxygen. This splitting is photolysis, and it is the source of the oxygen released.
- The hydrogen ions are taken up by NADP to form NADPH, the oxygen diffuses out, and the electrons provide the energy that joins ADP to inorganic phosphate to form ATP — a process called photophosphorylation.
- In the light-independent phase, the hydrogen carried by NADPH is combined with carbon dioxide, using energy from ATP, to form glucose. These reactions do not need light and run at the same time as the light reactions.
- Several glucose molecules are joined to form one molecule of starch, a process called polymerisation. Some plants convert glucose into sucrose or into oils instead.
- A leaf is adapted for photosynthesis by its large surface area, its arrangement at right angles to the light, its transparent cuticle and epidermis, its numerous stomata, its thinness, the position of its chloroplasts in the upper layers and its extensive vein system.
- The three end products are glucose, water and oxygen. Glucose is respired, stored as starch, converted to sucrose or used to build fats and proteins; the water is reused; most of the oxygen diffuses out through the stomata.
- Glucose is converted to insoluble starch for temporary storage in the leaf, changed back to soluble sugar at night, and translocated through the phloem to the rest of the plant, where it may be stored as starch again.
- Four external factors affect the rate of photosynthesis — light intensity, carbon dioxide concentration, temperature and water content — and three internal ones: chlorophyll, protoplasm and the structure of the leaf.
- The rate is set by whichever requirement is in shortest supply. Raising light alone lifts the rate only to a plateau; raising the carbon dioxide concentration then lifts it to a higher plateau.
- The rate roughly doubles for every 10°C rise up to the optimum temperature of about 35°C, falls above it, and stops above about 40°C, where the enzymes are destroyed.
- Only about 1 per cent of the water absorbed by the roots is used in photosynthesis; shortage of water acts mainly by closing the stomata and shutting out carbon dioxide.
- The rate rises from dawn to midday and falls towards dusk. Respiration goes on day and night, but in the light its effects are masked by the faster photosynthesis, so there is a net output of oxygen.
- Before any experiment on photosynthesis, a plant is destarched by keeping it in darkness for 24 to 48 hours, so that any starch found afterwards must have been made during the experiment.
- A leaf is tested for starch by killing it in boiling water, removing its chlorophyll in methylated spirit heated in a water bath, softening it in hot water and adding iodine solution: starch gives blue-black, its absence gives brown.
- A variegated leaf shows that chlorophyll is necessary; a leaf part-covered with black paper shows that light is necessary; a half-leaf enclosed with potassium hydroxide shows that carbon dioxide is necessary, the other half serving as the control.
- Gas collected from a water plant such as Hydrilla relights a glowing splinter, showing that the gas released in photosynthesis is oxygen.
- Every food chain begins with a green plant, the producer; the organisms that follow are consumers. Photosynthesis is the only doorway by which energy enters a living system.
- Free oxygen is present in the atmosphere because of photosynthesis, and almost every organism depends on it for respiration.
- In the carbon cycle, carbon is removed from the air by photosynthesis, passed along food chains, and returned to the air by respiration, decay, combustion of wood and fossil fuels, and the heating of limestone.
Flowchart
FC 6.7 — Photosynthesis From Raw Material To Stored Food Carbon dioxide enters through stomata and water arrives from the roots → both reach a chloroplast in a mesophyll cell → in the thylakoids, chlorophyll absorbs light and water is split (photolysis) → oxygen released, NADPH and ATP formed → in the stroma, hydrogen from NADPH combines with carbon dioxide using energy from ATP → glucose formed → polymerised to starch and stored in the leaf → changed to soluble sugar at night → translocated through the phloem to the rest of the plant
6.14 Key Terms
- ATP — adenosine triphosphate, the compound in which a cell stores energy in a form it can spend at once.
- Autotrophs — organisms that build their own food from simple, non-living raw materials.
- Carbon cycle — the series of changes by which carbon is taken from the air by living things and returned to it.
- Carnivore — an animal that feeds on the flesh of other animals.
- Chlorophyll — the green pigment of plants, which absorbs light energy for photosynthesis.
- Chloroplast — the green organelle inside a plant cell in which photosynthesis takes place.
- Consumer — an organism that lives on food made by some other organism.
- Control — the part of an experiment left untreated, against which the treated part is compared.
- Cuticle — the thin waxy layer covering the outer skin of a leaf.
- Decomposers — bacteria and fungi that break down the remains of dead organisms.
- Destarched — describing a plant whose leaves have been emptied of starch by keeping it in darkness.
- Endosmosis — the entry of water into a cell by osmosis.
- Exosmosis — the exit of water from a cell by osmosis.
- Flaccid — limp and soft, describing a cell that has lost water.
- Food chain — a series of organisms in which each is eaten by the next, beginning always with a green plant.
- Glucose — the simple, readily soluble sugar first formed in photosynthesis.
- Granum — a pile of thylakoids inside a chloroplast (plural, grana).
- Guard cells — the pair of chloroplast-containing cells that bound a stoma and open or close it.
- Herbivore — an animal that feeds on plants.
- Heterotrophs — organisms that cannot make their own food and take it ready-made from others.
- Light-dependent phase — the first phase of photosynthesis, occurring in the thylakoids and requiring light.
- Light-independent phase — the second phase of photosynthesis, occurring in the stroma and not requiring light.
- Mesophyll — the green tissue between the upper and lower skin of a leaf, where photosynthesis mainly occurs.
- NADP — nicotinamide adenine dinucleotide phosphate, the compound that carries hydrogen from the first phase to the second.
- Palisade cells — long, column-shaped mesophyll cells packed below the upper surface of a leaf, holding the most chloroplasts.
- Photolysis — the splitting of water into hydrogen ions, electrons and oxygen in the presence of light.
- Photon — the smallest unit, or packet, of light energy.
- Photophosphorylation — the formation of ATP from ADP and inorganic phosphate using energy that came from light.
- Photorespiration — the uptake of oxygen by leaf cells while they are in the light.
- Photosynthesis — the making of food from carbon dioxide and water in chlorophyll-containing cells, using light energy, with oxygen released as a by-product.
- Pigment — a substance that gives colour by absorbing some colours of light and reflecting the rest.
- Polymerisation — the joining of many small molecules into one large molecule, as glucose into starch.
- Producer — a green plant, which makes its own food and stands at the head of every food chain.
- Spongy cells — loosely arranged mesophyll cells with air spaces between them, lying towards the lower surface of a leaf.
- Starch — the insoluble carbohydrate in which a plant stores the glucose it makes.
- Stomata — minute pores in the skin of a leaf through which gases pass (singular, stoma).
- Stroma — the colourless ground fluid filling a chloroplast around the grana.
- Sucrose — cane sugar, a double sugar into which some plants convert glucose.
- Thylakoids — the flattened sacs inside a chloroplast whose membranes hold the chlorophyll.
- Translocation — the movement of prepared food in solution from the leaf to the other parts of the plant.
- Transpiration — the loss of water as vapour from the surface of a plant.
- Turgid — swollen and firm with water, describing a cell that has taken water in.
- Variegated — describing a leaf that carries both green and non-green areas on the same blade.
6.15 Points To Ponder
Chlorophyll reflects green light, which is one of the most plentiful colours in sunlight. A pigment that absorbed green as well would capture more energy. Why might a pigment that turns away so much of what reaches it still have been good enough for plants to have flourished with it for hundreds of millions of years?
Suppose a green plant and enough light were sealed inside a large airtight glass jar, with soil and water but nothing else. Which of the plant’s needs would the jar continue to supply, which would it not, and how long could such a plant keep itself going?
The light-independent phase does not need light, yet it stops soon after darkness falls. What does this tell you about the difference between what a process needs directly and what it depends on? Can you think of another process in the body that stops for a reason of this kind?
A leaf stores its food as insoluble starch rather than as soluble glucose. Storage substances in living things are very often insoluble ones. What advantages does insolubility give to a store, and what does it cost the organism when the store has to be used?
Carbon taken from the air by a leaf may return to the air within a day, through respiration, or after millions of years, through the burning of coal. If both routes end in the same place, what difference does the length of the journey make to the amount of carbon dioxide in the air at any one time?
Some plants of dry places have narrow leaves, which lose less water but also catch less light. Under what conditions would this trade be worth making, and under what conditions would a broad leaf win? How would you test your answer on real plants?
6.16 Objective Questions
Multiple Choice
Select the most appropriate option in each case.
- The phase of photosynthesis that takes place in the stroma of a chloroplast is the: (1 mark)
- photochemical phase
- light-dependent phase
- biosynthetic phase
- photolytic phase
- The oxygen given out during photosynthesis is obtained from: (1 mark)
- carbon dioxide
- water
- glucose
- NADPH
- Potassium hydroxide is used in one of the experiments on photosynthesis because it: (1 mark)
- dissolves out the chlorophyll
- absorbs carbon dioxide
- kills the cells of the leaf
- tests for the presence of starch
- The colours of light most effective in photosynthesis are: (1 mark)
- green and yellow
- violet and green
- blue and red
- orange and green
- Photosynthesis stops altogether at temperatures above about: (1 mark)
- 25°C
- 35°C
- 40°C
- 50°C
- A leaf containing no starch, when treated with iodine solution, turns: (1 mark)
- blue-black
- brown
- pale white
- bright green
- Which of the following is not one of the steps of the carbon cycle? (1 mark)
- decay of dead remains
- burning of fossil fuels
- photolysis of water
- heating of limestone
Assertion And Reason
In each of the following, an assertion (A) is followed by a reason (R). Choose from: (1 mark each) (a) Both A and R are true, and R is the correct explanation of A. (b) Both A and R are true, but R is not the correct explanation of A. (c) A is true but R is false. (d) A is false but R is true.
Assertion (A): Guard cells can change their shape, while the epidermal cells beside them cannot. Reason (R): The inner wall of a guard cell is thick and its outer wall is thin. (1 mark)
Assertion (A): A potted plant is kept in the dark for 24 to 48 hours before an experiment on photosynthesis. Reason (R): In darkness the starch in the leaves is used up in respiration or carried away to the storage organs. (1 mark)
Assertion (A): Beyond a certain brightness, adding more light does not increase the rate of photosynthesis. Reason (R): Light intensity is the only factor that decides the rate of photosynthesis. (1 mark)
Match, Fill, State And Name
- Match each item in Column A with the correct item in Column B. (5 marks)
| Column A | Column B |
|---|---|
| a. Thylakoid | i. carries hydrogen to the second phase |
| b. Methylated spirit | ii. site of the light-dependent phase |
| c. NADP | iii. carries prepared food down the stem |
| d. Phloem | iv. site at which glucose is formed |
| e. Stroma | v. dissolves the chlorophyll out of a leaf |
- Complete each statement with one suitable word. (5 marks)
- A pile of thylakoids inside a chloroplast is called a ______.
- The splitting of water molecules in the presence of light is called ______.
- The joining of several glucose molecules to form one starch molecule is called ______.
- According to the potassium ion concentration theory, potassium ions are pumped into the guard cells from the neighbouring ______ cells.
- The loss of water as vapour from the surface of a plant is called ______.
- State whether each statement is true or false. (4 marks)
- Chlorophyll is used up during photosynthesis.
- Photosynthesis is the only biological process that releases oxygen into the atmosphere.
- The light-independent phase can occur only after darkness falls.
- About 1 per cent of the water absorbed by the roots is used in photosynthesis.
- Name the following. (5 marks)
- The pigment that absorbs light energy in a leaf.
- The compound that carries hydrogen from the first phase of photosynthesis to the second.
- The tissue through which prepared food is carried down the stem.
- The cells of the leaf epidermis that contain chloroplasts.
- A water plant used to show that oxygen is released during photosynthesis.
- In each group, pick the odd one out and give a reason for your choice. (4 marks)
- Palisade cell, spongy cell, guard cell, root hair cell
- Coal, petroleum, natural gas, limestone
- Light intensity, temperature, carbon dioxide concentration, chlorophyll
- ATP, NADPH, oxygen, glucose
6.17 Check Your Concepts
Define photosynthesis. (2 marks)
Name the two phases of photosynthesis and state where in the chloroplast each of them occurs. (2 marks)
Give a reason why a destarched plant, and not any plant from the garden, is used for experiments on photosynthesis. (2 marks)
State any two features of a leaf that help it to absorb the greatest possible amount of light. (2 marks)
Explain why the oxygen released during photosynthesis comes from water and not from carbon dioxide. (3 marks)
Describe how the stomatal pore is opened during the daytime, according to the potassium ion concentration theory. (3 marks)
Give reasons for the following: a leaf converts glucose into starch during the day and changes that starch back into sugar at night. (3 marks)
Differentiate between the light-dependent phase and the light-independent phase, stating the basis of difference as (i) the site in the chloroplast and (ii) the products formed. (3 marks)
Differentiate between photosynthesis and respiration on the basis of: (i) time of occurrence, (ii) gases exchanged, (iii) what happens to food, and (iv) what happens to energy. (4 marks)
The rate of photosynthesis rises as the light grows brighter and then becomes steady, however much more light is given. Explain why it becomes steady, and state what must be supplied to raise it further. (4 marks)
Describe the steps by which a leaf is tested for starch, giving one reason for each step. (4 marks)
Describe an experiment to show that carbon dioxide is necessary for photosynthesis. State the chemical used, name the part of the leaf that serves as the control, and give the expected result of the starch test on both parts. (5 marks)
Carbon taken out of the air by a green plant is always returned to it. Explain, with reference to the carbon cycle, any four ways in which this return takes place. (5 marks)
6.18 Figure-Based Questions
- With reference to Fig. 6.3-1: (4 marks)
- Redraw the structure shown and label the granum, a thylakoid, the stroma and the envelope.
- State one function of the part labelled stroma.
- With reference to Fig. 6.4-1: (5 marks)
- State the condition of the guard cells in A and in B.
- Give two differences between the inner and the outer wall of a guard cell.
- Name the gas that enters the leaf through the pore shown.
- State at what time of day the condition shown in B is usually found.
- With reference to Fig. 6.5-1: (4 marks)
- Name the two kinds of mesophyll cells shown.
- Which of the two contains more chloroplasts, and why is it placed where it is?
- Name the structure through which water vapour leaves the leaf.
- With reference to Fig. 6.9-1: (4 marks)
- Name the quantity plotted on each axis of graph A.
- Explain why the curve in graph A becomes horizontal at the point S′.
- State what was altered in graph B to make the curve rise again beyond S′.
- With reference to Fig. 6.10-5: (5 marks)
- Name the process being demonstrated and the gas collected in the test tube.
- Describe the test by which that gas is identified.
- Explain why the water level in the beaker must stay above the top of the funnel stem.
- Name a substance that can be added to the water to make the bubbling faster.
- With reference to Fig. 6.12-1: (4 marks)
- Name the single process that removes carbon dioxide from the air.
- Name the group of organisms that release carbon dioxide during decay.
- Name two processes, other than respiration and decay, by which carbon dioxide is returned to the air.