Photosynthesis in Higher Plants: Class 11 Biology · Chapter 11

Stand at the edge of a wheat field in Punjab and you can study it in two ways. An ecologist counts the crop, the weeds, the aphids and the mynas feeding among them. A biochemist sees something else: countless chloroplasts turning sunlight, air and water into sugar. Both are biology. This unit takes the second view. It explains how a flowering plant makes its food (this chapter), releases energy from it (Chapter 12) and uses that energy to grow and develop (Chapter 13). Each time, we trace a whole-plant event back to cells and molecules, and we note where the surroundings speed it up or hold it back.

Trace the food on a lunch plate backwards and you always reach a green plant: the rice and dal directly, the curd through a cow that ate fodder. Green plants are autotrophs: they build their own organic food from simple inorganic raw materials. Animals, fungi and most bacteria are heterotrophs, which must eat food that an autotroph made first.

Photosynthesis matters for two reasons. It is the doorway through which the Sun's energy enters almost every food chain on Earth, and it keeps topping up the oxygen in the air that we and most other organisms need to breathe. In this chapter we open up the machinery: the pigments that catch light, the membranes that turn light into chemical energy, and the enzymes that spend that energy on fixing carbon.

11.1 What Do We Know?

From earlier classes you know three requirements: chlorophyll, light and carbon dioxide. Each was shown with the same trick. Deny one part of a leaf a single requirement, then ask the leaf whether it made starch. Iodine gives the answer, because it stains starch blue-black and leaves the rest of a decolourised leaf a pale brown-yellow.

In Fig. 11.1 a variegated leaf (green with white patches) has a strip of black paper clipped across it. After some hours in sunlight, only the parts that were both green and uncovered stain for starch. A second leaf is pushed halfway into a sealed bottle holding cotton soaked in potassium hydroxide ($\mathrm{K}\mathrm{O}\mathrm{H}$), which absorbs $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ from the air inside. Only the half left outside makes starch. Both halves had the same light, chlorophyll and water, so the missing $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ is the only difference that can explain the result.

11.2 Early Experiments

Our picture of photosynthesis was pieced together from a chain of simple experiments. Each one answered a question and raised the next.

Joseph Priestley (1733–1804), an English clergyman with a passion for chemistry, ran a famous series of bell-jar tests in the early 1770s. A sealed jar of air could keep a candle alight, or a mouse alive, for only a short time; after that it would support neither. Yet if a sprig of mint shared the jar for some days, the same air became fit for both again. Priestley's explanation: whatever flames and breathing animals take out of the air, green plants put back. In 1774 he went on to isolate the gas we now call oxygen.

Jan Ingenhousz (1730–1799), a Dutch-born physician working in England, asked a question Priestley had not: does it matter when the plant does this? Repeating the jar test in sunlight and in darkness, he found that the plant restores the air only in light (Fig. 11.2). Water plants made the point visible. In bright sun their green leaves and stems streamed tiny bubbles, in the dark they produced none, and non-green parts never did; the gas proved to be oxygen. So the air-restoring process needs sunlight and happens only in green tissue.

Around the middle of the nineteenth century the German botanist Julius von Sachs tied this gas exchange to food. Testing leaves with iodine, he found starch only in green tissue that had been in light, and he traced chlorophyll to discrete bodies inside the cell, the chloroplasts. The glucose that green tissue makes as a plant grows is mostly stored as that starch.

Which colours of light drive photosynthesis? In 1882 the German physiologist T. W. Engelmann (1843–1909) answered this with an elegant trick (Fig. 11.3). Through a prism he cast a small spectrum along a filament of the green alga Cladophora, in water containing aerobic bacteria. These bacteria swim towards oxygen, so they served as living oxygen detectors. They crowded under the blue and red parts of the spectrum and stayed sparse under green. A plot of photosynthetic output against wavelength is called an action spectrum. Engelmann's was the first ever measured, and you will see in Section 11.4 that its two peaks line up with the colours chlorophylls a and b absorb best.

By the second half of the nineteenth century the essentials were agreed: in light, plants make carbohydrate from $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ and water and give off oxygen. The overall change was written as

Where does the oxygen come from: the $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ or the water? The microbiologist Cornelius van Niel (1897–1985) cracked this by studying purple and green sulphur bacteria. They photosynthesise without releasing oxygen, using hydrogen sulphide ($\mathrm{H}{\phantom{X}}_2\mathrm{S}$) instead of water and leaving sulphur or sulphate behind. Comparing them with plants, he saw one general pattern: in every case light provides the energy for a hydrogen donor, $\mathrm{H}{\phantom{X}}_2\mathrm{A}$, to reduce $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ to carbohydrate, and the rest of the donor, A, is set free.

For a sulphur bacterium the donor is $\mathrm{H}{\phantom{X}}_2\mathrm{S}$ and the product A is sulphur. For a green plant the donor is water, so A must be oxygen: the $\mathrm{O}{\phantom{X}}_2$ a plant releases comes from water, not from $\mathrm{C}\mathrm{O}{\phantom{X}}_2$.11This was confirmed in 1941 by feeding algae water labelled with ${}^{18}\mathrm{O}$. NCERT calls this a radioisotope technique; ${}^{18}\mathrm{O}$ is in fact a stable heavy isotope, detected by its mass. Balancing the equation for one glucose, with every oxygen molecule coming from water, gives

Equation (11.3) sums up the inputs and outputs of dozens of separate steps; it is not one reaction.

11.3 Where Does Photosynthesis Take Place?

Leaves do most of the work, but they have no monopoly: any green tissue can photosynthesise. Young green stems, the green sepals of a flower bud, unripe fruits such as green chillies and raw mangoes, and the flattened green stems of cacti such as Opuntia all contain chloroplasts and fix carbon.

Within a leaf, most chloroplasts sit in the mesophyll (you met their structure in Chapter 8), lined up against the cell walls rather than floating in the middle of the cell. They can shift their position along the walls as the light changes (Fig. 11.4).

Each chloroplast is wrapped in two membranes, an outer and an inner one. Inside lies a third membrane system: flattened sacs called thylakoids, stacked like coins into grana and joined by unstacked stroma lamellae. Around them is the stroma, a protein-rich fluid that also holds ribosomes, starch grains and lipid droplets.

A chloroplast works like a factory with two rooms. The thylakoid membranes (grana and stroma lamellae) are the power room: they catch light and turn its energy into two portable forms, ATP and NADPH. The stroma is the assembly room: its enzymes spend ATP and NADPH on building sugar from $\mathrm{C}\mathrm{O}{\phantom{X}}_2$, and store some of the sugar as starch. The power room runs only while light falls on it, so its steps are called the light reactions, or photochemical reactions.

The assembly room has no light switch of its own; it simply stops when the power room stops sending ATP and NADPH. Out of habit its steps are still called the dark reactions, although carbon reactions is the better name: in a living leaf they run by day, alongside the light reactions, and halt soon after the light goes.

11.4 How Many Types of Pigments are Involved in Photosynthesis?

Crush a spinach leaf in a little acetone and you get a green liquid. Is that green one substance or several? Paper chromatography answers the question (Fig. 11.5): a solvent creeping up a strip of paper carries each pigment in the extract a different distance, so a mixture spreads out into separate bands.

A leaf extract separates into four bands. Chlorophyll a is bright or blue-green and chlorophyll b yellow-green; xanthophylls are yellow, and carotenes range from yellow to orange. NCERT calls this last band carotenoids; strictly, xanthophylls and carotenes are both kinds of carotenoid.

Pigments are coloured because they absorb some wavelengths of light and not others. Chlorophylls take in blue and red strongly and let much of the green bounce back, which is why leaves look green to us. No pigment is more plentiful in the plant world than chlorophyll a.

A graph of how strongly a pigment absorbs each wavelength is its absorption spectrum. A graph of how fast photosynthesis runs under each wavelength is the action spectrum. Fig. 11.6 puts them together. The match is striking: the action spectrum peaks in the blue and the red, exactly where chlorophyll a absorbs most strongly (near 430 and 660 nm). That is the evidence for calling chlorophyll a the chief pigment of photosynthesis.

The match is not perfect, though. Between about 450 and 500 nm, and in part of the green, photosynthesis runs faster than chlorophyll a's absorption alone would allow. The extra comes from the accessory pigments: chlorophyll b, the xanthophylls and the carotenes, which also sit in the thylakoid membranes. They absorb wavelengths that chlorophyll a uses poorly and hand the energy to chlorophyll a, widening the range of light a leaf can use. They also guard chlorophyll a against photo-oxidation, damage by absorbed light energy that cannot be used safely.

11.5 What is Light Reaction?

The light reactions, also called the photochemical phase, have light and water as inputs and three outputs: oxygen and two energy-rich carriers, ATP and NADPH. In between, the energy of absorbed light is used to split water and to push its electrons on to $\mathrm{N}\mathrm{A}\mathrm{D}\mathrm{P}{\phantom{X}}^{+}$.

The work is done by large protein complexes in the thylakoid membrane. The pigments are organised into two photosystems, known as PS I and PS II, each built around its own light-harvesting complex (LHC). Do not read the numbers as a running order: PS I got its name because it was discovered first, but in the light reactions PS II acts before it.

On its own, a chlorophyll molecule in full sunlight catches only a handful of photons each second, too few to keep the chemistry busy. A photosystem therefore works like a dish aerial (Fig. 11.7). Hundreds of pigment molecules of several kinds, held in place by proteins, form the antenna. Whichever of them absorbs a photon passes the energy on, molecule to molecule, towards the centre of the dish. There sits one special chlorophyll a molecule, the reaction centre, the only molecule in the photosystem that actually gives away an electron, which a primary acceptor captures. Because the antenna pigments absorb different wavelengths, the dish also collects a broader band of light than chlorophyll a could alone.

Each reaction centre is tuned to a slightly different red. PS I's reaction-centre chlorophyll a absorbs best at 700 nm, so it is called P700; the one in PS II absorbs best at 680 nm and is called P680.

11.6 The Electron Transport

Follow one electron through Fig. 11.8. Its trip has two climbs, each powered by light, and two descents. Climb 1: red light of 680 nm excites P680 in PS II, and the energised electron is captured by a primary acceptor. Descent 1: the acceptor hands it to an electron transport chain that includes cytochromes. Each carrier binds the electron a little more tightly than the one before (it lies lower on the redox-potential scale), so energy is released at every hand-over, and part of it is used to make ATP. The electron reaches PS I intact.

Climb 2: at PS I, light of 700 nm excites P700 and lifts an electron to a second acceptor, higher still. Descent 2: from there the electron runs downhill to $\mathrm{N}\mathrm{A}\mathrm{D}\mathrm{P}{\phantom{X}}^{+}$, which is reduced to $\mathrm{N}\mathrm{A}\mathrm{D}\mathrm{P}\mathrm{H}+\mathrm{H}{\phantom{X}}^{+}$. Plot every carrier on a redox-potential scale and the up–down–up–down path traces a Z lying on its side, hence the name Z scheme.

11.6.1 Splitting of Water

After P680 has given away an electron it is left electron-hungry, and it is a strong enough oxidising agent to take electrons from water, which almost nothing else in a cell can do. A water-splitting complex on PS II (it contains manganese) collects them four at a time from two water molecules:

So every $\mathrm{O}{\phantom{X}}_2$ molecule a leaf gives off began as two water molecules at PS II. The chain of replacement runs backwards through the scheme: water refills PS II, and electrons from PS II, after the transport chain, refill PS I.

Where do the protons and oxygen appear? The water-splitting complex sits on the inner face of the thylakoid membrane, facing the lumen (the space inside the thylakoid). So protons and oxygen are released into the lumen. The oxygen diffuses out of the chloroplast; the protons stay and, as Section 11.6.3 shows, are put to work.

11.6.2 Cyclic and Non-cyclic Photo-phosphorylation

Cells store energy released from oxidisable substances in the chemical bonds of molecules such as ATP. Making ATP from ADP and inorganic phosphate (${\mathrm{P}}_{\mathrm{i}}$) is called phosphorylation, and both mitochondria and chloroplasts do it. When the energy comes from light, the process is photophosphorylation.

The flow traced in the Z scheme is one-way. Electrons leave water, pass through PS II, the chain and PS I, and end up in NADPH; they never come back. ATP made during such one-way flow is the product of non-cyclic photophosphorylation, and it always comes with NADPH and released oxygen.

PS I has a second option. Its excited electron can be fed back into the transport chain instead of on to $\mathrm{N}\mathrm{A}\mathrm{D}\mathrm{P}{\phantom{X}}^{+}$, and so return to P700 to be excited again (step 6 of Fig. 11.8). Every lap through the chain still releases energy for ATP, but nothing is reduced and no water is split, so this cyclic photophosphorylation yields ATP alone. Two situations favour it. The stroma lamellae carry PS I but neither PS II nor the NADP reductase enzyme, so a loop is the only route open there; and light longer than 680 nm can excite P700 but not P680, so in such light PS I works alone.

11.6.3 Chemiosmotic Hypothesis

How does a flow of electrons actually make ATP? The answer, proposed by Peter Mitchell in 1961, is chemiosmosis. Energy from electron transport is first stored as a difference in proton concentration across a membrane, and protons flowing back down that gradient then drive ATP synthesis. Respiration uses the same idea (Chapter 12), but the protons collect in different places: in mitochondria they build up in the space between the two membranes, while in chloroplasts they build up inside the thylakoids, in the lumen.

Three processes in the light reactions build the gradient (Fig. 11.9):

1. Water splitting. It happens on the lumen side of the membrane, so its protons are released straight into the lumen.
2. A hydrogen carrier. The primary acceptor of PS II lies near the stroma side. It passes each electron not to a plain electron carrier but to a carrier of whole hydrogen atoms (plastoquinone). Taking the electron, the carrier also takes a proton from the stroma; when it hands the electron on near the lumen side, it releases that proton into the lumen.
3. NADP reductase. This enzyme sits on the stroma side. Reducing $\mathrm{N}\mathrm{A}\mathrm{D}\mathrm{P}{\phantom{X}}^{+}$ to $\mathrm{N}\mathrm{A}\mathrm{D}\mathrm{P}\mathrm{H}+\mathrm{H}{\phantom{X}}^{+}$ uses electrons from PS I's acceptor together with protons taken from the stroma.

Add the three together and the lumen gains protons while the stroma loses them. The lumen becomes measurably more acidic than the stroma, and the membrane between them now holds a store of energy, like water held behind a dam.

The dam has one spillway. Protons can cross back only through the enzyme ATP synthase, and the energy they give up on the way makes ATP. The enzyme has two parts with two jobs. The channel, $\mathrm{C}{\mathrm{F}}_0$, spans the membrane, and protons diffuse through it from lumen to stroma (facilitated diffusion). The catalytic head, $\mathrm{C}{\mathrm{F}}_1$, sits on the stroma face, which is why the ATP appears in the stroma, right where the Calvin cycle needs it. Protons streaming through $\mathrm{C}{\mathrm{F}}_0$ drive repeated changes in the shape of $\mathrm{C}{\mathrm{F}}_1$, and each change helps join ADP and ${\mathrm{P}}_{\mathrm{i}}$ into ATP.

In short, the dam needs a wall (a membrane that protons cannot cross freely), a pump (electron transport moving protons into the lumen), a head of water (the gradient) and a turbine (ATP synthase). The ATP it makes, with the NADPH from electron flow, is spent at once in the stroma on fixing $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ and making sugars.

11.7 Where are the ATP and NADPH Used?

Darken a leaf that has been photosynthesising and watch its $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ uptake. It does not stop at once: it carries on for a little while and then dies away, and it resumes when the light returns. The reactions that fix $\mathrm{C}\mathrm{O}{\phantom{X}}_2$, the biosynthetic phase, evidently need no light themselves. What they need is the ATP and NADPH that the light reactions keep supplying, and they run until that stock is spent. Oxygen, the third product of the light reactions, plays no part here: it simply diffuses out of the chloroplast.

Which compound does $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ enter first? Calvin's team answered this with radioactive ${}^{14}\mathrm{C}$ (see the box at the start of the chapter). After the briefest exposure of algal cells to labelled $\mathrm{C}\mathrm{O}{\phantom{X}}_2$, almost all the radioactivity was in one compound, 3-phosphoglyceric acid (PGA), an organic acid with three carbon atoms. His group went on to map every step of the route, which now bears his name.

Is PGA always first? Surveys of many species said no. In maize, sugarcane and their relatives the label turned up first in a four-carbon organic acid, oxaloacetic acid (OAA). So plants are sorted by the first stable product of $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ fixation: ${\mathrm{C}}_3$ plants make the three-carbon PGA first (the ${\mathrm{C}}_3$ pathway), and ${\mathrm{C}}_4$ plants make the four-carbon OAA first (the ${\mathrm{C}}_4$ pathway). The two groups differ in other ways as well (Section 11.8).

11.7.1 The Primary Acceptor of $\mathrm{C}\mathrm{O}{\phantom{X}}_2$

A little arithmetic sets the puzzle. If one $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ (one carbon) joins an acceptor and the first product, PGA, has three carbons, the obvious guess is an acceptor with two carbons. Researchers spent years hunting for one. The real acceptor came as a surprise: a five-carbon ketose sugar, ribulose-1,5-bisphosphate (RuBP). One $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ plus one RuBP gives six carbons, which split at once into two molecules of PGA.

11.7.2 The Calvin Cycle

RuBP is consumed in the first step, so the pathway could not continue unless something rebuilt it, and Calvin's group found that the later steps do exactly that. The pathway is therefore a loop, the Calvin cycle. Every photosynthetic plant runs it, ${\mathrm{C}}_3$, ${\mathrm{C}}_4$ or otherwise; ${\mathrm{C}}_4$ plants merely add a front end (Section 11.8). We split the loop into three stages (Fig. 11.10).

1. Carboxylation, the key step. RuBisCO attaches $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ to RuBP, and the unstable six-carbon product splits at once into two PGA (equation (11.5)), so $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ is now part of a stable organic compound. The enzyme's full name, RuBP carboxylase-oxygenase, records that it can attach $\mathrm{O}{\phantom{X}}_2$ as well (Section 11.9).
2. Reduction. PGA is phosphorylated with ATP and reduced with NADPH to a three-carbon sugar phosphate, triose phosphate. For every $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ fixed, this stage uses 2 ATP and 2 NADPH. Some triose phosphate is drawn off to make glucose, sucrose and starch.
3. Regeneration. The rest of the triose phosphate is rearranged to rebuild RuBP, so the cycle can keep accepting $\mathrm{C}\mathrm{O}{\phantom{X}}_2$. This needs 1 more ATP per $\mathrm{C}\mathrm{O}{\phantom{X}}_2$.

Now total the bill. Each turn fixes one carbon, so a six-carbon glucose takes six turns. Each turn costs 2 ATP in reduction and 1 ATP in regeneration, 3 ATP in all, plus 2 NADPH; equation (11.6) is the bill for a whole glucose. Notice the 3 : 2 ratio. One-way electron flow alone does not deliver ATP and NADPH in that proportion, and the extra ATP probably comes from cyclic photophosphorylation, which makes ATP without NADPH.

11.8 The ${\mathrm{C}}_4$ Pathway

Maize, sugarcane, sorghum and many grasses of hot, dry tropical regions are ${\mathrm{C}}_4$ plants. They make OAA as their first product, yet their main route to sugar is still the Calvin cycle. So what sets them apart? They produce more biomass, cope with heat, keep gaining from very bright light, show almost no photorespiration, and have an unusual leaf anatomy. We begin with the anatomy.

Cut across a maize leaf and look at a vein (Fig. 11.11). Around each vascular bundle sits a ring of unusually large cells, the bundle sheath, sometimes several layers deep. Look closely and three things stand out: these cells are crowded with chloroplasts, their walls are thick enough to stop gases passing, and they fit together with no air spaces. The mesophyll cells radiate outward from the ring, and the whole pattern looks like a wreath, which is what the German word Kranz means; hence Kranz anatomy.

Compare a ${\mathrm{C}}_3$ dicot leaf, such as a bean leaf. Its mesophyll forms palisade and spongy layers with air spaces, and the cells around each vein are small and pale. Even a ${\mathrm{C}}_3$ grass such as wheat, whose mesophyll is not layered, lacks the wreath.

The biochemistry was worked out by M. D. Hatch and C. R. Slack in Australia in the mid-1960s, so the ${\mathrm{C}}_4$ route is called the Hatch and Slack pathway. It too is a cycle. Follow it in Fig. 11.12:

1. Capture. In a mesophyll cell the enzyme PEP carboxylase (PEPcase) joins $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ to a three-carbon acceptor, phosphoenolpyruvate (PEP), giving the four-carbon acid OAA. RuBisCO is absent from these cells, so PEPcase has the $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ to itself.
2. Shipping. OAA is quickly turned into a more stable four-carbon acid (malic acid in maize, aspartic acid in some other species), which travels through plasmodesmata into the bundle sheath.
3. Release. In the sheath an enzyme strips $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ off the four-carbon acid (decarboxylation), leaving a three-carbon remnant, pyruvate in maize.
4. Return. Pyruvate goes back to the mesophyll, where it is rebuilt into PEP, closing the loop.

The division of labour is now clear. The mesophyll, armed with PEPcase but no RuBisCO, only captures $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ and ships it inward. The bundle sheath, rich in RuBisCO but without PEPcase, receives that $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ and runs the Calvin cycle on it. So both kinds of plant make their sugar by the same Calvin cycle: a ${\mathrm{C}}_3$ leaf runs it throughout its mesophyll, a ${\mathrm{C}}_4$ leaf only in its bundle sheath.

11.9 Photorespiration

Why do ${\mathrm{C}}_4$ plants go to the trouble of pumping $\mathrm{C}\mathrm{O}{\phantom{X}}_2$? The answer lies in a weakness of the enzyme that runs the first step of the Calvin cycle (equation (11.5)). No enzyme on Earth is more abundant than RuBisCO, partly because it works slowly, so a leaf needs a great deal of it.

Its active site accepts either gas, $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ or $\mathrm{O}{\phantom{X}}_2$ (Fig. 11.13); hence carboxylase-oxygenase. Offered equal amounts, it picks $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ far more often, but the choice is a competition decided by how many molecules of each gas are around. Change the mixture and you change the outcome.

Air holds far more $\mathrm{O}{\phantom{X}}_2$ than $\mathrm{C}\mathrm{O}{\phantom{X}}_2$, so in a ${\mathrm{C}}_3$ leaf a share of RuBisCO's encounters are with $\mathrm{O}{\phantom{X}}_2$, and each one is a $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ not fixed. Each time $\mathrm{O}{\phantom{X}}_2$ wins, RuBP is split into one PGA and one two-carbon phosphoglycolate instead of two PGA. This oxygen-consuming route, which runs only in light, is photorespiration.

For the plant it is pure loss. Carbon that had already been fixed is given off again as $\mathrm{C}\mathrm{O}{\phantom{X}}_2$, ATP is spent salvaging the phosphoglycolate, and no sugar, ATP or NADPH comes out of it. What purpose, if any, photorespiration serves is still not known.

This is where the ${\mathrm{C}}_4$ pump pays off. The mesophyll keeps delivering four-carbon acids to the bundle sheath, and their decarboxylation releases $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ faster than it can leak out through the thick walls. $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ around RuBisCO climbs far above its level in air, the competition is rigged in favour of $\mathrm{C}\mathrm{O}{\phantom{X}}_2$, and oxygenation all but disappears. With photorespiration negligible, ${\mathrm{C}}_4$ plants lose less of the carbon they fix, which is why they are so productive, and they keep this advantage in heat, when ${\mathrm{C}}_3$ plants photorespire most.

11.10 Factors Affecting Photosynthesis

Why does one field of wheat out-yield the next? Much of the answer is the rate of photosynthesis, and that rate depends on two sets of factors. External factors are the surroundings: sunlight, temperature, $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ concentration and water. Internal factors belong to the plant: how many leaves it has and how large, old and well angled they are; how many mesophyll cells and chloroplasts they hold; how much chlorophyll they carry; and how much $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ is inside them. These depend on the plant's genes and on how well it has grown.

All of these act at the same time, yet at any moment the rate is usually held back by one of them, the limiting factor: the one in shortest supply compared with what the leaf could use. A railway ticket queue is a fair picture. If only one counter is open, the queue moves at that clerk's pace, and printing more tickets will not speed it up. F. F. Blackman turned this idea into a law in 1905.

11.10.1 Light

Read the curve in Fig. 11.14 from left to right. At first (near W) the rate of $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ fixation climbs in direct proportion to light intensity: double the light, double the rate, so light is limiting. Then the climb flattens (X, Y), not because light stops mattering but because something else, usually $\mathrm{C}\mathrm{O}{\phantom{X}}_2$, now limits. Leaves reach this plateau surprisingly early (NCERT puts light saturation at about 10 per cent of full sunlight), so in nature light limits mainly plants in shade or under a dense forest canopy. At the far right (Z), light so strong that it breaks down chlorophyll pulls the rate down.

Intensity is not the whole story. The quality of light (its wavelengths, Section 11.4) and the duration of daily exposure matter too.

11.10.2 Carbon dioxide Concentration

Greenhouse growers of tomatoes and capsicum (bell pepper) pump extra $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ into the air around their crops and harvest more fruit. Why does that work? Because $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ is the factor that most often limits photosynthesis: air holds only about 0.04 per cent of it.22Global average $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ passed 420 µL per litre (0.042 per cent) in 2024 and is still rising. NCERT quotes the older range of 0.03–0.04 per cent. Up to about 0.05 per cent, more $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ means faster fixation, though levels much above that harm plants if kept up for long.

${\mathrm{C}}_3$ and ${\mathrm{C}}_4$ plants differ in how much extra $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ they can use. In dim light neither responds, because light is limiting. In bright light both do, up to a point: a ${\mathrm{C}}_4$ plant is saturated by about 360 $\mu \mathrm{L}{\mathrm{L}}^{-1}$, while a ${\mathrm{C}}_3$ plant keeps responding until beyond 450 $\mu \mathrm{L}{\mathrm{L}}^{-1}$. Today's air therefore holds a ${\mathrm{C}}_4$ crop close to its maximum but still holds a ${\mathrm{C}}_3$ crop back, and tomatoes and capsicum are ${\mathrm{C}}_3$ plants.

11.10.3 Temperature

Enzymes run the carbon reactions, and enzyme activity rises with warmth up to an optimum and falls beyond it, so temperature strongly controls these reactions; the light-driven steps are far less sensitive. Optima differ between plants. ${\mathrm{C}}_4$ plants, with their $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ pump, keep speeding up into the thirties (°C), while ${\mathrm{C}}_3$ plants peak at much cooler temperatures. Habitat matters too: a plant adapted to the tropics has a higher optimum than one adapted to a temperate climate.

11.10.4 Water

A wilting plant on a hot afternoon photosynthesises slowly, but not because it lacks water to split: the light reactions use only a tiny fraction of the water a plant takes up. The damage is indirect. To save water the plant shuts its stomata, which also shuts out $\mathrm{C}\mathrm{O}{\phantom{X}}_2$, and its drooping leaves present less surface to the light and slow their metabolism.