Photosynthesis in Higher Plants

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XI, Chapter 11.

Where does a tree's body come from?

Plant a small tree in a tub of soil and give it only water. Years later it weighs many kilograms, but the soil has barely lost any weight. A tree is built mostly from carbon dioxide taken out of thin air and water drawn up from the roots, stitched together using sunlight. That stitching is photosynthesis, a physico-chemical process by which green plants use light energy to make organic compounds. It is the primary source of all our food and of the oxygen in the air, because every heterotroph, directly or indirectly, depends on it. This chapter explains how it works, in the order a detective would: first the clues, then the machinery.

The detective story: how scientists found the ingredients

Simple school experiments already show the requirements. In a variegated leaf (green and white patches) or a leaf partly covered with black paper, starch appears only in the green, lit parts. In the half leaf experiment, a leaf kept in a tube containing KOH (which absorbs CO₂) makes no starch. Together these show that chlorophyll, light and CO₂ are necessary. The historical experiments added the rest.

Van Niel's general equation is 2H₂A + CO₂ → 2A + CH₂O + H₂O. In sulphur bacteria H₂A is H₂S and the product A is sulphur. In green plants H₂A is H₂O and A is oxygen. Putting it together, the overall plant equation is:

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂ (light, chlorophyll)

Water appears on both sides because 12 molecules are split to give the oxygen, while 6 new ones are made when carbon is reduced. This is the book keeping that proves O₂ comes from water, not CO₂. Later experiments with radioactive isotopes confirmed it.

Pause and predict: you supply a plant with water labelled with heavy oxygen (¹⁸O) and with ordinary CO₂. Where will the heavy oxygen appear in the released gas?

Check your answer: in the O₂. Water is the source of the oxygen that is released, which is exactly what van Niel inferred.

The factory: chloroplast and its two compartments

Photosynthesis happens in green leaves and other green parts. Inside the leaf, the mesophyll cells hold many chloroplasts arranged along the cell walls, which lets them catch the light falling on the leaf. The chloroplast has two regions that do two different jobs.

Beyond the textbook. For the exam: the second stage is called the dark reaction because it needs no light directly. In reality: the name is misleading. These reactions need the ATP and NADPH made in light, so they stop soon after the light goes off, and several Calvin cycle enzymes are themselves switched on by light. Why it matters: if a statement says the carbon reactions occur only at night, it is wrong.

Pigments: why leaves are green and light has a menu

Separate the pigments of a leaf by paper chromatography and you find four bands.

Chlorophyll a is the chief pigment. Its absorption spectrum peaks in the blue and red regions, and that matches the action spectrum of photosynthesis. The action spectrum is a graph of photosynthetic rate against wavelength, while the absorption spectrum is a graph of how much light a pigment absorbs at each wavelength. Engelmann's result is why the two curves are compared.

The accessory pigments (chlorophyll b, xanthophylls and carotenoids) do two jobs. They absorb wavelengths that chlorophyll a absorbs poorly, so more of the spectrum is used. They also protect chlorophyll a from photo-oxidation.

Beyond the textbook. For the exam: chlorophyll a is the chief pigment and carotenoids are accessory pigments. In reality: the yellow and orange of autumn leaves is not new. Carotenoids and xanthophylls were in the leaf all along, hidden by the larger quantity of chlorophyll. When chlorophyll breaks down in autumn, they show.

The light reactions: capturing light as chemical energy

Pigments are organised into two light harvesting complexes called Photosystem I (PS I) and Photosystem II (PS II). Each has hundreds of pigment molecules acting as antennae, and a single chlorophyll a molecule at the centre, the reaction centre. The antennae gather photons and funnel the energy to the reaction centre. The reaction centre of PS I absorbs best at 700 nm (P700) and that of PS II at 680 nm (P680).

Trap: the photosystems are numbered in the order they were discovered, not the order in which they work. In the light reaction, PS II acts first, then PS I.

The Z scheme (non-cyclic electron flow). Electrons move like water running down a staircase, with two uphill pushes from light. The path is:

PS II (P680) → primary acceptor → electron carriers (plastoquinone, cytochrome b6f, plastocyanin) → PS I (P700) → primary acceptor (ferredoxin) → NADP⁺ reduced to NADPH + H⁺.

Plotted on an energy scale, this path looks like the letter Z, hence the name. Light excites P680 and an electron leaves. Going downhill through the carriers, it drives ATP synthesis. In PS I, a second photon re-excites the electron, which then ends up on NADP⁺.

Where PS II gets its electrons. An electron that leaves PS II must be replaced, and the replacement comes from water. The water splitting complex sits on the inner (lumen) side of the thylakoid membrane, linked to PS II, and works with manganese, calcium and chloride ions:

2H₂O → 4H⁺ + O₂ + 4e⁻

This reaction supplies the oxygen we breathe, supplies protons to the lumen, and replaces the electrons of PS II.

Pause and predict: in cyclic flow, why is no oxygen released?

Check your answer: PS II is not working, so no water is split. Oxygen comes only from water splitting, so without PS II there is none, and since the electron returns to PS I rather than reaching NADP⁺, no NADPH forms either.

How ATP is made: chemiosmosis. The thylakoid membrane is a closed sac. Protons pile up inside it (the lumen), so the lumen becomes acidic and the stroma side low in protons. Three things build that gradient: water splitting releases H⁺ inside the lumen; as electrons pass through the carrier plastoquinone, it moves protons from the stroma into the lumen; and the reduction of NADP⁺ on the stroma side uses up H⁺ from the stroma. The protons then flow back out through the enzyme ATP synthase, which has two parts. CF₀ is the channel embedded in the membrane, and CF₁ is the headpiece that sticks out into the stroma and makes ATP. The energy released by the proton flow drives ATP synthesis. Respiration uses the same principle, as you will see in Respiration in Plants.

The Calvin cycle: making sugar from CO₂

Melvin Calvin used radioactive ¹⁴C in algal photosynthesis and discovered that the first stable product of CO₂ fixation is a three carbon acid, 3-phosphoglyceric acid (PGA). This pathway is the Calvin cycle or C₃ cycle, and it runs in the stroma of every photosynthetic plant.

It was expected that the acceptor of CO₂ would be a two carbon molecule, but it turned out to be RuBP (ribulose 1,5-bisphosphate), a five carbon ketose sugar. The cycle has three stages.

RuBisCO (ribulose bisphosphate carboxylase-oxygenase) is the most abundant enzyme on Earth, and its name tells you it can act on both CO₂ and O₂.

The bookkeeping. One turn of the cycle fixes one CO₂ and costs 3 ATP and 2 NADPH. A glucose has six carbons, so it needs 6 turns, which means 18 ATP and 12 NADPH per glucose. Note that it is the light reactions that supply these molecules, which is the link between the two halves of photosynthesis.

Why six turns, not one? Each turn fixes only one carbon, and the cycle must regenerate RuBP every time. Three turns fix three carbons, which gives one net triose phosphate that can leave the cycle. Two triose phosphates join to make a hexose, so one glucose takes six turns.

The C₄ pathway: a pump that fixes CO₂ before it is lost

In hot, dry regions a plant keeps its stomata partly closed to save water, and that restricts CO₂ entering the leaf. Plants of such regions, for example maize, sorghum and sugarcane, fit a CO₂ concentrating pump in front of the Calvin cycle. This is the C₄ (Hatch and Slack) pathway. In it the first stable product is a four carbon acid, oxaloacetic acid (OAA).

The steps follow the anatomy. C₄ leaves show Kranz anatomy, a "wreath" of large bundle sheath cells around the vascular bundles. These cells have many chloroplasts, thick walls impervious to gases and no intercellular spaces.

The pathway in one line: CO₂ + PEP (3C, in mesophyll cell, enzyme PEP carboxylase) → OAA (4C) → malate or aspartate → moves to bundle sheath cell → decarboxylation releases CO₂ for the Calvin cycle → the 3C product returns to the mesophyll to regenerate PEP.

PEP carboxylase works only in the mesophyll cells, which lack RuBisCO. The Calvin cycle, meanwhile, occurs only in the bundle sheath cells. This is the point NTA targets: C₄ plants still use the Calvin cycle, they simply add a pump.

Beyond the textbook. For the exam: C₄ plants are more productive in hot, dry conditions. In reality: the pump is not free. Running the C₄ pathway costs extra ATP, because PEP has to be regenerated each cycle, so in cool, shady conditions C₃ plants can do as well or better. Why it matters: C₄ is an advantage under heat and strong light, not a universally better design.

Photorespiration: RuBisCO's mistake

RuBisCO has a single active site that can bind either CO₂ or O₂, and the one that is more abundant near the site wins. When O₂ binds RuBP, the products are one molecule of 3-PGA and one of a two carbon compound, phosphoglycolate. The pathway that follows runs through the chloroplast, peroxisome and mitochondrion, releases CO₂, uses ATP, and makes no sugar, no ATP and no NADPH. In a sense, it undoes some of photosynthesis.

C₄ plants avoid this. Their bundle sheath cells are flooded with CO₂ from the decarboxylation of C₄ acids, so RuBisCO behaves as a carboxylase and its oxygenase activity is minimal. That is why photorespiration is absent in C₄ plants.

Beyond the textbook. For the exam: photorespiration releases CO₂, uses ATP and makes no sugar, so it is treated as a wasteful pathway whose biological function is not yet known. In reality: it does serve a purpose, which is to recover the carbon from phosphoglycolate, a compound that is harmful if allowed to build up, and return it to the Calvin cycle. Why it matters: it is a clean up route for an unavoidable side reaction, not a pointless extra.

What limits photosynthesis? Blackman's law

Internal factors include the number, size, age and orientation of leaves, the number of chloroplasts and the amount of chlorophyll. External factors are light, CO₂, temperature and water. In 1905 Blackman stated the Law of Limiting Factors: if a process depends on more than one factor, its rate is decided by the factor nearest its minimal value, and the others, even if plentiful, do not raise it.

Pause and predict: a plant is in bright light and the right temperature, but the air has very little CO₂. If you double the light, will photosynthesis rise?

Check your answer: no. By Blackman's law, CO₂ is the factor nearest its minimum, so only more CO₂ will raise the rate.

Exam traps

60-second revision

Where this chapter connects

• Previous: Cell Cycle and Cell Division ends the cell biology unit; Cell: The Unit of Life describes the chloroplast structure used here.
• Next: Respiration in Plants runs the opposite chemistry, burning the glucose made here, and uses the same chemiosmotic idea.
• Class XII link: Ecosystem builds on photosynthesis to explain primary productivity and energy flow.