NEET BiologyNCERT Class 11Chapter 11

Photosynthesis in Higher Plants: NEET notes

This chapter explains how green plants capture light energy and store it as chemical energy in sugar. It moves from the classic experiments that revealed the process, through the pigments and the light-driven reactions in the thylakoids, to CO₂ fixation in the stroma by the C₃ and C₄ routes, photorespiration, and the factors that limit the rate.

What NEET asks

NEET questions lean on the historical experiments, the two photosystems and their reaction centres, cyclic versus non-cyclic photophosphorylation, where protons build up in the chemiosmotic model, the ATP and NADPH bookkeeping of the Calvin cycle, and C₃ versus C₄ contrasts. Students most often lose marks by swapping PS I and PS II, by placing the proton gradient on the wrong side of the thylakoid, and by mixing up first products and primary acceptors.

Practise 10 NEET questions on this chapter

1. What do we know about photosynthesis

NCERT §11.1

Photosynthesis is the physico-chemical process by which green plants harness light energy to build organic compounds; every other organism ultimately relies on it for food.
Two things make it important: all food on earth comes primarily from it, and it releases oxygen into the atmosphere.
Simple school experiments show what is needed: a variegated leaf, or a leaf partly covered with black paper, forms starch only in the green and lit parts, showing chlorophyll and light are required.
In the half-leaf experiment, one part of a leaf is kept in a test tube containing KOH-soaked cotton, which absorbs CO₂; that part does not form starch, while the part exposed to air does, showing CO₂ is required.

2. Early experiments

NCERT §11.2

Joseph Priestley (1770) found that a candle or a mouse in a closed bell jar soon failed, but a mint plant placed inside restored the air so the candle burned and the mouse survived. He later discovered oxygen in 1774.
Jan Ingenhousz ran Priestley's set-up once in darkness and once in sunlight, and found that a plant can purify air fouled by a candle or an animal only when sunlight is present.
Working with an aquatic plant, Ingenhousz saw small bubbles form around its green parts in bright sunlight but not in the dark; he later identified the gas as oxygen, showing that only the green parts of a plant release oxygen.
Around 1854 Julius von Sachs gave evidence that growing plants make glucose, usually stored as starch, in their green parts; his later work placed chlorophyll inside special bodies in the cell, afterwards named chloroplasts.
T.W. Engelmann split light into a spectrum with a prism and shone it on Cladophora, a green alga, which sat in a suspension of aerobic bacteria; these gathered wherever O₂ was being released. Most of them collected in the blue and red bands, and this gave the first action spectrum of photosynthesis, which roughly matches the absorption spectra of chlorophylls a and b.
By the mid-nineteenth century the overall process was understood: green parts, in light, make carbohydrate from CO₂ and water and release oxygen.
Working with purple and green bacteria, Cornelius van Niel showed that photosynthesis depends on light, and that in it CO₂ is reduced to carbohydrate by hydrogen taken from a suitable oxidisable compound: 2H₂A + CO₂ → 2A + CH₂O + H₂O.
Green plants use H₂O as the hydrogen donor, oxidising it to O₂; bacteria that use H₂S produce sulphur or sulphate in place of O₂. So the O₂ that green plants release comes from water rather than CO₂, a point later confirmed with radioisotopic techniques.
The corrected overall equation for green plants is 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂ (in light).

3. Where photosynthesis takes place

NCERT §11.3

Photosynthesis occurs in the green leaves and also in other green parts; within the leaf, it happens mainly in the chloroplasts of mesophyll cells.
Mesophyll cells are packed with chloroplasts, which generally arrange themselves along the cell walls to catch an optimum amount of the light falling on them.
Inside the chloroplast, the membrane system (grana, stroma lamellae) traps light energy and makes ATP and NADPH.
In the stroma, enzymes reduce CO₂ to carbohydrates and the sugar is later converted to starch.
The membrane-bound events are directly light-driven and are called light reactions; the stroma events do not directly need light but use the ATP and NADPH made in light, and are called carbon reactions.
The older label 'dark reaction' is misleading: these reactions do not take place in darkness and are not independent of light, since they depend on the products of the light reactions.

4. Pigments involved in photosynthesis

NCERT §11.4

Separating leaf pigments by paper chromatography reveals four of them: chlorophyll a (bright or blue-green), chlorophyll b (yellow-green), carotenoids (yellow to yellow-orange) and xanthophylls (yellow).
Chlorophyll a is the chief pigment of photosynthesis.
An absorption spectrum shows chlorophyll a absorbs most strongly in the blue and red regions of light.
The action spectrum (rate of photosynthesis versus wavelength) peaks in the blue and red regions too, and roughly matches the absorption spectrum of chlorophyll a, confirming most photosynthesis occurs in blue and red light.
Some photosynthesis also occurs at other visible wavelengths because accessory pigments (chlorophyll b, xanthophylls, carotenoids) absorb them and pass the energy to chlorophyll a.
Accessory pigments widen the range of usable wavelengths and also protect chlorophyll a from photo-oxidation.

5. The light reaction and the photosystems

NCERT §11.5

The light reactions (the photochemical phase) cover light absorption, oxygen release, water splitting and the production of ATP and NADPH, the high-energy chemical intermediates.
Pigments are organised into two discrete light harvesting complexes (LHC) within Photosystem I (PS I) and Photosystem II (PS II).
Each LHC, which acts as an antenna, consists of hundreds of pigment molecules held by proteins; by absorbing different wavelengths it makes photosynthesis more efficient.
Every photosystem has all pigments except one chlorophyll a molecule that forms its reaction centre.
The reaction centre chlorophyll a of PS I has its absorption peak at 700 nm, hence the name P700; that of PS II peaks at 680 nm and is called P680.
The photosystems are numbered in the order they were discovered, not in the order they act during the light reaction.

6. Electron transport, splitting of water and photophosphorylation

NCERT §11.6–§11.6.2

In PS II, P680 absorbs red light of 680 nm; its electrons are excited and pass to a primary electron acceptor, then along an electron transport chain of cytochromes, moving downhill in redox potential.
Those electrons reach PS I. Meanwhile P700 in PS I is also excited by 700 nm light, and its electrons pass to another acceptor with a greater redox potential and finally reduce NADP⁺ to NADPH + H⁺.
Plotted on a redox scale, this path from PS II to PS I to NADP⁺ forms a Z shape, so it is called the Z scheme.
PS II refills its lost electrons by splitting water: 2H₂O → 4H⁺ + O₂ + 4e⁻. Water splitting is tied to PS II, which sits on the inner (lumen) side of the thylakoid membrane, so protons and O₂ are released into the lumen.
ATP synthesis from ADP and inorganic phosphate using light energy is called photophosphorylation.
Non-cyclic photophosphorylation uses both photosystems working in series; electrons flow from water to NADP⁺, and both ATP and NADPH + H⁺ are produced.
In cyclic photophosphorylation only PS I works; excited electrons circle back to PS I via the transport chain rather than reaching NADP⁺, so only ATP is made, with no NADPH.
The stroma lamellae are a likely site for cyclic flow: the grana membranes have both photosystems, while stroma lamella membranes have no PS II and no NADP reductase enzyme. Cyclic photophosphorylation also occurs if only light beyond 680 nm in wavelength is available.

7. Chemiosmotic hypothesis of ATP synthesis

NCERT §11.6.3

ATP synthesis in chloroplasts is linked to a proton gradient across the thylakoid membrane, with protons accumulating inside the thylakoid lumen rather than in the stroma.
Three things build the gradient: water splitting on the lumen side releases H⁺ into the lumen; a proton carrier in the membrane moves H⁺ from stroma to lumen as electrons pass through the photosystems; and NADP reductase on the stroma side takes up H⁺ from the stroma while reducing NADP⁺.
The result is fewer protons in the stroma and more in the lumen, so pH falls in the lumen.
The membrane is impermeable to protons, so they can return to the stroma only through the channel of ATP synthase.
ATP synthase has two parts: CF₀, embedded in the thylakoid membrane, forms the transmembrane channel for protons; CF₁ protrudes on the stroma-facing surface.
Proton flow through CF₀ causes a conformational change in CF₁ that makes ATP; energy from the gradient is used for this.
So chemiosmosis requires four things: ATP synthase, a proton pump, a proton gradient and a membrane. In the stroma, the ATP and NADPH produced are used straight away to fix CO₂.

8. Use of ATP and NADPH: the Calvin cycle (C₃ pathway)

NCERT §11.7–§11.7.2

Using radioactive ¹⁴C to trace carbon in algae, Melvin Calvin found that the first product of CO₂ fixation is a 3-carbon organic acid, 3-phosphoglyceric acid (PGA).
Later, a different group of plants was found whose first stable product is a 4-carbon acid, oxaloacetic acid (OAA). Plants of the first kind are called C₃ plants and the second kind C₄ plants.
In the Calvin cycle, CO₂ is first accepted by ribulose bisphosphate (RuBP), a ketose sugar with 5 carbons, and not by a 2-carbon molecule as was once expected.
The Calvin cycle occurs in all photosynthetic plants, whether C₃ or C₄. It runs in three stages: carboxylation, reduction and regeneration.
Carboxylation: CO₂ is fixed onto RuBP by RuBisCO (RuBP carboxylase-oxygenase), producing two molecules of 3-PGA. This is the most crucial step, and RuBisCO is described as the most abundant enzyme in the world.
Reduction: 3-PGA is converted to carbohydrate using 2 ATP (for phosphorylation) and 2 NADPH (for reduction) per CO₂ fixed.
Regeneration: RuBP is remade so the cycle can continue; this step needs one more ATP per CO₂ for phosphorylation.
So each CO₂ fixed costs 3 ATP and 2 NADPH. Making one glucose needs six turns of the cycle: 6 CO₂ in, 18 ATP and 12 NADPH used, 1 glucose out.

9. The C₄ pathway

NCERT §11.8

C₄ plants are adapted to dry tropical regions. Their first CO₂ fixation product is the 4-carbon OAA, but they still use the Calvin cycle as their main biosynthetic route.
Compared with C₃ plants they have a special leaf anatomy, tolerate higher temperatures, respond to high light intensity, lack photorespiration and have greater biomass productivity. Maize and sorghum are examples.
Kranz anatomy: around the vascular bundles lies a wreath-like layer of large bundle sheath cells (Kranz means wreath). These cells can be in several layers; they have many chloroplasts, no intercellular spaces, and thick walls that gases cannot pass through.
In mesophyll cells, the primary CO₂ acceptor is phosphoenol pyruvate (PEP), a 3-carbon molecule, and the enzyme is PEP carboxylase (PEPcase). Mesophyll cells lack RuBisCO.
The OAA formed in mesophyll is converted to other 4-carbon acids such as malic acid or aspartic acid, which move to the bundle sheath cells.
Inside bundle sheath cells the C₄ acids break down, releasing CO₂ plus a 3-carbon molecule, which goes back to the mesophyll to be converted into PEP again, so the cycle is completed.
The released CO₂ enters the Calvin cycle in the bundle sheath cells, which are rich in RuBisCO but lack PEPcase.
This pathway is also known as the Hatch and Slack pathway, and in effect it pumps CO₂ to where RuBisCO is, keeping CO₂ concentration high there.

10. Photorespiration

NCERT §11.9

RuBisCO can bind both CO₂ and O₂ at its active site, and the binding is competitive: whichever gas is relatively more concentrated tends to bind. RuBisCO has a much greater affinity for CO₂ when CO₂ and O₂ are present in similar amounts.
In C₃ plants some O₂ does bind RuBisCO. RuBP then combines with O₂ to give one molecule of phosphoglycerate and one of phosphoglycolate; this oxygenase route is photorespiration.
Photorespiration makes no sugar and no ATP or NADPH. Instead it uses ATP and releases CO₂, making it a wasteful process.
In C₄ plants photorespiration does not occur, because the mechanism concentrates CO₂ at the enzyme site in bundle sheath cells, so RuBisCO acts as a carboxylase and its oxygenase role is minimised.
Lacking photorespiration helps explain why C₄ plants give better productivity and yields. They also tolerate higher temperatures. The biological function of photorespiration is still not known.

11. Factors affecting photosynthesis

NCERT §11.10

Both internal (plant) and external factors affect the rate. Internal ones include how many leaves, mesophyll cells and chloroplasts there are and their size, age and orientation, the internal CO₂ concentration and the amount of chlorophyll; external ones are light, temperature, CO₂ concentration and water.
Blackman's Law of Limiting Factors (1905): when a chemical process depends on more than one factor, its rate is decided by the factor nearest its minimal value; changing that factor alone changes the rate.
Light: at low intensities the rate rises linearly with light; at higher intensities other factors limit it. Light saturation occurs at about 10 per cent of full sunlight, so light is rarely limiting in nature except for plants in shade or dense forests. Very high light can break down chlorophyll and lower the rate.
CO₂ is the major limiting factor. Its atmospheric level is very low (0.03 to 0.04 per cent); raising it up to about 0.05 per cent can increase fixation, but higher levels can be damaging over longer periods.
At low light neither C₃ nor C₄ plants respond to high CO₂; at high light both do. C₄ plants saturate at about 360 μL L⁻¹, while C₃ plants keep responding and saturate only beyond 450 μL L⁻¹, so present CO₂ levels limit C₃ plants.
Greenhouse crops such as tomatoes and bell peppers are grown in CO₂-enriched air to get higher yields.
Temperature: the enzymatic carbon reactions are temperature-controlled; the light reactions are also temperature-sensitive but much less so. C₄ plants have a higher temperature optimum than C₃ plants, and tropical plants have higher optima than temperate ones.
Water affects photosynthesis mainly through the plant: water stress closes stomata, cutting CO₂ supply, and makes leaves wilt, reducing leaf area and metabolic activity.

Must-know facts

Priestley (1770): plants restore air spoilt by a candle or mouse; he discovered oxygen in 1774.
Ingenhousz: sunlight is essential; only green parts release oxygen.
Sachs (about 1854): green parts make glucose, usually stored as starch; chlorophyll sits in bodies later called chloroplasts.
Engelmann: Cladophora plus aerobic bacteria gave the first action spectrum, with peaks in blue and red.
van Niel: O₂ released in photosynthesis comes from H₂O, not CO₂.
Overall equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂.
Light reactions: thylakoid membranes (grana, stroma lamellae). Carbon reactions: stroma.
Chlorophyll a is the chief pigment; accessory pigments widen the spectrum and protect chlorophyll a from photo-oxidation.
PS I reaction centre = P700; PS II reaction centre = P680; named in order of discovery.
Water splitting (2H₂O → 4H⁺ + O₂ + 4e⁻) is linked to PS II on the lumen side.
Non-cyclic: PS II + PS I, gives ATP and NADPH, releases O₂. Cyclic: PS I only, gives only ATP; likely site is the stroma lamellae.
Protons accumulate in the thylakoid lumen; CF₀ is the membrane channel, CF₁ sits on the stroma side and makes ATP.
Calvin: ¹⁴C tracer; first stable product in C₃ = 3-PGA; primary acceptor = RuBP (5C).
Per CO₂: 3 ATP + 2 NADPH. Per glucose: 6 turns, 18 ATP, 12 NADPH.
RuBisCO = RuBP carboxylase-oxygenase, the most abundant enzyme in the world.
C₄: first product OAA (4C), primary acceptor PEP (3C), enzyme PEPcase in mesophyll; Calvin cycle in bundle sheath.
Mesophyll of C₄ plants lacks RuBisCO; bundle sheath lacks PEPcase.
Photorespiration: RuBP + O₂ → phosphoglycerate + phosphoglycolate; no sugar, no ATP; absent in C₄ plants.
Light saturation at about 10% of full sunlight; C₄ saturate near 360 μL L⁻¹ CO₂, C₃ only beyond 450 μL L⁻¹.
Blackman (1905): the factor nearest its minimum limits the rate.

Common traps

Assuming PS I acts first because it is numbered 1.

Numbers reflect order of discovery. In the non-cyclic Z scheme, PS II (P680) acts first and passes electrons to PS I (P700).

Saying protons pile up in the stroma during the light reaction.

They accumulate in the thylakoid lumen and flow back out through CF₀ to the stroma; CF₁ faces the stroma.

Thinking the O₂ released comes from CO₂.

It comes from water, as van Niel inferred and isotope work later confirmed. Split water, get O₂.

Confusing the first product with the primary acceptor.

C₃: acceptor RuBP (5C), first product 3-PGA (3C). C₄: acceptor PEP (3C), first product OAA (4C).

Believing C₄ plants skip the Calvin cycle.

All photosynthetic plants run the Calvin cycle. In C₄ plants it runs in the bundle sheath cells.

Crediting cyclic photophosphorylation with NADPH or O₂ production.

Cyclic flow uses only PS I and makes only ATP; no water splitting, so no O₂ and no NADPH.

Taking 'dark reaction' literally, as reactions that run in darkness.

The carbon reactions need the ATP and NADPH made in light, so they depend on light indirectly; they carry on only briefly after light is withdrawn and then stop.

Assuming light is usually the limiting factor in the field.

Light saturates at about 10% of full sunlight, so except in shade or dense forest, CO₂ is the usual limiting factor.

Putting RuBisCO in C₄ mesophyll cells.

C₄ mesophyll has PEPcase and no RuBisCO; the bundle sheath has RuBisCO and no PEPcase.

Key terms

Action spectrum: A plot of photosynthetic rate against the wavelength of light used.
Absorption spectrum: A plot of how much light of each wavelength a pigment absorbs.
Accessory pigments: Chlorophyll b, xanthophylls and carotenoids, which absorb extra wavelengths and pass the energy to chlorophyll a.
Light harvesting complex: An antenna of hundreds of protein-bound pigment molecules feeding energy to a reaction centre.
Reaction centre: The single chlorophyll a molecule in a photosystem that actually loses an excited electron.
Z scheme: The zig-zag path of electrons from water through PS II and PS I to NADP⁺ when drawn on a redox scale.
Photophosphorylation: Making ATP from ADP and inorganic phosphate using light energy.
Photolysis of water: The PS II-linked splitting of water into protons, electrons and oxygen.
Chemiosmosis: ATP synthesis powered by protons flowing down a gradient through ATP synthase.
CF₀ and CF₁: The membrane proton channel and the stroma-facing catalytic head of chloroplast ATP synthase.
RuBP: Ribulose bisphosphate, the 5-carbon CO₂ acceptor of the Calvin cycle.
RuBisCO: The enzyme that joins CO₂ (or O₂) to RuBP; it has both carboxylase and oxygenase activity.
PEPcase: PEP carboxylase, the enzyme that fixes CO₂ onto PEP in C₄ mesophyll cells.
Kranz anatomy: The wreath of large, chloroplast-rich, thick-walled bundle sheath cells around veins in C₄ leaves.
Photorespiration: The oxygenase reaction of RuBisCO in C₃ plants that consumes ATP and releases CO₂ without making sugar.
Limiting factor: The factor, closest to its minimum, that sets the overall rate of a multi-factor process.

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All 10 questions on this chapter

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