BiologyISC Class 11 · Photosynthesis in Higher Plants

Photosynthetic Pigments and the Light Reactions: Z Scheme and Chemiosmosis

Updated 17 October 2026 · Fact-checked

The light reactions occur in the thylakoids: chlorophyll a and accessory pigments absorb light, PS II (P680) splits water releasing O₂, electrons pass through carriers to PS I (P700) and then to NADP⁺, and a proton gradient drives ATP synthesis. Non-cyclic flow forms ATP, NADPH and O₂; cyclic flow forms only ATP.

Understand Photosynthetic Pigments and Light Reactions

Pigments. Chlorophyll a (bright or blue-green) is the main pigment; chlorophyll b (yellow-green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange) are accessory pigments that absorb other wavelengths and protect chlorophyll from photo-oxidation. The *absorption spectrum* of chlorophyll a has peaks in the blue (about 430 to 470 nm) and red (about 660 to 680 nm) regions. The *action spectrum* (rate of photosynthesis at different wavelengths) resembles it; Engelmann's experiment with Cladophora and aerobic bacteria showed that most oxygen (and so most bacteria) appeared in the blue and red regions.

Photosystems. Light-harvesting complexes contain the pigments; the reaction centre is a special chlorophyll a molecule. PS I has P700 (absorbs at 700 nm, mainly on the stroma lamellae) and PS II has P680 (680 nm, in the grana).

Non-cyclic photophosphorylation (Z scheme). PS II absorbs light, P680 loses electrons to the primary acceptor; the electrons pass through plastoquinone, the cytochrome b₆f complex and plastocyanin to PS I. P680 is restored by electrons from water: 2H₂O → 4H⁺ + O₂ + 4e⁻ (photolysis, on the inner side of the thylakoid membrane, with Mn, Ca²⁺ and Cl⁻ as cofactors of the oxygen-evolving complex). PS I absorbs light, its electrons reach ferredoxin and then NADP⁺ reductase, which makes NADPH. Products: ATP, NADPH and O₂.

Cyclic photophosphorylation. Only PS I works: the electron is excited, passes to ferredoxin, cycles through the cytochrome complex and returns to PS I. Only ATP is made (on stroma lamellae); no NADPH and no O₂.

Chemiosmosis. The electron transport pumps H⁺ into the thylakoid lumen; photolysis of water adds H⁺ inside, and NADP⁺ reduction removes H⁺ from the stroma. The gradient drives H⁺ through the ATP synthase (CF₀–CF₁), forming ATP from ADP and Pi in the stroma.

Photosynthetic Pigments and Light Reactions: key facts

Photosystems: PS I: P700, stroma lamellae | PS II: P680, grana; Both contain chlorophyll a at the reaction centre.
Photolysis: $2 H_{2} O \to 4 H^{+} + O_{2} + 4 e^{-}$; On the lumen side of the thylakoid membrane.
Non-cyclic: 1.PS II
2.plastoquinone
3.cyt b₆f
4.plastocyanin
5.PS I
6.ferredoxin
7.NADP⁺; Products: ATP, NADPH, O₂.
Cyclic: $P S I \to f er r e d o x in \to cy t b_{6} f \to P S I$; Product: ATP only.

Photosynthetic Pigments and Light Reactions in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Why leaves look green

Suppose a student shines red, green and blue light on a leaf and measures oxygen release.

Predict the order of oxygen release and explain why leaves look green.

Try it first, then show the solutionSolution

Chlorophyll absorbs blue and red strongly, so oxygen release is highest in red and blue light.
Green light is reflected and transmitted, giving the green colour of the leaf, and photosynthesis in it is the lowest.
Accessory pigments (carotenoids, xanthophylls) absorb other wavelengths and pass the energy to chlorophyll a.

Answer: Red and blue light give most oxygen; green is reflected.

Memory hook: The action spectrum follows the absorption spectrum.

Tricks to remember Photosynthetic Pigments and Light Reactions

Short memory aids. Say each one aloud once, then use it on a question the same day.

Z-scheme order
PS II first, then PS I, despite the numbering.
Use it when: Non-cyclic electron flow.
Cyclic: only ATP
No water splitting, no NADPH, no O₂.
Use it when: Distinguishing the two pathways.

Photosynthetic Pigments and Light Reactions: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Reaction centre of PS I · 1 mark · MCQBoard level

The reaction centre chlorophyll of photosystem I is:
(a) P680
(b) P700
(c) P480
(d) P450

Try it first, then show the solutionSolution

PS I has P700; PS II has P680.

Answer: (b) P700

Example 2: Absorption and action spectra · 3 marks · Short answerBoard level

Distinguish between the absorption spectrum and the action spectrum of chlorophyll. What did Engelmann's experiment show?

Try it first, then show the solutionSolution

The absorption spectrum is a graph of the light absorbed by a pigment against wavelength. (1 mark)
The action spectrum is a graph of the rate of photosynthesis against wavelength. (1 mark)
Engelmann showed that bacteria accumulated at the blue and red parts of the spectrum where the alga released most oxygen. (1 mark)

Answer: Absorption by pigment versus photosynthetic rate; blue and red are most effective.

Example 3: Z scheme · 4 marks · Short answerHarder board level

Describe the non-cyclic flow of electrons from water to NADP⁺ in the Z scheme and state the products.

Try it first, then show the solutionSolution

PS II absorbs light; electrons from P680 go to plastoquinone, cytochrome b₆f and plastocyanin. (1 mark)
The electron hole in P680 is filled by electrons from the photolysis of water, releasing O₂ and H⁺. (1 mark)
PS I absorbs light and passes electrons via ferredoxin to NADP⁺ reductase, forming NADPH. (1 mark)
The products are ATP, NADPH and O₂. (1 mark)

Answer: PS II → carriers → PS I → NADP⁺; products ATP, NADPH, O₂.

Example 4: Photophosphorylation compared · 4 marks · Short answerHarder board level

Compare cyclic and non-cyclic photophosphorylation on photosystems used, electron source, location, and products. Calculate the number of NADPH and O₂ molecules from 24 electrons passing through the non-cyclic path.

Try it first, then show the solutionSolution

Non-cyclic: PS I and PS II, electrons from water, grana, ATP + NADPH + O₂. (1.3 marks)
Cyclic: PS I only, electrons recycle, stroma lamellae, ATP only. (1.3 marks)
NADPH needs 2 electrons each: 24/2 = 12 NADPH. (0.7 mark)
Each O₂ needs 4 electrons from 2H₂O: 24/4 = 6 O₂. (0.7 mark)

Answer: Non-cyclic gives NADPH and O₂, cyclic only ATP; 12 NADPH and 6 O₂.

Example 5: Chemiosmotic synthesis of ATP · 5 marks · Long answerHOTS / competency

Explain the chemiosmotic hypothesis in the chloroplast. State three reasons why protons accumulate in the lumen, where the ATP synthase is located and in which direction H⁺ moves.

Try it first, then show the solutionSolution

Electron transport pumps H⁺ from the stroma to the lumen; photolysis releases H⁺ inside the lumen; NADP⁺ reduction uses H⁺ from the stroma. (2 marks)
The gradient creates a proton-motive force across the thylakoid membrane. (1 mark)
The ATP synthase (CF₀–CF₁) lies in the thylakoid membrane, with the CF₁ headpiece on the stroma side. (1 mark)
H⁺ moves down the gradient from the lumen to the stroma through CF₀, making ATP from ADP and Pi in the stroma. (1 mark)

Answer: Three proton-accumulating steps; ATP synthase in the thylakoid; H⁺ flows lumen to stroma.

Where students lose marks in Photosynthetic Pigments and Light Reactions

The slips examiners see most often, and what to write instead.

Mistake: Saying oxygen comes from CO₂
A classic misconception.
Do this: Oxygen comes from the photolysis of water.
Usually costs: 2 marks.
Mistake: Reversing the order of photosystems
Starting the Z scheme at PS I.
Do this: Electrons flow from PS II to PS I.
Usually costs: 1 mark.
Mistake: Placing the light reactions in the stroma
Wrong location.
Do this: The light reactions happen in the thylakoids.
Usually costs: 1 mark.

Exam tips

Draw the Z scheme with the redox carriers.
Write photolysis as an equation.
Describe the three causes of proton accumulation in the chemiosmotic answer.
Link action spectrum to absorption spectrum.

Practice questions on Photosynthetic Pigments and Light Reactions

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The oxygen released in photosynthesis comes from:
(a) CO₂
(b) water
(c) glucose
(d) chlorophyll
Hint
Photolysis of water.
Answer
(b) water
Q2 · 2 marks · Short answerBoard level
Name the accessory pigments in chloroplasts and one function.
Hint
Think of carotenoids.
Answer
Chlorophyll b, xanthophylls and carotenoids; they absorb light of other wavelengths and pass the energy to chlorophyll a, and also protect against photo-oxidation.
Q3 · 3 marks · Short answerHarder board level
Where in the chloroplast do the light reactions and the dark reactions take place?
Hint
Grana and stroma.
Answer
The light reactions occur in the thylakoid membranes of the grana (and stroma lamellae); the dark (Calvin cycle) reactions occur in the stroma.
Q4 · 3 marks · Short answerHarder board level
What are the cofactors of the oxygen-evolving complex and what does it release?
Hint
It splits water.
Answer
Manganese, calcium and chloride ions; it splits water to release H⁺, electrons and O₂.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): Cyclic photophosphorylation does not release oxygen. Reason (R): It involves only PS I and no photolysis of water. Choose: (a) both true and R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.
Hint
Oxygen comes from water.
Answer
(a) Both true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a student isolates thylakoids, adds light and NADP⁺ and measures products.
(i) Name the gas released. (1 mark)
(ii) Name the reduced coenzyme formed. (1 mark)
(iii) State what would be formed if only PS I were active. (1 mark)
(iv) State the role of ATP synthase. (1 mark)
Hint
Compare the two pathways.
Answer
(i) Oxygen. (ii) NADPH. (iii) Only ATP (cyclic photophosphorylation). (iv) It makes ATP from ADP and Pi using the proton gradient.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Reaction centre of PS II?Show answer
P680.
2.Site of the light reaction?Show answer
Thylakoid membranes.
3.Products of cyclic flow?Show answer
ATP only.
4.Source of electrons in non-cyclic flow?Show answer
Water.

Photosynthetic Pigments and Light Reactions: frequently asked questions

Why is it called the Z scheme?

Plotted on a redox scale, the path of electrons from PS II to PS I looks like the letter Z.

Why is cyclic photophosphorylation useful?

It provides extra ATP when NADPH is plentiful, as needed by C4 plants and the Calvin cycle.

Why does chlorophyll appear green?

It absorbs red and blue light and reflects or transmits green.