BiologyCBSE Class 11 · Photosynthesis in Higher Plants

Light Reaction: Photosystems, Z Scheme and Photophosphorylation

Updated 17 October 2026 · Fact-checked

In the light reaction, chlorophyll and accessory pigments in photosystems I (P700) and II (P680) absorb light, and electrons pass along the Z scheme to NADP+ to make NADPH, while water is split to release oxygen. Proton gradient across the thylakoid membrane drives ATP synthase (chemiosmosis). Non-cyclic photophosphorylation yields ATP, NADPH and O2; cyclic involves only PS I and yields only ATP.

Understand Light Reaction and the Z Scheme

Pigments. Chlorophyll a (bright or blue-green), chlorophyll b (yellow-green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange) are in the thylakoid membranes. Chlorophyll a is the chief pigment; the others are accessory pigments that absorb other wavelengths and pass energy to chlorophyll a, and protect it from photo-oxidation. Absorption peaks of chlorophyll a are in the blue and red regions; the action spectrum of photosynthesis resembles the absorption spectrum of chlorophyll a, with some contribution from other pigments.

Photosystems. Pigments are organised into two discrete photochemical light harvesting complexes (LHC) within photosystem I (PS I) and II (PS II). Each LHC has hundreds of pigment molecules bound to proteins; each photosystem has a reaction centre (chlorophyll a) that differs in its absorption peak: PS I: P700 (absorbs at 700 nm); PS II: P680 (680 nm). The other pigments form the antennae.

Electron transport (Z scheme). In PS II, P680 absorbs 680 nm light, excites electrons and transfers them to an acceptor; electrons then pass down an electron transport chain (cytochromes, plastoquinone, plastocyanin) towards PS I, with energy released used to build ATP. In PS I, the electrons are excited again by 700 nm light, move to another acceptor and finally reduce NADP⁺ to NADPH + H⁺ via ferredoxin and NADP reductase. The path, when plotted by energy level, looks like the letter Z, hence the Z scheme (non-cyclic).

Splitting of water. PS II loses electrons and must replace them: water is split (photolysis) in the thylakoid lumen, with the help of Mn²⁺, Ca²⁺ and Cl⁻: 2H₂O → 4H⁺ + O₂ + 4e⁻. The electrons replace those lost by PS II; the protons accumulate in the lumen; oxygen is released as a by-product.

Cyclic and non-cyclic photophosphorylation.

Non-cyclic: both PS I and II work in series; products ATP, NADPH and O₂ (lost electrons are replaced by water).
Cyclic: only PS I works; excited electron is not passed to NADP⁺ but cycles back via the electron transport chain to PS I; only ATP forms, no NADPH, no O₂. It occurs in stroma lamellae, where PS II and NADP reductase are absent, when the cell needs ATP more than NADPH.

Chemiosmotic hypothesis (ATP synthesis). ATP forms because of a proton gradient across the thylakoid membrane. Protons accumulate in the lumen because (i) water splits in the lumen, (ii) plastoquinone moves protons from the stroma into the lumen while carrying electrons, (iii) NADP reductase uses protons from the stroma for NADPH, lowering stroma H⁺. The gradient is dissipated through ATP synthase (CF₀ forms a channel; CF₁ makes ATP), and the energy of the gradient makes ATP from ADP and Pi.

Products of the light reaction. ATP, NADPH and O₂, used in the biosynthetic phase.

Draw and label: the Z scheme with PS II (P680), PS I (P700), electron acceptors, ETC, water splitting, and NADP⁺ reduction.

Light Reaction and the Z Scheme: key facts

Photosystems: PS II: P680
PS I: P700
reaction centre chlorophyll a; Absorption peaks
Water splitting: $2 H 2 O - > 4 H + + O 2 + 4 e -$; In the lumen
Non-cyclic output: $A T P + N A D P H + O 2$; Both photosystems
Cyclic output: ATP only; PS I only; stroma lamellae

How to answer Light Reaction and the Z Scheme questions

Questions ask you to sequence events or distinguish the two phosphorylations.

1Identify the photosystem from the wavelength: 680 nm PS II, 700 nm PS I.
2Trace electrons from water to NADP+ for the non-cyclic path.
3State the source of protons for the gradient.
4For comparison with cyclic, list photosystems, products and location.
5Link the products to the Calvin cycle needs.

Light Reaction and the Z Scheme 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.

Leaf under a green lamp

Suppose a plant grown under red-blue light grows better than one under pure green light of the same brightness.

Explain using pigments.

Try it first, then show the solutionSolution

Chlorophyll a and b absorb most strongly in blue and red.
Green is reflected or transmitted, so less energy reaches the reaction centres.
So the plant under red-blue light makes more ATP and NADPH.

Answer: Red and blue match the absorption peaks; green is poorly absorbed.

Memory hook: Plants use what they absorb.

Tricks to remember Light Reaction and the Z Scheme

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

2 comes before 1
Z scheme: PS II (680) then PS I (700).
Use it when: Order
Cyclic = only ATP
C for cycle: ATP only.
Use it when: Cyclic
Photolysis in lumen
Water splits inside the thylakoid lumen.
Use it when: Water splitting

Light Reaction and the Z Scheme: solved examples

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

Example 1: Photosystems · 2 marks · Very short answerBoard level

State the reaction centre and wavelength of PS I and PS II.

Try it first, then show the solutionSolution

PS I: P700, 700 nm (1 mark).
PS II: P680, 680 nm (1 mark).

Answer: PS I P700; PS II P680.

Example 2: Z scheme · 3 marks · Short answerBoard level

Explain why the electron transport in the light reaction is called the Z scheme.

Try it first, then show the solutionSolution

Electrons are excited in PS II and passed down an electron transport chain (1 mark).
Electrons are excited again in PS I and reach NADP+ (1 mark).
Plotted by energy level, the pathway looks like the letter Z (1 mark).

Answer: Two excitation steps give a Z-shaped energy diagram.

Example 3: Cyclic vs non-cyclic · 3 marks · Short answerHarder board level

Compare cyclic and non-cyclic photophosphorylation on photosystems, products and location.

Try it first, then show the solutionSolution

Photosystems: PS I only versus PS I and II (1 mark).
Products: ATP only versus ATP, NADPH and O2 (1 mark).
Location: stroma lamellae versus grana thylakoids (1 mark).

Answer: Three differences.

Example 4: Oxygen and water · 4 marks · Short answerHarder board level

Using 2H2O -> 4H+ + O2 + 4e-, find the oxygen released and electrons supplied when 12 molecules of water are split. Where do the electrons and protons go?

Try it first, then show the solutionSolution

12 H2O is 6 sets of 2 H2O (1 mark).
O2 = 6 molecules, since each 2 H2O gives 1 O2 (1 mark).
Electrons = 6 x 4 = 24 (1 mark).
Electrons replace those lost by PS II; protons accumulate in the lumen to drive ATP synthase (1 mark).

Answer: 6 O2; 24 electrons.

Example 5: Proton gradient · 5 marks · Long answerHOTS / competency

Suppose a leaf is treated with a chemical that makes the thylakoid membrane leaky to H+. Predict the effect on ATP and on NADPH formation, and the later effect on the Calvin cycle, with reasons.

Try it first, then show the solutionSolution

Proton gradient cannot build, so ATP synthase makes little ATP (1 mark).
Electron transport may continue, so NADPH formation may continue at first (1 mark).
Less ATP means the Calvin cycle cannot run its reduction and regeneration steps (1 mark).
CO2 fixation then slows and sugar formation falls (1 mark).
Cells lack ATP although light is available, so photosynthesis falls despite NADPH (1 mark).

Answer: ATP falls; Calvin cycle stalls.

Where students lose marks in Light Reaction and the Z Scheme

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

Mistake: Oxygen from CO2
Source confused.
Wrong working
O2 comes from CO2.
Right working
O2 comes from water.
Do this: Photolysis.
Usually costs: 1 mark.
Mistake: Splitting in stroma
Site mixed.
Wrong working
Water splits in the stroma.
Right working
Water splits in the thylakoid lumen.
Do this: Lumen.
Usually costs: 1 mark.
Mistake: PS I first
Order mixed.
Wrong working
Electrons start in PS I.
Right working
Non-cyclic starts at PS II.
Do this: 2 then 1.
Usually costs: 1 mark.

Exam tips

Draw the Z scheme neatly with labels.
Mention LHC and reaction centre.
Give three sources of the proton gradient.
State products of each pathway.

Practice questions on Light Reaction and the Z Scheme

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
Oxygen released in photosynthesis comes from
(a) CO2 (b) water (c) glucose (d) ATP
Hint
Photolysis.
Answer
(b).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Cyclic photophosphorylation produces no NADPH.
Reason (R): The excited electron of PS I returns to the electron transport chain instead of reducing NADP+.
(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.
Hint
Oxygen from water at PS II.
Answer
(a).
Q3 · 2 marks · Very short answerHarder board level
Where in the chloroplast do the cyclic pathway and ATP synthase operate?
Hint
Membranes.
Answer
Cyclic photophosphorylation occurs in stroma lamellae; ATP synthase is in the thylakoid membrane, with CF1 facing the stroma.
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose an experiment shows ATP made but no NADPH or O2.
(a) Name the pathway. (1)
(b) Which photosystem is active? (1)
(c) Why no oxygen? (1)
(d) Where does it take place? (1)
Hint
Cyclic.
Answer
(a) Cyclic photophosphorylation. (b) PS I. (c) PS II is inactive, so water is not split. (d) Stroma lamellae.
Q5 · 3 marks · Short answerHOTS / competency
Suppose a plant needs more ATP than NADPH for a stage of growth. Which pathway and location would help, and why would the Calvin cycle be affected if only this pathway ran?
Hint
ATP/NADPH ratio.
Answer
Cyclic photophosphorylation in stroma lamellae yields ATP only. If only it ran, no NADPH would form, so the reduction step of the Calvin cycle would stall.

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.What is P700?Show answer
PS I reaction centre.
2.Where does cyclic occur?Show answer
Stroma lamellae.
3.What splits water?Show answer
Light, via PS II, with Mn, Ca and Cl.
4.Products of non-cyclic?Show answer
ATP, NADPH, O2.
5.What is CF1?Show answer
The part of ATP synthase that makes ATP.
6.What does NADP reductase do?Show answer
Reduces NADP+ to NADPH.

Light Reaction and the Z Scheme: frequently asked questions

What is the Z scheme?

Linked electron flow from PS II to PS I to NADP+.

What is photolysis?

Light-driven splitting of water.

What are accessory pigments?

Pigments that pass energy to chlorophyll a.

What is chemiosmosis?

ATP synthesis driven by a proton gradient.

Why is the cyclic pathway needed?

To supply extra ATP.