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Botany · Photosynthesis in Higher Plants

Cyclic vs Non-Cyclic Photophosphorylation

Section 11.6.2 of NCERT Class XI Biology introduces two distinct modes of ATP synthesis in the light reactions: cyclic and non-cyclic photophosphorylation. Together they determine not only how much ATP the chloroplast produces but also whether NADPH and oxygen are generated. NEET consistently probes this distinction — the photosystems involved, the electron source, and the products — making a precise mechanistic understanding essential for full marks in this chapter.

NCERT Grounding

NCERT Class XI Biology, Chapter 11 (Photosynthesis in Higher Plants), Section 11.6.2 reads: "When the two photosystems work in a series, first PS II and then PS I, a process called non-cyclic photo-phosphorylation occurs… When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs due to cyclic flow of electrons." The text explicitly locates cyclic photophosphorylation in stroma lamellae and identifies the Calvin cycle's 3:2 ATP:NADPH demand as the physiological rationale for its existence.

"The cyclic flow hence, results only in the synthesis of ATP, but not of NADPH + H+."

NCERT Class XI Biology, Section 11.6.2

Non-Cyclic Photophosphorylation

Overview and Photosystems Involved

Non-cyclic photophosphorylation is the dominant light-reaction pathway in all higher plants and cyanobacteria. It requires both Photosystem II (PSII) and Photosystem I (PSI) to operate in series — PSII first, then PSI — linked by an electron transport chain. The entire sequence is called the Z-scheme because, when plotted on a redox potential axis, the carriers trace a Z-shaped path from negative to positive potential.

The reaction centre of PSII is P680 (absorbs 680 nm red light); the reaction centre of PSI is P700 (absorbs 700 nm red light). Neither photosystem works in isolation during non-cyclic photophosphorylation.

Electron Pathway — Step by Step

Non-Cyclic Electron Flow (Z-Scheme)

7 stages · unidirectional
  1. 1

    Water Splitting

    2H₂O → 4H⁺ + 4e⁻ + O₂. Photolysis occurs on the inner (lumenal) face of the thylakoid membrane, associated with PSII.

    O₂ released
  2. 2

    PSII (P680) Excitation

    Absorbed 680 nm photon excites P680; energised electrons passed to primary acceptor (pheophytin), oxidising P680.

    High-energy e⁻
  3. 3

    Plastoquinone (PQ)

    PQ carries electrons and protons from stroma to lumen, contributing to the proton gradient that drives ATP synthesis.

    H⁺ gradient
  4. 4

    Cyt b₆f Complex

    Electrons pass through cytochrome b6f; energy released drives additional proton pumping into thylakoid lumen → ATP via CF₀-CF₁ ATPase.

    ATP synthesised
  5. 5

    Plastocyanin (PC)

    PC (copper protein) shuttles electrons from Cyt b6f to the oxidised P700 of PSI, restoring P700 to its ground state.

    Copper carrier
  6. 6

    PSI (P700) Excitation

    Absorbed 700 nm photon re-energises electrons in P700; electrons transferred to iron-sulphur protein primary acceptor, then to ferredoxin (Fd).

    2nd photon needed
  7. 7

    NADP⁺ Reduction

    Ferredoxin donates electrons to NADP⁺ reductase (FNR) on the stromal side → NADP⁺ + 2e⁻ + H⁺ → NADPH.

    NADPH formed

Products and Location

The three products of non-cyclic photophosphorylation are ATP, NADPH, and O₂. The process is localised in the grana thylakoid membranes, which contain both PSII and PSI complexes as well as NADP reductase. Electrons do not return to their original photosystem; the flow is strictly unidirectional.

Figure 1 Z-Scheme of Non-Cyclic Photophosphorylation Redox potential (more negative ↑) Electron flow direction → H₂O O₂ released PSII P680 Primary acc. 680 nm PQ Cyt b₆f ATP PC PSI P700 Primary acc. 700 nm Fd NADPH Photon excitation Electron flow

Figure 1. Simplified Z-scheme of non-cyclic photophosphorylation. Electrons flow from water (left) through PSII, the plastoquinone–cytochrome b6f–plastocyanin chain, PSI, and ferredoxin, terminating at NADP+ reductase to produce NADPH. ATP is synthesised at the cytochrome b6f step via chemiosmosis. O2 is released at water splitting (PSII side).

Cyclic Photophosphorylation

Mechanism and Electron Route

In cyclic photophosphorylation, only Photosystem I (PSI) is operational. P700 absorbs a photon and its excited electron is passed to an iron-sulphur protein and then to ferredoxin (Fd). Instead of proceeding to NADP+ reductase, the electron is diverted back through the cytochrome b6f complex and plastocyanin (PC), returning to the oxidised P700. The circuit is complete — no electron is lost, no external donor is consumed.

As electrons flow downhill through Cyt b6f, protons are pumped into the thylakoid lumen. This proton gradient drives ATP synthesis through CF0-CF1 ATP synthase exactly as in non-cyclic photophosphorylation. The sole energy-storing product is ATP.

Figure 2 Cyclic Photophosphorylation — Electron Cycle PSI (P700) Reaction centre 700 nm photon Ferredoxin Cyt b₆f ATP Plastocyanin e⁻ ✖ NADP⁺ not reduced ✖ No water splitting Electron cycle Photon input ATP produced

Figure 2. Cyclic photophosphorylation: only PSI (P700) operates. Excited electrons from P700 pass to ferredoxin but do not reduce NADP+. Instead, they flow through the cytochrome b6f complex (generating a proton gradient that drives ATP synthesis) and plastocyanin, returning to the oxidised P700. There is no water splitting and no NADPH formation.

Location and Triggering Conditions

Cyclic photophosphorylation occurs in the stroma lamellae (intergranal lamellae). NCERT notes that stroma lamellae membranes "lack PS II as well as NADP reductase enzyme." This structural segregation physically prevents the electron from reaching NADP+ even if conditions favour it.

Two conditions favour cyclic photophosphorylation: (1) when the cell has adequate NADPH but insufficient ATP, and (2) when only far-red light (wavelengths beyond 680 nm) is available — that wavelength can excite P700 but is too low in energy to activate P680 in PSII.

Side-by-Side Comparison

Feature Non-Cyclic Photophosphorylation Cyclic Photophosphorylation
Photosystems involved Both PSII (P680) and PSI (P700) Only PSI (P700)
Electron donor (source) Water (H₂O) — photolysis P700 chlorophyll itself (electrons cycle back)
Electron final acceptor NADP⁺ → NADPH P700 (electrons return to start)
Products ATP + NADPH + O₂ ATP only
O₂ release Yes (from water splitting) No
NADPH production Yes No
Location in chloroplast Grana thylakoid membranes Stroma lamellae (intergranal)
Electron flow direction Unidirectional (linear) Cyclic (returns to PSI)
NADP reductase present? Yes (on stromal face of grana) No (absent in stroma lamellae)
Primary purpose Generate ATP + NADPH for Calvin cycle Supplement extra ATP when ATP:NADPH ratio is low

The ATP:NADPH Problem — Why Cyclic PP Exists

The Calvin cycle (C3 pathway) has a precise stoichiometric demand for ATP and NADPH. Fixing 6 CO₂ to produce one glucose molecule requires:

18

ATP molecules per glucose

3 ATP per CO₂ fixed (carboxylation uses 1 ATP in regeneration; reduction uses 2 ATP per CO₂). Over 6 CO₂: 6 × 3 = 18 ATP.

:
12

NADPH molecules per glucose

2 NADPH per CO₂ fixed (reduction step). Over 6 CO₂: 6 × 2 = 12 NADPH. The ratio is 3:2 (ATP:NADPH).

Non-cyclic photophosphorylation produces roughly 1 ATP for every 1 NADPH (a 1:1 ratio in simplified terms). The Calvin cycle needs a 3:2 ratio. This shortfall — one extra ATP needed per two NADPH produced — is met by cyclic photophosphorylation running concurrently. NCERT states explicitly: "It is probably to meet this difference in number of ATP and NADPH used in the dark reaction that the cyclic phosphorylation takes place."

Energy currency produced per cycle

Non-Cyclic PP

ATP + NADPH

+ O₂ released

  • Supplies the reductant (NADPH) the Calvin cycle needs
  • Produces O₂ as a net product of photosynthesis
  • Requires two photons per electron (one at PSII, one at PSI)
  • ATP:NADPH ratio approximately 1:1
vs

Cyclic PP

ATP only

No NADPH, no O₂

  • Supplements ATP without increasing NADPH pool
  • Adjusts ATP:NADPH ratio to match Calvin cycle's 3:2 demand
  • Requires one photon per electron (only PSI excited)
  • Critical under far-red-only light conditions

Worked Examples

Worked Example 1

A student states: "Cyclic photophosphorylation produces ATP, NADPH, and O₂ just like non-cyclic, but the electrons cycle back." Identify the errors in this statement and correct them.

Errors: (1) NADPH is not produced in cyclic photophosphorylation — NADP+ reductase is absent in the stroma lamellae where cyclic PP occurs, so electrons never reduce NADP+. (2) O2 is not produced — O2 comes from photolysis of water at PSII, which is not involved in cyclic PP. Corrected statement: Cyclic photophosphorylation produces only ATP; electrons excited from P700 cycle back through Fd → Cyt b6f → PC → P700 without reducing NADP+ and without splitting water.

Worked Example 2

For the synthesis of one molecule of glucose by the Calvin cycle, how many molecules of ATP and NADPH are required? Given this, explain why cyclic photophosphorylation is necessary.

Requirement: 18 ATP and 12 NADPH per glucose (3 ATP and 2 NADPH per CO2, over 6 CO2). The ratio is 18:12 = 3:2. Problem: Non-cyclic photophosphorylation yields approximately 1 ATP per 1 NADPH. If only non-cyclic PP operated, the cell would have a proportional NADPH surplus and ATP deficit for the Calvin cycle. Solution: Cyclic photophosphorylation runs concurrently to synthesise additional ATP without making NADPH, correcting the ratio and ensuring the Calvin cycle is not ATP-limited.

Worked Example 3

A leaf is illuminated only with light of wavelength 710 nm (far-red). Which type of photophosphorylation will predominantly occur, and why?

Answer: Cyclic photophosphorylation. P680 of PSII has an absorption maximum at 680 nm; light at 710 nm is insufficient to excite P680 efficiently, so PSII remains largely non-functional. P700 of PSI absorbs at 700 nm and can be excited by 710 nm light. With PSII non-functional, electrons cannot flow from water through the full Z-scheme. Only PSI is active, electrons cycle through Fd → Cyt b6f → PC → P700, and only ATP is synthesised — the hallmark of cyclic photophosphorylation. This is the condition explicitly mentioned in NCERT Section 11.6.2.

Common Confusion & NEET Traps

NEET PYQ Snapshot — Cyclic vs Non-Cyclic Photophosphorylation

One confirmed PYQ linked to photosystem discovery; concept cards for direct mechanism questions not yet in official PYQ bank.

NEET 2016 — Q.62

Emerson's enhancement effect and Red drop have been instrumental in the discovery of:

  1. Two photosystems operating simultaneously
  2. Photophosphorylation
  3. Fixation of CO₂ in dark reaction
  4. Photolysis of water
Answer: (1)

Why: Emerson observed the red-drop (photosynthesis efficiency falls sharply beyond 680 nm) and the enhancement effect (combining 680 nm + 700 nm light gives more photosynthesis than either alone). These could only be explained if two photosystems — PSII (P680) and PSI (P700) — work in series. This cooperative action of PSII and PSI is the very basis of non-cyclic photophosphorylation. Option (1) is correct; the other options describe separate discoveries (Hill reaction, Calvin cycle, water splitting).

Concept

Which of the following is NOT a product of cyclic photophosphorylation?

  1. ATP
  2. NADPH
  3. Proton gradient across thylakoid membrane
  4. Re-oxidised P700
Answer: (2)

Why: Cyclic photophosphorylation produces only ATP (via the proton gradient created as electrons cycle through Cyt b6f). NADPH is not produced because electrons return to P700 rather than reducing NADP+. The proton gradient and re-oxidised P700 are both intermediates or outcomes of the cyclic pathway. NADPH (option 2) is the non-product.

Concept

Cyclic photophosphorylation occurs in the stroma lamellae because these membranes lack:

  1. Photosystem I and plastocyanin
  2. Photosystem II and NADP reductase
  3. Ferredoxin and cytochrome b6f
  4. ATP synthase and plastoquinone
Answer: (2)

Why: NCERT explicitly states that stroma lamellae lack PSII and NADP reductase. The absence of PSII means no water splitting and no linear electron flow. The absence of NADP reductase means electrons from ferredoxin cannot reduce NADP+ to NADPH. PSI, ferredoxin, Cyt b6f, plastocyanin, and ATP synthase are present — enabling the cyclic electron flow and ATP synthesis.

FAQs — Cyclic vs Non-Cyclic Photophosphorylation

Frequently asked questions on this subtopic from NEET aspirants and coaching forums.

What is the key difference between cyclic and non-cyclic photophosphorylation?

Non-cyclic photophosphorylation uses both PSII and PSI; electrons originate from water, travel in one direction, and ultimately reduce NADP+ to NADPH. Products are ATP, NADPH, and O2. Cyclic photophosphorylation uses only PSI (P700); excited electrons cycle back to PSI through the electron transport chain instead of reducing NADP+. The sole product is ATP — no NADPH, no O2, no water splitting.

Which photosystem is exclusively involved in cyclic photophosphorylation?

Only Photosystem I (PSI), with its reaction centre P700, is functional in cyclic photophosphorylation. Photosystem II is not involved, so water is not split and neither NADPH nor O2 is produced.

Where in the chloroplast does each type of photophosphorylation occur?

Non-cyclic photophosphorylation occurs in the grana thylakoid membranes, which contain both PSII and PSI as well as NADP reductase. Cyclic photophosphorylation occurs in the stroma lamellae (intergranal lamellae), which lack PSII and NADP reductase but contain PSI.

Why does cyclic photophosphorylation exist if non-cyclic already makes ATP?

The Calvin cycle consumes ATP and NADPH in a 3:2 ratio (18 ATP : 12 NADPH per glucose). Non-cyclic photophosphorylation produces roughly 1 ATP per 1 NADPH. Cyclic photophosphorylation generates additional ATP without producing NADPH, supplementing the extra ATP the Calvin cycle demands.

What is the electron path in non-cyclic photophosphorylation?

H2O → PSII (P680) → Plastoquinone (PQ) → Cytochrome b6f complex → Plastocyanin (PC) → PSI (P700) → Ferredoxin (Fd) → NADP+ reductase → NADPH. This unidirectional flow (the Z-scheme) simultaneously drives ATP synthesis via the proton gradient across the thylakoid membrane.

Under what conditions does cyclic photophosphorylation predominate?

Cyclic photophosphorylation predominates when the cell has sufficient NADPH but needs more ATP, and also when only wavelengths of light beyond 680 nm are available — because those wavelengths can excite PSI (P700) but not PSII (P680). NCERT explicitly states this condition.

How does Emerson's enhancement effect relate to the two photosystems?

Emerson found that photosynthesis was far greater when red light (680 nm) and far-red light (700 nm) were applied simultaneously than the sum of their individual effects (enhancement effect). Together with the red-drop (fall in efficiency beyond 680 nm), these observations established that two photosystems — PSII and PSI — operate simultaneously and cooperatively, the direct basis of non-cyclic photophosphorylation.

Related Topics
Photosynthesis in Higher Plants Back to the full chapter notes and overview. Light Reaction & Z-Scheme Full walkthrough of the Z-scheme electron transport and photon absorption. Chemiosmosis in Chloroplasts How the proton gradient drives ATP synthase (CF₀-CF₁) in both cyclic and non-cyclic pathways. Splitting of Water Photolysis at PSII: the electron source for non-cyclic photophosphorylation and origin of O₂. Calvin Cycle (C3 Pathway) Where the ATP and NADPH produced by photophosphorylation are consumed to fix CO₂. Action & Absorption Spectrum Emerson's experiments and the spectral basis for two-photosystem discovery.