explain that in cyclic photophosphorylation: only photosystem I (PSI) is involved, photoactivation of chlorophyll occurs, ATP is synthesised

13 Photosynthesis – Cambridge IGCSE/A‑Level (9700)

13.1 Overall description of photosynthesis

• Location: chloroplasts of green plant cells.
• Two linked stages:

  1. Light‑dependent reactions (thylakoid membranes) – capture light energy and convert it into chemical energy (ATP and NADPH) while oxidising water to O₂.
  2. Light‑independent reactions (Calvin‑Benson cycle) (stroma) – use ATP and NADPH to fix CO₂ into triose‑phosphates (C₃ sugars) and regenerate RuBP.

13.2 Structure of the chloroplast

• Envelope – outer and inner membranes control entry of metabolites.
• Stroma – fluid matrix containing:

  • Calvin‑Benson cycle enzymes
  • DNA, ribosomes, and soluble proteins

• Thylakoid system

  • Grana – stacks of flattened thylakoid discs.
  • Stroma lamellae – unstacked thylakoids that connect grana.
  • Membrane‑embedded complexes:

    • Photosystem II (PSII) – water‑splitting, P680 reaction centre
    • Photosystem I (PSI) – NADP⁺ reduction, P700 reaction centre
    • Cytochrome b₆f complex – proton pumping
    • ATP synthase – synthesises ATP from ADP + Pᵢ
    • Light‑harvesting complexes (LHCs) – pigment antennae

13.3 Pigments, absorption & action spectra

Action spectrum: the rate of photosynthesis versus wavelength mirrors the combined absorption spectra of the three pigments.

13.4 Chromatography of pigments (AO1)

Thin‑layer chromatography (TLC) of leaf extracts separates the pigments. Typical Rf values (in a suitable solvent system) are:

• Chlorophyll a ≈ 0.75
• Chlorophyll b ≈ 0.55
• β‑carotene ≈ 0.30

Students should link each Rf to the corresponding absorption peak.

13.5 Light‑dependent reactions

13.5.1 Non‑cyclic (linear) photophosphorylation – the “standard” pathway

1. Water splitting (PSII) – O₂‑evolving complex releases ½ O₂ + 2H⁺ + 2e⁻.
2. Excitation of P680 – photon → P680* → electron to primary quinone acceptor QA.
3. Plastoquinone pool – electrons pass QA → QB → PQ (reduced to PQH₂), picking up 2H⁺ from the stroma.
4. Cytochrome b₆f complex – transfers electrons to plastocyanin (PC) and pumps additional H⁺ into the lumen.
5. Plastocyanin → PSI – PC carries electrons to the P700 reaction centre.
6. Excitation of P700 – photon → P700* → electron to A₀.
7. Ferredoxin (Fd) → NADP⁺ reductase – electrons reduce NADP⁺ to NADPH; H⁺ taken from the stroma.
8. Proton‑motive force – H⁺ accumulated in the lumen drives ATP synthase (ADP + Pᵢ → ATP).

Overall products (per 2 photons): ATP, NADPH and O₂.

13.5.2 Cyclic photophosphorylation – ATP‑only pathway

• When is it used?

  • ATP demand of the Calvin‑Benson cycle exceeds supply from linear flow.
  • NADPH is already abundant (e.g., very high light, low CO₂).

• Step‑by‑step mechanism

  1. Photo‑activation of P700 (chlorophyll a) → P700*.
  2. Electron transferred to primary acceptor A₀.
  3. Passes through phylloquinone (A₁) → iron‑sulphur proteins (FX, FA, FB).
  4. Electrons reduce plastocyanin (PC) in the lumen.
  5. Reduced PC donates the electron back to oxidised P700⁺, closing the loop.
  6. As electrons traverse the cytochrome b₆f complex, additional H⁺ are pumped into the lumen, strengthening the proton gradient.
  7. ATP synthase uses the gradient to form ATP (ADP + Pᵢ → ATP).

• Overall reaction:

  \$\text{Light energy} + \text{ADP} + \text{P}_i \;\longrightarrow\; \text{ATP}\$

13.5.3 Comparison of linear and cyclic pathways

13.5.4 Balancing the ATP : NADPH ratio

The Calvin‑Benson cycle consumes 9 ATP and 6 NADPH for every 3 CO₂ fixed (≈ 3 : 2). Linear photophosphorylation delivers roughly the same ratio, but the actual output varies with light intensity. Cyclic flow adds ATP without extra NADPH, allowing the plant to reach the required 3 : 2 ratio.

Sample calculation (AO2):

If each cyclic turn moves 2 electrons and pumps 2 H⁺, and 14 H⁺ are needed for one ATP (including the 3 H⁺ for Pi transport), then:

1. H⁺ per turn = 2
2. H⁺ required per ATP = 14
3. Turns needed = 14 ÷ 2 = 7

Thus, seven complete cycles through PSI generate one ATP molecule.

13.6 Carbon‑fixation pathways

13.7 Investigations of limiting factors (AO3)

13.8 Experimental methods to distinguish cyclic from non‑cyclic photophosphorylation (AO3)

• Leaf‑disc buoyancy assay with DCMU – DCMU blocks electron flow from PSII; O₂ evolution stops but ATP can still be generated via cyclic flow, allowing students to infer the presence of cyclic photophosphorylation.
• Chlorophyll fluorescence (PAM fluorometer) – measuring changes in PSI‑specific fluorescence when an artificial electron acceptor (e.g., methyl viologen) forces electrons to recycle.
• ATP assay (luciferin‑luciferase) – compare ATP accumulation under high‑light conditions with and without PSII inhibitors; a higher ATP yield in the presence of the inhibitor indicates cyclic flow.

13.9 Quick exam checklist (what you need to know for the 9700 exam)

• Define photosynthesis and name its two stages.
• Identify the main structural parts of a chloroplast and state where each stage occurs.
• List the three major pigments, their absorption peaks and typical Rf values.
• Explain the water‑splitting reaction at PSII and the role of the oxygen‑evolving complex.
• Describe the linear electron flow, the role of the cytochrome b₆f complex and the products formed.
• State why cyclic photophosphorylation uses only PSI and does NOT produce NADPH or O₂.
• Outline the six steps of the cyclic pathway and indicate where protons are pumped.
• Calculate the ATP yield from a given number of cyclic turns (use 14 H⁺ per ATP).
• Sketch a labelled thylakoid membrane showing both linear and cyclic electron flows, and the direction of proton movement.
• Give one experimental method that can distinguish cyclic from non‑cyclic photophosphorylation.
• Describe the three carbon‑fixation pathways (C₃, C₄, CAM) and give an example plant for each.
• Summarise how light intensity, CO₂, temperature and water affect the rate of photosynthesis.

13.10 Suggested diagram (placeholder)