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

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology 9700 – Photosynthesis as an Energy Transfer Process

Photosynthesis as an Energy Transfer Process

Photosynthesis converts light energy into chemical energy stored in organic molecules. The light‑dependent reactions generate a proton‑motive force that drives ATP synthesis, while the light‑independent (Calvin‑Benson) cycle uses ATP and NADPH to fix CO₂.

Cyclic Photophosphorylation

Cyclic photophosphorylation is a pathway that uses only Photosystem I (PSI) to produce ATP without the formation of NADPH or O₂. It is especially important when the plant requires extra ATP, for example during the Calvin cycle.

Key Features

  • Only PSI is involved; Photosystem II (PSII) is not used.

  • Chlorophyll a in the reaction centre (P700) is photo‑activated by absorption of a photon.

  • Excited electrons are transferred to a series of carriers and then return to the PSI reaction centre, forming a closed (cyclic) loop.

  • The electron flow drives the pumping of protons into the thylakoid lumen, creating a proton gradient.

  • The proton gradient powers ATP synthase, synthesising ATP from ADP and inorganic phosphate (\$ADP + P_i \rightarrow ATP\$).

Step‑by‑Step Mechanism

  1. Photoactivation of P700: Absorption of a photon excites chlorophyll a (P700) to P700*.

  2. Primary electron donor: The excited electron is transferred to the primary acceptor A₀.

  3. Electron transport chain: The electron moves through a series of carriers (A₀ → phylloquinone → iron‑sulphur proteins) and finally to the plastocyanin pool.

  4. Return to PSI: Plastocyanin donates the electron back to the oxidised P700⁺, completing the cycle.

  5. Proton pumping: As electrons pass through the chain, protons are translocated from the stroma into the thylakoid lumen, building a proton‑motive force.

  6. ATP synthesis: ATP synthase uses the proton gradient to convert ADP + \$P_i\$ into ATP.

Overall Reaction

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

Comparison with Non‑Cyclic Photophosphorylation

AspectCyclic PhotophosphorylationNon‑Cyclic Photophosphorylation
Photosystems involvedOnly PSIBoth PSII and PSI
Electron flowClosed loop (cyclic)Linear flow from H₂O to NADP⁺
ProductsATP onlyATP, NADPH, O₂
Source of electronsElectrons recycled from PSIWater oxidation at PSII
Role in plant metabolismSupplemental ATP when demand is highPrimary source of both ATP and NADPH for the Calvin cycle

Biological Significance

Cyclic photophosphorylation provides a flexible mechanism for balancing the ATP/NADPH ratio required by the Calvin‑Benson cycle. When NADPH is abundant but ATP is limiting, the plant can divert electrons through the cyclic pathway to generate additional ATP without over‑producing NADPH.

Suggested diagram: Schematic of cyclic photophosphorylation showing PSI, electron carriers, plastocyanin, and ATP synthase within the thylakoid membrane.

Summary

In cyclic photophosphorylation, only PSI participates. Light excites chlorophyll a (P700), and the resulting electron flow is cyclic, leading to proton pumping and ATP synthesis. No NADPH or O₂ is produced, making this pathway essential for meeting the plant’s variable ATP requirements during photosynthesis.