state that cyclic photophosphorylation and non-cyclic photophosphorylation occur during the light-dependent stage of photosynthesis

Photosynthesis as an Energy Transfer Process

Learning Objective

State that cyclic photophosphorylation and non‑cyclic photophosphorylation occur during the light‑dependent stage of photosynthesis.

Key Concepts

• The light‑dependent reactions take place in the thylakoid membranes of chloroplasts.
• These reactions convert light energy into chemical energy in the form of ATP and NADPH.
• Two distinct pathways of photophosphorylation exist:

  • Cyclic photophosphorylation – produces ATP only.
  • Non‑cyclic photophosphorylation – produces both ATP and NADPH and evolves O₂.

• Both pathways involve the flow of electrons through photosystems, but the routes differ.

Non‑Cyclic Photophosphorylation (Linear Electron Flow)

In non‑cyclic photophosphorylation, electrons travel from water to NADP⁺:

\$\text{H}2\text{O} \xrightarrow{\text{PSII}} \text{PQ} \rightarrow \text{Cyt b}6\text{f} \rightarrow \text{PC} \rightarrow \text{PSI} \xrightarrow{\text{Fd}} \text{FNR} \rightarrow \text{NADP}^+\$

Key outcomes:

1. Oxidation of water releases O₂, protons and electrons:
   

   \$2\text{H}2\text{O} \rightarrow \text{O}2 + 4\text{H}^+ + 4e^-\$

2. ATP synthesis via chemiosmosis driven by the proton gradient.
3. NADPH formation by the reduction of NADP⁺.

Cyclic Photophosphorylation

In cyclic photophosphorylation, electrons from PSI are returned to the plastoquinone pool, generating a proton gradient without producing NADPH or O₂:

\$\text{PSI} \rightarrow \text{Fd} \rightarrow \text{PQ} \rightarrow \text{Cyt b}_6\text{f} \rightarrow \text{PC} \rightarrow \text{PSI}\$

Key outcomes:

• Only ATP is produced.
• The pathway is activated when the cell requires additional ATP relative to NADPH.
• No water is split; therefore, O₂ is not released.

Comparison of Cyclic and Non‑Cyclic Photophosphorylation

Why Both Pathways Are Important

The Calvin‑Benson cycle consumes ATP and NADPH in a ratio of approximately 3:2. The coexistence of cyclic and non‑cyclic photophosphorylation allows the chloroplast to adjust the relative production of these carriers, ensuring the correct balance for carbon fixation.