explain that in non-cyclic photophosphorylation: photosystem I (PSI) and photosystem II (PSII) are both involved, photoactivation of chlorophyll occurs, the oxygen-evolving complex catalyses the photolysis of water, ATP and reduced NADP are synthesis

Photosynthesis as an Energy Transfer Process

Objective

Explain that in non‑cyclic photophosphorylation:

• Both photosystem I (PSI) and photosystem II (PSII) are involved,
• Photoactivation of chlorophyll occurs,
• The oxygen‑evolving complex (OEC) catalyses the photolysis of water,
• ATP and reduced NADP⁺ (NADPH) are synthesised.

1. Overview of Non‑Cyclic Photophosphorylation

Non‑cyclic photophosphorylation is the linear electron flow that links the light reactions to the synthesis of the two main energy carriers, ATP and NADPH. The process proceeds through the following sequence:

1. Absorption of light by PSII and PSI.
2. Water splitting at the oxygen‑evolving complex of PSII.
3. Electron transport through the plastoquinone pool, cytochrome b₆f complex, and plastocyanin.
4. Generation of a proton gradient used by ATP synthase to produce ATP.
5. Reduction of NADP⁺ to NADPH at the end of the chain.

2. Role of Photosystem II (PSII)

PSII is the entry point for photons into the electron transport chain. Its reaction centre (P680) is excited by light, causing the following events:

• Photoactivation of the chlorophyll a molecule (P680 → P680*).
• Transfer of an excited electron to the primary electron acceptor (pheophytin).
• Replacement of the lost electron by oxidation of water at the OEC.

3. Photoactivation of Chlorophyll

When a chlorophyll a molecule absorbs a photon of suitable wavelength (≈680 nm for PSII, ≈700 nm for PSI), an electron is promoted from the ground state to an excited state:

\$\text{Chl} + h\nu \rightarrow \text{Chl}^*\$

The excited chlorophyll (Chl*) then donates an electron to the reaction centre, initiating charge separation.

4. Oxygen‑Evolving Complex (OEC) and Water Photolysis

The OEC, a Mn₄CaO??

cluster associated with PSII, catalyses the four‑step oxidation of water:

\$2\,\text{H}2\text{O} \;\xrightarrow{\text{light}}\; 4\,\text{H}^+ + 4\,e^- + \text{O}2\$

Key points:

• Four photons are required to extract four electrons from two water molecules.
• The released protons contribute to the lumenal proton gradient.
• Molecular oxygen is a by‑product that diffuses out of the thylakoid.

5. Electron Transport to Photosystem I (PSI)

Electrons from PSII travel via plastoquinone (PQ), the cytochrome b₆f complex, and plastocyanin (PC) to the reaction centre of PSI (P700). This transport also pumps protons into the thylakoid lumen, reinforcing the electrochemical gradient.

6. Role of Photosystem I (PSI)

PSI absorbs longer‑wavelength light (≈700 nm). The excited chlorophyll (P700*) transfers an electron to its primary acceptor (A₀) and ultimately to ferredoxin (Fd). The electron is then used to reduce NADP⁺:

\$\text{NADP}^+ + H^+ + 2e^- \rightarrow \text{NADPH}\$

7. ATP Synthesis (Photophosphorylation)

The proton gradient generated by electron transport drives ATP synthase (CF₁CF₀). Protons flow back into the stroma, providing the energy for phosphorylation of ADP:

\$\text{ADP} + Pi + \text{H}^+{\text{out}} \;\xrightarrow{\text{ATP synthase}}\; \text{ATP} + \text{H}^+_{\text{in}}\$

8. Summary of Key Steps

9. Overall Light‑Dependent Reaction

The combined outcome of non‑cyclic photophosphorylation can be expressed as:

\$\text{2 H}2\text{O} + \text{NADP}^+ + \text{ADP} + Pi + \text{light} \;\longrightarrow\; \text{O}_2 + \text{NADPH} + \text{ATP}\$

10. Suggested Diagram