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

Learning Objective

Explain that in non‑cyclic photophosphorylation:

• Both photosystem I (PSI) and photosystem II (PSII) are involved.
• Chlorophyll molecules undergo photo‑activation.
• The oxygen‑evolving complex (OEC) catalyses the photolysis of water.
• ATP and reduced NADP (NADPH) are synthesised.

Overview of Non‑Cyclic Photophosphorylation

Non‑cyclic photophosphorylation is the linear flow of electrons from water to NADP⁺, producing both chemical energy (ATP) and reducing power (NADPH). The process occurs in the thylakoid membranes of chloroplasts and can be divided into three major phases:

1. Light‑dependent reactions (photo‑activation, water splitting, electron transport).
2. Generation of a proton gradient and synthesis of ATP.
3. Reduction of NADP⁺ to NADPH.

1. Photo‑Activation of Chlorophyll

When photons of appropriate wavelength strike the pigment‑protein complexes of PSII and PSI, the energy is transferred to the central chlorophyll a (P680 in PSII, P700 in PSI), raising it to an excited state (P*). The excited chlorophyll then donates an electron to a primary electron acceptor.

2. Water Splitting at the Oxygen‑Evolving Complex (OEC)

The loss of electrons from PSII creates a charge deficit that is replenished by the oxidation of water at the OEC, a Mn₄CaO??

cluster associated with the D1 protein of PSII.

The overall photolysis reaction is:

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

Key points:

• Four electrons are extracted from two water molecules.
• Oxygen is released as a by‑product.
• Protons contribute to the thylakoid lumenal pH gradient.

3. Electron Transport Chain (ETC)

Electrons from PSII travel through a series of carriers:

1. Plastoquinone (PQ) – transports electrons to the cytochrome b₆f complex.
2. Cytochrome b₆f – pumps additional protons into the lumen.
3. Plastocyanin (PC) – delivers electrons to PSI.

Simultaneously, the energy released during electron transfer is used to pump protons from the stroma into the thylakoid lumen, establishing a proton motive force.

4. ATP Synthesis (Photophosphorylation)

The proton gradient drives ATP synthesis via the enzyme ATP synthase (CF₁CF₀). Protons flow back into the stroma through ATP synthase, providing the energy for the phosphorylation of ADP:

\$\text{ADP} + \text{P}i + \text{H}^+{\text{out}} \;\xrightarrow{\text{ATP synthase}}\; \text{ATP} + \text{H}_2\text{O}\$

5. NADP⁺ Reduction at PSI

Excited P700* in PSI transfers an electron to ferredoxin (Fd). Ferredoxin‑NADP⁺ reductase (FNR) then catalyses the transfer of two electrons to NADP⁺, together with a proton, to form NADPH:

\$\text{NADP}^+ + 2\,e^- + \text{H}^+ \;\longrightarrow\; \text{NADPH}\$

Two photons are required for each NADPH molecule (one for PSII, one for PSI).

Summary Table

Key Take‑aways

• Non‑cyclic photophosphorylation is a linear electron flow that couples light energy to the synthesis of both ATP and NADPH.
• Both PSI and PSII are essential; PSII provides electrons by splitting water, while PSI provides the high‑energy electrons needed to reduce NADP⁺.
• The oxygen‑evolving complex is the source of atmospheric O₂.
• The proton gradient generated across the thylakoid membrane powers ATP formation via chemiosmosis.