Light-Dependent Stage: Photolysis and Photophosphorylation

The light-dependent stage is where the energy of sunlight is actually captured as chemical energy. OCR specification module 5.2.1(f)(i) requires you to describe this stage in detail — from the absorption of a photon by photosystem II, through photolysis and the electron transport chain, to the reduction of NADP and the synthesis of ATP by chemiosmosis. This is one of the most conceptually demanding topics at A-Level, but if you understand the sequence and the role of each component, the exam questions become straightforward.

Key Definitions:

• Photosystem — a protein–pigment complex in the thylakoid membrane consisting of hundreds of light-harvesting pigment molecules surrounding a central chlorophyll a "reaction centre".
• Photolysis — the light-driven splitting of water molecules (2H₂O → 4H⁺ + 4e⁻ + O₂).
• Photophosphorylation — the production of ATP using light energy (via chemiosmosis).
• Reduced NADP (NADPH) — an electron and hydrogen carrier that supplies the Calvin cycle with reducing power.
• Chemiosmosis — ATP synthesis driven by the flow of protons down their electrochemical gradient through ATP synthase.

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Overview

The light-dependent stage takes place in and on the thylakoid membrane. It has three linked outcomes:

1. Splitting water (photolysis) to release O₂, H⁺ and electrons.
2. Synthesising ATP by photophosphorylation (chemiosmosis).
3. Reducing NADP to form reduced NADP (NADPH).

The ATP and reduced NADP produced are then used in the stroma by the Calvin cycle (light-independent stage). The O₂ is a waste product that diffuses out of the leaf.

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Step-by-Step: Non-Cyclic Photophosphorylation

Non-cyclic photophosphorylation is the main pathway in normal photosynthesis. It uses both photosystem II and photosystem I, and the electrons travel in a linear (non-cyclic) path from water all the way to NADP.

flowchart LR
    H2O[H2O] -->|Photolysis| PS2[PSII<br/>P680]
    PS2 -->|Excited e-| ETC1[Electron Transport Chain]
    PS2 -. H+ pumping .-> TSP[Thylakoid space H+]
    ETC1 --> PC[Plastocyanin]
    PC --> PS1[PSI<br/>P700]
    Light2[Photon] --> PS1
    Light1[Photon] --> PS2
    PS1 -->|Excited e-| FD[Ferredoxin]
    FD --> NADP[NADP reductase]
    NADP -->|Reduced NADP| Stroma[Stroma]
    TSP -->|Flows down gradient| ATPS[ATP synthase]
    ATPS --> ATP[ATP]

1. Light Absorption by PSII

A photon of light is absorbed by a pigment molecule in the PSII antenna complex. The energy is passed from pigment to pigment until it reaches the reaction centre chlorophyll P680. A chlorophyll a molecule at the reaction centre becomes excited, and one of its electrons gains enough energy to leave the molecule altogether — it is "photoactivated" and captured by the primary electron acceptor of PSII.

2. Photolysis of Water

The loss of an electron from P680 leaves it positively charged and highly oxidising. To replace the lost electron, water is split by the oxygen-evolving complex associated with PSII:

$2\,\text{H}_2\text{O} \rightarrow 4\,\text{H}^+ + 4\,e^- + \text{O}_2$2H2​O→4H++4e−+O2​

• The electrons replace those lost by P680.
• The protons (H⁺) accumulate in the thylakoid space, contributing to the proton gradient.
• The oxygen is a waste product that diffuses out of the chloroplast and eventually out of the leaf through the stomata.

Photolysis is the source of all the oxygen released during photosynthesis — this is an important OCR point.

3. Electron Transport Chain (Thylakoid Membrane)

The excited electrons from PSII are passed along a series of electron carriers in the thylakoid membrane, including plastoquinone, the cytochrome b6f complex, and plastocyanin. As they pass through the cytochrome b6f complex, the energy released is used to actively pump H⁺ ions from the stroma into the thylakoid space. This builds up a steep proton gradient.

4. Light Absorption by PSI

The electrons arrive at PSI (reaction centre chlorophyll P700), which has also absorbed a photon of light. The absorbed energy boosts the electrons to a higher energy level again (they were losing energy as they passed along the ETC). These re-excited electrons are picked up by ferredoxin.

5. Reduction of NADP

From ferredoxin, the electrons (together with H⁺ from the stroma) are used by the enzyme NADP reductase to reduce NADP to reduced NADP (NADPH):

$\text{NADP}^+ + 2\,e^- + \text{H}^+ \rightarrow \text{reduced NADP}$NADP++2e−+H+→reduced NADP