Light-Dependent Stage: Photolysis and Photophosphorylation

Spec Mapping — OCR H420 Module 5.2.1 — Photosynthesis, content statements covering the light-dependent stage of photosynthesis, including the absorption of light by photosystems, the role of photolysis, the electron transport chain in the thylakoid membrane, photophosphorylation by chemiosmosis, and the reduction of NADP (refer to the official OCR H420 specification document for exact wording).

The light-dependent stage is where the energy of sunlight is actually captured as chemical energy. OCR specification module 5.2.1 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.

The architecture of the Z-scheme — two photosystems acting in series — was deduced not by a single experiment but by the convergence of three mid-twentieth-century discoveries. Robert Hill (1939) showed that illuminated isolated chloroplasts could evolve O₂ in the presence of a non-physiological electron acceptor; Robert Emerson (1957) showed that combining red and far-red light gave more photosynthesis than the sum of each alone (the "enhancement effect"); and Daniel Arnon (1954) demonstrated that isolated chloroplasts could synthesise ATP under illumination (photophosphorylation). Paraphrasing the synthesis offered in the late 1950s by the British plant biochemists Robert Hill and Fay Bendall (who proposed the "Z-scheme"), the data could only be reconciled if two distinct photosystems — one good at oxidising water, another good at reducing NADP — passed electrons in series, each requiring its own photon. The chemiosmotic mechanism that converts the resulting proton gradient into ATP was then proposed by the British biochemist Peter Mitchell (1961), for which he received the 1978 Nobel Prize — paraphrasing his school of thought, the energy of light is first stored as an electrochemical gradient across the thylakoid membrane, then harvested by ATP synthase.

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.

Learning objectives — by the end of this lesson you should be able to:

• Sequence the events of non-cyclic photophosphorylation from photon absorption at PSII to the reduction of NADP⁺, naming every carrier in order (PSII → PQ → cyt b₆f → PC → PSI → Fd → NADP⁺ reductase).
• Explain photolysis at the oxygen-evolving complex and state precisely where the O₂, H⁺ and electrons go.
• Describe photophosphorylation by chemiosmosis (paraphrasing Mitchell) and explain why the sealed thylakoid lumen is essential.
• Account quantitatively for the number of photons and the ATP/NADPH stoichiometry of the light-dependent stage.

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The Oxygen-Evolving Complex and the Four-Photon Water-Splitting Mechanism

Photolysis is often written glibly as $2\,\text{H}_2\text{O} \rightarrow 4\,\text{H}^+ + 4\,e^- + \text{O}_2$2H2​O→4H++4e−+O2​, but the underlying mechanism is one of the most remarkable pieces of bioinorganic chemistry in the living world, and understanding it is a genuine A* discriminator. The reaction is catalysed by the oxygen-evolving complex (OEC), a cluster of four manganese ions, one calcium ion and five oxygen bridges ($\text{Mn}_4\text{CaO}_5$Mn4​CaO5​) bound to the lumenal face of PSII.

The problem the OEC solves is a counting problem. Each photon absorbed by P680 removes only one electron. But splitting two water molecules to release one O₂ requires four electrons to be removed. A single-electron oxidant cannot strip four electrons from water in one step without generating dangerous partially-oxidised intermediates (such as hydroxyl radicals). The American biophysicist Bessel Kok (1970) resolved this by proposing that the OEC acts as a charge-accumulating capacitor: it steps through five oxidation states, labelled S₀ to S₄ (the "S-state" or "Kok" cycle). Each photon that reaches P680 advances the OEC by one S-state, storing one positive charge. Only when the fourth charge has accumulated (state S₄) does the OEC have enough oxidising power to remove four electrons from two waters simultaneously — releasing O₂ in a single concerted step and resetting to S₀.

The elegant experimental evidence (paraphrasing Kok's finding) came from illuminating dark-adapted chloroplasts with brief, evenly spaced flashes of light and measuring O₂ release per flash. Rather than a steady output, O₂ appeared in a striking period-four oscillation: almost none on flashes 1 and 2, a burst on flash 3, then peaks every fourth flash thereafter, gradually damping. This "flash-four periodicity" is direct evidence that four sequential photon-driven oxidation events are needed to produce each O₂ — a beautiful confirmation of the charge-accumulation model.

Worked example — photon accounting

If four photons are required per O₂ released at PSII, and one glucose requires 6 O₂ to be released (equivalently 6 CO₂ fixed), then the PSII photon demand for one glucose is:

$4\,\text{photons/O}_2 \times 6\,\text{O}_2 = 24\,\text{photons at PSII}$4photons/O2​×6O2​=24photons at PSII

Because non-cyclic flow is a two-photosystem process — each electron is re-energised once at PSI as well as once at PSII — the minimum total photon requirement is:

$24\,\text{(PSII)} + 24\,\text{(PSI)} = 48\,\text{photons per glucose (theoretical minimum)}$24(PSII)+24(PSI)=48photons per glucose (theoretical minimum)

Real quantum requirements are higher (typically 9–10 photons per CO₂, so ~55–60 per glucose) because of losses to fluorescence, heat and cyclic flow. This kind of photon-budget calculation is exactly the sort of quantitative reasoning OCR uses to test genuine understanding of the Z-scheme rather than rote recall.

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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

Reduced NADP is the main source of reducing power for the Calvin cycle — it carries hydrogen and electrons to be used when reducing GP to TP.

6. Chemiosmosis and ATP Synthesis

The H⁺ ions accumulating in the thylakoid space form a steep electrochemical gradient across the thylakoid membrane. They can only cross back into the stroma through ATP synthase enzymes embedded in the membrane. As H⁺ flows down its gradient through ATP synthase, the energy released drives the phosphorylation of ADP + Pi → ATP. This is chemiosmosis, and because it is driven by light, it is called photophosphorylation.

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Summary Table: Inputs and Outputs

	From	To
Electrons	Water (photolysis)	NADP (reduced NADP)
Protons (H⁺)	Water + pumped by ETC	Stroma (via ATP synthase)
ATP	ADP + Pi (chemiosmosis)	Calvin cycle
Reduced NADP	NADP + H⁺ + 2e⁻	Calvin cycle
Oxygen	Water (photolysis)	Diffuses out of leaf

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Where It All Happens

Component	Location
Photosystems I and II	Thylakoid membrane
Electron transport chain	Thylakoid membrane
Photolysis	Lumen (thylakoid space) side of PSII
Proton accumulation	Thylakoid space
ATP synthase	Thylakoid membrane
ATP production	Stroma side of thylakoid
NADP reduction	Stroma side of thylakoid

The thylakoid membrane is therefore doing everything at once: harvesting light, shuttling electrons, pumping protons, making ATP and reducing NADP. Its huge surface area (provided by the stacking of thylakoids into grana) makes this possible.

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Why Is the Proton Gradient Essential?

If the thylakoid membrane were freely permeable to H⁺, no gradient could form and no ATP could be made. The thylakoid space must be a sealed compartment. Protons enter this space both by being pumped in by the ETC and by being released by photolysis of water. They leave only through ATP synthase, releasing energy as they pass through. This is the same principle used in respiration, but in mitochondria the protons accumulate in the intermembrane space instead of the thylakoid lumen.

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Exam Tip

Be precise with electron fate. In non-cyclic photophosphorylation: electrons leave water, pass to PSII, to the ETC, to PSI, to ferredoxin, to NADP. That is the complete sequence — you may be asked to list every step in order. Also, always mention where the H⁺ comes from: some from photolysis, some pumped across by the ETC. Missing the source of protons is a common lost mark.

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Common Exam Mistakes

• Saying ATP is used to make reduced NADP. No — NADP is reduced by electrons from PSI and H⁺ from the stroma. ATP is a separate product of chemiosmosis.
• Putting photolysis in the stroma. Photolysis happens at PSII in the thylakoid membrane/lumen.
• Saying water is split to make ATP. Water is split to replace electrons at PSII and to release H⁺ and O₂. ATP is made later, by chemiosmosis.
• Forgetting that PSI and PSII are activated by separate photons. Each requires its own photon input.
• Writing that NADP is reduced in the thylakoid lumen. It is reduced on the stromal side — otherwise the Calvin cycle in the stroma could not access it.
• Confusing the names P680 and P700. These refer to the wavelengths of peak absorption for the reaction centre chlorophyll a molecules in PSII and PSI respectively.
• Saying oxygen comes from CO₂. Oxygen always comes from photolysis of water.

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The Z-Scheme as a Detailed SVG