Photosynthesis: Light-Dependent Reactions

This lesson is mapped to AQA 7402 Section 3.5.2 — Photosynthesis: light-dependent reactions (refer to the official AQA specification document for exact wording). The light-dependent reactions are the first stage of photosynthesis. They take place on the thylakoid membranes within chloroplasts, harness light energy to lift electrons to high reduction potentials, and use those electrons together with a chemiosmotic proton gradient to make ATP and the reducing agent NADPH (reduced NADP). Oxygen is released as a by-product of water splitting (photolysis). The ATP and NADPH so produced are then exported to the stroma to drive the Calvin cycle, where atmospheric CO₂ is fixed into organic compounds (covered in lesson 6).

The history of photosynthesis is the history of two photosystems, two cycles, and the slow accumulation of evidence that ultimately revealed an extraordinary biochemical machine. Robert Hill demonstrated in 1937 (paraphrased) that isolated chloroplasts could split water and reduce an artificial electron acceptor in the light — proving that water is the electron donor and oxygen the by-product of photosynthesis (the "Hill reaction"; revisited in lesson 8 in the context of Required Practical 7). Robert Emerson showed in 1957 that the efficiency of photosynthesis dropped sharply at wavelengths beyond ~680 nm, but could be restored by simultaneously illuminating with shorter wavelengths — the "Emerson enhancement effect" — which led to the recognition that photosynthesis requires two photosystems acting in series. Theodor Engelmann (1882–83) had decades earlier illuminated different bands of the spectrum onto the alga Cladophora and observed where aerobic bacteria clustered most densely, mapping the action spectrum of photosynthesis and showing it matched the absorption spectrum of chlorophyll.

Key Definition: The light-dependent reactions of photosynthesis are those that require light energy directly. They occur on the thylakoid membranes of chloroplasts and produce ATP, NADPH, and O₂.

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Chloroplast Architecture: Where the Reactions Happen

The chloroplast is bounded by two membranes (the chloroplast envelope), enclosing the stroma — a fluid-filled compartment analogous to the mitochondrial matrix. Within the stroma sits an internal membrane system: stacks of disc-shaped thylakoids (each stack is a granum; plural grana) connected by membrane bridges called stroma lamellae or intergranal thylakoids. Each thylakoid encloses a small fluid-filled space — the thylakoid lumen — analogous to the intermembrane space of the mitochondrion.

The thylakoid membrane is where the light-dependent reactions occur. Embedded in it are:

• Photosystem II (PSII) — predominantly in the grana.
• The cytochrome b₆f complex — throughout the thylakoid system.
• Photosystem I (PSI) and the ATP synthase — predominantly in the stromal lamellae and grana margins, where they have access to the stroma for delivering NADPH and ATP.

This compartmentalisation is functionally identical in principle to mitochondrial architecture: protons are pumped into one compartment (the thylakoid lumen here; the intermembrane space in mitochondria), the membrane is impermeable to H⁺, and protons return through ATP synthase to make ATP.

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Photosystems: Structure and Function

Photosystems are large multi-protein complexes embedded in the thylakoid membrane. Each photosystem contains:

• An antenna complex (light-harvesting complex, LHC) — several hundred accessory pigment molecules (chlorophyll b, carotenoids — including β-carotene and xanthophylls) that absorb light at a range of wavelengths and funnel the absorbed energy via resonance transfer to the reaction centre.
• A reaction centre — a special pair of chlorophyll a molecules that receives the funnelled energy and undergoes photoionisation, releasing high-energy electrons to a primary electron acceptor.

Photosystem II (PSII)

• Contains the reaction-centre chlorophyll pair P680 (absorbs light maximally at 680 nm — the red region).
• PSII is the first photosystem to be excited in non-cyclic photophosphorylation.
• Associated with the oxygen-evolving complex (OEC), the manganese-containing cluster that catalyses photolysis of water.

Photosystem I (PSI)

• Contains the reaction-centre chlorophyll pair P700 (absorbs light maximally at 700 nm — far-red).
• PSI is the second photosystem to be excited in non-cyclic photophosphorylation; it can operate alone in cyclic photophosphorylation.
• Electrons from PSI are used to reduce NADP⁺ to NADPH.

Exam Tip: Despite the numbering, PSII acts before PSI in the non-cyclic pathway. The numbering reflects the order of discovery (PSI was characterised first); the order of function in non-cyclic flow is PSII → ETC → PSI → NADP⁺. Always state this explicitly in an exam.

Photosynthetic Pigments

The thylakoid membrane contains several classes of pigment, each with a characteristic absorption spectrum:

• Chlorophyll a — absorbs strongly at ~430 nm and ~662 nm; the principal pigment of both reaction centres.
• Chlorophyll b — absorbs at ~453 nm and ~642 nm; an antenna pigment.
• Carotenoids (β-carotene, xanthophylls) — absorb in the blue and green regions where chlorophyll absorbs poorly; an antenna role and a photoprotective role (quenching reactive singlet oxygen).
• Phycobilins (in cyanobacteria and red algae) — supplement chlorophylls in low-light or deep-water habitats.

The combined absorbance covers most of the visible spectrum apart from a notable trough in the green region (520–570 nm), which is why most leaves reflect green light and appear green. This combined spectrum is the action spectrum of photosynthesis as mapped historically by Engelmann.

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Photoionisation (Photoactivation)

When light energy is absorbed by a reaction-centre chlorophyll (P680 or P700), an electron pair is excited to a higher energy level (a higher molecular orbital). The excited electrons leave the chlorophyll molecule, which becomes photoionised — it has been oxidised and now carries a positive charge. The electrons are captured by the primary electron acceptor of the photosystem, beginning a downhill cascade through the ETC.

Key Definition: Photoionisation is the process by which light energy causes electrons to be emitted from a chlorophyll molecule, leaving the chlorophyll in an oxidised (ionised, electron-deficient) state.

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Non-Cyclic Photophosphorylation

This is the main pathway of the light-dependent reactions and involves both PSII and PSI. It is "non-cyclic" because the electrons follow a one-way path: from water → PSII → ETC → PSI → NADP⁺. They do not return to the original photosystem.

Step-by-Step Process

1. Light absorption by PSII. Light energy is absorbed by the antenna pigments in PSII and transferred (by resonance energy transfer) to the reaction centre P680. An electron pair in P680 is excited to a higher energy level and captured by the primary electron acceptor (pheophytin).

2. Electron transport chain (first chain). The excited electrons pass along a chain of carriers: pheophytin → plastoquinone (PQ, lipid-soluble, mobile) → cytochrome b₆f complex → plastocyanin (PC, water-soluble, mobile, on the lumen side). As electrons pass through cytochrome b₆f, energy is released and used to pump H⁺ from the stroma into the thylakoid lumen, contributing to the proton gradient.

3. Photolysis of water. The electrons lost from P680 must be replaced. They are supplied by the oxygen-evolving complex (OEC), which uses its manganese cluster to extract electrons from water:

   2H₂O → 4H⁺ + 4e⁻ + O₂

   • The 4 electrons replace those lost from P680 (the photosystem cycles four times per O₂ released).
   • The 4 H⁺ are released into the thylakoid lumen, contributing to the proton gradient.
   • Oxygen is released as a by-product, diffuses out of the chloroplast, and ultimately leaves the cell — this is the source of atmospheric O₂.

4. Light absorption by PSI. The electrons (now at a lowered energy level after their downhill travel through the first ETC) reach PSI via plastocyanin. PSI absorbs light, and the electrons in P700 are re-excited to an even higher energy level than they reached after PSII.

5. Reduction of NADP⁺. The re-excited electrons pass via ferredoxin (Fd, a small iron-sulphur protein in the stroma) to the enzyme NADP⁺ reductase (also called ferredoxin-NADP⁺ reductase, FNR), which catalyses:

   NADP⁺ + 2H⁺ + 2e⁻ → NADPH + H⁺

   The H⁺ used here come from the stroma, reducing stromal [H⁺] still further and steepening the proton gradient.

6. Chemiosmosis and ATP synthesis. The accumulated H⁺ in the thylakoid lumen (from photolysis at PSII and proton pumping by cytochrome b₆f) creates a steep proton motive force across the thylakoid membrane. The thylakoid membrane is otherwise impermeable to H⁺. Protons return to the stroma only through ATP synthase complexes embedded in the membrane; their flow drives the synthesis of ATP from ADP + Pi. The mechanism is identical in principle to mitochondrial chemiosmosis (Mitchell's hypothesis — paraphrased).

graph LR
    A["H₂O<br/>(in lumen)"] -->|"2H₂O → 4H⁺ + 4e⁻ + O₂<br/>photolysis at OEC"| B["PSII<br/>P680"]
    B -->|"light: e⁻ excited"| C["Plastoquinone (PQ)"]
    C --> D["Cytochrome b₆f<br/>pumps H⁺ to lumen"]
    D --> E["Plastocyanin (PC)"]
    E --> F["PSI<br/>P700"]
    F -->|"light: e⁻ re-excited"| G["Ferredoxin (Fd)"]
    G -->|"NADP⁺ reductase<br/>+ NADP⁺ + H⁺"| H["NADPH"]
    I["Lumen H⁺ accumulate"] -->|"flow through ATP synthase"| J["ATP"]

    style B fill:#27ae60,color:#fff
    style F fill:#3498db,color:#fff
    style H fill:#e67e22,color:#fff
    style J fill:#9b59b6,color:#fff

Products of Non-Cyclic Photophosphorylation

• ATP — used in the Calvin cycle.
• NADPH — used in the Calvin cycle as the source of reducing power.
• O₂ — released as a waste product; the source of atmospheric oxygen.

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Cyclic Photophosphorylation

Process

• Only PSI (P700) is involved.
• Light excites electrons in P700, which are passed to ferredoxin.
• Instead of being passed to NADP⁺ reductase, the electrons are passed back to the cytochrome b₆f complex in the ETC.
• As the electrons pass through cytochrome b₆f, H⁺ are pumped into the thylakoid lumen.
• Electrons return via plastocyanin to PSI (hence "cyclic" — they cycle back to where they started).
• The proton gradient drives ATP synthesis via ATP synthase.

Products of Cyclic Photophosphorylation

• ATP only — no NADPH is produced, no photolysis of water occurs, and no O₂ is released.

Why Cyclic Photophosphorylation Matters

• The Calvin cycle requires 3 ATP and 2 NADPH per CO₂ fixed (covered in lesson 6). For every 3 CO₂ fixed, 9 ATP and 6 NADPH are needed.
• Non-cyclic photophosphorylation produces ATP and NADPH in a fixed stoichiometric ratio that does not quite match the Calvin cycle's demand.
• Cyclic photophosphorylation supplies the shortfall of ATP without simultaneously producing more NADPH that the cell does not need.
• Cyclic flow is also up-regulated in plants under stress (e.g. drought, high light), where additional ATP is required for non-photosynthetic processes.

Key Definition: Cyclic photophosphorylation involves only PSI. Electrons cycle from PSI through cytochrome b₆f and back to PSI, generating ATP but not NADPH or O₂.

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Photolysis of Water: A Closer Look

Key Definition: Photolysis is the light-driven splitting of water molecules, catalysed by the oxygen-evolving complex (OEC) associated with PSII.

2H₂O → 4H⁺ + 4e⁻ + O₂

• The 4 electrons replace those lost from P680.
• The 4 H⁺ ions accumulate in the thylakoid lumen, contributing to the proton gradient.
• Oxygen is released as a by-product.

The OEC contains a Mn₄CaO₅ cluster (four manganese ions, one calcium, five oxygens). The cluster cycles through five oxidation states ("S-states", S₀ to S₄) as it accumulates the four oxidising equivalents needed to extract four electrons from two water molecules. This is one of the most studied — and still partially mysterious — catalytic mechanisms in biology; it is unique to oxygenic photosynthesis and is the molecular machine that produced essentially all the O₂ in the modern atmosphere.

Synoptic link: Magnesium is the central ion in the porphyrin ring of chlorophyll (AQA 7402 Section 3.1.1, inorganic ions). Mg²⁺-deficient plants develop chlorosis (yellow leaves) because they cannot synthesise functional chlorophyll. Manganese deficiency similarly disables the OEC and stops photolysis.

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Comparison of Cyclic and Non-Cyclic Photophosphorylation

Feature	Non-Cyclic	Cyclic
Photosystems involved	PSII and PSI	PSI only
Electron pathway	Linear (water → PSII → ETC → PSI → NADP⁺)	Circular (PSI → ETC → PSI)
ATP produced	Yes	Yes
NADPH produced	Yes	No
O₂ released	Yes (from photolysis)	No
Photolysis of water	Yes	No
Purpose	Produces ATP, NADPH, and O₂	Produces extra ATP to supplement Calvin cycle
Source of electrons	Water (replaces those lost from P680)	PSI itself (electrons cycle back)

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The Z-Scheme

The light-dependent reactions are commonly represented as the Z-scheme, which plots electron energy (y-axis, increasing upwards) against the order of carriers (x-axis):

1. Electrons start at low energy level in P680 (PSII).
2. Light excites them to a high energy level (upward arrow).
3. They lose energy as they pass downhill along the first ETC (downward slope).
4. They arrive at P700 (PSI) at a lower energy level than they reached after PSII (lower than the top of the first peak).
5. Light excites them again, even higher (second upward arrow).
6. They are used to reduce NADP⁺ to NADPH (a small further drop in energy).

The resulting graph resembles the letter Z (read left-to-right) or a sideways N. The two upward arrows represent the two light-absorption events; the downward slopes represent the energy released to pump protons.

Exam Tip: You may be asked to draw or interpret the Z-scheme. Ensure you label: PSII (P680), PSI (P700), the primary electron acceptors of each, the cytochrome b₆f complex, the points where H⁺ is pumped, where photolysis occurs (replacing electrons lost from PSII), where ATP is generated, and where NADP⁺ is reduced. Indicate the two light-driven upward energy transitions.

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Synoptic Links

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