Photosynthesis: Light-Dependent Reactions

The light-dependent reactions are the first stage of photosynthesis and take place on the thylakoid membranes within chloroplasts. These reactions use light energy to produce ATP and reduced NADP (NADPH), and release oxygen as a by-product. The ATP and NADPH are then used to drive the light-independent reactions (Calvin cycle) in the stroma.

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

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Photosystems

Photosystems are large complexes of photosynthetic pigments and proteins embedded in the thylakoid membranes. Each photosystem contains:

• Antenna complex (light-harvesting complex) — hundreds of accessory pigment molecules (chlorophyll b, carotenoids) that absorb light energy and funnel it to the reaction centre.
• Reaction centre — a special pair of chlorophyll a molecules that receives the funnelled energy and releases excited electrons.

Photosystem II (PSII)

• Contains the reaction centre chlorophyll P680 (absorbs light maximally at 680 nm).
• PSII is the first photosystem to be excited in non-cyclic photophosphorylation.
• The site of photolysis of water.

Photosystem I (PSI)

• Contains the reaction centre chlorophyll P700 (absorbs light maximally at 700 nm).
• PSI is the second photosystem to be excited in non-cyclic photophosphorylation.
• Electrons from PSI are used to reduce NADP⁺.

Exam Tip: Despite the numbering, PSII acts before PSI in the non-cyclic pathway. They are numbered in order of discovery, not in order of function.

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Photoionisation (Photoactivation)

When light energy is absorbed by the reaction centre chlorophyll (P680 or P700), a pair of electrons is excited to a higher energy level. These high-energy electrons leave the chlorophyll molecule, which becomes photoionised (it has lost electrons and now carries a positive charge). The electrons are captured by an electron acceptor (the first carrier in the electron transport chain).

Key Definition: Photoionisation is the process by which light energy causes electrons to be emitted from a chlorophyll molecule, leaving it in an oxidised (ionised) 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 called "non-cyclic" because the electrons follow a one-way path and 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 to the reaction centre P680. A pair of electrons in P680 is excited to a higher energy level and captured by the primary electron acceptor.

2. Electron transport chain (first chain) — The excited electrons pass along a chain of electron carriers (including plastoquinone, the cytochrome b6f complex, and plastocyanin). As electrons pass through the cytochrome b6f complex, energy is released and used to pump H⁺ ions from the stroma into the thylakoid lumen, contributing to the proton gradient.

3. Photolysis of water — The electrons lost from P680 must be replaced. This is achieved by the photolysis (light-splitting) of water:

   2H₂O → 4H⁺ + 4e⁻ + O₂

   • The electrons replace those lost from P680.
   • The H⁺ ions accumulate in the thylakoid lumen (adding to the proton gradient).
   • Oxygen is released as a by-product (it diffuses out of the chloroplast and out of the cell).

4. Light absorption by PSI — The electrons from the first ETC reach PSI (via plastocyanin). At PSI, light energy excites the electrons in P700 to an even higher energy level.

5. Reduction of NADP⁺ — The re-excited electrons are passed via ferredoxin to the enzyme NADP⁺ reductase. This enzyme catalyses the reduction of NADP⁺:

   NADP⁺ + 2H⁺ + 2e⁻ → NADPH

   The H⁺ ions are taken from the stroma, further reducing the H⁺ concentration in the stroma relative to the lumen, enhancing the proton gradient.