5.1.2 The Light Dependent Reaction

Steps in the Light-Dependent Reaction

1. Photoionisation in Photosystem II (PSII):

• Photons of light are absorbed by chlorophyll molecules in PSII, exciting electrons to a higher energy level.
• This process, known as photoionisation, causes the electrons to leave the chlorophyll, generating a positive charge.

2. Photolysis of Water:

• Light energy is used to split water molecules into oxygen (O₂), protons ($$H⁺), and electrons (e⁻).
• Equation:

$$2H_2O \rightarrow 4H^+ + 4e^- + O_2$$

• The oxygen is either released via stomata or used in respiration, while the electrons replace those lost from PSII.

3. Electron Transport Chain (ETC):

• Excited electrons from PSII pass through a series of protein complexes (electron carriers) embedded in the thylakoid membrane.
• Energy from the electrons is used to pump protons (H⁺) from the stroma into the thylakoid space, creating a proton gradient.

4. Photoionisation in Photosystem I (PSI):

• Electrons reaching PSI are re-energised by light absorbed in PSI.
• These high-energy electrons are used to reduce NADP by combining with protons (H⁺), forming reduced NADP (NADPH).

5. Chemiosmosis and ATP Synthesis:

• The proton gradient formed by pumping H⁺ ions into the thylakoid space causes protons to diffuse back into the stroma via ATP synthase (a stalked particle).
• The energy released as protons move through ATP synthase drives the process of photophosphorylation, where ATP is synthesised from ADP and Pi.

Key Products of the Light-Dependent Reaction

• ATP: Used in the Calvin Cycle (light-independent reactions).
• Reduced NADP (NADPH): Provides reducing power for carbon fixation in the Calvin Cycle.
• Oxygen (O₂): A by-product released into the atmosphere.

Key Concepts to Remember

• The light-dependent reaction generates energy and reducing power (ATP and NADPH) required for the synthesis of carbohydrates in the next stage of photosynthesis.
• The proton gradient is crucial for ATP production, and the process relies on chemiosmosis.