Light-Dependent Reactions (Photochemical Phase)
The light reactions are the first stage of photosynthesis. They occur in the thylakoid membranes of chloroplasts and convert light energy into chemical energy in the form of ATP and NADPH, together known as assimilatory power. Oxygen is released as a by-product.
Photosystems – Functional Units of Light Absorption
Photosynthetic pigments are organised into two distinct complexes called photosystems. Each photosystem has:
- a reaction centre (special chlorophyll a molecule)
- an antenna complex (chlorophyll b + carotenoids) that collects light and transfers energy
1. Photosystem II (PS II):
- Reaction centre: P680 (absorbs light at 680 nm)
- Location: Mainly in grana thylakoids
- Special role: Photolysis of water and initiation of electron flow
2. Photosystem I (PS I):
- Reaction centre: P700 (absorbs light at 700 nm)
- Location: Stroma lamellae and grana margins
- Special role: Reduction of NADP⁺ to NADPH
Key Point: Although named PS II, it functions before PS I in the electron transport chain.
💧 Photolysis of Water (Oxygen Evolution)
Photolysis occurs at PS II in the presence of light:
2H₂O → 4H⁺ + 4e⁻ + O₂
Importance of this reaction:
- O₂ is released into the atmosphere
- Electrons replace those lost by P680
- Protons (H⁺) accumulate inside the thylakoid lumen, helping form a proton gradient
Key Point: Oxygen released in photosynthesis comes from water, not CO₂.
Electron Transport Chain (ETS)
The excited electrons from PS II move through a series of carriers:
PS II → Primary acceptor → Plastoquinone (PQ) → Cytochrome b₆f complex → Plastocyanin (PC) → PS I
From PS I:
- Electrons go to Ferredoxin (Fd)
- Then to NADP⁺ reductase
- Finally reduce NADP⁺ to NADPH
Key Concept: Electron flow is downhill in energy, releasing energy used for proton pumping.
🔋 Photophosphorylation (ATP Formation)
Photophosphorylation means light-driven synthesis of ATP.
1. Non-cyclic photophosphorylation:
- Uses both PS II and PS I
- Electron flow: H₂O → PS II → PS I → NADP⁺
- Products: ATP + NADPH + O₂
2. Cyclic photophosphorylation:
- Uses only PS I
- Electrons cycle back to cytochrome b₆f
- Products: ATP only
- Occurs when the cell needs extra ATP
Chemiosmotic Hypothesis
ATP synthesis is explained by the chemiosmotic hypothesis:
How the proton gradient is formed:
- Photolysis releases H⁺ into lumen
- ETS pumps H⁺ from stroma to lumen
ATP Synthase:
- F₀: Proton channel in membrane
- F₁: Synthesises ATP in stroma
As H⁺ flows from lumen (high concentration) to stroma (low concentration), ATP is formed.
Key Point: ATP synthesis depends on proton gradient, not directly on light.
Memory Capsules – Light Reaction in One Shot
Photosystems:
- PS II → P680 → Water splitting → O₂
- PS I → P700 → NADPH
Electron Flow (Z-scheme): H₂O → PS II → PQ → Cyt b₆f → PC → PS I → Fd → NADP⁺
Photophosphorylation:
- Non-cyclic → ATP + NADPH + O₂
- Cyclic → ATP only
ATP Logic: High H⁺ in lumen → Flow through ATP synthase → ATP in stroma
One-Line Recall: Light reaction gives ATP, NADPH and Oxygen.
💡 Questions and Answers
Q1. Why is Photosystem II essential for oxygen evolution?
A1. Photosystem II contains the oxygen-evolving complex that splits water molecules during photolysis. This reaction releases oxygen, electrons, and protons, making PS II essential for oxygen evolution.
Q2. Why does cyclic photophosphorylation not produce NADPH?
A2. In cyclic photophosphorylation, electrons released from PS I return back to the electron transport chain instead of reducing NADP⁺. Since NADP⁺ is not reduced, NADPH is not formed.
Q3. What is the role of the proton gradient in ATP synthesis?
A3. The proton gradient stores energy. When protons move from the thylakoid lumen to the stroma through ATP synthase, this energy is used to convert ADP and Pi into ATP.
Q4. Why is photophosphorylation called light-dependent?
A4. Photophosphorylation depends on light because light energy is required to excite electrons in photosystems and initiate electron transport, which ultimately leads to ATP formation.