Quick Summary
Photosynthesis converts light energy into chemical energy stored in glucose. It occurs in two stages: light-dependent reactions (thylakoids) and light-independent reactions/Calvin cycle (stroma). Understanding the two photosystems, the Z-scheme of electron flow, and the difference between C3, C4, and CAM plants is essential for NEET.
Overview
Overall equation: 6CO₂ + 12H₂O + light → C₆H₁₂O₆ + 6H₂O + 6O₂
Site: Chloroplast Pigments: Chlorophyll a (primary), chlorophyll b, carotenoids, xanthophylls (accessory)
Absorption maxima:
- Chlorophyll a: 680 nm (red) and 700 nm
- Chlorophyll b: 640 nm (red) and 480 nm (blue)
- Carotenoids: blue-violet region
Light Reactions (Photo-reactions)
Site: Thylakoid membranes
Photosystems
| Photosystem | Reaction centre | Wavelength | Role |
|---|---|---|---|
| PS II (P680) | Chlorophyll P680 | 680 nm | Water splitting (photolysis), releases O₂ |
| PS I (P700) | Chlorophyll P700 | 700 nm | NADP⁺ reduction to NADPH |
Z-scheme (non-cyclic photophosphorylation): Light → PS II → electron carrier chain → PS I → NADPH Water split at PS II → O₂ released → electrons replace those lost by PS II
Products of light reactions: ATP + NADPH + O₂
Cyclic vs Non-cyclic Photophosphorylation
| Feature | Cyclic | Non-cyclic |
|---|---|---|
| Photosystem | PS I only | PS II + PS I |
| Products | ATP only | ATP + NADPH + O₂ |
| Water splitting | No | Yes |
Calvin Cycle (Dark / Light-independent Reactions)
Site: Stroma Also called: C3 cycle (first stable product is 3-PGA, a 3-carbon compound)
Three Stages
1. Carbon fixation: CO₂ + RuBP (5C) → 2 × 3-PGA (3C) — catalysed by RuBisCO 2. Reduction: 3-PGA → G3P (glyceraldehyde-3-phosphate) — uses ATP + NADPH 3. Regeneration of RuBP: G3P → RuBP — uses ATP
Net output per 3 CO₂ fixed: 1 molecule of G3P (for 6 turns, 1 glucose equivalent)
ATP and NADPH used: 9 ATP + 6 NADPH per 3 CO₂ fixed
C4 Pathway (Hatch-Slack Cycle)
C4 plants: Maize, sugarcane, sorghum, Amaranthus
Key feature — Kranz anatomy: Bundle sheath cells (thick-walled) surround vascular bundles; mesophyll cells outside.
Process: 1. CO₂ fixed in mesophyll cells by PEP carboxylase → OAA (4C) → malate 2. Malate transported to bundle sheath cells → CO₂ released → enters Calvin cycle 3. PEP regenerated in mesophyll cells
Advantages:
- No photorespiration (PEP carboxylase has no affinity for O₂)
- More efficient at high temperatures and low CO₂
Photorespiration
- Occurs in C3 plants when RuBisCO fixes O₂ instead of CO₂
- Occurs in peroxisomes and mitochondria; consumes ATP, releases CO₂ — wasteful process
- Suppressed in C4 plants (high CO₂ concentration in bundle sheath cells)
Factors Affecting Photosynthesis
| Factor | Effect |
|---|---|
| Light intensity | Rate increases up to light saturation point |
| CO₂ concentration | Limiting factor at normal levels (0.04%) |
| Temperature | Optimum 25–35°C; dark reactions more temperature-sensitive |
| Water | Stomata close → CO₂ entry reduced; direct role in photolysis |
| Chlorophyll | More pigment → more light absorbed |
Blackman's Law of Limiting Factors: Rate of photosynthesis is determined by the factor present in least amount relative to the minimum needed.
Important Questions
One Mark
- Name the enzyme that catalyses CO₂ fixation in the Calvin cycle.
- What is the first stable product of the C4 cycle?
- Which photosystem is responsible for splitting of water?
- What is photorespiration?
Short Answer (3 marks)
- Explain non-cyclic photophosphorylation (Z-scheme) briefly.
- Compare C3 and C4 plants on the basis of four characters.
- State Blackman's Law of Limiting Factors. Give one example.
Long Answer (5 marks)
- Describe the Calvin cycle (C3 cycle). Name the enzyme, reactants, and ATP/NADPH requirement.
- Explain the light reactions of photosynthesis. Describe the roles of PS I and PS II.
Common Mistakes
- O₂ in photosynthesis comes from water, NOT from CO₂ — this is a classic NEET trap.
- PS II comes BEFORE PS I in the Z-scheme, despite the naming.
- C4 plants have TWO carboxylation steps — first by PEP carboxylase (mesophyll), then RuBisCO (bundle sheath).
- Photorespiration: it occurs in C3 plants in the presence of light and O₂, not darkness.
Board Exam Tips
- Calvin cycle diagram with three stages (fixation, reduction, regeneration) is a standard 5-mark question.
- Z-scheme (non-cyclic photophosphorylation) diagram with PS I and PS II is another 5-mark staple.
- C3 vs C4 comparison (4 characters) appears as a 3-mark question every year.
- NEET: Photosynthesis contributes 3–5 questions — light reactions, Calvin cycle, and C4 pathway are all tested.