Photosynthesis — Concept Core
Overview
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy)
Occurs in chloroplasts. Two stages: Light Reactions (thylakoid membrane) and Dark Reactions / Calvin Cycle (stroma).
Light Reactions — Z-Scheme
Photosystem II (PS II / P680): Absorbs 680 nm light. Water is split (photolysis): 2H₂O → 4H⁺ + 4e⁻ + O₂. Oxygen is released here (NOT from CO₂).
Electron transport chain: Electrons move from P680* → Plastoquinone (PQ) → Cytochrome b6f → Plastocyanin (PC) → Photosystem I.
Photosystem I (PS I / P700): Absorbs 700 nm light. Re-energised electrons go to Ferredoxin → NADP reductase → NADPH formed.
ATP synthesis: Chemiosmosis through ATP synthase (CF₀–CF₁ complex) as H⁺ gradient builds up across thylakoid membrane.
Dark Reactions — Calvin Cycle (C3 plants)
Three stages:
1. Carboxylation: CO₂ + RuBP (5C) → 2 molecules of 3-PGA (3C) via enzyme RuBisCO (most abundant enzyme on Earth).
2. Reduction: 3-PGA → G3P using ATP and NADPH.
3. Regeneration: G3P → RuBP using ATP. RuBP regenerated to accept next CO₂.
Net: 3 CO₂ + 9 ATP + 6 NADPH → 1 G3P (used to make glucose)
C4 Plants — Hatch-Slack Pathway
CO₂ acceptor: PEP (Phosphoenolpyruvate, 3C) → OAA (4C) via PEP carboxylase in mesophyll cells.
CO₂ shuttle: OAA → Malate → Bundle sheath cells → CO₂ released → enters Calvin cycle.
Kranz anatomy: Two types of cells — mesophyll (primary CO₂ fixation) + bundle sheath (Calvin cycle). Prominent in C4 plants.
Advantage: PEP carboxylase has higher CO₂ affinity, no photorespiration, better at high temperatures. Examples: Maize, Sugarcane, Sorghum, Amaranthus.
CAM Plants
Crassulacean Acid Metabolism. Stomata open at NIGHT (fix CO₂ as malate), close in daytime. CO₂ released from malate during day → enters Calvin cycle. Adaptation to dry/desert environments. Examples: Opuntia, Agave, Cacti.
Photorespiration
In C3 plants at high O₂ / low CO₂: RuBisCO acts as oxygenase → fixes O₂ instead of CO₂ → releases CO₂ without ATP synthesis (wasteful). C4 plants avoid photorespiration by concentrating CO₂ in bundle sheath cells.
Photosynthesis — Worked Examples
Q1. In the Z-scheme of light reactions, which of the following is the primary electron donor to Photosystem I?
A) Water
B) Ferredoxin
C) Plastocyanin
D) NADPH
Electron flow in Z-scheme: PSII (P680) → PQ → Cyt b6f complex → Plastocyanin (PC) → PSI (P700) → Ferredoxin → NADP⁺ → NADPH. So Plastocyanin donates electrons TO PSI. Water donates electrons to PSII (via OEC). Ferredoxin accepts electrons from PSI.
Q2. Which of the following is NOT a characteristic of C4 plants?
A) Kranz anatomy
B) PEP carboxylase in mesophyll cells
C) Photorespiration at normal O₂ levels
D) High photosynthetic efficiency at high temperatures
C4 plants do NOT undergo significant photorespiration because: CO₂ is concentrated in bundle sheath cells via C4 cycle, so RuBisCO always encounters high CO₂ and acts as carboxylase, not oxygenase. This makes C4 plants more efficient than C3 in hot, dry conditions. All other options ARE characteristics of C4 plants.
Q3. In the Calvin cycle, how many ATP and NADPH molecules are required to fix 3 molecules of CO₂?
A) 6 ATP and 6 NADPH
B) 9 ATP and 6 NADPH
C) 6 ATP and 9 NADPH
D) 12 ATP and 12 NADPH
Per CO₂ fixed: 3 ATP + 2 NADPH. For 3 CO₂: 9 ATP + 6 NADPH. These produce one molecule of G3P (glyceraldehyde-3-phosphate). Two G3P molecules are needed to form one glucose (so for 1 glucose from 6 CO₂: 18 ATP + 12 NADPH). The extra 3 ATP are used for RuBP regeneration.
Photosynthesis — Mistake DNA
❌ "Oxygen in photosynthesis comes from CO₂"
✅ WRONG. O₂ released in photosynthesis comes from WATER (H₂O), not CO₂. Proved by Ruben and Kamen (1941) using heavy oxygen isotope ¹⁸O₂. The reaction at PSII: 2H₂O → 4H⁺ + 4e⁻ + O₂. This is photolysis of water.
❌ "Dark reactions don't need light at all"
✅ MISLEADING. Dark reactions don't directly use light, but they depend on ATP and NADPH produced by light reactions. Without light, these products deplete and dark reactions stop. Modern nomenclature calls them "light-independent reactions" rather than "dark reactions" — they can occur in light too.
❌ "C4 plants have a different Calvin cycle than C3 plants"
✅ WRONG. C4 plants still use the same Calvin cycle (C3 cycle) in bundle sheath cells. What's different is the ADDITIONAL C4 cycle (Hatch-Slack pathway) in mesophyll cells that concentrates CO₂. C4 = C4 cycle + C3 cycle. C3 = only C3 cycle.
❌ "Photosystem I absorbs shorter wavelength light than Photosystem II"
✅ WRONG. PSI (P700) absorbs 700 nm, PSII (P680) absorbs 680 nm. P700 absorbs LONGER wavelength. In the Z-scheme, PSII acts first (absorbs 680 nm, splits water), then PSI (absorbs 700 nm, forms NADPH). The numbering I and II is historical, not based on reaction sequence.
Photosynthesis — Chapter Intelligence
AVG QUESTIONS
3–4
Steady across NEET 2020–2025
NCERT CHAPTER
Ch 13: Photosynthesis in Higher Plants (Class 11)
TOP QUESTION TYPES
Z-scheme electron flow, C3 vs C4 comparison, ATP/NADPH requirements
2025Emerson enhancement effect, origin of O₂ in photosynthesis (water), Kranz anatomy definition
2024Calvin cycle ATP requirements (9 ATP per 3 CO₂), PEP carboxylase in C4 plants, CAM plant examples
2023Z-scheme: plastocyanin donates to PSI, photorespiration in C3 plants, RuBisCO as most abundant enzyme
2022C4 plants list (maize, sugarcane), dark reaction location (stroma), absorption spectrum of chlorophyll
🎯 High-Probability 2026 Topics
✦ Z-scheme: sequence of electron carriers (PSII → PQ → Cyt b6f → PC → PSI → Fd → NADPH)
✦ C3 vs C4 plant comparison (first product, carboxylase enzyme, anatomy)
✦ Number of ATP and NADPH for Calvin cycle (3 ATP + 2 NADPH per CO₂)
✦ Photorespiration — why it occurs and how C4 plants avoid it