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How many ATP and NADPH molecules are required to make one molecule of glucose through the Calvin pathway?
Options
1
18 ATP and 12 NADPH
2
6 ATP and 12 NADPH
3
24 ATP and 18 NADPH
4
12 ATP and 18 NADPH
Correct Answer
Option 1 : 18 ATP and 12 NADPH
Solution
1

Per CO₂ fixed in Calvin cycle: 3 ATP + 2 NADPH required

2

To make 1 glucose (C₆H₁₂O₆): 6 CO₂ molecules must be fixed

Total ATP = 6 × 3 = 18 ATP

Total NADPH = 6 × 2 = 12 NADPH

Per CO₂: 3 ATP + 2 NADPH
For 1 glucose (6 CO₂): 18 ATP + 12 NADPH
Theory: Photosynthesis
1. Calvin Cycle — Overview

Calvin cycle (dark reactions / light-independent reactions / C3 cycle) occurs in stroma of chloroplast. Three stages: (1) Carbon fixation: CO₂ + RuBP →(RuBisCO) 2× 3-PGA (3-phosphoglycerate, C3). (2) Reduction: 3-PGA → G3P (glyceraldehyde-3-phosphate) using ATP + NADPH. (3) Regeneration of RuBP: G3P → RuBP using ATP. Per turn (1 CO₂): 3 ATP + 2 NADPH. For 1 glucose (6 CO₂): 18 ATP + 12 NADPH. Net G3P produced per 6 CO₂: 2 G3P (used to make 1 glucose). 10 G3P go back to regenerate 6 RuBP.

2. Light Reactions — Inputs and Outputs

Light reactions in thylakoid membrane. Products: ATP (by photophosphorylation via ATP synthase), NADPH (by reduction of NADP⁺ at PSI), O₂ (from water splitting at PSII). Photosystems: PSII (P680): absorbs 680nm light. Water → O₂ + 2H⁺ + 2e⁻ (water splitting/photolysis). PSI (P700): absorbs 700nm light. Reduces NADP⁺ → NADPH. Z scheme: PSII → PQ (plastoquinone) → cytochrome b6f complex → PC (plastocyanin) → PSI → Fd → NADP⁺ reductase → NADPH. Cyclic photophosphorylation: PSI only → only ATP (no O₂, no NADPH).

3. RuBisCO — Most Abundant Enzyme on Earth

RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase): enzyme that catalyses CO₂ fixation in Calvin cycle. Most abundant enzyme on Earth (~0.5 kg/m² of leaf). Has two activities: (1) Carboxylase: fixes CO₂ onto RuBP → 3-PGA (useful). (2) Oxygenase: fixes O₂ onto RuBP → 2-phosphoglycolate (photorespiration — wasteful). C3 plants: most plants. First product = 3-PGA (3 carbon). Photorespiration occurs. Less efficient. C4 plants: CO₂ concentrated around RuBisCO → minimal oxygenase activity → more efficient. First product = oxaloacetate (4C) in mesophyll cells. Examples: maize, sugarcane, sorghum.

4. C4 Pathway and Kranz Anatomy

C4 plants: two types of cells. Mesophyll cells: CO₂ fixed by PEP carboxylase (high affinity for CO₂) → OAA (C4) → malate or aspartate → transported to bundle sheath cells. Bundle sheath cells: decarboxylation releases CO₂ → high CO₂ concentration → RuBisCO operates as carboxylase only → no photorespiration. Kranz anatomy: ring of bundle sheath cells around vascular bundle, surrounded by mesophyll cells. C4 plants: maize (corn), sugarcane, sorghum, amaranth, Atriplex. More efficient in hot, dry, high-light conditions. C4 plants have higher water use efficiency. CAM (Crassulacean Acid Metabolism): night CO₂ fixation (stomata open at night) → stored as malate → day decarboxylation → Calvin cycle. Examples: cacti, agave, pineapple.

5. Photorespiration

Photorespiration: RuBisCO's oxygenase activity. High O₂/low CO₂ → RuBP + O₂ → 3-PGA + 2-phosphoglycolate. 2-phosphoglycolate → recycled (phosphoglycolate pathway) in peroxisome and mitochondria → CO₂ released (photorespiratory CO₂). Net effect: wasteful — consumes ATP and O₂, releases CO₂ without producing sugar. C3 plants: significant photorespiration (~25% of fixed C is lost). C4 and CAM plants: CO₂ concentration mechanism suppresses photorespiration. Temperature increases photorespiration (RuBisCO's oxygenase activity increases faster with T than carboxylase). Genetic engineering to reduce photorespiration is an active research area.

6. Factors Affecting Photosynthesis

Blackman's law of limiting factors: rate of photosynthesis is limited by the factor present in minimum quantity. Key factors: Light intensity: increases photosynthesis until saturation point. Beyond saturation: CO₂ becomes limiting. CO₂ concentration: rate increases with CO₂ (especially in C3 plants). Temperature: optimum ~25-30°C for most plants. Increases rate up to optimum, then denatures enzymes. Water: needed for photolysis; water stress closes stomata (reduces CO₂ entry). CO₂ enrichment: greenhouses use CO₂ enrichment to boost crop yields. Light compensation point: light intensity where photosynthesis = respiration (no net O₂ exchange). Below this: respiration > photosynthesis (net CO₂ release).

7. Electron Transport in Photosynthesis

Z-scheme (non-cyclic photophosphorylation): PSII (P680) absorbs light → chlorophyll excited → electron to PQ. H₂O → [Mn₄Ca] water-splitting complex → 2e⁻ to P680 + 2H⁺ (to lumen) + ½O₂. PQ carries 2e⁻ + 2H⁺ → cytochrome b6f complex → 2H⁺ pumped to thylakoid lumen → proton gradient → ATP synthesis. PC (plastocyanin) → PSI (P700) → ferredoxin → NADP⁺ reductase (FNR) → NADPH. Chemiosmosis: H⁺ gradient (built up in thylakoid lumen by water splitting and PQ) → flows through CF₁-CF₀ ATP synthase → ATP synthesis. Similar to mitochondrial ATP synthesis but in reverse direction (H⁺ flows from lumen to stroma).

8. Products of Photosynthesis

Net equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Products: Glucose (immediate product is G3P → sucrose for transport, starch for storage). Oxygen (from water — confirmed by O¹⁸ isotope experiments). ATP (consumed in Calvin cycle). NADPH (consumed in Calvin cycle). The O₂ produced comes entirely from water (H₂O), NOT from CO₂. Confirmed by van Niel (using purple sulphur bacteria: H₂S + CO₂ → sugar + S — parallel to H₂O + CO₂ → sugar + O₂). Radioactive O¹⁸ labelling: H₂O¹⁸ → O¹⁸₂ (not CO¹⁸₂ → O₂).

Frequently Asked Questions
1. Where does the Calvin cycle occur? ⌄
The Calvin cycle occurs in the stroma of the chloroplast (not the thylakoid membrane). The light reactions occur in/on the thylakoid membrane. Stroma = aqueous phase surrounding thylakoids. Contains: RuBisCO (most abundant protein here), all other Calvin cycle enzymes, ribosomes (70S — chloroplast has its own), DNA (circular, like prokaryotes — supports endosymbiont theory), starch granules. The ATP and NADPH produced by light reactions in thylakoids diffuse into stroma and are used in Calvin cycle.
2. Why exactly 3 ATP and 2 NADPH per CO₂? ⌄
Per CO₂ fixed in Calvin cycle: Step 1: CO₂ + RuBP (C5) → 2 × 3-PGA (C3) [no ATP/NADPH needed]. Step 2: Each 3-PGA → G3P requires: 1 ATP (for phosphorylation: 3-PGA → 1,3-bis-PGA) + 1 NADPH (for reduction: 1,3-bis-PGA → G3P). 2 × 3-PGA → 2 G3P = 2 ATP + 2 NADPH. Step 3: Regeneration of RuBP from G3P: complex series, net 1 ATP per CO₂. Total per CO₂: 2+1 = 3 ATP + 2 NADPH. For 6 CO₂: 18 ATP + 12 NADPH.
3. What is the first stable product of C3 photosynthesis? ⌄
3-PGA (3-phosphoglycerate) is the first stable product. It's a 3-carbon compound — this is why these plants are called C3 plants. CO₂ + RuBP →(RuBisCO) → unstable 6C compound → 2 × 3-PGA. In C4 plants: first stable product = OAA (oxaloacetate, 4C) in mesophyll cells. Then OAA → malate/aspartate. In CAM plants: same as C4 (OAA, 4C), but carbon fixation happens at night. In all cases, glucose (6C) is ultimately produced in the Calvin cycle in bundle sheath cells (C4) or during the day (CAM).
4. What is the difference between cyclic and non-cyclic photophosphorylation? ⌄
Non-cyclic (Z-scheme): involves BOTH PSI and PSII. Products: ATP + NADPH + O₂. Electrons: from water → PSI → NADP⁺. Linear electron flow. PSII is included — water is oxidised, O₂ is released. This is the main pathway. Cyclic: involves ONLY PSI. Products: ATP only (no NADPH, no O₂). Electrons: from PSI → Fd → cytochrome b6f → PSI (cycle). No PSII, no water splitting. Function: produces extra ATP when ATP/NADPH ratio needs adjustment (Calvin cycle needs more ATP than NADPH — cyclic tops up ATP). Also operates when NADPH accumulates.
5. How is glucose made from G3P? ⌄
G3P (glyceraldehyde-3-phosphate, C3) is the direct product of Calvin cycle. Two G3P molecules combine to form fructose-1,6-bisphosphate → fructose-6-phosphate → glucose-6-phosphate → glucose (by gluconeogenesis-like reactions). These occur in the chloroplast stroma and cytoplasm. Alternatively, G3P is used for: sucrose synthesis in cytoplasm (for phloem transport). Starch synthesis in chloroplast (when excess). Fatty acid and amino acid synthesis. In practice: glucose is NOT directly exported from leaf — sucrose is the main transport form in phloem. Sucrose = glucose + fructose.
6. Why are C4 plants more efficient than C3 plants? ⌄
C4 plants use a CO₂ concentration mechanism: PEP carboxylase in mesophyll has very high affinity for CO₂ and does not react with O₂. Concentrates CO₂ in bundle sheath cells → RuBisCO acts only as carboxylase (not oxygenase) → no photorespiration. Results: (1) No photorespiration loss (~25% of fixed C saved). (2) Better performance at high temperatures (RuBisCO's oxygenase activity increases faster with T in C3 plants). (3) Better water use efficiency (stomata can be more closed, reducing water loss). Disadvantage: C4 cycle uses extra ATP (5 ATP + 2 NADPH per CO₂ vs 3 ATP + 2 NADPH for C3). So in cool climates, C4 advantage disappears — C3 plants like wheat/rice dominate.
7. What is the Hill reaction? ⌄
Robin Hill (1937) showed that isolated chloroplasts could produce O₂ in the presence of an artificial electron acceptor (DCPIP = dichlorophenolindophenol) even without CO₂. Reaction: 2H₂O + 2DCPIP → O₂ + 2DCPIPH₂ (DCPIP is reduced = changes from blue to colourless). Significance: (1) Proved that O₂ comes from water, NOT CO₂. (2) Proved light reactions can occur in isolated chloroplasts. (3) Separated light reactions from dark reactions. The Hill reaction is now PSII: water splitting + electron transfer to artificial acceptors.
8. What is quantum yield of photosynthesis? ⌄
Quantum yield = number of CO₂ fixed (or O₂ produced) per photon absorbed. Theoretical maximum for C3 plants: 1/8 (8 photons needed per CO₂ fixed). 4 photons for PSII (2H₂O → O₂ + 4H⁺ + 4e⁻) + 4 photons for PSI (4e⁻ → 2 NADPH). Total 8 photons for one CO₂. Actual quantum yield is lower due to energy losses. Red light has higher quantum yield than blue (blue photons have more energy but excess converted to heat). Emerson enhancement effect: quantum yield of photosynthesis with red + far-red light together > red alone + far-red alone. This proved that two photosystems work together.
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