Calvin Cycle (C3 Pathway)

• How CO₂ is fixed into organic molecules in the stroma of chloroplasts.
• The three stages of the Calvin cycle: carboxylation, reduction, and regeneration of RuBP.
• The stoichiometry: how many ATP and NADPH are consumed per CO₂ fixed and per glucose produced.
• The role and dual nature of the enzyme RuBisCO.
• Why photorespiration occurs and how it reduces photosynthetic efficiency.

Level 1 — Three Stages of the Calvin Cycle

The Calvin cycle (also called the C3 pathway or dark reactions) runs in the stroma of the chloroplast using ATP and NADPH from the light reactions. It has three stages:

1. Carboxylation — CO₂ is attached to a 5-carbon molecule called RuBP (ribulose-1,5-bisphosphate) by the enzyme RuBisCO. This produces an unstable 6-carbon compound that immediately splits into two molecules of 3-PGA (3-phosphoglycerate), a 3-carbon acid — hence the name "C3 pathway."

2. Reduction — 3-PGA is reduced using ATP and NADPH (from the light reactions) to form G3P (glyceraldehyde-3-phosphate), a 3-carbon sugar. Some G3P is exported to make glucose and other organic compounds.

3. Regeneration — Most G3P molecules are used to regenerate RuBP (using ATP) so the cycle can continue.

Level 2 — Stoichiometry, RuBisCO, and Photorespiration

Stoichiometry (per 3 CO₂ fixed — one "turn" of the full cycle):

Net output: 1 G3P exported (the other 5 of 6 G3P are used to regenerate 3 RuBP). To make 1 glucose (6C): 18 ATP + 12 NADPH consumed; 6 CO₂ fixed; 12 G3P produced, 2 exported.

RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase):

• Most abundant enzyme on Earth.
• Carboxylase activity: CO₂ + RuBP → 2× 3-PGA (productive, leads to sugar synthesis).
• Oxygenase activity: O₂ + RuBP → 1× 3-PGA + 1× 2-phosphoglycolate (2C) → triggers photorespiration.
• RuBisCO has a much higher affinity for CO₂ than O₂, but in high-O₂ / low-CO₂ conditions (e.g., hot, sunny days when stomata partially close), O₂ competes.

Photorespiration:

• 2-phosphoglycolate is metabolised in peroxisomes and mitochondria, releasing CO₂ and consuming O₂ — the opposite of photosynthesis.
• Photorespiration wastes up to 25–30% of fixed carbon in C3 plants.
• C4 and CAM plants suppress photorespiration by concentrating CO₂ around RuBisCO.

Key enzymes and regulation:

• RuBisCO is activated by high Mg²⁺, high pH, and high CO₂ (all conditions present in illuminated stroma).
• Phosphofructokinase (in glycolysis, not Calvin) — mentioned here only to contrast: the Calvin cycle has its own regulatory enzymes (phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase).

NEET-style Question:
How many molecules of ATP and NADPH are required to fix 6 molecules of CO2 via the Calvin cycle
and produce 1 molecule of glucose?

Step 1 — Establish per-CO2 cost
  Per 3 CO2 fixed: 9 ATP + 6 NADPH consumed (from stoichiometry above).
  For 6 CO2: 2 × (9 ATP + 6 NADPH) = 18 ATP + 12 NADPH.

Step 2 — Check what is produced
  6 CO2 fixed → 12 G3P produced.
  10 G3P used to regenerate 6 RuBP.
  2 G3P exported → combined to form 1 glucose (C6H12O6).

Step 3 — Verify the regeneration step
  Regeneration: 10 G3P → 6 RuBP, using 6 ATP (3 ATP per 3 CO2, so 6 ATP per 6 CO2). ✓
  This is already included in the 18 ATP total.

Step 4 — State the answer
  To produce 1 glucose from 6 CO2 via the Calvin cycle:
  ATP required = 18
  NADPH required = 12

Answer: 18 ATP and 12 NADPH

• Write the net reaction of the Calvin cycle for 3 CO₂ fixed (with ATP and NADPH as reactants).
• Calculate the total ATP and NADPH consumed to synthesise one molecule of glucose via the Calvin cycle.
• Explain why RuBisCO is described as having both carboxylase and oxygenase activity.
• Define photorespiration and state in which organelles it occurs (chloroplast, peroxisome, mitochondria).
• Name the first stable product of CO₂ fixation in the C3 pathway.
• Why does photorespiration increase on hot, sunny days when stomata close?
• Distinguish between the Calvin cycle products G3P and 3-PGA (structure, stage at which formed, role).

• Figure 13.4 — The Calvin cycle (dark reaction) showing three stages: carboxylation (CO₂ + RuBP → 2 PGA), reduction (PGA → G3P using ATP + NADPH), and regeneration (G3P → RuBP using ATP). NCERT Class 11, Chapter 13.
• Summary diagram of the chloroplast showing the light reactions (thylakoid) feeding ATP and NADPH to the Calvin cycle (stroma).
• Comparison diagram of C3 vs. C4 plants showing where RuBisCO operates and how CO₂ concentration differs.

• Name the enzyme that catalyses CO₂ fixation in the Calvin cycle.
• What is the first stable product of the Calvin cycle? How many carbons does it contain?
• How many ATP molecules are consumed per CO₂ fixed in the Calvin cycle?
• True or False: The Calvin cycle takes place in the thylakoid membrane. (Answer: False — it occurs in the stroma)
• What is photorespiration and why is it considered wasteful?
• If NADPH supply is cut off (e.g., light reactions are blocked), which stage of the Calvin cycle is directly halted first?
• Stretch: A scientist engineers a mutant RuBisCO that cannot bind O₂. Predict the effect on (a) photorespiration, (b) photosynthetic efficiency in C3 plants grown at high temperatures, and (c) the relative advantage of C4 plants over this mutant C3 plant.

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