How many molecules of ATP and NADPH does the Calvin cycle use to make one molecule of glucose?
- One glucose (C6H12O6) has 6 carbon atoms, so 6 CO2 must be fixed.
- ATP: 6 × 3 = 18.
- NADPH: 6 × 2 = 12.
Answer: 18 ATP and 12 NADPH.
Photosynthesis
MDCAT Biology · Biology · Bioenergetics. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The idea
Photosynthesis uses light energy to make glucose from carbon dioxide and water. It has two stages inside the chloroplast.
The light-dependent reactions take place on the thylakoid membranes. Pigments in photosystems I and II absorb light, water is split (photolysis), oxygen is released, and ATP and NADPH are made. Chlorophyll a is the reaction centre pigment; chlorophyll b and carotenoids are accessory pigments that pass energy to it.
The Calvin cycle takes place in the stroma. It needs no light directly, but it uses the ATP and NADPH from the light stage to reduce carbon dioxide to sugar. The rate is set by whichever factor is in shortest supply, the limiting factor.
Rules to remember
- 6CO2 + 12H2O + light → C6H12O6 + 6O2 + 6H2O; the oxygen released comes from water
- Chlorophyll absorbs mainly violet-blue and orange-red light and reflects green. Absorption spectrum = light absorbed at each wavelength; action spectrum = rate of photosynthesis at each wavelength
- Photolysis: 2H2O → 4H+ + 4e− + O2, at photosystem II (P680)
- Non-cyclic photophosphorylation: both photosystems; electrons go water → PS II → PS I → NADP+; products ATP, NADPH and O2
- Cyclic photophosphorylation: photosystem I (P700) only; electrons return to P700; product ATP only
- Calvin cycle: carbon fixation (CO2 + RuBP → two 3-carbon PGA, enzyme RuBisCO) → reduction (PGA → G3P, uses ATP and NADPH) → regeneration of RuBP (uses ATP)
- Each CO2 fixed uses 3 ATP and 2 NADPH. Limiting factors: light intensity, CO2 concentration, temperature, water
Common mistake
Writing that the oxygen given off comes from carbon dioxide, or that the Calvin cycle works in the dark. The oxygen comes from water, and the cycle stops soon after the light does because ATP and NADPH run out.
Worked example
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