What is the Calvin Cycle?
The Calvin Cycle, also called the light-independent reactions, is the second stage of photosynthesis where plants use energy from ATP and NADPH to convert CO₂ into glucose. It occurs in the stroma of chloroplasts and doesn't need light directly.
The Calvin Cycle is a series of three phases—carbon fixation, reduction, and regeneration—that transforms CO₂ and water into glucose using the ATP and NADPH produced in the light reactions.
- 1.Carbon Fixation — CO₂ (3 molecules) combine with RuBP (5-carbon sugar) via the enzyme RuBisCO, forming 3-phosphoglycerate (3-PG).
- 2.Reduction — ATP and NADPH (from light reactions) reduce 3-PG to glyceraldehyde-3-phosphate (G3P). One G3P exits the cycle; five continue.
- 3.Regeneration — Five G3P molecules use ATP to regenerate three RuBP molecules, restarting the cycle. Cycle repeats to produce glucose.
Step-by-step worked examples
How many times must the Calvin Cycle turn over to produce one glucose molecule?
Glucose has 6 carbons. Each cycle fixes 1 CO₂. Calvin Cycle must turn 6 times to fix 6 CO₂ atoms. This produces 1 glucose (C₆H₁₂O₆).
What is RuBisCO and its role?
RuBisCO = ribulose-1,5-bisphosphate carboxylase/oxygenase. It catalyzes carbon fixation by joining RuBP and CO₂. It is the most abundant protein on Earth.
Why does the Calvin Cycle need ATP and NADPH?
ATP and NADPH are produced in the light reactions. They provide energy and reducing power (electrons) to reduce 3-PG to G3P. Without them, 3-PG cannot be converted to sugar.
Flashcards
Quick quiz
Q1.How many times must the Calvin Cycle turn to make one glucose?
Q2.Which of these is NOT a phase of the Calvin Cycle?
Q3.Why is it called 'light-independent'?
Q4.What does RuBisCO bind in carbon fixation?
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Common mistakes
The Calvin Cycle requires light to proceed. — Correct: It does not directly require light, but it needs the ATP and NADPH from light reactions.
RuBP is the final product of the Calvin Cycle. — Correct: RuBP is regenerated at the end; the net product is G3P, which forms glucose.
One turn of the Calvin Cycle produces one glucose. — Correct: The cycle must turn 6 times (fixing 6 CO₂) to net one glucose molecule.
The Calvin Cycle produces O₂. — Correct: O₂ is produced only in the light reactions (photolysis of water).
FAQ
What is the difference between the light reactions and the Calvin Cycle?
Light reactions occur in the thylakoid membrane and use light to split water and produce ATP and NADPH. The Calvin Cycle occurs in the stroma and uses that ATP and NADPH to fix CO₂ into glucose.
Can the Calvin Cycle run without the light reactions?
No, because the light reactions produce ATP and NADPH, which are essential for the reduction phase of the Calvin Cycle.
Is the Calvin Cycle the same in all plants?
Most plants use the standard Calvin Cycle (C₃ pathway). Some plants use alternative pathways like C₄ or CAM to be more efficient in hot or dry climates.
What happens to the G3P made in the Calvin Cycle?
One of every six G3P molecules exits and is used to synthesize glucose and other organic molecules. The other five regenerate RuBP to continue the cycle.