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Biology · Photosynthesis · Class 11

Calvin Cycle — The Light-Independent Reactions Explained

The Calvin cycle is the second stage of photosynthesis. It occurs in the stroma of chloroplasts, uses ATP and NADPH from the light reactions, and fixes CO2 into glucose. It does not need light directly, but it depends on light reaction products.

Explanation

Imagine the Calvin cycle as a factory assembly line. Raw material (CO2) enters, gets assembled into a useful product (glucose), and the assembly machinery (RuBP) gets recycled for the next round.

The cycle starts when RuBisCO grabs a CO2 molecule and attaches it to a 5-carbon sugar called RuBP. This creates an unstable 6-carbon compound that immediately splits into two 3-carbon molecules (3-PGA). Using ATP and NADPH from the light reactions, 3-PGA is reduced to G3P (glyceraldehyde-3-phosphate). Most G3P molecules are recycled back into RuBP so the cycle can continue. For every 3 turns of the cycle (3 CO2 molecules fixed), one net G3P molecule is produced. Two G3P molecules combine to form one glucose.

Step-by-Step Breakdown

  1. Carbon Fixation

    RuBisCO enzyme combines CO2 with RuBP (5C) to form an unstable 6C compound that immediately splits into two molecules of 3-PGA (3C). This is the carbon fixation step.

  2. Reduction

    ATP provides energy and NADPH provides electrons to convert 3-PGA into G3P (glyceraldehyde-3-phosphate). This is where the energy from light reactions is actually used to build organic molecules.

  3. Regeneration of RuBP

    Most G3P molecules (5 out of every 6) are rearranged using ATP to regenerate RuBP, so the cycle can fix more CO2. Only 1 out of 6 G3P is the net product.

Key Definitions

TermDefinition
RuBPRibulose-1,5-bisphosphate. The 5-carbon acceptor molecule that combines with CO2 in the Calvin cycle.
3-PGA3-phosphoglycerate. The first stable 3-carbon product of carbon fixation.
G3PGlyceraldehyde-3-phosphate. The reduced 3-carbon sugar that exits the Calvin cycle. Two G3P molecules make one glucose.
Carbon FixationThe process of incorporating inorganic CO2 into organic molecules. Catalyzed by RuBisCO.

Common Mistakes Students Make

1

Calling it the 'dark reaction' and assuming it happens at night. It happens during the day — it just does not directly need light. It needs ATP and NADPH, which are only produced when light is available.

2

Thinking one turn of the Calvin cycle produces one glucose. It takes 3 turns (3 CO2 fixed) to produce one net G3P. It takes 6 turns (6 CO2) to make one glucose.

3

Forgetting the regeneration step. Students describe fixation and reduction but skip the regeneration of RuBP, which uses ATP and is essential for the cycle to continue.

4

Confusing G3P with glucose. G3P is a 3-carbon sugar. Two G3P molecules are needed to build one 6-carbon glucose molecule.

Exam Focus — How This Is Asked

CBSE often asks: 'Describe the three stages of the Calvin cycle.' Expect a 3-mark or 5-mark question. For NEET, know the exact inputs and outputs per turn: 3 CO2, 9 ATP, 6 NADPH → 1 G3P (net). The question 'How many CO2 molecules are needed to produce one glucose?' appears frequently (answer: 6).

Practice Questions

Q1.How many turns of the Calvin cycle are needed to produce one molecule of glucose?2m
6 turns. Each turn fixes one CO2 and produces one G3P (net: 1 G3P per 3 turns). Two G3P molecules combine to form one glucose, so 6 CO2 fixations = 6 turns.
Q2.What is the role of RuBisCO in the Calvin cycle?2m
RuBisCO catalyzes carbon fixation — it combines CO2 with RuBP (5C) to form an unstable 6C compound that splits into two 3-PGA (3C) molecules. It is the most abundant enzyme on Earth.

Photosynthesis — 30-Second Revision

  • •Calvin cycle occurs in stroma (not thylakoid)
  • •Three stages: fixation → reduction → regeneration
  • •RuBisCO fixes CO2 + RuBP → 3-PGA
  • •ATP + NADPH reduce 3-PGA → G3P
  • •Per glucose: 6 CO2, 18 ATP, 12 NADPH
  • •Does not directly use light, but needs light reaction products
  • •G3P is the exit product (two G3P = one glucose)

Related Topics

Frequently Asked Questions

Why is the Calvin cycle also called C3 cycle?
Because the first stable product of carbon fixation is a 3-carbon molecule (3-PGA). Plants that primarily use this pathway are called C3 plants.
Can the Calvin cycle run without light?
Technically, the reactions themselves do not need light. But they need ATP and NADPH, which are only produced during the light reactions. So in practice, the Calvin cycle stops shortly after light is removed.

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