📌Definition Table

Term	Definition
Calvin cycle	Light-independent pathway in the chloroplast stroma that fixes CO₂ into carbohydrates using ATP and NADPH.
Carbon fixation	Incorporation of atmospheric CO₂ into organic molecules.
Rubisco	Enzyme that catalyzes the first step of carbon fixation by attaching CO₂ to RuBP.
RuBP (ribulose-1,5-bisphosphate)	Five-carbon compound that reacts with CO₂ in the Calvin cycle.
GP (3-phosphoglycerate)	First stable product of CO₂ fixation.
TP (triose phosphate)	Three-carbon sugar produced in the Calvin cycle, used for carbohydrate synthesis.
Regeneration	Process of reforming RuBP from TP so the cycle can continue.

📌Introduction

The Calvin cycle, also called the light-independent reactions, occurs in the stroma of chloroplasts. It uses the ATP and NADPH generated in the light-dependent stage to fix CO₂ and synthesize carbohydrate precursors. This cycle does not directly require light but is indirectly dependent on it, since its energy and reducing power come from the light reactions.

📌 Stages of the Calvin Cycle

1. Carbon Fixation
   • CO₂ combines with RuBP (5C) to form an unstable 6C compound.
   • Reaction catalyzed by Rubisco, the most abundant enzyme on Earth.
   • The 6C intermediate immediately breaks down into two molecules of GP (3C).
2. Reduction
   • GP is reduced to TP (triose phosphate) using:
     • ATP (energy source).
     • NADPH (reducing agent).
   • TP is the key output: it can form glucose, amino acids, fatty acids.
3. Regeneration of RuBP
   • Most TP molecules are recycled to regenerate RuBP using ATP.
   • Ensures the cycle continues.

• Products per 3 turns (fixing 3 CO₂):
  • 6 TP formed, but only 1 TP exits to contribute to glucose synthesis.
  • 5 TP recycled to regenerate 3 RuBP.
• Net result per glucose (6 CO₂ fixed):
  • 2 TP used → 1 glucose.
  • Requires 18 ATP and 12 NADPH.

🧠 Examiner Tip: Many students wrongly state that Calvin cycle produces glucose directly. Always emphasize that TP is the actual product; glucose forms after multiple turns.

📌 Regulation and Dependency

• Calvin cycle depends on ATP and NADPH from light-dependent reactions.
• If light is absent, ATP and NADPH are unavailable → cycle halts.
• Rubisco is sensitive to CO₂ and O₂ levels; high O₂ can trigger photorespiration, reducing efficiency.

🧬 IA Tips & Guidance: Students can measure the effect of light intensity or CO₂ concentration on photosynthetic rate using leaf disk assays, directly linking data to Calvin cycle activity.

📌 Outputs of the Calvin Cycle

• TP → forms glucose, sucrose, starch, cellulose (carbohydrates).
• TP also provides precursors for lipids and amino acids.
• ATP and NADPH are converted back into ADP, Pi, and NADP⁺, which return to the light reactions.

🌐 EE Focus: An EE could analyze Rubisco activity under varying environmental conditions, or model Calvin cycle efficiency under elevated CO₂ (climate change relevance).

📌 Role of Chloroplast Structure in Calvin Cycle

• Stroma contains enzymes of Calvin cycle.
• Close proximity to thylakoids ensures rapid supply of ATP/NADPH.
• Double membrane maintains suitable internal environment.

❤️ CAS Link: Students could design community projects on sustainable farming, showing how plant photosynthesis underpins global food security.

📌 Integration of Calvin Cycle

• Links with light-dependent stage via ATP and NADPH supply.
• Provides organic molecules feeding into cellular respiration, biosynthesis, and growth.

🔍 TOK Perspective: Rubisco’s inefficiency raises a TOK question: How do scientific explanations account for imperfections in natural systems, and does “efficiency” reflect human bias in evaluating biology?

📝 Paper 2: Be prepared to outline the stages of Calvin cycle, identify key molecules (RuBP, GP, TP), explain ATP/NADPH use, and state the net requirements for glucose synthesis. Data questions may test effects of light, CO₂, or enzyme activity.