Preview
IB DP Biology — C — Interaction and Interdependence | SL | C1.3 Photosynthesis
📋 What You Will Learn
- Identify the inputs, outputs, and location of the Calvin cycle within the chloroplast
- Explain the three stages of the Calvin cycle: carbon fixation, reduction, and regeneration of RuBP
- Analyse how limiting factors — light intensity, CO₂ concentration, and temperature — affect the rate of photosynthesis using experimental data
- Explain how C4 and CAM plants are structurally and biochemically adapted to reduce photorespiration in hot, dry, or high-light environments
- Apply knowledge of the Calvin cycle to interpret graphs showing changes in GP, G3P, and RuBP concentrations under altered conditions
🔍 Guiding Question How does the Calvin cycle convert inorganic carbon into organic molecules, and how have photosynthetic organisms adapted this process to survive in diverse environments?
🌍 Why This Matters Every carbon atom in your body was once CO₂ in the atmosphere — pulled into organic form by a single enzyme, RuBisCO, in the Calvin cycle. Maize, sugarcane, and sorghum — crops feeding billions — use a modified version of this cycle that is up to 40% more efficient in hot climates, a difference that shapes global food security.
📋 What You Will Learn
- Identify the inputs, outputs, and location of the Calvin cycle within the chloroplast stroma
- Explain the three stages of the Calvin cycle: carbon fixation, reduction, and regeneration of RuBP
- Analyse how limiting factors affect the rate of photosynthesis using experimental data
- Explain the structural and biochemical adaptations of C4 and CAM plants
- Apply Calvin cycle knowledge to interpret graphs of GP, G3P, and RuBP concentrations
The Problem the Chloroplast Had to Solve
The light-dependent reactions have done their work. ATP and NADPH are loaded and ready. But energy alone is useless — the cell needs carbon skeletons. Sugars, amino acids, lipids: all of them begin as CO₂ floating in from the atmosphere. The challenge is to take that inorganic gas and stitch it, atom by atom, into something a cell can use.
That is the job of the Calvin cycle — a cyclic series of enzyme-catalysed reactions in the stroma of the chloroplast that uses the ATP and NADPH from the light reactions to fix atmospheric CO₂ into organic molecules. It runs continuously in the light, consuming the products of the thylakoid reactions as fast as they are made.
📖 Key Definition — Carbon Fixation Carbon fixation is the incorporation of inorganic carbon dioxide into an organic molecule. In the Calvin cycle, CO₂ is fixed by the enzyme RuBisCO, which catalyses its addition to the five-carbon acceptor molecule ribulose bisphosphate (RuBP).
📖 Key Definition — The Calvin Cycle The Calvin cycle (also called the light-independent reactions or carbon fixation cycle) is a cyclic series of reactions occurring in the stroma of the chloroplast. It uses ATP and NADPH from the light-dependent reactions to reduce CO₂ into glycerate-3-phosphate (GP) and then into triose phosphate (G3P), which can be used to synthesise organic compounds including glucose.
The Calvin Cycle — Stage by Stage
The cycle has three distinct stages. Each stage is a chapter in the same story: carbon arrives as a gas, gets reduced using chemical energy, and the acceptor molecule is rebuilt so the cycle can run again. Follow the carbon atoms — they are the protagonists.
Stage 1 — Carbon Fixation
Three molecules of CO₂ enter the cycle and are each attached to a molecule of ribulose