The Calvin Cycle
Track the fixation of Carbon Dioxide by Rubisco and the energy transfer from Light-Dependent ATP/NADPH to synthesize G3P.
THE LIGHT-INDEPENDENT REACTIONS
The **Calvin Cycle** (or light-independent reactions) occurs in the **stroma** of the chloroplast. It uses the chemical energy (ATP) and reducing power (NADPH) generated in the light reactions to "fix" carbon dioxide into organic molecules, eventually forming G3P, a precursor to glucose.
THREE STAGES OF THE CYCLE
1. **Carbon Fixation**: The enzyme **RuBisCO** attaches to a 5-carbon sugar called RuBP. 2. **Reduction**: ATP and NADPH are used to convert the molecules into G3P (a 3-carbon sugar). 3. **Regeneration**: Some G3P molecules leave the cycle to make glucose, while others are recycled using ATP to regenerate RuBP.
HOW TO USE THIS VISUALIZATION
1. **Track Carbon Atoms**: Follow the carbon molecules as they are rearranged from to G3P. 2. **Monitor Energy Use**: See where ATP and NADPH enter the cycle and are converted back to ADP and NADP+. 3. **Vary Levels**: Observe how the rate of the cycle responds to changes in carbon availability. **Try This**: Block the light reactions (stop ATP/NADPH flow). Watch the Calvin cycle slow down and stop. Since it's "light-independent," why can't it run in the dark for long?
AP EXAM CONNECTION
Unit: Unit 3: Cellular Energetics (Topic 3.5)
Learning Objective: ENE-1.I
COMMON MISCONCEPTIONS
- Thinking glucose is the direct product (it's actually G3P).
- Believing the cycle happens in the thylakoid (it happens in the stroma).
- Assuming the cycle only happens at night (it mostly happens in the light when energy is available).
KEY TAKEAWAYS
- Occurs in the stroma.
- Fixes CO2 into G3P.
- Uses ATP and NADPH.
- RuBisCO is the key enzyme.
PRACTICE QUESTIONS
Q1 (CONCEPTUAL): Which enzyme is responsible for the initial fixation of in the Calvin cycle?
Show Answer & Explanation
Answer: RuBisCO.
Explanation: Ribulose bisphosphate carboxylase/oxygenase (RuBisCO) is the most abundant enzyme on Earth and catalyzes the first step of carbon fixation.
Q2 (QUANTITATIVE): How many molecules of are required to produce one net molecule of G3P?
Show Answer & Explanation
Answer: Three.
Explanation: The cycle must turn three times (fixing 3 molecules) to produce one net molecule of the 3-carbon sugar G3P.
