SECTION 6: The Calvin Cycle & Carbon Fixation

Q: Where do the light-independent reactions (Calvin Cycle) occur?

A: In the stroma of the chloroplast.

Q: What are the inputs and outputs of the Calvin Cycle?

A: Inputs: CO₂, ATP, NADPH; Outputs: Glucose (C₆H₁₂O₆), ADP, NADP⁺.

Q: What is the main purpose of the Calvin Cycle?

A: To convert inorganic CO₂ into organic sugars using the chemical energy (ATP and NADPH) produced in the light reactions.

Q: Why is the Calvin Cycle called "light-independent"?

A: It doesn’t directly require light, but depends on ATP and NADPH generated by the light-dependent reactions.

Q: What enzyme catalyzes the first step of carbon fixation?

A: RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase).

Q: What molecule does RuBisCO fix CO₂ to?

A: RuBP (ribulose bisphosphate), a 5-carbon compound.

Q: What is the product of the carbon fixation step before it becomes sugar?

A: A short-lived 6-carbon molecule that splits into two 3-carbon molecules (3-PGA, 3-phosphoglycerate).

Q: How is 3-PGA converted into G3P (glyceraldehyde-3-phosphate)?

A: Through reduction using ATP and NADPH from light reactions.

Q: What happens to G3P molecules produced in the Calvin Cycle?

A: Some are used to regenerate RuBP; others are combined to form glucose and other carbohydrates.

Q: Why is CO₂ fixation considered a “reduction” process?

A: CO₂ (oxidized carbon) gains hydrogen and electrons to form glucose (reduced carbon).

Q: What happens if stomata close due to water stress?

A: CO₂ levels in leaves drop, limiting the Calvin Cycle and reducing photosynthesis.

Q: 🔴Why is RuBisCO considered inefficient?

A: 🔴It can also bind O₂ instead of CO₂, leading to photorespiration, which wastes energy and reduces sugar production.

Q: 🔴What adaptation do some plants have to overcome RuBisCO’s inefficiency?

A: 🔴C₄ and CAM photosynthetic pathways concentrate CO₂ around RuBisCO or separate processes temporally/spatially to minimize photorespiration.