AP Biology: Calvin Cycle (Light?Independent Reactions) – Carbon Fixation, Rubisco, G3P

Calvin Cycle (Light?Independent Reactions) – Carbon Fixation, Rubisco, G3P

Concept Summary

• Calvin Cycle: Light-independent reactions in the stroma that fix CO? into organic molecules (G3P) using ATP and NADPH from the light reactions.
• Carbon Fixation: Incorporation of inorganic CO? into organic molecules (3-PGA) via Rubisco, the first step of the Calvin Cycle.
• Rubisco (RuBP carboxylase/oxygenase): Enzyme that catalyzes CO? fixation to RuBP; most abundant protein on Earth but inefficient (can also bind O?, causing photorespiration).
• G3P (Glyceraldehyde-3-phosphate): 3-carbon sugar product of the Calvin Cycle; used to synthesize glucose and other carbohydrates (2 G3P-1 glucose).
• Regeneration of RuBP: 5 of every 6 G3P molecules are recycled to regenerate RuBP, enabling continuous CO? fixation.

Core Questions

WHAT (definitional)

Q: What is the Calvin Cycle? A: A metabolic pathway in the stroma that converts CO? into G3P using ATP and NADPH, independent of light. Trap/Clarification: It is not the "dark reactions"—it occurs in light or dark but depends on ATP/NADPH from light reactions.

Q: What is Rubisco? A: The enzyme that catalyzes the carboxylation of RuBP (adding CO?) to form 3-PGA, initiating the Calvin Cycle. Trap/Clarification: Rubisco also binds O? (oxygenase activity), leading to photorespiration, a wasteful process.

WHY (causal/explanatory)

Q: Why is the Calvin Cycle important? A: It produces G3P, the precursor for glucose, starch, cellulose, and other organic molecules essential for plant growth and energy storage. Trap/Clarification: It does not directly produce glucose—2 G3P molecules are needed to form 1 glucose.

Q: Why is Rubisco’s oxygenase activity problematic? A: It competes with carboxylation, wasting energy (ATP/NADPH) and releasing CO? via photorespiration, reducing photosynthetic efficiency. Trap/Clarification: Photorespiration increases in hot/dry conditions when stomata close, raising O?:CO? ratios.

HOW (process/application)

Q: How does the Calvin Cycle fix carbon? A: CO? + RuBP (5C)-2 × 3-PGA (3C) via Rubisco; 3-PGA is then phosphorylated (ATP) and reduced (NADPH) to G3P. Trap/Clarification: Only 1 of every 6 G3P molecules exits the cycle; the rest regenerate RuBP.

Q: How is RuBP regenerated? A: 5 G3P (3C each)-3 RuBP (5C each) via a complex series of reactions requiring ATP. Trap/Clarification: The cycle must turn 3 times to produce 1 net G3P (3 CO? fixed).

CAN (conditions/possibilities)

Q: Can the Calvin Cycle occur without light? A: Yes, but only temporarily—it requires ATP/NADPH from light reactions, which are depleted in the dark. Trap/Clarification: "Light-independent" refers to direct light use, not independence from light-dependent products.

Q: Under what conditions does photorespiration increase? A: High temperatures, low CO?:O? ratios (e.g., closed stomata in drought), or high light intensity (increases O? production). Trap/Clarification: C4/CAM plants minimize photorespiration by spatially/temporally separating CO? fixation from Rubisco.

Quick Facts & Traps

• Fact: 3 turns of the Calvin Cycle fix 3 CO? to produce 1 net G3P (6 G3P total, 5 recycled).
• Trap: "The Calvin Cycle produces glucose directly."-Reality: It produces G3P, which is later converted to glucose (2 G3P-1 glucose).
• Fact: Rubisco is slow (~3 CO? fixed/sec) but compensates by being highly abundant (up to 50% of leaf protein).
• Trap: "Photorespiration is beneficial."-Reality: It wastes energy and releases CO?, reducing photosynthetic efficiency.
• Fact: C4 plants (e.g., corn) use PEP carboxylase to fix CO? first, then deliver it to Rubisco, minimizing photorespiration.
• Trap: "All plants use the Calvin Cycle the same way."-Reality: C4/CAM plants modify it to concentrate CO? around Rubisco.

Rapid-Fire True/False

• Statement: The Calvin Cycle occurs in the thylakoid membrane. Answer: FALSE Why the common mistake happens: Confusion with light reactions (thylakoid) vs. Calvin Cycle (stroma).

• Statement: Rubisco can only fix CO?, not O?. Answer: FALSE Why the common mistake happens: Overlooking its oxygenase activity, which causes photorespiration.

• Statement: G3P is the direct output of the Calvin Cycle used to make glucose. Answer: TRUE Why the common mistake happens: Assuming glucose is the immediate product (it’s synthesized from G3P).