Lesson 4: Light-independent Reactions

Calvin Cycle

Takes place in the stroma of chloroplasts.

fixation, reduction, and regeneration

The light-independent reactions of the Calvin cycle can be organized into three basic stages:_______.

Stage 1: Fixation

CO₂ enters leaves through stomata, diffuses to the chloroplast stroma. Enzyme RuBisCO and 3 molecules of RuBP (Ribulose bisphosphate).

Stage 1: Fixation

RuBisCO catalyzes the reaction between CO₂ and RuBP.

Produces two 3-carbon molecules of 3-PGA (3-Phosphoglycerate) per CO₂ molecule.

Maintains carbon balance (3 CO₂ + 3 RuBP → 6 3-PGA).

Stage 2: Reduction

Inputs: 6 molecules of ATP and NADPH.

Stage 2: Reduction

3-PGA is converted to G3P (Glyceraldehyde-3-phosphate). NADPH becomes NADP⁺ and returns to light-dependent reactions. Electrons are gained from NADPH. ATP loses a phosphate (converted to ADP).

Stage 3: Regeneration

Output: 1 G3P molecule is exported to the cytoplasm for forming carbohydrates.

Stage 3: Regeneration

5 remaining G3P molecules are recycled to regenerate RuBP (requires 3 ATP). 3 turns of the cycle produce 6 G3P, but only 1 is exported.

Photosynthesis

Converts CO₂ and sunlight into carbohydrates, releasing O₂.

Respiration

Breaks down carbohydrates using O₂, releasing CO₂.