Apentix/AP Biology/Unit 2: Cell Structure and Function/Question
AP Biologymediummcq1 pt

The carbon-fixing step that incorporates CO2 into three-carbon sugars depends on rubisco, an enzyme that floats freely rather than sitting in any membrane. In which chloroplast compartment does this enzyme act?

A.In the stroma, the fluid surrounding the thylakoid membranes
B.In the thylakoid lumen, where stored protons drive the chemistry
C.Embedded in the inner envelope membrane facing the cytosol
D.In the cytosol outside the chloroplast entirely

Explanation

Core Concept

Rubisco catalyzes the Calvin cycle in the stroma, the soluble phase around the thylakoids. That placement lets the enzyme draw ATP and NADPH straight from the light reactions occurring in nearby thylakoid membranes. The three-carbon product then leaves for further synthesis elsewhere in the cell.

Correct Answer

AIn the stroma, the fluid surrounding the thylakoid membranes

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