Correct Answer:
B. Stroma
The correct answer is Stroma. The stroma is the dense, colorless fluid that fills the inner space of chloroplasts, surrounding the thylakoid membranes. It is within this semi‑aqueous matrix that the Calvin cycle takes place—the set of light‑independent reactions that fix carbon dioxide and produce sugar.
Why the Stroma, Not Other Parts?
- Site of the Calvin Cycle: The enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) captures CO₂ in the stroma. Through a series of reactions, carbon is reduced and ultimately converted into glucose and other carbohydrates. This is the true "sugar factory" of the chloroplast.
- Lumen – Proton Reservoir: The thylakoid lumen is the space inside the thylakoid sacs. Its primary role is to accumulate protons during the light reactions to generate a gradient for ATP synthesis, not carbon fixation.
- Thylakoid – Light Reactions: The thylakoid membrane houses the photosystems and electron transport chains that convert light energy into ATP and NADPH. These energy carriers are then consumed in the stroma during the Calvin cycle.
- Outer Membrane – Barrier Only: The outer membrane of the chloroplast is a porous envelope that allows passive transport of small molecules. It plays no direct role in photosynthesis or sugar manufacture.
Thus, CO₂ is fixed and sugar is manufactured in the stroma of the chloroplast.