5.5—Chloroplast structure and photosynthesis

Chloroplasts organise photosynthesis by separating reactions. The envelope contains the organelle, the stroma provides enzymes for carbon fixation, and thylakoid membranes hold photosystems, electron carriers and ATP synthase.

Grana provide extensive membrane area for absorbing light and building proton gradients, while the small thylakoid space allows a gradient to form quickly. The stroma is the right compartment for the Calvin cycle because its enzymes and substrates are outside the thylakoid lumen.

A defect in ATP synthase would affect ATP production at the thylakoid membrane first; a defect in a stromal Calvin-cycle enzyme would impair carbon fixation even if light absorption continued.

Photosystems are embedded in thylakoid membranes, not freely dissolved in the stroma. Structure supports function, but it does not mean every chloroplast component performs a photosynthetic reaction.