13.1.1—Chloroplast structure-function relationship

A chloroplast is compartmentalised so light energy is captured at an internal membrane system while carbon-fixation reactions occur in the surrounding stroma. Its structure therefore supports the transfer of light energy into chemical energy.

• Envelope: a double phospholipid-bilayer envelope surrounds the chloroplast and separates its internal reaction spaces from the cytoplasm.
• Stroma: the fluid-filled stroma surrounds the internal membranes and is the compartment for the light-independent reactions; carbon dioxide, enzymes and products are dissolved there.
• Thylakoid membranes: flattened fluid-filled thylakoids form an internal membrane system. The membranes contain photosynthetic pigments, electron carriers and enzymes for light-dependent reactions.
• Grana and connections: stacks of thylakoids form grana, linked by stroma lamellae. The extensive membrane area provides many sites for light capture and electron-transfer reactions.
• Functional boundary: pigments are located in the thylakoid membrane system, whereas the stroma provides the separate reaction environment for carbon fixation. This is a location/function boundary, not a complete description of every photosynthetic step.

Do not treat the chloroplast as one undivided space, place pigments in the stroma, or assume the double envelope itself is the light-capturing surface. Detailed pigment spectra, chromatography, photophosphorylation pathways and Calvin-cycle carbon accounting belong to neighbouring cards.