13.1.3—Sites within chloroplast

A chloroplast separates photosynthetic work between an internal thylakoid membrane system and the surrounding stroma: thylakoids provide the organised membrane surface for light-dependent reactions, while the stroma provides the fluid reaction space for the Calvin cycle.

• Thylakoid system: flattened, fluid-filled thylakoids contain pigments, enzymes and electron carriers needed for the light-dependent stage. Pigments are arranged in photosystems within the thylakoid membranes, so light energy is captured at this membrane surface.
• Grana and stroma lamellae: stacks of thylakoids form grana, with connecting membrane regions between stacks. The membrane arrangement gives a large surface area and many organised sites for light-dependent reactions.
• Thylakoid spaces: the internal spaces of the thylakoids are part of the light-dependent system; keeping them enclosed allows the membrane system to separate reaction spaces.
• Stroma: the fluid surrounding the thylakoids contains dissolved carbon dioxide, sugars, enzymes and other molecules. It is the site of the light-independent stage, including the Calvin cycle.
• Functional boundary: thylakoid membranes capture light and transfer energy, whereas the stroma provides the separate environment for carbon fixation. The ATP and reduced NADP made by the light-dependent stage connect these regions without making them the same reaction space.

Do not place photosynthetic pigments in the stroma or treat grana as the site of the Calvin cycle. This card establishes structure and location; detailed pigment spectra, chromatography and individual photochemical steps belong to neighbouring cards.