Van Helmont, chloroplasts and the light stage

Photosynthesis happens in green plastids called chloroplasts, mostly in leaf cells. Plastids are small organelles in plant cells. Chromoplasts hold coloured pigments, as in plum skin and petals. Leucoplasts are colourless, as in a potato. A chloroplast has a double membrane around it. Inside is a fluid, the stroma, which contains enzymes for the light-independent reactions. Floating in the stroma are stacks of flat sacs called grana, each sac a thylakoid, whose membranes hold chlorophyll. The light-dependent reactions happen in the grana. We cannot draw a chloroplast honestly as a photograph here, so we use a diagram of boxes. Chloroplasts are found in leaves and green stems, and also in green algae, but not in roots buried in soil. A leaf is a good place for photosynthesis: it is flat and thin, so light and carbon dioxide reach every cell.