Chloroplast. Coloured scanning electron micrograph (SEM) of a section through a plant’s chloroplast. Chloroplasts are the site of photosynthesis, the process that synthesises carbohydrates from carbon dioxide and water using sunlight. The pigment responsible for photosynthesis, chlorophyll, is found on stacks of parallel thylakoids (membranes, brown lines) called grana.

Credit: DR DAVID FURNESS, KEELE UNIVERSITY/SCIENCE PHOTO LIBRARY