Chloroplast. Cutaway computer illustration showing the internal structure of a chloroplast. Chloroplasts are the organelles in plant cells responsible for photosynthesis. The cell membrane encloses the granum - stacks of thylakoids (green discs), the plastidial DNA (deoxyribonucleic acid, yellow loop), ribosomes (brown) and starch grains (magenta). The thylakoid stacks are the site of photosynthesis - the capturing of energy from sunlight using the green pigment chlorophyll. This energy can be stored in the form of starch for later use.