Animation showing changing light levels within a chloroplast. Chloroplasts are the organelles within plant cells responsible for photosynthesis. Inside each chloroplast are stacks called grana, made up of disc_shaped thylakoids linked by bands called lamellae. The membrane of the thylakoid is the site of the photosynthetic reactions, where light energy is captured by the green pigment chlorophyll and converted into chemical energy. Plants use this energy to capture atmospheric carbon dioxide and use it to build up the sugars it uses as food.