Chloroplast

Double-membrane organelle containing an internal system of flattened membrane-bound sacs called thylakoids, which are arranged into stacks called grana. The fluid-filled region surrounding the thylakoids is called the stroma and contains enzymes, circular DNA and 70S ribosomes. Chlorophyll pigments are embedded within the thylakoid membranes.

Site of photosynthesis. The light-dependent reactions occur in the thylakoid membranes, where light energy is converted into chemical energy in the form of ATP and NADPH. The Calvin cycle occurs in the stroma, where carbon dioxide is fixed into organic molecules using ATP and NADPH.

Photosynthetic plant cells and many algae. Not present in animal cells or prokaryotic cells.

Chloroplasts enable photosynthetic organisms to convert light energy into chemical energy stored in organic compounds. These compounds provide carbon skeletons and chemical energy for growth and metabolism, while photosynthesis also releases oxygen.

Thylakoid membranes contain photosystems, electron carriers and ATP synthase for light dependent reactions; stroma contains enzymes for carbon fixation.