Online Biology Dictionary

Chlorplasts Chlorplasts within the cells of Plagiomnium affine, Many-fruited Thyme-moss. Image: Kristian Peters

cyanobacterium Chroococcus, a cyanobacterium
Image: X. Vazquez

Chloroplasts belong to a class of organelles known as plastids. They carry out photosynthesis in eukaryotic cells, the essential process that plants use to convert carbon dioxide into water and sugar in the presence of sunlight. As are other types of organelles, a chloroplast is enclosed in a lipid bilayer that forms a limiting membrane. Within is the stroma, a cytoplasm-like fluid.

All plastids of this type contain chlorophyll a (which is why most plants are green). But in some organisms they are not themselves green because they contain additional pigments affecting their color.

These sugar-producing organelles are thought to be the descendants of endosymbiotic cyanobacteria, and have circular chromosomes like those of prokaryotes. This circular genome, termed the plastome, codes mostly for redox proteins involved in electron transport during photosynthesis.

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Within the stroma are the thylakoids, the hollow, nummiform bodies where photosynthesis takes place, which are arranged in a network composed of stacks called grana (sing. granum). Additional structural details of this organelle are shown in the enlargeable diagram at right.

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