chloroplasts

Chloroplasts are organelles found in the cells of plants and green algae, and in some other protists. They capture light energy and convert it into chemical energy via photosynthesis, producing ATP and NADPH that power the synthesis of sugars from carbon dioxide.

Chloroplasts are bounded by two membranes and contain an internal fluid called the stroma. They also house thylakoid membranes organized into stacks known as grana, with spaces between the membranes called the thylakoid lumen. The chloroplast contains its own circular DNA, ribosomes, and pigments such as chlorophyll a and chlorophyll b, along with carotenoids that assist in light harvesting and protection from excess light.

The light-dependent reactions occur in the thylakoid membranes, where absorbed light drives electron transport and the production of ATP and NADPH. The Calvin cycle, which fixes carbon dioxide into sugars, takes place in the stroma. In addition to photosynthesis, chloroplasts synthesize fatty acids, certain amino acids, and plant pigments, and they participate in signaling and various stress responses.

Chloroplasts originated from cyanobacteria through endosymbiosis. Their genomes are circular and encode a subset of essential genes; most chloroplast proteins are encoded in the nucleus and imported into the organelle. Chloroplasts divide by binary fission and are inherited through cellular generations in patterns that vary among species, often exhibiting maternal inheritance in plants.

In non-photosynthetic parasitic plants, chloroplasts can be reduced. Among algae and other groups, chloroplasts may arise from secondary or tertiary endosymbiosis and can display additional membranes or multiple plastid lineages, reflecting diverse evolutionary histories.