Cloroplaste

Cloroplaste (chloroplast) is an organelle found in the cells of plants and many algae. It contains the green pigment chlorophyll and is the site of photosynthesis, the process that converts light energy into chemical energy stored as ATP and NADPH. Through the Calvin cycle, chloroplasts also help fix carbon dioxide into sugars, supporting plant growth and metabolism. In addition to photosynthesis, they contribute to the synthesis of fatty acids, amino acids and various secondary metabolites.

Structurally, a cloroplaste is enclosed by a double membrane. Inside, thylakoid membranes form flattened sacs that organize into stacks called grana, connected by stroma lamellae. The fluid surrounding the thylakoids is the stroma, where the Calvin cycle takes place. The internal compartments house photosystems, electron transport chains, and the chemical environment needed for light reactions. Chloroplasts contain their own circular DNA and ribosomes, and they may store starch as granules in the stroma. They also harbor other pigments besides chlorophyll, such as carotenoids, which broaden light absorption and protect against damage.

Functionally, chloroplasts capture light energy and drive the light-dependent reactions to produce ATP and NADPH. These energy carriers power the fixation of CO2 into sugars in the Calvin cycle. They are semi-autonomous, relying on many nuclear-encoded proteins imported into the organelle, and they can divide by binary fission to increase their numbers as the plant grows.

Evolutionarily, chloroplasts are widely believed to have originated from an ancient endosymbiotic cyanobacterium. They retain a reduced genome and specialized protein synthesis machinery, reflecting a long coevolution with the host cell. Chloroplasts differentiate into other plastid types, such as chromoplasts and leucoplasts, depending on tissue and developmental stage.