kloroplast

Kloroplasts are organelles within the cells of plants and many algae that carry out photosynthesis, the process by which light energy is converted into chemical energy. In these organelles, light-dependent reactions occur in the thylakoid membranes, organized into stacks called grana. These reactions produce ATP and NADPH, which are used by the Calvin cycle in the stroma to synthesize sugars from carbon dioxide. Chlorophyll and other pigments in the chloroplasts capture light energy and contribute to the green color of many plants.

Structurally, kloroplasts are bounded by a double membrane and contain an internal set of membranes and compartments. The stroma is the fluid-filled space outside the thylakoids and contains enzymes, ribosomes, and a circular DNA molecule. They are semi-autonomous and can grow and divide within the cell; many proteins are encoded in the chloroplast genome, but most are imported from the nucleus via TOC/TIC translocon complexes.

Kloroplasts are believed to have originated from endosymbiotic cyanobacteria and are classified as plastids, a broader family of related organelles. They are most abundant in photosynthetic tissues, especially leaf mesophyll cells, but occur in other green tissues and in many algae. In addition to photosynthesis, chloroplasts participate in the synthesis of fatty acids, amino acids, and various signaling pathways that respond to environmental conditions. Light conditions influence their development, movement, and gene expression, enabling adaptation to changing light environments.