Grænukornets

Grænukornets (chloroplasts) are essential organelles in the cells of plants and algae that carry out photosynthesis. They capture light energy and convert it into chemical energy, producing sugars from carbon dioxide and water while releasing oxygen. In addition to photosynthesis, chloroplasts synthesize fatty acids, amino acids, and pigments, and they can store starch.

Chloroplasts are bounded by a double membrane envelope. Inside is the stroma, a fluid containing enzymes, small ribosomes, and the chloroplast DNA. Dispersed within the stroma are thylakoid membranes, organized into stacked grana connected by lamellae. The chlorophyll and other pigments reside in the thylakoid membranes, where the light-dependent reactions generate ATP and NADPH, which drive the Calvin cycle in the stroma to fix CO2 into sugars. Most chloroplast proteins are encoded in the plant nucleus and imported into the organelle via the TOC/TIC translocon complexes; the chloroplast retains a small, circular genome and ribosomes for a limited set of internal gene expression.

Chloroplasts are believed to have originated from cyanobacteria through endosymbiosis and subsequently evolved to lose many ancestral genes to the host nucleus, becoming semi-autonomous. They replicate by division and are typically inherited maternally in many plants, though inheritance patterns vary.

Different plastid forms arise from chloroplasts in different tissues, such as amyloplasts that store starch or chromoplasts containing colored pigments in fruits and flowers.