chloroplast

Chloroplasts are organelles in plant cells and many algae that carry out photosynthesis, converting light energy into chemical energy stored as sugars. They contain chlorophyll and other pigments that capture light. Chloroplasts are a type of plastid and are typically green, though pigment variation occurs.

Structure: Chloroplasts are enclosed by a double membrane and contain a circular genome and ribosomes. The interior space, the stroma, surrounds thylakoid membranes arranged into grana. The thylakoid membranes host the photosystems and electron transport chain; the space inside thylakoids is the lumen. The stroma contains enzymes of the Calvin cycle.

Function: The primary role is photosynthesis. Light-dependent reactions produce ATP and NADPH in the thylakoids, which power carbon fixation by the Calvin cycle in the stroma to synthesize sugars. Chloroplasts also synthesize fatty acids, amino acids, and pigments, and they can store starch.

Origin and inheritance: Chloroplasts are thought to originate from cyanobacteria via endosymbiosis. They divide by binary fission and rely on many nuclear-encoded proteins imported into the organelle. In many plants, chloroplast DNA is inherited maternally.

Diversity and evolution: Chloroplast structure and function vary among photosynthetic lineages; some algae have different thylakoid arrangements. Non-photosynthetic plastids occur in parasitic species, highlighting plastid evolution and adaptation.