In which cellular organelle does photosynthesis occur?

Photosynthesis occurs in the chloroplast, a specialized organelle found in plant and algal cells.

Chloroplasts contain chlorophyll and other photosynthetic pigments, allowing them to capture light and transform its energy into chemical energy. Inside the chloroplast, the light-dependent reactions take place on flattened membranes called thylakoids, which are often arranged in stacks known as grana.

The chloroplast's surrounding fluid, called the stroma, is where the Calvin cycle uses ATP and NADPH from the light reactions to help build carbohydrate molecules from carbon dioxide. Thus, photosynthesis is not one single reaction in one compartment; its stages are organized within different parts of the chloroplast.

Chloroplasts are especially abundant in leaf mesophyll cells, although photosynthesis can also occur in green stems and algae. They are thought to have evolved from ancient cyanobacteria engulfed by an early eukaryotic cell, and they retain their own DNA and ribosomes. Mitochondria make much of a cell's usable energy through respiration, but they are not the organelles where plant photosynthesis occurs.