Chloroplast: the organelle in plant cells (and algae) where photosynthesis occurs. Chloroplasts contain chlorophyll — the pigment that absorbs light energy — organized in flattened membrane structures called thylakoids, which are often stacked into grana. The thylakoid membranes are the site of the light-dependent reactions, where light energy drives the splitting of water molecules and the generation of ATP and NADPH. These energy carriers then fuel the Calvin cycle in the surrounding stroma, fixing atmospheric carbon dioxide into organic sugars. The entire process converts light energy into chemical energy, providing the foundation for nearly all food webs on Earth.
Like mitochondria, chloroplasts have their own DNA, their own ribosomes, and they divide independently within the cell. This is strong evidence that chloroplasts originated as free-living cyanobacteria that entered into an endosymbiotic relationship with an ancestral eukaryotic cell — an event that occurred roughly 1.5 billion years ago. Over evolutionary time, most of the cyanobacterial genome was transferred to the host cell’s nucleus, but chloroplasts retain enough of their own genetic machinery to make their origin unmistakable. This is one of the most consequential instances of symbiosis in evolutionary history — it made photosynthetic eukaryotes possible and ultimately gave rise to all plant life.
Chloroplasts are concentrated in the mesophyll cells of leaves, where light is most available, and their distribution within the cell is itself regulated — chloroplasts move toward or away from light depending on its intensity, optimizing photosynthesis while avoiding photodamage. The chloroplast is not a static component of the cell but a dynamic, semi-autonomous partner whose activity is coordinated with the cell’s metabolic demands and the plant’s environmental circumstances.
Related terms
- Cell — the chloroplast as a defining organelle of the plant cell
- Symbiosis — endosymbiosis as the origin of chloroplasts
- Photosynthesis as Relation — the process chloroplasts perform
- Leaf — the organ where chloroplasts are most densely concentrated
- Stomata — regulate the carbon dioxide supply that chloroplasts require