Organelle

An organelle is a specialized structure inside a cell that performs a particular job, much as an organ does for the body. The name means "little organ." Most are enclosed by membranes, though some, like ribosomes, are not.

The nucleus holds the chromosomes. Mitochondria make most of a cell's ATP, and chloroplasts in plants and algae carry out photosynthesis. The endoplasmic reticulum and Golgi apparatus make and sort proteins and lipids, lysosomes digest waste, and peroxisomes break down fatty acids and hydrogen peroxide. Plant cells also have a large central vacuole.

Ribosomes, the centrosome and the nucleolus lack a surrounding membrane. A newer view recognizes membraneless compartments, called condensates, that form when proteins and RNA separate out of solution like oil droplets in water. Prokaryotes have few if any membrane-bound organelles, though bacteria contain protein-walled compartments such as carboxysomes.

Mitochondria and chloroplasts were once free-living bacteria, according to endosymbiotic theory, whereas the other membrane-bound organelles probably arose from folds of the cell's own membranes, part of the endomembrane system. Scientists separate organelles by spinning broken cells in a centrifuge, a method developed in the mid-twentieth century. The overall layout of cells is described in cell theory and architecture.

• Cell — the unit that contains them
• Mitochondria — the energy organelle
• Endomembrane System — the connected set of membrane organelles