Organelles allow eukaryotes to carry out complex pathways efficiently by keeping all of the pathways' workers and materials together. They also can serve a number of other specialized purposes.

All eukaryotic cells have organelles called mitochondria which carry out the long pathways that generate most of the cell's energy. Plants also contain chloroplasts that contain the workers that let them trap the energy in light to make food. Curiously, both mitochondria and chloroplasts contain their own tiny Instruction Books, distinct from the cell's main set of Instruction Books.

Some cells contain other special-purpose organelles: lysosomes are miniature stomachs that digest food. Fat cells consist largely of an organelle that stores fats for later use.