To most people, the fundamental division between living organisms is that between plants and animals. However, there is a far older and more fundamental separation in the history of life, compared to which plants versus animals becomes only a difference in life styles. This is the division between procaryotes and eucaryotes, that is, between cells without nuclei and those with nuclei. The procaryotes (pre-nuclei) include bacteria and blue-green algae. Their DNA is clustered in the cell fluid without any surrounding boundary or membrane. The metabolic machinery is similarly spread out in the cell: glucose breakdown and energy extraction, photosynthesis if present, and all other processes. There is little that could be called internal structure in a bacterial cell.

The eucaryotes (good nuclei) include green algae, fungi, protozoa, and all other plants and animals. In these organisms the DNA is organized into chromosomes and is confined within a nucleus except during cell division. The initial breakdown of glucose to pyruvic acid takes place in the cell fluid, or cytoplasm, but respiration (combustion with 0₂) occurs in special organelles within the cell called mitochondria. Similarly, if photosynthesis is present, it takes place in other cell organelles known as chloroplasts (next page). Eucaryotes represent a more recent and more developed organization for living cells.