There are two major types of cells: prokaryotes and eukaryotes. Eukaryotic cells, whose name derives from the Greek eu, meaning “good,” and karyon, “kernel” or “nucleus,” have a nucleus and membrane-bound organelles. Prokaryotic cells, whose name derives from the Greek pro, meaning “before,” contain neither nucleus nor organelles. As the names imply, prokaryotic cells are less evolutionarily advanced than eukaryotic cells.

Prokaryotes include some of the most primitive forms of life: bacteria and blue-green algae (also known as cyanobacteria). Prokaryotic organisms are generally single-celled.

Prokaryotes have a cell membrane, and they are made up of generally undifferentiated fluid, called the cytoplasm, in which floats a circular ring of DNA that controls the functioning of the cell. Prokaryotes maintain their shape through a cytoskeleton and have ribosomes that float in the cytoplasm. In addition, some prokaryotes have a special type of cell wall made of a protein-sugar combination called peptidoglycan. A few prokaryotes possess whiplike tails called flagella that help propel the cells through water.

Though less complex and less efficient than eukaryotes, prokaryotes are hardy because of their simplicity. They are able to survive environmental extremes that would kill higher life forms.

All living things besides bacteria and cyanobacteria consist of eukaryotic cells, which are larger and structurally more complex than prokaryotic cells. Like prokaryotes, eukaryotes are surrounded by a lipid bilayer cell membrane and have cytoplasm and ribosomes. However, unlike prokaryotes, eukaryotes also contain organelles and a defined nucleus containing DNA.

Eukaryotes benefit enormously from the presence of membrane-bound organelles. Each organelle creates an additional compartment in the cell that can specialize in particular activities or processes, increasing productivity as a result. The structure of eukaryotic cells and the specific functions of the various organelles are often tested by the SAT II Biology.