3.1—Cells as the basis of life

Cell theory states that all living organisms consist of one or more cells, cells are the basic functional units of life, and new cells arise from pre-existing cells.

All cells share a cell-surface membrane, cytoplasm, DNA and ribosomes. The rest of the ultrastructure differs: eukaryotes have membrane-bound organelles, while prokaryotes lack internal membrane compartments and have smaller ribosomes.

A bacterium and a plant cell look very different, but both contain DNA, ribosomes, cytoplasm and a boundary membrane. Those shared features support the common-cell principle; they do not imply identical cell functions.

Cell theory is a unifying model, not a claim that every cell has the same organelles. Viruses are not cells and therefore are not explained as living cellular organisms by this theory.