A cell is the smallest biological unit that can maintain a controlled internal chemical environment while carrying out the processes needed for its continued existence and reproduction.

Every cell is bounded by a plasma membrane. The membrane separates the cell interior from the surrounding environment while allowing selected matter and information to cross.

Inside the membrane is an aqueous interior containing ions, small molecules and larger biological molecules. The fluid portion is called the cytosol.

Despite enormous diversity, all known cells share several basic features:

• a plasma membrane that defines an inside and an outside;
• DNA, a nucleotide polymer that stores hereditary sequence information;
• ribosomes, molecular assemblies that build proteins according to encoded sequences;
• catalysts and reaction networks that transform matter and energy;
• mechanisms that maintain internal conditions different from the environment.

The last point is essential. A cell is not merely a bag of molecules at equilibrium with its surroundings. Cells continually exchange matter and energy while using membrane transport, reactions and regulation to maintain nonequilibrium states such as ion gradients and unequal small-molecule concentrations.

Compartment boundaries create chemical control

A membrane can keep some substances at different concentrations on its two sides. That allows reactions to be localized, incompatible processes to be separated and gradients to be used as stored thermodynamic driving forces.

In cells with little internal membrane compartmentalization, many reactions occur in the cytosol or at the plasma membrane. Eukaryotic cells contain additional membrane-bounded compartments that create several chemically distinct internal environments.

The concept of a cell is therefore fundamentally organizational: a cell combines a selectively bounded chemical system, encoded information and reaction networks into a unit capable of maintaining and reproducing that organization.