Cells

A cell is the smallest unit that exhibits all of the qualities associated with the living state. Cells must obtain energy from an external source to carry on such vital processes as growth, repair, and reproduction. All of the chemical and physical reactions that occur in a cell to support these functions constitute its metabolism. Metabolic reactions are catalyzed by enzymes. Enzymes are protein molecules that accelerate biochemical reactions without being permanently altered or consumed in the process. The structure of each enzyme (or any other protein) is encoded by a segment of a deoxyribonucleic acid (DNA) molecule referred to as a gene.

Molecular and cell biology are the sciences that study all life processes within cells and at the molecular level. In doing so, these sciences draw upon knowledge from several scientific disciplines, including biochemistry, cytology, genetics, microbiology, embryology, and evolution.