The fundamental unit of life is the cell. Cells are the smallest structural and functional units of living organisms, capable of performing all life processes. There are two main types of cells: prokaryotic and eukaryotic. Prokaryotic cells, found in single-celled organisms like bacteria, lack a nucleus, whereas eukaryotic cells, which make up multicellular organisms—including plants and animals—contain a nucleus and various membrane-bound organelles. Cells can carry out functions such as metabolism, energy production, and reproduction. Understanding cell structure and function is crucial in biology, as it forms the basis for all higher levels of organization in living organisms.