n.
In biology, the basic unit of which all living things are composed; the smallest structural unit of living matter that is able to function independently.
A single cell can be a complete organism in itself, as in bacteria and protozoans . Groups of specialized cells are organized into tissues and organs in multicellular organisms such as higher plants and animals. There are two distinct types of cells: prokaryotic cells, found only in bacteria (including blue-green algae, or cyanobacteria ), and eukaryotic cells, composing all other life-forms. Though the structures of prokaryotic and eukaryotic cells differ (see prokaryote , eukaryote ), their molecular compositions and activities are very similar. The chief molecules in cells are nucleic acids , proteins , and polysaccharides . A cell is bounded by a membrane that enables it to exchange certain materials with its surroundings. In plant cells, a rigid cell wall encloses this membrane.