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Cell, in biology, smallest unit that possesses all the essential properties of a living organism: metabolism, reproduction, differentiation, regeneration, and excitability (response to stimulus). A living cell can also be described as having a flow of matter: Chemicals come into the cell; they are broken down or transformed into other chemicals that then leave the cell. There is also a flow of energy: Energy comes into a living system either as chemical energy or as radiant energy from the sun. However, a third element is crucial: information. A living system is a directed and responsive system in which there is control and regulation of chemical reactions. The cell contains a specific information system: the nucleic acids and the proteins, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). These molecules contain and transmit the genetic information of the cell. All cells have an outer membrane, or cell boundary, cytoplasm (the contents of the cell excluding the nucleus), a cell nucleus, more or less spherical, containing specific molecules, a nucleolus, and organelles (cytoplasmic structures) to which the diverse functions are attributed.

See also: Biology.

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