The chemical complexity required to pass from prebiotic chemistry to life requires the concentration of a large number of compounds into a small space. The cell avoids these molecules being scattered within the solvent. The cell membrane separates these molecules from the exterior medium. At the centre of a cell, DNA molecules store the genetic information; they are surrounded by a solution of proteins and amino acids, and are protected by the cellular membrane, itself consisting of lipids and proteins, carbohydrates and amino acids. The cell is the basic structure of all living organisms that we know about, whether we are dealing with monocellular organisms or complex systems such as humans, who each contain some 1012 individual cells. Viruses do not consist of cells, but parasitize them to achieve their own life-cycle.
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