ABSTRACT
Every living organism is an open system that exchanges matter and energy with the environment. In various ways the organism takes all the chemical substances it needs for the composition of its body from the environment and casts off into the environment the substances that are no longer useful for its functions. A feature of organisms is that they are composed of one or more cells, which are either similar to or different from each other and perform different functions. All the single-celled organisms, along with organisms composed of more than one cell but without differentiated functions, are characterised by a small degree of complexity. Proteins are polymeric compounds, usually of great complexity, with high molecular weight. The nucleic acids are extremely complex compounds composed of hundreds to thousands of nucleotides. The energy produced by the energy reactions is transported in the form of chemical energy successively to the various organic compounds, generally with small thermal losses.
