The cytoplasm of living cells is separated from the environment by means of membranes. To accomplish this there have to be defined pores in the membranes. In microorganisms the regulation of the exchange of metabolites depends on the very near surroundings of the cell. In multicellular organisms there is a transport problem between different organs over longer distances, too. In animals the energy supply substance glucose has to be excreted by one kind of cells into the circulatory system, other cells have to import it, both in well tuned regulation.
Plants have to transport sucrose and amino acids between different tissues.
Transport of substances across membranes often requires energy to overcome a concentration gradient. The water balance of higher organisms is actively regulated by the aquaporins, shifting in man in excess of 150 liters per day. In many organisms the genetics of proteins enabling diffusion or transport of all kind of substances through membranes are well understood. Some data were obtained by nuclear magnetic resonance investigations of solubilized proteins. Another rule is typing by the substance moved, e.g. glucose transporter.