<P> The movements of most solutes through the membrane are mediated by membrane transport proteins which are specialized to varying degrees in the transport of specific molecules . As the diversity and physiology of the distinct cells is highly related to their capacities to attract different external elements, it is postulated that there is a group of specific transport proteins for each cell type and for every specific physiological stage . This differential expression is regulated through the differential transcription of the genes coding for these proteins and its translation, for instance, through genetic - molecular mechanisms, but also at the cell biology level: the production of these proteins can be activated by cellular signaling pathways, at the biochemical level, or even by being situated in cytoplasmic vesicles . </P> <P> While studies on membrane permeability and osmosis dates back to the 18th century, works on membrane transporters or carriers begun in the 1930s . </P> <P> Thermodynamically the flow of substances from one compartment to another can occur in the direction of a concentration or electrochemical gradient or against it . If the exchange of substances occurs in the direction of the gradient, that is, in the direction of decreasing potential, there is no requirement for an input of energy from outside the system; if, however, the transport is against the gradient, it will require the input of energy, metabolic energy in this case . For example, a classic chemical mechanism for separation that does not require the addition of external energy is dialysis . In this system a semipermeable membrane separates two solutions of different concentration of the same solute . If the membrane allows the passage of water but not the solute the water will move into the compartment with the greatest solute concentration in order to establish an equilibrium in which the energy of the system is at a minimum . This takes place because the water moves from a high solvent concentration to a low one (in terms of the solute, the opposite occurs) and because the water is moving along a gradient there is no need for an external input of energy . </P> <P> The nature of biological membranes, especially that of its lipids, is amphiphilic, as they form bilayers that contain an internal hydrophobic layer and an external hydrophilic layer . This structure makes transport possible by simple or passive diffusion, which consists of the diffusion of substances through the membrane without expending metabolic energy and without the aid of transport proteins . If the transported substance has a net electrical charge, it will move not only in response to a concentration gradient, but also to an electrochemical gradient due to the membrane potential . </P>

When does transport across a biological membrane require energy
find me the text answering this question