Active Transport In Biological Systems

Energy-requiring process by which molecules or ions are transported across the membranes of living cells against a concentration gradient .It is particularly important in the uptake of food across the gut lining, in the re-absorption of water and salts from the urine in the kidney before excretion, in secretion of substances from gland cells, in the transmission of nerve impulses and in the uptake of minerals by the plant root. The energy is supplied by the chemical Adenosine Tri Phosphate (ATP), which is broken down by an enzyme at the site of active transport to form Adenosine Di Phosphate (ADP), with the release of energy. Active transport enables cells to maintain an internal chemical environment which is of a different composition from that of their surroundings.

Active transport can be defined as the energy-requiring activity of cells which involves movement of a metabolite or an inorganic ion across a membrane against a gradient of concentration. Nearly 70 % of the available metabolic energy is utilized for active transport processes.

Different functions of Active Transport Systems:

Active-transport systems have many important functions in addition to the transport of fuels and essential nutrients from the surrounding medium. They participate in the maintenance of metabolic steady states by their ability to maintain the internal concentration of organic nutrients and metabolites relatively constant in the face of various changes in the environment.

Active-transport systems are involved in the transmission of information by the nervous system and in the excitation and relaxation cycle of muscle tissue and they also function in the absorptive activity of intestinal epithelia and secretory activity of the kidneys. They also have a very important role in the conversion of the energy of electron transport into the chemical energy of ATP during oxidative and photosynthetic phosphorylation. Active transport systems also maintain constant and optimal internal concentrations of inorganic electrolytes.

Most active-transport systems in cell membranes can drive transport against rather high gradients of concentration as in the case of sugars and amino acids.

Characteristics of Active Transport:

• All mediated-transport processes across biological membranes share the three characteristic properties of saturability, substrate specificity and specific inhibition.
• The first and important requisite is whether a transport system occurs up or down the gradient.
• The second criterion of an active-transport process is its dependence on metabolic energy.
• Active-transport systems are unidirectional in their normal energy requiring function-they will move the substrate across the membrane in only direction.

Much progress has been into the active-transport systems in bacteria as the transport systems are very powerful due to the high rate of metabolism.