Active transport is the process by which cells expend energy to move atoms ormolecules across membranes, requiring the presence of a protein carrier, which is activated by ATP. Cotransport is active transport that uses a carrier that must simultaneously transport twosubstances in the same direction. Countertransport is active transport that employs a carrier that must transport two substances in opposite directions at the same time.

Biologists in nearly every field of study have discovered that one of the major methods by which organisms regulate their metabolisms is by controlling themovement of molecules into cells or into organelles such as the nucleus.

This regulation is possible because of the semipermeable nature of cellularmembranes. The membranes of all living cells are fluid mosaic structures composed primarily of lipids and proteins. The lipid molecules are aliphatic, which means that their molecular structure exhibits both a hydrophilic (water-attracted) and a hydrophobic (water-repelling) portion.

These aliphatic molecules form a double layer: The hydrophilic heads are arranged opposite one another on the inner and outer surfaces, and the hydrophobic tails are aligned across from one another within the interior, sandwiched between the hydrophilic heads. The protein in the membrane is interspersed periodically throughout the lipid bilayer.

Some of the protein, referred to as peripheral protein, penetrates only one of the lipid layers. The integral protein, as the remaining protein is called, extends through both layers of lipid to interface with the environment on both the internal and external surfaces of the membrane. These integral proteins can serve as transport channelsand carriers.

Cellular Energy
 

Transport across the membrane is accomplished by three different mechanisms: simple diffusion, facilitated diffusion, and active transport. The first two mechanisms are referred to as passive processes because they do not require the direct input of cellular energy, and they involve transport down a concentration gradient, that is, from the side with a higher concentrationto the side with a lower concentration of the substance being transported.

In many instances, however,substances are transported across amembrane from the side with a low concentration to the side containing a greater concentration. This "uphill" movement across membranes is called active transport, and it requires the expenditure of cellular energy.

Cellular energy, produced by the biological oxidation of fuels such as carbohydrates, is stored as adenosine triphosphate (ATP). When this high-energy phosphate is hydrolyzed, the stored energy is released to drive cellular reactions such as active transport. The ATPase protein located in membranes belongs to one of the groups of enzymes which hydrolyze ATP.

The mechanism has not been completely deciphered, but it appears as though a protein carrier molecule binds with the substance to be transported at