Active Transport (Cellular Biology).

Biologists in nearly every field of study have discovered that one of the major methods by which organisms regulate their metabolisms is by controlling the movement of molecules into cells or into organelles such as the nucleus. This regulation is possible because of the semipermeable nature of cellular membranes. 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 channels and carriers.