Transport across the plasma membraneThe plasma membrane covers all living cells, allowing the contents of cells to be held together and controls the movement of substances into and out of the cell. Plasma membranes are made up of phospholipids, proteins and carbohydrates. Phospholipids are essentially made up of two chains of fatty acids and a phosphate-glycerol group. They are arranged in a bilayer with the hydrophilic phosphate head facing outward and the hydrophobic fatty acid chains facing inward and toward each other in the middle of the bilayer. This effectively provides a barrier against all but the smallest molecules. Within the membrane are proteins that have a number of functions. Those that span the entire bilayer (from hydrophobic head to hydrophobic head) act as channels and carriers to transport substances across the membrane in active or facilitated diffusion. Those on the surface act as receptors with specific binding sites where hormones and other chemicals can bind. This binding in turn triggers other events in the cell. Finally, those found on the inner surface maintain the shape and motility of the cell, such as enzymes. Glycoproteins and glycolipds are found on the surface of the cell. They form a cell envelope (glycocalyx) and are important in cell protection and recognition. The bilayer is described as fluid since the lipids it contains are able to slide and exchange places freely. This feature allows processes such as phagocytosis to occur. There are five main ways of transport across cells: Simple diffusion; The only substances that can diffuse directly into the middle of the paper......near the cell, the membrane forms a slight depression or pit. When close enough, they are surrounded by a fold of membrane, which then closes to form a closed vesicle. As the vesicle enters the cell, the molecules are digested and the molecules produced are released. When the materials are small particles and often liquid, endocytosis is known as pinocytosis. When materials consist of large particles and often solids, the process is called asphagocytosis. Exocytosis is the transport of materials out of a cell. This is exactly the opposite of endocytosis. The molecules to be transported must first be enclosed in a membrane vesicle, usually originating from the Golgi apparatus and the RER. It then moves to the membrane and fuses with it, expelling the substances. These processes are controlled and require energy..