Explanation:
Membranes
Recall that phospholipids have a hydrophobic end and a hydrophilic end and that when placed in water they will orient themselves accordingly (5.11 pg 79). This is the basis for the plasma membrane of a cell. The cell membrane consists of a phospholipid bilayer with embedded proteins. We refer to the modern conceptual model of the cell membrane as the "fluid mosaic" model since the phospholipids are able to move about across the surface of the membrane (fluid) and the proteins are many and varied (mosaic) (5.12).
Attached to the some proteins and to some of the phospholipids are oligosaccharides (short polysaccharides). When a protein has an oligosaccharide attached it is called a glycoprotein. Glycolipids are phospholipids with the sugar chains added. These oligosaccharides are found on the outside of the membrane and are used in cell to cell recognition. They differ among species, among individuals and within individuals.
Membrane proteins can have a number of functions, such as transport proteins, enzymes (more on these shortly), receptor sites, cell adhesion, attachment to the cytoskeleton. (5.13)
The most important thing about membranes is that they regulate what moves in and out of a cell. The membrane is selectively permeable because substances do not cross it indiscriminately.
Some molecules, such as hydrocarbons and oxygen can cross the membrane. Many large molecules (such as glucose and other sugars) cannot. Water can pass through between the lipids. Ions such as H+ or Na+ cannot.
Transport proteins make passage possible for molecules and ions that would not be able to pass through a plain phospholipid bilayer. Some transport proteins have a hydrophilic tunnel through them which allows polar molecule or ions to pass. Others actually bind to the molecules and move them across the membrane. In either case transport proteins are very specific.
Passive Transport
