Phospholipids are a type of lipid (a lipid being a fat, a sterol, or a wax among other things), which constitute a major part of a cell membrane. Most phospholipids are made up of two fatty acid molecules (hydrophobic end) and a glycerol (hydrophilic end). Many times, the glycerol molecule is attached to choline. The presence of the glycerol and a phosphate group make one end of the phospholipid water loving (hydrophilic), while the other end that has fatty acid chains works to repel water (hydrophobic).
Because of its lipid bilayer, as it is known, phospholipids perform many functions in a cellular atmosphere, including a role in the mitochondria, an organelle within the cell. Phospholipids perform an important function inside of the cell as it can line the outside of organelles, such as the mitochondria, but also the nucleus. The presence of these phospholipids allows for the compartmentalization of what is inside of it. By keeping things inside of the organelle, the phospholipids allow for whatever process is going on in the organelle, to stay in the organelle where it belongs.
For the most part, the phospholipid membrane functions to keep the cell separate from its outer environment. This membrane is semi-permeable, allowing for only certain substances to enter or leave the cell. Much of the phospholipid membrane’s functioning is owed to the membranes permeability and its ability to move fluently. The arrangement of water-loving heads (hydrophilic) and water-hating (hydrophobic) tails of the lipid bi-layer prevent such things as amino acids, carbohydrates, nucleic acids, and proteins from moving across the membrane by diffusion. Proteins present in the phophoslipid molecule also help to regulate what goes in and out of the cell. However, the membrane usually allows for the passive diffusion of certain other molecules. Because the phospholipids form a bi-layer (two layers), the hydrophilic and hydrophobic layers line up together facing the same way, making the membrane barrier. The lipid bi-layer is usually held together by non-covalent bonds, helping to prevent adjacent molecules from sticking to each other.
Hydrophobic molecules, those that repel water, can consist of non-polar organic molecules such as alkanes and fats. These molecules are greasy, can easily repel water, form groups known as micelles and are non-polar. Hydrophilic molecules on the other hand are polar, easily dissolvable in water and contain hydrogen bonds. The phospholipid membrane is also important in maintaining the normal pH, which keeps the cell functioning properly. Again, this is accomplished by its selective permeability.