Phospholipids are just one type of a large group of organic compounds called lipids. The main role of phospholipids in living organisms is that they make up the cell membrane. Lipids form a large and varied group of compounds that contain carbon (C), oxygen (O) and hydrogen (H) and are insoluble in water, or they do not dissolve in water. All lipids fall into one of two groups – fats or oils. The only difference between these groups is that fats are solid at room temperature while oils are a liquid.
The chemical structure of lipids varies between the many different types. Most are triglycerides, or they are formed from three fatty acid chains attached to a glycerol molecule. All lipids contain the same glycerol molecule, so it is the nature of the fatty acids that determines a lipid’s characteristics. All fatty acids are made up of a carboxyl group (-COOH) attached to a hydrocarbon chain, made up of carbon and hydrogen atoms. The hydrocarbon chains can vary in length and are hydrophobic, or repel water.
Phospholipids are not triglycerides. There is a small but very important difference in the structure of phospholipids compared to triglycerides. Instead of having three fatty acids attached to the glycerol molecule, one is replaced by a phosphate group, made up of phosphorous, oxygen and hydrogen. The phosphate group is hydrophilic, or it is attracted to water, in contrast to the rest of the molecule. The fact that one part of the molecule attracts water while the rest repels it affects the role of phospholipids in the cell membrane.
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In a water-based solution, the phospholipids automatically arrange themselves into a double layer, or a phospholipid bilayer. The fatty acids, which repel water, are found in the middle of the bilayer. The hydrophilic heads, which contain the phosphate group, face outwards along the bilayer and line either side of it. Due to this formation, some substances will be able to pass through the bilayer and others will not, so that it acts as a natural border.
The function of a cell membrane is to act as a boundary between the inside of the cell and its surrounding environment. It is made up almost entirely of a phospholipid bilayer and proteins. Molecules that are water soluble, or dissolve in water, are unable to pass through the phospholipid bilyer due to the hydrophobic interior. The role of some of the proteins found within the bilayer is to allow transport across it, both into and out of the cell. In this way, the phospholipids and proteins control what enters and leaves the cell.