Phospholipids make up the cell membrane and are different from triglycerides due to the presence of a phosphate group. They arrange themselves into a bilayer with hydrophobic fatty acids in the middle and hydrophilic phosphate groups facing outward. The cell membrane controls what enters and leaves the cell.
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 diverse group of compounds that contain carbon (C), oxygen (O), and hydrogen (H) and are insoluble in water, or 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 liquid.
The chemical structure of lipids varies among many different types. Most are triglycerides or are made up of 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 the characteristics of a lipid. All fatty acids consist of a carboxyl group (-COOH) bonded to a hydrocarbon chain, made up of carbon and hydrogen atoms. Hydrocarbon chains can vary in length and are hydrophobic or water repelling.
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 phosphorus, oxygen and hydrogen. The phosphate group is hydrophilic, meaning 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.
In a water-based solution, phospholipids automatically arrange themselves into a bilayer, or a phospholipid bilayer. Fatty acids, which repel water, are found in the middle of the bilayer. The hydrophilic heads, which contain the phosphate group, face outward along the bilayer and line up on either side of it. Because of this formation, some substances will be able to pass through the bilayer and some will not, so that it serves as a natural boundary.
The function of a cell membrane is to serve as a boundary between the cell’s interior and its surroundings. It consists 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 enable transport across it, both into and out of the cell. In this way, phospholipids and proteins control what enters and leaves the cell.
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