Structure of the Cell Membrane

Phospholipid Bilayer

The cell membrane is made up of a phospholipid bilayer. Phospholipids are lipid molecules made up of a phosphate group head and two fatty acid tails. Importantly, the properties of phospholipid molecules allow them to spontaneously form a double-layered membrane.

The phosphate group head of a phospholipid is hydrophilic, whereas the phospholipid tail is hydrophobic. This means that the phosphate group is attracted to water, whereas the tail is repelled by water.

When in water or an aqueous solution (including inside the body) the hydrophobic heads of phospholipids will orient themselves to be on the inside, as far away from the water as possible. In contrast, the hydrophilic heads will be on the outside, making contact with the water. The result is that a double layer of phospholipids is formed, with the hydrophobic heads clustering together in the center, and the hydrophilic tails forming the outside of the structure. The technical term for this double layer of phospholipids that forms the cell membrane is a phospholipid bilayer.

Structure of the cell membrane and its associated components

Membrane-Associated Factors

In addition to the phospholipid bilayer, the cell membrane also contains lipid molecules, particularly glycolipids and sterols. One important sterol is cholesterol, which regulates the fluidity of the cell membrane in animal cells. When there is less cholesterol, membranes become more fluid, but also more permeable to molecules. The amount of cholesterol in the membrane helps maintain its permeability so that the right amount of molecules can enter the cell at a time.

The cell membrane also contains many different proteins. Proteins make up about half of the cell membrane. Many of these proteins are transmembrane proteins, which are embedded in the membrane but stick out on both sides (i.e., they span across the entire lipid bilayer).  Some of these proteins are receptors, which bind to signal molecules. Others are ion channels, which are the only means of allowing ions into or out of the cell. Scientists use the fluid mosaic model to describe the structure of the cell membrane. The cell membrane has a fluid consistency due to being made up in large part of phospholipids, and because of this, proteins move freely across its surface. The multitude of different proteins and lipids in the cell membrane give it the look of a mosaic.