3.1 - Structure of Plasma Membrane

Plasma membrane plays an important role in most of the biochemical reactions in the body. It regulates all substances moving in and out of a cell. The fluid mosaic model describes the plasma membrane as having a mosaic pattern. The plasma membrane is seemingly floating about while containing molecules such as phospholipids, cholesterols and proteins.

Figure 1 - The plasma membrane is a biological membrane that separates the outer and inner environment. It protects the cell from its environment. It is able to maintain its fluidity at various temperatures.

Each phospholipid molecule is made up of a hydrophilic (water-liking) head and a hydrophobic (water-repelling) tail. The head likes water and therefore faces the outside while the tail faces inside because it dislikes water.

Figure 2 - The presence of phosphate group which is negatively-charged makes the head hydrophilic. The fatty acid chains are uncharged and non-polar which make the tail hydrophobic.

Many different types molecules can be found embedded in the phospholipid bilayer. For example, protein molecules which include channel protein and carrier proteins. Channel proteins are proteins that open a channel to allow molecules to flow into and out of the cell. Carrier proteins only bind to specific molecules and transport them through facilitated diffusion or active transport.

Figure 3 - Channel protein (left) forms a tunnel for solutes to move across the plasma membrane. Carrier protein (right) can change shape to move solutes from one side to the other.

Glycoproteins and glycolipids can also be found at the surface of the plasma membrane. They are oligosaccharides that act as receptors to hormones. They can also help to stabilize the membrane by forming hydrogen bonds with water molecules. Cholesterol is another molecule that can be found between the phospholipid molecules. Cholesterol makes the membrane less permeable to water-soluble substances. It also helps to stabilize, but gives a flexible structure. This is what gives the structure the 'fluid' characteristic.

Figure 4 - The glycoprotein (left) and glycolipid (right) protruding from the surface of the cell membrane helps in cell-cell recognition.

Permeability of a Plasma Membrane

Plasma membrane is a selectively permeable membrane, which means it only allows certain molecules to pass through. Its permeability is determined by the phospholipid bilayer and protein molecules.