The plasma membrane is the boundary between the cell and its environment. It regulates what enters and exits the cell. Cells must maintain an appropriate amount of molecules to function inside them. They must also have a way to keep things out or to allow things to enter. This is the job of the plasma membrane. The plasma membrane is like the guard at a gated community. The guard must inspect those who enter and those who leave to make sure that only the people and things needed in the community are there.
To perform these roles, the plasma membrane needs lipids, which make a semi-permeable barrier between the cell and its environment. It also needs proteins, which are involved in cross-membrane transport and cell communication, and carbohydrates (sugars and sugar chains), which decorate both the proteins and lipids and help cells recognize each other.
Here, we’ll take a closer look at the different components of the plasma membrane, examining their roles, their diversity, and how they work together to make a flexible, sensitive, and secure boundary around the cell.
Fluid mosaic model
The currently accepted model for the structure of the plasma membrane, called the fluid mosaic model, was first proposed in 1972. This model has evolved over time, but it still provides a good basic description of the structure and behavior of membranes in many cells.
According to the fluid mosaic model, the plasma membrane is a mosaic of components—primarily, phospholipids, cholesterol, and proteins—that move freely and fluidly in the plane of the membrane. In other words, a diagram of the membrane (like the one below) is just a snapshot of a dynamic process in which phospholipids and proteins are continually sliding past one another.
Interestingly enough, this fluidity means that if you insert a very fine needle into a cell, the membrane will simply part to flow around the needle; once the needle is removed, the membrane will flow back together seamlessly.
- • A phospholipid is a lipid made of glycerol, two fatty acid tails, and a phosphate-linked head group. Biological membranes usually involve two layers of phospholipids with their tails pointing inward, an arrangement called a phospholipid bilayer.
- • Cholesterol, another lipid composed of four fused carbon rings, is found alongside phospholipids in the core of the membrane.
- • Membrane proteins may extend partway into the plasma membrane, cross the membrane entirely, or be loosely attached to its inside or outside face.
- • Carbohydrate groups are present only on the outer surface of the plasma membrane and are attached to proteins, forming glycoproteins, or lipids, forming glycolipids.