What is Plasma Membrane?
Plasma membrane, also known as the cell membrane, is a protective layer of all living cells. It acts as a barrier between the interior of the cell and its environment. It is selectively permeable membrane which lets only certain substances in and keeps other out. The primary function of the membrane is to protect the cell from its surroundings. It not only protects the cell from potentially harmful substances but also act as an attachment surface for various organelles including glycocalyx, cell wall, and the cytoskeleton. Unlike animals, plants, bacteria and fungi also have an additional protective layer called cell wall.
Function of Plasma Membrane
- As plasma membrane is selectively permeable, thus it facilitates the transport of materials needed for cell’s survival. This transport of materials may be active or passive. Active transport requires the cell to expend energy.
- The plasma membrane also helps the in maintaining cell’s potential. The plasma membrane thus acts as a filter to let only certain things enter or leave the cells.
- It helps in the diffusion of carbon dioxide and oxygen, which is a passive transport process.
- It helps in the endocytosis (the process in which cells absorb surrounding molecules by engulfing them) and exocytosis (the process in which cells extrude its contents to the surrounding environment).
Structure of Plasma Membrane
The cell membrane is basically composed of a mixture of proteins and lipids. Percentage composition of proteins and lipids depends on the location of the membrane. Lipids composition can vary from 20 to 80 percent. Lipids help in the cell’s flexibility while proteins are responsible for various biological activities of the cell including cell to cell contact, cytoskeleton contact, surface recognition, and transporting substances through the membrane.
Fluid Mosaic Model
Fluid mosaic model, proposed in 1972, is currently most accepted model for the structure of plasma membrane. According to the fluid mosaic model, the plasma membrane is made up of a mosaic of phospholipids, proteins, cholesterol, and carbohydrates which can move around freely in the membrane, giving membrane fluid-like characteristics.
A lipid made up of a phosphate group head, two fatty acid tails, and glycerol. Plasma membranes have two layers of phospholipids with their heads pointing to the outer environment and the tails facing each other inside. These phospholipids are the main component of the plasma membrane.
Cholesterol is another lipid found in the plasma membrane along with phospholipids. Cholesterol molecules are placed between the hydrophobic tails of phospholipids.
Membrane proteins may be present on the outer surface or on the inner surface of the membrane, or they may cross the membrane completely. There are two types of proteins found in plasma membrane; (i) integral proteins which may or may not cross the membrane entirely and are embedded in the phospholipid bilayer, (ii) peripheral proteins which are not embedded but are found in the inner or outer surface of the membrane.
Carbohydrates are found only on the outer surface of the membrane. When attached to proteins they form glycoproteins and when attached to lipids form glycolipids.
Plasma membrane serves a variety of functions most important of them is performing barrier functions. They keep the out what is outside and keep in what is inside, allowing only certain things to move on, relay messages or cross the membrane.