Every type of living cell, either simple prokaryote or advanced eukaryote, has a membrane that separates the cell constituents from the external environment. This membrane is known by different names - cell membrane, plasma membrane or simply, plasmalemma. In some bacterial strains and plants, the plasma membrane is enclosed within the rigid cell wall, which is protective in function. Whereas in case of animals cells, the cell wall is absent and the protoplasm is outlined by the plasma membrane.
The plasma membrane does the major function of regulating transportation of substances from inside the cell to the outside and vice versa. The specificity of plasma membrane structure plays a crucial role in the overall functioning of the cell. In simple terms, it acts in a similar manner to the skin of animals. Various scientific hypotheses have been proposed to explain the structure of the plasma membrane, out of which the most popularly accepted theory is the fluid mosaic model.
Plasma Membrane Structure: The Fluid Mosaic Model
The fluid mosaic model was developed by S.J. Singer and Garth Nicolson in 1972. As per this theory, the plasma membrane consists of carbohydrates and different types of lipids and proteins. And the model is named in accordance to the structure of the plasma membrane; the membrane is not rigid, but more of a fluid type, containing various molecules like a mosaic pattern. These molecules are arranged in a proper manner, which helps in selective movement of the substances. Discussed below is a brief illustration of the fluid mosaic model:
Lipid Bilayer: The fundamental part of the plasma membrane structure is the lipid bilayer. Types of lipids present in the plasma membrane are phospholipids, cholesterol and glycolipids. However, as majority of the molecules are of phospholipid type (containing a phosphate group), the two lipid layers are better known as phospholipid layers.
The lipid tails are water repelling (hydrophobic), while phosphate heads are water-attracted (hydrophilic). The phospholipid bilayer is arranged in a specific fashion, with the hydrophobic tails orienting towards the inside (facing each other) and the hydrophilic head aligning to the outside. Thus, both sides of the plasma membrane, one that faces the cytosol and the other facing the outside environment, are hydrophilic in nature.
Membrane Proteins: Another key component of the plasma membrane is proteins, which help in selective transport of the macromolecules like sucrose, amino acids and ions. Based on the location of proteins with reference to the phospholipid bilayers, there are two types of proteins:
• Integral membrane proteins attach to the lipids of the bilayered structure. And integral proteins that traverse the phospholipid bilayer are called transmembrane proteins.
• Peripheral membrane proteins are indirectly or loosely attached to the membrane. They are non-covalently connected with the lipids or ends of the integral proteins.
Carbohydrates: In addition to phospholipids and proteins, the cell membrane also consists of carbohydrates, basically glycoproteins and glycolipids. These molecules are exclusively arranged in the outer side of the cell membrane, wherein the carbohydrate portions are exposed to the external surface of the cell.
While speaking about the plasma membrane function, the primary roles are cell recognition and regulation of cellular transport. Small molecules like oxygen, nitrogen, etc. are allowed to enter or leave the cell freely (passive transport), while larger molecules like amino acids are passed selectively through the membrane against the concentration gradient (active transport). For example, through active transport waste materials are expelled, while allowing other useful substances to enter inside the cell. Know more on cell membrane function.
With this information on plasma membrane structure and function, I hope you have cleared your doubts regarding the arrangement of membrane molecules and their roles.