Every type of living cell, either a simple prokaryote or an 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.
Besides separating the cell components from the outside, 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 cell membrane structure also 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 its structure, of which the most popularly accepted theory is the fluid mosaic model.
Plasma Membrane Structure: The Fluid Mosaic Model
The fluid mosaic model was proposed by S.J. Singer and Garth Nicolson in 1972. As per this theory, the cell 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, which is not rigid, but more of a fluid type, containing various molecules like a mosaic pattern. Nevertheless, these molecules are arranged in a specific manner, so as to allow selective movement of the substances from inside to the outer side of the cell, and vice versa. Discussed below is a brief illustration of the fluid mosaic model.
The fundamental part of the cell membrane structure is the lipid bilayer. Types of lipids present in the plasma membrane are phospholipids, cholesterol, and glycolipids. However, majority of molecules are of the phospholipid type (containing a phosphate group). Hence, the two lipid layers of the cell membrane are better known as phospholipid layers.
The lipid tails are water repelling (hydrophobic), while the phosphate heads are water-attracted (hydrophilic). Accordingly, the phospholipid bilayer is arranged in a specific fashion, with the non-polar, hydrophobic tails orienting towards the core (facing each other) and the polar hydrophilic heads aligning to the outer side of the layers. Thus, both sides of the plasma membrane, one that faces the cytosol and the other facing the outside environment, are hydrophilic in nature.
Another key component of the plasma membrane is proteins, which help in selective transport of the macromolecules like sucrose, amino acids, and ions. They remain embedded in the lipid layer. Based on the actual location of proteins with reference to the phospholipid bilayer, two types of proteins are identified.
- Integral membrane proteins attach to the lipids of the bilayered structure. Those 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.
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 attaching to the proteins or phospholipids, wherein the carbohydrate portions are exposed to the external surface of the cell.
Plasma Membrane Function: Cell Transport
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), with the expense of energy. For example, through active transport waste materials are expelled, while allowing other useful substances to enter inside the cell.
This way, lipids, proteins, and carbohydrates make up the cell membrane. The lipids and membrane proteins can move freely in the membrane, as if they are suspended in a fluid. In fact, the consistency of plasma membrane at normal body temperature is similar to that of vegetable oil. With this brief information on cell membrane structure and function, I hope you have cleared your doubts regarding the arrangement of membrane molecules and their roles in the overall cell transport and functioning.