Cell Membrane: Definition

The cell membrane, mainly comprising lipids and proteins, is essential for transporting nutrients throughout the cell. It also regulates the substances that are exiting or entering a cell. The cell membrane can be defined as a plasma membrane that separates the internal contents of the cell from its environment.

This model depicts that the cell or the plasma membrane comprises phospholipids, proteins, cholesterol, and carbohydrates. It was proposed in 1972 by Singer and Nicolson and explained the detailed structure of the plasma membrane. These components present in the cell membrane are responsible for its fluidity. 

The cell membrane is a semi-permeable membrane surrounding the cytoplasm in the cell. It helps maintain the cell’s structural integrity and thus maintains its shape and size. It also helps regulate the growth of cells through the process of endocytosis and exocytosis.

Composition of the cell membrane

 The major composition of the cell membrane is lipids and proteins. It also contains carbohydrates. The contents entering or leaving the cell are regulated by two processes, namely, exocytosis and endocytosis. The phospholipids present in the cell membrane form the basic fabric of the cell membrane. The cholesterol is present between the layers of phospholipids. However, the proteins are an integral part of the cell membrane, present in the phospholipid layers. 

The outside layer of the cell membrane is attached to the carbohydrates. Together, phospholipids, cholesterol, proteins form the fluid character of the cell membrane. The phospholipids are amphiphilic. However, hydrophobic (water-hating) and hydrophilic (water-loving) molecules are present in the cell membrane. Approximately 30% of the total composition of a cell membrane is lipids.

Function of the cell membrane 

The main functions or roles of the cell membrane are listed below.

• Separating the cell contents from the external environment, which may contain toxins that may be harmful to the growth of the cell
• Maintaining the shape and size of the cell, thereby holding it together like a skeleton
• Regulating the substances entering and leaving the cell as it is a selectively permeable membrane that allows necessary materials to enter that are essential for the cell’s survival
• Releasing the waste materials

Cell membrane structure

1. The cell membrane structure can be explained based on the fluid mosaic model, which says that the cell membrane is a two-dimensional liquid wherein the lipid and protein molecules diffuse.
2. The cell membrane consists of a bilayer of lipid molecules comprising phospholipids arranged in a hydrophobic tail and a hydrophilic head. These layers of lipid molecules allow the passage of ions/molecules. 
3. These phospholipids form the structural basis of the cell membrane. The proteins in the cell membrane support the cell and can be of three types. These are the transport proteins, the glycoproteins, and the receptor proteins.
4. The extracellular part of the cell membrane has carbohydrates called the glycocalyx. The cytoplasm present in the cell consists of a cytoskeleton in the cell membrane, which allows proteins to pass through the cell. 
5. Inside the cell in the intracellular membrane, organelles such as the mitochondria, chloroplasts, Golgi apparatus, and the nuclear membrane, each with their specifications. 
6. The amphiphilic nature of the phospholipids implies that these contain both the hydrophilic and the hydrophobic parts. 
7. The hydrophilic part of the phospholipid is made of glycerol and a phosphate group, while the hydrophobic tail is made of fatty acids. 
8. The phospholipid layers are formed as the head, which is polar, gets attached to the hydrophobic tail, and these tails get attached. The heads form hydrogen bonds amongst them, and so the layers are formed in the cell membrane.  

Conclusion

A membrane separating the cell’s interior from the exterior environment is known as the cell membrane. The cell membrane is a semi-permeable membrane that allows the release of toxins from within the cell and allows necessary nutrients to enter the cell. The structural integrity of the cell is maintained by the cell membrane, also known as the plasma membrane. It also regulates the growth of the cells. The cell membrane is a two-dimensional layer that separates the cell from the external environment. The major composition of the cell membrane is lipids and proteins. It also contains carbohydrates. The contents entering or leaving the cell are regulated by two processes, namely, exocytosis and endocytosis. The phospholipids present in the cell membrane form the basic fabric of the cell membrane.