Profile
Settings
Refer your friends
Sign out
Every living cell is externally covered by a thin, transparent, elastic, regenerative, microscopic, and semi-permeable membrane. This membrane is called a Cell membrane. It is the outermost covering of a cell. It is also known as Plasma membrane or Plasmalemma. This membrane is present in both prokaryotic and eukaryotic cells. A distinctive functional property of cell membrane is it is semi-permeable in nature.
A cell membrane may be defined as a biomembrane that collectively covers all various cell organelles present inside the cell that have the same ultrastructure. Such cell organelles play vital functions for a living cell. This membrane is required for the specific movement of material from outside the cell to the inner side and vice-versa. It is the only layer that is responsible for the separation of the outer environment of the cell with respect to the inner environment.
Functions of Cell membrane
The cell membrane is primarily meant to regulate the exchange of materials between the cytoplasm and extracellular environment. Therefore, it maintains homeostasis inside the cell. Homeostasis refers to the process by which living organisms actively maintain the stable conditions mandatory for survival. In other words, the cell membrane maintains a balanced state, which is required for the survival and existence of a cell. In addition to this, the cell membrane also helps in the synthesis of many macromolecules and the response of the cells to a variety of physical and chemical stimuli. Without the cell membrane, the cell would be vulnerable and susceptible to germs and diseases.
Structural components of the Cell membrane
The cell membrane consists of several components such as Lipids, proteins, and carbohydrates. It does not matter whether a cell membrane exists in animals, plants, bacteria, or fungi. It is made up of the same basic and essential components. The essential components of the cell membrane are detailed below one by one.
1)- Lipids
Lipid molecules are arranged in a bilayer in the cell membrane. Being of an amphipathic nature, lipid molecules are arranged in such a way that the polar head towards the outer side, and the hydrophobic tails towards the inner side. This arrangement ensures that the nonpolar tail of saturated hydrocarbons is protected from the aqueous environment, and results in the formation of a water-resistant barrier. Lipids are mainly formed of phospholipids and cholesterol.
Phospholipids contribute about 55-75% of the lipids. The most common types of phospholipids which are found in the cell membrane are cephalin, phosphatidylserine, phosphatidylinositol, lecithin, and phosphatidic acid. These molecules of phospholipids form the structural framework of the plasma membrane and allow the faster movement of fat-soluble molecules.
Other than phospholipids, cholesterol molecules are also found in the cell membrane. They constitute about 25-30% of total lipids. They are also known as sterols. These cholesterol molecules are always present in between the phospholipid molecules. The function of the cholesterol molecule is that they stabilise the cell membrane.
2)- Proteins
Protein occupies half of the space of the cell membrane and constitutes about 60% of the cell membrane. Proteins that are found in the cell membrane are of two types. One is Peripheral or extrinsic proteins, and the other is Intrinsic or integral proteins.
Peripheral or extrinsic proteins lie outside the outer and inner surfaces of the phospholipid bilayer. These proteins are attached by weak electrostatic bonds and easily separated by small treatment with salt solutions. Examples of peripheral proteins include spectrin.
Intrinsic or integral proteins are completely embedded in the phospholipid bilayer. These proteins are not easily separable. They provide strength to the cell membrane. Examples of intrinsic proteins include Rhodopsin in retinal rod cells.
Some structural proteins form the backbone of the membrane and provide elasticity and mechanical stability.
3)- Carbohydrates
Carbohydrates are present in a very small amount in the cell membrane and constitute about 1-3% of chemical composition. Major carbohydrates found in the cell membrane are fucose, hexose, sialic acid, etc. Some glycoproteins in the cell membrane help in blood grouping, cellular recognition, and immune response.
A major model that describes all the components of the cell membrane in a better and more understandable way is the Fluid mosaic model of the plasma membrane, which was proposed by Singer and Nicolson in 1972. It states that a chemical cell membrane is composed of Lipids, Proteins, and carbohydrates.
Conclusion
The cell membrane is defined as a biological membrane that separates the inner atmosphere of the cell from the outer environment. It is semi-permeable in nature. It also maintains homeostasis inside the cell. It overall regulates and monitors the transport of various materials, and ions into and out of a cell. There are several components that are found in the cell membrane, such as Lipids, Proteins, and carbohydrates. All of the components play a significant role in the structural as well as the functional property of the cell membrane.
