Cell membrane: Structure and Function

The cell membrane is the part of a cell that separates the interior and external environment of the cell. It acts as the barrier which protects the internal constituent of the cell. It is also called the plasma membrane, plasmalemma, or biological membrane. Nageli and Cramer, in 1855, gave the term cell membrane. It is composed of lipids, proteins, and carbohydrates. 

The cell membrane regulates the transportation of the molecules in and outside of the cell. They are of two types: cytoplasmic membrane and internal membrane. The cytoplasmic membrane surrounds the whole protoplasm. The internal membrane surrounds the various cellular organelles and the vacuoles.  

Structure of the cell membrane

Two important models that describe the cell membrane structure are the sandwich model (lamellar model) and the fluid mosaic model.

Sandwich model (lamellar model)

In 1935, James Danielli and Hugh Davson proposed the sandwich model of the cell membrane. The sandwich model states that the cell membrane is a solid and stable structure.

According to this model, the cell membrane consists of four molecular layers: two phospholipid layers and two protein layers. The center part has the phospholipid bilayers. Then at its sides, tail globular ɑ-protein molecules are present in it. It makes the pattern of the P-L-L-P. i.e., P: Protein, L: Lipid.  Each phospholipid bilayer consists of a head and two tails. The head is polar ( hydrophilic) made up of glycerol, and the tail is non-polar ( hydrophobic), made up of fatty acids. The hydrophilic head faces the protein in the periphery, but the hydrophobic faces the center. Strong electrostatic forces hold the hydrophilic head, and the weak Vander Waals forces hold the hydrophobic tails.

Limitations of the Sandwich model

The sandwich model of the cell membrane is the rejected model due to its various limitations, which are described below:

• According to the sandwich model, the cell membrane is the stable structure, but it is a dynamic structure. This model also describes it as a solid structure, but the membrane is a semi-solid (quasi fluid) structure.
• It cannot explain the changeability of the membrane because different biomembranes differ in form, composition, and thickness.
• It cannot explain the transport of the solutes, solvents, and the active and bulk transports of material through the membrane.
• Lipid: protein ratio does not favor the model too.

Fluid Mosaic Model

In 1972, Singer and Nicolson developed the fluid mosaic model of the cell membrane. According to the fluid mosaic model, the cell membrane is the dynamic and the quasifluid structure. It consists of the protein icebergs in the sea of phospholipids. The head is polar (hydrophilic), and the tail is non-polar (hydrophobic). The head faces towards the two surfaces, and the tail faces towards the center. Protein molecules are present in two forms: floating forms and suspended forms. Membrane proteins are of two types: Extrinsic proteins and intrinsic proteins.

• Extrinsic proteins are the peripheral proteins present in the two surfaces of the membrane in the floating form.

• Intrinsic proteins are the integral or tunnel proteins in the fluid phospholipids bilayer in the complete or partial suspended forms.

• Waters, ions, and water-soluble solutes pass through the channels. Hydrophobic interaction also takes place in the cell.

The different membrane proteins present in the cell and their functions are given below:

• Structural protein: for stability
• Channel protein: transport of water and the dissolved substances
• Carrier protein: active transport
• Enzymes: perform the different metabolic activities
• Receptor proteins: transport of the hormones and the conduction of the nerve impulses

Lipids and the extrinsic proteins on the outer surface join the oligosaccharides forming the glycolipid and the glycoprotein, respectively.

Advantages of the Fluid Mosaic Model

• The fluid mosaic model is an accepted model because it explains the limits the sandwich model could not describe.
• It is a dynamic structure and can be repaired quickly.
• It defines the membranes as semi-solid (quasi-fluid) structure.
• Variability of the membrane is explained because different biomembranes are different in their forms, compositions, and thickness.
• It explains the transport of the solutes, solvents, and the active and bulk transport of the substances through the membrane.
• Lipid: protein ratio supports this model.

Difference between the Sandwich model and the Fluid Mosaic Model

• Cell membrane structure

The fluid model states the membrane as the less rigid and dynamic structure whereas the sandwich model states the membrane as the rigid and stable structure.

• Variability of the cell membrane

The sandwich model does not clarify variability of the membrane, but the fluid model explains it.

• Phospholipid bilayer

The fluid model defines it as a semi-solid (quasi fluid) structure, but the sandwich model defines the phospholipid bilayer is solid.

• Transport

The sandwich model doesn’t explain the active and bulk transport of materials, and the passage of the electrolytes and non-electrolytes from the membrane, but the fluid model explains these processes properly.

Functions of the cell membrane

The cell membrane protects the internal cell organelles like mitochondria, nucleus, etc., in the cell. Besides, it performs various other vital roles, which are mentioned below:

• Glycolipids and glycoproteins are present in the cell membrane, which helps in cell recognition. 
• Antigens in the membrane helps in the blood groupings, immune response, acceptance, and rejection of the transplants.
• Microvilli, the modification in the membrane, help digest food materials.
• Pseudopodia is the modification in the membrane that helps in locomotion.
• Carrier proteins help in the active transport of the materials.
• Endocytosis and pinocytosis help in the bulk transport of the materials.
• Exocytosis removes the waste materials.
• Osmosis help to transport the water, and diffusion helps to exchange gases in the membrane.

References

• Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (First edition). S . Chand and Company Ltd
• Alberts, B. (2004). Essential cell biology. New York, NY: Garland Science Pub.
• Shakya, M., Mehta, D. K. R., Gautam, M., Pokharel, K. R., & Khanal, K. (2077). Principles of Biology (First edition). Asmita Books Publisher and Distributors Ltd.