The cell membrane is the part of cell which separates the interior and external environment of the cell. It acts as the barrier which protects the internal constituent of the cell. The cell membrane is also called the plasmalemma or biological membrane. Nageli and Cramer, in 1855, gave the term cell membrane. The cell membrane is composed of lipids, proteins, and carbohydrates.
The biological membrane regulates the transportation of the molecules in and outside of the cell. It is the outermost layer of the animal cell. These are of two types: cytoplasmic membrane and internal membrane. This membrane surrounds the whole protoplasm .The internal membrane surrounds the various cellular organelles and the vacuoles.
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Structure of the cell membrane
Two important models that describes 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 plasma membrane is a solid and stable structure.
The plasma membrane consists of four molecular layers: two phospholipid layers and two protein layers. The center of the plasma membrane consists of 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. ie 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 the plasma membrane is the dynamic structure. This model also describes the plasma membrane as the solid structure, but the plasma membrane is a semi-solid ( quasifluid) structure.
- This model cannot explain the changeable of the plasma membrane because different biomembranes differ in form, composition, and thickness.
- It cannot explain the transport of the solutes and the solvents through the biological membrane.
- It cannot explain the active and bulk transports of material through the biological membrane.
- Lipid: protein ratio does not favor the model too.
Fluid Mosaic Model of Cell Membrane
In 1972, Singer and Nicolson developed the Fluid Mosaic Model of Cell Membrane. According to the Fluid Mosaic Model, the biological 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.
There are five types of membrane proteins:
- 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 cell membrane model because it explains the limits the sandwich model could not describe.
- The cell membrane is a dynamic structure and can be repaired quickly.
- It is a semi-solid (quasi fluid) structure.
- Variability of the biological membrane is explained because different biomembranes are different in their forms, compositions, and thickness.
- It explains the transport of the solutes and solvents through the biological membrane.
- It explains the active and bulk transport of the substances through the plasma membrane.
- Lipid: protein ratio supports this model.
Difference between the Sandwich and the Fluid Mosaic Model
- Cell membrane structure: The sandwich model states the cell membrane as the rigid and stable structure, but the fluid model states it as the less rigid and dynamic structure.
- Variability of the cell membrane: The sandwich model does not explain the variability of the plasma membrane, but the fluid model explains it.
- Phospholipid bilayer: The sandwich model explains the phospholipid bilayer is solid, but the fluid model explains it as a semi-solid (quasifluid) structure.
- Transport: The sandwich model doesn’t explain the active and bulk transport of materials from the cell membrane, but the fluid model explains it. The sandwich model doesn’t explain the passage of the electrolytes and non-electrolytes, but the fluid model explains it.
Functions of the cell membrane
Cell membrane is responsible for protecting the internal cell organelles 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 with cell recognition.
- Antigens in the membrane help in the blood groupings, immune response, acceptance, and rejection of the transplants.
- Microvilli, the modified membrane, help in the digestion of food materials.
- Pseudopodia is the modified plasma 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 plasma membrane.
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