The plasma membrane is an outer, thin, semi-permeable membrane that surrounds the cytoplasm and the other constituents of the cell. The Fluid Mosaic model proposed by Singer and Nicholson in 1976, describes the plasma membrane as a structure in which a mosaic of proteins is discontinuously embedded in or attached to a fluid lipid bilayer (membrane).
The fluidity of the plasma membrane is maintained by the lipids present in the membrane such as phospholipids, glycolipids, and sterols. These lipids play a vital role in enhancing the flexibility and mechanical stability of the membrane.
Lipid in Plasma membrane:
According to the fluid mosaic model, the plasma membrane is made up of two parts i.e. Lipids and Proteins. Lipids form the structural backbone of the membrane and are arranged in a bimolecular layer (or bilayer). They are amphipathic in nature i.e. they contain both hydrophobic and hydrophilic parts.
Types of lipids:
There are three kinds of lipid molecule i.e. phospholipids, glycolipids, and sterols. Among these, the phospholipids are the most abundant lipid molecules. The further types of these lipids are as follow:
- Phospholipids: Glycerophospholipids and Sphingolipids.
- Glycolipids: Glycosphingolipids and Galactolipids.
- Sterols: Cholesterol (In Animals), Stigmasterol (In Plants), and Hopanoids (structural analogs in bacteria).
The fluidity of the Plasma Membrane:
The fluidity of the membrane is a primary feature of the Fluid mosaic model (proposed by Singer and Nicholson in 1976). It means that most of the membrane lipids are in constant motion and are capable of lateral movements (i.e. movement parallel to the surface of the membrane).
Apart from lateral movements, they can show four other types of movements such as Bobbing, Rotation, Flexion, and Flipping.
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Role of lipid molecules in maintaining fluidity:
Role of Unsaturated fatty acids (a constituent of lipid molecule)
The presence of double bonds in unsaturated fatty acid chains increases the fluidity of the membrane. This is because each double bond introduces a 45-degree bend (also called Kink) in the hydrocarbon chains. Such bends make it more difficult to pack these chains together in an ordered crystalline structure. Thus, plasma membranes form a fluid-structure.
Role of Cholesterol:
The plasma membrane contains a large number of cholesterol molecules in its structure. These cholesterol molecules orient themselves in the lipid bilayer in such a way that their hydroxyl groups remain close to head groups of phospholipids. Due to this interaction, the rigid plate-like sterol rings of cholesterol partly immobilize the polar head groups of phospholipids, leaving the rest of the hydrocarbon chain flexible. Hence, it promotes the fluidity of the plasma membrane.
Cholesterol also prevents the crystallization of the membrane by preventing hydrocarbon chains from coming together and crystallizing. Thus, cholesterol enhances the flexibility of the plasma membrane.
So, now you know about the fluidity of the plasma membrane and how lipids help in maintaining it. Also, the above article tells you about the role cholesterol plays in maintaining the fluidity of the membrane and preventing its crystallization. So, that is all for now and meet you in my next article. Keep Reading, Keep Exploring, and Keep Sharing your Knowledge and above all BE CURIOUS. 😉
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- Cdn.kastatic.org. 2020. Available at: <https://cdn.kastatic.org/ka-perseus-images/01398d30422f5d4f8b1e70b312619b3740d51072.svg>
- Ibbiologyhelp.com. 2020. Available at: <http://ibbiologyhelp.com/Cells/cholesterolfluidity.png>.
- Commons.wikimedia.org. 2020. File: Cell Membrane Detailed Diagram En.Svg – Wikimedia Commons. Available at: <https://commons.wikimedia.org/wiki/File:Cell_membrane_detailed_diagram_en.svg>.