Plasma Membrane Structure and Function Notes and Lecture, Fluid Mosaic Model Explained

The plasma membrane, a flexible yet sturdy barrier that surrounds and contains the cytoplasm of a cell, is best described by using a structural model called the fluid mosaic model. According to this model, the molecular arrangement of the plasma membrane resembles an ever-moving sea of fluid lipids that contains a mosaic of many different proteins. Some proteins float freely like icebergs in the lipid sea, whereas others are anchored at specific locations like boats at a dock. The membrane lipids allow passage of several types of lipid-soluble molecules but act as a barrier to the entry or exit of charged or polar substances. Some of the proteins in the plasma membrane allow movement of polar molecules and ions into and out of the cell.

Check out about the Structure and Function of Plasma Membrane in below mention lecture 

Other proteins can act as signal receptors or adhesion molecules Cholesterol molecules are weakly amphipathic and are interspersed among the other lipids in both layers of the membrane. The tiny —OH group is the only polar region of cholesterol, and it forms hydrogen bonds with the polar heads of phospholipids and glycolipids. The stiff steroid rings and hydrocarbon tail of cholesterol are nonpolar; they fit among the fatty acid tails of the phospholipids and glycolipids.

The carbohydrate groups of glycolipids form a polar “head”; their fatty acid “tails” are nonpolar. Glycolipids appear only in the membrane layer that faces the extracellular fluid, which is one reason the two sides of the bilayer are asymmetric, or different.

The Lipid Bilayer
The basic structural framework of the plasma membrane is the lipid bilayer, two back-to-back layers made up of three types of lipid molecules—phospholipids, cholesterol, and glycolipids. About 75% of the membrane lipids are phospholipids, lipids that contain phosphorus. Present in smaller amounts are cholesterol (about 20%), a steroid with an attached !OH (hydroxyl) group, and various glycolipids (about
5%), lipids with attached carbohydrate groups.

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The bilayer arrangement occurs because the lipids are amphipathic molecules, which means that they have both polar and nonpolar parts. In phospholipids, the polar part is the phosphate containing “head,” which is hydrophilic. The nonpolar parts are the two long fatty acid “tails,” which are hydrophobic hydrocarbon chains. Because “like seeks like,” the phospholipid molecules orient themselves in the bilayer with their hydrophilic heads facing outward. In this way, the heads face a watery fluid on either side—cytosol on the inside and extracellular fluid on the outside. The hydrophobic fatty acid tails in each half of the bilayer point toward one another, forming a nonpolar, hydrophobic region in the membrane’s interior.

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