The plasma membrane surrounds the cell and is crucial to cell life: it must control what enters and leaves the cell. It must allow sufficient amounts of food molecules, such as glucose, to pass into the cell and must also allow for the quick removal of waste products from the cell.
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Most of the lipids in a membrane are phospholipids. Phospholipids contain glycerol, two fatty acids, and a phosphate group. The phosphate group is polar (hydrophilic), enabling it to interact with water. The fatty acid tails are nonpolar (hydrophobic) and do not interact with water.
Phospholipids form a bilayer in a water. They arrange themselves so that the polar heads are oriented toward the water and the fatty acid tails are oriented toward the inside of the bilayer. The bilayer arrangement shown below enables the nonpolar fatty acid tails to remain together, avoiding the water. The polar phosphate groups are oriented toward the water.
In animals, cholesterol is a major membrane lipid. At low temperatures cholesterol keeps the phospholipids apart. At high temperatures it attracts the phosphoipids and stabilized the membrane.
Proteins Embedded in the Membrane
Proteins are scattered throughout the membrane and are capable of moving around in the bilayer. They may be attached to inner surface, embedded in the bilayer, or attached to the outer surface.
Proteins have different functions. Some serve as special carriers or transport channels for molecules that are too large or too hydrophilic to pass through the phopholipid bilayer. Other membrane proteins have sugar chains attached to them. These carbohydrate and protein combinations are called glycoproteins, act as attachment sites for molecules that need to enter or carry a message to the cell.
The Fluid Mosaic Model
The term “fluid” is used because the phosopholipid molecules and proteins that make up the membrande are free to drift around in fluid motion. The term “mosaic” is used to describe the position of the protein molecules. The molecules are placed randomly and there is no set pattern.
Cell membranes are called selectively permeable because they are able to control what passes through them.