Structure of the plasma membrane
Fluid mosaic model
- A phospholipid is a lipid made of glycerol, two fatty acid tails, and a phosphate-linked head group. Biological membranes usually involve two layers of phospholipids with their tails pointing inward, an arrangement called a phospholipid bilayer.
- Cholesterol, another lipid composed of four fused carbon rings, is found alongside phospholipids in the core of the membrane.
- Membrane proteins may extend partway into the plasma membrane, cross the membrane entirely, or be loosely attached to its inside or outside face.
- Carbohydrate groups are present only on the outer surface of the plasma membrane and are attached to proteins, forming glycoproteins, or lipids, forming glycolipids.
- At cooler temperatures, the straight tails of saturated fatty acids can pack tightly together, making a dense and fairly rigid membrane.
- Phospholipids with unsaturated fatty acid tails cannot pack together as tightly because of the bent structure of the tails. Because of this, a membrane containing unsaturated phospholipids will stay fluid at lower temperatures than a membrane made of saturated ones.
The components of the plasma membrane
|Phospholipids||Main fabric of the membrane|
|Cholesterol||Tucked between the hydrophobic tails of the membrane phospholipids|
|Integral proteins||Embedded in the phospholipid bilayer; may or may not extend through both layers|
|Peripheral proteins||On the inner or outer surface of the phospholipid bilayer, but not embedded in its hydrophobic core|
|Carbohydrates||Attached to proteins or lipids on the extracellular side of the membrane (forming glycoproteins and glycolipids)|