STRUCTURE OF THE CELL MEMBRANE.
CELL MEMBRANE → also called “plasma membrane”. It envelops the cell and is a thin, pliable, elastic structure (7.5 – 10 nm thick).
MEMBRANE COMPOSITION.
It is composed almost entirely of proteins and lipids. The approximate composition is:
- Proteins 55%
- Phospholipids 25%
- Cholesterol 13%
- Other lipids 4%
- Carbohydrates 3%
Cell lipid barrier impedes penetration by water-soluble substances.
The basic structure of the cell membrane is a LIPID BILAYER → thin double-layered film of lipids that is continuous over the entire cell surface. Interspersed in this lipid film are large globular proteins. It is composed of 3 main types of lipids:
- PHOSPHOLIPIDS → most abundant, they have one hydrophilic head – phosphate end (which is soluble in water) and one hydrophobic tail – fatty acid portion (which is soluble only in fats).
- SPHINGOLIPIDS → they also have hydrophobic and hydrophilic groups and are present in small amounts in the cell membranes, specially in nerve cells. Functions:
- Protection of the cell from harmful environmental factors.
- Signal transmission.
- Adhesion sites for extracellular proteins.
- CHOLESTEROL → helps to determine the degree of permeability (or impermeability) of the bilayer to water-soluble constituents of body fluids. It also controls much of the fluidity of the membrane.
Integral and peripheral cell membrane proteins.
Proteins in the cell membrane are mainly glycoproteins. There are two types:
- INTEGRAL PROTEINS → protrude all the way through the membrane. They can be:
- CHANEL PROTEINS (PORES).
- CARRIER PROTEINS.
- TRANSPORTERS.
- ENZYMES.
- RECEPTOR PROTEINS.
- PERIPHERAL PROTEINS → attached only to one surface of the membrane and do not penetrate all the way through. They act as:
- ENZYMES.
- CONTROLLERS OF TRANSPORT (through pores).
Membrane carbohydrates – “Glycocalix”.
Membrane carbohydrates occur almost invariably in combination with proteins (glycoproteins) or lipids (glycolipids) → most of the integral proteins are glycoproteins, and 1/10 of the membrane lipid molecules are glycolipids. The “glyco” part of these molecules usually protrudes outside the cell.
Other carbohydrate compounds are proteoglycans (carbohydrate substances bound to small protein cores) → they are loosely attached to the outer surface of the cell.
THE ENTIRE OUTSIDE SURFACE OF THE CELL HAS A LOOSE CARBOHYDRATE COAT → GLYCOCALIX.