Biomembranes - composition, types and functions. Membrane lipids
Subject and importance of the biological membranology, types of membranes. Functions and basic chemical composition of the biomembranes. Membrane phospholipids, glycolipids and cholesterol.
A biological membrane or biomembrane is an enclosing or separating membrane that acts as a selectively permeable barrier within living things. Biological membranes, in the form of cell membranes, often consist of a phospholipid bilayer with embedded, integral and peripheral proteins used in communication and transportation of chemicals and ions.
Biomembrane functions:
- 1. Compartmentalization
- 2. Transport of substances in and out
- 3. Enzymatic activity
- 4. Signal transduction
- 5. Cell-cell recognition and interaction - tissue and organ formation-
- 6. Energy transformation and transduction.
- 7. Propagation of nerve impulses
- 8. Cell division
Chemical composition of biomembranes: Lipids: 15% - 80 % Proteins: 20% - 85% Carbohydrates: 5% - 10% (typically bound to the membrane glycolipids and glycoproteins).
Lipids: Lipids constitute a large and diverse group of compounds with complex classification. Lipids are insoluble in water, but soluble in fats and organic solvents.
Lipid classification: 1. Phospholipids:
Phospholipids are amphiphilic compounds composed of a polar head group which is attached to a backbone (glycerol) through a phosphate group and two fatty acid chains (saturated/unsaturated) which are linked to glycerol backbone via ester or ether bonds.
2. Glycolipids
Lipids with polar head groups which contain carbohydrates - one or more sugar residues such as glucose, galactose & others. Glycolipids are found in the external leaflet of plasma membranes and play important roles in the processes of cellular recognition and communication. Example: The blood group types of A, B, AB, O are determined by the type of glycolipids present in the external leaflet of the erythrocyte membrane.
3. Cholesterol
Composition = alkyl chain, sterol nucleus and polar group. Cholesterol supports biomembranes in their functional liquid crystalline state.
At elevated temperatures: Cholesterol reduces membrane fluidity by restricting the motion of the phospholipid molecules.
At low temperatures: Maintains membrane fluidity by preventing dense lipid packing and membrane transformation into solid (gel) phase.
There are two mechanisms by which phase changes of membrane lipids are avoided and the biomembranes are maintained in their functional liquid crystalline state:
- 1. Cholesterol is abundant in membranes, such as plasma membranes, that include many lipids with long-chain saturated fatty acids. Cholesterol blocks transition to the low-temperature crystalline state.