Cell Membrane and its Functions
CELL MEMBRANE OR PLASMA MEMBRANE
THICKNESS:
70-100 Angstrom (A) or 7-10 nanometer (nm) (1 nm = 10^-9 mts; 1A = 10^-10 mts).
STRUCTURE (FLUID MOSAIC MODEL)
1. All membranes consist of a double layer of lipid molecules in which proteins are embedded.
2. Proteins make up 50% of the mass of the membrane and is of 2 types:
(i) Lipoproteins (proteins containing lipids): function as enzymes and ion channels.
(ii) Glyco-proteins (proteins containing carbohydrates): function as receptors for hormones and neurotransmitters.
Some proteins are located in the inner surface of the membrane (intrinsic proteins); some are located in the outer surface of the membranes (extrinsic or peripheral proteins); while some extend through the membrane (transmembrane proteins):
(i) Intrinsic proteins serve mainly as 'enzymes'.
(ii) Extrinsic proteins contribute to the cytoskeleton (framework of the cell).
(iii) Transmembrane proteins serve as:
(a) Channels, through which ions or small water soluble substances can diffuse.
(b) Carriers, which actively transport materials across the lipid layer
(c) Pumps, which actively transport ions across the lipid layer.
(d) Receptors, which when activated initiate intracellular reactions. The number of receptors in a cell are not constant but their number increases and decreases in response to various stimuli, and their properties change with change in physiological condition. For example, when a hormone or neurotransmitter is present in excess, the number of active receptors decreases (called down regulation); whereas during deficiency, the number of active receptors increases (called up regulation). These effects on receptors are of physiological significance in explaining the phenomenon of denervation hypersensitivity and tolerance to certain drugs.
3. The clear area formed of a bimolecular thickness of lipid molecules (phospholipids, cholesterol and glycolipids) is arranged as follows:
(i) Head end: contains phosphate portion, is positively charged and quite soluble in water (i.e. polar or hydrophilic). Polar groups of lipid molecules have affinity for water (water loving) and faces the aqueous phase i.e. exterior of the cell on one side (ECF) and cytoplasm on the other (ICF).
(ii) Tail end: quite insoluble in water (no affinity for water/water fearing) (i.e. non-polar or hydrophobic), contains two fatty acid chains. The hydro phobic ends facing each other meet in the water-poor interior of the membrane.
The proteins in the cell membrane are different from those in organelle membranes. Thus, the special functions of the different types of cells depend primarily on their membrane proteins.
The bimolecular lipid layer in the membrane has the characteristics of a fluid due to presence of cholesterol. This fluidity makes the membrane quite flexible, thus allows cells to undergo considerable changes in shape without disruption of their structural integrity.
FUNCTIONS
1. Protective - it forms outermost boundary of the cell organelles.
2. Digestive - takes in food and excrete waste products.
3. Property of selective permeability:
(i) Non-polar molecules (gases like O₂, CO₂ and N₂, lipids, steroid hormones, alcohol) can dissolve in the non-polar regions of the membrane and thus move rapidly across the membrane; polar molecules (water soluble substances: ions, glucose, urea etc.) have much lower solubility, therefore, penetrate the membrane much very slowly.
(ii) Chemical and physical characteristics of the membrane controls the free passage of ions and molecules into and out of the cell. This property of selective permeability of the cell membrane helps in maintaining the difference of composition between ECF and ICF
4. It provides a framework for the arrange ment of an ordered sequence of protein molecules (enzymes,pumps, receptors, ions, channels, co-factors, carriers) in a functionally meaningful pattern.
5. It links adjacent cells together by junctional complexes to form tissues.
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