Published on March 27, 2014
M.Prasad Naidu MSc Medical Biochemistry, Ph.D.Research Scholar
MEMBRANES Plasma membrane is an envelop surrounding the cell. Separates & protects the cell from the external hostile environment. Besides being a protective barrier, PM provides a connecting system b/n the cell & its environment.
Chemical composition Membranes are composed of lipids, proteins & carbohydrates. Among the lipids, amphipathic lipids ( containing both hydrophilic & hydrophobic groups) namely PL, GL and Cholesterol are found in the animal membranes.
Structure of membranes Lipid bilayer model-- Davson&Danielle(35) Fluid mosaic model-Singer&Nicolson(1972 FM model is more recent and acceptable Thickness---5-8nm A membrane is essentially composed of a lipid bilayer. The hydrophobic (non-polar) regions face each other at the core of the bilayer while the hydrophilic (polar) regions fact outward Globular proteins are irregularly embedded in the lipid bilayer.
Membrane proteins Membrane proteins are categorized into two groups. 1. Extrinsic (peripheral)membrane proteins Eg: Cyt.c of mitochondria 2. Intrinsic (integral)membrane proteins. Eg: hormone receptors, Cyt.P450 The membrane is asymmetric due to the irregular distributions of proteins.
Transport across membranes The biological membranes are relatively impermeable. The membrane therefore forms a barrier for the free passage of compounds across it. 3 distinct mechanisms have been identified for the transport of solutes (metabolites) through the membrane. 1. Passive diffusion 2. Facilitated diffusion 3. Active Transport
Passive diffusion A simple process which depends on the concentration gradient of a particular substance across the membrane. Passage of water & gases through the membrane occurs by passive diffusion. The process does not require energy.
Facilitated diffusion This is somewhat comparable with PD, since the solute moves along the concentration gradient (higher to lower) and no E is needed. FD occurs through the mediation of carrier or transport proteins. Specific carrier proteins for the tpt of glu, gal, leu, phe etc have been isolated and characterized.
Factors affecting rate of diffusion 1. Permeability of the cell membrane 2. Temparature 3. Conc.gr / electrical gr of the substance across the cell membrane 4. Solubility of the substance 5. Thickness of the cell membrane 6. Size of the molecules 7. Size of the ions. 8. Charge of the ions
Active transport AT occurs against a conc. gradient & this is dependent on the supply of metabolic E (ATP) This is also a carrier mediated process like FD. Movement of substances against the chemical or electrical or electrochemical gradient is called AT. It is like swimming in the opp direction of water flow in a river( also called uphill transport) The E required is liberated mostly by break down of high E compounds like ATP.
Active transport AT occurs with the help of carrier proteins as in the case of FD. (mech .is diff) Each CP can carry only one substance or more than one across the membrane. Those CPs transporting only 1 substance are called uniports/uniport pumps. Those CPs carrying more than one substance are called symports/ antiports
Mechanism of active transport When a substance to be transported across the cell membrane comes near the cell, it combines with the CP of the membrane and a Sub-Pro Complex is formed. Now this SP complex moves towards the inner surface of the membrane. Now the substance is released from the CPs The same CP moves back to outer surface of the membrane to transport another molecule of the substance.
Substances transported by AT Both ionic form and non-ionic forms Ionic forms:- Na+ , K+ , Ca2+ , H+ , Cl- & I- Non-ionic forms:- Glu, AAs, Urea
Types of AT 1. Primary AT:- In this type of AT, the E is liberated directly from the break down of ATP. By this method Na+ , K+ , Ca2+ , H+ , Cl- are transported across the membrane 2. Secondary AT:- When Na+ is transported by a CP, another substance is also transported by the same protein simultaneously, either in the same direction or in the opp direction. This type of tpt of a sbustance with Na+ by means of a CP is called the secondary AT. i)CP tpts 2 diff molecules in the same direction (symport) Ii)CP tpts 2 diff molecules in opp.direction (antiport)
Primary active transport of Na+ & K+ (Na+ - K+ Pump) Na+ & K+ are transported across the membrane by means of common mechanism called Na+ -K+ Pump. This tpts Na+ from inside the cell to outside and K+ from outside into the cell. This pump is present in all parts of the body. Cells have a high intracellular K+ conc and a low Na+ conc. This is essentially needed for the survival of the cells. High cellular K+ is needed for the optimal glycolysis & for protein biosynthesis. Further Na+ & K+ gradients across the membranes are needed for the transmission of nerve impulse.
CP of Na+ -K+ Pump The CP involved in Na+ -K+ Pump has got 6 sites 3 receptor sites for Na+ . These are on the inner (towards the cytoplasm) surface of the protein molecule 2 receptor sites for K+ . These are on the outer (towards extra cellular fluid) surface of the protein molecule 1 site for the enzyme ATPase (mol.wt250000) which is near the sites for Na. ATPase consists of 2α & 2β subunits and represented as (αβ)2.
Mechanism of action of Na+ -K+ Pump 3 Na ions from the cell get attached to the receptor sites of Na+ on the inner surface of the CP. 2 K ions outside the cell bind to the receptor sites of K+ located on the outer surface of the CP The binding of Na+ & K+ to the CP immediately activates the ATPase. ATPase causes the break down of ATP to ADP with the release of one high E phosphate.
Mechanism of action of Na+ -K+ Pump Now the E liberated causes some sort of conformational change in the CP. Bcos of this, the outer surface of the CP with K+ , now faces the inner side of the cell. And the inner surface of the CP with Na+ faces the outer side of the cell. Now dissociation and release of the ions takes place so that the Na+ are released out side the cell and the K+ are released inside the cell. The exact mechanisms involved in the dissociation and release of the ions are not yet known.
Electrogenic activity of Na+ -K+ Pump Na+ -K+ Pump moves 3 Na+ outside the cell and 2 K+ inside the cell. Thus, when the pump works once, there is a net loss of one +vely charged ion from the cell. The continuous activity of these Na+ -K+ Pump causes reduction in the no.of +vely charged ions inside the cell. Leading to the development of –ve potential inside the cell. This is called electrogenic activity of Na+ -K+ Pump.
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