Microbiology Procedure

Index >> Membrane Transport >> Passive Diffusion OR Active Diffusion

Passive Diffusion OR Active Diffusion

	Passive Diffusion OR Active Diffusion Transport of metabolites across the cell membrane along the concentration gradient and without the use of a carrier molecule is called passive or simple diffusion. Diffusion occurs down a concentration gradient from a high concentration to a low concentration region and the concentration gradient disappears as diffusion proceeds. Simple diffusion shows nonsaturation kinetics. The rate of diffusion is proportional to the difference in solute concentration on the inner and outer sides of the membrane (Fick's first law)
When equilibrium is reached the concentration is the same on both sides of the membrane. Passive diffusion does not involve stereo specificity (i.e. both Land D isomers move across at equal rates) It takes place as a result of random molecular movement, and is a slow process. It is not believed to be an important mechanism for transport across cell membranes. Diffusion through the membrane can take place through pores or channels in the membrane or by dissolving in the lipid phase.
In the Danielli Davson model it was assumed that passage of substances took place through small (7A) rigid, protein lined pores in the membrane. According to the Singer Nicolson fluid mosaic model the pores may not be stable, but may be constantly appearing and disappearing (statistical pore concept) They are believed to be formed by the appearance of gaps in the highly fluid lipid bilayer because of random movement of membrane phospholipids. Small polar molecules could cross the membrane through the gaps (pores) which arise in a random manner and arc transitory
Experiments with natural and artificial membranes show that there is great increase in permeability when antibiotics are incorporated into membranes. Four rod-shaped molecules of the antibiotic nystatin become arranged in the membrane to form a cylindrical channel or pore. Water, urea and chloride, which have a diameter of less than 4A, pass through the membrane, while larger molecules cannot penetrate the Pore. A l00, 000 fold increase in chloride ion permeability takes place when nystatin is incorporated into artificial membrane
	Solute molecules can cross the membrane by dissolving in the lipid bilayer and diffusing across the membrane. Before a solute can, leave the aqueous phase and enter the lipid phase it must break its hydrogen bonds with water. The more the number of -OH groups present in a solute the more will be its H-bonding capacity with water, and therefore the less its lipid solubility. Thus hexanol with only one -OH group is, poorly soluble in water, while mannitol with six -OH groups is highly soluble. Hexanol therefore diffuses across the lipid of the membrane much more readily than mannitol. For breaking each hydrogen bond with water, about 5 kcal of energy is required.

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