Tuesday, June 28, 2011

Preservation by High Osmotic Pressure

Osmotic pressure is the force with which water tends to move across a wall or membrane, the relative concentration of solute molecules in either side of the membrane drives water flow towards the side with the higher solute concentration.

Bacteria reach osmotic equilibrium by two means:
1. In hypertonic environments (environment having an osmotic pressure higher than that of the bacterial cell) the volume of the protoplasts will shrink. Water from the interior of the cell goes into the surrounding medium resulting in the shrinkage of the cell – phenomenon called plasmolysis.

2. In hypotonic environments (an environment that solution having an osmotic pressure lower than that of the bacteria cell) the rigid wall will resist increase in protoplasts volume at a limiting volume of water; equilibrium results from turgor against the wall. Water from surrounding media can enter the cell resulting in swelling- the phenomenon is known as plasmoptysis.

One of the major functions of the bacteria cell envelope is to present, or at least slow down, the influx of deleterious compounds from the environment.

The rigid wall present in the bacteria cells enables most bacteria to tolerate even extremely dilute environments. Osmotic equilibrium is achieved by the development of turgor pressure against the wall. The cell wall of gram positive species, such as mycobacteria acts as an effective permeability barrier. The wall of gram positive micrococci can withstand 22 atm of pressure.

Although the walls of gram-negative rods have lower tensile strengths, the wall is sufficiently strong to retain the turgor pressure if the cell is suspended in water.

Turgor against a rigid wall is apparently a much simpler development than the mechanism utilized by many protozoa for similar purposes.

Turgor pressure is almost essential to growth and consequently must be almost maintained despite variations in the external osmolarity.

Cell membranes are semi-permeable that contain various system (typically called pumps) to help attenuate the effects of osmotic pressure.

Protozoa contractile a vacuole to pump out the water in adapting to osmotic pressure in order to maintain equilibrium.
Preservation by High Osmotic Pressure

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