The osmotic behaviour of a solution is a colligative property, related to the
number of molecules of solute(s) present. It will be influenced by the degree of
impermeability of the membrane to the solutes, its permeability to water and differences in hydrostatic pressure between the two sides .
Electro-osmosis is water movement that results from the application of an electric field to a solution containing electrically charged substances. If the charged
materials are a fixed membrane, then water will move relatively to this structure
and across it. Such water flow has been often considered in biological systems but
does not appear to playa major role in substantial movements of water.
c) Evaporation
The change from a liquid (or solid) into a vapour or gas is called vaporization
or evaporation. Kinetically this can be looked at as the escape of molecules, with
a greater than average kinetic energy, across a boundary from a liquid to a gas phase.
The excess kinetic energy of the molecules will be lost to the solution and this will
be translated as heat loss . When water evaporates at 100 0 the loss of heat is 539
cal/g ; at more physiological temperatures, it is considered for practical purposes
to be 580 callg. Heat lost as a result of evaporation of water plays an important
role in thermoregulation of birds and mammals.
Evaporation of water takes place more rapidly when it is at a higher temperature
(higher kinetic energy), and when its concentration (vapour pressure) in the gas
phase is low. Evaporation will cease when this phase is saturated with the vapour
(saturated vapour pressure), a concentration that becomes greater with increasing
temperature. If the vaporized water mixes slowly with the surrounding atmosphere, then the rate of evaporation will be slowed, while if mixing is hastened, the
process will be facilitated. The disposition and arrangement of cutaneous structures
such as scales, feathers and hairs may thus be expected to alter the rates of evaporative water loss. For more practical purposes the rate of evaporation of water from
a surface , like the skin, will depend on the difference between the predicted saturated vapour pressure of the air at skin temperature and the actual vapour pressure of water that is present. This is called the 'saturation deficit' of the air.
d) Facilitated Diffusion
When molecules move down their electro-chemical gradients more rapidly than
the properties and structure of the dividing membrane would indicate is likely to
occur by simple diffusion, we may suspect that some interaction is taking place
with the cell. This may be confirmed by such phenomena as a reduced rate of movement in the presence of other similar molecules (due to competitive antagonism)
and, instead of strict adherence to the laws of diffusion, maximal rates of transfer
in the presence of relatively small differences in concentration. In addition low concentrations of diverse substances, such as heavy metal ions, which are known to
react with cell constituents, may reduce the rate of change. A definitive criterion
is the absence of an energ y requirement, except for the small amount that may be
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number of molecules of solute(s) present. It will be influenced by the degree of
impermeability of the membrane to the solutes, its permeability to water and differences in hydrostatic pressure between the two sides .
Electro-osmosis is water movement that results from the application of an electric field to a solution containing electrically charged substances. If the charged
materials are a fixed membrane, then water will move relatively to this structure
and across it. Such water flow has been often considered in biological systems but
does not appear to playa major role in substantial movements of water.
c) Evaporation
The change from a liquid (or solid) into a vapour or gas is called vaporization
or evaporation. Kinetically this can be looked at as the escape of molecules, with
a greater than average kinetic energy, across a boundary from a liquid to a gas phase.
The excess kinetic energy of the molecules will be lost to the solution and this will
be translated as heat loss . When water evaporates at 100 0 the loss of heat is 539
cal/g ; at more physiological temperatures, it is considered for practical purposes
to be 580 callg. Heat lost as a result of evaporation of water plays an important
role in thermoregulation of birds and mammals.
Evaporation of water takes place more rapidly when it is at a higher temperature
(higher kinetic energy), and when its concentration (vapour pressure) in the gas
phase is low. Evaporation will cease when this phase is saturated with the vapour
(saturated vapour pressure), a concentration that becomes greater with increasing
temperature. If the vaporized water mixes slowly with the surrounding atmosphere, then the rate of evaporation will be slowed, while if mixing is hastened, the
process will be facilitated. The disposition and arrangement of cutaneous structures
such as scales, feathers and hairs may thus be expected to alter the rates of evaporative water loss. For more practical purposes the rate of evaporation of water from
a surface , like the skin, will depend on the difference between the predicted saturated vapour pressure of the air at skin temperature and the actual vapour pressure of water that is present. This is called the 'saturation deficit' of the air.
d) Facilitated Diffusion
When molecules move down their electro-chemical gradients more rapidly than
the properties and structure of the dividing membrane would indicate is likely to
occur by simple diffusion, we may suspect that some interaction is taking place
with the cell. This may be confirmed by such phenomena as a reduced rate of movement in the presence of other similar molecules (due to competitive antagonism)
and, instead of strict adherence to the laws of diffusion, maximal rates of transfer
in the presence of relatively small differences in concentration. In addition low concentrations of diverse substances, such as heavy metal ions, which are known to
react with cell constituents, may reduce the rate of change. A definitive criterion
is the absence of an energ y requirement, except for the small amount that may be
10