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Diffusion

Diffusion refers to the process by which molecules intermingle as a result of theirkinetic energy of random motion. Consider two containers of gas A and B separated by a partition. The molecules of both gases are in constant motion and make numerous collisions with the partition. If the partition is removed as in the lower illustration, the gases will mix because of the random velocities of their molecules. In time a uniform mixture of A and B molecules will be produced in the container.
The tendency toward diffusion is very strong even at room temperature because of the high molecular velocities associated with the thermal energy of the particles.
external image diff1.gif
|| Rate of diffusion || Osmosis || Thermal energy ||

Energetics of solute diffusion
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Rate of Diffusion

Since the average kinetic energy of different types of molecules (different masses) which are at thermal equilibrium is the same, then their average velocities are different. Their average diffusion rate is expected to depend upon that average velocity, which gives a relative diffusion rate
external image diff2.gif

where the constant K depends upon geometric factors including the area across which the diffusion is occuring. The relative diffusion rate for two different molecular species is then given by
external image diff3.gif
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Osmosis

If two solutions of different concentration are separated by a semi-permeable membrane which is permeable to to the smaller solvent molecules but not to the larger solute molecules, then the solvent will tend to diffuse across the membrane from the less concentrated to the more concentrated solution. This process is called osmosis.
Osmosis is of great importance in biological processes where the solvent is water. The transport of water and other molecules across biological membranes is essential to many processes in living organisms. The energy which drives the process is usually discussed in terms ofosmotic pressure.
external image osm1.gif

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Osmotic Pressure

Osmosis is a selective diffusion process driven by the internal energy of the solvent molecules. It is convenient to express the available energy per unit volume in terms of "osmotic pressure". It is customary to express this tendency toward solvent transport in pressure units relative to the pure solvent.
If pure water were on both sides of the membrane, the osmotic pressure difference would be zero. But if normal human blood were on the right side of the membrane, the osmotic pressure would be about seven atmospheres! This illustrates how potent the influence of osmotic pressure is for membrane transport in living organisms.
The decision about which side of the membrane to call "high" osmotic pressure is a troublesome one. The choice made here is the opposite of that made in many biology texts, which attribute "high" osmotic pressure to the solution and zero osmotic pressure to pure water. The rationale for the choice is that the energy which drives the fluid transfer is the thermal energy of the water molecules, and that energy density is higher in the pure solvent since there are more water molecules. The thermal energy of the solute molecules does not contribute to transport, presuming that the membrane is impermeable to them. The choice is also influenced by the observed direction of fluid movement, since under this choice the fluid transport is from high "pressure" to low, congruent with normal fluid flow through pipes from high pressure to low. The final rationale has to do with the measurement of osmotic pressure by determining how much hydrostatic pressure on the solution is required to prevent the transport of water from a pure source across a semi-permeable membrane into the soluton. A positive pressure must be exerted on the solution to prevent osmotic transport, again congruent with the concept that the osmotic pressure of the pure solvent is relatively "high".
Nevertheless, the dialog continues on this issue since the discussion of osmosis is most relevant to the biological and life sciences and perhaps the logic stated above should yield to the conventions of the field in which the phenomena are most relevant.
external image osm2.gif
|| Measuring osmotic pressure ||
|| Calculating osmotic pressure ||

Osmotic pressure example: egg in syrup
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Measuring Osmotic Pressure

One approach to the measurement ofosmotic pressure is to measure the amount of hydrostatic pressure necessary to prevent fluid transfer by osmosis.
external image osm3.gif
Membrane transport

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Membrane Transport

The transport of water and other types of molecules across membranes is the key to many processes in living organisms. Many of these transport processes proceed by diffusion through membranes which are selectively permeable, allowing small molecules to pass but blocking larger ones. These processes, including osmosis and dialysis, are sometimes called passive transport since they do not require any active role for the membrane. Other types of transport, called active transport, involve properties of a cell membrane to selectively "pump" certain types of molecules across the membrane.
The transport of gases across membranes depends upon diffusion and the solubility of the gases involved. In life science applications such transport is characterized by Graham's Law and Fick's Law.
More on Cell Membrane transport

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