Difference between Diffusion and Osmosis.

Quora digest posed the question “What is the difference between Diffusion and Osmosis?”

We reviewed Guyton’s Textbook of Medical Physiology.   He notes three modes for transport of ions and molecules through a cell membrane: diffusion, osmosis, and active transport.    We have previously looked at active transport.     The active transport models were  developed to explain movement of large molecules through a cell membrane.   We have difficulty understanding that this complexity exists in all cell membranes.

The question today is between Diffusion and Osmosis.  We suggest that they are identical processes.   The apparent difference is due to the method of discovery.   Diffusion is demonstrated by removing a barrier between two different concentrations of a gas.  Gas moves from a high concentration area to a lower concentration area until the entire volume is at the same concentration.   From an engineering/physics point of view we look for a driving force for movement.    The partial pressure of a gas is directly related to its concentration.     Therefore,  gas moves from a high partial pressure to a lower partial pressure area.      The total pressure of a gas is the sum of the partial pressure of each component of the gas.    For air, atmospheric pressure (total pressure) is the sum of the partial pressures of nitrogen, oxygen, carbon dioxide, etc.

Osmosis was discovered during an experiment with water and wine separated by a membrane.  It was expected that the column of wine would be filtered as it passed through the membrane.  The effect of the filtration would be obvious in the clear water below the membrane.    This was supposed to be a bulk flow experiment.   The column of wine would force flow through the membrane.   Instead, water flowed upwards through the membrane into the wine and a high pressure applied to the wine was needed to stop the flow.    It was more than 100 years later that others discovered that the wine could be replaced by a sugar/water solution and that the solute sugar in the sugar/water solution lowered the vapor pressure of water.      Thus, vapor pressure was the driving force for pure water moving through the membrane into the sugar/water solution.   And,  membrane properties allowed the passage of water molecules but not the larger sugar molecules.           Osmotic pressure applied to the sugar/water solution raised the water vapor pressure in the wine to equal the vapor pressure of pure water to stop the flow.       

 Note: Pressure is defined as the force per unit area produced by molecules bouncing from a surface.

©   Larry Howlett     HTMD Engineering     2019

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