Biological Osmosis

The osmosis experiment demonstrated that water flows uphill through a membrane from the pure water side into the wine. A large pressure applied to the wine stopped the flow of water into the wine.  Later experiments showed that a sugar water solution could replace the wine.  This led to Van’t Hoff’s theory for osmosis that related osmotic pressure to the concentration of sugar.

We have shown that free water molecules pass through the membrane from the high-water vapor pressure side (pure water) to the lower water vapor pressure side (sugar water solution).   Sugar does not pass through the membrane because the molecules are much larger than the membrane pore size.  

Pressure applied to the sugar water solution increases the water vapor pressure in the sugar water to equal the water vapor pressure on the other side of the membrane to stop the flow across the membrane.  Early experimenters found that sugar water solutions produced the best results.  

Recently we discussed osmosis with saltwater.   Saltwater is different because salt (NaCl) is a strong electrolyte.     Sodium chloride dissociates into positive (Na+) and negative (Cl-) ions.   The question is “what drives the flow through the membrane?”    Is it still vapor pressure or  is  its electrical potential? 

Unless, we apply an electric potential across the membrane,  vapor pressure difference is still the driving force.   Without an applied electric potential,   flow of ions still proceeds from a high vapor pressure region to a lower vapor pressure region.   And since vapor pressure is related to the concentration of free molecules,  flow proceeds from a high concentration area to a low concentration area.

Early experiments used manmade membranes due to the large values of osmotic pressure.  Smaller molecules can pass through the membrane, but larger molecules are blocked. 


Sugar molecules are much larger than the water molecules.  Salt,  sodium, and chlorine molecules are only slightly larger than water molecules.   The original man-made membranes worked much better with sugar water solutions than with saltwater solutions.

Flow proceeds from high a concentration of water molecules to lower concentration of water molecules.    Note that we have proposed that osmotic flow occurs as free molecules rather than bulk water flow across the membrane. 

With the saltwater solution, vapor pressure still provides the driving force.   If an electrical potential is imposed,  across the membrane then the electrical potential difference will drive the flow of ions across the membrane.

We have discussed osmosis from the basis of the osmosis experiment and have discussed reverse osmosis and some applications to living organisms.      Cells are the fundamental building blocks of living organisms.    Nutrients and waste products must pass through a cell membrane.  Flow through the membrane is governed by the same rules as the osmosis experiment.    

In plants, cells in leaves, use energy from sunlight to  convert carbon dioxide into oxygen.  Vapor pressure difference due to solute concentrations is most probably the driving force for nutrient and waste transport across a plant cell membrane.  

Animal cells convert nutrients and oxygen from body fluids into energy and carbon dioxide.   Molecular flow across the animal cell membrane is probably driven by a vapor pressure difference also if the energy produced in the cell  (ATP)  is produced as a chemical.     If the energy is produced as an electrical potential;  ionic flow may be an important factor for osmotic flow across an animal membrane.    

The question now focuses on the net form of energy produced in the cell.   If it’s a chemical energy,  then vapor pressure driven osmosis is primary.    If the net energy produced by the cell is primarily in electrical form,   then,  ionic flow through the membrane becomes important.   IE Is the  ATP molecule  “energy of oxidation”  neutral  or is it charged?  Blogs are supported by HTMD Engineering.  We provide Quality Engineering Services and  will appreciate the opportunity to bid on your projects.   Larry Howlett ©   2019

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