Osmosis flow for Vesicles and Cells

I recently learned of the 2013 Nobel Prize for physiology or medicine shared by James Rothman, Randy Schekman and Thomas Sudhof, for their work on improving the understanding of structure and function of cell vesicles.    A vesicle is a small fluid-filled membrane sac that occurs both within and outside of cells.    Problems within these vesicles may be related to Alzheimer’s, diabetes, and other recognized maladies.     

Questions about the formation, purpose, and behavior of these vesicles are still under study. 

Vesicles and cells transfer fluids through their membrane walls.     Several models have been proposed for this fluid transfer.    One of these models is osmosis.

The textbook definition for osmosis is: the net movement of water molecules from a region of high concentration to a region of lower concentration. The pressure required to stop osmotic flow is named osmotic pressure.   

For the water wine experiment, water flows from the pure water side through the membrane into the wine, diluted water side. Sugars in the wine reduce the concentration of water in the wine.  Thus, flow proceeds from the pure water, high water concentration, to the lower water concentration.   

Several models for osmosis flow details have been proposed. We will not discuss them here instead we will explain our proposed model for osmosis, molecular flow model.  

We examined the molecular models for solids, liquids, and gases. Solids and liquids are modeled as crystalline structures. Strong intermolecular bonds create the characteristic rigid shapes of Solids. Weaker intermolecular bonds allow liquids to wet the sides of the container and exhibit a free service.

Maxwell proposed a kinetic theory model for gases.   Each molecule moves freely.  Elastic collisions occur with other molecules and the container walls. The speed distribution of the molecules is predicted by Maxwell’s equations.  The root mean square (Vrms) speed varies with temperature and with the gas.    

We investigated condensation of a kinetic theory gas.  We assumed that high speed molecules did not condense into liquids or solids.  The total kinetic energy of all molecules moving faster than approximately three times the root mean square molecular speed (V ≥ 3*Vrms) is equal to the published values of vapor pressure, (vapor pressure is a specific measure of energy per unit volume of a gas).   

We conclude:  

  1. The molecular model proves that free gas molecules exist in solids and liquids and the number of free molecules is proportional to vapor pressure.
  2. Net flow through a membrane proceeds from a high concentration of free molecules to a low concentration of free molecules i.e. from a high vapor pressure to a lower vapor pressure.
  3. These free molecules provide the mechanism for osmosis flow through a membrane. We propose that movement of these free molecules in liquids and solids provides osmosis flow through the membrane rather than the bulk flow suggested by the text book model. Osmotic pressure is a bulk property.

This model for molecular flow through a membrane applies to cells, vesicles, and more.   It differs from the textbook model for osmosis because it introduces molecular flow as the mechanism for osmotic flow.  

©   Larry Howlett     HTMD Engineering     2018            Osmosis: The Molecular Theory      

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