Osmotic Pressure and Water Availability:

• Procedure (NaCl)
• Procedure (Sucrose)

Osmotic pressure is the force water exerts on the semi-permeable membrane (plasma membrane) surrounding the cell.  Water moves across the plasma membrane in response to an unequal distribution of dissolved solutes in the environment.  When a microorganism is placed into a hypotonic environment/medium in which the concentration of solute is lower outside the cell than inside the cell, water flows into the cell.  If this influx of water is uncontrolled, the cell may eventually burst.  Most bacteria, algae and fungi have rigid cell walls that allow them to tolerate and even enjoy a somewhat hypotonic environment.  Many microorganisms keep the solute concentration slightly higher in their protoplasm by synthesizing solutes that are compatible with their metabolism and growth, thus keeping their membrane pushed tightly against the cell wall.  By contrast, when microorganisms are placed into a hypertonic environment where the concentration of solute is higher outside the cell, water is lost from the cell resulting in dehydration, shrinkage of the plasma membrane and eventual death.  This is a process known as plasmolysis.

As can be seen, the osmotic concentration of the environment affects the water that is available to a microorganism.  The greater the solute concentration of the environment, the less readily available the water.  Some prokaryotes can maintain the availability of water in environments with high solute concentrations (hypertonic environments) by increasing the solute concentration within the cell.  Microorganisms that can do this and thus tolerate hypertonic environments are osmotolerant.  Osmotolerant bacteria, such as Staphylococcus aureus can grow in a wide variety of environments with varying osmotic pressures.  In fact this bacteria can be cultured in media containing sodium chloride (NaCl) concentrations as high as 3M.

Some bacteria specifically require an environment with a high concentration of sodium chloride.  These organisms are called halophiles.

Rachel Watson, M.S.
AG 5010
766-3524
Cell: 307-760-2942
rwatson@uwyo.edu