OSMOSIS AND CHEMICAL POTENTIAL

 

WATER MOVEMENT IS A FUNCTION OF THE DIFFERENCE IN CHEMICAL POTENTIAL ( m)BETWEEN THE WATER

IN A CELL AND THE CELLS INTERNAL ENVIRONMENT.

 

WATER MOVES FROM A REGION OF HIGH CHEMICAL POTENTIAL TO AN AREA OF LOW CHEMICAL POTENTIAL.

 

CHEMICAL POTENTIAL IS INFLUENCED BY:

   1. CONCENTRATION

   2.   PRESSURE

   3.   ELECTRICAL POTENTIAL

   4.   GRAVITY (SOMETIME)

 

m = m* + RT ln a + zZE + VP +mgh

 

m* IS THE CHEMICAL POTENTIAL IN A REFERENCE STATE (HARD TO MEASURE AND NOT IMPORTANT FOR US)

 

 

RT ln a IS THE CHEMICAL ACTIVITY

R = UNIVERSAL GAS CONSTANT

T = ABSOLUTE TEMPERATURE

 

CONCENTRATION CAN REPLACE a

C = CONCENTRATION

 

zZE = ELECTRIC POTENTIAL

z = NET CHARGE; SINCE THE NET CHARGE OF WATER IS 0, zZE = 0

 

VP = EFFECT OF PRESSURE ON CHEMICAL POTENTIAL

 

V = PARTIAL MOLAL VOLUME (VOLUME OF ONE MOLE OF CHEMICAL SPHERES)

 

1 L OF H2O CONTAINS 55.5 MOLES = 1000/55.5 = 18 ML PER MOLE OF WATER.

 

P = PRESSURE

 

mgh = CONTRIBUTION OF GRAVITATIONAL FIELD;

 

m = MASS OF THE CHEMICAL SPECIES

g = ACCELERATION DUE TO GRAVITY

h = HEIGHT (OF WATER COLUMN)

 

mgh IS NOT SIGNIFICANT AT THE CELLULAR LEVEL SO IT MAY BE IGNORED.

 

   m    = m* + RT ln C + VP

 

THE RELATIONSHIP BETWEEN THE DIFFUSION OF WATER BY OSMOSIS, AND CHEMICAL POTENTIAL OF WATER IS THAT AS SOLUTES ARE ADDED AN OSMOTIC PRESSURE (p) IS GENERATED.

 

WHEN SOLUTES ARE ADDED TO THE SOLUTION THERE IS A DECREASE IN MOLE FRACTION OF WATER AND AN INCREASE IN OSMOTIC PRESSURE.

 

 

     p  = - RT   ln aW

              Vw

 

      RT ln aw = -Vw

 

EQUATION 1 -

 

   m = m* + RT ln C + VP

 

EQUATION 2 -

 

   RT ln aW = -Vw

 

SUBSTITUTE EQUATION 2 INTO EQUATION 1 TO GET:

 

   mw = mw* -Vw  + VwP

 

THE CONCEPT OF WATER POTENTIAL

 

THE CHEMICAL POTENTIAL OF WATER IN A SYSTEM CAN BE EXPRESSED AS WATER POTENTIAL

 

 

 

WATER POTENTIAL IS DETERMINED BY MEASURING HYDROSTATIC PRESSURE AND OSMOTIC PRESSURE

 

THE COMPONENTS OF WATER POTENTIAL ARE:

 

   1. YP IS THE PRESSURE                     POTENTIAL

 

   2. YS IS THE SOLUTE POTENTIAL

 

  

 YP = P

 

 YS = S

 

SO Y = P + S

 

MATRIX POTENTIAL IS ALSO PART OF THE WATER POTENTIAL EQUATION BUT, ITíS EFFECT IS RELATIVELY MINOR SO CAN BE IGNORED AT THIS TIME.

 

MEASURING WATER POTENTIAL AND ITS COMPONENTS

 

1.   TISSUE WEIGHT CHANGE METHOD

 

   PRINCIPLE:EQUILIBRATE    PREWEIGHED SAMPLES IN    SOLUTIONS OF KNOWN OSMOTIC    POTENTIAL

 

   DETERMINE WHICH SOLUTION HAS A    WATER POTENTIAL CLOSEST TO THE    TISSUE SAMPLE

 

      a.   CUT TISSUE SAMPLES

      b.   WEIGH TISSUE SAMPLES

     

 

 

      c.   PLACE IN SOLUTIONS OF

         VARYING OSMOTIC                      POTENTIAL FOR 1-2 HOURS

      d.   WEIGH TISSUE SAMPLES

      e.   CALCULATE WEIGHT GAIN OR

         LOSS

      f.   GRAPH WEIGHT CHANGE

         AGAINST OSMOIC (OR

         WATER) POTENTIAL

 

 

 

 

 

 

 

      g.   WATER POTENTIAL IS WHERE

         THE LINE CROSSES THE X

         AXIS

 

 

 

 

 

 

 

2.   THERMOCOUPLE PSYCHOMETRY

 

   a.   MEASURED WITH A VAPOR

      PRESSURE OSMOMETER

 

 

 

 

THE THISTLE TUBE EXPERIMENT EXPLAINED USING WATER POTENTIAL

 

DISSOLVED SUCROSE IN THE THISTLE TUBE GENERATES AN OSMOTIC POTENTIAL WHICH DROPS THE WATER POTENTIAL (Y < 0) SO IT IS LOWER THAN THE WATER POTENTIAL OF PURE WATER (Y = 0). WATER MOVES FROM A HIGH WATER POTENTIAL TO A LOWER WATER POTENTIAL AND WATER RISES IN THE THISTLE TUBE.