Describe how the properties of water make it an
ideal medium for life on earth. (8)
Osmosis is the diffusion of water through a
membrane.
Water moves from one area to another due to a
pressure called water potential.
Water potential is reduced by dissolved solutes.
Pure water has the highest water potential. Its value is
zero.
A solution has a lower water potential e.g. -1.2 KPa.
Water potential can be opposed by other pressures
such as blood pressure and the pressure of a cell wall.
Algal cell
1
2
3
4
5
6
Initial  values
(MPa)
solution
wall
-0.4
0.0
-0.8
0.0
-0.6
0.3
-0.5
0.5
-0.8
0.4
-1.0
0.4
 of Bathing
liquid (MPa)
0.0
0.0
0.0
0.0
-0.6
-0.7
Rate of Diffusion a
surface area x concentration difference
distance
Molarity of sucrose solution
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
Water potential (Kpa)
-130
-260
-410
-540
-680
-860
-970
-1120
-1280
-1450