3.
Adhesion, Cohesion, and Surface Tension
Hydrogen
bonding creates many unique physical properties of water and many organisms
depend on these constants for life functions. Adhesion, cohesion, and surface tension, are results of
hydrogen bonding.
Surface
tension of water is its surface resistance to stretching or breaking. Water has an exceptionally high surface
tension when compared to other liquids. Where water and air interface there is an ordered
arrangement of water molecules that are hydrogen bonded to one another and to
the water below. The pull on the
surface molecules by molecules below and beside (cohesion), creates an elastic
surface where the molecules are pulled in towards themselves assuming the least
amount of area they can (Eugene Hecht 2003).
Cohesion,
simply, is water sticking
to itself from hydrogen bonding. Adhesion is when water adheres to
another substance. A great example
of these two functions working together is found in plants. A tree undergoes a daily process of
transpiration, where water is pulled from the roots to shoot tips, and then
evaporated into the atmosphere.
This process provides the treeÕs vital regions with water and aids in
thermoregulation.
Evaporation from leaves and cohesion pulls water up from the roots
through microscopic tubes called xylem vessels. The transport of the water is helped by adhesion. The walls of the xylem cells are
hydrophilic which means they have the ability to hydrogen bond with water. Water moving up the tubes adheres to
the walls and in this manner is able to combat the downward pull of gravity
(Fig. 6)