General ---------Biology ------------------Online----------------------- Ch. 3 Water and the fitness of the environment

Water columns.  The cohesive nature of water molecules causes them to stick together.  You can suck liquid through a straw, and even trap the liquid in a cohesive column by sealing the top end of the straw with your finger; release the finger and the liquid pours out the bottom.  You could not do this with sand, sawdust or any other non cohesive substance.

This same property allows water to rises up hollow xylem or cellulose tubes in the trunks of trees.  See Fig 3.2, Water transport in plants.  This movement of water also demonstrates adhesion of water to the xylem or cellulose tubes.  You may have seen the TV ad for paper towels, where the water is shown slowly rising up two competing brands.  Cellulose is called hydrophilic (hydro=water; philic=loving) because water will adhere to it.  Oils are hydrophobic, because they are repelled by water. 

Surface tension.  Fig. 3.2 Walking on water shows an insect gliding over a pond without breaking through the surface. Both phenomena are the result of hydrogen bonding among water molecules.  Trees, insects and other organisms have evolved to exploit both of these characteristics of water.

Summary:  write summary statements as bullet points below. 
 
 

Test your knowledge:  Practice Quiz # 29, 34.  Label this figure.