How do Trees lift Water From the Ground?

Handout 21 of Biology 101
Professor Mayim Etzman


How do trees raise water from their roots to the leaves? Some of the power lifting water in trees comes from the sun. Solar radiation vaporizes water inside leaves. That makes the water left in the leaves more concentrated so that osmotic pressure pulls water up into the leaves. This process is called transpiration. Another force that pulls water up the trees comes from hydrogen bonding. Trees contains long columns called xylem. The walls of the xylem are made of cellulose to which water binds strongly and so there is a force pulling the water up the cellulose. You can see this force in action if you dip a paper towel in a bowl of water. Water will travel up the cellulose in the paper towel. The water that climbs the cellulose hydrogen bonds with other water molecules and pulls them up with it. Water descends through other parts of the tree via the phloem. The weight of the water in the phloem probably helps counterbalance the weight of the water in the xylem the way heavy weights help counter balance the weights of elevators. (This is just professorial speculation.)



There are many mysteries with unconvincing explanations regarding how a tree can lift water so high.  If suction created by transpiration were too lift water as high as a tree calculations show that the pressure gradient would have to be such that water would vaporize.  In addition calculations have shown that the height to which capillaries can lift water depends inversely on the diameter of the capillaries.  Calculations show that Xylem, whose minimum diameter is 20 micrometers should only be able to lift water 3 feet.  The pores in leaves however are a lot smaller than 20 micrometers calculations show that at their diameter a capillary could lift water a kilometer.  The pore in leaf explanation doesn't hold water (pun intended) however when explaining how a tree can lift water.

The redwood tree in the picture below is over 300 feet high.  It is truly remarkable that redwood trees can lift water so high.