Defying gravity - how pressure difference in a tree trunk transfers water from the bottom to the top

By Mohan Raj.

Some trees grow up to 100 meters in height, but how do these trees actually transport the water from their roots all the way up to the leaves at the top?

Even in a perfect vacuum, the water can be only pumped up to about 10.3 meters. Trees create immense negative pressures of 10's of atmospheres. This is achieved by evaporating water from nanoscale pores. For every molecule of CO2 taken in, there is a lot of water molecules which evaporates. Only 1% of the water that has been sucked to the top of the trees are used for photosynthesis and other functions. What happens to the balance 95% of the water? The answer is that it evaporates.

How a tree’s pressure difference drives water up the trunk

How a tree’s pressure difference drives water up the trunk

 By sucking water up 100m in a vacuum state, one would assume that the water would start to boil due to the immense pressure. However, it does not.  This is due to the perfect xylem tubes which do not contain any air bubbles. The vacuum works because of the pressure difference between the top of the trees and the bottom of the roots. This pressure difference can amount to a staggering 16 atmospheres, where the top of the tree has an immense pressure of 15 atmospheres and the bottom has a negative pressure of negative 1 atmospheres, creating a vacuum and driving the water up the trunk.