Internal cohesion

 One of the biggest differences between a gas and a liquid is the tendency for liquid molecules to stick together or be cohesive. In contrast, gas molecules bounce off each other in elastic collisions. When they are very close they are attracted momentarily, but not strongly enough to stick them together. The molecular attractions in a liquid such as water can be quite strong and are the result of hydrogen bonding between the water molecules. This attraction will pull a glob of water floating in the space station into a sphere or hold a column of water together such that you can “pull” on the column at one end, and suck the liquid up a straw or the xylem of a plant.

The internal cohesion of liquids has several implications for biological systems. It is particularly important for plants which need to move fluids, but don’t have pumps with moving parts to do it.  We will discuss how much fluid can be moved when we talk about flow. For now, we just note that water moves up the xylem of a plant due to its internal cohesion. The movement is driven by the evaporation of water from the leaves, causing an effective negative pressure, sucking the water from the roots up through the body of the plant to those leaves, based solely on water’s internal cohesion.

Julia Gouvea 8/21/13