Physics in Chemistry – Water Potential

Physics in Chemistry

Did you know that the tallest of the trees are redwoods and they grow up to around 100 meters (that is 1/5th the height of Taipei 101)? But how do they manage to pump water all way up to nourish their leaves, fruits and branches. The fact that xylem conducts water, doesn’t point at the driving force enabling this phenomenon. Let’s see what physics has got to say about this.

Chemical potential is used to explain reactions that take place in nature and in our bodies. It is not much different from the potential energy that is discussed in textbooks. Chemical potential is the change in potential energy of each of the molecules associated with a change in the number of molecules, of a particular type, in a given system. For example let’s assume that you take a glass of salt water and measure the potential energy of each of the water molecules present in system. Now add a little more water to the glass and measure the potential energies again; assuming everything else is constant the change in potential energies of the water molecules for one unit of water molecules added is called as chemical potential and in the case of water it’s called water potential. So this tells us that when water molecules are surrounded by a lot of water molecules they have more potential energy.

The apple that has been rumored to have fallen on Newton’s head is affected by gravity. It can also be interpreted as the virtue of the apple to lose its gravitational potential energy. An apple placed up in the air has more potential energy than that of an apple placed on ground. And hence falls onto the ground in the process of which it loses gravitational potential energy. Similarly, a water molecule that has a lot of chemical potential energy tries to reduce it, by all means.

Water escapes from the leaves of a plant in the form of vapor and is released into the atmosphere and this process is called transpiration. This water is generally evaporated from the cells that are present on the surface of the leaves. Now the cells below the surface have more water in them. The water molecules present in the cells underneath have more chemical potential energy, as there are more water molecules present in the inner layer cells compared to the cells present on the surface. The general tendency to reduce potential energy forces them to move to the upper layers. The water that moves to the upper layers is evaporated into the atmosphere as the number of water molecules in the atmosphere is much less than the number of water molecules in the leaf cells.

Veinlets on the leaf supply water to the inner layers of the leaf. These veinlets in turn are connected to the veins that are in turn connected to the xylem of the trunk. These xylem cells are connected all the way down to the root of a plant, which absorbs water from soil. Thus water potential drives water all the way up to 100 meters and more. Now, imagine how much electricity is consumed in pumping water all the way up to 100 meters and give thought to how evolved plants have developed an energy efficient mechanism to do the same.

How’s that for applications of the concepts taught in Physics tuition classes? : )