Transpiration: How Plants Defy Gravity, among other things

labeled for reuse link

Plants. Yup plants. I actually just heard that a scientist hooked up a plant to a lying machine, and supposedly showed that they can faint from pain. Think about that next time you eat a salad. Just a bowl full of passed out lettuce. Bon Appetit.

But that is not what we are here to learn about. As usual I digress. So transpiration. The process starts off in the roots of plants. So extend your mind beneath the soil. You there yet? Alrighty. As you probably know soil is rich in minerals. If you didn't you know now. So, in the roots of the plant these minerals are actively transported into the roots. It is active transport because there are more minerals in the roots than there is in the soil. If you've forgotten, active transport is the movement of substances against the concentration gradient. So now that we have a high concentration of ions in the roots we can get water to come into them via osmosis. Remember, osmosis is the movement of water molecules, across a semipermeable membrane, from an area of low solute concentration, to an area of high solute concentration. So now the picture comes together. The active transport of minerals into the root allow for the passive movement of water molecules into it.

Step one complete. Now that the water is in the root of the plants it will enter the xylem. Xylem is tube like structure that brings water and minerals up to the leaves of the plant. It can only go up, not down. The xylem is made up of dead cells called parenchyma and sclerenchyma. The structures themselves are known as tracheids and vessel elements. Now the water enters the xylem. Following the law of gravity the water would just remain there, it would fall. Like the apple on Newton's head. But due to two concepts called adhesion and cohesion, water is kept in a line going up the xylem. Cohesion is an attractive force between two or more like molecules. Whereas, adhesion is an attractive force between two unlike molecules. So we see adhesion between two water molecules, between a negatively polarized oxygen atom of one water molecule and a positively polarized hydrogen of another water molecule. This keep them in a straight line. Adhesion, occurs between water molecules and the walls of the xylem. This also keeps them in a line. Like good school children.

This still doesn't explain why water moves against gravity! You cry in frustration. Never fear, your answer is near. Not rhyme intended. It just sort of happened. Well in the leaves of a plant we have small pores called stomata. These stomata open in order to let carbon dioxide in (to conduct photosynthesis) and to let oxygen out (waste product of photosynthesis). However, these open pores also result in the evaporation of water (called transpiration/evapotranspiration). This force of transpiration pulls on the stream of water molecules, as they evaporate out of the plant.

And there you have it. That is how plants defy gravity. You can't say the same I'm afraid.