UCSB Science Line
Sponge Spicules Nerve Cells Galaxy Abalone Shell Nickel Succinate X-ray Lens Lupine
UCSB Science Line
How it Works
Ask a Question
Search Topics
Our Scientists
Science Links
Contact Information
How does Transpiration work?
Question Date: 2014-02-09
Answer 1:

Transpiration is basically evaporation with a plant added in. It’s called evapotranspiration once we add in the evaporation. As you know, when liquid water turns to water vapor, that’s evaporation. The molecules of water don’t change when they become vapor, they’re just moving a lot more. The energy it takes to move more comes from heat. You probably know that things evaporate faster when you add more heat. You don’t have to boil water to make it evaporate, that just makes evaporation go even faster.

Now let’s look at the plant. Plants have many tiny tubes inside. Some of these go all the way from the roots to tiny holes in the plant leaves. These carry things around a plant, sort of like our blood vessel carry things around our bodies. One major difference is that the plant has no heart to pump fluid around. When sap made in the leaves is going down to the roots, gravity does the job. But how does water get from the roots to the leaves? The energy comes from the sun.

Water molecules are attracted to each other because they have a semi-positive end and a semi-negative end. You know what they say, “opposites attract.” You know how water beads up on a piece of glass, or how you can carefully add drops of water to a container so that it actually bulges over the top? That’s because water molecule are attracted to each other. Imagine that they like to hold hands.

Okay, water molecules don’t have hands, so imagine the aliens from Toy Story holding hands. The claw comes down and grabs one of them, and lifts it up. All of the ones who are holding hands get pulled up with it.

Now imagine that I pull on the water molecule at the top of the tubes in the plant. Not only do I pull on that molecule, I pull on all the ones it’s “holding hands” with. The tiny tubes inside the plant are also very “sticky” for water. As a water molecule evaporates out through the tiny leaf holes into the air, more water molecules get pulled up the tiny tube behind it. The pull goes all the way down to the roots. The water below is also pushing into drier places. It’s like a big solar pump taking water from the ground and putting it into the air. The water also has nutrients in it that the plant needs.

One thing that doesn’t come up from the roots is carbon dioxide. Plants need carbon dioxide to do photosynthesis. See if you can figure out where that comes from.

If you are interested in plants, you may want to study botany.

Thanks for asking,

Click Here to return to the search form.

University of California, Santa Barbara Materials Research Laboratory National Science Foundation
This program is co-sponsored by the National Science Foundation and UCSB School-University Partnerships
Copyright © 2020 The Regents of the University of California,
All Rights Reserved.
UCSB Terms of Use