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Transpiration: Biology Lab

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  • 0:02 What Is Transpiration?
  • 2:52 What Affects Transpiration?
  • 5:26 Lesson Summary
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Instructor: Jennifer Szymanski

Jen has taught biology and related fields to students from Kindergarten to University. She has a Master's Degree in Physiology.

Transpiration, movement of water from the roots to shoots of a plant, is crucial to a plant's ability to maintain homeostasis. This lab examines both the mechanism of transpiration and the effects of the environment on this important process.

What Is Transpiration?

Humans sweat - a lot. Whether we jog up the stairs, are stuck in a crowded elevator, or just have too many layers of clothing on, we release water in an effort to keep our body temperature relatively constant. This relatively stable set of internal conditions is called homeostasis. Plants also release water as part of their need to maintain homeostasis. However, instead of perspiration, it's called transpiration, technically defined as the loss of a plant's water to its environment through evaporation.

Let's start by reviewing a few terms crucial to understanding properties of water. First, recall that water moves from an area of high concentration to an area of low concentration through osmosis. Water molecules also exhibit the properties of cohesion, in which water molecules stick to one another, and adhesion, in which water molecules stick to surfaces. One way to remember the difference between these two terms is to think that in adhesion you 'add' a surface, while cohesion works when water molecules 'cooperate' with one another. Together, cohesion and adhesion cause capillary action, the movement of a liquid due to forces between molecules of that liquid. Capillary action and surface tension, the pressure on the surface of a liquid due to intermolecular forces, allow water to move up a narrow tube, like xylem in plants, against the force of gravity.

But how does this process really work? First - think 'TACT': transpiration, adhesion, cohesion, and tension. All of these things combine to draw water from the root up and through the shoot. How? Well, first, recall that gas exchange takes place primarily in openings in the leaves called stomata. Although the guard cells that surround these stomata have some control over how much water can escape through the stomata, a significant amount of water enters the atmosphere by evaporation. This is the technical definition of transpiration, and the first 'T' in 'TACT.'

As this happens, water that's already in the leaves rushes in to fill the space left behind by the evaporating water molecules, which stick to one another and to the inner surfaces of vessels in the leaves and stem. These are the 'A' and 'C' of 'TACT.' These moving water molecules, in turn, pull the water from the xylem. Tension, the final 'T,' between the water molecules pulls water up the xylem from the roots. It's not unlike pulling on a line of people holding hands.

What Affects Transpiration?

What are some variables that you think might affect transpiration? Let's start at the top. If you poke one hole in a bucket of water, it will leave the bucket very slowly. If you poke 20 holes, on the other hand, it will empty the bucket pretty quickly. Likewise, it's reasonable to expect that the number of stomata per leaf will have an effect on the rate of transpiration. Creating microscope slides using dermal tissue squares of equal size from the upper and lower epidermis, counting the number of stomata that appear, and estimating the number of stomata per plant per unit of surface area will give you an estimate of that plant's ability to complete transpiration.

A better and quantitative way to measure the rate of transpiration is to create a potometer, a set-up designed to measure the rate of transpiration. Water loss is measured by using a pipette, and the rate of transpiration is determined by dividing the measured water loss by a specific period of time. You can then subject the plants of equal leaf surface area to more than one potometer with different environmental conditions to see what variables affect the rate of transpiration.

What are some of these variables? Let's consider four set-ups:

Set-up number Condition
1 room conditions (control)
2 increased humidity
3 increased light
4 increased wind (fan)

Think about what you would predict for each of these conditions, considering what is going on in the leaves. How would each of these conditions compare to the control condition? These graphs show some data you might get from such an experiment:

Experiment data
three graphs of different conditions for transpiration experiment

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