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Understand Transpiration

Worksheet Overview

Did you know that plants only use about 5 - 6% of all the water they absorb for photosynthesis? So what happens to all that water?  Let's find out more below. 


Image of photosynthesis



Plants can’t help but lose water continually to the air. This is called transpiration. 

Water is constantly lost from the leaves of a plant through pores called stomata. When a plant opens its stomata to allow carbon dioxide in for photosynthesis, water will evaporate and diffuse out of the stomata. More water is drawn up from the stem and the roots to replace the lost water. As water moves from the roots to the leaves, more water is drawn up from the soil into the root hair cells. This process is known as the transpiration stream.

Although transpiration is inevitable, it's also quite useful! It helps the plant remain cool and allows minerals to be drawn up the plant along with the water.


Factors affecting transpiration

Transpiration is affected by many factors:

Temperature - increasing the temperature makes transpiration happen faster - the plant loses more water from its leaves.

Humidity - if it's really humid, it means there's a lot of moisture in the air - the plant doesn't transpire as much, so doesn't lose as much water.

Wind - if it's really windy, water vapour is blown away from the leaf. This causes the leaf to transpire faster, so the plant loses more water from its leaves.

Light intensity - if its really sunny, the stomata will open to let in more carbon dioxide for photosynthesis -  this causes the plant to lose water.


Plant adaptations


Plants have adaptations that allow it to do a particular job.


Image of root hair cell


Root hair cells are specialised cells found at the roots of a plant (see image above). These cells are long and thin, making them useful to manoeuvre between soil particles in search of water. The large surface area of the root hair cell allows a greater chance of contact with water.​ 


Image of leaf anatomy


Another adaptation of the plant is found in the leaf. The lower epidermis layer contains the stomata (stoma for one pore). These stomata allow gases in and out of the underside of the leaf. The stomata are found between guard cells, which open or close the stomata. The stomata will open to allow in carbon dioxide for photosynthesis during the day, but will close during the night when there's no sunlight. ​


 Image of xylem and phloem vessels


Water moves through xylem vessels.  A xylem vessel is a continuous hollow tube that transports water in one direction and minerals from a plant's roots to the plant's leaves, via the stem.

Another vessel the plant has is called the phloem. The phloem vessels move food substances that the plant has made by photosynthesis to where they are needed (for example in growing parts of the plant and storage). Food travels up and down the stem. 


Measuring transpiration


Image of a Potometer  Image of a potometer



Scientists have been able to work out how to measure transpiration in the science lab. A potometer (shown in the picture above) is used to measure how much water is being taken up by the plant and the rate at which it does so.

Here's how to use a simple potometer:


1) The potometer is filled with water.

2) A leafy plant is connected to it underwater through a rubber tube at one end of the potometer.

3) A bubble of air passes up the potometer when held upright and then placed back into the water.

4) The bubble is observed and the distance travelled by the bubble over a set period of time is measured.

5) The faster the bubble moves, the greater the rate of water uptake and so the greater the rate of transpiration.

The conditions can be varied depending on what you want to investigate, for example, the effect of changing temperature, humidity,  light intensity or wind speed (for example, with a fan).

In the following activity, you will describe how transpiration occurs in plants.

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