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Explain Transpiration

Worksheet Overview

QUESTION 1 of 10

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. This is because a concentration gradient exists - there's more water in the roots than the leaves. Water moves by osmosis from a high concentration in the roots to a lower one in the leaves. As water moves from the roots to the leaves, more water is drawn up from the soil into the root hair cells. This occurs because the concentration of water in the soil is greater than in the roots, so water moves by osmosis 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. It also gives plants structure and support.


 

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. This is because water evaporates at a faster rate when it's warmer.  
Humidity - if it's really humid, it means there's a lot of moisture in the air. The concentration of water outside the plant is higher than inside the plant, so the plant doesn't transpire as much and doesn't lose as much water. 
Wind - if it's really windy, water vapour is blown away from the leaf.  The concentration of water outside the plant is lower than inside the plant, causing the plant to lose more water from its leaves.
Light intensity - if it's 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.

 

Root Hair Cell

 

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 for a greater chance of contact with water.​ Root hair cells contain lots of mitochondria which provide the cell with energy. This energy is essential for active transport, which is needed to take in minerals from the soil.

 

Stomata

 

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 depending on how turgid (full of water and swollen - stomata open) or flaccid (lacking water and shrunken - stomata closed) the guard cells are. 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. 


Xylem

 

 Image of xylem and phloem vessels

 

 

Water moves through xylem vessels.  A xylem vessel is a continuous hollow tube that transports water and minerals in one direction from a plant's roots to the plant's leaves, via the stem. This allows water to travel uninterruptedly up the stem to the leaves. Xylem vessels are made of lignin that strengthens the walls. This helps to support the plant.

 

Phloem

Another transport vessel the plant has is called the phloem. The phloem vessels move sucrose (a form of sugar) that the plant has made by photosynthesis to where it's needed (for example in growing parts of the plant and storage). Food travels up and down the stem using energy. This is known as translocation.


 

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 image 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 explain how transpiration occurs in plants.

The xylem vessel doesn't have end walls between cells.

 

How does this adaptation help the plant?  

 

Image of xylem and phloem vessels

It allows food to be transported quickly

It allows water to be transported quickly

It allows carbon dioxide to be transported quickly

Root hair cells have an important job to do - they allow water and minerals to be absorbed from the soil.


What two features of the root hair cell allow water and minerals to be absorbed by the plant?

 

Image of root hair cell

 

Thin walls to pass through

Thick walls to pass through

Small surface area to take up less space

Large surface area to absorb more water and ions

Which environmental factors can affect the rate of transpiration? 

Wind speed

Minerals

Humidity

Temperature

Light intensity

What does the term translocation mean?

The movement of nutrients made through photosynthesis to where it's needed

The movement of water through the plant

The movement of gases in and out of the stomata

Fill in the blanks below explaining why transpiration is normally slower at night than during the day. 

 

This is because the ...A... are closed during the ...B... as plants can't ...C..., so plants won't lose ...D...

Column A

Column B

A
Night
B
Stomata
C
Water
D
Photosynthesise

The phloem vessel allows substances to be transported up and down the plant.

 

 How does this adaptation help the plant?

 

Image of xylem and phloem vessels

 

It allows water to be transported quickly

It allows oxygen to be transported quickly

It allows substances to be transported to wherever they are needed most

Explain why transpiration happens at a slower rate if it's very humid outside?

Column A

Column B

High humidity means there's a lot of
is higher than inside the plant
The concentration of water outside the plant
water vapour in the air
The plant doesn't lose as much water to the air be...
water normally diffuses from a high concentration ...
The rate of transpiration
decreases

Helen and Aisha are investigating transpiration in their science lesson. They'll be setting up equipment to measure the transpiration rate of a plant. Their teacher gives them instructions to follow but they soon realise they're not in the correct order.

 

Help Helen and Aisha place the instructions in the correct order.

 

  Image of a potometer

 

Step

 Instructions to measure transpiration rate

1  The faster the bubble moves, the greater the rate of water uptake and so the greater the rate of transpiration.
2  A leafy plant is connected to the potometer underwater through a rubber tube at one end of the potometer.
3  The bubble is observed and the distance travelled by the bubble over a set period of time is measured.
4  The potometer is filled with water.
5   A bubble of air passes up the potometer when held upright and then placed back into the water.

 

 

4, 2, 5, 3, 1

4, 5, 1, 2, 3

5, 4, 1, 3, 2

Explain what the transpiration stream is by placing the following into the order in which they occur.

 12345
More water is drawn up from the stem and the roots to replace the lost water as there's a higher concentration of water in the roots than in the leaves
Water travels up the stem to the leaves
As the plant opens its stomata to allow carbon dioxide in for photosynthesis water evaporates and diffuses out of the stomata
The concentration of water in the soil is greater than in the roots so water moves by osmosis into the root hair cells
As water moves from the roots to the leaves more water is drawn up from the soil into the root hair cells

Explain how the guard cells open and close the stomata to allow water to enter and leave the plant. 

The stomata are found between guard cells, which absorb water in the light and become turgid, opening the stomata

The stomata are found between guard cells, which absorb oxygen in the light and become turgid

In the dark, the guard cells lose water and become flaccid, closing the stomata

In the light, the guard cells lose water and become flaccid, closing the stomata

  • Question 1

The xylem vessel doesn't have end walls between cells.

 

How does this adaptation help the plant?  

 

Image of xylem and phloem vessels

CORRECT ANSWER
It allows water to be transported quickly
EDDIE SAYS
Can you remember what the xylem carry inside them? Xylem cells transport water and minerals from the roots of a plant up to the leaves. As the xylem forms one continuous tube, water can flow directly to the leaves where it's used for photosynthesis.
  • Question 2

Root hair cells have an important job to do - they allow water and minerals to be absorbed from the soil.


What two features of the root hair cell allow water and minerals to be absorbed by the plant?

 

Image of root hair cell

 

CORRECT ANSWER
Thin walls to pass through
Large surface area to absorb more water and ions
EDDIE SAYS
Root hair cells have thin walls which allow water and mineral ions to pass through really easily and quickly. The tiny projections of the root hair cell increase the surface area, allowing more water to be in contact with the cell. This means more water and minerals which are needed for the process of photosynthesis, can pass into the root hair cell.
  • Question 3

Which environmental factors can affect the rate of transpiration? 

CORRECT ANSWER
Wind speed
Humidity
Temperature
Light intensity
EDDIE SAYS
There were four correct options to tick here - did you get them all? These four factors are often asked about in exam questions, so it's useful to remember them. Minerals are important to a plant but have no affect on the rate of transpiration.
  • Question 4

What does the term translocation mean?

CORRECT ANSWER
The movement of nutrients made through photosynthesis to where it's needed
EDDIE SAYS
All three options were describing vital movements of substances through plants - but which one refers to translocation?! Translocation is the movement of nutrients - mainly sugars and amino acids - around the plant. It is a process that is carried out by the phloem cells. The movement of water through the plant takes place in the xylem cells and is part of the transpiration stream. The movement of oxygen and carbon dioxide happens through the stomata in the leaves by diffusion.
  • Question 5

Fill in the blanks below explaining why transpiration is normally slower at night than during the day. 

 

This is because the ...A... are closed during the ...B... as plants can't ...C..., so plants won't lose ...D...

CORRECT ANSWER

Column A

Column B

A
Stomata
B
Night
C
Photosynthesise
D
Water
EDDIE SAYS
How did you get on with this one? The plant's stomata don't need to open at night, since there's no sunlight to power photosynthesis. This allows plants to reduce water loss from transpiration.
  • Question 6

The phloem vessel allows substances to be transported up and down the plant.

 

 How does this adaptation help the plant?

 

Image of xylem and phloem vessels

 

CORRECT ANSWER
It allows substances to be transported to wherever they are needed most
EDDIE SAYS
Did you get a bit muddled with this and the previous question? It is important to get the xylem and the phloem sorted out! This question is asking about the phloem vessels, which are the ones that transport substances around the plant. A two-way flow helps the plant to get access to substances such as sucrose and amino acids, which are needed for growth or storage.
  • Question 7

Explain why transpiration happens at a slower rate if it's very humid outside?

CORRECT ANSWER

Column A

Column B

High humidity means there's a lot...
water vapour in the air
The concentration of water outsid...
is higher than inside the plant
The plant doesn't lose as much wa...
water normally diffuses from a hi...
The rate of transpiration
decreases
EDDIE SAYS
Don't worry if you found this a little tricky! It's easy to get mixed up with humidity and heat/temperature. Remember that humidity is about how much moisture is in the air and not the actual temperature. If there is a high humidity, this means that there is a lot of water in the air - a higher concentration of water in the air than in the plant's leaves. Since water moves from a high concentration to a low concentration by osmosis, water will not travel from the plant into the air so the transpiration rate will be low.
  • Question 8

Helen and Aisha are investigating transpiration in their science lesson. They'll be setting up equipment to measure the transpiration rate of a plant. Their teacher gives them instructions to follow but they soon realise they're not in the correct order.

 

Help Helen and Aisha place the instructions in the correct order.

 

  Image of a potometer

 

Step

 Instructions to measure transpiration rate

1  The faster the bubble moves, the greater the rate of water uptake and so the greater the rate of transpiration.
2  A leafy plant is connected to the potometer underwater through a rubber tube at one end of the potometer.
3  The bubble is observed and the distance travelled by the bubble over a set period of time is measured.
4  The potometer is filled with water.
5   A bubble of air passes up the potometer when held upright and then placed back into the water.

 

 

CORRECT ANSWER
4, 2, 5, 3, 1
EDDIE SAYS
It's important to set up the potometer correctly, especially when attaching the plant. Attaching the plant is usually done underwater so that air does not get trapped in the plant's xylem vessels. Once you are familiar with how to measure the rate of transpiration, it is possible to adjust certain factors (such as temperature and light intensity) to see how these will affect the rate.
  • Question 9

Explain what the transpiration stream is by placing the following into the order in which they occur.

CORRECT ANSWER
 12345
More water is drawn up from the stem and the roots to replace the lost water as there's a higher concentration of water in the roots than in the leaves
Water travels up the stem to the leaves
As the plant opens its stomata to allow carbon dioxide in for photosynthesis water evaporates and diffuses out of the stomata
The concentration of water in the soil is greater than in the roots so water moves by osmosis into the root hair cells
As water moves from the roots to the leaves more water is drawn up from the soil into the root hair cells
EDDIE SAYS
How are you getting on so far? This is a great activity to consolidate your knowledge. If you didn't get it spot on first time, you can always give it another go or two!
  • Question 10

Explain how the guard cells open and close the stomata to allow water to enter and leave the plant. 

CORRECT ANSWER
The stomata are found between guard cells, which absorb water in the light and become turgid, opening the stomata
In the dark, the guard cells lose water and become flaccid, closing the stomata
EDDIE SAYS
Have you grasped how the guard cells control the opening and closing of the stomata? The guard cells surround and control the stomata - which is why they have the name guard cells! In the light, the guard cells absorb water and swell up (become turgid) which opens the stomata. In the dark, they lose water and become flaccid which closes the stomata. You've reached the end of this activity. Great work!
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