Transpiration
In this topic you will:
- find out how water vapour is lost from plant leaves
- summarise why plants need water
Key words
- transpiration
- wilted
How water moves through leaves
In the last topic, we saw how water goes into a root, across the root to its centre, and then up through the root and stem towards the leaves. Now we will think about what happens to the water when it reaches the leaves.
This diagram shows a section through part of a leaf. The arrows show how water moves out of the xylem vessels, and into the leaf.
When water arrives at a leaf, it moves out of the xylem vessels and into the leaf cells. The cells that have chloroplasts use some of the water for photosynthesis. But they do not need very much water for that, and most of the water does not stay in the cell. The liquid water in the cell soaks into the cellulose cell wall, and then changes to water vapour – it evaporates. The water vapour diffuses into the air spaces between the cells.
These air spaces connect with the air outside the leaf through tiny holes in the underside of the leaf – the stomata. The water vapour can diffuse through these holes and into the air.
The loss of water vapour from leaves is called transpiration.
Quick Fact
Transpiration not only helps move water through the plant, but also cools the leaf and helps draw nutrients up from the roots.
Important Concept
Water moves through a leaf by evaporation and diffusion. Most of it does not stay in the cells but exits the leaf through the stomata as water vapour. This process is vital for cooling the plant and maintaining water and nutrient flow.
Common Mistake
Don’t confuse transpiration with photosynthesis. Although water is used in both processes, transpiration is mainly about water loss and movement, while photosynthesis is about making food using light energy.
Think Like a Scientist
Investigating transpiration
In this experiment, you will investigate water loss from a plant growing in a pot.
You will need:
- two very similar plants, both growing in the same size pots
- two big, transparent polythene bags
- a balance for measuring the mass of the plants in their pots
Steps to follow:
Step 1: Check that the soil in both plant pots is moist. If not, give both of them the same volume of water. Leave the pots for about five minutes, so that any excess water can drain away.
Step 2: Take the first plant, and completely cover the plant and the pot with a plastic bag. Tie the bag very firmly at the top.
Step 3: Take the second plant, and cover the pot and soil, but not the leaves of the plant, with a plastic bag. Tie the bag tightly around the plant stem – but take care not to damage the stem.
Step 4: Measure the mass of both plants in their pots. Record the masses.
Step 5: Each day for at least a week, measure the masses of both plants. Record all of your readings in a results table. (You could put the date in the first column, and the masses of the two plants in the next two columns. Remember to include the units in the column headings.)
Step 6: Use your results to draw a line graph. You can draw two lines on the same graph – one line for each plant.
Questions:
Show Answer
Whether or not the leaves of the plant were exposed to air.
Show Answer
Same type of plant, same pot size, same soil moisture, same environment, same measuring method.
Show Answer
The plant with exposed leaves lost mass faster and more than the one with leaves covered.
Show Answer
The plant with uncovered leaves lost more water through transpiration, causing a faster decrease in mass.
Think Like a Scientist
Which side of a leaf loses most water?
In this experiment, you will hang a number of leaves up to dry. The leaves will be treated differently, so that the surfaces that can lose water vapour are not all the same. When the leaves lose water, they lose mass. You can compare the mass lost from each leaf to find out which treatment is best at helping the leaf to conserve (keep) its water.
You will need:
- eight leaves, all from the same kind of plant, and all roughly the same size
- some petroleum jelly
- at least eight paperclips, or another way of fixing the leaves to your ‘washing line’
- a ‘washing line’ that you can hang your leaves from
- a balance to measure the mass of the leaves
- some filter paper or other paper to place on the balance during measurement
Steps to follow:
Step 1: Choose eight leaves that are as similar to each other as possible.
Step 2: Line the leaves up on a piece of paper. Write a letter next to each leaf, in order: A, B, C, D, E, F, G and H. Make sure each leaf stays next to its letter.
Step 3: Apply petroleum jelly to some of the leaves:
- Leaves A and B: no jelly
- Leaves C and D: jelly on upper surface only
- Leaves E and F: jelly on lower surface only
- Leaves G and H: jelly on both surfaces
Step 4: Measure and record the mass of each leaf carefully.
Step 5: Hang all eight leaves on your washing line in order. Do not label them physically; maintain their order based on your paper list.
Step 6: Make a prediction about which leaves will lose mass fastest. Write it down with reasons.
Step 7: Allow the leaves to dry for at least one day. Then remeasure their masses.
Step 8: Record your new readings in a results table.
Step 9: Identify and decide what to do about any anomalous results.
Step 10: Calculate the mass lost by each leaf and record it in your table.
Step 11: Calculate the mean change in mass for each treatment pair (A+B, C+D, etc.) and add that to your table.
Questions:
Show Answer
Most water conserved: G/H (both sides coated) > E/F (lower side coated) > C/D (upper side coated) > A/B (no coating).
Show Answer
Most stomata are on the lower surface of the leaf. Blocking this side reduces water loss more effectively. Blocking both sides prevents almost all transpiration.
Show Answer
No, slight variations in leaf thickness, size, or jelly coverage may have caused differences in mass loss.
Show Answer
Using two gives a basic average, but more leaves would improve reliability and reduce the effect of anomalies.
Show Answer
Suggestions could include using a more accurate balance for better mass readings, and drying the leaves in a controlled environment to keep temperature and humidity consistent.
Self-assessment
How well did your results chart work? Do you think you designed it well? Was it easy to write your results into it?
Why plants need water
In this topic and the previous one, you have learnt how plants absorb water and transport it to their leaves. But why do plants actually need all this water?
Water for support
The photograph shows one reason why plants need water – it helps them to stand upright.
The plant on the left has been given plenty of water. The plant on the right has not been watered for several days.
Plant cells contain a lot of water, especially inside their vacuoles. A plant cell that has plenty of water is strong and firm. When all the cells in a plant are like this, they press on one another and make the whole plant firm and well supported.
When a plant cell doesn’t contain enough water, it becomes soft and floppy. When all the cells in a plant are like this, the plant collapses. We say that it has wilted.
Water for transport
As water moves through the xylem vessels, it carries dissolved mineral salts. This is how these mineral salts are transported from the roots to all the other parts of the plant.
Water for cooling
When water evaporates, it takes heat away with it. So when water evaporates from the cell wall of a cell in a leaf, it cools the cell. This is important for plants that live in very hot places.
Water for photosynthesis
Water is one of the reactants in photosynthesis. In photosynthesis, plants change water and carbon dioxide to glucose and oxygen.
Quick Fact
A plant can lose up to 95% of the water it absorbs through transpiration, yet it still plays a vital role in temperature control and nutrient transport.
Important Concept
Water helps maintain turgor pressure in plant cells. This internal pressure keeps the plant upright and firm, especially in non-woody stems and leaves.
Common Mistake
Some students think that water is the plant's food. In fact, plants use water in photosynthesis to make their own food—glucose—from carbon dioxide and sunlight.
Questions
Show Answer
The plant becomes soft and floppy as cells lose turgor pressure. Eventually, it may collapse and wilt.
Show Answer
Water is needed to transport minerals through the xylem and is used in photosynthesis to produce glucose and oxygen.