The shape of the lesson
By the end of the lesson, learners can
- Evaporation, then diffusion: The sim shows a block of leaf: palisade and spongy mesophyll cells with air spaces between them and stomata in the lower surface.
- Measuring it: the potometer: A potometer measures how fast a leafy shoot takes up water.
- The transpiration pull, and wilting: Extended.
- Steps: water evaporates from mesophyll cell surfaces into air spaces, then water vapour diffuses out through the stomata.
- Faster with: higher temperature, higher wind speed; slower with higher humidity (Extended explanations: kinetic energy, gradient).
- Potometer: measures water uptake; rate = distance ÷ time; change one factor at a time.
- Extended: transpiration pull on a column held together by attraction between water molecules; wilting = flaccid cells.
One picture every idea returns to
Transpiration is the loss of water vapour from leaves. Water evaporates from the surfaces of the mesophyll cells into the air spaces and then diffuses out of the leaves through the stomata as water vapour. Higher temperature and higher wind speed increase the rate. Extended: the loss of water pulls a continuous column of water molecules up the xylem (the transpiration pull), because water molecules are attracted to each other; high humidity slows transpiration; and when water is lost faster than it is taken up, cells become flaccid and the plant wilts.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Leaves lose water, roots replace it.. A sunflower can lose more than a litre of water on a hot day, almost all of it as water vapour from its leaves. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Evaporation, then diffusion | The sim shows a block of leaf: palisade and spongy mesophyll cells with air spaces between them and stomata in the lower surface. Project tab 4 of the lab (Transpiration in a leaf) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Measuring it: the potometer | A potometer measures how fast a leafy shoot takes up water. Project tab 5 of the lab (Potometer investigation) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: The transpiration pull, and wilting | Extended. Project tab 6 of the lab (Transpiration pull and wilting) and let a student drive it while the class predicts. | Jigsaw |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 22 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in The transpiration pull, and wilting, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Transpiration is the loss of water vapour, and the vapour leaves by diffusion through the stomata. | Liquid water does not flow out of the stomata, and the stomata are not where the water evaporates: it evaporates inside the leaf from the mesophyll cell surfaces. |
| A potometer measures water uptake, not water loss. | A small amount of the water taken up is used in the plant, for example in photosynthesis, so uptake is only approximately equal to transpiration. |
| The transpiration pull is a pull from the top, caused by water loss from the leaves. | Do not describe it as the roots pushing water up, and do not say the molecules are "sucked" up: the key idea is the attraction between water molecules holding the column together. |
Transpiration: worksheet
6 questions, 22 marks. Written in the style of the papers; no past paper question is reproduced.
Describe how water leaves a leaf during transpiration.
A student used a potometer to time an air bubble moving 50 mm at different temperatures.
| Temperature / °C | Time / s |
|---|---|
| 15 | 500 |
| 25 | 300 |
| 35 | 170 |
(a) Calculate the rate of water uptake in mm per minute at each temperature, to one decimal place. (b) Describe the trend.
Plan an investigation, using a potometer, to find the effect of wind speed on the rate of transpiration.
Extended. Explain why the rate of transpiration is higher on a hot, dry, windy day than on a cool, humid, still day.
Extended. Explain how water moves up the xylem from the roots to the leaves of a tall tree.
Extended. A pot plant left on a sunny windowsill without water wilts by the afternoon. Explain how and why wilting occurs, and suggest how wilting can help the plant.
Answer key and marking notes
- Water evaporates from the surfaces of the mesophyll cells.
- Into the air spaces inside the leaf.
- Water vapour then diffuses out of the leaf through the stomata.
- (a) 15 °C: 50 ÷ 500 × 60 = 6.0 mm per minute.
- 25 °C: 50 ÷ 300 × 60 = 10.0 mm per minute.
- 35 °C: 50 ÷ 170 × 60 = 17.6 mm per minute.
- (b) As temperature increases, the rate of water uptake (and so transpiration) increases, and it rises more steeply at higher temperatures.
- Cut a leafy shoot under water, fit it to the potometer under water and seal the joints with petroleum jelly.
- Place a fan at a fixed distance and use several settings (off, low, medium, high) to vary wind speed.
- For each setting, time how long the bubble takes to move a fixed distance (or measure distance moved in a fixed time) and calculate the rate; reset the bubble with the reservoir between readings.
- Keep temperature, light intensity and the same shoot constant; repeat each setting and calculate a mean.
- Higher temperature: water molecules have more kinetic energy, so they evaporate from mesophyll surfaces and diffuse out faster.
- Dry air (low humidity): less water vapour outside, so a steeper concentration gradient from leaf to air.
- Wind: removes water vapour from around the stomata, keeping the concentration gradient steep.
- Water evaporates from mesophyll cells and diffuses out of the leaves as water vapour (transpiration).
- Water moves from the xylem in the leaf into the mesophyll cells to replace it, creating a pull (tension) on the water in the xylem.
- Water molecules are held together by forces of attraction between them, forming a continuous column.
- So the whole column of water is drawn up the xylem from the roots: the transpiration pull.
- Water is lost by transpiration faster than it is taken up from the dry soil.
- The cells lose water, so their vacuoles shrink and they no longer press outwards on their cell walls: they become flaccid.
- Flaccid cells cannot support the leaves and stem, so the plant droops.
- Drooping leaves expose less surface to the sun and the stomata close, reducing further water loss.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
During transpiration, from where does water evaporate inside a leaf?
- AThe surfaces of the mesophyll cells
- BThe inside of the xylem vessels
- CThe outer surface of the upper epidermis
- DThe inside of the chloroplasts
Through which structures does most water vapour leave a leaf?
- AStomata
- BXylem vessels
- CPhloem sieve tubes
- DPalisade cells
The graph shows the distance moved by the air bubble in a potometer. What is the rate of water uptake?
- A12 mm per minute
- B60 mm per minute
- C5 mm per minute
- D0.083 mm per minute
Four identical leafy shoots were kept for one hour in: warm, windy air; warm, still air; cool, still air; cool, still air inside a plastic bag. The table shows the mass each shoot lost. Which shoot was most likely in cool, still air?
- AS
- BR
- CP
- DQ
When setting up a potometer, why is the leafy shoot cut and fitted under water?
- ATo stop air entering the xylem and breaking the water column
- BTo keep the leaves cool so the results are fair
- CTo wash away sucrose that leaks out of the phloem
- DTo make the stomata close before the experiment starts
Extended: why does the rate of transpiration fall when the air around a plant becomes very humid?
- AThe water vapour concentration gradient between the air spaces and the air is smaller
- BWater molecules have less kinetic energy in humid air
- CThe humid air pushes water back up the xylem into the leaf
- DThe mesophyll cells absorb water vapour from the air instead
Extended: an air bubble enters a xylem vessel in a stem. Why does the water above the bubble stop rising in that vessel?
- AThe column is broken, so the pull from the leaves is not passed down to the water below
- BThe air bubble uses up the energy that pumps the water upwards
- CAir in the xylem makes the stomata close at once
- DThe water is now pulled down by the phloem instead
Extended: a lettuce plant wilts on a hot afternoon but recovers in the evening without being watered. Which explanation is best?
- AIn the afternoon water was lost faster than it was taken up; in the evening transpiration slowed and cells regained turgor
- BIn the afternoon the roots stopped working; in the evening the roots started again because it was dark
- CIn the afternoon the xylem melted in the heat; in the evening it hardened again
- DIn the afternoon the cells became turgid; in the evening they became flaccid again
Quiz key
Water evaporates from the wet surfaces of the mesophyll cells into the air spaces. Water reaches the leaf in the xylem but does not evaporate inside it. The upper epidermis is covered by a waxy cuticle and loses very little water. Chloroplasts are inside the cells, not at a surface exposed to air.
Water vapour diffuses out of the air spaces through the stomata. Xylem brings liquid water into the leaf, phloem carries sucrose and amino acids, and palisade cells are inside the leaf, so none of these is the exit.
The bubble moves 60 mm in 5 minutes, so rate = 60 ÷ 5 = 12 mm per minute. 60 is the total distance, not a rate. 5 is the total time. 0.083 is time divided by distance, the wrong way round.
Mass lost is mainly water lost by transpiration. Ranked: warm and windy loses most (R, 3.4 g), then warm and still (P, 1.9 g), then cool and still (S, 0.8 g), and the humid air in the bag slows it most (Q, 0.3 g). So S was in cool, still air. R and P lost too much water for cool still air, and Q lost so little that it must have been in the bag.
Cutting under water stops air getting into the xylem. An air lock would break the continuous column of water, so the shoot could not take up water properly. The leaves stay in air, so this does not cool them. Sucrose leakage is not the concern, and cutting under water does not close the stomata.
Humid air already contains a lot of water vapour, so the difference between the nearly saturated air spaces and the outside air is small, and less vapour diffuses out. Humidity does not change the kinetic energy of the molecules; temperature does. Air does not push water up the xylem, and leaves do not switch to absorbing water vapour as their main supply.
Water molecules are held together by forces of attraction, forming a continuous column. A bubble breaks the column, so the transpiration pull from the leaves is no longer transmitted to the water below the gap. There is no pump using energy in xylem. The bubble does not directly close the stomata, and phloem does not pull water down.
On a hot afternoon transpiration is fast and the roots cannot keep up, so cells lose water, become flaccid and the plant droops. In the cooler evening transpiration slows, uptake catches up, the cells become turgid again and the plant recovers. Roots do not switch off in daylight, xylem does not melt, and wilted cells are flaccid, not turgid, so the last option has the terms the wrong way round.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.