Lesson overview
Transpiration: Why does a plant lose more water on a hot, windy day?
Time an air bubble in a potometer to measure how fast a leafy shoot takes up water, then change one thing at a time: moving air from a fan, warmer air, and humid air in a clear bag over the leaves. Find out where the water goes, how it is lost from the leaves, what pulls it up the xylem and why plants wilt.
Suggested sequence
- Lesson 1 · Water vapour leaves the leaves. Transpiration as the loss of water vapour from leaves; how a potometer measures water uptake; evaporation from the mesophyll cells and diffusion out through the stomata; the internal surface area and the stomata (Supplement). Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Hot, windy or humid. Investigating the effects of temperature and wind speed on the rate of transpiration; explaining the effects of temperature, wind speed and humidity (Supplement). Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 3 · Pulled up the xylem, and wilting. Supplement: the transpiration pull on a column of water molecules held together by attraction; how and why wilting occurs. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- End of module · final check. 9 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 10 questions, 15 min, lesson closed.
Learning goals
Part 1: Water vapour leaves the leaves
- Describe transpiration as the loss of water vapour from leaves (8.3.1)
- Measure the rate of water uptake with a potometer: time an air bubble, read the same edge, and work out the rate in mm per minute (part of 8.3.3)
- State that 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 (8.3.2)
- Supplement: explain how water vapour loss is related to the large internal surface area provided by the interconnecting air spaces between mesophyll cells and the size and number of stomata (8.3.4)
Part 2: Hot, windy or humid
- Investigate and describe the effects of variation of temperature and wind speed on transpiration rate (8.3.3)
- Supplement: explain the effects on the rate of transpiration of varying temperature, wind speed and humidity (8.3.6)
Part 3: Pulled up the xylem, and wilting
- Supplement: explain the mechanism by which water moves upwards in the xylem in terms of a transpiration pull that draws up a column of water molecules, held together by forces of attraction between water molecules (8.3.5)
- Supplement: explain how and why wilting occurs (8.3.7)
Misconceptions to listen for
- “Transpiration is water moving up the stem.” — Water does move up the stem, but transpiration is the loss of water vapour from the leaves.
- “Transpiration is the loss of liquid water from leaves.” — The water leaves as vapour, a gas, not as drops of liquid.
- “The droplets dripped out of the leaves as liquid water.” — The leaves lose water vapour, a gas. It turns into liquid only on the plastic.
- “The bag made the water out of the air.” — A bag cannot make water. The water came from the leaves.
- “C is opened to push the bubble back.” — C is the reservoir that holds the water. D is the tap that lets it flow.
- “The bubble moves away from the shoot, because the shoot pushes water out.” — The shoot takes water in. The water in the tube moves towards it.
- “Time the run with the tap open.” — Water from the reservoir would push the bubble too. Close the tap before timing.
- “Read the right edge of the bubble at the start and the left edge at the end.” — Read the same edge each time, or the bubble's length is added to the distance.
- “The rate is 232 mm per minute.” — 232 mm is where the bubble stopped. Subtract to find how far it moved.
- “The rate is 18 mm per minute.” — 18 mm is the distance in 2 minutes. Divide by 2 for the rate per minute.
Scope
Transpiration as the loss of water vapour from leaves (8.3.1); evaporation from the surfaces of the mesophyll cells into the air spaces and diffusion out through the stomata (8.3.2); investigating the effects of temperature and wind speed (8.3.3); the internal surface area and the size and number of stomata (Supplement 8.3.4); the transpiration pull (Supplement 8.3.5); the effects of temperature, wind speed and humidity explained (Supplement 8.3.6); wilting (Supplement 8.3.7). The distances in the experiment come from a teaching model.