top of page

B4.8 Evaporation and Transpiration

GCSE Combined Science · AQA · Chapter B4: Organising Animals and Plants

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: stomata and guard cells, the transpiration stream and why plants need it. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe the process of transpiration and the transpiration stream
  • Explain how guard cells open and close stomata, and why this matters
  • Explain why plants need a transpiration stream
  • Calculate a mean and the number of stomata per mm² from a leaf peel

Key words

  • Transpiration: The loss of water vapour from the leaves, by evaporation and diffusion out through the stomata.
  • Transpiration stream: The continuous flow of water from the roots, up the xylem, to the leaves.
  • Stoma (plural stomata): A small pore in the leaf surface, mostly on the lower side, that lets gases in and out.
  • Guard cells: A pair of cells around each stoma that change shape to open or close it.
  • Turgid: Describes a plant cell that is swollen and firm because it is full of water.
  • Flaccid: Describes a plant cell that is soft because it has lost water.

Stomata and guard cells

Leaves need carbon dioxide for photosynthesis, which enters by diffusion through stomata. Each stoma is surrounded by two guard cells. When the guard cells take in water they become turgid and bend apart, opening the stoma. When they lose water they become flaccid and the stoma closes.

  • Stomata usually open in the light, so carbon dioxide can get in for photosynthesis.
  • They close in the dark (no photosynthesis, so no need for carbon dioxide) and when the plant is short of water, to reduce water loss.
  • Most stomata are on the lower surface of the leaf, which is cooler and shaded, so less water evaporates.

Exam tip: Open stomata are a trade-off: the plant gains carbon dioxide but loses water vapour. Guard cells balance the two.

Transpiration and the transpiration stream

Water evaporates from the surfaces of the mesophyll cells into the air spaces of the leaf. The water vapour then diffuses out through the stomata, from a high concentration inside the leaf to a lower concentration in the air. This loss of water vapour is transpiration.

As water leaves the leaf cells, water moves in from the xylem to replace it. This pulls a column of water up the xylem from the roots, and the roots absorb more water from the soil. The whole flow is the transpiration stream.

Key fact: Transpiration (water loss) and the transpiration stream (water movement through the plant) are linked: the faster water is lost, the faster it is pulled up.

Why plants need the transpiration stream

  • It supplies water for photosynthesis.
  • It carries mineral ions from the soil up to the leaves.
  • It keeps cells turgid, which supports the leaves and stem (a plant without enough water wilts).
  • Evaporation of water cools the leaves on hot days.

Worked example: percentage mass lost

A cut leafy shoot has a mass of 16.0 g. After 5 hours on a sunny windowsill its mass is 14.2 g. Calculate the percentage loss in mass.
mass lost = 16.0 − 14.2 = 1.8 g
percentage loss = (1.8 ÷ 16.0) × 100 = 11.25% ≈ 11%

Answer: about 11% (almost all of this is water lost by transpiration)

Comparing leaf surfaces and counting stomata

You can compare water loss from the two surfaces of a leaf. Smear waterproof petroleum jelly on the upper surface of one leaf, the lower surface of a second leaf, both surfaces of a third and neither surface of a fourth (seal every leaf stalk). Weigh each leaf, hang them up in the same place, and reweigh after a day. The leaves that lose least mass are the ones with the lower surface covered, because that is where most stomata are.

Paint clear nail varnish on a leaf, let it dry, peel it off with sticky tape and look at it under a microscope. Count the stomata in several fields of view and work out a mean. To find the number per mm², divide by the area of the field of view (area of a circle = πr²).

Worked example: stomata per mm²

A student counts 21, 18, 24, 19 and 23 stomata in five fields of view on a lower leaf surface. The field of view has a diameter of 0.50 mm.
mean = (21 + 18 + 24 + 19 + 23) ÷ 5 = 105 ÷ 5 = 21 stomata
radius = 0.25 mm, so area = π × 0.25² = 0.1963 mm²
stomata per mm² = 21 ÷ 0.1963 = 106.9…

Answer: about 110 stomata per mm² (2 s.f.)

Common mistakes

  • Saying transpiration is "water lost through the roots" or "plants sweating". It is water vapour lost from the leaves, mostly through stomata.
  • Mixing up turgid and flaccid: turgid guard cells open the stoma.
  • Saying stomata are there to let water out. Their main job is gas exchange (carbon dioxide in, oxygen out); water loss is a side effect.
  • Using the diameter instead of the radius in πr² when working out the area of a field of view.

Revision questions

Try these, then check your answers below each one.

Question 1. What is transpiration? (Choose from: The loss of water vapour from the leaves, The movement of sugars in the phloem, The uptake of water by root hair cells, The release of oxygen in photosynthesis.)

Answer: The loss of water vapour from the leaves

Working: Transpiration is the loss of water vapour from the leaves by evaporation and diffusion.

Question 2. The diagram shows two pairs of guard cells. Which statement is correct? (Choose from: A: guard cells turgid, stoma open, A: guard cells flaccid, stoma open, B: guard cells turgid, stoma closed, B: guard cells turgid, stoma open.)

Answer: A: guard cells turgid, stoma open

Working: In A the guard cells are swollen with water (turgid) and bowed apart, so the stoma is open. In B they are flaccid and the stoma is closed.

Question 3. Name the cells that open and close the stomata.

Answer: guard cells

Working: Guard cells control the size of each stoma.

Related lessons

Continue in the full course

This lesson is section B4.8 (AGCSC0408) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

bottom of page