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Exchange and Transport

Exchange and Transport. 13.9 Transpiration and factors affecting it. Learning outcomes. Students should be able to understand the following: Why transpiration occurs What factors affect the rate of transpiration How to measure the transpiration rate Candidates should be able to:

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Exchange and Transport

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  1. Exchange and Transport 13.9 Transpiration and factors affecting it

  2. Learning outcomes Students should be able to understand the following: • Why transpiration occurs • What factors affect the rate of transpiration • How to measure the transpiration rate Candidates should be able to: • Explain transpiration and the effects of light, temperature, humidity and air movement

  3. Why do plants need transpiration? • Lots of water is lost from the leaves of a plant because: • Leaves have a large surface area • Leaves have stomata • Therefore a transpiration pull effect is needed to transport large volumes of water up the stems of large plants/trees to the leaves • As well as providing water for photosynthesis, dissolved ions, sugars and hormones are also moved about in the transpiration stream Giant Redwood trees can lose 700 litres of water per day

  4. Is the rate of transpiration constant? Transpiration is a bit like a straw, pulling water up the plant. Sometimes the pulling force will be stronger and the plant will lose more water. The speed at which a plant loses water is called the rate of transpiration. This varies depending on the plant’s environment. What environmental factors will affect the rate of transpiration? • humidity (amount of moisture in the air) • light intensity • temperature • air movement (wind).

  5. Why does transpiration vary?

  6. Measuring water loss • Enclose the pot of a potted plant in a plastic bag and put the plant on a top-pan balance. The mass gradually falls as water evaporates off the surface s of the plant. • Class Practical • Estimating the rate of transpiration from a plant cutting • Set up the cuttings and record the starting volume and mass for each one • Make a second set of readings and answer questions in the next lesson

  7. Measuring uptake of water Transpiration can be measured using a potometer. A cut plant stem is sealed into the potometer using a rubber bung. An air bubble is introduced to the capillary tube. The distance the bubble travels shows how much water the stem has taken up. This gives an indirect measurement of the rate of transpiration.

  8. Extension and Homework AQA AS Biology textbook pg 198 Summary questions 1-3 2. Read ‘Measurement of water uptake using a potometer’ on page 198-199 then answer application questions 1-5 2. Complete the exam style question about transpiration rate in plants

  9. Mark scheme - exam style question Answers – transpiration rate questions 1.(a)(i) distance moved by bubble in specified time period; diameter / radius of capillary (lumen); 2 (ii) surface area/ mass of leaves/ plant; 1 (b) rapid loss then decreases; 1 loss in mass as water loss is not replaced; initially stomata are open / later the stomata close; higher water potential of leaf cells compared with atmosphere / diffusion of water from leaf to atmosphere; water potential of leaf reduced / diffusion gradient decreased; max 3 [7]

  10. Learning outcomes Students should be able to understand the following: • Why transpiration occurs • What factors affect the rate of transpiration • How to measure the transpiration rate Candidates should be able to: • Explain transpiration and the effects of light, temperature, humidity and air movement

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