Using a potometer to investigate the rate of water loss from leaves

Transcription

1 Using a potometer to investigate the rate of water loss from leaves TEACHERS /TECHNICIANS NOTES Specification reference: B3.1.3 Exchange systems in plants (d) Plants mainly lose water vapour from their leaves. Most of the loss of water vapour takes place through the stomata. evaporation is more rapid in hot, dry and windy conditions if plants lose water faster than it is replaced by the roots, the stomata can close to prevent wilting. Learning objectives To understand how a potometer can be used to measure the uptake of water in plants To learn that most of the evaporation takes place through the stomata To investigate the conditions that affect evaporation from leaves Description of activity This is a practical activity that can be carried out in groups of up to three individuals observer, timekeeper and scribe. A pre-bought potometer is not required, just a simple capillary tube and some rubber tubing. Decision on lesson organisation will depend on how capable the students or technicians are as to constructing a potometer. In this activity the rate of water uptake, due to evaporation from a shoot of a woody plant, is measured by timing how long a bubble takes to move a set distance in the capillary tube of a potometer. Students could collate class data to draw a table and then construct a graph to show results The method can be adapted to look how different factors affect the rate of transpiration How Science Works The following aspects can be addressed: Appreciating that, unless certain variables are controlled, the results may not be valid (4.3.2a) Choosing the most appropriate form of presentation (4.4.2c) Identifying causes of variation in data (4.5.2a) Resources Cut leafy shoots from a woody plant or shrub Capillary tubes marked with a scale Pieces of rubber tubing Secateurs / scalpel Fans Lamps Vaseline Stopwatches Coloured dye Syringe Boss head and clamps Page 1 of 5

2 Practical tips 1 Use shoots from a shrub with leaves that have thin waxy cuticles, eg beech, lilac, spirea. Leaves with thicker waxy cuticles (very glossy) do not work well. 2 Fill a washing up bowl with water. Using secateurs or a scalpel, cut a large leafy shoot under water, and then cut again a few cm above the first cut. If air gets into the xylem vessels of the plant, it can form air locks that will prevent the plant taking up water and so prevent steady transpiration. 3 Submerge the potometer completely in the water. Make sure that all air bubbles are removed from the apparatus. You may need to flush it through with a syringe. Make sure that the rubber connection for the stem is completely filled with water. 4 (Steps 4 to 6 ideally need two people.) Do not wet the leaves of the shoot. Re-cut the base of the stem in water at a point on the stem where it is slightly wider than the rubber tubing. You will need a shoot with about six to twelve leaves on it. 5 Push the cut stem firmly into the rubber connection of the potometer, under water. Lift the potometer out of the bowl and use a clamp, boss and stand to hold the cut shoot steady. Put Vaseline on the connection between the rubber tubing and the stem. 6 Use a paper towel to gently dry the capillary tubing. Dip the capillary in the coloured dye to introduce an air bubble. If necessary, gently blot the end of the tube with a piece of paper towel and an air bubble should appear. 7 When changing the conditions under which to test the potometer allow the plant's shoot 5 minutes or so to settle down to the new conditions. 8 A strong halogen or fluorescent lamp is better to use than a bench lamp to look at light intensity. Health and safety Take care while using the secateurs or scalpel when cutting the leafy shoot. Take care when handling glassware as the capillary tube is quite fragile. Timing Approximately 45 minutes + time to collate results and draw the graph Different approaches Support students by assembling the potometers before the lesson, as there is an art in setting them up. This will give the leaves time to dry and give technicians a chance to check that they are working before the students begin to take measurements. Extend students by getting them to calculate the rate. Rate of movement (cm/s) = Distance moved by the air bubble (cm) Time taken (s) They could also be asked to design a further investigation to discover the effects of changing temperature or light intensity on the rate of water evaporation. Page 2 of 5

3 Answers to evaluation questions 1 Independent variable is time 2 This variable is continuous 3 Dependent variable is distance air bubble moved 4 This variable is continuous 5 Control variable: type of plant, temperature, distance of plant from fan 6 Precautions: done under water to avoid air bubbles, do not block stomata with Vaseline 7 Moving air maintains diffusion gradient, water molecules blown away 8 Stomata are also found on the lower surface, so putting Vaseline on this surface will lead to a slower uptake of water as more stomata are blocked 9 See student's results Useful links This is a useful website which looks at factors that affect transpiration. This deals with all levels of learning and includes animations and a simulation. This website also has an animation on how the guard cells work, plus an investigation on how to calculate the surface area of a leaf and relate this to transpiration rate. Page 3 of 5

4 Using a potometer to investigate the rate of water loss from leaves STUDENTS' WORKSHEET Aim To use a potometer to measure the uptake of water by a cut shoot To investigate different conditions that could affect the rate of transpiration Taking measurements with the potometer The potometer is used to measure the rate of water uptake by a cut shoot. As the cut shoot takes up water from the wide glass tube, air is pulled into the capillary tube from its open end. An air bubble can be introduced and the distance it moves over a set period of time can be recorded in centimetres per minute (cm min -1 ). Health and safety Take care while using the secateurs or scalpel when cutting the leafy shoot. Take care when handling glassware as the capillary tube is quite fragile. Method You are provided with: Secateurs/scalpel Washing up bowl Leafy shoot Potometer 100 cm 3 beaker Vaseline Stand, boss and clamp Paper towels Electric fan Stop watch These diagrams show two different sorts of potometer. Your teacher will demonstrate how to set up the apparatus. Page 4 of 5

5 How to set up the potometer 1 Fill a large bowl with water. Using a secateurs or scalpel, cut a large leafy shoot under water, and then cut again a few cm above the first cut. This is to stop an airlock in the stem. 2 Submerge the potometer completely in the water. Make sure that all air bubbles are removed from the apparatus. Do not wet the leaves of the shoot. 3 Push the cut stem firmly into the rubber connection of the potometer, under water. Put Vaseline on the connection between the rubber tubing and the stem. 4 Use a paper towel to gently dry the capillary tubing. 5 Tip the end of the capillary tube into some coloured dye. Gently blot the end of the tube with a piece of paper towel and an air bubble should appear. 6 Use a stopwatch to record the distance the bubble has moved every 2 to 3 minutes. 7 Change the conditions by using a fan to gently blow cold air over the same shoot system. 8 Allow the plant's shoot 5 minutes or so to settle down to the new conditions. Results Record your results in a suitable table. Collect class data and calculate a mean. Draw a suitable graph of results Evaluation questions 1 What was the independent variable in this investigation? 2 Is this variable categoric or continuous? 3 What is the dependent variable in this investigation? 4 Is this variable categoric or continuous? 5 Name one control variable you used. 6 What precautions did you have to take in setting up the potometer? 7 Explain why the rate of uptake of water is usually quicker in moving air than still air. 8 Putting Vaseline on the lower epidermis of the leaves as well as on the upper epidermis would lead to a slower uptake of water than if only the upper epidermis was coated. Why is this? 9 What was the average rate of uptake of water in still and moving air? Extension Design an experiment using the potometer to investigate the effect of changing the temperature or amount of light on water uptake. Page 5 of 5