7.2 Transport mechanisms
KC5
KC6
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7.2.1 State that some mineral ions and organic compounds can be transported within plants dissolved in water.
7.2.2 Describe the transport of water from the soil to the xylem through the:
apoplast pathway, including reference to lignin and cellulose
symplast pathway, including reference to the endodermis, Casparian strip and suberi.
7.2.3 Explain that transpiration involves the evaporation of water from the internal surfaces of leaves followed by diffusion of water vapour to the atmosphere.
7.2.4 Explain how hydrogen bonding of water molecules is involved with movement of water in the xylem by cohesion-tension in transpiration pull and by adhesion to cellulose in cell walls.
7.2.5 Make annotated drawings of transverse sections of leaves from xerophytic plants to explain how they are adapted to reduce water loss by transpiration.
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Resource Plus
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Carry out the Investigating the effect of carbon dioxide concentration on stomatal density experiment referring to the Teaching Pack for lesson plans and resources.
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In this task, learners will use microscopy to find the stomatal density of plant leaves grown in high, atmospheric and low carbon dioxide concentrations. As part of this task, they develop their practice using a light microscope and carry out activities requiring knowledge of eyepiece graticules and stage micrometers as well as evaluating the conclusions of investigations using statistical tests.
Bring contextual relevance to the study of transport in plants. Learners research how gold mining is now partly dependent on analysing the contents of leaves such as eucalyptus. After allowing 2–3 minutes for them to undertake internet research, ask a number of questions, including ‘How did the gold particles get there?’ Develop the discussion by showing the short clip of David Attenborough, high up next to a tree, discussing how water can be brought upwards: www.youtube.com/watch?v=Qwb6mVeMpW8
Provide a series of cut-out statements that describe how water moves up a plant. Learners arrange the statements in order. Learners describe the mechanism in a stepwise fashion, starting at the top of the plant and working to the bottom. A useful animation that shows the movement of water in the xylem vessels of plants is at: www.saps.org.uk/animations/plant_biology/index.html?video=1 Emphasise how hydrogen bonding of water molecules is involved in this process. (I)
Learners undertake a practical activity to explore how to set up a potometer to measure the rate of transpiration from a young branch. This can be constructed using a long piece of capillary tubing that has a short length of rubber tubing attached at one end. The whole apparatus can be supported vertically. Learners actually measure the rate water is taken up by a shoot and make the assumption that all the water that is taken up is lost by the leaves. Learners carry out an investigation into the effect of a factor on the rate of transpiration of a plant, such as humidity, temperature or carbon dioxide concentration. A useful website is: www.nuffieldfoundation.org/practical-biology/measuring-rate-water-uptake-plant-shoot-using-potometer.
Help learners to produce annotated drawings of transverse sections of leaves from xerophytic plants to explain how they are adapted to reduce water loss by transpiration. Demonstrate on the class whiteboard how this is done, and encourage learners to follow your guidance. Provide learners with clear success criteria, and give an opportunity for self- or peer-assessment.
Ask a carefully-chosen series of hinge questions (a point in a lesson when you check whether or not learners have grasped a key concept and are ready to move on to study another) to elicit higher-order thinking skills among learners. One option is to ask them to compare key terms, to reinforce their knowledge of key definitions, including: lignin/ cellulose (low demand), apoplast/symplast (intermediate demand) and cohesion/tension (high demand). (F)
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