4.2 Movement into and out of cells
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4.2.1 Describe and explain the processes of simple diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis
4.2.2 Investigate simple diffusion and osmosis using plant tissue and non-living materials, including dialysis (Visking) tubing and agar.
4.2.3 Illustrate the principle that surface area to volume ratios decrease with increasing size by calculating surface areas and volumes of simple 3-D shapes.
4.2.4 Investigate the effect of changing surface area to volume ratio on diffusion using agar blocks of different sizes.
4.2.5 Investigate the effects of immersing plant tissues in solutions of different water potentials, using the results to estimate the water potential of the tissues.
4.2.6 Explain the movement of water between cells and solutions in terms of water potential and explain the different effects of the movement of water on plant cells and animal cells.
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Write the six methods of transport on the board: diffusion, facilitated diffusion, osmosis, active transport, exocytosis and endocytosis. Initiate a ‘think, pair, share’ activity between pairs of learners to consider all associated key terms they can think of that relate to them. (F)
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Carry out the Investigating the effect of changing surface area-to-volume ratio on diffusion experiment referring to the Teaching Pack for lesson plans and resources.
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In this task, learners use agar jelly containing the indicator cresol red to investigate how varying the surface area to volume ratio of a cell affects the diffusion of a small molecule. There are opportunities to practise mathematical and graphing skills, and to evaluate the method, with a particular emphasis on the types of error (random or systematic).
Learners carry out practical activities to investigate and explain the movement into and out of cells in terms of water potential. These include:
Learners estimate the water potential of potato tuber cells by placing pieces of potato tuber into solutions with different water potentials. They find the percentage change in mass for a range of solutions of known concentration and plot a graph. The concentration at which the potato cells neither gain nor lose water can be read from the graph. Useful information is at:
www.saps.org.uk/secondary/teaching-resources/286-measuring-the-water-potential-of-a-potato-cell
Leaners estimate the water potential equivalent to that of onion epidermal cells. They determine the point at which half of a population of onion cells look normal and half are plasmolysed to identify the point of incipient plasmolysis. Useful information is at:
www.kscience.co.uk/animations/plasmolysis.htm Challenge learners to explain the different effects of the movement of water on plant cells and animal cells.
To model exocytosis, learners fill a balloon with strips of paper on which revision questions have been written. A The balloon, representing a vesicle, is blown up and tied. For exocytosis, the learners show how the balloon is passed from the Golgi body (perhaps represented by the individual who prepared the balloon) to the end of a line of learners to reach an individual at the end, who represents the plasma membrane. One or more learners role play the bilayer and burst the balloon ‘behind their backs’ to show the fusion of the vesicle and exocytosis of the contents. Challenge learners to suggest how a similar role play could be used to illustrate endocytosis.
Share animations and interactive platforms with learners that illustrate the various types of movement into and out of cells. A good example is https://phet.colorado.edu/en/simulation/legacy/membrane-channels
Ask questions of learners ‘in reverse.’ Give them a series of answers and challenge them to suggest a question for each. This engages learners in higher-order thinking skills. To add an extra degree of challenge, learners decide on the most appropriate command term (taken from the syllabus) for each of their responses. For example, provide the term ‘by active transport, using ATP from the mitochondria’ to learners, for which they identify a question (e.g. ‘How does a cell absorb substances against the concentration gradient’) and the most appropriate command word (‘Explain’). (F)
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