The mechanisms by which substances can move into and out of cells, across cell membranes, needs to be clearly understood by GCSE Biology students. Students need to be able to explain what diffusion is, examples of substances transported in and out of cells by diffusion and the factors that affect the rate of diffusion. Similarly students need to explain how water may move across the cell membrane via osmosis. With active transport students need to explain how substances are moved against a concentration gradient and that this requires energy from respiration. Having an understanding of all three processes should allow students to compare the processes and explain the differences between them.
The transport of substances is often a difficult and abstract concept for students and as such a number of misconceptions occur. As this is a fundamental concept which is then seen in many other GCSE Biology topic areas, it is vital that students understand the mechanisms fully.
Confusion is often seen in terms of the differences between diffusion and osmosis, so it is important to stress this difference clearly, and reinforce and revise it in other topics,ensure that students clearly understand that osmosis is only ever the movement of water.
Similarly students have difficulty understanding the term net movement and concentration gradient, as well as the difference between active and passive movement. Using animations and getting students to do "the voice over" or getting students to give presentations to articulate the processes can help to ensure that misconceptions do not occur.
Links and Resources
This issue of the Big Picture, from the Wellcome Trust, focuses on cell structure and function. There are a whole range of interesting articles. The ones which relate specifically to this topic are 'Mind your Membranes' (page seven) and 'Researching membrane proteins' (page six).
These articles could be copied and laminated for use as extension activities in class for students who may complete a set task before others. They could also be given as reading for homework, with additional research tasks linked to them.
This Catalyst article provides a very quick and easy reminder for students of the processes of osmosis. Students could undertake the investigation, and it would be possible to extend the activity in a variety of ways.
This brief summary could be used at the start of this topic to confirm students' understanding of osmosis. It would be interesting to see how many students answer the question underneath the photograph of the potato chips correctly.
There are many ways to make effective use of Catalyst articles, a booklet has been produced which provides ideas on how Catalyst articles can be used with students and includes six exemplar activities. This booklet can be accessed at: https://www.stem.org.uk/elibrary/resource/27308>
Although quite a dated publication, which was written against previous GCSE specifications, there are some very useful activity student sheets within this publication, which are presented with detailed teacher notes (with answers to all the questions).
The student sheets are a mix of practical and written activities. Section 2, starting on page 20, is most relevant to this topic, with student sheets 2.1, 2.2 and 2.3 focusing on "What is osmosis?, How does osmosis work?" and a practical activity on osmosis and living cells.
This resource from Science and plants for schools (SAPS) is designed for post 16 students, however the first practical activity: A method for measuring water potential of fruit and vegetables, is useful for GCSE Biology students, it will allow them to produce graphs of both % change in weight and change in length of the potato cylinder.
Students can then use these graphs to consider overall direction of water movement in relation to the concentration of sugar.
This is an alternative practical activity, again from SAPS; this again measures osmotic potential of plant tissues using potatoes. Two methods are given here, the standard protocol for measuring weight change and also the Chardakov method, this is a challenging investigation, but would work well as a demonstration with flex/web cams on the whiteboard to groups of GCSE Biology students
This resource describes a visual way of demonstrating diffusion through a semi-permeable membrane. It can be used as a model for the human gut or for investigating the effect of amylase on starch. Two standard tests are used. The first uses iodine to test for starch and the second test uses Benedict’s reagent to test for the presence of reducing sugars, such as glucose.
Students can draw diagrams to illustrate why glucose can escape the model gut whilst starch is retained within the gut. Relate this to the digestion of starch in the diet. Remind students that the cell surface membrane is also semi-permeable and will act in the same manner to control substances getting into and out of cells.
Students can be challenged to assess the validity of using Visking tubing to model absorption in the small intestine. It could also be used to investigate factors such as the effect of temperature, or initial solute concentrations on the speed of diffusion.