Cell transport
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.
Whilst this list provides a source of information and ideas for experimental work, it is important to note that recommendations can date very quickly. Do NOT follow suggestions which conflict with current advice from CLEAPSS, SSERC or recent safety guides. eLibrary users are responsible for ensuring that any activity, including practical work, which they carry out is consistent with current regulations related to Health and Safety and that they carry an appropriate risk assessment. Further information is provided in our Health and Safety guidance.
Secrets of Cells *suitable for home teaching*
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.
Water Movement in Potatoes *suitable for home teaching*
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>
Controlling Change
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.
Water Potential During Ripening and Storage
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.
Visking Tubing
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.
ABPI interactive website
This is an animation showing active transport, diffusion and osmosis. It can be found by scrolling to the bottom of the page. Active transport can be looked at first by reminding students that diffusion sees molecules move down a concentrations gradient.
Suggest that there are times when cells need to move molecules up a concentration gradient. What is moving up a gradient likely to need? The process is called active transport as it requires energy. The animation can be used to point out how the transport protein carries the molecule into the cell where there is already a high concentration present. Stress that this carrier protein needs energy to do this. Without energy, the process will stop.
Students can be reminded about the process of cellular respiration and that this is the process that provides the energy for active transport.
The processes of diffusion, active transport and osmosis can be summarised by having students produce a revision table that contains the similarities and differences of each process. This page is also useful in reminding students of the key features when they construct their table.
Membrane channels
This is a great simulation although it does take some setting up. Firstly, Java needs to be installed on the computers being used. The simulation can then be downloaded and used off-line.
It looks at the movement across a membrane through membrane channels. In this way it is a model for diffusion across a semi-permeable membrane.
Open the simulation and drag some green leakage channels into the cell membrane. These will allow green circles through but will not allow blue diamond molecules to pass through. Describe this as being the same as the semi-permeable cell membrane. Introduce green molecules above the membrane by selecting them and pressing the large red button on the dispenser. Note how they cross through the channels to the other side of the membrane. Select the blue diamond molecules and introduce them above the membrane. Note how these are unable to pass through the membrane.
Set students the task of investigating the movement of molecules across the membrane. Ask the following whilst they work with the simulation
• Explain how the simulation relates to movement in and out of a cell.
• What is the effect of differing concentrations of molecules and rate at which they get from one side of the membrane to the other?
• How does the number of channels (permeability) of the membrane effect movement?
• Do molecules move only in one direction through the channels or do they move back-and-forth? What is the overall effect? How does this relate to the definition of diffusion?
• What happens if molecules are introduced on both sides of the membrane?