Transport across membranes
At A level, biologists need to understand that the cell membranes is integral to the cell theory. Students need to be familiar with and be able to explain in detail the fluid mosaic model of membrane structure, this includes being confident in representing phospholipids accurately in diagrams, using the correct terms such as hydrophilic and hydrophobic and describing the role of all membrane components. Students need to carry out and be familiar with investigations into factors affecting membrane structure and permeability.
The level of detail and understanding required in terms of movement across membranes is significantly increased from the requirements at Key Stage 4: simple diffusion and facilitated diffusion need to be explained in detail in terms of passive methods, and also the limitations of these methods. Facilitated diffusion (involving the roles of carrier proteins and channel proteins) and active transport (involving the role of carrier proteins and the importance of the hydrolysis of ATP) need to be explained in terms of active transport processes.
Understanding of osmosis, endocytosis and exocytosis are also a requirement for A level.
Many students even at this level still struggle with the concepts involved in the transport across membranes. Diffusion is proportional to the difference in concentrations between two regions—the concentration gradient. Students still lose marks as they say it is proportional to the concentration. Similarly with osmosis students often lose marks because they are confused about water potential values. With active transport students do not always appreciate the specific nature of the transport proteins or they refer to them as having active sites like enzymes, which is incorrect.
Often questions involving transport across membranes are similarity/difference type questions, so ensuring students have lists/tables of these to consolidate learning is always worthwhile.
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- ALL
- Poster
- Article
- Experiment
- External link
Poster
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.
Article
Water Movement in Potatoes *suitable for home teaching*
This is a very quick and easy reminder for students of the processes of osmosis. There is no need for A level students to undertake the investigation, although it would be possible to extend it in a variety of ways.
This brief summary could be used at the start of this topic to confirm students' understanding of osmosis from Key Stage Four. It would be interesting to see how many students answer the question underneath the photograph of the potato chips correctly!
This article could be used to further extend students understanding of water potential, which causes frequent mistakes in A level exams.
Experiment
Water Potential During Ripening and Storage
This activity, produced by Science & Plants for Schools (SAPS), is specifically designed for post 16 students. Although there are several practical’s included in the investigation, it is the first practical which is most useful in terms of this topic on transport across membranes. In this practical cylinders of a vegetable (potato is the easiest to use) are placed in different sucrose solutions. Depending on the concentration of the solution, the potato cylinder either gains or losses weight due to the movement of water in or out of the potato cells.
Students can calculate the percentage change in weight for each potato cylinder and plot this data on a graph. Using the graph students should then be able to explain the overall water movement.
External link
Investigating Effect of Temperature on Plant Cell Membrane
This experiment allows for the collection of both qualitative and quantitative data and in-depth evaluation of both the procedure used and the reasons for the results seen, making it a suitable practical at A level.
The practical uses beetroot and is relatively easy to set up. There is some time during the investigation for students to start to consider and discuss expected results and reliability of the investigation.
The practical procedure provided here includes some detailed technician and teachers notes. It is worth downloading the student sheet as the questions provide a suitable level of challenge for A level students (but make sure you don't give them the answers as these are also included on the third page!).
Transport Across Membrane Animation
This website page includes an animation showing active transport, diffusion and osmosis. It can be found by scrolling to the bottom of the page. Although this page is aimed at 14-16 aged students it would be a useful starter activity for A level.
Students could be asked to watch the animation and then prepare a voice over that includes all the relevant explanation and terms required at A level.
Students could watch the animation as a quick recap and then be asked to prepare a revision table that contains all the similarities and differences between the different processes. They could be asked to add an additional column of examples of these processes within plants or animals.
Membrane channels animation
This is an excellent animation, but it does require a bit of setting up in order for it to work successfully. Java needs to be installed on the computers being used, though the simulation can be downloaded and used off-line.
The animation shows the movement across a membrane through membrane channels. Students "drag/dispense" different coloured channels and molecules into the simulation. Students can be set a range of challenging questions linked to this simulation. These can include the effect of altering concentrations of molecules, rate of movement, energy required for this movement to take place and how this relates to the fluid mosaic model of membrane structure.
