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.
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 other 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.
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 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.
This detailed text book was produced in 1971 by the Institute of Biology. It is something which teachers could use to reinforce their own knowledge and understanding of this topic before delivering it to students.
There are undoubtedly ways that information and particularly data within the book could be extracted and used to set interesting and challenging questions. For example on page 14, the data provided on the percentage of different phospholipids in red blood cell membranes of sheep and man could be used to provide some stimulus material and then some challenging questions on membrane structure and research into why this difference might be an advantageous adaptation.
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.
This video is a demonstration of diffusion through a semi-permeable membrane and was originally aimed at teachers. 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. This is a Key Stage Four practical as presented in the demonstration.
The video could be used in a variety of ways with A level students:
a) as a very basic starter activity at the beginning of this topic to confirm their understanding (what they remember!) from Key Stage Four in relation to transport across membranes
b) use the video without the sound and ask different students to provide the voice over for various sections of the video
c) watch the video all the way through, and then ask students to devise an A level standard practical that will test the transport across the membrane of different concentrations of a given solution.