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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