According to both responses and views on my YouTube channel, the A level topics that are causing the most difficulties are the kidneys (reabsorption and the Loop of Henle) and epistasis. I think what these two areas have in common is that both of them require students to hold at least two contrasting ideas simultaneously in their heads in order to get to the correct explanation for what is happening. It’s quite straightforward for students to understand concentration gradients for example, to know that as a solute is diffusing in one direction that osmosis will effectively be occurring in the opposite direction is for many a step too far and the brain melt-down sirens begin to wail. They think this stuff is harder than a chimera of Chuck Norris and a Honey badger. Likewise, epistasis requires the idea of dominant and recessive genes to be understood (easy GCSE stuff) but to then have another set of genes that are required to be expressed (or not) in order for the other set to be expressed has added another layer of complexity.
I remember years ago being told by one of the county Science coordinators that KS3 maths levels could be more or less seen as adding in another layer of complexity, for example an operation, each time. That stuck with me as a simple way to assess the complexity of an idea. So taking apart the Loop of Henle for example, here are some of the things that I think you should be able to understand in order to give an explanation of what is happening:
Diffusion is occurring of ions down electrochemical gradients
Osmosis is occurring in the medulla in response to ion concentration
The descending limb is partially impermeable to ions
The ascending limb is impermeable to water
The descending limb comes first!
The fluid is moving, and so the effect of concentration gradients will differ along the length of the limbs
Both Na+ and Cl– are in play, resulting in Osmolar units
That’s a whole lotta stuff. None of these things are particularly complicated on their own, but the mental juggling required to hold all of these things together and then build them into something cohesive is difficult. This is definitely an area I feel benefits from modelling, using coloured discs etc. and setting up the model with a few simple rules (water moves towards where there is the most discs, water molecules move down the tube 1 space each time if you use squared paper). This can visibly show the differences in diffusion rates as the concentration changes each time. I also prefer to keep the descending and ascending limbs as separate tubes initially since it I think the idea of the two tubes carrying different concentrations of fluid an easier concept at first than showing that the fluid stream is continuous. So the process becomes like a flow chart or even a program, IF THEN . The repeating nature of checking each step can reinforce the rules of the program rather than it having to be recalled simultaneously. Something similar can be run with epistasis, where rules such as ‘check if the first set of alleles are expressed, and only then look at the other alleles’ removes the idea of trying to keep both parts in mind at the same time which students tend to do.
Incidentally, the most watched video on my channel was one of a summary of a GCSE unit. Looks like most students would still prefer the quick fix rather than identify a particular single idea. So it goes.