When trying to predict the synoptic question, you must look through the content and find threads that run through the two years. They must be sweeping and they must act as a large umbrella under which many substantial topics may fit happily. We have it easier than those before us because there are only so many of these threads to find and I doubt the examiners will repeat themselves.
My bet is on diffusion. You've got Fick's law, absorption in the small intestine, gas exchange, lung disease (and how it reduces efficiency of gas exchange by decreasing SA, reducing diffusion gradients and increasing length of diffusion pathways) and transpiration to name a few. However, one problem with this topic is that there isn't anything all that substantial from unit 5 that pertains to it.
Any thoughts on this idea?