Hydrophobia rules

I structure the Year 12 SoW to build a foundation of important fundamental principles as quickly as possible. So we start with water, making interesting and topical links to the possibility of life on exo-planets, or Europa, and establishing all those vital properties that make it essential for Life As We Know It.

I then focus in on its role as a solvent. How can we tell if something is going to be soluble in water? We build the important vocabulary and concepts and I get them to work out whether any given molecule is likely to be water-soluble.

But is it soluble…

I get them to think of oxygen as the child at a party who won’t share its electrons nicely, so that not only is water polar, but so are -OH groups. And we explore why lipids cannot dissolve – they bring nothing to the water party, and quickly get jostled out of the way so that the water molecules can resume their endless round of speed dating, meeting new and exciting molecules every nano-second.

We then have an imaginative role play where I get them to imagine the very first origins of life on earth, a little corner at the bottom of the ocean where some organic molecules spontaneously form and can start reacting becaue they are soluble, and are therefore free to move and collide. The students are the organicĀ  molecules, standing in the corner of the lab (the deep sea vent), but what will happen?

This takes them a while to figure out. Go on, I urge, what will you do? In solution, in the deep sea? Sure enough, they start to drift apart, diffusing to the far corners of the lab. Why is this a problem for kick-starting life? They get it – they’re too far apart to ever collide. Life will never get beyond the odd nucleotide.

So what do we need? We discuss the concept of a barrier, something that hold these molecules in one place, to prevent them diffusing into oblivion, and allow the very first reactions of life to take hold. And I ask them, what property must this barrier NOT HAVE??? This takes them a little longer as they’re not quite sure what I’m asking. But then the light dawns – any barrier intended to prevent water-soluble molecules diffusing away into the big blue sea MUST NOT BE SOLUBLE ITSELF. In other words, it must be hydrophobic. Can they think, off hand, of any potential hydrophobic molecule that might serve as the basis for this barrier, this, for want of a better word, MEMBRANE?

We look at lipids, figure out why a simple lipid can’t work, introduce the idea of a phospho-lipid, and try drawing diagrams of how they would interact with water, and how you could arrange them so that could associate happily with water while yet remaining insoluble. There’s a lovely lightbulb moment when they figure out the phospho-lipid bi-layer.

Still with me?

OK, so now they can start to work out some of the properties of this MEMBRANE thing. It’s a really nice example of how even a largely theoretical topic can be turned into a journey of discovery, where students figure things out for themselves, rather than just telling them what’s going on.

I set them this exercise.

Movement across cell membranes

A whole load of molecules/ions that must cross membranes if cells are to survive – but can they cross a phospholipid bi-layer? It’s back to the very first principles of water and solubility. If it’s soluble in water it must be hydrophilic, but the membrane itself is hydrophobic. So can water cross it? No. Can ions cross it? No! Can glucose cross it? No, no and thrice over no!!!!

I start introducing bigger ideas. Using this animated PP as an illustration.

channel proteins

Na+ cannot cross the membrane, it will be repelled by the hydrophobic core, and yet it must cross. Your entire nervous system depends on it. Every thought you have, every thing you feel and see and hear and believe is based on Na+ crossing cell membranes. Cl- cannot cross membranes. But it must. If Cl- doesn’t cross membranes you have cystic fibrosis and your lungs fill up with mucus and you are very ill. Even water itself, by definition, cannot cross a hydrophobic barrier!!! And yet clearly it must, not just for simple rehydration purposes, but the whole basis of osmo-regulation in the kidney is built on the water permeability of cells in the collecting duct.

So it must be more complicated than this…. Can they suggest anything?

The suggestions come in, slowly, tentatively, until someone eventually (and they always do) wonders whether there could be another pathway through the membrane?

What, like this? A click on the PP and a channel protein inserts itself. And slide by slide, we build up the properties of these rather wonderful structures. Specific, gated, facilitative of diffusion… It’s a wonderful moment as they start to understand the first basic principles of selective permeability. And it’s why Membranes are one of my favourite topics at A-level.

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