hi guys My name is Harry and I did biochemistry undergraduate at Oxford university as the class of 2010. I am here to share with all the oxford biochemistry applicants what kind of interview questions to expect in december Alright so I prepared what I thought was a difficult question at my interview which is 'Given that the selectivity filter is lined with carbonyl oxygens, how might potassium ion channel allow potassium ion through but not sodium ion' so to help you guys picture the question, i drew out this diagram where carbonyl oxygens are pointing towards the incoming potassium ions and the filter width is just wide enough to fit in one potassium ion. So the real question is why can’t sodium ion which has the same effective nuclear charge of 1+ pass through when it’s smaller? Interviewers wouldn’t expect you to know the whole answer so we can start by stating the obvious: that there is electrostatic attraction between positive potassium ions and negatively charged carbonyl oxygens and that potassium ions are bigger than sodium. From this point onwards this is where you want to ask for some hints as I did because there is nothing disadvantageous with asking for help and you are expected to ask for hints because you wouldn’t have learnt this at school. The hint would be to think about what potassium ions are interacting with before entering the filter, i.e. water Water molecules surround potassium ions to form hydration shells and you could add that water solvation exists to stabilise the high energy state ion. We need to catch out the point that when a potassium ion enters the channel, it’s losing its hydration shell for new interaction with carbonyl oxygens and that the energy gain from this new interaction is big enough to compensate the loss of hydration. Now the answer. Because sodium ion is smaller than potassium ion, its reduced proximity to carbonyl oxygens means that the resulting electrostatic interaction is weaker than potassium ions and therefore insufficient to compensate loss of sodium ion’s hydration. Quite interesting right? So there we go. Our answer to the ion channel question. We hope that you enjoyed our intellectually stimulating ‘what to expect in the Oxbridge interviews’ video and would like to encourage you to get in touch with us to learn more about biochemistry interviews. Thanks！