architecture portfolio

I'm an A-level student applying to do architecture at university and have just submitted my full UCAS application. One thing I'm slightly concerned about is that the universities have mentioned that alongside the interview we also have to submit a portfolio of some art work. Now my A-level subjects aren't traditional architecture subjects as I do sociology, psychology and biology (the university requirements stated there were no subject specific A-levels) and I wanted to know the process or general idea of what is usually included in these portfolios. I understand it's a submission of some art work but any advice or ideas as to what they're looking for will help as I want to prepare in advance.