Optional Units in a Degree course?

I've been looking at courses at University, and the one I have my eye on is a Microbiology degree at Bristol. They say in the course content section the content of years 1 and 2, but then say that in year 3 the teaching is more free, and you can choose what units you take (any that the university offers that can fit into your timetable). My questions are: Do you need any other previous qualifications to take some of these optional units in an area like Psychology. Also what recognition do you get out of taking these units? i.e. will it say on your degree?