In this unit we explore how proteins are the 'doers' of the cell. They are huge in number and variety and diverse in structure and function, serving both the structural building blocks and the functional machinery of the cell. Just about every process in every cell requires specific proteins. The basic principles of protein structure and function which are reviewed in this unit are crucial to understanding how proteins perform their various roles. This study unit is just one of many that can be found on LearningSpace, part of OpenLearn, a collection of open educational resources from The Open University. Published in ePub 2.0.1 format, some feature such as audio, video and linked PDF are not supported by all ePub readers.