Research that has focused on external representations in biochemistry has uncovered student difficulties in comprehending and interpreting external representations. This study focuses on students' understanding of three external representations (ribbon diagram, wireframe, and hydrophobic/hydrophilic) of the potassium ion channel protein. Analysis of the interview data demonstrates that students were able to use the ribbon structures and polarity of the cell membrane to help support claims about the protein's orientation and interactions within the cell membrane. Students expressed fragmented understandings of the interactions between the potassium ion and the aqueous solution outside/inside of the cell membrane. Suggestions for instruction are to probe student understanding to help students activate prior knowledge and to help them build a more connected set of concepts pertaining to protein structure and function.