Scientific research generates the new knowledge that moves a field like biochemistry forward. Consequently, the Department of Biochemistry requires its majors to spend at least a year doing undergraduate research and prefers that they do at least two years. It is to the student's advantage to select a research advisor from the Department's faculty, since that will make it easier for the student to become well-acquainted with several faculty and with the other students in the Department. However, there is no requirement that Biochemistry majors do their research within the Department, and many choose advisors in other Departments based on their research interests. If a research mentor from another Department is chosen, the project must involve modern molecular biological approaches so that the student gets experience with applications of those approaches and acquires knowledge of some of the principal tools of modern biochemistry.
The best way of deciding on a research advisor is to go to the faculty member's web page. Every department has a link on its web site for "faculty". See Biochemistry's Faculty Research page for one example. Usually that leads to a concise description of the kind of projects a faculty member is researching. Clicking on that statement opens a much larger description of the particular project which is being researched. A student looking for a mentor should make a list of five or six faculty doing research in areas of that student's interests and then go and talk to the faculty members. The trick is to find the right persons in other departments.