While there are several available lessons for teaching introductory biology students about diffusion, facilitated diffusion, and active transport, fewer materials exist to support upper-division students' understanding of the proteins that mediate these forms of transport. In the 1970s, mitochondrial pyruvate carrier (MPC) proteins were predicted to import pyruvate from the cytoplasm into mitochondria for cellular respiration. Yet it was not until 2012 that the identity of the proteins responsible for this transport was confirmed in two seminal publications. In this Lesson, students will use their background knowledge of transport mechanisms to analyze data from those papers to determine which of the predicted MPC proteins are actually part of the mitochondrial pyruvate transporter. Student will also learn how scientists test whether a protein is necessary and sufficient. The Lesson is written in the style of process-oriented guided inquiry learning (POGIL). POGIL is a teaching approach that requires students to work collaboratively in small groups to answer a set of questions based on scientific data. Questions in the POGIL activity, called the problem set, are structured so that each question leads to the next, helping to guide students to a deeper understanding of the content. During this Lesson, the instructor acts as a facilitator to guide student learning. Several forms of assessment are included within the Lesson, allowing instructors to assess learning gains. This Lesson has been used multiple times by over 10 faculty in an upper-division Cell Biology course and can also be used in other upper-division biology courses.