The compound that is a reactant in cellular respiration is oxygen. To arrive at this answer, you can review the process of cellular respiration. Cellular respiration is the process by which cells convert glucose and oxygen into carbon dioxide, water, and energy in the form of ATP (adenosine triphosphate). In this process, glucose is broken down in the presence of oxygen, and carbon dioxide and water are produced as byproducts. Therefore, as a reactant, oxygen is needed for cellular respiration to occur.
