The compound that acts as a reactant in both cellular respiration and dehydration synthesis is glucose (C6H12O6).
In cellular respiration, glucose is broken down in the presence of oxygen to produce energy (ATP), carbon dioxide, and water.
In dehydration synthesis (also known as condensation reaction), glucose can be used to form larger carbohydrates such as starch or glycogen, by removing a water molecule during the process of linking glucose molecules together.