Final answer:
The concentration of oxygen and the amount of ADP fell during the investigation with isolated mitochondria because oxygen was consumed in respiration, and ADP was converted into ATP.
Step-by-step explanation:
The concentration of oxygen and the amount of ADP fall during the investigation with isolated mitochondria because oxygen is consumed in respiration, and ADP is converted into ATP. The process of cellular respiration involves the breakdown of glucose in the presence of oxygen to produce ATP. Oxygen acts as the final electron acceptor in the electron transport chain of aerobic respiration, and during this process, ADP is phosphorylated to form ATP.