Answer:
(a) oxygen; (b) energy
Step-by-step explanation:
Adenosine triphosphate (ATP) is the main source of energy for most cellular processes. ATP is known as 'the energy coin of the cell' because the hydrolysis of its high-energy phosphate bonds provides energy which is then used by the cells in metabolic processes. Moreover, the process of cellular respiration requires oxygen as a substrate to generate ATP. In consequence, cells with a higher energy requirement require more oxygen supply to generate more ATP.