During exercise, energy demand in the muscle increases. which other body system directly provides for this demand

During the course of normal exercise,  the muscle system will utilize the glucose and glycogen to support the energy levels.

As a next step, the body dissolves the fat storage of the body to power up your muscles. This is called the beta-oxidation reaction

the way that the mass of carbon can be conserved through cellular respiration is through a process/law called the law of conservation of mass which even tho it undergoes a process, mass is not be destroyed or changed