It follows the law of conservation of matter
Cellular respiration involves many steps such as glycolysis, krebbs cycle,oxidative phosphorylation etc. but we can consider the overall equation as it gives the sum total of all the processes.
C6H12O6 + 6O2 = 6CO2+6H2O + 38ATP
In the equation,6 atoms of carbon give all those 6 back likewise, number of atoms of every element is same on both sides.
Hence it follows the law of conservation of energy which states that matter can niether be created nor be destroyed.