To better illustrate this, let us first show the balanced chemical equation of the two processes.
6CO2 + 6H2O + Light Energy -> C6H12O6 + 6O2
For cellular respiration:
C6H12O6 + 6O2 -> 6CO2 + 6H2O + ATP (energy)
Law of conservation of mass says that the mass of any closed system/s remains constant at all times. Mass can neither created nor destroyed, but cab be transformed into different forms or rearranged in space.
Obviously, we can see that photosynthesis is the reverse reaction of the cellular respiration. Exactly 6 moles of carbon dioxide and 6 moles of water is used by plants (and other photosynthetic organisms), with the help of sunlight as the source of energy to produce one mole of glucose and 6 moles of oxygen.
Humans and other organism consume the glucose and oxygen that the plants are producing to produce energy in the form of adenosine triphosphate (ATP).
This shows that the two biological phenomena show that the law of conservation of mass is true.