Hi!
In order for cellular respiration to occur in a cell, the cell needs to have glucose molecules to utilize in the reaction which is as follows:
C6H12O6 + 6 O2 --> 6 CO2 + 6 H2O + ATP (energy)
Cellular respiration cannot occur in the absence of glucose. Plant cells acquire this glucose from photosynthesis:
6CO2 + 6H2O + light energy = C6H12O6 + 6O2
Animal cells do not need to photsynthesize because they acquire the amount of glucose for cellular reaction from their diet/consumption. Herbivores acquire this glucose from plants, carnivores from feeding off other animals, and omnivores from both feeding off plants and animals.
Hope this helps!