Given that co2 is produced by cellular respiration, why does the amount of co2 in the atmosphere remain relatively constant? (when answering this question, exclude the impact of human activities on atmospheric co2.)

Given that CO2 is produced by cellular respiration, the amount of CO2 in the atmosphere remain relatively constant because CO2 in the atmosphere is also being used by plants to produce not only oxygen but also to produce carbohydrates through the process of photosynthesis.