What is the major factor that affects the process of cellular respiration?

Photosynthesis first, Cellular Respiration second.

Explanation:

Photosynthesis:

the intensity of the light being absorbed by the chloroplasts the concentration and availability of CO2 (Carbon Dioxide) the temperature

Cellular Respiration

the availability of oxygen (determines whether or not fermentation will be used) the availability and amount of glucose (the food is broken down) affected by the temperature of the cell environment