Oxygen and CO2 cycle

1. Water + co2 ---> sugar + o2 photosynthesis 

2. Sugar + o2 ---> water + co2 cellular respiration 

O2 levels drop at night because of no sun. 

Animals use o2 at night and make it drop more. 

When the sun comes out photosynthesis begins again. 

The more light, the more o2.