O2 represents molecular oxygen, which is a diatomic molecule consisting of two oxygen atoms bonded together. It is necessary for respiration and combustion processes.
NO3 represents the nitrate ion, which consists of one nitrogen atom bonded to three oxygen atoms. It is often involved in redox reactions due to its ability to donate or accept electrons.
CO2: CO2 represents carbon dioxide gas, which is formed during the process of cellular respiration and is involved in atmospheric processes.