Answer:
air
Step-by-step explanation:
The main source of oxygen molecules is the air around us. Air is made up of 21% of oxygen gas which is the second most abundant gas found in the atmosphere.
- Starch, Proteins and food are all made up of complex organic molecules.
- Air is a mixture of many gases.
- Gaseous exchange is made possible when air supplies oxygen and takes carbon dioxide away.
- Oxygen is an essential gas in the metabolic process of animals.