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For use in (aerobic) respiration, polypeptides would first be hydrolysed to their amino acid monomers. These are each deaminated, i.e. have their amino groups removed, producing an organic acid, which can be fed into the Kreb's Cycle, and a waste product, ammonia (NH₃), which is moved to the liver and removed from the blood.
Answer:The answer is tiny organisms known as cyanobacteria, or blue-green algae.
Explanation:
These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and oxygen increase. Additionally, some sources you could use that I used to answer this question was
https://www.scientificamerican.com/article/origin-of-oxygen-in-atmosphere/