Final answer:
Oxygen is a reactant in cellular respiration and is needed for the production of ATP. It is also a by-product of photosynthesis in plants, hence it is both a waste product and an essential element for life.
Step-by-step explanation:
Oxygen is a critical element in many chemical and biological processes on Earth. In the context of cellular respiration, the question "Is oxygen a product or a reactant of respiration?" seeks to understand its role. Oxygen is a reactant in cellular respiration; it is needed to help convert glucose into energy, specifically adenosine triphosphate (ATP) in the presence of oxygen - a process known as aerobic respiration.
Besides being a reactant in respiration, oxygen is indeed a waste product of the light reactions of photosynthesis in plants. This dichotomy illustrates the balance in Earth's ecosystem: plants release oxygen as a by-product, which animals and other organisms use in respiration.
Thus, in the air we breathe, oxygen is far from a waste product. It is essential for life, playing a key role in energy production within cells. Oxygen's role is so central that it is considered an oxidizing agent, helping to drive the chemical reactions that release energy from organic molecules.