Almost all of the oxygen (\(\mathrm{O_2}\)) one consumes in breathing is converted to:

• A. glucose (\(\mathrm{C_6H_{12}O_6}\))
• B. carbon dioxide (\(\mathrm{CO_2}\))
• C. water (\(\mathrm{H_2O}\))
• D. lactic acid

Answer: (C)

Verified step by step guidance

1. Step 1: Begin by understanding the process of cellular respiration, which is the primary mechanism by which oxygen (\(\mathrm{O_2}\)) is utilized in the body. Cellular respiration occurs in the mitochondria and involves the breakdown of glucose (\(\mathrm{C_6H_{12}O_6}\)) to produce energy in the form of ATP.
2. Step 2: Recognize that oxygen (\(\mathrm{O_2}\)) plays a critical role in the electron transport chain, the final stage of cellular respiration. Here, oxygen acts as the final electron acceptor, combining with electrons and protons to form water (\(\mathrm{H_2O}\)).
3. Step 3: Recall the overall chemical equation for cellular respiration: \(\mathrm{C_6H_{12}O_6} + 6\mathrm{O_2} \rightarrow 6\mathrm{CO_2} + 6\mathrm{H_2O} + \text{energy (ATP)}\). This equation shows that oxygen is directly involved in the production of water as a byproduct.
4. Step 4: Understand that while carbon dioxide (\(\mathrm{CO_2}\)) is also produced during cellular respiration, it is generated during earlier stages (glycolysis and the citric acid cycle) and is not directly linked to the consumption of oxygen. Oxygen is specifically converted into water (\(\mathrm{H_2O}\)) during the electron transport chain.
5. Step 5: Eliminate the other options: glucose (\(\mathrm{C_6H_{12}O_6}\)) is a reactant, not a product; lactic acid is produced under anaerobic conditions (absence of oxygen), not aerobic respiration. Therefore, the correct answer is water (\(\mathrm{H_2O}\)).