Final answer:
Oxygen is essential for cellular respiration, where it helps convert glucose into water, carbon dioxide, and ATP.
Step-by-step explanation:
Besides sugar, oxygen is required for cellular respiration to occur. In cellular respiration, glucose and oxygen are converted into carbon dioxide and water, producing ATP along the way as chemical energy for the cell's use. Water is another important byproduct of this process, but it's not a required input like oxygen is. While cellular respiration primarily 'starts' with glucose derived from carbohydrates, other molecules such as fats and amino acids can also feed into the cellular respiration pathways at various points. This process occurs in a series of stages, where each needs oxygen to efficiently produce ATP.