Here's why:

* Cellular Respiration: This process breaks down glucose in the presence of oxygen to produce ATP, which is the primary energy currency of cells. It involves four main stages: glycolysis, pyruvate oxidation, the Krebs cycle, and oxidative phosphorylation.

* Fermentation: This process occurs in the absence of oxygen and uses glycolysis to generate a small amount of ATP. However, fermentation doesn't involve the other stages of cellular respiration, and it produces different end products depending on the type of fermentation (e.g., lactic acid or ethanol).

In summary:

* Cellular respiration: Glucose + Oxygen → ATP + Carbon dioxide + Water

* Fermentation: Glucose → Small amount of ATP + Lactic acid/Ethanol + Carbon dioxide

ATP is the main energy source produced by cellular respiration and is essential for various cellular functions, while fermentation only produces a limited amount of ATP.