* Glucose: The most common and readily available source of energy for cells.
* Fatty acids: Can be broken down into acetyl-CoA, which enters the Krebs cycle.
* Amino acids: Can be used as an energy source, though this is less efficient than glucose or fatty acids.
In summary:
* Glucose is the main substrate.
* Fatty acids and amino acids can also be used as fuel.
It's important to note that cellular respiration is a complex process with many steps, and the specific substrates used can vary depending on the organism and its metabolic needs.
