* Glucose: The most common and well-known substrate. It's a simple sugar that provides the energy for the majority of cellular processes.

* Fatty acids: These are long chains of hydrocarbons that can be broken down to produce energy. They are a significant source of energy, especially during prolonged fasting or exercise.

* Amino acids: While primarily used for building proteins, amino acids can also be broken down for energy. This is less common than glucose or fatty acids.

Other potential substrates:

* Glycerol: A component of triglycerides (fats).

* Pyruvate: A product of glycolysis (the first stage of cellular respiration).

* Lactate: Can be used as a fuel source under anaerobic conditions.

It's important to note that oxygen is a crucial reactant in cellular respiration, but it is not technically a substrate, as it does not get directly incorporated into any products.

Substrate usage depends on factors like:

* Availability: The most readily available fuel source is typically used first.

* Energy needs: Different tissues and organs may use different fuel sources based on their energy demands.

Cellular respiration is a complex process with multiple steps, and different substrates enter at different points in the process.