In biology, cellular fuel is generally referred to as ATP, which stands for adenosine triphosphate.

Here's why:

* ATP is the primary energy currency of cells. It's a molecule that stores and releases energy in a way that cells can easily use.

* Cells use ATP for a variety of functions, including muscle contraction, nerve impulse transmission, protein synthesis, and active transport.

* ATP is constantly being produced and used by cells. This process is known as cellular respiration.

While other molecules can be used as fuel sources (like glucose, fatty acids, and amino acids), they are ultimately broken down to produce ATP, making it the "universal energy currency" of cells.