Here's why:

* ATP is the "energy currency" of cells: It stores chemical energy released from the breakdown of food molecules (like glucose) and then releases it when needed to power cellular processes.

* ATP powers many vital functions: These include muscle contraction, nerve impulse transmission, protein synthesis, and active transport of molecules across cell membranes.

While ATP is the primary energy supplier, it's important to remember that cells also utilize other energy sources in specific situations:

* Glucose: The main fuel source for cellular respiration, the process that generates ATP.

* Fatty acids: Can be broken down to produce ATP, especially during prolonged periods of fasting or exercise.

* Amino acids: While not the primary source, amino acids can be used for energy production.

Therefore, ATP is the direct energy supplier for cells, while glucose, fatty acids, and amino acids are the sources of energy that are converted into ATP.