Here's why:

* Primary Energy Source: The primary energy source for most living organisms is glucose, a simple sugar.

* Secondary Energy Source: Carbohydrates, which are made up of long chains of sugar molecules, can be broken down into glucose. This makes them a readily available secondary energy source.

* Other Sources: While fats and proteins can also be used for energy, they are not as readily available as carbohydrates.

Let me know if you have any other questions!