1. Glucose: This is the most common and preferred energy source for cells. Glucose is a simple sugar that is broken down through a process called cellular respiration. This process involves a series of chemical reactions that release energy stored in the glucose molecules and convert it into a usable form called ATP (adenosine triphosphate).

2. Fatty Acids: These are long chains of hydrocarbons that can be broken down to provide energy. Fatty acids are typically stored as triglycerides in cells and are broken down through a process called beta-oxidation. This process generates acetyl-CoA, which enters the citric acid cycle (part of cellular respiration) to generate ATP.

Other Less Common Energy Sources:

* Amino acids: While not the primary energy source, amino acids can be broken down into intermediates that enter the citric acid cycle. However, this is usually a last resort when glucose and fatty acids are limited.

* Ketone bodies: These are produced in the liver from fatty acids during prolonged fasting or starvation. They can serve as an alternative fuel source for the brain and other tissues.

Important Note:

While cells can utilize different energy sources, their preference for glucose is due to its readily available nature and its ability to be quickly broken down for energy production.