The internal (cellular) energy reserve in the cells of animals isGlycogenProteinStarchFatty acids

The internal (cellular) energy reserve in the cells of animals is Glycogen.

Here's a step-by-step explanation:

1. Animals, including humans, store excess glucose in the form of a polysaccharide called glycogen.

2. When the body needs a quick boost of energy or when the body isn’t getting glucose from food, glycogen is broken down to release glucose into the bloodstream to be used as fuel for the cells.

3. Glycogen is stored in the liver and the muscles. The liver glycogen is used for maintaining blood sugar levels, while the muscle glycogen is used for muscle activities.

4. Proteins, starch, and fatty acids can also be used as energy sources, but they are not the primary energy reserves. Proteins are typically used for building and repairing tissues, not for energy. Starch is a form of energy storage in plants, not animals. Fatty acids stored in adipose tissue can be used for energy but it's a long-term energy source, not a quick, readily available source like glycogen.

So, the primary internal energy reserve in animal cells is glycogen.