1 Answer

Nutrients are locked up within foods. When you pick up an apple, it is not in the form of nutrients that can be absorbed by the animal and used in its body.

So, between ingesting the food and using the nutrients stored within it, we need an intermediate step (digestion), that prepares the nutrients and converts into a form that can be used by the animal body. The following are the molecules that each macromolecule must be broken into before they can be used by the animal body:

Starch/cellulose/glycogen should be broken down into glucose
Proteins must be broken into amino acids
Fats and oils should be broken into fatty acids and glycerol

The vitamins and minerals should be extracted from the food too and absorbed into the animal's circulatory system before they are of any use to the animal.

Therefore, digestion is the bridge between food and the ability of animals to use it.