Answer:
Macromolecules such as starches, proteins, and fats store great amount of energy (ATP) by making comlpex chemical bonding. This energy cannot be extracted unless they are digested (broken) first. Therefore, organisms with the help of enzymes break the big molecules to make them simpler compounds. Thereon, energy is extracted through series of chemcial reactions occuring within the animal body, i.e. glycolysis, electron transport chain, etc.