Final answer:
Animals convert the macromolecules in food into the simple molecules needed for cellular function through digestion and absorption. Essential nutrients, which cannot be produced by the body, must be obtained from food. This ensures that the body has the necessary building blocks and compounds for its structures.
Step-by-step explanation:
Animals must convert the macromolecules in the food they consume, such as proteins, fats, and carbohydrates, into the simple molecules required for maintaining cellular function. This process involves digestion and absorption, where food particles are broken down into smaller components and then absorbed by the body. Essential nutrients, which the body cannot produce, need to be consumed from food to provide the necessary building blocks and compounds for the body's structures.