Anytime we ingest macromolecules, for instance, proteins, carbohydrates or lipids, we have to break these macromolecules down into their individual form. So we break them down into the amino acids, the individual sugar monomers glucose molecules and into fatty acids. And ultimately, these three different types of molecules, amino acids, glucose molecules and fatty acids are brought into the cell and inside the cell, the cell metabolizes breaks them down into ATP molecules via process known as aerobic cellular respiration. Now, these ATP molecules are used as energy molecules to carry out different types of processes and reactions that take place inside the cells. Now, the problem with aerobic cellular respiration is it produces a waste byproduct, namely carbon dioxide. And carbon dioxide cannot actually be used by the cell in any useful manner.