The reactions are reversed in the alveoli of the lungs, where the concentration of carbon dioxide is lower than in the capillary beds of the tissues. Now that we have the oxygen in and the carbon dioxide out, let's bring some fuel to our metabolizing cells. As we have stressed at several earlier points in this semester, heterotrophs need food. This is what he meant. There are interesting variations on the theme of ingestion.
All ingested food is reduced by enzymatic action to monomers.
This is the process of digestion. Specific enzymes reduce carbohydrates to monosaccharides, proteins to amino acids, fats to small fatty acids and monoglycerides, and nucleic acids to nucleotides and ultimately to bases and monosaccharides. Digestion in animals typically is extracellular, taking place in the lumen or hollow opening of the gut. Some animals, however, rely on intracellular digestion. In this case tiny food particles are phagocytized into cells and packaged in vesicles with digestive enzymes.