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.
Extracellular digestion is much more common, and is typical of mammals, including humans.
Intracellular digestion is much rarer, being characteristic of highly specialized animals such as the sponges. Intracellular digestion is similar to the process by which heterotrophic protistans get their nutrients. Many animals have simple tube digestive systems. The nematode is an example. Without compartments, specialized processing cannot readily occur.