Mini MCAT passage: Why rabbits can digest cellulose

Cellulose, an important structural component of the plant cell wall, is a polysaccharide consisting of a linear chain of multiple β($1-4$‍) linked glucose units, as shown in Figure 1. Humans are unable to digest cellulose due to the lack of the enzyme needed to cleave its β($1-4$‍) glycosidic bond.

Rabbits, however, are herbivores that need to derive nutrition from the large amounts of cellulose in their diet. Therefore, rabbits have a special mechanism for digesting cellulose. After the cellulose passes through the small intestine, it enters a chamber called the cecum. The cecum contains microorganisms that are able to cleave the β($1-4$‍) glycosidic bond and break down the cellulose through hindgut fermentation. The resulting food is then excreted via nutrient-rich droppings. The rabbits then re-ingest the droppings, a behavior referred to as coprophagy. This allows the rabbit’s digestive system to further extract nutrients and vitamins that were not absorbed during the first round of digestion.

Figure 1. Molecular structure of cellulose.