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MCAT

Unit 2: Lesson 3

Foundation 3: Organ Systems

Why rabbits can digest cellulose

Problem

Cellulose, an important structural component of the plant cell wall, is a polysaccharide consisting of a linear chain of multiple β(1, minus, 4) linked glucose units. Cellulose is difficult for most animals to digest. Humans are unable to digest cellulose due to the lack of the enzyme needed to cleave its β(1, minus, 4) glycosidic bond (see molecular structure in Figure 1).
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, minus, 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.
Assuming rabbits have an identically structured alimentary tract to humans, where along the tract is digested cellulose most likely absorbed?
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