2.11: Not all carbohydrates are digestible.

Despite their general importance as a fuel source for humans, not all carbohydrates can be broken down by our digestive system. Two different complex carbohydrates—both indigestible by humans—serve as structural materials for invertebrate animals and plants: chitin (pronounced kite-in) and cellulose (FIGURE 2-26). Chitin forms the rigid outer skeleton of most insects and crustaceans (such as lobsters and crabs). Cellulose forms a huge variety of plant structures that are visible all around us. We find cellulose in trees and the wooden structures we build from them, in cotton and the clothes we make from it, in leaves and in grasses. In fact, it is the single most abundant compound on earth.

Surprisingly, cellulose is almost identical in composition to starch. Nonetheless, because of one small difference in the chemical bond between the simple sugar units, cellulose has a slightly different three-dimensional structure. Even tiny differences in the shape of a molecule can have a huge effect on its behavior. In this case, the difference in shape makes cellulose a sturdy structural material and, unlike starch, also makes it impossible for humans to digest. Consequently, the cellulose we eat passes right through our digestive system unused.

Although it is not digestible, cellulose is still important to human diets. The cellulose in our diet is known as “fiber” (FIGURE 2-27). It is also appropriately called “roughage” because, as the cellulose of celery stalks and lettuce leaves passes through our digestive system, it scrapes the wall of the digestive tract. Its bulk and the scraping stimulate the more rapid passage of food and the unwanted, possibly harmful products of digestion through our intestines. That is why fiber reduces the risk of colon cancer and other diseases (but it is also why too much fiber can lead to diarrhea).

Unlike humans, termites have microorganisms living in their gut that are able to break down cellulose. That’s why they can chew on wood and, with the help of the cellulose-digesting boarders in their gut, can break down the cellulose and extract usable energy from the freed glucose molecules.