Energy -- Workshop 5

Digestion is not easy for animals larger than tiny worms. It requires a major investment of time and energy, and requires an extensive system of digestive organs, secretions, and some surprising microscopic helpers.
In order to break down food to absorbable molecules, the digestive system first has to physically break apart and grind down food items. In humans, who evolved to eat somewhat easily digested fruits, seeds, and some meat, the teeth are the primary breaking and grinding structures.
Chemical secretions including saliva, stomach acid, and a variety of enzymes digest the more complex nutrients into simpler components.
Our small intestine also harbors beneficial bacteria that take part in digestion in exchange for a free meal.
The digested nutrients are absorbed into the blood through the thin walls of the small intestine.
The large intestine completes the process by resorbing water and forming excess bacteria and indigestible foodstuff into excretable feces.
Some animals have evolved digestion strategies quite different from humans.
Animals that live on high cellulose plants, such as grass, have to process their food more thoroughly than humans. Cattle, deer and other ruminants harbor cellulose-digesting bacteria in their many-chambered stomachs. After the bacteria have broken down the cellulose sufficiently, the cow "chews the cud," reprocessing the grass before further digestion.
Birds lack teeth for grinding. Ostriches and other birds that live on plant material swallow stones, which stay in the bird's muscular gizzard. As the gizzard churns, the stones grind the food. There is evidence that some dinosaurs, which are close relatives to birds, also used gizzard stones.
Wood is almost impossible to digest. Wood-eating termites depend on symbiotic protozoa in their guts to produce the enzymes for wood digestion. In some termites, this is carried a step further. The protozoa contain symbiotic bacteria of their own that digest the wood.