Digestion occurs when the body breaks down food
in to small particles--molecules--which can be delivered to individual
cells. Only when the food particles are small enough will they pass
through the membranes that line the small intestines. At this point
the blood carries these nutrients to every cell in the body.
- 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
- Some animals have evolved digestion strategies quite different
- 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
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