why can cows digest grasses while persons don't?
The digestive system of cows is quite complex, you can find details in a
library or by an internet search. Their systems have adapted through
evolution over many generations to take advantage of that particular food.
Cows don't actually digest grass, which contains cellulose. They have a
ruminant stomach, which means that their stomach is compartmentalized. Some
would say they have more than one stomach. But in one of the "stomachs"
they have bacteria which are able to break a specific chemical bond in
cellulose that breaks it down into glucose. This is an example of
mutualistic symbiosis, where both organisms benefit from a relationship.
The cow benefits by being able to take advantage of a food source, and the
bacteria get a ready supply of food and protection. Did you know that
termites can't digest wood? They also have organisms living in their guts
that break down the cellulose in wood.
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Update: June 2012