why can cows digest grasses while persons don't?
They have a complex digestive system designed for slow digestion and
also have a microbe that breaks down the cellulose sugar that makes up
grass. Humans posess neither the digestive system nor the enzyme to
live off of grass and such.
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