Date: September 2008
Why herbivore can digest cellulose but humans cannot?
Cellulose is a long molecule called a polymer, which, like DNA, consists of
subunits connected together by covalent bonds. In the case of cellulose the
subunits are the simple sugar glucose and they are connected by a very
specific bond, called a "beta-1,4-glycosidic bond". This bond makes the
molecule stiff and able to pack together to create structure in the cell
walls of plants.
Humans lack enzymes to break this kind of chemical bond and therefore cannot
digest cellulose. Cows, and other ruminants like deer, camels, and water
buffalo, have microorganisms in their guts which express enzymes capable of
breaking down cellulose. These animals go through cycles of chewing,
swallowing, and regurgitating, in order to reduce the particle size of the
plant matter so that it can be digested and broken down with the aid of
microorganisms in a process known as "ruminating".
Herbivores and people both have digestive systems. These systems produce
chemicals that digest the food that is eaten. The digestive systems of
herbivores, such as cows and sheep, include enzymes (digestive chemicals)
that break down cellulose (a large chemical) into its building block chemicals.
The human digestive system does not include the chemical factories that
produce the cellulose-digesting enzymes.
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Update: June 2012