Most plants synthesize cellulose which they use to give strength to their bodies and make them withstand the vagaries of storms. It gives the shape to the trees and branches then expand the tree. Cellulose becomes the wood when trees become mature. Furniture is made of this material. It is obvious that cellulose is insoluble in water otherwise no one will use it in construction and furniture.
Cellulose and starch and other carbohydrates consist of glucose molecules which are arranged in chains of different styles. In starch, two chains are intertwined. These chains get separated on boiling the starch in water. This caused the chains to disperse in the water increasing its viscosity. Such starch is called pre-gelatinized starch.
On the other hand, cellulose structure is such that chains are not dispersed and wood remains unaffected in water. Glucose is the basic unit of sugar which is used by animals like humans for obtaining energy to keep the body running.
How do then we assimilate higher sugars like starch. They have to broken down to glucose units. The enzymes found in humans and other animals allow them to digest and metabolize many, but not all, biomolecules. Cellulose is one example of a molecule that defies digestion in many animals.
But the slight difference in the way the glucose molecules are hooked together in starch compared with how they are hooked together in cellulose makes a big difference in their digestibility.
Humans and many other higher animals have the enzyme required to break the bonds in starch, releasing glucose. The particular enzyme is called alpha-amylase.But because the shape of the linkage is different in cellulose, the same enzyme will not work. In fact, where cellulose is concerned, humans do not have an enzyme that will work.
As it turns out, most humans eat a fair amount of cellulose in the form of fruits and vegetables. Although we cannot digest it, the cellulose serves as roughage or fiber that gives food bulk and keeps it moving through the digestive system. In the end, all of the undigested material ends up being eliminated as feces.
Maybe you are wondering how animals such as cattle, sheep, deer, and goats thrive on a diet of grass or other cellulose-rich food. Can they digest cellulose when humans cannot? The answer is no. None of these animals have the enzymes required to digest cellulose. Instead they rely on colonies of microorganisms living in their digestive systems.
These simple microorganisms have the correct enzymes to digest the cellulose and to reassemble the products into starches and proteins. From these products, grazing animals acquire their nutrients. The special relationship between these animals and their resident microbes is called symbiosis—two organisms living with each other to the benefit of both.