Difference Between Cellulose and Starch

Cellulose vs Starch

Starch and Cellulose belong to the same group of carbohydrates. Carbohydrates are one of the common forms of energy sources in food. They have the molecular formula CH2O. They are macromolecules made up of several monomer units of glucose. They have a high molecular weight and are built from these entities repeated and interconnected through chemical linkages.

Cellulose

Cellulose is the polymeric form of glucose units linked together by glycoside linkages. It is the most abundant organic molecule and the major structural unit of plants. Cotton and paper are some forms of pure cellulose. It is made of about 4000-8000 glucose molecules with beta bonds between the 1st C of the first unit and the 4th carbon of the next glucose unit. Thus it forms beta 1,4 linkages.

There are two forms of cellulose, hemicellulose and lignin. Cellobiose is another form which results from the hydrolysis of cellulose. It is a disaccharide made of two glucose molecules linked by beta 1,4 linkage. Cellulose is hydrolyzed by cellulases.

Starch

Starch is essentially similar to cellulose in the composition. They are polymeric forms of glucose molecules lnked by alpha 1,4 linkage. The number of molecules making up a starch molecule can vary from 4000- 8000. The chain of glucose can be either linear, branched or a mix of both depending on the source and site where the form is stored. It is the primary storage form of carbohydrate.

Starch is a storage form of carbohydrates in plants. Properties of starch can vary from one to another depending on the source from which it is isolated. The properties also depend on the nature of branching and the number of alpha 1,4 glycosidic bonds. There are two forms of starch, amylase and amylopectin.

Difference Between Cellulose and Starch

Although both starch and cellulose are polymeric forms of glucose, they differ in their chemical and physical properties. These differences are mainly attributed to the difference in linkage.

Structure

Cellulose is mostly linear chains of glucose molecules linked together by beta 1,4 glycosidic bonds whereas starch is found in both linear and branched chains.

The orientation of the glycosidic linkages in cellulose makes the glucose rings to be arranged in a flip flop fashion which contributes to the rigidity. There are no branching chains in cellulose. Cellulose also owes its rigidity to the numerous hydrogen bonds in the structure which in turn makes a good structural polysaccharide.

Chemical bond

Cellulose has beta 1,4 linkage between the glucose units while starch has alpha 1,4 linkages

Forms

Cellulose occurs in nature as pure cellulose , hemicellulose or lignin. Starch occur in two forms- amylase and amylopectin. Amylose is simple linear form where as amylopectin is complex and branched. Glycogen is the storage form of starch in animals and more branched than amylopectin.

Function

Cellulose is a structural polysaccharide. Starch is mainly a storage polysaccharide.

Hydrolysis

Cellulose on hydrolysis form cellobiose and ultimately to glucose monomers by the action of celulases. These enzymes are less common and found in certain protozoa and bacteria in small quantities. Starch is acted upon by amylases which break it into glucose units.

Digestibility

Starch can be broken down to maltose and then finally to glucose by the enzymes present in humans. Cellulose cannot be digested properly in the absence of the cellulase enzymes.

Conclusion

Although starch and cellulose are both polymeric forms of glucose, they differ in the properties. These differences tend to be the result of the difference of a single chemical bond between the monomeric units. The varied nature makes the carbohydrates to play both energy providing function as well as structural roles.