Question

In what ways are cellulose and starch similar to each other and in what ways are they different? Be specific in your comparison, being sure to discuss both structure and function.

Answer 1

Plants store surplus glucose as starch granules within plastids, including chloroplasts, and withdraw it as needed for energy or carbon.

http://www.course-notes.org/Biology/Outlines/Chapter_5_The_Structure_and_Function_of_Macromolecules
Animals that feed on plants, especially parts rich in starch, have digestive enzymes that can hydrolyze starch to glucose.

Animals store glucose in a polysaccharide called glycogen.

Glycogen is highly branched like amylopectin.
Humans and other vertebrates store a day’s supply of glycogen in the liver and muscles.

Cellulose is a major component of the tough wall of plant cells.

Plants produce almost one hundred billion tons of cellulose per year. It is the most abundant organic compound on Earth.

Like starch, cellulose is a polymer of glucose. However, the glycosidic linkages in these two polymers differ.

The difference is based on the fact that there are actually two slightly different ring structures for glucose.
These two ring forms differ in whether the hydroxyl group attached to the number 1 carbon is fixed above (beta glucose) or below (alpha glucose) the plane of the ring.

Starch is a polysaccharide of alpha glucose monomers.
Cellulose is a polysaccharide of beta glucose monomers, making every other glucose monomer upside down with respect to its neighbors.
The differing glycosidic links in starch and cellulose give the two molecules distinct three-dimensional shapes.

While polymers built with alpha glucose form helical structures, polymers built with beta glucose form straight structures.
The straight structures built with beta glucose allow H atoms on one strand to form hydrogen bonds with OH groups on other strands.
In plant cell walls, parallel cellulose molecules held together in this way are grouped into units called microfibrils, which form strong building materials for plants (and for humans, as lumber).

The enzymes that digest starch by hydrolyzing its alpha linkages cannot hydrolyze the beta linkages in cellulose.

Cellulose in human food passes through the digestive tract and is eliminated in feces as “insoluble fiber.”
As it travels through the digestive tract, cellulose abrades the intestinal walls and stimulates the secretion of mucus, aiding in the passage of food.