Question

Write the structural difference between starch and cellulose

Answer 1

Starch is formed from alpha glucose, while cellulose is made of beta glucose. 

Answer 2

In starch, the sugar molecules are linked together such that they are all in the same orientation. In cellulose, they are linked together such that alternating molecules are rotated 180 degrees from each other. This seemingly minor change makes cellulose much stronger than starch, because parallel cellulose fibers stack up just like corrugated sheets piled on top of each other. The fibers hydrogen-bond to each other, providing enormous structural strength. The fibers of starch, by contrast, bind to each other much more weakly and thus fall apart more easily.

Some kinds of starch (glycogen and amylopectin) are formed out of branching chains, while other kinds (amylose) are linear polymers. The latter are much harder for the body to break down, because they coil tightly into a helical form. Cellulose is only formed out of linear chains, because branches would substantially reduce its strength by disrupting the strong bonds between parallel fibers.

The alternating structure of cellulose also makes it much more difficult to digest. In fact, most organisms lack this capacity entirely whereas almost all organisms can digest starch. I’m not sure exactly why there is such a huge difference between the stability of alpha (starch) vs. beta (cellulose) glycosidic bonds, but this difference has enormous consequences for the ecology of Earth.