Question

Cellulose and starch are both polymers of glucose but fulfil very different functions in cells. Explain the roles and properties of the two polymers and speculate which of their properties may have aided the evolution of their current function. Use the following terms in your explanation: Glycosidic bonds; Starch; Cellulose; Cell wall; Energy storage.

Answer 1

Starch and cellulose are two very similar polymers, the both made up of the same monomer, glucose, and have the same glucose-based repeat units. To understand the difference betwenn starch and cellulose please understand the structure of glucose fiirst (figure 1)

Fgui- Glucose, Al tohel group a. Cup alshel utup allachad bo up etta had to Starah cH20H 따 4 14 A o4 H 6H 4ala upsidta

There are two forms of glucose (i.e.) alpha glucose and beta glucose (figure 2 (a), (b)) When two molecules of glucose are inked together they share oxygen and the structure (starch or cellulose) is determined by what form of glucose is linked together.

Starch contains alpha glucose polymers and the glucose moleucles are linked by a-1,4 glycosidic links. On the other hand cellulose contains beta glucose polymers linked by b-1,4 glycosidic links. The only difference between starch and cellulose is that, all the glucose repeat units are oriented in the same direction in starch. On the other hand in cellulose, each successive glucose unit is rotated 180°C relative to the last repeat unit, around the axis of the polymer backbone chain (figure 2).Yes, as you think the difference may seem minor but it plays a huge role in the three-dimensional shape of the molecule.

Hydrogen bonding between two cellulose molecules, forms cables known as micro fibrils of parallel chains, which give it a rigid structure. (e.g.) wood composed of cellulose and it is a useful building material.

Starch is a storage form of carbohydrate mainly found in plants, such as potatoes and cereal grains. Starch is practically useless as a material, but cellulose is strong enough to make fibers an therefore hence rope, clothing, etc.

Cellulose does not dissolve in water and it will not breakdown easily but starch will dissolve in water and will breakdown easily. Breaking down or dissolving in water would be a little too inconvenient for something we use to make clothes, buildings etc.

If that happens a good soaking rain would wash away all the wooden houses, park benches etc.

We can eat starch and digest it, but we can't digest cellulose. Our body contains enzymes that can break starch down into glucose to fuel your body. But we don't have enzymes that can break down cellulose but some animals do (e.g.) termites and cattle.

Cellulose is a lot stronger than starch. It is the supporting part of plants such as the stems of flowers or the wood of trees.

from the above mentioned differences between starch and cellulose it is clear that during evolution the strong cellulose has become the supporting material for plants, woods and trees.