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I'm studying "Campbell Biology, 10th Edition" and in chapter 5 page 71 there's a statement I can't understand. according to book:

In starch, all the glucose monomers are in the α configuration.

and then says:

In contrast, the glucose monomers of cellulose are all in the β configuration, making every glucose monomer “upside down” with respect to its neighbors.

My question is if all glucose monomers of cellulose are all in the β configuration (according to figures) why each monomer in cellulose is “upside down” with respect to its neighbors and is not just all the same as in glucose monomers.

Starch and cellulose structures

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The reason the monomer units are shown as alternating orientation in the cellulose case and not for starch is due to the angles required for the bonds between the atoms involved.

Note that in α-glucose the OH groups of the #1 and #4 carbons are shown on the same side of the ring. When these two groups are changed into a single O joining two α monomers into starch, they have to remain in the same position. When a single O joins them together, that oxygen requires the angle between the bond be less than 180 degrees, so the two glucoses have to be on one side of the oxygen. That's fine in the drawing because in α glucose both OH groups are pointing down.

On the other hand, in β-glucose the OH groups of carbons 1 and 4 are on opposite sides of the glucose ring. Joining two β monomers to make cellulose requires that these two OH groups point in the same direction so that when changed into bonds to the single O, the angle which the oxygen requires can be provided. To get the two OH groups pointing the same way requires one of the two β-glucoses to flip relative to the other. Hence you get the monomers of cellulose alternating in orientation.

Part of the problem may be trying to visualize these 3-D relationships from the 2-D diagrams. On this α-/β- glucose webpage there are some rotatable images of the molecules which may help visualize the relationships.

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I'm having trouble understanding the last part of your question: "why each monomer in cellulose is “upside down” with respect to its neighbors and is not just all the same as in glucose monomers."

I assume you want to know more about the beta configuration of cellulose. Cellulose is made of monomers of glucose linked together via B 1-4 glycosidic linkages. In contrast, "starch" more specifically amylose is made of monomers of glucose linked together via A 1-4 glycosidic linkages. Our bodies have glycosidases that can break down the alpha linkage but not the beta linkage.

This is the first time that I have seen cellulose polymers drawn that way. I am more accustomed to them being drawn like this: http://chemwiki.ucdavis.edu/Core/Biological_Chemistry/Carbohydrates/Polysaccharides/Cellulose

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