Welcome to the Carbohydrates debriefing.

Answer the questions in the spaces provided. If necessary, click on the Check buttons when you are done with each question.


1. In the following groups of molecules, select the ones that could be carbohydrates. Click on the Check button when you are finished.

A. C6H12O6 B. C2H6O C. CH2O
C12H22O11 C3H6O3 C2H4O2
C3H9N C2H2O C7H14O7
All of the above All of the above All of the above

Correct!

Correct! But are there any others?
Remember, the formula for a carbohydrate is Cx(H2O)y.

2. Label the following monosaccharides and disaccharide as α or β anomers or having an α or β glycosidic bond.

α β
α β
α β
α β

Correct!

Remember, if C6 and the hydroxyl group on C1 are on the same side, it's the β anomer. Otherwise, it's the α anomer.

Enzymes are biological molecules that catalyze very specific chemical reactions on other molecules. Enzymes are so specific that very subtle changes in the molecule they act on (substrate) can make a large difference in whether the reaction will go forward or not. For example, some enzymes will not catalyze a reaction if a single methyl group is added to the substrate. (See the Enzymes module for more information about enzymes.) Starch is broken down by α-amylase and α-dextrinase, while glycogen is broken down by phosphorylase. Animals have all three enzymes.

3. Why do you think animals can't break down cellulose, which is also composed of glucose units linked by glycosidic bonds?

A. Because β glycosidic bonds are more stable than α glycosidic bonds.
B. Because animals do not possess a digestive enzyme specific for the β glycosidic bonds between glucose units in cellulose.
C. Because cellulose is linear, while glycogen and starch are branched.
D. A and C.

Correct!

Sorry, try again.

The enzyme that breaks down cellulose is called cellulase. It relies on the specific orientation of hydroxyl groups around a β glycosidic bond, which is why phosphorylase, α-amylase, and α-dextrinase cannot break down cellulose. Cellulase is produced by bacteria in the intestines of termites, cows, and other herbivores. Some of the glucose released by the breakdown of cellulose is absorbed by the bacteria's host. This is why cows can eat grass but humans cannot.


Congratulations!

You have completed the Carbohydrates module.

Click on the Carbohydrates icon below to return to the Biomolecules gateway page.