Free Response

Describe the similarities and differences between glycogen and starch.


Glycogen and starch are polysaccharides. They are the storage form of glucose. Glycogen is stored in animals in the liver and in muscle cells, whereas starch is stored in the roots, seeds, and leaves of plants. Starch has two different forms, one unbranched (amylose) and one branched (amylopectin), whereas glycogen is a single type of a highly branched molecule.

Why is it impossible for humans to digest food that contains cellulose?


The β 1-4 glycosidic linkage in cellulose cannot be broken down by human digestive enzymes. Herbivores such as cows, koalas, and buffalos are able to digest grass that is rich in cellulose and use it as a food source because bacteria and protists in their digestive systems, especially in the rumen, secrete the enzyme cellulase. Cellulases can break down cellulose into glucose monomers that can be used as an energy source by the animal.

Draw the ketose and aldose forms of a monosaccharide with the chemical formula C3H6O3. How is the structure of the monosaccharide changed from one form to the other in the human body?


The human body switches carbohydrates between their aldose and ketose forms using a family of enzymes called isomerases. The ketose triose is called dihydroxyacetone, and has the oxygen double-bonded to the center carbon:

The aldose is called glyceraldehyde, and can have the oxygen double-bonded to the first or third carbon of the molecule: