Topic Test : Small Biomolecules -                                    [answers given below]
                    Simple Sugars and The Orientation of Atoms in Space

True/False
1.  The disaccharide lactose is made of the monosaccharides glucose and galactose.
2.  Cellulose is a polysaccharide, made of many glucose units, in which the bond between them has a
        different orientation than the same bond between the glucose molecules which are linked to form
        the polysaccharide, starch?
3.  All the isomeric forms of a compound have the same physical and chemical properties?

Multiple Choice
4.  The chemical formula which reveals the orientation of the atoms in a molecule is called its?
        a) empirical (molecular) formula      b)  structural formula      c)  monosaccharide formula
        d) branched formula                                                    e) polysaccharide formula
5. Which of the following is (are) biological polymers ?
        a)  starch     b)  amylose     c) chitin      d) cellulose      e) all of these are polymers
6. Which of the following pairs are isomers?
        a) ¹⁴C and ¹²C     b) alanine & glycine     c) glucose & galactose      d) glycogen & fructose
       e) choose this answer if none of the above is correct

Short Answer/Problem Solving
7.      What would have to happen to allow humans to be able to digest the cellulose in leafy salads they
            eat.
8.  The common polysaccharides found in living cells are only made from one monomer. Why do you
        suppose that there are no polysaccharides made of random sequences of the isomers of glucose
        (glucose, galactose, mannose, etc.)?

Topic Test 1:  Answers

1. True.    Lactose is a disaccharide that consists of β-D-galactose and β-D-glucose fragments bonded through a β1-4 glycosidic linkage.

2.  True.   Cellulose is made with a beta-oriented glycosidic bond between the glucose units, while starch is made with an alpha-oriented glycosidic bond between the glucose monomers. We can digest starch, but not cellulose.

3.  False.  Isomers of a compound have a different structural formulas and orientations of their atoms, which results in them having different chemical and physical properties. Glucose, which has its C4-OH oriented downward is perceived by humans to be, much sweeter tasting than galactose, which differs only in having its C4-OH group oriented upward.

4.  b.        It is the structural formula of molecules, which reveals the orientation of its atoms in space.

5.  e.        All of these are biological polymers: Starch is a linear polymer of simple glucose  monomers. Amylose is another name for starch.  Chitin is cellulose which has had amine groups linked to its glucose units. It makes up the exoskeletion of insects.  Cellulose is the beta-OH polymer of glucose units, which makes up the structural pattern of plants.

6.   c.       Glucose and galactose are isomers.  ¹⁴C and ¹²C are isotopes of carbon, i.e., elements with the same number of protons, but a different number of neutrons.  Alanine and glycine are amino acids and each has a different chemical structure to its residue group, and glycogen is a polymeric polysaccharide, while fructose is a simple sugars monomer.

Short Answer/Problem Solving

7.  To be able to digest the cellulose in their dietary leafy salads humans would have to be able to digest cellulose with its beta-OH glycosidic bond.  Humans lack the enzyme, cellulase, to be able to do this so they would have to evolve a symbiotic relationship, like that found in ruminant animal,  with those organisms able to digest cellulose, which is not likely.  A second option might be to use genetic engineering to incorporate into the human genome the gene for the enzyme which could digest cellulose, thereby allowing students the chew on their pencils for nourishment.

8.  Random sequence polymers of all the isomeric forms of hexose sugars would have no energetic
     advantage over polymers of single glucose units, so it is unlikely they would have evolved. Even if
     enzymes to do this did evolve previously during cellular evolution, the product they could make by
     linking glucose, mannose, and galactose together in random order wouldn't have been selected for
     any better than the polysaccharides of amylose and glycogen. In fact it would probably be more
     energetically expensive to make a random polymer, as each glycosidic bond formed between the
     different monomers would need a unique enzyme. Thus at least up to 3 different enzymes, or more,
     would have been needed to make random polysaccharide polymers, whereas starch needs only one
     enzyme.

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