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					TLW 7e Chapter 3 Answers
Molecules of Life

Apply Your Understanding
1. Figures 3.6 and 3.13
The molecule below, on the left, is a peptide made from monomers of amino acids. The molecule
below, on the right, is a disaccharide made from monomers of simple sugars. Both molecules
were synthesized using a common chemical reaction. What is the chemical reaction that formed
these molecules and what is the common by-product of both these reactions?

The chemical reaction that formed these molecules is called dehydration synthesis. This type of
reaction removes two hydrogens and an oxygen in formation of a bond, and so the common by-
product is water.


2. Figure 3.15
The following are two lipid molecules. The lipid on the left is a saturated fat and the one on the
right is an unsaturated fat. What is the difference in the chemical structure of their fatty acid tails
and how does this affect their physical properties?

Unsaturated fatty acids have one or more double bonds between carbon atoms, while saturated
fatty acids do not. This means that in a saturated fatty acid, each internal carbon atom is bonded
to two hydrogens, and therefore contain the maximum number of hydrogens possible (hence the
name “saturated”). In an unsaturated fatty acid, there are fewer hydrogens. Fats composed of
unsaturated fatty acids tend to be more liquid at room temperature, while fats composed of
saturated fatty acids are more solid. Most plant fats are unsaturated, while animal fats are
saturated.

Synthesize What You Have Learned
1. How many molecules of water are used up in the breakdown of a polypeptide that is 15 amino
acids in length?

The breakdown of this polypeptide takes 14 molecules of water: one for each of the 14 peptide
bonds that links the 15 amino acids in the polypeptide chain.

2. The enzyme present in the cells of your body can break the bonds between the glucose
monomers in starch but it cannot break the bonds between the glucose monomers in cellulose.
You learned that enzymes are very specific in what molecules they bind to.
After examining the chemical structures of starch and cellulose in table 3.2, explain why the
same enzyme that breaks down starch cannot break down cellulose.

Cellulose is a structural polysaccharide found in the cell walls of plants; its glucose subunits are
joined in a way that cannot be broken down readily. Cleavage of the links between the glucose
subunits in cellulose requires an enzyme most organisms lack. Some animals, such as cows, are
able to digest cellulose by means of bacteria and protists they harbor in their digestive tract,
which provide the necessary enzymes.

				
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