Biology 107 (Dr. Harrington) Problem set 1 answers

BIOL 107 B2 - Answers to Problem set 1

Lecture 1 Macromolecules

Question 2 (exam) Why does our digestive enzyme Amylase break down starch but not cellulose?

C. the sugars in cellulose bond together differently than in starch

Question 3 (thinking) Peanut butter and margarine are made from plant oils and yet both are semi-solid at room temperature. What must occur during the manufacturing of these products?

These products are produced by hydrogenation (addition of hydrogens) which changes C=C bonds into C-C bonds. Essentially the oils, which are liquid at room temperature, are converted into fats, which are solid at room temperature. The problem with this process is that some of the C=C double bonds are converted from the normal cis arrangement into the trans arrangement. Trans fatty acids have several detrimental effects on our health. I have more information on this on my webpage.

Question 4 (exam) A peptide bond in a protein connects a _____ to a _____. (Hint: draw it)

C. carbon ... nitrogen

Question 5 (exam) A molecule that contains only carbon, hydrogen, and oxygen may be a:

D. lipid or a sugar

Question 6 (thinking) Carbohydrates are commonly used in cooking. For example dextrose is the common name for glucose, table sugar is crystalline sucrose, molasses is concentrated sucrose, and icing sugar is powdered sucrose.

This question is for your information - it is not representative of exam questions.

a) Corn syrup (great on pancakes) is a solution of glucose made from corn starch. By what steps can corn starch be converted into corn syrup?

Corn starch is broken down into oligosaccharides (oligo=few) and maltose molecules by an enzyme. Then this mixture is further broken down into glucose monomers by a second enzyme. We will discuss enzymes in lecture 8.

b) Glucose-fructose (known as high fructose corn syrup or HFCS in the US) is a commonly used sweetener found in pop, candy, etc. It is a mixture of about half glucose and half fructose that is also made from corn starch. How is it made?

The glucose produced by the above process is treated with a third enzyme which converts most of the glucoses into fructoses. The before mixture (mostly glucose) and the after mixture (mostly fructose) are combined. A mixture containing 42% fructose is mildly sweet and is used to sweeten products such as canned fruit. A mixture containing 55% fructose is quite sweet tasting and is used to sweeten ice cream, pop, etc. The advantages glucose-fructose has over regular sugar (obtained from beets or sugar cane) is that it is cheaper, lasts longer, and tastes sweeter. However the product is vilified, particularily in the US, as the cause of the obesity epidemic.

c) Honey is a mixture of glucose and fructose that bees make using nectar (~sucrose) from flowers. How is it made?

Bees eat the nectar made by flowers. The nectar is broken down into glucose and fructose in their guts. Back in the hive the bees regurgitate the monosaccaride mixture into honeycombs. Then the bees fan the honeycomb with their wings to evaporate much of the water. This thickens the mixture into honey. By decreasing the moisture content of the mixture the bees prevent bacteria and other microorganisms from spoiling it. We will discuss how in lecture 3.

Question 7 (exam) Which of the following structures is NOT a carbohydrate?

Answer: A (it has no carbonyl group and its formula is not C_nH_2nO_n)

For your information: A = glycerol, B = glyceraldehyde, C = fructose, D = ribose

Question 8 (exam) Which of these is the structure of aspartic acid (an acidic amino acid)?

Answer: C

For your information: A is valine, a non-polar amino acid, B is not an amino acid because it has an extra carboxyl group where there should be a hydrogen, and D is not an amino acid because it has an extra carboxyl group where there should be an amino group. I like this question because it asks students to be able to recognize what is an amino acid and to be able to recognize what category they belong in.

Question 9 (thinking) People, like other animals, use lipids for most energy storage. This is because lipids store six times as much energy per kg as carbohydrates do. Only a small, though significant, proportion of our body's energy is stored as glycogen. Hypothetically, if a person was to use glycogen exclusively they would have to be much larger in order to store the same amount of energy. How much heavier would the person be? Assume this individual is 60 kg and has 20% body fat*. (*For the purposes of this question disregard the fact that only some of our fat tissues are actual fat molecules.)

Our 'before' person has 12 kg of fat. To store the same amount of energy would require 72 kg of carbohydrates. Thus our person has lost 12 kg by dispensing with fat but has gained 72 kg by the addition of carbohydrates. Our 'after' person weighs 120 kg - twice what they weighed before. This is obviously a hypothetical situation but it does illustrate the benefit animals gain by using lipids rather than carbohydrates as their predominant energy supply.