Key to Review Worksheet for Exam 1
Chem 103, Winter 2006
1. Circle and label all of the functional groups (besides alkanes) in hirsutic acid (an
antibiotic metabolite from a fungus):
O
carboxylic
acid
HO
CH2
alkene
H H 3C
C
OH
alcohol (secondary)
H 3C
cyclic ether
O
H
2. Circle all of the chiral carbons in hirsutic acid:
O
HO
C
CH2
H H 3C
OH
H 3C
H
O
3. Glucose has the molecular formula C6H12O6.
a) What is the molar mass of glucose (mass of one mole of glucose in grams/mol)?
180.16 g/mol
b) How many glucose molecules are in 0.100 g of glucose?
0.100 g x (1 mol/180.16 g) x (6.02 x 1023 molecules/1 mol) = 3.34 x 1020 molecules
c) How many moles of glucose are in 10.0 L of a 0.100 M glucose solution?
10.0 L x (1 L/106 L) x (0.100 mol/1 L) = 1.00 x 10-6 mol
d) How many grams of glucose are in 5.00 L of a 5.00% glucose solution?
5.00 L x (1 mL/103 L) x (5.00 g/100 mL) = 2.50 x 10-4 g
4. Give an example (draw the Fischer projection and write the name) of each of the
following classes of monosaccharides:
a) a D aldohexose
H
H
b) an L aldotriose
O
OH
HO
H
HO
H
H
H
OH
HO
CH2 OH
D-Galactose
O
H
CH2 OH
L-Glyceraldehyde
c) an L ketohexose
d) a D ketotetrose
CH2 OH
O
HO
H
HO
H
CH2 OH
OH
H
CH2 OH
L-Fructose
O
H
OH
CH2 OH
D-Erythrulose
5. Which saccharide is the main energy source for cells of the central nervous system?
D-Glucose
6. Galactose is an important component of the cell membranes of the nervous system.
a) What is our main dietary source of galactose (what type of food)?
Milk products.
b) Do we obtain the galactose directly from the food? If not, how do we get it?
No, we obtain lactose from the milk products. The lactose is then hydrolyzed
with enzymes to produce D-galactose and D-glucose.
c) How else might we obtain galactose besides through food?
When needed, we can obtain galactose by enzymatic conversion of glucose.
6. Give the structure and the name of the product (or products) for each of the
following reactions:
HO
CH2 OH
O
HO
H
a)
H
OH
H
OH
H
Benedict's
reagent
OH
HO
H
OH
H
OH
CH2 OH
CHO
CH2 OH
OH
H
OH
HO
H
H2
HO
H
HO
H
Pt
HO
H
b)
H
D-gluconic acid
H
CH2 OH
H
O
D-galacitol
OH
H
CH2 OH
OH
CH2 OH
CH2 OH
O
HO
H
CH3OH
OH
Acid Cat.
c)
H
H
CH2 OH
CH2 OH
O
H
OH
OH
H
H
methyl alphaD-fructoside
OCH3
OH
CH2 OH
CH2 OH
O
H
H
OH
d)
Sucrose
Invertase
H
alpha-D-glucose
H
OH
OH
H
+
OH
CH2 OH
beta-D-fructose
OH
O
H
OH
OH
H
H
e)
Fructose
Yeast
2
OH
ethanol
+
2CO 2
carbon dioxide
CH2 OH
7. D-Glucose in solution is mainly found in the pyranose form. However, a small
amount is also present in the furanose form. Draw the Haworth structure for the
furanose form of -D-glucose.
H
HOH 2C
OH
O
HO
OH
H
H
H
OH
H
8. Many adults experience an intolerance to lactose.
a) Draw the Haworth structure for -lactose.
CH2 OH
H
OH
CH2 OH
O
OH
H
OH
O
H
beta-1,4-glycosidic bond
H
H
OH
O
H
OH
H
H
H
H
OH
b) What type of glycosidic bond does -lactose have? Label it on your structure.
Beta-1,4-glycosidic bond
c) Is lactose a reducing sugar? Explain your answer.
Yes, lactose is a reducing sugar. Because the glucose is linked to the galactose by
a 1,4 linkage, the glucose is still a hemiacetal, and can undergo mutorotation. During
mutorotation, the glucose spends some time in the open-chain form. In this form, the
aldehyde can be oxidized by reducing agents such as Tollen’s or Benedict’s.
d) What are the two methods for hydrolyzing lactose (give the specific reagents needed
for each)?
1. Aqueous acid (such as aqueous HCl) and heat.
2. Enzymes, such as lactase, at body temperature.
e) Write the products (Haworth structures and names) of the hydrolysis products of
lactose.
CH2 OH
CH2 OH
O
OH
H
OH
OH
H
H
H
H
OH
D-galactose
O
H
H
OH
H
H
OH
H
OH
OH
D-glucose
f) Pure -lactose has a specific rotation of +92.6 at 20C. However, when it is
dissolved in water, the optical rotation decreases over time until it reaches +52.3.
Explain why this occurs.
When the lactose is in solution, the glucose hemiacetal ring can undergo
mutorotation, giving both the alpha and beta anomers. An equilibrium between the
two anomers is established over time. So, as the concentration of the alpha anomer goes
down, and the concentration of the beta anomer goes up, the specific rotation becomes
less positive. When equilbrium is reached, the rotation levels out at +52.3.
10.
Amylose, amylopectin, glycogen and cellulose are all polymers of what
monosaccharide?
D-glucose
11. What is the main difference in the chemical structures of amylose and cellulose?
Amylose has -1,4-glycosidic bonds and cellulose has -1,4-glycosidic bonds.
Because of this, amylose forms coiled strands, while cellulose does not. Also, our
enzymes only recognize the -1,4-glycosidic bonds.
12. The iodine test is used to detect the presence of starch. Explain why iodine
complexes strongly with starch, but not with glycogen or cellulose.
Starch contains amylose, which forms coiled strands. Rows of iodine atoms stack
in the core of the -helix of amylose, forming strong complex. Because glycogen and
cellulose do not form -helices, they do not complex well with iodine.
13. What characterizes a biomolecule as being a lipid?
Lipids are soluble in nonpolar solvents, and not soluble (or very sparingly
soluble) in water.
14. Why are unsaturated fatty acids liquid at room temperature, while saturated fatty
acids are solid at room temperature?
Unsaturated fatty acids have cis double bonds that make it hard for them to stack
well together, while saturated fatty acids can stack closely together. So, the unsaturated
fatty acids experience weaker intermolecular forces (dispersion forces) than the
saturated fatty acids, making them liquid at room temperature. The saturated fatty
acids, which experience stronger dispersion forces, are solid at room temperature.
15. Write the structure for a cis monounsaturated fatty acid with 12 carbons.
O
HO
16. What is the main function of triacylglycerols in animals?
Energy storage.
17. What is the difference between a fat and an oil?
In general, fats are solid at room temperature and usually come from animals,
while oils are liquid at room temperature and usually come from plants.
18. Give the products (structures) for each of the following reactions:
O
H 2C
O
C
O
O
(CH2)7 C
CH(CH2) 7CH3
a)
CH
H 2C
O
O
C
O
C
(CH2)7 C
CH(CH2) 7CH3
(CH2)7 C
CH(CH2) 7CH3
(CH2)7 C
CH(CH2) 7CH3
H 2C
H2
O
CH
Ni
O
H 2C
O
C
O
(CH2)16CH3
C
O
C
(CH2)16CH3
(CH2)16CH3
O
H 2C
H 2C
O
C
O
3KOH
b)
CH
H 2C
O
O
C
O
C
(CH2)7 C
OH
O
CH
OH
+
CH(CH2) 7CH3
(CH2)7 C
CH(CH2) 7CH3
(CH2)7 C
CH(CH2) 7CH3
H 2C
OH
H 2C
OH
3 K O
C
(CH2)7 C CH(CH2) 7CH3
O
H 2C
O
C
O
lipase
c)
CH
O
C
O
(CH2)7 C
O
CH
CH(CH2) 7CH3
+
H 2C
H 2C
O
C
(CH2)7 C
OH
O
C
O
H 2C
O
O
C
O
C
(CH2)7 C
+
3 HOOC(CH2)7 CH3
CH(CH2) 7CH3
O
(CH2)7 C
H 2C
CH(CH2) 7CH3
O
O2
d)
CH
C
CH(CH2) 7CH3
O
H 2C
3 HO
OH
(CH2)7 C
(CH2)7 C
CH(CH2) 7CH3
CH(CH2) 7CH3
CH
H 2C
O
O
C
O
C
O
C
(CH2)7 COOH
(CH2)7 COOH
(CH2)7 COOH
19. Why do oils and fats turn rancid over time if left out at room temperature in an
open container? Explain in terms of chemical reactions that take place.
Fats and oils can become rancid due to bacterial hydrolysis of the ester groups or
air oxidation of the double bonds in unsaturated fatty acids. The products of these
reactions include short-chain carboxylic acids, which taste and smell horrible.
20. What are the main structural differences between glycerophospholipids and
sphingolipids? Draw a schematic of the structure for each.
Glycerophospholipids have a glycerin backbone, while sphingolipids have a
sphingosine backbone. In addition, the sphingolipids have only one fatty acid, while
the glycerophospholipids have two.
Fatty acid
Sphingosine
Glycerol
Fatty acid
PO4
Amino
alcohol
Fatty Acid
PO4
Amino
alcohol
21. Where are cerebrosides and gangliosides mainly found? What are their main
functions?
They are both glycosphingolipids, and are found in cell membranes. They are
involved in cell recognition and cell signaling. The saccharide portions are on the
outside of the cell, and can act as receptors or can be recognized by immune cells.
22. Draw the chemical structure of the steroid nucleus.
C
A
D
B
23. Can your body synthesize cholesterol? If so, where is it made?
Yes, your liver can synthesize cholesterol.
24. Name three important biological functions of cholesterol:
Cholesterol has a number of important functions, including: as part of cell
membranes and myelin and as a precursor to biosynthesis of steroid hormones, bile
salts and vitamin D.
25. You will often hear people speak about “good cholesterol” and “bad cholesterol”.
Are these terms accurate? Explain.
No, there is no such thing as “good cholesterol” and “bad cholesterol”. What
they really mean is “good lypoprotein” or “bad lypoprotein”. What’s referred to as
“good cholesterol” is high-density lipoprotein (HDL), which transports excess
cholesterol from the tissues to the liver for biosynthesis. What’s referred to as “bad
cholesterol” is low-density lipoprotein (LDL), which transports cholesterol from the
liver to the tissues. It’s considered bad because when there is excess cholesterol in the
tissues, the LDL will deposit the cholesterol on the arterial walls.
26. From what lipid are bile salts biosynthesized? What is the main function of bile
salts?
Bile salts are biosynthesized from cholesterol in the liver. They are used to aid
digestion of fats and oils. The bile salts are stored in the gall bladder and released into
the small intestine where they help to dissolve fats and oils so they can be digested.
Since they have both a polar and a nonpolar region, they work like soap.
27. What is a steroid hormone?
Steroid hormones are biosynthesized from cholesterol in the endocrine glands
and act as chemical messengers between different parts of the body.
28. What are the roles of aldosterone and cortisone (two corticosteroids produced in the
adrenal glands)?
Aldosterone acts in the kidneys to control electrolyte balance and water
retention. Cortisone raises the blood glucose level by limiting glucose metabolism to
the nervous system and by stimulating glycogen synthesis.
29. Why are cell membranes made up mainly of phospholipids?
Phospholipids contain both a polar head group and nonpolar tails. This
structure allows them to form bilayers with the nonpolar tails pointing towards each
other and the polar head groups facing both inside and outside the cell. The
phospholipid bilayer provides a semipermeable membrane.
30. Does adding cholesterol make cell membranes more rigid or less rigid? Explain.
Adding cholesterol makes cell membranes more rigid. Cholesterol also has a
polar group and a nonpolar region. It is relatively flat, and so there are relatively strong
dispersion forces between cholesterol molecules. They also interact reasonably well
with the nonpolar tails of the phospholipids. The increased intermolecular forces add
both strength and rigidity.
31. Explain the difference between active transport and facilitated transport.
Active transport requires energy from ATP and moves substances across the
membrane against concentration gradients. Facilitated transport requires no energy
from ATP and moves substances across the membrane with concentration gradients.