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Microbiology - Problem Drill 03: The Chemical Basis of Life
Question No. 1 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems on paper as
needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
Question 1. At a high pH the amino acid lysine, shown below, will have:
H
H2N
C
COOH
(CH2)4
Question
NH2
A. A negative charge
B. A positive charge
C. No charge
D. Net two positive charges
A. Correct!
At high pH, the [H+] is low, so the amino groups will be uncharged and the deprotonated
carboxyl group will have a single negative charge.
B. Incorrect!
At high pH, the amino groups will remain uncharged while the carboxyl group has a negative
charge.
C. Incorrect!
At high pH the amino groups will remain uncharged while the carboxyl group has a negative
charge.
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D. Incorrect!
At high pH the amino groups will remain uncharged while the carboxyl group has a negative
charge.
To solve this type of problem, look at the functional groups one by one, assess their charges at
the given pH, and add them together.
Question
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Question No. 2 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems on paper as
needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
Question 2. Given the following reaction, which of the following is not true?
HCl + H20 Æ H30+ + Cl-, ∆G= -34.2 kJ/mol
Question
A.
B.
C.
D.
The exergonic reaction will be spontaneous.
The reverse reaction is not spontaneous.
At equilibrium, the pH will be less than 7.0.
Addition of a catalyst will shift the reaction equilibrium backwards.
A. Incorrect!
Exergonic reactions are spontaneous (∆G<0) so this statement is true.
B. Incorrect!
∆G for the reverse reaction is >0, so this statement is true.
C. Incorrect!
HCl is a strong acid that would form an acidic solution in H2O, so this statement is true.
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D. Correct!
Catalysts only change Ea for a given reaction; the concentrations of reactants or products at
equilibrium are unaffected, so this statement is false.
For the reaction shown you should consider how ∆G relates to reaction spontaneity, your
knowledge of common strong acids and bases, and the role of catalysts in chemical reactions.
Question
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Question No. 3 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems on paper as
needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
Question 3. Given a molecule with the following functional groups, what is most likely to be true
for this compound in a living cell?
R-OH
Question
A.
B.
C.
D.
RO-PO32-
R-NH2
R-O-R’
It will be embedded within a membrane.
Its polymer may be localized in the nucleus.
Its unmodified polymer will be a polysaccharide.
It will not be susceptible to an oxidoreductase.
A. Incorrect!
There are several hydrophilic functional groups in this molecule making it unlikely that it would
be found embedded in a hydrophobic membrane.
B. Correct!
This compound may be a nucleotide, so its polymer could reasonably be found in the nucleus (it
might also be found in the mitochondrion or chloroplast).
C. Incorrect!
Unless modified, this compound cannot be a monosaccharide. They contain only C, H, and O.
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D. Incorrect!
Several of the functional groups listed here can participate in oxidation-reduction reactions.
Refer to your knowledge of functional groups, which macromolecules could possibly contain
those functional groups, and review what you know about enzymatic reactions. Consider also
the hydrophobic and hydrophilic microenvironments that exist inside cells.
Question
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Question No. 4 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems on paper as
needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
Question 4. Proteins are composed of which of these monomers?
A. glucose
B. amino acids
C. fatty acids
D. nucleotides
Question
A. Incorrect!
Glucose is a carbohydrate monomer.
B. Correct!
Amino acids are the building blocks of proteins.
C. Incorrect!
Fatty acids are a component of lipids and fats, not proteins.
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D. Incorrect!
Nucleotide polymers for nucleic acids.
You should recall that all macromolecules are made from smaller building blocks, or monomers.
Proteins and peptides are made from amino acids.
Glucose and fatty acids often modify proteins in the cell but are not part of the primary
composition of a protein.
Question
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Question No. 5 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems on paper as
needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
Question 5. Which of the following is true?
A. Hydrofluoric acid, HF, is capable of hydrogen bonding.
B. Enzymes can reduce the ∆G of a reaction, consequently accelerating it.
C. NaOH would make an appropriate buffer for biochemical reactions.
D. The isotopes of Hydrogen have different atomic numbers.
Question
A. Correct!
A Hydrogen atom bound to an electronegative atom like Fluorine will be a polar covalent bond
capable of hydrogen bonding.
B. Incorrect!
Enzymes bind to reactants and bring down the activation energy, but they don’t change the
overall ∆G of a reaction.
C. Incorrect!
NaOH is a strong base that completely dissociates in aqueous solution, so it is a poor choice for
a buffer.
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D. Incorrect!
Isotopes have the same number of protons, but different numbers of neutrons, and it is the
number of protons that determines the atomic number.
To solve this question, use the process of elimination.
A. Remember that molecules containing H atoms bound to such atoms as N, F, or O are good
candidates for hydrogen bonding.
B. The concept outlined in answer choice B. is quite important to remember for the MCAT.
Enzymes are catalysts, and just like all other catalysts, they do not change ∆G.
C. The most important feature of buffers is that they are made from weak acids and bases. It is
a good idea to become familiar with some common acids and bases. Ex. Strong acids/bases
would include NaOH or HCl, while weak acids/bases would include H2CO3 (carbonic acid) or
CH3COOH (acetic acid).
D. Atomic number is assigned by the number of protons in the nucleus.
Question
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Question No. 6 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems on paper as
needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
Question 6. Which of the following covalent bonds would require the most energy to break?
A. C-C
B. C=C
C. CΞC
D. C-H
Question
A. Incorrect!
Single carbon-carbon bonds involve a single pair of electrons and store less energy than multiple
bonds between carbon atoms.
B. Incorrect!
The C=C bond involves two pairs of electrons and stores more energy than C-C, but it is a lower
energy bond than a triple bond.
C. Correct!
The CΞC bond is the highest energy bond shown above. It involves three shared pairs of
electrons and stores the most energy, so it would take the most energy to break it apart.
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D. Incorrect!
The C-H bond is a single bond and stores less energy than the C-C bond.
Covalent bonds form when two atoms share their outer shell, or valence, electrons. The energy
is stored in these bonds. The more electron pairs they share, the harder it is to break the bond.
Question
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Question No. 7 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems on paper as
needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
Question 7. What type of bonding occurs between two water molecules?
A.
B.
C.
D.
A covalent bond
An ionic bond
A hydrogen bond
A polar bond
Question
A. Incorrect!
Covalent bonds are strong bonds that involve the sharing of electrons.
B. Incorrect!
Ionic bonds involve the transfer of electrons.
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C. Correct!
The unequal sharing of electrons between the O and H atoms creates dipoles. The overall
molecule is considered to be polar because of its net dipole, and the partial positive ends of one
molecule (at the H atoms) will weakly attract the partial negative end of another molecule (at the
O atom). This weak attraction is termed a hydrogen bond.
D. Incorrect!
There is no such thing as a polar bond. The term polar bond or non-polar bond actually refers to
covalent bonds. A polar covalent bond involves the unequal sharing of electrons that arises when
atoms with unequal electronegativities are involved. A truly polar bond would actually be an ionic
bond.
The unequal sharing of electrons between the O and H atoms creates dipoles within the
molecule. The net dipole creates regions of partial charge in the water molecule that can lead to
intermolecular attraction. Remember, however, that having polar molecules around isn’t enough
to create hydrogen bonds. They must also come into close proximity with each other.
In general, expect to see hydrogen bonds form between molecules that have hydrogen and any
of the following: O, N, or F. Although they are weak, hydrogen bonds have an additive strength
and are important in certain structures, such as forming the three-dimensional structure of a
protein enzyme.
Question
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Question No. 8 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems on paper as
needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
Question 8. How many more [H+] are in a solution of pH 5 than a solution of pH 7?
A.
B.
C.
D.
50
100
200
1000
Question
A. Incorrect!
Hint: pH = -log [H⁺]
B. Correct!
Use pH = -log [H⁺]
C. Incorrect!
Hint: pH = -log [H⁺]
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D. Incorrect!
Hint: pH = -log [H⁺]
pH is measured on a logarithmic scale, meaning that a single unit increase in pH means a 10fold increase in [H+].
1) At pH 5, [H+]=10-5
2) At pH 7, [H+]=10-7
3) To calculate the fold difference, take 10-5/10-7= 100
Remember that pH = -log [H+].
Question
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Question No. 9 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems on paper as
needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
Question 9. What is true of an aqueous acetic acid solution?
H3COOH + H20 H30+ + H3COO–
Question
A.
B.
C.
D.
A small addition of HCl would shift the equilibrium to the right.
The reaction requires an enzyme.
At equilibrium, the pH will be strongly acidic.
A small addition of KOH would shift the equilibrium to the right.
A. Incorrect!
A small addition of the acid HCl would shift the equilibrium to the left.
B. Incorrect!
Electrolytes release ions in water without the need for enzymes.
C. Incorrect!
Acetic acid is a weak acid in aqueous solution.
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D. Correct!
A small addition of the base KOH will shift the equilibrium to the right.
HA ↔ H+ + A–
More H+
HA
H+ + A-
More OH–
HA
H+ + A-
If we consider the above equation, there are two possibilities:
Question
1) Adding small amounts of an acid such as HCl shifts the equilibrium left, forming more HA to
keep the [H+] constant.
2) Adding small amounts of a base such as NaOH or KOH shifts the equilibrium to the right, to
keep the [H+] constant.
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Question No. 10 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems on paper as
needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
Question 10. Which of the following will not favor the formation of a chemical bond?
Question
A.
B.
C.
D.
The ∆G for the reaction is less than zero.
A less stable electron arrangement.
The valence shell will be filled.
The atoms or molecules are in close proximity.
A. Incorrect!
If the change in free energy for formation of a particular bond is less than zero it will be
spontaneous (energetically favorable).
B. Correct!
Chemical bonds are formed only when the electron arrangement will become more favorable
(e.g., filling the valence shell).
C. Incorrect!
Creating a filled an outer (valence) shell with electrons will favor chemical bonding because it
promotes a more stable electron arrangement.
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D. Incorrect!
In order to create products, the reactants must be in close proximity and in a favorable
orientation.
Remember that this question asks which factor does NOT favor chemical bonding. Electron
number is an important concept in determining whether bonds are formed. Any bond that will
produce a more stable electron arrangement is favorable. A more stable electron arrangement
would occur if the outer valence shell were completely filled.
Question
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