MCAT-Prep GS-1 Test – Questions and
Answers
The (anabolic) pathway in Figure 1 occurs spontaneously:
(A) as written
(B) only if coupled with a sufficiently exergonic reaction or series of reactions
(C) only if coupled with a sufficiently endergonic reaction or series of reactions
(D) in conditions that cannot be determined with the information provided - ANSWER-
(B) The biosynthetic pathway is anabolic: subunits are used to build more complex
molecules. Building requires energy and thus without energy cannot proceed. Figure 1
must be coupled with reactions that release sufficient energy to drive the synthesis of
lanosterol. The release of free energy (Gibbs free energy) is signified by a negative
change in Gibbs free energy, termed an exergonic reaction. Of course, an endergonic
reaction requires free energy to occur (e.g., Figure 1) and has a positive Gibbs free
energy. Adding the ∆G values of the endergonic and exergonic reactions must give a
negative ∆G in order for the reaction to be spontaneous.
An important function of the pentose phosphate pathway is the generation of:
(A) NAD+, which is used for glycolysis.
(B) NADH for the production of ATP via the electron transport chain.
(C) NADP, which is a substrate in amino acid metabolism.
(D) NADPH for fatty acid synthesis. - ANSWER-(D) The pentose phosphate pathway
(PPP) generates NADPH, which is used for reductive biosynthesis reactions within cells
(i.e., fatty acid synthesis and the synthesis of other compounds such as cholesterol and
other steroids). PPP is responsible for almost two-thirds of cellular NADPH. PPP does
not produce NAD, NADP, or NADH.
Going Deeper: You should have a clear "big picture" for each of the biochemistry
pathways that are on the AAMC syllabus. Here is the big picture for the pentose
phosphate pathway:
PPP (aka the phosphogluconate pathway; aka the hexose monophosphate shunt)
functions in parallel with glycolysis and generates NADPH and pentoses (= 5-carbon
sugars, which include ribose-5-phosphate required for the synthesis of RNA and DNA).
There are two distinct phases: 1) the oxidative phase, in which NADPH is generated;
and 2) the non-oxidative synthesis of 5-carbon sugars. For most organisms, the pentose
phosphate pathway takes place in the cytosol. Note: Although PPP does begin with the
oxidation of glucose-6-phosphate, PPP's primary role is anabolic rather than catabolic.
The Ksp for calcium oxalate is 2.7 x 10^-9. The typical concentration of calcium in urine
is 1.5x the plasma concentration, which is 2.0 mmol/L. Determine the minimum oxalate
ion concentration in urine that would be required to form kidney stones.
,(A) 9.0 x 10^-7 M.
(B) 9.0 x 10^-4 M.
(C) 6.0 x 10^-7 M.
(D) 6.0 x 10^-4 M. - ANSWER-(A) The inference is that the precipitate (s = solid)
calcium oxalate forms the stone, i.e., Ca(OOCCOO)(s). The Ksp for calcium oxalate is
given in the passage, and its value can only refer to the reverse of the reaction (Ca++ +
-OOCCOO-(aq) <-> Ca(OOCCOO)(s)), i.e., the product of the ion concentrations. The
concentration of calcium ions in urine must be the concentration in blood multiplied by
1.5 (see Table 1) and then converted from mmol/L to mol/L.
Ksp for calcium oxalate = [calcium ion][oxalate ion]
2.7 × 10^-9 = [2.0 mmol/L x 1.5 x (1 mol/1000 mmol)][oxalate ion] = [3.0 x 10^-3][oxalate
ion]
Thus,
[oxalate ion] = (27 × 10^-10)/( 3.0 x 10^-3) = 9.0 x 10^-7 mol/L
Going Deeper: Technically, precipitation occurs when the product of [calcium ion]
[oxalate ion] is above the Ksp by any amount, which may be a minuscule amount that is
not obvious from the number of significant figures in the question. Given the large
differences between the answer choices, the answer provided is by far the best option.
Consider the following sequence of reactions:
1) HOOCCOOH <-> HOOCCOO- + H+
2) HOOCCOO- <-> -OOCCOO- + H+
3) Ca++ + -OOCCOO-(aq) <-> Ca(OOCCOO)(s)
When the body's intake of calcium is low, in order to maintain homeostasis, there is an
increased renal absorption of calcium. Based on the information provided, which of the
following changes in urine is most consistent with a decrease in calcium intake in a
person with kidney stones?
(A) Decreased oxalate ions
(B) Increased solubility product
(C) Decreased pH
(D) Increased pH - ANSWER-(D) We learn from the question stem that a decrease in
calcium intake is associated with increased renal absorption of calcium, which means
there will be less calcium in the urine. Now we just apply Le Chatelier's principle:
Beginning with reaction (3), if calcium is low (i.e., being removed), the reaction shifts to
the left, producing more oxalate ions; reaction (2) shifts to the left; and then reaction (1)
shifts to the left. The net result of the latter two reactions is the net removal of hydrogen
ions, which means that the pH increases.
Organic chelating agents have been used to bind calcium ions thereby dissolving
calcium oxalate kidney stones. What can be inferred regarding the interaction between
the chelating agent and the the calcium ions?
(A) The calcium ions bond ionically to the chelating agent.
(B) The coordination of calcium ions is the carbon and hydrogen.
(C) The binding is via coordinate covalent bonds.
, (D) The chelating agent must be a strong conjugate acid. - ANSWER-(C) On the
Surface: Chelation is the bonding of ions and molecules (= the ligand) to metal ions.
Chelation involves two or more separate coordinate covalent bonds between a
polydentate (poly = multiple bonded; dent = teeth) ligand and a single central atom (in
our case, calcium cations; the ligand bites the metal!). Usually these ligands are organic
compounds, and are called chelators (AKA: chelating agents, sequestering agents).
Note that the bonds are covalent, not ionic (answer choice A). Since calcium is
positively charged (cation), the attraction must be to a negative charge (either a formal
negative or partial negative charge as one typically sees for oxygen or nitrogen in
organic compounds; thus answer choice B is dubious). And since the charge should be
negative, a conjugate base (e.g. Cl-) would be more likely than a strong conjugate acid
(e.g. HCl; answer choice D is upside down).
At a given temperature, T in Kelvin, the relationship between the three thermodynamic
quantities including the change in Gibbs free energy (ΔG), the change in enthalpy (ΔH),
and the change in entropy (ΔS) can be expressed as follows:
ΔG = ΔH - TΔS
The sublimation of carbon dioxide occurs quickly at room temperature. What might be
predicted for the three thermodynamic quantities for the reverse reaction?
(A) Only ΔS would be positive.
(B) Only ΔS would be negative.
(C) Only ΔH would be negative.
(D) Only ΔG would be positive. - ANSWER-(D) ΔG = ΔH - TΔS
"The sublimation of carbon dioxide occurs quickly at room temperature" hopefully
reminds you that solid carbon dioxide ("dry ice") spontaneously (ΔG must be negative
by definition) converts to the gaseous state (fog machines in nightclubs, theaters, etc.)
because carbon dioxide cannot exist in liquid form at room temperature (reason: no H-
bonds, nonpolar molecule, only held together by very weak van der Waals
forces).Sublimation means:
Solid CO2 + heat -> vapor
Entropy (randomness) is clearly increasing (thus, positive ΔS) because we are moving
from a structured, ordered solid to randomly moving gas particles. Heat is required, so it
is endothermic, meaning ΔH is positive. The question is asking about the reverse
reaction, so all three signs are reversed: ΔG is now positive; ΔH is now negative; ΔS is
now negative.
Going Deeper: Notice that a negative ΔS multiplied by a -T (see the Gibbs free energy
equation) creates a positive term that overshadows the effect of the negative ΔH, and
thus, ΔG is still positive. Also, please keep in mind that sometimes the MCAT will
provide the Gibbs free energy equation, but sometimes it will not.
Doppler ultrasound is a noninvasive test that can be used to estimate blood flow
through vessels by bouncing high-frequency sound waves (ultrasound) off circulating
red blood cells (RBCs). A probe can be positioned such that blood in a vessel under
investigation may appear to be moving toward or away from the probe.
Answers
The (anabolic) pathway in Figure 1 occurs spontaneously:
(A) as written
(B) only if coupled with a sufficiently exergonic reaction or series of reactions
(C) only if coupled with a sufficiently endergonic reaction or series of reactions
(D) in conditions that cannot be determined with the information provided - ANSWER-
(B) The biosynthetic pathway is anabolic: subunits are used to build more complex
molecules. Building requires energy and thus without energy cannot proceed. Figure 1
must be coupled with reactions that release sufficient energy to drive the synthesis of
lanosterol. The release of free energy (Gibbs free energy) is signified by a negative
change in Gibbs free energy, termed an exergonic reaction. Of course, an endergonic
reaction requires free energy to occur (e.g., Figure 1) and has a positive Gibbs free
energy. Adding the ∆G values of the endergonic and exergonic reactions must give a
negative ∆G in order for the reaction to be spontaneous.
An important function of the pentose phosphate pathway is the generation of:
(A) NAD+, which is used for glycolysis.
(B) NADH for the production of ATP via the electron transport chain.
(C) NADP, which is a substrate in amino acid metabolism.
(D) NADPH for fatty acid synthesis. - ANSWER-(D) The pentose phosphate pathway
(PPP) generates NADPH, which is used for reductive biosynthesis reactions within cells
(i.e., fatty acid synthesis and the synthesis of other compounds such as cholesterol and
other steroids). PPP is responsible for almost two-thirds of cellular NADPH. PPP does
not produce NAD, NADP, or NADH.
Going Deeper: You should have a clear "big picture" for each of the biochemistry
pathways that are on the AAMC syllabus. Here is the big picture for the pentose
phosphate pathway:
PPP (aka the phosphogluconate pathway; aka the hexose monophosphate shunt)
functions in parallel with glycolysis and generates NADPH and pentoses (= 5-carbon
sugars, which include ribose-5-phosphate required for the synthesis of RNA and DNA).
There are two distinct phases: 1) the oxidative phase, in which NADPH is generated;
and 2) the non-oxidative synthesis of 5-carbon sugars. For most organisms, the pentose
phosphate pathway takes place in the cytosol. Note: Although PPP does begin with the
oxidation of glucose-6-phosphate, PPP's primary role is anabolic rather than catabolic.
The Ksp for calcium oxalate is 2.7 x 10^-9. The typical concentration of calcium in urine
is 1.5x the plasma concentration, which is 2.0 mmol/L. Determine the minimum oxalate
ion concentration in urine that would be required to form kidney stones.
,(A) 9.0 x 10^-7 M.
(B) 9.0 x 10^-4 M.
(C) 6.0 x 10^-7 M.
(D) 6.0 x 10^-4 M. - ANSWER-(A) The inference is that the precipitate (s = solid)
calcium oxalate forms the stone, i.e., Ca(OOCCOO)(s). The Ksp for calcium oxalate is
given in the passage, and its value can only refer to the reverse of the reaction (Ca++ +
-OOCCOO-(aq) <-> Ca(OOCCOO)(s)), i.e., the product of the ion concentrations. The
concentration of calcium ions in urine must be the concentration in blood multiplied by
1.5 (see Table 1) and then converted from mmol/L to mol/L.
Ksp for calcium oxalate = [calcium ion][oxalate ion]
2.7 × 10^-9 = [2.0 mmol/L x 1.5 x (1 mol/1000 mmol)][oxalate ion] = [3.0 x 10^-3][oxalate
ion]
Thus,
[oxalate ion] = (27 × 10^-10)/( 3.0 x 10^-3) = 9.0 x 10^-7 mol/L
Going Deeper: Technically, precipitation occurs when the product of [calcium ion]
[oxalate ion] is above the Ksp by any amount, which may be a minuscule amount that is
not obvious from the number of significant figures in the question. Given the large
differences between the answer choices, the answer provided is by far the best option.
Consider the following sequence of reactions:
1) HOOCCOOH <-> HOOCCOO- + H+
2) HOOCCOO- <-> -OOCCOO- + H+
3) Ca++ + -OOCCOO-(aq) <-> Ca(OOCCOO)(s)
When the body's intake of calcium is low, in order to maintain homeostasis, there is an
increased renal absorption of calcium. Based on the information provided, which of the
following changes in urine is most consistent with a decrease in calcium intake in a
person with kidney stones?
(A) Decreased oxalate ions
(B) Increased solubility product
(C) Decreased pH
(D) Increased pH - ANSWER-(D) We learn from the question stem that a decrease in
calcium intake is associated with increased renal absorption of calcium, which means
there will be less calcium in the urine. Now we just apply Le Chatelier's principle:
Beginning with reaction (3), if calcium is low (i.e., being removed), the reaction shifts to
the left, producing more oxalate ions; reaction (2) shifts to the left; and then reaction (1)
shifts to the left. The net result of the latter two reactions is the net removal of hydrogen
ions, which means that the pH increases.
Organic chelating agents have been used to bind calcium ions thereby dissolving
calcium oxalate kidney stones. What can be inferred regarding the interaction between
the chelating agent and the the calcium ions?
(A) The calcium ions bond ionically to the chelating agent.
(B) The coordination of calcium ions is the carbon and hydrogen.
(C) The binding is via coordinate covalent bonds.
, (D) The chelating agent must be a strong conjugate acid. - ANSWER-(C) On the
Surface: Chelation is the bonding of ions and molecules (= the ligand) to metal ions.
Chelation involves two or more separate coordinate covalent bonds between a
polydentate (poly = multiple bonded; dent = teeth) ligand and a single central atom (in
our case, calcium cations; the ligand bites the metal!). Usually these ligands are organic
compounds, and are called chelators (AKA: chelating agents, sequestering agents).
Note that the bonds are covalent, not ionic (answer choice A). Since calcium is
positively charged (cation), the attraction must be to a negative charge (either a formal
negative or partial negative charge as one typically sees for oxygen or nitrogen in
organic compounds; thus answer choice B is dubious). And since the charge should be
negative, a conjugate base (e.g. Cl-) would be more likely than a strong conjugate acid
(e.g. HCl; answer choice D is upside down).
At a given temperature, T in Kelvin, the relationship between the three thermodynamic
quantities including the change in Gibbs free energy (ΔG), the change in enthalpy (ΔH),
and the change in entropy (ΔS) can be expressed as follows:
ΔG = ΔH - TΔS
The sublimation of carbon dioxide occurs quickly at room temperature. What might be
predicted for the three thermodynamic quantities for the reverse reaction?
(A) Only ΔS would be positive.
(B) Only ΔS would be negative.
(C) Only ΔH would be negative.
(D) Only ΔG would be positive. - ANSWER-(D) ΔG = ΔH - TΔS
"The sublimation of carbon dioxide occurs quickly at room temperature" hopefully
reminds you that solid carbon dioxide ("dry ice") spontaneously (ΔG must be negative
by definition) converts to the gaseous state (fog machines in nightclubs, theaters, etc.)
because carbon dioxide cannot exist in liquid form at room temperature (reason: no H-
bonds, nonpolar molecule, only held together by very weak van der Waals
forces).Sublimation means:
Solid CO2 + heat -> vapor
Entropy (randomness) is clearly increasing (thus, positive ΔS) because we are moving
from a structured, ordered solid to randomly moving gas particles. Heat is required, so it
is endothermic, meaning ΔH is positive. The question is asking about the reverse
reaction, so all three signs are reversed: ΔG is now positive; ΔH is now negative; ΔS is
now negative.
Going Deeper: Notice that a negative ΔS multiplied by a -T (see the Gibbs free energy
equation) creates a positive term that overshadows the effect of the negative ΔH, and
thus, ΔG is still positive. Also, please keep in mind that sometimes the MCAT will
provide the Gibbs free energy equation, but sometimes it will not.
Doppler ultrasound is a noninvasive test that can be used to estimate blood flow
through vessels by bouncing high-frequency sound waves (ultrasound) off circulating
red blood cells (RBCs). A probe can be positioned such that blood in a vessel under
investigation may appear to be moving toward or away from the probe.
