Princeton MCAT Exam #2
Questions and Answers (Graded
A)
What single change to the experimental system would reduce the electric field
strength necessary to maintain the oil drop in static equilibrium? - ANSWER -
Decreasing the mass of the drop
-When the oil drop (with charge -q) is in static eqbm, the electric force it feels, qE
upward, is balanced by the gravitational force, mg downward. Thus, qE = mg, so E
= mg / q. Don't be confused by the electric field strength in a capacitor, E = V / d.
Which of the following equations gives the voltage V necessary to maintain an oil
droplet of mass m and charge -q suspended in the region between Plates A and B,
which are separated by a distance d? - ANSWER - V = mgd / q
-The voltage V & the electric field strength E, between Plates A and B are related by
the equation V = Ed, where d is the distance between the plates. The oil drop is
suspended in the region between the plates when qE = mg or in terms of V, when
q(V / d) = mg. Solving this last equation for V gives V = mgd / q.
Which one of the following graphs best illustrates the relationship between a, an oil
drop's acceleration perpendicular to the plates, and E, the strength of the electric
field between the plates? - ANSWER - The net force on an oil drop (charge -q)
between Plates A and B is the difference between the electric force and the
gravitational force: Fnet = qE - mg. Since Fnet = ma, the acceleration of the oil drop
is given by the equation a = (q/m)E - g. This is an equation of the form "y = mx +
b," whose graph is a straight linear line with positive slope rising upwards
What best explains the variation among the values of the floating voltage for the
different droplets? - ANSWER - The charge of the droplets differs because the
amount of ionization due to the x-rays is random
,-there is no way to control the exact number of excess electrons in the ionized air
that will cling to a falling oil droplet, but the net charge must be a whole number
multiple of the elementary charge.
What impact do the results shown in Figure 1 have on the hypothesis that leptin
resistance secondary to obesity reduces all effects of leptin? - ANSWER - Weakens
the hypothesis due to the increase in neural activity with injection of leptin in obese
mice
-given the dramatic increase in nerve activity with increasing leptin dosage, leptin
does have an impact on RSNA. Thus leptin resistance secondary to obesity does
NOT reduce ALL effects of leptin weakening q-stem hypothesis
A researcher proposed to block the leptin receptor in order to treat hypertension in
obese patients. Which of the following detection methods would work best to
identify the receptor in the different tissues affected by leptin? - ANSWER - western
blot
-The leptin receptor is a protein and located in the cell membrane of different tissue
types. Detection of a protein can be done using a Western blot
Ghrelin, the "hunger hormone," is released when the stomach is empty. It increases
hunger and promotes secretion and motility in the digestive tract. The receptor for
ghrelin is found on the same cells in the brain as the receptor for leptin. Which of
the following is a true statement? - ANSWER - There is no interaction between leptin
and ghrelin because they operate on separate receptors
-they bind to the SAME cells in the brain, the fact that leptin and ghrelin bind to
separate receptors means that there cannot be any kind of inhibition or interaction
between them. They have seperate receptors, prevents any competitive inhibition.
Brown fat will increase energy expenditure in the form of heat better than any other
adipose tissue in the body. In brown fat, leptin indirectly activates uncoupling
protein 1 (UCP-1) located in the inner mitochondrial membrane. UCP-1 activation
will dissipate the proton gradient across the inner mitochondrial membrane and
STOP which of the following reactions in mitochondria? - ANSWER - Oxidative
phosphorylation
,-absence of the proton gradient across the inner mitochondrial membrane no
electrochemical gradient has been built and the formation of ATP by ATP synthase,
will not proceed. The ETC will continue and pass electrons along the transport chain,
but the pumped protons will return to the matrix of the mitochondria with the help
of UCP-1 activation
The structure of (+)-fructose is shown below:
How many chiral centers does (+)-fructose contain? - ANSWER - 3 chiral centers
-Numbering from the TOP carbon down, C-3, C-4, and C-5 are chiral centers because
they each have four different substituents
Creatine phosphate is a high-energy buffer that helps maintain the level of available
high-energy phosphates such as ATP. During intense muscular exertion, creatine
phosphate replenishes the muscle's ATP by releasing a relatively larger amount of
energy in transferring its phosphate group to ADP. ATP is able to accept this
phosphate group from creatinine phosphate and serve as a convenient carrier of
energy because: - ANSWER - it has phosphate groups with an intermediate transfer
potential
-ATP is good energy carrier bc it has an intermediate transfer potential. This allows
higher energy phosphate carriers (creatine phosphate) to drive the synthesis of ATP
by transferring their P-groups to ADP
The sound level of a sound wave that is striking an eardrum is increased from 35 dB
to 55 dB. By what factor has the intensity of the sound increased? - ANSWER - 100
-each increase by 10 in the decibel level, the intensity of the sound increases by a
factor of 10. the decibel level has increased by 2 units of 10 (since 55 - 35 = 20 =
10 + 10), so the intensity increases by two factors of 10, 10^2 = 100.
The ratio of products to reactants at equilibrium for Reaction 1 will change with a
change in the: - ANSWER - temperature
, ATP - ANSWER - a particularly good energy carrier bc it has an intermediate transfer
potential which allows higher energy phosphate carriers (like creatine phosphate) to
drive the synthesis of ATP by transferring their phosphate groups to ADP
has a varied role in the body, as an energy carrier and also as a nucleotide for RNA
and DNA
is present in every cell in the body
Rank the following bonds in order of increasing polarity:
C—Cl
Cl—Cl
Si—Cl
P—Cl - ANSWER - Cl-Cl < C-Cl < P-Cl < Si-Cl
-polarity of a bond depends on the difference in EN between the 2 bonding atoms:
The larger the difference, the more unequal the sharing of electrons, the more polar.
the Cl—Cl bond is completely nonpolar. Carbon is more EN than Si, therefore Cl,
which is the most EN element depicted, will show the greatest EN difference with Si.
Therefore the Si—Cl bond is most polar
-only way to change a reaction's equilibrium constant is to change the temperature.
in the forward direction, Reaction 1 may be considered:
ATP + glutamate + NH3 => ADP + phosphate + glutamine - ANSWER - favorable,
because the negative ΔG° for Reaction 2 more than compensates for the positive
ΔG° for Reaction 3.
-ΔG°2 + ΔG°3 < 0, so the overall reaction would be favorable and still be negative
when added together
G2= -3I
G3= 14
Which of the following is true for Reaction 2, once it is allowed sufficient time to
achieve equilibrium?
ATP + H2O => ADP + phosphate ΔG° = -30.5 kJ/mol - ANSWER - ΔG > ΔG°
