NEXTSTEP MCAT DIAGNOSTIC, CHEM/PHYS

At which nucleophilic site is serine most likely to be acylated by penicillin?

Good nucleophiles are electron-rich species. The likely nucleophile would also need to minimize steric hindrance to be acylated by penicillin. The side chain hydroxyl group fits both of these criteria. nucleophile (Lewis base) electrophile (Lewis acid)

Which of the following is most likely to be the precursor of the group found linked to the N-terminus of a Wnt protein?

correct CH3(CH2)10COOH the passage states, "Wnts are unusual as secreted proteins in that they have a fatty acid chain covalently attached to their N-terminus, which increases their binding to cell surfaces" find the FA, FA are long carbon chains with a CA in the end

Tolazamide is an aromatic drug with a similar sulfonylurea structure to glipizide. Which of the following is most likely tolazamide?

correct tolazamide is aromatic. Next, the passage states that glipizide is a sulfonylurea antidiabetic drug. As its name implies, a sulfonylurea compound contains both sulfonyl and urea groups. know the structure for urea and sulfonyl

The two steps of the Vitamin D synthesis are summarized below. Step 1: 7-dehydrocholesterol + UV light ⇒ previtamin D3 ΔG = -134 kJ/mol Step 2: previtamin D3 ⇒ cholecalciferol (Vitamin D) ΔG = +58 kJ/mol What is the ΔG for Vitamin D synthesis?

correct -76 kJ/mol

On the penicillin heteroatom ring, what is the absolute configuration of the carbon labeled 1?

correct 1) Identify the four substituents bonded to the carbon in question. 2) Assign priorities to those substituents (atomic number ~ priority). 3) Count 1-2-3 and record what direction you move in. Counterclockwise = S, clockwise = R (remember the mnemonic "clockwise is the Right way for a clock to run"). 4) Determine which direction the lowest-priority (#4) group is facing. If #4 faces away from you, keep your answer. If #4 is coming out at you, invert your designation. nitrogen is DIRECTLY attached to carbon 1. although C1 is also attached to a CA, it is directly attached to a carbon. Nitrogen is more electronegative and has a higher priority

How many grams of compound X (MM = 292.81 g/mol) are required to create a 1.5 M solution of the mixture used to test HCN current as shown in Figure 3?

correct 2g The passage states that 5 mL solutions were used to test HCN currents. 5 mL is equivalent to 0.005 L. The question asks for a 1.5 M solution of compound X, meaning 1.5 mol/L. We are given the molar mass of 292.81 g/mol. Using dimensional analysis and simplifying the numbers, we can calculate the required mass as follows: (1.5 mol/L)(0.005 L)(292.81 g/mol) = (1.5 mol/L)(5 x 10-3 L)(3 x 102 g/mol) = 22.5 x 10-1 g = about 2 g (Note that we rounded 292.81 to 300.)

Based on the observed effects, what is the minimum dose of Compound X required to reduce the HCN current to half the normal current?

correct 5 x 10-2 μmol Passage states, "Solutions of 5 mL each were prepared with differing amounts of Compound X. " A closer look at Figure 3 shows that the y-axis represents the inhibition of Ih as a proportion of Ihmax. The question is asking us to find the X dosage that would cut the Ih in half. On Figure 3 we can see that ΔIh/Imax = 0.5 at approximately 10-5 M on the x-axis. The passage tells us that solutions of 5 mL were prepared. To find the number of moles, we simply multiply volume by concentration: 10-5 M x 5 x 10-3 L = 5 x 10-8 mol 5 x 10-8 mol x (1 μmol / 10-6 mol) = 5 x 10-2 μmol

In the figure below, the black line represents an enzyme-catalyzed reaction and the grey line represents that same reaction after adding 0.05 mM of factor X. Factor X can best be described as:

correct A competitive inhibitor will increase the Km of a given reaction while leaving the Vmax unchanged at high substrate concentrations.

Modern MRI machines use electricity to generate their magnetic fields inside a circular chamber instead of using permanent magnets. Which of the following would NOT increase the strength of the MRI field?

correct Increased radius of the MRI chamber power = IV. Field strength is directly proportional to current and inversely proportional to radius. Increasing the radius of the chamber will lower the strength of the field at the center of the chamber.

Compound X and lidocaine can be separated using ether extraction because:

correct Lidocaine is lipophilic while compound X is hydrophilic "Lipophilic" refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and nonpolar solvents such as hexane. The axiom that "like dissolves like" generally holds true. Thus, lipophilic substances tend to dissolve in other lipophilic substances, while hydrophilic (H2O-loving) substances tend to dissolve in water and other hydrophilic solvents. Lidocaine has a large, nonpolar aromatic ring, alkyl groups, and a few polar groups (the carbonyl, 2 amino groups), so it will be lipophilic (i.e. hydrophobic) overall. Compound X has a charged group, along with several polar amino groups. This makes compound X hydrophilic. Charged species are virtually always more polar than similar uncharged molecules. i got this question right because I saw compound X and saw that it had a (+) charge, indicating that it is hydrophilic. I should review to understand the terms lipophilic

In vivo, PO43- molecules utilize proton-coupled transport mechanisms to enter the cell. Which statement explains why this is necessary?

correct PO43- is hydrophilic. The structure of phosphate shows it to be negatively charged and polar, which makes it hydrophilic. Thus, it cannot cross the hydrophobic interior of the membrane.

Which of the following must be true if the penicillin-serine reaction "locks" in the ester formation as described in the passage?

correct The thermodynamic cost of breaking the penicillin-serine ester is larger than the thermodynamic release of amide formation. the passage states, "During the acylation of serine, the amide on penicillin is converted to an ester. This causes binding and locking of the molecules, which prevents serine from activating the cell wall-synthesizing enzymes. " The passage tells us that once penicillin forms an ester with serine, it prevents the further reactions by the enzyme because the serine residue is now locked into the ester with penicillin. For this to be the case, the ester must be stable. It must be so stable that breaking it apart would be too "expensive" - or too hard to do given the subsequent thermodynamic release that would occur from amide formation. it would be easier to form the serine ester than keeping the amide form. so breaking up the ester compound would cost more energy for it to be done.

What is the mechanical work done by the initial load placed on a fiber if the fiber is linearly deformed by 3 nm?

correct W=Fdcos(theta) Mechanical work can be calculated using work = Fdcos θ, where F is the force applied, d is the distance over which the force is applied, and θ is the angle between the force applied and the direction of the object's movement. Here, we have linear deformation, so θ = 0º. Paragraph 5 states the initial load as 10 N, and the question gives the distance as 3 nm. Work = 10 N (3 x 10-9 m)(cos 0) = 10 (3 x 10-9)(1) = 3 x 10-8 J

32P has a very short half-life of 14 days and decays into sulfur as shown below. This nuclear reaction is an example of:

correct beta minus decay. good but go back to physics chapter and memorize this

Which functional group is NOT found on penicillin?

correct imine aminde (nh2-c=o) imine C=N carboxylic ac. (cooh) thioether (-s-)

If all other factors are held constant, which of the following scaffolds would have the highest productivity?

correct look at passage equations Equation 2 shows that productivity is directly proportional to efficiency and inversely proportional to area and spin time. Combining the equations, we see: E=Mm/Me productivity= E/At(spin) Here, we can see that productivity is inversely proportional to Me. To obtain the largest productivity, we want the lowest Me.

A student preparing for the experiments inadvertently adds an additional 400 mL of the same acid solution to the dissolution vessel. What will be the new pOH of this solution?

correct First, note that the question asks for pOH, which should be greater than 7 for a solution of concentrated acid. To decide between the remaining options, remember that solution chemistry tells us that if concentration remains the same, then volume changes will not affect pH. The pH of a solution is calculated by taking the negative log of the proton concentration. For this experiment, the dissolution vessel contains 0.1 M HCl, whether it is the original 900 mL or the 1300 mL present in this question. For the MCAT, we can assume that a strong acid will completely dissociate, meaning the effective [H+] = 0.1 M = 1 x 10-1 M. Calculating the pH gives -log[1 x 10-1] = 1. We should also remember that at standard conditions, pH + pOH = 14 for any aqueous solution. Thus, we can calculate the pOH = 14 - pH = 14 - 1 = 13.

Which of the following correctly lists the floating lag times for the three trials in increasing order? (note: assume that the mixing of the drug and polymer does not change the density of either component)

correct The passage defines lag time as the time taken for the tablet to rise to the surface of the dissolution media. Object flotation is determined only by density - a less dense object will rise quickly in a fluid and will then float with more of its volume above the fluid surface. A more dense object will rise more slowly and will float with more of its volume submerged.

The figure shown below shows the pressure and volume changes of the left ventricle during the cardiac cycle for two different patients.

correct The work done by patient 2's left ventricle is greater recall PV loops show the relationship between Work, Pressure, and Volume you can find work by finding the area under the curve.

Assuming a mass of 0.04 g, what is the power exerted by the electrical forces on a fragment of fiber if the fragment has a velocity of 200 mm/s, 10 ms after ejection?

correct Work = ΔKE = KEf - KEi Thus, power can be calculated as ΔKE/time. Remember also that KE = ½ mv2. Power = [½ (0.04 x 10-3 kg)(200 x 10-3 m/s)2] J / (10 x 10-3) s = (0.5 x 4 x 10-5 x 4 x 10-2) / (1 x 10-2) = 8 x 10-5 J/s

Which of the following waves would have a wavelength larger than any standard electrospun fiber?

correct we are simply looking for the choice with the largest wavelength. Be familiar with the electromagnetic spectrum! Visible light (ROYGBIV) ranges from approximately 7 x 10-7 m to 4 x 10-7 m in wavelength. X-rays are smaller than all visible light, with wavelengths around 1 x 10-11 m. Finally, microwaves are larger than visible light waves, with wavelengths of 1 x 10-3 m. As the longest-wavelength option, microwaves must be the correct answer. good but go back and memorize electromagnetic spectrum!

How many chiral carbons are present in compound X?

correct, 0 A chiral carbon is a tetrahedral atom that has four different substituents. There are no such carbons in compound X.

Early research into the mechanism of antibiotics theorized that it was implausible that the amide in penicillin would convert to an ester. Which of the following explains this doubt?

incorrect Amides are less reactive than esters. Good leaving groups are usually weak bases. For the MCAT, you can simply remember the relative reactivities of these carboxylic acid derivatives, starting with the most reactive: acyl halides > anhydrides > esters and acids > amides. It is not thermodynamically favorable for a molecule to transform from a less reactive (more stable) form, like an amide, to a more reactive (less stable) form, like an ester. understand that an amide is less reactive (more stable) than an ester, so it wouldnt make sense to convert an already stable compound to something unstable. I couldnt relate reactivity with stability.

Which of the following best explains why, at a constant voltage, warmer solutions are ejected more quickly than cooler ones?

incorrect The surface tension is lower for solutions at higher temperatures. Paragraph 1 states that "electrospinning occurs when the electrical forces at the surface of a polymer solution overcome the surface tension and causes an electrically charged jet to be ejected." If warmer solutions are being ejected sooner, this equilibrium imbalance must be reached sooner. This imbalance can be caused by either an increase in electrical forces or a decrease in surface tension. Electrical forces, which are generated by the voltage source and by the intrinsic electrical properties of the material, will be unaffected by increased temperature. Surface tension is a contractive tendency of the surface of a liquid that allows it to resist an external force. Surface tension decreases as temperature rises, such that as the average kinetic energy of the fluid molecules increases, they are more likely to be able to break the intermolecular bonds holding the molecules together at the surface. Surface tension also decreases as the surface area of the fluid rises due to the molecules being stretched further apart, resulting in a thinner film of surface tension. review

When in vivo studies were performed on the three drug-polymer combinations, it was found patients in the polaxemer-188 group experienced the most stomach pains after administration. Given the results of the study, this is most likely due to:

incorrect tablet fragments causing inflammation of the gastric mucosa we are looking in particular to polaxemer-188 which is trial 3. use process of elimination D can be ruled out because PH and POH was not mentioned in the passage regarding the stomach If we look at the graph, it shows that the glipizide/polaxemer matrix demonstrated the lowest percent of drug release, eliminating choice A, which implies the opposite (that elevated drug concentration is the cause of the pain). we can eliminate choice c. The passage states that small matrices lead to increased drug release, but the figure indicates that the polaxemer tablets released the drug the most slowly, implying that the polaxemer tablets have large matrices and contradicting choice C. Choice B is the only remaining option. Since the glipizide/polaxemer-188 pill displayed the lowest solubility, the pill must remain partially undissolved for a longer time than the other medications, and the resulting solids could cause the most inflammation. I got this question wrong because I chose A, I did not make the connection to go back to the graph and look at specifically trial 3. I need to read the questions more carefully.

The flow rate of stomach content emptying is 100 cm3/sec. Patients who undergo gastric bypass surgery will increase this rate to almost 1600 cm3/sec. Assuming the flow of stomach contents approximates Poiseuille's Law, what change to their gastrointestinal connection would explain this, provided no other changes occur in the conditions of stomach content flow?

incorrect The new connection radius is 2 times larger. Flow = ΔPπr4 / 8Lη The question states that the flow increased from 100 to 1600 cm3/sec, a 16-fold change. Of all the answers provided, only doubling the radius would result in a 24 increase in the flow. I identified the equation correctly, but was mistaken that the radius was to the power of 2 and not 4.

What alteration to an HCN channel will reduce the transport of K+ in favor of Na+ transport?

incorrect Decreasing the diameter of the channel by removing amino acids Paragraph 1 states that HCN channels are permeable to K+ and Na+ ions. Since Na+ is chemically similar to K+, one can assume the difference must be due to the size of the atom. The K+ ion is larger than the Na+ ion, so reducing the diameter of the channel can allow Na+ to enter while preventing K+ entry. review i guessed

Which of the following properties explains why sound wave velocity increases as it travels from air to a liquid medium at the same temperature?

incorrect Bulk modulus of the medium In general, sound travels fastest in solids and slowest in gases. The equation for sound velocity is given by v = √(Κ/ρ), where K is a coefficient of stiffness termed the bulk modulus (or the modulus of bulk elasticity for gases) and ρ is the density. From this equation, we can see that the speed of sound increases with the stiffness (the resistance of an elastic body to deformation by an applied force) of the material; since solids are considerably more stiff than liquids and especially gases, sound travels fastest through them. Since density is present in the denominator of the equation, sound waves moving through two equally-stiff solids will travel faster in the less dense material. But when comparing solids to gases, density has a smaller effect than stiffness, because the extent to which a typical solid is more stiff than a gas far exceeds the ratio of their densities. review sound waves

Which of the following explains why the pitch of person's resonant voice harmonics rises when they inhale helium instead of air?

incorrect The change in timbre results in a higher perceived frequency of the voice. The pitch of a perceived sound is dependent on its frequency. In the case of the human voice, there are many overlapping waves with various frequencies and higher harmonics of those frequencies. Nonetheless, the basic fact remains that pitch depends on perceived frequency. Thus, when helium atoms are inhaled, the density of the air is being changed, which alters the timbre (an overall quality of voice that results from overlapping waves) to emphasize the higher-pitched harmonics. We perceive that as a high-pitched, "squeaky" voice. However, note that even without knowing the definition of timbre, this question can be solved by eliminating the other options. A decrease in frequency would result in a lower-pitched, deeper voice, not a high-pitched squeaky voice. review waves, i didnt know how to answer this question, make sure you review

A scientist concludes that the phosphorylated β-catenin is the form that is degraded. Why do the experimental results support this conclusion?

incorrect The levels of nuclear 32P decrease faster and remain lower than 32P levels in the cytoplasm. Figure 1 shows that the levels of radioactive P drop off for both cell mixtures. However, 32P activity drops off sooner and nearly reaches 0 in the nuclear extracts. Paragraph 3 states that intact β-catenin accumulates in the nucleus, implying that β-catenin is degraded in the cytoplasm and that there is a mechanism for moving β-catenin between the nucleus and the cytoplasm. Since the nucleus accumulates intact β-catenin (as indicated by the passage) that is non-phosphorylated (as indicated by the drop-off in radiolabeling activity, in contrast to the steady state that seems to be reached in the cytoplasm), it can be concluded that phosphorylated β-catenin is degraded. review