MCAT Misses
ATP is formed by ATP synthase, which is embedded in the inner mitochondrial membrane between the intermembrane space and the mitochondrial matrix. All of the following statements accurately describe this process EXCEPT that:
it is energetically favorable The formation of ATP from ADP and an inorganic phosphate group is not energetically favorable, and it must be powered by the electrochemical gradient formed by the difference in proton concentrations between the intermembrane space and the mitochondrial matrix. ATP formation is energetically unfavorable for exactly the same reasons that it is a good way to store energy: once energy is "invested" into ATP by compressing together three phosphate groups that do not really "want" to be together, that energy can be liberated by breaking ATP down, allowing other reactions to be powered.
Which pair incorrectly matches an enzyme category to its function?
ligase -> cleaves bonds non-hydrolytically Ligases join molecules together covalently, while the similarly sounding lyases cleave bonds non-hydrolytically (think of the ly- root in glycolysis).
Which of the following are functions of the smooth endoplasmic reticulum? Select all that apply.
lipid synthesis, detoxification, steroid hormone synthesis The smooth endoplasmic reticulum is involved in lipid synthesis, steroid hormone synthesis, detoxification, and calcium storage in muscles.
Research has found that certain amino acid residues in histones can be phosphorylated through the enzyme-catalyzed addition of a PO32− group. If DNA is negatively-charged, which of the following is the most likely effect of histone phosphorylation?
loosening the DNA coiled around histones The question stem indicates that phosphorylation adds a strongly negatively charged group to histones. These groups will experience electrostatic repulsion from the negatively charged phosphate groups on the DNA backbone, because like charges repel. This means that the histones will associate more loosely with the DNA, providing more room for transcription factors
Which quantum number describes the spatial orientation of a specific orbital within a given electronic subshell?
magnetic quantum number
which organelle is capable of replicating itself
mitochondria mitochondria have their own DNA genome, separate from the cell's nuclear genome. This allows mitochondria to replicate independently of the cell cycle
where do lysosomal enzymes originate
they're synthesized at the ER. Lysosomal enzymes are proteins that are synthesized in the rough endoplasmic reticulum. After synthesis, they are packaged within the Golgi apparatus into lysosomes, where they break down waste.
Which of the following are possible values for angular momentum quantum numbers in the n = 4 shell?
0,1,2,3 The angular momentum quantum number (l), also called the azimuthal quantum number, describes the shape of the orbital. This tells us what subshell the electron is located in within a shell, where L can range from 0 to n-1 for a given principal quantum number. A shell of n = 4 will have angular momentum quantum numbers 0 to 3 which includes 0, 1, 2, and 3.
According to the data in Table 1, what is the turnover number for variant V245F?
3x10^-11 min^-1 C is correct. The turnover number is also known as kcat, or the time it takes one enzyme to turnover a maximum amount of substrate molecules per unit time. No kcat values were given, but it can be obtained by using the equation kcat = Vmax / [Et]: kcat = (1.5 x 104 μM/min ) / (0.05 pM). Changing concentrations to molarity we obtain (1.5 x 10-2 M/min) / (5 x 10-14 M) which is equal to 3 x 1011 min-1.
The mRNA transcribed from a dsDNA molecule has a partial sequence of 5'-AUGAAUC-3'. The corresponding sequence on the sense DNA strand is:
5'-ATGAATC-3' The sense DNA strand should have the same sequence as the mRNA strand produced, except the DNA strand should contain thymine instead of uracil. Here, choices B and C represent mRNA, not DNA, and choice A is the sequence of the antisense DNA strand.
Glucose 6-phosphate dehydrogenase catalyzes a reaction involving glucose 6-phosphate. What is the most likely product formed in this reaction?
6-phosphogluconolactone Even if the pentose phosphate pathway is unfamiliar, focusing on the type of enzyme in conjunction with process of elimination can help you arrive at the correct answer. Phosphogluconolactone contains word stems that can be used as clues, such as 'phospho' and 'gluco'. The lactone ending suggests a C=O bond resulting from oxidation of an O-H, i.e. a redox process catalyzed by the oxidoreductase enzyme glucose 6-phosphate dehydrogenase.
A horizontal force of 6 N is applied to a box that is simultaneously lifted with a force of 4 N. What would be the magnitude of the net force applied to the box?
7.2 N This answer choice correctly applies the Pythagorean theorem, whereby the net force applied is equal to the hypotenuse: a^2 + b^2 = c^2 (6N)^2 + (4N)^2 = c^2 36 + 16 = c^2 c = √52 = about 7.2 N
Assuming that the pKa of the side chain of histidine is approximately 6, which of the following is closest to the isoelectric point of this amino acid?
7.5 To calculate isoelectric point, you must consider the pKa values of the amino acid in question. Histidine has a carboxylic acid group (pKa ~ 2), an amino group (pKa ~ 9), and its side chain (pKa ~ 6). Note that we should not average all three of these values; instead, since histidine is a basic amino acid, we should average the two most basic pKa values. (6 + 9) / 2 = 7.5.
Which of the following orbitals cannot exist in an s subshell? Select all that apply.
+1,-1,+2 The question stem asks for orbitals that cannot exist. The magnetic quantum number, ml, ranges from -l to +l for a given subshell. The s subshell (l = 0) has just 1 orbital, so ml = 0. Thus, there is only one possible orientation in space for this subshell.
In eukaryotic cells, DNA replication takes place in the: I. nucleus. II. cytosol. III. mitochondria.
I and III only Both the nucleus and the mitochondria contain their own DNA genomes. In eukaryotes, mitochondrial and nuclear DNA replication occurs independently of each other and within their respective organelles.
Prefixes to know for physics
The prefixes of the metric system are as follows: tera = 10^12 pico = 10^-12 giga = 10^9 nano = 10^-9 mega = 10^6 micro = 10^-6 kilo = 10^3 milli = 10^-3 hecto = 10^2 centi = 10^-2 deca = 10^1 deci = 10^-1
True or false: Protonating aniline (C6H5NH2) to form C6H5NH3+ would improve its solubility in hexane, a nonpolar solvent.
false This statement is false. As the question indicates, protonating aniline would make it positively charged. Charged substances are poor solutes in nonpolar solvents such as hexane.
True or false: Steps in a pathway where the reactants and products have approximately the same energetic favorability are especially important targets for regulation.
false This statement is false. Instead, highly energetically favorable or unfavorable steps tend to be closely regulated, because they are more likely to be committed steps and/or steps that require investment of ATP.
Which of the following are most likely NOT potential functions of cytoskeletal capping proteins? Select all that apply.
promoting cytoskeletal polymer degradation and promoting the Krebs cycle Capping proteins inhibit cytoskeletal protein degradation by preventing actin polymer breakdown. Aerobic respiration (including the Krebs cycle) occurs in mitochondria to provide cellular energy as ATP. As the Krebs cycle occurs in the mitochondria, it is likely unrelated to cytoplasmic cytoskeletal proteins, such as actin and microtubules. POTENTIAL FUNCTIONS ARE: muscle contraction and cell movement The cytoskeletal protein actin is strongly implicated in cell movement. Actin polymerizes into lamellipodia (cellular projections that aid in cell motility), which can be facilitated through capping proteins that inhibit the breakdown of actin polymers. The cytoskeletal protein actin is heavily implicated in muscular contraction and found in sarcomeres (the contractile unit of muscles). Capping proteins thus likely play a role in altering muscle contraction by affecting the cytoskeletal proteins found in the sarcomer
A microfilament capped at the plus (+) end would tend to:
shrink Capping prevents the addition or removal of monomers to the capped end of the filament. Actin filaments capped at the plus (+) end would likely shrink, as actin monomers readily fall off the minus (-) end, shrinking the filament size.
Which of the following characteristics regarding enzyme-substrate complexes can be explained by the induced fit, but not the lock-and-key, model?
stabilization of the transition state The stabilization of the transition state is explained by the induced fit model, but not by the lock-and-key model. The induced fit theory posits that the active site of the enzyme is shaped by interactions between the active site itself and the substrate. The gradual molding of the active site to the substrate is what eventually stabilizes the transition state.
Which of the following covalent bonds is the shortest?
the C-H bond in methanol Bond length is primarily determined by the atomic radii of the bonded atoms. Therefore, bonds formed with hydrogen atoms tend to be shorter than bonds formed with any other atom.
In DNA, A-T base pairs involve two hydrogen bonds, whereas G-C pairs have three hydrogen bonds. Based on this, which of the following sequences would be the most stable and could be isolated from a single-cell organism living in very hot anaerobic conditions
CCATCGCTACG Hydrogen bonds are strong non-covalent intermolecular attractions that raise the melting and boiling points of a substance. As such, the DNA sequence with the most GC pairs would be expected to be the most heat-stable - and therefore, the most suitable to this environment. Of the answer choices, CCGATCGCTACG has the most C and G nucleotides.
The reaction of magnesium sulfide with hydrogen peroxide produces both magnesium sulfate and water. What is the balanced equation for this process, and which element is reduced?
MgS + 4 H2O2 -> MgSO4 + 4 H2O On the reactant side, magnesium and sulfide ions have +2 and -2 charges, respectively. Magnesium maintains this +2 charge on the product side, while sulfur changes its oxidation state to +6. Oxygen, however, initially has an oxidation state of -1, but becomes -2 as a component of water. For this reason, oxygen, not magnesium, is reduced.
Order the following functional groups from most polar to least polar
RCOOH > ROH > R2C=O > RNH2 > RH The carboxylic acid is the most polar, followed in order of decreasing polarity by the alcohol, the carbonyl of the ketone, the amine, and the alkyl functional groups. A carboxylic acid functional group (-COOH) has a strong dipole and can engage in multiple hydrogen bonding interactions. Alcohols have a highly polar functional group that can hydrogen bond, and a ketone has a carbonyl functional group that has a significant dipole. Nitrogen is less electronegative than oxygen, but an amine does have a dipole because of the lone pair of electrons on nitrogen. Finally, RH is a completely nonpolar alkane.
In liquid chromatography, a mobile phase contains various low molecular weight organic solutes identified as an alcohol (ROH), a secondary amine (R2NH), and an alkyl halide (RCl). The solutes are run through a stationary phase that has primary amine (RNH2) groups. Assuming no solvent interaction with the stationary phase, arrange the solutes by their order of elution from first to last.
RCl, R2NH, ROH The solute that will elute first is the one that has the weakest interaction with the stationary phase. The alkyl halide will have no hydrogen bonds with the amine, so it will be the first to elute. The secondary amine can form a hydrogen bond to the primary amine that is a component of the stationary phase, but the alcohol will form a stronger hydrogen-bonding interaction with the stationary phase because of its greater polarity. Consequently, the secondary amine will be second to elute and the alcohol will be the last to elute from the column.
How are the C-H bonds in methane (CH4) different from the K-Cl bond in potassium chloride (KCl)?
The character of the bonds in CH4 is the result of a smaller electronegativity difference than for the bond in KCl Potassium chloride (KCI) has ionic bonding as there is attractive force between the K+ cation and Cl- anion, and potassium has donated one electron of its outermost orbit to Iodine to complete its octet. We can infer this due to the large left-right difference between the two elements on a periodic table. Electronegativity increases up and to the right on the periodic table, and Cl is just below the most electronegative element, fluorine. On the other hand, methane (CH4) contains nonpolar covalent bonds because H and C are much closer together, and the electronegativity difference between hydrogen (2.20) and carbon (2.55) is lower than 0.5.
In the reaction shown below, heroin is formed from morphine by acetylation. Based on this reaction, which prediction about the relative effects of heroin versus morphine is most reasonable?
The psychoactive effects of heroin will occur more quickly because heroin can pass more easily through the epithelial cells that form the blood-brain barrier The two -OH groups on morphine are highly polar and engage in hydrogen bonding, thereby impeding (to some extent) the ability of morphine to pass freely through the plasma membrane. In heroin, these groups are acetylated. They become esters (although knowledge of this term is not needed to answer the question), which are considered less polar because they cannot engage in hydrogen bonding. This means that heroin is overall less polar than morphine, and will pass more freely through the plasma membrane. Choices A and C state that compounds must pass through epithelial cells to reach the brain, so we can predict that the rapidity of onset of psychoactive effects will correlate with how quickly they pass through the plasma membrane of these cells. Heroin is predicted to pass through the membrane more rapidly, so its effects are predi
If a person had a genetic mutation that impaired the microtubules from forming efficiently, which of the following would NOT occur?
all motile cells would have difficulty moving While flagellated and ciliated cells would have trouble moving, many cells (like the immune system cells) rely on actin reorganization for motility, and would thus be unaffected.
Which of the following would be significantly increased by mutations that negatively affect microtubule function? Select all that apply.
birth defects, mucous buildup in the throat, and male sterility Mutations affecting microtubules would cause birth defects by impacting the ability for chromosomes to be "pulled apart" during anaphase of meiosis Mutations affecting microtubules would cause mucus buildup in the throat by inhibiting throat epithelial cells' ciliary action, which normally clears debris in the throat Mutations affecting microtubules could cause male sterility in at least two ways: 1) chromosomal nondisjunction might affect cellular functionality, and 2) flagellum dysfunction would inhibit sperm motility
Which of the following atoms are most likely to participate in ionic bonding with one another
calcium and bromine Ionic bonding occurs between atoms that differ substantially in electronegativity. Of the choices given, calcium and bromine lie on opposite ends of the periodic table and, therefore, have the greatest difference in electronegativity of the choices given. Note that options A and B each consist of two nonmetals, meaning they will not participate in ionic bonding at all. Choice C can form an ionic bond, but the electronegativity difference between chlorine and aluminum is less than that between calcium and bromine.
A novel hormone is found to require assistance from a transport protein to move throughout the circulatory system due to its poor solubility in aqueous solutions. Which other feature(s) can it be predicted to have? Select all that apply
molecular weight less than 500 g/mol; interaction with nuclear receptors; ability to freely diffuse thru the plasma membrane The question stem states that this hormone is not soluble in aqueous solutions, meaning that it must be nonpolar. This means that it is a steroid hormone. They are also relatively small, with a common four-ring lipid structure derived from cholesterol. In contrast, peptide hormones are relatively large proteins. Therefore, we can also predict that the molecular weight of this hormone will be relatively small. The question stem states that this hormone is not soluble in aqueous solutions, meaning that it must be nonpolar. This means that it is a steroid hormone. Steroid hormones freely diffuse through the plasma membrane and interact with nuclear receptors.
Which of the following steps in a pathway is LEAST likely to be a target for regulation?
moving an -OH from one carbon to another on a 6 carbon substrate Moving an -OH group from one carbon to another on a six-carbon substrate is a relatively small structural adjustment that is unlikely to be extremely energetically favorable or unfavorable, meaning that it is easily reversible. The function of such a reaction (technically known as an isomerization) is likely to set the stage for a more important reaction; as such, it is unlikely to be a target for regulation.
Which of the following pure substances is expected to have the highest boiling point
water Of the given choices, only water and ammonia are capable of intermolecular hydrogen bonding, one of the strongest intermolecular forces. Hydrogen sulfide and carbon tetrachloride molecules are attracted to each other only by dipole-dipole interactions and London dispersion forces. Covalent bonds between oxygen and hydrogen are more polar than covalent bonds between nitrogen and hydrogen, because oxygen is more electronegative than nitrogen. Consequently, the intermolecular hydrogen bonds between water molecules are stronger than those between ammonia molecules. Alternatively, you could eliminate choice C if you knew that ammonia (unlike water) is a gas at room temperature, meaning that it must have a boiling point lower than room temperature.
Rank the following molecules in order of increasing bond angle.
water < ammonia < methane The correct answer is choice C. All of these molecules have four electron groups, so according to VSEPR theory, they have the same electron group geometry (tetrahedral). Molecular geometry, though, describes the geometry of only the bonded atoms. Water has two lone pairs of electrons that are only attracted to one nucleus, so they occupy more space than a bonded pair. Consequently, the bond angles deviate from an ideal bond angle of 109.5°. The bonded atoms have a bent molecular geometry with a bond angle of 104.5°. In ammonia, nitrogen has a lone pair of electrons that also occupies more space. The structure has three bonded pairs; its molecular geometry is trigonal pyramidal with bond angles of 107°. Methane has four symmetrical bonds, so it is tetrahedral and has the ideal bond angles of 109.5°.
