MCAT Practice questions and answers

Consider a biochemical reaction A to B, which is catalyzed by A-B dehydrogenase. Which of the following statements is true? A. The reaction will proceed until the enzyme concentration decreases. B. The reaction will be more favorable at 0 degrees C. C. A component of the enzyme transferred from A to B. D. The free energy change (delta G) of the catalyzed reaction is the same as for the uncatalyzed reaction. D. -In an enzyme-catalyzed reaction, the rate of a reaction is increased by a decrease in the activation energy. Furthermore, enzymes are not changed or consumed during the course of the reaction. Also, the overall free energy change of the reaction, delta G, remains unchanged in the presence of an enzyme. Which of the following statements about enzyme kinetics is false? A. An increase in the substrate concentration (at constant enzyme concentration) leads to proportional increases in the rate of the reaction. B. Most enzymes operating in the human body work best at a temperature of 37 degree C. C. An enzyme-substrate complex can either form a product or dissociate back into the enzyme and substrate. D. Maximal activity of many human enzymes occurs around pH 7.2. A. -Most enzymes in the human body operate a maximal activity around a temperature of 37 degrees C and a pH of 7.2, which is the pH of most body fluids. In addition, as characterized by the Michaelis-Menten model, enzymes form an enzyme-substrate complex, which can either dissociate back into the enzyme and substrate or proceed to form a product. We can eliminate B, C, and D. An increase in the substrate concentration, while maintaining a constant enzyme concentration, leads to a proportional increase in the rate of the reaction only initially. However, once most of the active sites are occupied, the reaction rate levels off, regardless of further increases in substrate concentration. At high concentrations of substrate, the reaction rate approaches its maximal velocity and is no longer changed by further increases in substrate concentration. Therefore statement A is not entirely true. 00:43 01:29 At which pH would pancreatic enzymes work at maximum activity? A. 5.3 B. 6.7 C. 7.2 D. 8.5 D. -Pancreatic enzymes work optimally in the alkaline condition of the small intestine. It is not necessary to know the exact pH at which these enzymes work because the only very basic pH is seen in (D), which is 8.5. Some enzymes require the presence of a non-protein molecule to behave catalytically. An enzyme devoid of this molecule is called a(n): A. holoenzyme B. apoenzyme C. coenzyme D. zymoenzyme B. -An enzyme devoid of its necessary cofactor is called an apo-enzyme and is catalytically inactive. Which of the following factors determine an enzyme's specificity? A. The 3D shape of the active site. B. The Michaelis constant. C. The type of cofactor required for the enzyme to be active. D. The prosthetic group on the enzyme. A. -An enzyme's specificity is determined by the 3D shape of its active site. Regardless of which theory of enzyme specificity we are discussing (lock and key or induced fit), the active site determines which substrate the enzyme will react with. Enzymes increase the rate of a reaction by: A. decreasing the activation energy. B. increasing the overall free energy of the reaction. C. both A and B. D. none of the above. A. -Enzymes increase the rate of a reaction by decreasing the activation energy. They do not affect the overall free energy, delta G, of the reaction. Bonding between atoms of an enzyme such as trypsin is best described as: A. peptide B. saccharide C. ionic D. van der Waals A. -Enzymes are proteins. Proteins are composed of amino acids linked together by peptide bonds. Choice (B) - Saccharide, is a type of bond found in polysaccharides. Choice (C) - Ionic, is a chemical bond formed through electrostatic interaction between positive and negative ions. Choice (D) may be formed in secondary or tertiary structures of an enzyme but is not as good an option as (A). In the equation below, substrate C is an allosteric inhibitor to enzyme 1. Which of the following is another mechanism caused by substrate C? A -- enzyme 1 -- B -- enzyme 2 -- C. A. Competitive inhibition B. Irreversible inhibition C. Feedback enhancement D. Negative feedback D. -By limiting the activity of enzyme 1, the rest of the pathway is slowed, which is the definition of negative feedback. Choice (A) is incorrect because there is no competition for the active site with allosteric interactions. There is not enough information for (B) to be correct because we aren't told whether the inhibition is reversible. In general, allosteric interactions are temporary. Choice (C) is incorrect because is it the opposite of what occurs when enzyme 1 activity is reduced. When lactose hydrolyzes its substrate, lactose, which of the following occurs? A. Lactase retains it structure after the reaction. B. Lactose retains its structure after the reaction. C. Lactase increases the activation energy of the reaction. D. Lactose decreases the activation energy of the reaction. A. -Choice (A) is correct answer because, by definition, an enzyme remains unchanged by the reaction that is catalyzes. Choice (B) is incorrect because a substrate is changed by an enzymatic reaction. Choice (C) is not true, an enzyme would decrease the activation energy. Choice (D) is also incorrect since a substrate does not affect the activation energy. Discuss why competitive inhibition can be overcome with increasing substrate concentration but non-competitive inhibition cannot be overcome. -Competitive inhibitors are structurally similar to the substrate of an enzyme. This means that in the course of catalyzing reactions the enzyme can bind the inhibitor instead of the substrate. Imagine there are an equal number of inhibitor and substrate molecules. The chance that the enzyme will bind the inhibitor is equal to the chance that it will interact with the substrate. Thus, the rate of catalysis will be decreased, effectively inhibiting the enzyme. However, if the concentration of substrate if 1,000 or 100,000 times greater than the concentration of inhibitor, then the chance the enzyme will randomly interact with the inhibitor instead of the substrate falls dramatically (practically to zero). -Noncompetitive inhibitors do not associate with the enzyme at the active site. Instead, they bind at an allosteric site. When a noncompetitive inhibitor is bound, the enzyme cannot catalyze bind the substrate. Increasing the concentration of substrate will not decrease the likelihood of a noncompetitive inhibitor associating at an allosteric site. Why is negative feedback inhibition more common than positive feedback loops? Are there biological examples when positive feedback is necessary? -Most biological conditions, such as blood pH, body temperature, and blood pressure, must be kept within a narrow range. Negative feedback allows for the maintenance of these conditions. If a homeostatic system moves too far to one extreme, negative feedback will brings the levels back within biologically acceptable conditions. Positive feedback, however, has the opposite effect. The more a process moves in a particular direction, the more positive feedback will keep it moving in that same direction. Two examples in which positive feedback is important are blood clotting and childbirth. Compare and contrast the the concept of allosteric effectors with non-competitive inhibition. -First, all noncompetitive inhibitors bind at allosteric sites. The binding of noncompetitive inhibitors, however, does not affect the binding of substrate to the active site (and vice versa). What noncompetitive inhibitors do affect is the ability for the enzyme to catalyze the reaction and release product. In contrast, allosteric effectors allow the enzyme to exist in two different confirmations (T and R), in which one state binds substrate more readily than the other. Again, this is in direct contract to noncompetitive inhibitors, which do not influence the binding of substrate. Consider the pathway: A --1- B --2- C --3- D --4- E --5- X --6- Y An increase in [E] leads to the inhibition of enzyme 3. All of the following are results of the process except: A. an increase in [B]. B. an increase in [Y}. C. decreased activity of enzyme 4. D. increased activity of enzyme 6. A. -Looking at the pathway, we notice that if enzyme 3 is inhibited, everything that is controlled by it will decrease in concentration and activity. That is, the concentration of D and E will eventually decrease. The activity of enzyme 4 will also decrease, because it will be acting on a decreased amount of substrate. In addition, if enzyme 3 is inhibited, more of C will be converted to X through enzyme 5. As such, the concentration of X and Y is expected to increase, as is the activity of enzyme 6. Anything before enzyme 3 should remain unaffected. Therefore, the concentration of B will remain the same. An infectious agent is probably a virus instead of a bacterium if it: a) lacks membrane-bound organelles b) cannot reproduce in acellular culture c) has a length of 1?m, about the size of a mitochondrion d) contains DNA Answer: B Viruses possess DNA or RNA but not both, require a host cell's reproductive machinery in order to reproduce, and have a size 10-100nm. For the MCAT: eukaryotic cell bacterium (roughly size of mitochondrion) virion (roughly size of protein, roughly 100x smaller than bacterium, 1000x smaller than eukaryotic cell, cannot be seen with optical microscope). a) lacks membrane-bound organelles, incorrect, Both viruses and bacteria lack membrane-bound organelles. b) cannot reproduce in acellular culture, correct. Viruses require host cells in order to reproduce. c) has a length of 1μm, about the size of a mitochondrion, incorrect, This is the size of a bacterium. d) contains DNA, incorrect, Both bacteria and viruses may contain DNA. Which of the following is NOT true regarding lymphatic cells and their function(s)? Please click on the correct answer: A . Neutrophils are phagocytic cells. B. Eosinophils secrete enzymes that fight against parasites. C. Basophils secrete the antidiuretic hormone to aid in vasodilation. D. B cells secrete antibodies. Answer: C Basophils secrete histamine, not ADH, as one of its functions to fight off pathogens. In doing so, the histamine increases the permeability of the capillaries to white blood cells (as well as some proteins), and this allows for the white blood cells to interact with pathogens present in infected tissues. A compound consists of approximately 72g carbon and 6g hydrogen. What is the empirical formula? a) C6H6 b) CHO c) CH d) C6H12 Answer: C A is incorrect because its the molecular formula not the empirical formula. Convert grams into moles and then change ratio to smallest possible to solve this equation. Glen brings cookies to work. Although you have not yet tasted them, you say to another coworker, "Glen is such a great guy; I'm sure these cookies are fantastic!" What type of bias is this? A. Reliance on central traits B. Direct benefits C. Halo effect D. Similarity Answer: C The halo effect is a cognitive bias in which judgments of an individual's character can be affected by the overall impression of the individual. Antidiuretic hormone (ADH) is a peptide hormone produced in the hypothalamus and released by the posterior pituitary gland in response to an increase in serum osmolarity. Diabetes insipidus is a disease characterized by a physical or functional lack of ADH and production of dilute urine. Given the symptoms of diabetes insipidus, one would expect ADH to act on: A. Na/K/Cl channels in the loop of Henle. B. NaCl channels in the distal convoluted tubule. C. H2O channels in the distal convoluted tubule and collecting duct. D. Na/H channels in the proximal tubule. Answer: C ADH acts to concentrate urine by encouraging reabsorption of water in the distal convoluted tubule and collecting duct of the nephron. It has no effect on any of the other channels in choices (A), (B), and (D). The photoelectric effect, caused when some photons and electrons collide, helps radiologists distinguish tissue types. An electron in a metal object is struck by a photon whose energy is exactly equal to the metal's work function. The electron will: A. be ejected from the atom and will move faster than its previous velocity. B. be ejected from the atom and move at its previous velocity. C. be ejected from the atom and move slower than its previous velocity. D. not be ejected from the atom, but will be accelerated to a greater velocity. Answer: C In the photoelectric effect, an electron escapes from a metal if it is struck by a photon whose energy is at least as high as the metal's work function. After it is ejected, its kinetic energy is equal to the difference between the photon's energy and the work function. Since the photon in this question has an energy that is equal to the work function, the electron's kinetic energy after ejection will be equal to zero. This means that the energy of the photon is just enough to let the electron escape; afterward, its velocity will be approximately zero, making choice (C) the correct answer.