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MCAT Content / AAMC MCAT Practice Exam 3 Bb Solutions


Exam 3 Biology/Biochemistry Section Passage 1


  1. Before we consider the relative amount of work performed by each person, it’s important to note that work is independent of time. Work is the force exerted over some distance. Right off the bat this answer choice is a poor answer choice. On to the work: if work is the force over some distance and the two individuals lift the same amount of weight, they are exerting the same force (weight=force=mass*acceleration). However, the question stem says that individual M has longer limbs, which suggests they are taller than individual N. If both individuals lift the weight from the ground to their shoulders, the taller individual, M, is exerting the same force over a longer distance, and is doing more work. This answer is incorrect because it says the two individuals do the same amount of work and because it tries to justify the amount of work done using time.
  2. As noted above, individual M has longer limbs and is presumably taller than individual N, so they move the same weight over a longer distance and do more work than individual N, not less. Work and distance are positively proportional.
  3. This is correct. There is a positive relationship between distance and work performed. When the weight is moved over a shorter distance as is done by the individual with shorter limbs, the amount of work performed is less.
  4. The speed at which the muscle contracts, which takes time into account, is relevant for the power produced. The question is asking about the work performed. Answer choice C is the best answer.

2) According to the passage, muscles that have similar diameters will also have similar strengths.

  1. Based on the passage, it is correct that the two fibers in the question stem with similar diameters will produce similar forces. However, recall that power is the force produced multiplied by the velocity at which that force is generated. As the name implies, fast-twitch fibers contract at faster velocities than slow-twitch fibers. If the force is the same and the velocity is greater for fast-twitch fibers, then the fast-twitch fibers generate more power.
  2. This contradicts the passage which says similar muscle diameters produce similar strengths and strength is the production of force.
  3. As noted in the explanation for answer choice A, the power produced by fast-twitch fibers is greater than the power produced by slow-twitch fibers because the two will produce the same force when they are the same diameter, but the fast-twitch fibers do so with a greater velocity. This is the correct answer.
  4. Like answer choice B, this directly contradicts the passage. Muscles of similar diameter should produce similar strengths and forces. Answer C remains the best answer.


  1. The fermentation of glucose to lactic acid is a type of anaerobic metabolism. This would be seen, making it incorrect for a LEAST question.
  2. The electron transport chain relies on oxygen to function. Under anaerobic conditions, the ETC will be off so phosphorylation of ADP would not occur. This is the correct answer.
  3. Anaerobic glucose metabolism produces less ATP than aerobic glucose metabolism, so more glucose needs to be metabolized to meet the same energy needs. Glucose will need to be mobilized from its glycogen storage form. Depletion of these glycogen stores could certainly occur under anaerobic conditions.
  4. Lactic acid fermentation under anaerobic conditions will, as the name suggests, generate acidic products which will acidify the cytoplasm. Answer choice B is the only answer which would not be seen under anaerobic conditions.

4) Power is equal to work performed divided by the time over which it is performed. The question stem specifies that the duration of contraction is the same, so time is constant across all four points. This means that we can estimate the work by looking at the power curve. The work performed will be lowest when power is lowest which will be when either the force or velocity approach zero.

  1. At point A, the power curve is virtually zero. Given a constant time, this means that work is also approaching zero. We’re off to a great start.
  2. The power produced at point B is greater than the power produced at point A. /the time of contraction is held constant, the power can be used to directly estimate the work performed. Since the power is greater at point B, the work performed is also greater at point B.
  3. Point C corresponds to the greatest power which, given our approximation, corresponds to the greatest work done by the muscle, not the least.
  4. Like point B, point D corresponds to a greater power than point A meaning it also corresponds to a greater work. Point A corresponds to the least amount of work done by the muscle.

5) One way to answer this question is by using the final statement in the passage about slow-twitch fibers, that they are adapted to increase oxidative capacity. An answer choice that would promote increased oxidative capacity would correspond to slow-twitch fibers and would be incorrect. Another approach to this question is to find an answer that corresponds to greater contractile strength for fast-twitch fibers.

  1. Greater Ca2+-pumping capacity would improve contractile strength which corresponds to fast-twitch fibers. Remember that the movement and subsequent binding of calcium to troponin is what allows for the binding of actin and the myosin heads, ultimately producing contraction. Also, it does not directly correlate with increased oxidative capacity which would be a marker for slow-twitch fibers. This is a great answer choice.
  2. Increased capillary density would promote blood flow and oxygen delivery to the slow-twitch muscle fibers, thereby improving oxidative capacity. The question stem asks for a property of fast-twitch fibers so this is incorrect.
  3. More mitochondria correspond to greater oxidative capacity for slow-twitch muscle fibers.
  4. Like answer choices B and C, this answer choice would improve oxidative capacity for ATP production in slow-twitch fibers. Only answer choice A would be a property of fast-twitch muscle fibers.


Exam 3 Biology/Biochemistry Section Passage 2

6) Below you will find a flowchart to help you determine the most likely mechanism of inheritance when looking at a pedigree. Please note that it was created with the pedigree from this passage in mind and as such is not exhaustive.

  1. This aligns with our pedigree analysis and mechanism of inheritance flowchart. Males are not truly disproportionately affected (see above) so autosomal is more likely than sex-linked. Also, two affected parents had unaffected offspring which means that it is a dominant mechanism of inheritance. If it were recessive, II-1 and II-2 would only be able to pass down recessive alleles and all of their children would be homozygous recessive and affected. Because III-1 and III-3 are not affected, the parents must both be heterozygous and only have one copy of the dominant allele.
  2. As noted in the explanation for answer choice A, this is not possible because two unaffected offspring are born to two affected parents.
  3. This would be more likely if males were disproportionately affected. Before you yell “wait a moment!” at your screen, note that both generations II and III have mostly male offspring to begin with. Also, II-2 “married” into the family; they are not offspring of I-1 and I-2 so they should not be included in our rough assessment of the number of males versus females affected. Because the male-to-female ratio is already biased at birth, we cannot say that males are disproportionately affected.
  4. Females are affected indicating that it cannot be Y-linked; only males have Y chromosomes. A is the correct answer choice.

7) Take a look at the 9th band from the top in the normal and then in the tumor gel. The original nucleotide was T in the normal tissue and C in the tumor tissue.

  1. Both thymine (normal) and cytosine (mutation/tumor) are pyrimidines. This is the opposite of what we’re looking for.
  2. While cytosine is indeed a pyrimidine, thymine is not a purine so the first half of this answer is incorrect.
  3. Thymine is indeed a pyrimidine, but so is cytosine. The second half of this answer is incorrect.
  4. This is it; both thymine (normal) and cytosine (mutation/tumor) are pyrimidines making the mutation a pyrimidine to pyrimidine mutation.

8) Halfway through the second paragraph, the author notes the amino acid substitution is an arginine for a histidine. The question asks for the amino acid that is replaced so the correct answer should be the structure of histidine. You’ve heard this before but if you haven’t yet memorized the structures of the amino acids, add that to your MCAT to-do list!

  1. This is the structure for proline, not histidine. Don’t let the ring confuse you.
  2. Not quite, if we see this structure we should think of tyrosine.
  3. Here’s histidine folks! This is the correct structure.
  4. This is phenylalanine, not histidine. Answer choice C it is.

9) The second sentence of the passage states that in HGF, a disulfide bond joins the α subunit to the β subunit. The correct amino acid should be able to form a disulfide bond.

  1. Alanine does not contain a sulfur atom to form a disulfide bond.
  2. Cysteine contains a sulfur atom and is capable of forming a disulfide bond.
  3. Don’t let the single letter abbreviation fool you; serine does not contain any sulfur atoms and cannot form disulfide bonds.
  4. Tyrosine cannot form disulfide bonds; only cysteine can form the bond that joins the α and β subunits of HGF making answer choice B the correct answer.

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