We’re on the last part of bio/biochem and we’re finishing up with a set of discrete questions. Once again, we’re joined by Clara from Next Step Test Prep as we dive in right into these questions and walk you through getting the correct answer. So you can get the MCAT score that you want!
Meanwhile, be sure to check out all our other podcasts on the MedEd Media Network and listen to more resources to help you along your journey towards becoming a physician!
[02:52] Question 57
patients with Ehler’s Danlos syndrome commonly suffer from hypermobile, unstable joints and fragile, easily bruised skin. Which of these loss-of-function mutations is most likely to cause Ehler’s Danlos syndrome”
- (A) A mutation in actin
- (B) A mutation in keratin
- (C) A mutation in elastin
- (D) A mutation in collagen
The correct answer here is D. While C, however, is the most picked wrong answer. But what can save you from picking C is looking at that line in the passage saying “loss-of-function mutations.” If that’s the case in elastin that means elastin is no longer working and your joints wouldn’t be hypermobile, but not mobile at all. And you’d be stiff essentially.
Actin is one of those proteins in the muscle, keratin is in the skin, and so these two don’t make sense too.
[04:54] Question 58
In skeletal myocytes, calcium is normally released from the sarcoplasmic reticulum after:
- (A) the muscle begins to relax.
- (B) myosin releases actin.
- (C) the membrane depolarizes.
- (D) physical damage to the muscle occurs.
Calcium gets troponin and tropomyosin and they’re blocking the binding site that myosin would typically use to attach to actin. Then when calcium comes out, it causes a shift that will expose that binding site and that’s what allows myosin to bind to actin. ATP is what triggers the release. ATP binds to the myosin head and calcium binds to get troponin and tropomyosin out of the way. C is the right answer here since the membrane depolarizes and that’s the signal to start the muscle contraction which causes the calcium to flow out of the sarcoplasmic reticulum.
Extra tip: When you review your test, review not only what you got wrong, but also, what you got right. Sometimes, you’d get the right answer out of just guessing or for the wrong reason. And even sometimes, you might get the right answer for the right reason, but maybe the explanation will have an alternate way to do it or there may be content facts you weren’t thinking about. So review everything!
[10:15] Question 59
In aerobic organisms, glycolysis:
- (A) is upregulated when blood glucose levels fall.
- (B) directly supplies the citric acid cycle with acetyl-CoA.
- (C) requires oxygen as the final electron acceptor.
- (D) is inhibited by a high rate of intracellular ATP:AMP.
B can be tempting as it’s talking about the citric acid cycle and that’s what we typically think of as coming next after glycolysis. But it’s wrong because it says, “directly supplies the citric acid cycle with acetyl-COA” and glycolysis doesn’t produce acetyl-CoA directly. Instead, it produces pyruvate, which is decarboxylated into acetyl-CoA in a later stage in pyruvate decarboxylation.
C is wrong too because glycolysis isn’t what has the final electron acceptor, but the electron transport chain. So glycolysis doesn’t involve oxygen at all. Then A is another wrong answer because if blood glucose levels are falling then you don’t want to be using up the glucose you have in glycolysis.
The right answer is D. Basically, ATP and AMP are activators of a lot of these metabolic processes because they are signals of how much energy the cell has. So if we have a high ratio of ATP:AMP, that means we have a lot of ATP and so we have a lot of energy already. There’s no need to run these processes just to make more energy if we already have plenty. High ATP concentrations typically inhibit metabolic processes and high AMP is a sign that we don’t have a lot of ATP so high AMP stimulates these processes.
[14:30] Next Step Test Prep
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