Another week of MCAT full-length breakdown with Clara from Next Step Test Prep. We’re covering Passage 4 from NSTP full-length 10 bio/biochem section.
[02:02] Passage 4
The compensations that led to an evolution of endothermic physiology are still being argued. A widely accepted model is that directional selection for incremental increases in aerobic capacity and sustained activity led to the evolution of endothermy, the aerobic model. The greater scope for physical activity means that endotherms tire less quickly and are therefore better able to forage, escape predators, and migrate. The parental care model suggests that increased non-shivering thermogenesis permitted parents to control incubation temperatures and thereby providing a selective advantage leading to endothermy.
Thyroid hormone (TH) is a regulator of thermogenesis in humans. Thyroid receptors regulate gene expression by bonding thyroid response elements in the promoters of target genes. TH can also regulate protein function. An experiment was performed on two groups of Wistar Coyote rats, Group A rats were kept in an environment with an ambient temperate of 27 degrees Celsius while Group B was kept in a 9 degrees Celsius environment.
In both groups, pharmacologically manipulated thyroid levels cause changes in relative sodium, potassium, ATP’s activity, scales to muscle PGC-1 alpha, a transcriptional coactivator that regulates the genes involved in energy metabolism, mRA levels, and cardiac sarcoendoplasmic reticulum, calcium ATPase or circa activity levels. TH regulates these levels of physiology in ectothermic vertebrates as well, specifically in response to cold. The net result of these actions is a TH that buffers mammalian movement from the thermodynamic effects of low temperature. It is likely therefore that TH regulatory activity was under strong positive selection because it would improve fitness in variable environments. Even incremental increases in TH-mediated locomotor performance and physical activity under cold conditions would represent clear, selective advantages for accessing food and energy assimilation, parental care, and territorial gains. Additionally, ectotherms have restricted activity times that are closely tied to the operative thermal environment.
[04:40] Question 18
Which of the following statements regarding endotherm development is most supported by the information presented in the passage?
- (A) Their designs became more advantageous because the physiological performance of early endotherms were increasingly uncoupled from environmental variation.
- (B) Their designs became less advantageous because the physiological performance of early endotherms were increasingly uncoupled from environmental variation.
- (C) Their activity was more restricted compared with ectotherms which allowed lesser environmental adaptation.
- (D) Their activity was restricted compared to ectotherms which led to an evolutionary disadvantage.
The passage talked more about how it was an advantage and how everything helped like accessing food, energy assimilation, and territorial gain. So we’d probably throw out D right away as well as C. So now we’re left with A and B. But B also talks about being “less advantageous” so B, C, D all talk about the disadvantage except for A. So through the process of elimination, we pick A. Again, the passage was clearly talking about advantages over ectotherms, so A is the ideal answer.
[08:00] Question 19
Localized radiation therapy of the thyroid would most likely affect another endocrine gland that has which of the following characteristics?
- (A) It consists of multiple cell types secreting a number of different steroid hormones.
- (B) High calcium stimulates the release of its major hormone product.
- (C) Hyperactivity of the gland can cause osteoporosis.
- (D) It synthesizes its hormone from cholesterol ester using enzymes found in the smooth endoplasmic reticulum and mitochondria.
The correct answer here is C. We can cross out A and D as they’re both talking about, or at least referring to a steroid. And parathyroid hormone is not a steroid, but a peptide hormone. So we’re left with B and C. Just remember that parathyroid hormone causes a decrease in blood calcium. And when you say hyperactivity of the gland causes osteoporosis, indicates blood calcium being taken away from the body.
[10:57] Question 20
A medical student taking an important exam notes that her heart rate is significantly elevated due to thyroid activity. According to the experiment conducted in the passage, which of the following changes would not be expected in this student?
- (A) Increased cellular hydrolysis of ATP
- (B) Increased rate of gluconeogenesis
- (C) Increased passive transport of thyroid hormone through cell membranes
- (D) Increased translation of PGC-1a protein
This question combines being passage-based but also requiring some content knowledge. This can be confusing for students since we see PGC-1a protein and we thin we can get the answer from going back to the passage alone. Unfortunately, the passage is unhelpful with this. What you want to keep in mind here is you need to know the endocrine system inside and out for the MCAT. For C, thyroid hormone is not going to be transported passively through cell membranes, since TH is derived from tyrosine. So it’s going to be too polar to pass through cell membranes. So, C is the right answer here.
[15:27] Question 21
Thyroid storm is an acute life-threatening hypermetabolic state induced by excessive release of thyroid hormones with a 90% mortality rate in adults. It is observed that a hypermetabolic rate causes release of thyroid hormone which then further elevates the metabolic rate. This cycle indicates what kind of mechanism may be the cause?
- (A) Negative feedback loop
- (B) Positive feedback loop
- (C) Negative forward loop
- (D) Positive forward loop
There is a type of feedback loop called the feed-forward loop where something that is upstream, which is something earlier. A chemical pathway is going to act on layer steps. So it’s like the opposite of a feedback pathway. But C is not a thing. So they’re trying to mix up here. That being said, this is a classic case of a positive feedback. We can spot it when you see a cycle that seems to be going more and more out of control, where it’s stimulating itself and it’s going to increase and increase until something else stops it.
[17:35] Next Step Test Prep
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