Confused about cholesterol

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Apr 29, 2014
  1. Pre-Medical
    I know it can make membranes more fluid at low temp and less fluid at high temps. So which one is it at 37C?
    Hi @salemstein !

    The Q as you have presented it is not a very AAMC like question. The test makers do not expect you to be an expert in all kinds of gray areas, so I would not take this question too seriously, unless the info was in the passage, which you say it was not. For example knowing that ideal gases deviate at low T and high P is fair game for the MCAT, but they would not expect you to decide if 250 K or 150 K is considered "low" Temperature. They will give you extreme numbers to relate and analyze relationships with. In this case, the AAMC would probably perform an experiment to determine how cholesterol acts above and below physiological temperature. There are actually very few properties you need to memorizing about the human body. The test makers prefer to ask about relationships, cause and effect, not just boring recall. The area I have seen them repeatedly ask about physiological conditions is determining the charge on a given amino acid at physiological pH (~7.4). This is fair because to some side chains, this pH is about the same, and to many others, this pH will be significantly different from the pKa's. Even in these examples the questions relate to when an AA is in an environment more basic or more acidic than the overall AA.

    People have posted screenshots of every test prep company there is, including the AAMC. So long as you are not egregious or pasting whole sections of exams, you should be ok posting the relevant Q or figure you are asking about. It makes it infinitely easier to get a response on SDN.

    Hope this helps, good luck!

    Altius Premier Tutor

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    1. Pre-Medical

      When you hear the classic principle that cholesterol "increases fluidity at lower temps and decreases fluidity at higher temps" that is referring specifically to temperatures HIGHER or LOWER than normal physiological temperatures. In other words, the principle is describing temperature changes RELATIVE TO normal physiological temp. It is helpful to think of cholesterol as a sort of membrane "buffer" that helps maintain homeostatic membrane fluidity. At above-average temps the membrane could become too fluid, but cholesterol resists this, and at lower temperatures the membrane could become too rigid, and cholesterol resists that as well.

      With all due respect, @NextStepTutor_2 has done you a disservice in his response: "The Q as you have presented it is not a very AAMC like question. The test makers do not expect you to be an expert in all kinds of gray areas, so I would not take this question too seriously..."

      This question is actually a superb representation of AAMC trends and testing style and there's a lot you can learn from it. I searched our materials for "37°C" and found the following question, which must be the one you are referring to:

      What differences would be predicted in the fatty acids making up the plasma membrane of a hypothetical bacterium living at 50°C compared to a bacterium living at 37°C?

      AAMC Trends Illustrated By This Question:
      1. This is an example of a common AAMC question blueprint and critical thinking requirement. It requires you to make a prediction about a biological scenario.
      2. Further, it requires you to compare two different biological states and infer or intuit differences between them. Both 1 & 2 are FREQUENT AAMC reasoning blueprints.
      3. Contrary to what you were told, being familiar with 37°C is a logical and fair expectation on the MCAT because it is normal human body temp (98.6°F). The AAMC has a proven track record of expecting recognition/knowledge of a few basic physiological parameters. This is much like needing to know that the blood has a pH of 7.4, or that standard temperature and pressure is 0°C and 1 atm.

      This question doesn't require you to have anything memorized about membrane fluidity at that temperature per se, only to recognize it as normal body temp and compare it to the MUCH higher temp of 50°C. It would be very AAMC-like to ask similar questions. For example, they might reference a thermophile (a bacteria capable of living at VERY high temperatures in places like hot springs) and ask you to intuit something about cholesterol and membrane fluidity in that special case, or compare the cholesterol content of thermophiles to that of normal bacteria.

      All of our questions are tagged in our system with both the PhD author who wrote the question and the PhD author who double-blind peer-reviewed it. This question was written by a tenured professor at a major research university who has written for the AAMC previously and reviewed by another tenured PhD. If you have future questions about Altius materials feel free to PM me or use the @ symbol to get my attention. We are able to submit questions for clarification to the original author of each question.
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