Re-Cap of Biology III

Hi all,

This email is a bit of a monster, but it’s got a lot of great stuff. 

Remember that MSCT III will be on Monday, December 13 from 6 pm-9 pm (slash whenever we decide to pack up and go home; as late as you guys want!). 

I need to correct something I said incorrectly in class.  When I referred to T3 and T4, I called T3 "triiodothyroxine"; the actual name is triiodothyronine.  T4 is called thyroxine (although chemists or endocrinologists could refer to it as tetraiodothyronine too).  The "3" and "4" in the hormone's names, by the way, refer to the number of iodine atoms in the molecule.

Let’s take a look at your homework for MSCT III and what to expect:

• You MUST complete Full-Length 2 before our last teaching session.  Some of you neglected to finish Full-Length 1 before MSCT II; do NOT make that same mistake before this session.  Schedule time between now and our next class to sit and take the full-length test, no “ifs,” “ands,” or “buts”!
• I will discuss the medical school application process after the Lesson Book portion of the class, in particular discussing the timeline of the process, what to expect, and my two biggest pieces of advice.  You don’t have to stay for this, but I strongly encourage you to.  There are a LOT of misconceptions out there about the process, and – trust me – you want the honest story.
• After discussing the medical school application process, I’ll stay to discuss any content questions you all have.  Common topics in the past have included inclined planes, the oxyhemoglobin curve, and electrochemical cells.  I will stay as long as any of you would like to stay; we can order in pizza if we need to!  But use this opportunity to leave the class on a high note – having had a chance to ask any questions you have!

Helpful Hints:

• It can not be stressed enough how important a strong understanding of molecular genetics is for success in the Biological Sciences section on Test Day.  Be sure to spend sufficient time reviewing the key concepts and processes of DNA and RNA which can be found in the review notes and in a comprehensive (and difficult) workshop.  Learn to apply your understanding of molecular genetics to MCAT passages and questions by working through the molecular genetics topical test listed above.
• The key principles of the Endocrine system include:  (1) the hierarchy of command (the hypothalamus is the master gland, the anterior pituitary is next in line, then all the other endocrine glands); (2) hormones – different classes, solubilities, modes, onset and duration of action.
• Now that we have completed our content review lessons, continue to plug away at the full-length practice tests.  If you have not completed all of the required homework, be sure to do so to maintain your HSG eligibility.  Remember, the required homework only includes the online workshops and quizzes, the review notes (and the review note end-of-chapter review questions) and the five scheduled full-length practice tests.

Biology III in review:

• Homeostasis
  • The major example of homeostasis we discussed was the kidney’s role in osmoregulation.  In class we discussed the role of the vasa recta in maintaining the renal hyperosmotic gradient, but we didn’t diagram it (I describe the vasa recta below – they’re the capillary bed that exchanges materials with the filtrate in the nephron).  It’s not easy to find great diagrams, but I’ve spliced together two figures from two different textbooks into the image called “osmoregulation”.
  • Figure 26-3 depicts the relevant anatomy.  The entire meshwork of peritubular capillaries is referred to as the vasa recta.  One unique feature of the renal portal system is that it features two capillary systems connected in series: the glomerular capillaries are connected in series to the vasa recta via the efferent arteriole.  Recall that the efferent arteriole is the site where reabsorption from the proximal convoluted tubule will occur, i.e. this is the site where we reabsorb salt (approximately 65% of the filtered load), and 100% of the glucose, amino acids and vitamins that got shoved into the lumen through the glomerulus (water comes along for the ride into the efferent arteriole to maintain isotonicity). 
  • Figure 20-10 zooms in on the relationship between the loop of Henle and the vasa recta.  The U-shape of the vasa recta is critical: the descending limb of the vasa recta reabsorbs salt (approx 25% of the filtered load) and loses some water to match the increasing renal gradient as it descends deeper into the medulla.  The ascending limb of the vasa recta needs to become more dilute, and it primarily does so by reabsorbing water as shown in the diagram (a little salt is lost).  Notice that water is coming from two sources: the descending limb of vasa recta and the descending limb of loop of Henle.  This last point is key:  the renal hyperosmotic gradient would be diluted when water is lost from the descending limb of the loop of Henle, but the water that leaves the lumen of the descending limb of the loop of Henle is picked up by the vasa recta.  In short, the U-shape of the vasa recta facilitates the maintenance of the hyperosmotic renal gradient.
  • To help provide greater context on this issue I’ve created a few more hybrid images (see attachments).  The captions are fairly self-explanatory but I’ll add a few comments:
    • Body Fluids image: these figures succinctly summarize the ideal steady-state of human bodily fluids.  Of course you are not responsible for any of the numbers; the purpose of these images is to help you understand the greater context of renal physiology.
    • Filtration image: Figure 26-8 is a nice cartoon that represents the activities of the kidney.  Figure 26-12 focuses in on the Starling Forces at play at the glomerulus.  As we discussed in class, there’s a pressure gradient that forces fluid out of the capillary and into the nephron.  These numbers merely specify the magnitude of that pressure.  You are not responsible for the numbers, but do keep in mind that the glomerular filtration rate (GFR) is a function of the net filtration pressure so if you are presented with a scenario where the net filtration pressure is decreased (e.g. via kidney stone raising Bowman’s capsule pressure) then you should be able to predict GFR will decrease.  On the other hand, if net filtration pressure is increased (e.g. hypertension) then GFR will increase.
    • Reabsorption image: these two figures illustrate the mechanisms behind reabsorption in the PCT.  Unfortunately neither figure depicts the flow of water, but don’t forget that water will be following the solutes to maintain isotonicity.  One useful aspect of this image is its depiction of the brush border, tight junctions, and the basement membrane.  Many introductory biology courses fail to review this type of cellular terminology, but we’ve seen the MCAT throw out these phrases before so if you’re unfamiliar with the terminology this image helps you get a visual on it.  As an FYI, the “apical surface” refers to the luminal side of the epithelial cell while the “basolateral surface” refers to the side of the epithelial cell exposed to the interstitial fluid.
• Hormones
  • We discussed nearly all of the hormones you need to know for the test, with a particular focus on the menstrual cycle.  Make sure to know what’s made when and where, and what it does PLUS make sure to know the menstrual cycle.
• Transcription and Translation
  • This video is fantastic at illustrating the process of protein synthesis.  I’m pretty sure these people are on LSD, but it’s still a surprisingly good illustration (considering how old it is) of the process.  It starts getting good at 3:10.

To reinforce the concepts we discussed in Biology III complete the following:

• Digestive & Excretory Systems Test 1
  • Comment:  The first passage discusses how the kidney regulates the concentration of H⁺.  The second passage relates to our passage in MSCT II of how saturated fats are packaged into chylomicrons (aka lipoproteins) for transport.  You’ll probably like this passage as it relates cholesterol to LDL and HDL.  This passage also falls under the category of “don’t have to know it for the exam, but it’s good to know.”
• Reproductive System and Development Test 1
  • Comment: solid test.  One of the passages talks about anti-Müllerian hormone, which may become a standard test for fertility in the near future. 
• Endocrine Test 1
  • Comment: another solid test.  The second passage talks about parathyroid hormone, and this is a good test to ensure you understand how the negative feedback loops regulate the concentration of calcium ion in the blood.  By the way, it’s easy to get confused and pick an opposite answer on this stuff so don’t get down on yourself if that happens.
• Respiratory & Skin Systems Test 1
  • Comment: we didn’t discuss these systems in depth, but both could be the main concept of an MCAT passage.  Make sure you know all the terms (tidal volume, etc.), and use these questions to learn more about these systems.  When you take the AAMC PTs keep an eye out for how they test these concepts as well.
• Molecular Biology Test 1
  • Comment: Fantastic topical test.  All high-yield.
• Molecular Genetics Test 1

That was quite the email!   Let me know if you have any questions, and get ready for MSCT III!