EK Lecture 3 question #240

[Sammy1024]

Passage: Glycerol-based phospholipids arrange themselves into micelles under suitable conditions and serve as the basis for biological membranes. These phospholipids are free to: 1) diffuse along the plane of the bilayer - lateral rotation, 2) alternate between the surfaced (from inner to outer or vice versa) - flip flop, and/or 3) spin on their axis - axis rotation....

240. It can be inferred from the passage that which of the following is a possible movement type associated with proteins found in the plasma membrane?

I. Motion along the plane of the membrane
II. Sinking in and out of the membrane
III. Flip-flop in and out of the membrane

A. I only
B. I and II only
C. II and III only
D. I, II, and III

I thought it would be D, but the answer is B. The explanation says that proteins can't flip flop but I thought that was what the passage was saying when it said "flip flop". Can someone just explain why III is false?

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[DrknoSDN]

I just read the passage and I agree with you. It is a poorly worded passage with insufficient information to eliminate 3 as a possibility.

The answer provided is saying that 1 and 2 are correct because proteins behave similarly to phospholipids (no rationalization for that given), however they specifically reference surface receptors saying they cannot flip-flop. That's true but the question asks if it is possible, and if you either use information provided in the passage (no external knowledge), or just know that some proteins can flip-flop then D would be the most correct answer.

The question should have asked what is a common movement type. Flipping is not very common.

 

[Sammy1024]

Thank you so much for your very detailed answer! :]

 

[Sammy1024]

I thought that phospholipids were a protein? I guess the thing to know is that proteins don't usually flip flop?

 

[DrknoSDN]

I thought that phospholipids were a protein? I guess the thing to know is that proteins don't usually flip flop?

I think proteins are usually described as something translated in the usual mRNA > Protein fashion.

I don't recall ever reading that translation of an mRNA could create something like a lipid. Also the structure and scale is much different, membrane phospholipids usually contain 20-30 carbons in the hydrophobic chain making them about the size of a single digit polypeptide protein. Also proteins are built from the 20 amino acids.

Might need to brush up on cell bio if this isn't sounding remedial. Hope it is. GL

 

[inasensegone]

I thought that phospholipids were a protein? I guess the thing to know is that proteins don't usually flip flop?

No phospholipids are not proteins... They're lipids with a phosphate group attached. Similarly, glycoproteins are proteins with carbohydrate portions attached. Notice the importance of the suffix. What this question seems to be referring to is the basis of the fluid mosaic model. Proteins within the phospholipid bilayer are free to sort of slide around (lateral movement) and depending on their amino acid sequence, they may "sink in and out" but a flat out inversion is highly unlikely. The orientation of the protein is fairly specific to its function. Think about the proteins on our cell membranes that act as receptors. What good would they do if they were inverted to have their active site facing the cytosol? Also, these proteins tend to have a large hydrophilic portion which extends above the surface of the membrane and this would make inversion even less likely due to the repulsion with the hydrophobic core of the bilayer.

I can understand why the phospholipids might be able to flip flop in place on the membrane, because this would not change the function of the phospholipid. Remember, it is a bilayer... There is another phospholipid head facing the inner portion of the cell so it would not be detrimental if a phospholipid whose head was facing the ECF suddenly flipped and is now facing the cytosol. No net change in functionality.

 

[Sammy1024]

So basically this question required outside knowledge. They tell you what phoapholipids can do but the question asks about proteins and while proteins do many of the same things that phospholipids do like glide or whatever they said in the passage, proteins have a special atructure and would not "flip flop" which is why III is incorrect and he answer is B?

 

[inasensegone]

So basically this question required outside knowledge. They tell you what phoapholipids can do but the question asks about proteins and while proteins do many of the same things that phospholipids do like glide or whatever they said in the passage, proteins have a special atructure and would not "flip flop" which is why III is incorrect and he answer is B?

Sort of... but you should be familiar with the structure and function of the membrane and its components anyway, it is pretty important information as it applies to many other topics (resting membrane potential, neurotransmitter receptors, myelination, etc.). This question seems in accordance with something the AAMC might throw at you... they tell you how one molecule works and then ask you how something seemingly related might work. In this case you must use your deductive reasoning to determine that the protein is unlikely to suddenly flip flop as a phospholipid molecule would. Many (maybe even the majority) of the passage questions can be answered based off of outside knowledge... they're sort of like pseudo-discretes.

 

[DrknoSDN]

Sort of... but you should be familiar with the structure and function of the membrane and its components anyway, it is pretty important information as it applies to many other topics (resting membrane potential, neurotransmitter receptors, myelination, etc.). This question seems in accordance with something the AAMC might throw at you... they tell you how one molecule works and then ask you how something seemingly related might work. In this case you must use your deductive reasoning to determine that the protein is unlikely to suddenly flip flop as a phospholipid molecule would. Many (maybe even the majority) of the passage questions can be answered based off of outside knowledge... they're sort of like pseudo-discretes.

The question actually asked "which of the following is a possible movement type?", So I'm going to go out on a limb and say I would have gotten this wrong 10/10 times because it is possible.

Based on basic outside knowledge of biology we know it happens, and there are classes of proteins that DO flip. Additionally from the passage it did not give any reason as to why it would be IMpossible. So I just have to disagree with this answer. It happens and if this stuff comes up on MCAT, that's the reason why people don't score 15's. In reality, just settle for a 13 and move on. =p

 

[inasensegone]

The question actually asked "which of the following is a possible movement type?", So I'm going to go out on a limb and say I would have gotten this wrong 10/10 times because it is possible.

Based on basic outside knowledge of biology we know it happens, and there are classes of proteins that DO flip. Additionally from the passage it did not give any reason as to why it would be IMpossible. So I just have to disagree with this answer. It happens and if this stuff comes up on MCAT, that's the reason why people don't score 15's. In reality, just settle for a 13 and move on. =p

Oh I don't disagree that it very well might happen... that's why I said "...the protein is unlikely to suddenly flip..." but the MCAT doesn't generally test the exceptions. Also, I agree that in previous MCATs at least, the AAMC has sometimes given scientifically incorrect answers. For example I took issue with one of their questions (in FL 4) that said that the bacteria in our colon provide us with vitamin B-12. They don't. Anyway, I would just remember the rules, not the exceptions. The exceptions will serve you better in medical school, not so much the MCAT. If you look through the AAMC FL CBT threads, you will find other examples where the AAMC could be faulted... but it won't do you any good to fight their reasoning, even if it is wrong 😉

 

[mcloaf]

The question actually asked "which of the following is a possible movement type?", So I'm going to go out on a limb and say I would have gotten this wrong 10/10 times because it is possible.

Based on basic outside knowledge of biology we know it happens, and there are classes of proteins that DO flip. Additionally from the passage it did not give any reason as to why it would be IMpossible. So I just have to disagree with this answer. It happens and if this stuff comes up on MCAT, that's the reason why people don't score 15's. In reality, just settle for a 13 and move on. =p

You're really over thinking this. All they are asking you to know is that transmembrane proteins aren't spontaneously flipping between membrane leaflets. Thinking about MCAT questions in more depth than that is rarely, if ever, a worthwhile use of your time or beneficial to your score.