BR Bio, Section 3, Passage 9, Question 56

[sillyjoe]

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A student is purifying an enzyme in the laboratory by overnight dialysis. The student tests for successful dialysis using an enzyme activity assay. What would the student find if the dialysis tubing was punctured with a pin before the experiment?

A. No enzyme activity in the bag.
B. Increased enzyme activity in the bag compared to the activity measured before the experiment.
C. There is no effect caused by the pinhole, and the enzyme activity in the bag is the same as before.
D. Slightly decreased enzyme activity in the bag compared to the activity measured before the experiment.

A is correct, no enzyme activity in the bag. Although enzymes are large compared to atoms, they are small compared to a pinhole. This is why you test your dialysis tubing for holes before you begin enzyme purification. Since there is no way the amount of enzyme can increase, choice B is incorrect. Since there is an effect caused by the pinhole, choice C is in correct. Choice D is in correct since the enzyme would all (or almost all) move out during an overnight dialysis. The correct choice is A.


How are you supposed to differentiate between choices A and D with a scenario like this. The passage gives no indication of what would happen so how are you supposed to be able to discern the variables that this question would entail without prior knowledge or specific knowledge of the "experiment" i.e. concentration, rate of diffusion, etc. ?

 

[Alucard6]

Answer choices A and D are pretty distinct. Essentially the answer choices go like this:

A.) No enzyme
B.) Increased effect
C.) No Effect
D.) Decreased effect

If you think about it, poking a hole in a bag with really small materials will just let it all slip away. You can think about it like water, even a small hole will eventually make it so all the material flushes out. Without any enzyme, everything leaves the bag and there is nothing to be seen.

Even if something DID happen before the enzyme left the bag, the product would also flush out of the bag anyway. So even if the reaction went to completion, the bag would be empty. Thus, the variables you talk about like "concentration" and "diffusion" do not play a factor if they will not even be present the next day.

 

[el_duderino]

Wait. Why would there be NO enzyme in the bag? Wouldn't the bag have some enzyme left?

 

[Alucard6]

I assume the pinhole is enough to let everything escape from the bag. It is not exactly the greatest question because it is vague, but I feel like if anything A is the best answer.

 

[DrknoSDN]

This is probably not representative of the type of answer choices found on AAMC MCATs.

Unless the passage explicitly said the dialysis process removed ALL solutes able to pass through the membrane tubing. Then you could assume that all enzyme was removed too.
It would be more likely to give you a data table and have you determine the answer based on measured values.

 

[sillyjoe]

Unless the passage explicitly said the dialysis process removed ALL solutes able to pass through the membrane tubing. Then you could assume that all enzyme was removed too.
It would be more likely to give you a data table and have you determine the answer based on measured values.

Can you please explain what you mean here?

 

[el_duderino]

Not to speak for DrknoSDN, but I think he means that the question is a bit ambiguous and not well-worded. I find that this is typical of the "Third party" questions and you don't find something like this on the MCAT itself.

 

[sillyjoe]

Not to speak for DrknoSDN, but I think he means that the question is a bit ambiguous and not well-worded. I find that this is typical of the "Third party" questions and you don't find something like this on the MCAT itself.

Thank you, I meant clarification on the specific conditions he stated that about the dialysis tubing

 

[DrknoSDN]

I just meant that if you encounter a problem like this on the MCAT it will probably tell you what the expected "normal" results for the dialysis were. And if the expected normal result was to remove everything except the enzyme, then puncturing the tubing would remove everything and the enzyme so there would be zero enzyme left.

In general jonnythan summed up what I was getting at. If you see something like this on the MCAT, it will be presented differently in a way that can clearly be answered without having to make assumptions.

 

[el_duderino]

If there's a hole in the tubing, then over time the concentration of everything in the solutions will slowly equalize. I guess the implication in the question is that the volume of solvent or whatever outside the tubing is so much larger than the volume in the tubing, the concentration of enzyme inside the tubing will drop so low as to be almost undetectable.

 

[DrknoSDN]

To clarify why "No enzyme activity in the bag" is indicative of 3rd party test prep style questions, if you look at concentrations of undesirable solute removal (Urea etc) using dialysis most of the time the simplified formula follows an inverse log function so there will never be zero enzyme activity in the bag. Jonnythan is right that you need to make assumptions about the dialysis process/efficiency to arrive at the given answer of A.

This type of answer choice (especially with choice D also available) seems too ambiguous to appear on an AAMC exam the way it is worded.