NextStep FL NS 3 C/P #20

[jeep1010]

A student theorizes that the differences in ATP-to-ADP ratios between cells detected in the experiment could be due to variations in Y32 expression rather than differences in the cells’ metabolic conditions. Does information presented in the passage support this possibility?

C.

No, the shape of the fluorescence excitation spectrum of Y32 is influenced by the bound state.

D.

No, the spectral ratio intrinsically normalizes for the amount of biosensor.

I don't understand the reasoning behind D after reading the answer choices, can someone help to clarify? I thought that the bound/unbound state of ATP/ADP are what dictate the fluorescence. I do not understand the reasoning for answer choice D. Thanks!

@NextStepTutor_2 @NextStepTutor_3

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[NextStepTutor_3]

This is a tricky one! You are entirely correct in that the bound/unbound state of ATP and ADP is what dictates the particular fluorescence pattern observed. The difficult part is to figure out what the question is actually asking. It's very helpful to simplify complex question stems in your head before attacking the answer choices. Let me break it down:

"A student theorizes that the differences in ATP-to-ADP ratios between cells detected in the experiment could be due to variations in Y32 expression rather than differences in the cells’ metabolic conditions."

...is simply saying "A student believes that the results observed in the experiment could be due to the amount of Y32 expressed, rather than to actual differences in the amounts of ATP or ADP bound."

So now we just need to decide whether the student is correct. Could the results simply vary due to the amount of Y32 expressed? In other words, would it make a difference whether a cell expresses a lot of the Y32 biosensor or only a little bit? The answer is no, because we're measuring a ratio (the ATP/ADP ratio, to be exact). If we have a cell that expresses a very large amount of Y32 biosensor, we'd expect it to bind more ATP than a cell that only expresses a small amount of Y32. But it would also bind more ADP, causing the ratio to be exactly the same. This is what choice D means when it says the "ratio intrinsically normalizes for the amount of biosensor."

If that's still confusing, think about it in terms of numbers. Let's say that Cell A expresses twice as much Y32 as cell B. Cell A, then, might bind 500 ATP molecules and 250 ADP molecules. Cell B, since it has less Y32 biosensor available, might only bind 250 ATP molecules and 125 ATP molecules. Regardless of the amount of Y32, the ratio doesn't change. That's what is meant by "normalizing" for the amount of biosensor.

Looking at option C (which is tempting), we now see that it's incorrect simply because it doesn't answer the question. Yes, the shape of the curve is influenced by the bound state, but how does that statement explain why the amount of Y32 expression has no effect on the results? It's a classic case of a "true statement that doesn't answer the question" - one of the hardest types of wrong answers to eliminate.

Let me know anytime if you need further clarification