#19 - Light is registered in the retina when photons hit

[Jeff Wren]

Hi Dancingbambarina,

I'm not sure that I entirely follow your question.

As far as I can tell, the word "normal" is being used to describe "standard or typical" molecular motion. "Normal" is not referring to the normal amount of molecular motion or to the variability, it is simply referring to the typical molecular motion that happens in the retina.

In the context of the sentence in which it appears, the key distinction is that the change in molecular shape from this normal molecular motion is different from the change in molecular shape that happens when photons hit the rhodopsin molecules and that this change from the normal molecular motion creates visual errors.

The final sentence of the stimulus states that the amount of this normal molecular motion is directly proportional to the temperature of the retina, meaning that as the temperature increases, so does the amount of normal molecular motion.

To solve the question, you simply need to link the temperature to the normal molecular motion and then the motion to the visual errors and find an answer that links these.

[Dancingbambarina]

Would you say molecular motion CAUSES Rhodopsin molecules to sometimes change shape? Would the "sometimes" make it a probable cause?

[Dave Killoran]

They are clearly allowing for different causes. The first line makes it clear that light changes the shape: "Light is registered in the retina when photons hit molecules of the pigment rhodopsin and change the molecules' shape."

Then the second line says that even without light, "rhodopsin molecules sometimes change shape because of normal molecular motion." So yes, the motion is sometimes making it happen as part of the normal course of operations. I don't think to myself this is a "probable" cause because I don't know how often it happens or at what volume--there's no need to classify it as such. I just know it can happen, which is good enough.

Thanks!

[Dancingbambarina]

They are clearly allowing for different causes. The first line makes it clear that light changes the shape: "Light is registered in the retina when photons hit molecules of the pigment rhodopsin and change the molecules' shape."

Then the second line says that even without light, "rhodopsin molecules sometimes change shape because of normal molecular motion." So yes, the motion is sometimes making it happen as part of the normal course of operations. I don't think to myself this is a "probable" cause because I don't know how often it happens or at what volume--there's no need to classify it as such. I just know it can happen, which is good enough.

Thanks!

Thanks for your help Dave,

I have managed to jot down EXACTLY why this question was problematic for me. Do you have clarification on the following?

So at the end, a correlation is established. I have no problem with that. Where uncertainty creeps in is the assumed correlation between "normal" molecular motion and the "sometimes" changing shape.

I have a MAJOR problem here:


Who says that due to molecules changing shape "sometimes", they would somehow change shape more frequently when the requirement is increased? Surely we don't know this because the "sometimes" could depend on the requirement (motion) being NORMAL motion. NOT increased or extreme or dynamic motion. The necessary requirement here is NORMAL MOTION.

How Question #19's correct answer makes me think of this is that there are many assumptions going on here that simply implausible.

"Normal motion" assumed to stay normal when increased
"Sometimes" assumed to be increased to a lot more of the time with more "normal" motion

Thanks very much