Gel Electrophoresis Question

[pnoybballin]

Protein molecules run during gel electrophoresis have a net:

A. negative charge, and hence migrate to the anode.
B. negative charge, and hence migrate to the cathode.
C. positive charge, and hence migrate to the anode.
D. positive charge, and hence migrate to the cathode.

The answer is B. But shouldn't it be A!? I thought the anode would be the (+) end?

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[dragon529]

should be anode...

 

[sv3]

nm

 

[wait4me]

What?? i thought cathode was the + end?? Dont the negatively charged particles move to the positive end?

Protein molecules run during gel electrophoresis have a net:

A. negative charge, and hence migrate to the anode.
B. negative charge, and hence migrate to the cathode.
C. positive charge, and hence migrate to the anode.
D. positive charge, and hence migrate to the cathode.

The answer is B. But shouldn't it be A!? I thought the anode would be the (+) end?

 

[sv3]

anode and cathode signs are switched when its an electrolytic cell like gel electro is. Wish i knew more though as I know nothing about lab techniques.......kills if your a non trad

 

[Hemichordate]

Yup, should be anode.

http://en.wikipedia.org/wiki/Gel_electrophoresis

 

[thebillsfan]

okay so gel electro is an electrolytic cell. cathode accepts CATIONS in a galvanic cell, so that means it accepts anions in an electrolytic cell? but I thought the cathode was considered positive in an galvanic cell and negative in an electrolytic cell?

so confused.

also, why are proteins considered to be negative? cant they be positive?

 

[raab32]

also, why are proteins considered to be negative? cant they be positive?

When proteins are folded in their native conformations, they can possess whatever variety of charge and acidity/basicity. But for electrophoresis, proteins are denatured with SDS/detergent and this process basically gives them an overall negative charge, which allows for migration to the positive terminal.

 

[bakanoisha]

okay so gel electro is an electrolytic cell. cathode accepts CATIONS in a galvanic cell, so that means it accepts anions in an electrolytic cell? but I thought the cathode was considered positive in an galvanic cell and negative in an electrolytic cell?

so confused.

also, why are proteins considered to be negative? cant they be positive?

It's been a while since I did this (1yr 3 months since the MCAT!) so forgive me if my details get mixed up, but I'm pretty sure this is an easy way to go about remembering the info: Just remember the spontaneous galvanic cell. Then remember that the electrolytic cell runs backward b/c a current is being forced through it.

 

[bakanoisha]

When proteins are folded in their native conformations, they can possess whatever variety of charge and acidity/basicity. But for electrophoresis, proteins are denatured with SDS/detergent and this process basically gives them an overall negative charge, which allows for migration to the positive terminal.

Proteins are negatively charged in general, without detergents. I think the SDS/detergent are just to essentially standardize the shape of the proteins since conformation can haev a large effect on how fast the protein can move through the gel matrix. This way you can accurated determine protein size via electrophoresis without having to consider conformation too much.

 

[sv3]

Proteins are negatively charged in general, without detergents. I think the SDS/detergent are just to essentially standardize the shape of the proteins since conformation can haev a large effect on how fast the protein can move through the gel matrix. This way you can accurated determine protein size via electrophoresis without having to consider conformation too much.

is knowledge of lab techniques needed for success on the MCAT? I'm a non trad using TPR and Ek and none of them really go over this stuff. Ek mentions some methods at a very high level and i've done some research on my own but curious if i need to spend more time here? For example, should i have known SDS page makes things negative?

thanks

 

[bakanoisha]

I'd spend more time learning the techniques, not necessarily the exact chemicals in them. Those are pretty fundamental lab techniques, but the concepts are the more important thing. And, I just read that SDS does in fact increase the negativity of the (already negative) protein. I wouldn't worry about knowing that SDS does that, but it's useful to know proteins are negatively charged.

Usually the MCAT provides you details in a passage that require you to put large concepts to use regarding that specific instance. So, in other words, if you know the concept, you can usually answer all of the questions.

 

[inaccensa]

Protein molecules run during gel electrophoresis have a net:

A. negative charge, and hence migrate to the anode.
B. negative charge, and hence migrate to the cathode.
C. positive charge, and hence migrate to the anode.
D. positive charge, and hence migrate to the cathode.

The answer is B. But shouldn't it be A!? I thought the anode would be the (+) end?

Although anode is positive, but it is still performing oxidation, loosing electrons. I'm really confused with galvanic and electrolytic cells. Galvanic are spontaneous and anode is negative and cathode is positive attracting anions and cations respectively. Electrolytic cells are nonspontaneous. Anode is positive and cathode is negative, but is anode now attracting cations and cathode attracting anions?? Based on the disscusions above, it seems so.

Never mind. I got it. DNA, proteins are -vely charged and migrate to anode!!

 

[inaccensa]

the cathode attracts cations because thats where reduction takes place, and the anode attracts anions because thats where oxidation takes place...

in a galvanic cell, yes the anode is negative. But when you're trying to run a gel, for like proteins or DNA, you're making an electrolytic cell, in which the anode is actually positive. Therefore, the negative DNA will move toward the anode

This was a reply in another post by Doodl3s. Anyone want to refute this? There are contradicting answers.

 

[ChemEngSoonMD]

Protein molecules run during gel electrophoresis have a net:

A. negative charge, and hence migrate to the anode.
B. negative charge, and hence migrate to the cathode.
C. positive charge, and hence migrate to the anode.
D. positive charge, and hence migrate to the cathode.

The answer is B. But shouldn't it be A!? I thought the anode would be the (+) end?

Yup, should be anode.

http://en.wikipedia.org/wiki/Gel_electrophoresis

Thank you hemi, I swear, they should have a "how to wikipedia" section on the MCAT.

 

[ThirdEye]

Here's my take on it.

Electrochemists like to confuse people.

What never changes is that oxidation occurs at the anode and reduction occurs at the cathode. Only the signs flip when you add a power supply.
The best way I've found to memorize this is that when you have the power supply, elecrons are being pulled out off the anode making it positive instead of negative. Thus, the anode is an extension of the positve end of the battery, and the cathode is an extension of the negative end of the battery.

The answer should be A.

And remember when doing a gel. It "Runs to Red" (+)