pnoybballin

Junior Member
10+ Year Member
5+ Year Member
Nov 29, 2005
96
0
Status
Pre-Medical
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?
 

wait4me

10+ Year Member
Apr 1, 2009
104
1
Status
Pre-Medical
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

10+ Year Member
Feb 24, 2009
575
0
Status
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
 

thebillsfan

Unseasoned Veteran
10+ Year Member
5+ Year Member
Dec 22, 2008
778
0
Status
Pre-Medical
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

10+ Year Member
Jan 9, 2008
106
7
Status
Pre-Medical
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

5+ Year Member
Apr 23, 2009
198
2
Status
Resident [Any Field]
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

5+ Year Member
Apr 23, 2009
198
2
Status
Resident [Any Field]
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

10+ Year Member
Feb 24, 2009
575
0
Status
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

5+ Year Member
Apr 23, 2009
198
2
Status
Resident [Any Field]
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

10+ Year Member
Sep 5, 2008
511
1
Status
Medical Student
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??:confused: Based on the disscusions above, it seems so.

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

inaccensa

10+ Year Member
Sep 5, 2008
511
1
Status
Medical Student
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

Removed
Jul 7, 2009
596
2
SoCal
Status
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?
Thank you hemi, I swear, they should have a "how to wikipedia" section on the MCAT.
 

ThirdEye

10+ Year Member
Apr 14, 2009
362
6
Status
Pre-Medical
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" (+)