gel electrophoresis clarification

[Oh_Gee]

so in TBR CHEMII, I read
"Biochemists focus on the flow of ions through electrical fields (gel electrophoresis occurs between the charged plates of a capacitor). They assign charges to the anode and cathode of a charged capacitor according to the type of ions they attract. Anions migrate to the anode because of the anode's positive charge
build-up (the anode has lost electrons). A biochemist concludes that the anode of a capacitor carries a positive charge (attracting anionic molecules). Cations migrate towards the cathode, because of the cathode's negative charge build-up (the cathode has gained electrons). A biochemist concludes that the cathode of a capacitor carries a negative charge (attracting cationic molecules)."

In gel electrophoresis, isn't the black wire above the red wire meaning the anode is up and the cathode is down? If DNA/proteins(after SDS) are negatively charged, why does TBR say anions migrate toward the anode?

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[Blueprint MCAT Tutor]

You've got to separate out two concepts here that get confused all the time.

First, anions migrate to the anode.

Forever and ever to death 'til us part. Anions migrate to the anode.

That is the very definition of what the anode is. "The thing the anions migrate towards [or conversely that cations migrate away from]"

Okay so lock that in. For the MCAT, that's an unshakeable rule.

Now, what about "which electrode is positive and which electrode is negative?"

Here's the weird part: that's an entirely separate question from which is the anode. So anions migrate to the anode, but then the anode could be positive or negative.

The mnemonic I use here is G.A.I.N. = galvanic anode is negative

So just remember those two separate facts:

GAIN and "anions migrate to the anode".

If you're answering any question on the MCAT don't break either of those rules. They can create some counter-intuitive situations in your mind but this is one of those places where our intuitions can end up just hurting us.

Now I'm sure someone with a masters in electrochemistry can come in here and be horrified by my brute generalizations, but they get the job done on the MCAT

 

[Oh_Gee]

so in a gel tank, the anode is actually on the "bottom" (red cable)?

 

[Cawolf]

Yes.

The bottom electrode with the red/positive wire is the anode.