Oxidation and Reduction

[MedPR]

I get that reduction is gain of electrons, and oxidation is loss of electrons. I also get that the reducing agent is oxidized and the oxidizing agent is reduced. I have serious trouble applying these to physiological reactions. I'm not talking about HCl + NaOH or other straightforward chem reactions, I'm talking about things like glycolysis, ATP->ADP, etc.

Can someone give me some typical physiological redox reactions that are likely to be on the MCAT?

Is glycolysis the oxidation of glucose to pyruvate? Or is it reduction of glucose to pyruvate? I'm pretty sure I've read it both ways.

I know oxidation doesn't necessarily involve oxygen, but is it valid to say that glycolysis is a reduction reaction because it is anaerobic?

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[MT Headed]

In biology, oxidation/reduction is the loss/gain of electrons.
In gen chem, oxidation/reduction is the increase/decrease of the oxidation number.
In biochem and ochem, oxidation is the insertion of an O or removal of 2 H's, and reduction is the deletion of an O or the addition of 2 H's.

These are all trying to say the same thing.

Usually in a biochemistry setting, the carbons are analyzed by what they are attached to.
A carbon starts off with a score of 0.
Every time it is attached to another carbon, that's a tie, so the score doesn't change.
Every time it has a bond to a hydrogen, well carbon is more electronegative so its score decreases by one.
Every time it has a bond to an oxygen, carbon is the loser in that tug of war so its score increases by one. A double bond to oxygen increases its score by two.

An entire process, or the individual steps of a process can be declared an oxidation or a reduction by analyzing what happens to those carbons.

Specifically, glycolysis is an oxidation of glucose to pyruvate. How do I know this?

Clue #1: Glycolysis is an energy harvesting step, not an energy storing step. Oxidation is harvesting, reduction is storage.

Clue #2: The reaction, in addition to producing ATP from ADP and Pi, also creates NADH,H+ from NAD+. This is obviously a reduction, because two H's (and their electrons) got added to NAD+. Well, if NAD+ was reduced, I know something got oxidized.

Clue #3: The oxidation number of all those carbons in glucose adds up to zero. Yeah the aldehyde side has an extra bond to an oxygen (+1), but the alcohol terminal has an extra hydrogen (-1), so the whole molecule adds up to 0. But now look at the resulting molecule, pyruvate. The CH3 carbon is (-3). The C=O carbon is (+2). The acid carbon is (+3). Wow, that's a total of +2! Somebody's oxidation number went up... must be an oxidation going on here.

 

[Ssina]

C6H12O6 + 6O2 --> 6CO2 + 6H2O

so you know that glucose is eventually oxidized to CO2 (C: 0-->+4) and Oxygen is reduced to water (O: 0-->-2)... so through all the steps glucose is being oxidized... this is catabolic reactions, so stuff are broken down (as oppose to anabolic that bonds are reduced, essentially storing potential energy in the chemical bonds to be released later upon oxidation later by the cell).

Now cytoplasm is a reducing environment, but glycolysis is the only step that is oxidizing glucose to pyruvate, while the rest of the oxidation happens inside the mitochondria.

So in glycolysis:
Glucose is oxidized through bunch of steps to pyruvate, and in the process ADP is phosphorylated to ATP and NAD+ is reduced to NADH (essentially gaining H-, which is reduction from OIL RIG, and since glucose is oxidized something else has to be reduced)... NADH is essentially electron carrier till it gets to ETC... and you know the rest.

If you look at one of the steps that is making NADH, e.g. GADP to 1,3BPG, GADP has an aldehyde group that is essentially oxidized to carboxylic acid (although it's really COOPhosphate, but you get the idea) so now NAD+ is reduced (gains the H-) and forms NADH.

so by the end, 6C glucose is broken down to 3C, which then through PDC into a 2C unit that is fed through krebs. In krebs, acetyl coAs are again oxidized to CO2 and NAD+ and FAD are reduced to NADH and FADH2 and GDP is phoshphorylated to GTP.

Then NADH and FADH2, which are the reduced form, get to the ETC... they are electron carriers and are oxidized to NAD+ and FAD... giving off their electron and a H+... H+ is pumped into the intermembrane and the electrons are passed through electron the complexes till it gets to O2 + 2e- +2H+ --> H2O

Inside each complex, there is a redox reaction where NADH originally gives it's electrons to e.g. FMN that is reduced to FMNH2 (I think?!)... then cycle's back with FeS... and essentially passing electrons down.

I hope at least I made some sens, and this is how I know it not much more details than this... but I'm sure you can fill in between.

EDIT: MT HEADED beat me to it!