Fermentation etc

Other Carbon Sources
Obviously, you don't just eat sugar. Where would other macromolecules feed into your metabolism. Well, it can get complicated.
  • Fats…the long-chain fatty acids can be separated 2 carbons at a time (attached to coenzyme A) and fed into the citric acid cycle. The glycerol can be converted to pyruvate.
  • Carbohydrates other than glucose get modified and fit into glycolysis somewhere.
  • Amino acids…the nitrogen is often just excreted (in urine, for us). But, the rest of the molecules are contain carbon, hydrogen and oxygen and can be fed into the citric acid cycle after several steps found on the Big Scary Chart.


As usual, I’ve taken all these images from Wikipedia…because I can.
We have been discussing how aerobic (in the presence of O2) respiration works. Importantly, there are some odd organism that use other electron acceptors when there is no oxygen. There are some iron and sulfur compounds used by some odd bacteria in anaerobic (no oxygen) conditions. Their electron transport systems are a little different, but the concept is the same. All eukaryotes use Oxygen as the electron acceptor if they use mitochondria at all. However, Archaea and Bacteria are far more versatile.

The problem: running out of NAD+

If Eukaryotes have no oxygen, then what? Well, we can still run glycolysis to obtain ATP by breaking down glucose and other molecules. It’s not very efficient and generates only small amounts of ATP (though it is rapid). But, there is a bigger problem: what to do with the NADH?
Glycolysis uses NAD
+ as an oxidizing agent, generating the reduced form: NADH + H+. That’s a good thing if there is oxygen around because we transport that to the mitochondria and use it to feed electrons into the ETS (which oxidizes it and generates the NAD+ again). If there is no oxygen, we run out of NAD+ and we cannot even continue glycolysis. We also have a build up of pyruvate we really cannot use.
We “solve” these problems by reducing pyruvate to lactic acid, using NADH as the reducing agent. That regenerates the NAD
What follows is far more detail than you would be expected to know. I will bold the important stuff.
This is pyruvic acid, (since the proton is still attached to the carboxyl in this diagram, it’s named as the acid):
Pyruvic-acid-3D-balls. “Pyruvate” is the name for the form that has lost it’s H+, also known as the conjugate base. It looks like this, with a negative charge on the oxygens on the right. Along with some ATP and NADH, this is the product of glycolysis. We need to convert the NADH back to NAD+. For that, we ferment the pyruvate to Lactate.
This is Lactic acid: 753px-Lactic-acid-3D-balls. Notice that the middle carbon was a carbonyl in pyruvate and now is a hydroxyl. That’s where the reduction is taking place. The electron to cause that came from NADH, which regenerated the NAD+ for glycolysis.
Unfortunately for you, the lactate really makes your muscles hurt and not work so well.
Another organism that does this form of fermentation are “Lactobacilli.” These are the strains of bacteria that make yogurt.

Beer, Bread and Wine:

But, there is still another form of fermentation that does one additional step. While there are other organisms that do this form of fermentation, by far the most successful are strains of yeast. They are successful because what they end up making is very popular among humans. The first step in this fermentation is to remove that carboxyl group from the end of pyruvate. This releases CO2 as a gas (good to make your bread rise).
You are then left with a toxic and bad-smelling molecule called acetaldehyde:
You really want to get rid of this. It gets reduced to ethanol (as before, the carbonyl is converted to a hydroxyl.
Ethanol-3D-balls and NADH is oxidized back to NAD+. The widespread use of yeast to carry out this reaction on sugars from various grains or fruit has given us bread (the CO2 is what causes bread to rise), beer and wine (the alcohol is very popular in some circles). As a side benefit, the yeast is able to make a wide range of nutrients that humans need, notably B vitamins. Bread made this way has a lot more nutritive value than the starting flour. Beer also has a lot of nutrients.
Although it does not portray the mitochondrial
outer membrane, this is a good general diagram of what we have been studying:


A couple of names: “Proton motive force” is a name given to the electrochemical gradient of protons that drives the ATP synthase. Similarly “chemiosmosis” is another name for the protons driving the ATP synthase. Electron Transport and Chemiosmosis together comprise "Oxidative phosphorylation."