M.Ed., Stanford University
Winner of multiple teaching awards
Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.
Anaerobic respiration is the process of producing cellular energy without oxygen. Anaerobic respiration is a relatively fast reaction and produces 2 ATP, which is far fewer than aerobic respiration. Anaerobic respiration happens in the cytoplasm where glycolysis releases energy from glucose and fermentation recycles NADH back to NAD+.
The metabolic process of respiration which is the release of energy from food comes in 2 distinct flavors anaerobic respiration and aerobic respiration. Anaerobic respiration is that break down of food that does not require oxygen and in fact the name anaerobic refers to that an means without aero means air and what's important in air? Oxygen gas, so this process has got a few advantages over aerobic respiration one you don't need oxygen, two it's very fast. Unfortunately that speed, that simplicity comes at a cost. It comes with a cost that it only produces a net gainer a profit of 2ATP per glucose molecule.
Let's take a look at this diagram here, and see how that works out. Here we see glucose coming into the cell but unfortunately in the beginning stage of glycolsis you actually have to spend energy from the cell, why do that? Well glucose is a 6 carbon molecule by putting a pair of phosphates on either end of that 6 carbon molecule it makes it unstable and easier to break. Additionally by putting those phosphates on either end it makes it negatively charged so it can no longer go out through the glucose channels that the glucose enter through the cell.
Now ultimately you wind up converting 4 of your adenosine diphosphates into 4 ATPs so that gives you 4 you've already spent 2 that's a profit or net gain of 2 ATP now the other molecule that is produced during glycolysis is a molecule called NADH. Where do that come from? There's a special kind of a molecule called NAD positive, NAD positive is what's known as an electron carrier. It's a molecule that can absorb high energy electrons and carry them to some place else. Now for aerobic creatures the NADH is a great molecule because you can use the energy of the high energy electrons in your mitochondria to generate a ton of ATP. But for anaerobic creatures that NADH is, in fact it's a waste it is pointless.
And let's take a look why, if we take a look for them their NADH is useless it can't be used for anything and in fact it means that you no longer have the NAD positive to continue doing glycolysis that's why anaerobic creatures came up with a system called fermentation. Fermentation doesn't generate any energy only glycolysis does an anaerobic respiration but the fermentation allows you to recycle the NAD positive to NADH it has another side benefit because that pyruvate acid that's produced at the end of glycolysis that's actually pretty toxic for the cell. Now there's 2 versions of fermentation, there's other alcohol fermentation and lactate fermentation.
Now let's take a look at the one that produces lactate, because that's the one that your cells do if you ever need to get a lot of energy very, very quickly. Let's say for example you're being chased by a Tiger and you have to sprint you only have enough energy reserved sitting in your muscle cells for say 10 to 15 seconds worth of operation at max speed, but you can't stop after 15 seconds and say please Mr. Tiger time out. Because there's no time out in Tiger hunts so instead your muscles will switch to lactic acid fermentation in order to generate at least some ATPs so you can keep going. If you've ever done a heavy work out you knew this causes problems because while that lactate acid or lactate is less toxic than pyruvate it does have some problems your muscles start to feel sore and they start becoming less and less efficient. So let's see what's happening, so we do glycolysis and we produce the pyruvate but then with the NADH we dumped the high energy electrons that are on NADH back on the pyruvate. It undergoes a few changes and becomes a different 3 carbon molecule called lactate.
That gives us our NAD positive so that we can at least continue doing glycolysis. It's a way of recycling as you can see here the NADH back in the NAD positive. There are some cells like yeast that will do a different form of fermentation called alcoholic fermentation, now what they do is instead of converting the pyruvate into a lactate they convert the pyruvate into ethyl alcohol a 2 carbon molecule and carbon dioxide a gas. And this is basically how people make booze, you just take a bunch of food you mix it together with some bacteria or yeast close it off in a barrel or in a bottle so that no oxygen gets in and they all happily turn to anaerobic respiration and alcoholic fermentation, there'll go, eat all the glucose and pee out a bunch of alcohol.
The carbon dioxide forms the bubbles that you may have noticed before in alcohol that your parents are drinking, so that's it that's anaerobic respiration. Begins with glycolysis and ends with fermentation.