What molecule does fermentation replenish that is necessary for glycolysis to occur?

This article relies largely or entirely on a single source. Relevant discussion may be found on the talk page. Please help improve this article by introducing citations to additional sources.
Find sources: "Anaerobic glycolysis" – news · newspapers · books · scholar · JSTOR (June 2021)

Anaerobic glycolysis is the transformation of glucose to lactate when limited amounts of oxygen (O2) are available.[1] Anaerobic glycolysis is only an effective means of energy production during short, intense exercise,[1] providing energy for a period ranging from 10 seconds to 2 minutes. This is much faster than aerobic metabolism.[2] The anaerobic glycolysis (lactic acid) system is dominant from about 10–30 seconds during a maximal effort. It replenishes very quickly over this period and produces 2 ATP molecules per glucose molecule,[3] or about 5% of glucose's energy potential (38 ATP molecules).[4][5] The speed at which ATP is produced is about 100 times that of oxidative phosphorylation.[1]

Anaerobic glycolysis is thought to have been the primary means of energy production in earlier organisms before oxygen was at high concentration in the atmosphere and thus would represent a more ancient form of energy production in cells.

In mammals, lactate can be transformed by the liver back into glucose using the Cori cycle.

Fates of pyruvate under anaerobic conditions:

Pyruvate is the terminal electron acceptor in lactic acid fermentation
When sufficient oxygen is not present in the muscle cells for further oxidation of pyruvate and NADH produced in glycolysis, NAD+ is regenerated from NADH by reduction of pyruvate to lactate.[4] Lactate is converted to pyruvate by the enzyme lactate dehydrogenase.[3] The standard free energy change of the reaction is -25.1 kJ/mol.[6]
Ethanol fermentation
Yeast and other anaerobic microorganisms convert glucose to ethanol and CO2 rather than pyruvate. Pyruvate is first converted to acetaldehyde by enzyme pyruvate decarboxylase in the presence of Thiamine pyrophosphate and Mg++. Carbon-dioxide is released during this reaction. Acetaldehyde is then converted to ethanol by the enzyme alcohol dehydrogenase. NADH is oxidized to NAD+ during this reaction.

Aerobic glycolysis
Lactate shuttle hypothesis
Lactic acidosis

^ a b c Stojan, George; Christopher-Stine, Lisa (2015-01-01), Hochberg, Marc C.; Silman, Alan J.; Smolen, Josef S.; Weinblatt, Michael E. (eds.), "151 - Metabolic, drug-induced, and other noninflammatory myopathies", Rheumatology (Sixth Edition), Philadelphia: Content Repository Only!, pp. 1255–1263, ISBN 978-0-323-09138-1, retrieved 2020-11-02
^ Pigozzi, Fabio; Giombini, Arrigo; Fagnani, Federica; Parisi, Attilio (2007-01-01), Frontera, Walter R.; Herring, Stanley A.; Micheli, Lyle J.; Silver, Julie K. (eds.), "CHAPTER 3 - The Role of Diet and Nutritional Supplements", Clinical Sports Medicine, Edinburgh: W.B. Saunders, pp. 23–36, doi:10.1016/b978-141602443-9.50006-4, ISBN 978-1-4160-2443-9, retrieved 2020-11-02
^ a b Bender, D. A. (2003-01-01), Caballero, Benjamin (ed.), "GLUCOSE | Function and Metabolism", Encyclopedia of Food Sciences and Nutrition (Second Edition), Oxford: Academic Press, pp. 2904–2911, ISBN 978-0-12-227055-0, retrieved 2020-11-02
^ a b Kantor, PAUL F.; Lopaschuk, GARY D.; Opie, LIONEL H. (2001-01-01), Sperelakis, NICHOLAS; Kurachi, YOSHIHISA; Terzic, ANDRE; Cohen, MICHAEL V. (eds.), "CHAPTER 32 - Myocardial Energy Metabolism", Heart Physiology and Pathophysiology (Fourth Edition), San Diego: Academic Press, pp. 543–569, doi:10.1016/b978-012656975-9/50034-1, ISBN 978-0-12-656975-9, retrieved 2020-11-02
^ Engelking, Larry R. (2015-01-01), Engelking, Larry R. (ed.), "Chapter 24 - Introduction to Glycolysis (The Embden-Meyerhoff Pathway (EMP))", Textbook of Veterinary Physiological Chemistry (Third Edition), Boston: Academic Press, pp. 153–158, doi:10.1016/b978-0-12-391909-0.50024-4, ISBN 978-0-12-391909-0, retrieved 2020-11-02
^ Cox Michael M, Nelson David L (2008). "Chapter 14: Glycolysis, Gluconeogenesis, and the Pentose Phosphate Pathway". Lehninger Principles of Biochemistry (5 ed.). W H Freeman & Co. pp. 527–568. ISBN 978-1429222631.

Retrieved from "https://en.wikipedia.org/w/index.php?title=Anaerobic_glycolysis&oldid=1069258735"

If you’re sitting and reading this tutorial on a computer or some other device, you’re doing it while performing aerobic respiration. If you got up and got on a bicycle and cycled at a leisurely pace, you’d still be doing aerobic respiration. Aerobic respiration is done when you can supply your muscles and other organs with enough oxygen to completely oxidize glucose. Aerobic respiration is sustainable. For example (assuming that you’re in reasonable shape), if you’re walking at an aerobic pace you’ll never need to stop to catch your breath.

Back to our bike ride. You reach a long, steep hill. Your muscles strain to keep the bike moving upward and forward. You start breathing harder to get more oxygen into your lungs. Your heart is beating faster to deliver that oxygen to your muscles.

After a while, your breathing rate and your heart rate reach their maximum. Yet even this maximum isn’t delivering enough oxygen to your system. At that point, you switch over to anaerobic respiration.

Anaerobic respiration is about continuing glycolysis in the absence of oxygen. Why? Simply, it’s because two ATPs are a lot better than no ATP. Anaerobic activities include sprinting (whether running, biking, or swimming) and weightlifting. You can’t sustain these activities, but you can do them in short bursts. During these bursts, your muscles are going to need every ATP they can get.

Look again at glycolysis

To move past the “cleavage” phase (phase 2), G3P (at “F”) needs to be oxidized. That oxidation can only happen if NAD+ is present (at “G”). But glycolysis converts NAD+ to NADH. So, in order for glycolysis to continue to create its two ATPs/glucose, there needs to be a way for the cell to resupply itself with NAD+. That way is called fermentation. Fermentation oxidizes NADH, converting it to NAD+ so that glycolysis can continue.

Fermentation happens in people in a process called lactic acid fermentation. Lactic acid fermentation is also used by the bacteria that make yogurt. And yeast carry out fermentations that produce alcohol. Let’s start with that one.

3. Alcohol Fermentation

“A” is glycolysis. Glucose is converted to two molecules of pyruvate (C3H3O3), with a net yield of two ATP and two NADH.

In step “B,” enzymes break the carboxyl group off of pyruvate, producing the two carbon molecule acetaldehyde. The carboxyl group becomes a CO2 molecule. This CO2 becomes the bubbles in beer or champagne or bread. In other alcohol fermentation processes, such as in wine-making, the CO2 is allowed to escape.

Step “C” is about regenerating NAD+ so that glycolysis can continue. The cell does this by a redox reaction in which acetaldehyde is reduced to ethanol (a two carbon alcohol), while NADH is oxidized. Thise oxidation and reduction is paired together, with the electrons (and hydrogens) flowing from NADH (which is being oxidized) to acetaldehyde (which is being reduced). You can see this by comparing the formulas for acetaldehyde (C2H3O) and ethanol (C2H6O). Ethanol, with all of those hydrogens, is a more reduced (and more energetic compound). Ethanol, in fact, is a fuel. You can drive a car with it. In California, where I live, up to 10% of the fuel that I put in my car is ethanol (with the other 90% being petroleum-derived gasoline).

4. Lactic Acid Fermentation

Lactic acid fermentation occurs in yogurt making bacteria, and in our very own muscle cells. Whenever we’re low on oxygen (every time we sprint, weightlift, etc). our cells will temporarily shift to lactic acid fermentation. This allows us to keep on producing two ATPs/glucose for a short period of time by continuing glycolysis, without oxidative phosphorylation. Here’s how it works.

As with alcohol fermentation, the process begins with glycolysis (“A”). In the absence of oxygen, enzymes take pyruvate and reduce it to lactate, or lactic acid. This reduction is accompanied by the simultaneous oxidation of NADH to NAD+. With NAD+ available, glycolysis can continue, at least for a short while.

As you move towards the top of your aerobic zone, your body will try to maximize oxygen delivery to your cells through increasing your heart and breathing rate. As you exceed your aerobic capacity, anaerobic respiration begins. You’ll accumulate lactic acid in your muscle tissue, leading to what athletes experience as a “lactic acid burn.” At a certain point, as lactic acid and other waste products build up, you’ll have to slow down or stop altogether. As you do, you’ll reduce oxygen demand. You’re heart rate and breathing rate will decrease as you shift back to aerobic respiration (with its more efficient production of ATP). The lactic acid, a high energy compound, diffuses out of your muscles into your blood, which carries it to your liver. Liver cells, in turn, convert lactic acid into glucose, which can then diffuse back into the bloodstream to power cellular respiration.

yogurt: a product of lactic acid fermentation

If you’re interested in making yogurt, the process is quite simple. You add yogurt making bacteria such as Lactobacillus and Acidophilus to pasteurized milk (pasteurization lowers the amount of other bacteria in the milk). The Lactobacillus and Acidophilus will take lactose (milk sugar) and use it as a fuel to create ATP. Lactic acid is a metabolic waste product. As it accumulates, the pH of the milk drops to where it causes milk proteins to change from a liquid form to more a a gel, giving yogurt its texture and sourness.

This tutorial ends this series on cellular respiration. From here, you can go back to the Cellular respiration main menu, or choose a new tutorial from the menu above.