Does Glycolysis produce lactate, or pyruvate?

EDIT- Somebody suggested that this is the same question as this, it isn't. This one is asking about the definition of glycolysis. That one was asking about the definition of fermentation.

Does Glycolysis produce lactate, or pyruvate?

I'm aware that ultimately in the human body, after sugar is converted into pyruvate, then if fermentation happens it will be converted into lactate, or if aerobic respiration happens then it won't.

My question is on the term Glycolysis

I notice that most sources seem to say glycolysis ends with pyruvate


Glycolysis… is the metabolic pathway that converts glucose… into pyruvate

The only time lactate comes into it is After glycolysis. (so in this wikipedia page on Glycolysis, in the Post Glycolysis part it mentions this) "pyruvate is converted to lactate "

However, on the other hand, I see some sources, even another wikipedia article titled "anaerobic glycolysis", which the page says isn't well sourced, it has glycolysis as ending in lactate. "Anaerobic glycolysis is the transformation of glucose to lactate when limited amounts of oxygen (O2) are available"

Also here "we contend that La− is always the end product of glycolysis"
(putting aside their controversial claim that it is always the end product, i'm interested here in the idea of theirs that it is ever an end product, so, their use of the term glycolysis)

So it seems that there are those two positions

For the purposes of this question i'll call one position the lactate position and the other position the pyruvate position.

One position, call it the lactate position, which is those sources that place lactate, as the end product of glycolysis… thus counting not just sugar->pyruvate, but sugar->pyruvate + the whole fermentation process, as glycolysis.

While others, call this the Pyruvate position, count purely sugar->pyruvate those sources count just that, as Glycolysis.

I'm wondering if both definitions are correct… / both usages are valid… Or if one of those e.g. if the lactate one, is an odd one out and if most academic texts wouldn't use that definition, and would use the pyruvate position for their definition of Glycolysis.

Note- I had written that glycolysis ends with pyruvate {or pyruvic acid which dissociates into pyruvate}, and the lactic acid fermentation ends in lactate {or lactic acid which dissociates into lactate}. But what is in those curly braces there is wrong and I have been corrected on that one. Glycolysis produces pyruvate, and Lactic Acid Fermentation produces lactate. The reason (answerer David explains), why Lactic acid fermentation bears that name, is it is named after what are called "lactic acid bacteria" "Lactic acid bacteria are named for their effect on the medium in which such bacteria grow, not for the ionization state of lactic acid in the cell and when bound to enzymes (about which the namers could have had no knowledge)." and they do that type of fermentation that produces lactate. There are two types of "lactic acid fermentation", homolactic fermentation, and heretolactic fermentation. Humans do homolactic fermentation that produces lactate and no ethanol, as opposed to heterolactic fermentation that produces lactate and ethanol

Also, note that Muscle cells do what is called "lactic acid fermentation", but the idea that they produce lactic acid is a myth that has been commonly propagated in sports science(I guess perhaps partly as a result of what I think is biology's poor nomenclature, the fact that the process is called lactic acid fermentation). Muscles don't produce lactic acid, in fact, it's not lactic acid and they don't produce it. Muscle cells use lactate, they don't produce lactic acid. This is verifiable from googling humans produce lactate not lactic acid eg the first line here mentions that myth and calls it out as a myth. The body of the following question and its answers here are related and very interesting.

I think you will find all text books (e.g. Berg et al. Ch 16) describe glycolysis as the conversion of glucose to pyruvate, as this is how it has been defined and considered in countless biochemical papers. The subsequent reactions of pyruvate are regarded as separate metabolic steps or pathways.

The title of the short review article you cite (“Lactate is always the end product of glycolysis”) has mislead you - it was obviously meant to be controversial. It is the ambiguous term “end product” that is the (deliberate?) cause of the problem. What the article suggest is that the product of glycolysis - pyruvate - is always, at least partially, converted to lactate in animal cells. It would have been better entitled “Lactate is always produced from the pyruvate generated in glycolysis”. Whether or not that is true (and that is not your question as I understand it), the conversion of pyruvate to lactate is not considered to be part of glycolysis any more than its conversion to acetate.

There may be ambiguity in the use of the ancient term 'fermentation', but not with glycolysis and other metabolic pathways established in the twentieth century.

Lactate, not pyruvate, is the end-product of glycolysis. If pyruvate was the end-product, there would be a major problem: glycolysis would stop.

I am talking here about how cells 'obtain' ATP from the splitting of glucose under strict anaerobic conditions (glycolysis).

Glycolysis is the splitting of glucose, and occurs anaerobically without any net oxidation or reduction, and the free energy sequestered in the form of ATP is derived from the splitting reaction. In thermodynamic terms, we can envisage that entropy makes a contribution to the overall (favorable) Gibbs free energy change: splitting a molecule into two parts makes the system more disordered (to take a crude view of what entropy means). But of course the rearrangement of chemical bonds will also make a (major?) contribution.

Let's be blunt about the role of oxidation and reduction: as stated, there is no net oxidation or reduction in glycolysis. But in this last statement lies a major paradox, and therein is the reason why we must consider lactate (and in alcoholic fermentation, ethanol) the end-product of glycolysis.

Although glycolyis proceeds without net oxidation or reduction, a supply of oxidied NAD (i.e., NAD+) is required in order for the process to continue.

I am of course talking about the glyceraldehyde-3-phosphate dehydrogenase (GAPdh) reaction, that ubiquitous enzyme of glycolysis.

$$ mathrm{ ext{glyceraldehyde-3-phosphate},+,NAD^{+},+,P_i ext{ = } ext{1,3-bisphosphoglycerate} +,,NADH,+,H^+}$$

Pyruvate production from glucose is an oxidation - we have 'removed' a pair of electrons (strictly speaking, 2 pairs), but for glycolysis to proceed we (paradoxically) need a reduction - we must (somehow) get rid of the electrons from NADH, and one way (crazily at first sight) is to 'give' the electrons back to the carbon skeleton from whence they came - to pyruvate in the case of muscle (lactate) glycolysis.

One of the key steps to understanding glycolysis is to ask the question: how is the NADH produced in the GAPdh reaction oxidized back to NAD+?

In lactic acid fermentation and in muscle glycolysis, this it done with lactate dehydrogenase (LDH), which catalyzes the NADH -linked reduction of pyruvate to lactate, with concomitant production of NAD+, thus allowing glycolysis to continue. If we could 'knock out' the LDH gene, there would be no glycolysis (a lethal mutation), unless the cell could somehow find another method of regenerating NAD+.

The splitting of glucose into lactate may be represented stoichiometrically as follows (one glucose is split into two lactic acid molecules):

$$ mathrm{ ext{1 }C{_6}H_{12}O_6 ext{ = 2 }CH_{3},CHOH,COOH}$$

We may note that there is no oxidation or reduction. In aerobic respiration, glucose yields 12 pairs of electrons to the respiratory chain, and all are 'held' in either carbon-carbon or carbon-hydrogen bonds. Each lactate acid molecule (or more precisely, each lactate molecule) has six (C-C and C-H) bonds: they are all there, nothing has changed.

Furthermore, it is the above reaction that yields the free energy, no matter how we view the intricacies of the molecular processes that lead to it. That is, fundamentally, lactic acid fermentation (or muscle glycolysis) is a sequestration of Gibbs free energy, in the form of ATP, obtained by the splitting of glucose into two molecules of lactate.

In the case of pyruvate, we would need to write the equation (balanced in terms of electrons but not hydrogens) as follows:

$$ mathrm{ ext{1 }C{_6}H_{12}O_6,+,2,NAD^{+} ext{ = 2 }CH_{3},(C ext{=}O),COOH +,2,NADH}$$

If we count the number of (C-C and C-H) bonds in pyruvate, we see there are five: one pair of electrons is 'missing' and is now 'held' by the nicotinamide cofactor.

So what of alcoholic fermentation? How is NADH re-oxidized here? In this case, pyruvate is first decarboxylated to acetaldehyde and CO2 by the enzyme pyruvate decarboxylase, and NADH is reoxidized by the enzyme alcohol dehydrogenase acting as an aldehyde reductase. That is, the NAD+ necessary for the GAPdh reaction is 'regenerated' by the reduction of acetaldehdye to ethanol. As far as anaerobic fermentation in yeast is concerned, ethanol is a 'waste' product (but very useful to us humans).

$$ mathrm{CH_{3}CHO,+,NADH,+,H^{+} ext{ = }CH_{3}CH_{2}OH,+,NAD^{+}}$$

If we count the number of (C-H and C-C) bonds in acetaldehyde, there are five, but in ethanol there are six. As two molecules of ethanol are produced for each glucose, the total is twelve and we have accounted for everything: once again there is no net oxidation and reduction, and the stoichiometric splitting of glucose in alcoholic fermentation may be written as follows (one glucose gives two molecules of ethanol and two molecules of carbon dioxide),

$$ mathrm{ ext{1 }C{_6}H_{12}O_6 ext{ = 2 }CH_{3}CH_{2}OH,+,2,CO_2}$$

and it is the Gibbs free energy 'released' in this reaction that is used to 'make' ATP.

NADH produced in glycolysis may also be regenerated 'aerobically', that is by reoxidation in mitochondria via the respiratory redox chain. The problem here is that the inner mitochondrial membrane is impermeable to NAD, and a shuttle system, such as the aspartate-malate shuttle, is required to get the electrons across the inner membrane. If NADH is regenerated via mitochondrial respiration, this is of course an alternative to the LDH reaction and lactate need not be formed (but we are no longer talking about anaerobic glycolysis). This, I suspect, is where the confusion often arises.

To make one final point about lactate glycolysis in humans. A muscle cell, perhaps obtaining as much energy as possible from glycolysis during vigorous exercise, regards lactate as a waste product. But this is not true of the organism as a whole: the lactate is taken to the liver where it may be converted back to glucose (via pyruvate) as part of the Cori cycle (which is way beyond the scope of this answer).

So why the apparent paradox of NAD oxidoreduction during glycolysis? And why oxidize a carbon skeleton with NAD+ and then regenerate the oxidizing agent by reducing the carbon skeleton at a later step? I have no explanation, but this is surely a great example of what Albert Lehninger called the molecular logic of living organisms.

Glycolysis Pathway

Carbohydrates are the first cellular constituents formed by photosynthetic organisms and result from the fixation of CO2 on the absorption of light. The carbohydrates are metabolized to yield a vast array of other organic compounds, many of which are subsequently utilized as dietary constituents by animals. The animals ingest great quantities of carbohydrates that can be either stored, or oxidized to obtain energy as ATP, or converted to lipids for more efficient energy storage or used for the synthesis of many cellular constituents.

The major function of carbohydrates in metabolism is as a fuel to be oxidized and provide energy for other metabolic processes. The carbohydrate is utilized by cells mainly as glucose. The 3 principal monosaccharides resulting from digestive processes are glucose, fructose, and galactose. Much of the glucose is derived from starch which accounts for over half of the fuel in the diets of most humans. Glucose is also produced from other dietary components by the liver and, to a lesser extent, by the kidneys. Fructose results in a large intake of sucrose while galactose is produced when lactose is the principal carbohydrate of the diet. Both fructose and galactose are easily converted to glucose by the liver. It is thus apparent that glucose is the major fuel of most organisms and that it can be quickly metabolized from glycogen stores when there arises a sudden need for energy. Pentose sugars such as arabinose, ribose and xylose may be present in the diet. But their fate after absorption is, however, obscure.

Glycolysis (glycosG = sugar (sweet) lysis = dissolution) is the sequence of 10 enzyme-catalyzed reactions that convert glucose into pyruvate with the simultaneous production of ATP. Moreover, glycolysis also includes the formation of lactate from pyruvate. The glycolytic sequence of reactions differs from one species to the other only in the mechanism of its regulation and in the subsequent metabolic fate of the pyruvate formed. In aerobic organisms, glycolysis is the prelude to the citric acid cycle and the electron transport chain which together harvest most of the energy contained in glucose. In fact, glycolysis is the central pathway of glucose catabolism

Glycolysis takes place in the extramitochondrial part of the cell (or the soluble cytoplasm). It is frequently referred to as Embden-Meyerhof-Parnas or EMP pathway, in honour of these pioneer workers in the field, and still represents one of the greatest achievements in the field of biochemistry. Other illustrious investigators, who contributed significantly to the final elucidation of the glycolytic pathway, include Fritz A. Lipmann, Harden and Young, A.V. Hill, Carl Neuberg, Otto Warburg, and Carl F. Cori and his wife Gerty T. Cori.

There are 3 important routes taken by pyruvate after glycolysis, depending on the organism and the metabolic conditions

  • In aerobic organisms, the pyruvate so formed then enters mitochondria where it is oxidized, with the loss of its carboxyl group as CO2, to form the acetyl group of acetyl-coenzyme A. Later, the acetyl group is completely oxidized to CO2 and H2O by the citric acid cycle with the intervention of molecular oxygen. This pathway is followed by aerobic animal and plant cells.
  • If the supply of oxygen is insufficient, as in vigorously contracting skeletal muscles, the pyruvate cannot be oxidized further for lack of oxygen. Under such conditions, it is then reduced to lactate, a process called anaerobic glycolysis. Lactate is also produced from glucose in anaerobic microorganisms that carry out lactic acid fermentation.
  • In some microorganisms (e.g., brewer’s yeast), the pyruvate formed from glucose is transformed anaerobically into ethanol and CO2, a process called alcoholic fermentation. Since living organisms first arose in an atmosphere devoid of oxygen, anaerobic breakdown of glucose is the most ancient type of biological mechanism for obtaining energy from organic fuel molecules (Lehninger AL, 1984)

Gluconeogenesis is the process of synthesizing glucose or glycogen from non-carbohydrate precursors. Gluconeogenic precursors are molecules that can be used to produce a net synthesis of glucose. Indeed, the most important gluconeogenic precursors are glycerol, lactate, and the α-keto acids obtained from the metabolism of glucogenic amino acids.

Gluconeogenesis occurs mainly in the cytosol, although some precursors are produced in the mitochondria. It involves several enzymes of glycolysis, but it is not a reversal of glycolysis. The irreversible steps in glycolysis are circumvented by four enzymes which are designated as the key enzymes of gluconeogenesis.

Glycolysis VS gluconeogenesis

Glycolysis and gluconeogenesis are two pathways of glucose metabolism. One is the breakdown of glucose while the other is the synthesis of glucose. Gluconeogenesis closely resembles the reversed pathway of glycolysis, although it is not a complete reversal of glycolysis.

I wrote a separate article on glycolysis vs gluconeogenesis where I discuss similarities and differences between glycolysis and gluconeogenesis. If you would like to check, see: Glycolysis vs gluconeogenesis

Clinical Use and Interpretation of Lactic Acidosis

We understand that the term “lactic acidosis” has been used in clinical research and practice for more than 100 years. With the duration of this use comes considerable engrained misunderstanding and misapplication, and to expect a rapid change from any engrained convention may be unrealistic. However, given that the terminology is wrong based on incorrect understanding of metabolic biochemistry and acid-base chemistry, that clinical practice involves treating illnesses and saving lives from premature mortality, and that correct treatment most often requires a correct understanding of the true mechanisms of disease and symptomology, one would hope that clinical professionals would prefer to base their practice on empirical truths rather than engrained convention.

It has been encouraging to see many physicians altering their view of a lactic acidosis based on revised explanations consisting of expressions of elevated blood lactate (hyperlactatemia) and an associated (or not) systemic acidosis (3, 5, 7). For example, considerable research of hyperlactatemia occurs for the condition of sepsis (3, 5, 7) and also metformin toxicity (1). For sepsis, hyperlactatemia is predictive of disease severity and premature mortality, with more than a threefold increase in mortality when hyperlactatemia is accompanied by tissue hypo-perfusion (5). The prior conventional interpretation of sepsis-associated hyperlactatemia accompanied by acidosis is framed on belief in a causal connection between the disease state, altered perfusion causing a localized hypoxia, stimulation of glycolysis, and lactic acid-induced metabolic acidosis. This is false knowledge, since there is no such condition as lactic acid-induced metabolic acidosis. The increased lactate presumably occurs due to increased stimulation of energy catabolism, causing increased substrate flux through glycolysis, which will therefore also increase lactate production and/or compromise blood lactate removal. For many patients, there is no accompanied acidosis (3, 5, 7), which is consistent with the metabolic biochemistry of the combined production of lactate and the retained function of mitochondrial respiration, since a continual H + supply is needed as a substrate for each aspect of energy catabolism. For patients with a systemic acidosis, there could be a localized or systemic inflammatory response that triggers altered mitochondrial function and a metabolic milieu now consistent with metabolic acidosis (3, 7). Such a scenario is more aligned with altered mitochondrial respiration (normally a H + sink) accompanied by increased glycolytic stimulation, the consequence of the two conditions causing increased net H + release and an eventual acidosis.

Cellular lactate production occurs to facilitate sustained glycolysis by regenerating cytosolic NAD + , consuming a near stoichiometric H + per lactate produced (Table 1), and allowing for both lactate and H + efflux from the metabolically active tissue via the monocarboxylate transport proteins (12).

6.2: Fermentation

  • Contributed by OpenStax
  • General Biology at OpenStax CNX
  • Define fermentation and explain why it does not require oxygen
  • Describe the fermentation pathways and their end products and give examples of microorganisms that use these pathways
  • Compare and contrast fermentation and respiration

There are two mechanisms by which chemoheterotrophs can generate ATP: respiration and fermentation. Although respiration relies on the generation of a proton gradient and ATP synthesis by oxidative phosphorylation, ATP synthesis in fermentation is entirely through substrate-level phosphorylation in metabolic pathways. In general, the amount of ATP produced through fermentation is less than respiration, but there are situations where fermentation is necessary or preferable. Many prokaryotes, such as E. coli, are facultative, meaning that should the environmental conditions change to provide an appropriate inorganic final electron acceptor for respiration, organisms containing all the genes required to do so will switch to respiration because respiration allows for much greater ATP production. Whereas lack of an appropriate inorganic final electron acceptor is environmentally dependent, some organisms lack the ability to respire altogether. Many prokaryotes, including members of the clinically important genera Streptococcus and Clostridium, rely entirely on fermentation for ATP generation.

If respiration does not occur, NADH must be reoxidized to NAD + for reuse as an electron carrier for glycolysis and other catabolic pathways to continue. Some living systems use a metabolite produced through the cell's metabolite (such as pyruvate) as a final electron acceptor through a process called fermentation. Because all the NADH produced must be reoxidized to NAD+, the net NADH of any fermentation pathway must be zero (0). Considering that the purpose of fermentation is to generate ATP, there must also be a net gain of ATP in these metabolic pathways.

Fermentation does not involve an electron transport chain and does not directly produce any additional ATP beyond that produced during glycolysis by substrate-level phosphorylation. Organisms carrying out fermentation typically produce a maximum of two ATP molecules per glucose during glycolysis. Table (PageIndex<1>) compares the final electron acceptors and methods of ATP synthesis in aerobic respiration, anaerobic respiration, and fermentation. Note that the number of ATP molecules shown for glycolysis assumes the Embden-Meyerhof-Parnas pathway. The number of ATP molecules made by substrate-level phosphorylation (SLP) versus oxidative phosphorylation (OP) are indicated.

Electron transport and chemiosmosis (OP):

Electron transport and chemiosmosis (OP):

In all bacterial fermentations, at least one of the waste products produced is an organic acid. This feature of bacterial fermentations is frequently exploited in metabolic tests used to identify bacteria. For example, E. coli can ferment lactose, forming gas, whereas some of its close Gram-negative relatives cannot. The ability to ferment the sugar alcohol sorbitol is used to identify the pathogenic enterohemorrhagic O157:H7 strain of E. coli because, unlike other E. coli strains, it is unable to ferment sorbitol. Last, mannitol fermentation differentiates the mannitol-fermenting Staphylococcus aureus from other non&ndashmannitol-fermenting staphylococci.

The simplest fermentation, which is used by some bacteria, like those in yogurt and other soured food products, and by animals in muscles during oxygen depletion, is homolactic or lactic acid fermentation (Figure (PageIndex<1>). In homolactic fermentation the electrons on NADH produced during glycolysis are reoxidized to NAD+ by donating their electrons to the end product of glycolysis, pyruvate. The resulting waste product is lactate (lactic acid).

Figure (PageIndex<1>): Homolactic (lactic acid) fermentation. Note that the 2 NADH produced in glycolysis are reoxidized to NAD+ when their electrons are added to pyruvate to make the waste product lactate (lactic acid) (2021 Jeanne Kagle)

Bacteria of several Gram-positive genera, including Lactobacillus, Leuconostoc, and Streptococcus, are collectively known as the lactic acid bacteria (LAB), and various strains are important in food production. During yogurt and cheese production, the highly acidic environment generated by lactic acid fermentation denatures proteins contained in milk, causing it to solidify. When lactic acid is the only fermentation product, the process is said to be homolactic fermentation such is the case for Lactobacillus delbrueckii and S. thermophiles used in yogurt production. However, many bacteria perform heterolactic fermentation, producing a mixture of lactic acid, ethanol and/or acetic acid, and CO2 as a result, because of their use of the branched pentose phosphate pathway instead of the EMP pathway for glycolysis. One important heterolactic fermenter is Leuconostoc mesenteroides, which is used for souring vegetables like cucumbers and cabbage, producing pickles and sauerkraut, respectively.

Lactic acid bacteria are also important medically. The production of low pH environments within the body inhibits the establishment and growth of pathogens in these areas. For example, the vaginal microbiota is composed largely of lactic acid bacteria, but when these bacteria are reduced, yeast can proliferate, causing a yeast infection. Additionally, lactic acid bacteria are important in maintaining the health of the gastrointestinal tract and, as such, are the primary component of probiotics.

Another familiar fermentation process is alcohol fermentation by yeast, which produces ethanol. The ethanol fermentation reaction is shown in Figure (PageIndex<2>). You may notice that unlike bacterial fermentations this fungal (eukaryotic) fermentation does not produce an acid as a waste product. The ethanol fermentation of pyruvate by the yeast Saccharomyces cerevisiae is used in the production of alcoholic beverages and also makes bread products rise due to CO2 production. Outside of the food industry, ethanol fermentation of plant products is important in biofuel production.

Figure (PageIndex<2>): The chemical reactions of alcohol fermentation are shown here. Ethanol fermentation is important in the production of alcoholic beverages and bread.

Beyond lactic acid fermentation and alcohol fermentation, many other fermentation methods occur in microbes, all for the purpose of ensuring an adequate supply of NAD + for glycolysis (Table (PageIndex<2>)). Without these pathways, glycolysis would not occur and no ATP would be harvested from the breakdown of glucose. It should be noted that most forms of fermentation besides homolactic fermentation produce gas, commonly CO2 and/or hydrogen gas. Many of these different types of fermentation pathways are also used in food production and each results in the production of different organic acids, contributing to the unique flavor of a particular fermented food product. The propionic acid produced during propionic acid fermentation contributes to the distinctive flavor of Swiss cheese, for example.

Several fermentation products are important commercially outside of the food industry. For example, chemical solvents such as acetone and butanol are produced during acetone-butanol-ethanol fermentation. Complex organic pharmaceutical compounds used in antibiotics (e.g., penicillin), vaccines, and vitamins are produced through mixed acid fermentation.

In addition to fermentation ability, fermentation products are used in the laboratory to differentiate various bacteria for diagnostic purposes. For example, enteric bacteria are known for their ability to perform mixed acid fermentation, reducing the pH, which can be detected using a pH indicator. Similarly, the bacterial production of acetoin during butanediol fermentation can also be detected. Gas production from fermentation can also be seen in an inverted Durham tube that traps produced gas in a broth culture.

Table (PageIndex<2>): Common Fermentation Pathways
Pathway End Products Example Microbes Commercial Products
Acetone-butanol-ethanol Acetone, butanol, ethanol, CO2 Clostridium acetobutylicum Commercial solvents, gasoline alternative
Alcohol Ethanol, CO2 Candida, Saccharomyces Beer, bread
Butanediol Formic and lactic acid ethanol acetoin 2,3 butanediol CO2 hydrogen gas Klebsiella, Enterobacter Chardonnay wine
Butyric acid Butyric acid, CO2, hydrogen gas Clostridium butyricum Butter
Lactic acid Lactic acid Streptococcus, Lactobacillus Sauerkraut, yogurt, cheese
Mixed acid Acetic, formic, lactic, and succinic acids ethanol, CO2, hydrogen gas Escherichia, Shigella Vinegar, cosmetics, pharmaceuticals
Propionic acid Acetic acid, propionic acid, CO2 Propionibacterium, Bifidobacterium Swiss cheese

When would a metabolically versatile microbe perform fermentation rather than respiration?


Identification of a microbial isolate is essential for the proper diagnosis and appropriate treatment of patients. Scientists have developed techniques that identify bacteria according to their biochemical characteristics. Typically, they either examine the use of specific carbon sources as substrates for fermentation or other metabolic reactions, or they identify fermentation products or specific enzymes present in reactions. In the past, microbiologists have used individual test tubes and plates to conduct biochemical testing. However, scientists, especially those in clinical laboratories, now more frequently use plastic, disposable, multitest panels that contain a number of miniature reaction tubes, each typically including a specific substrate and pH indicator. After inoculation of the test panel with a small sample of the microbe in question and incubation, scientists can compare the results to a database that includes the expected results for specific biochemical reactions for known microbes, thus enabling rapid identification of a sample microbe. These test panels have allowed scientists to reduce costs while improving efficiency and reproducibility by performing a larger number of tests simultaneously.

Many commercial, miniaturized biochemical test panels cover a number of clinically important groups of bacteria and yeasts. One of the earliest and most popular test panels is the Analytical Profile Index (API) panel invented in the 1970s. Once some basic laboratory characterization of a given strain has been performed, such as determining the strain&rsquos Gram morphology, an appropriate test strip that contains 10 to 20 different biochemical tests for differentiating strains within that microbial group can be used. Currently, the various API strips can be used to quickly and easily identify more than 600 species of bacteria, both aerobic and anaerobic, and approximately 100 different types of yeasts. Based on the colors of the reactions when metabolic end products are present, due to the presence of pH indicators, a metabolic profile is created from the results (Figure (PageIndex<2>)). Microbiologists can then compare the sample&rsquos profile to the database to identify the specific microbe.

Figure (PageIndex<2>): The API 20NE test strip is used to identify specific strains of gram-negative bacteria outside the Enterobacteriaceae. Here is an API 20NE test strip result for Photobacterium damselae ssp. piscicida.

Cellular Respiration Stage I: Glycolysis

The first stage of cellular respiration is glycolysis. It does not require oxygen, and it does not take place in the mitochondrion - it takes place in the cytosol of the cytoplasm.

When was the last time you enjoyed yogurt on your breakfast cereal, or had a tetanus shot? These experiences may appear unconnected, but both relate to bacteria which do not use oxygen to make ATP. In fact, tetanus bacteria cannot survive if oxygen is present. However,Lactobacillus acidophilus (bacteria which make yogurt) and Clostridium tetani (bacteria which cause tetanus or lockjaw) share with nearly all organisms the first stage of cellular respiration, glycolysis. Because glycolysis is universal, whereas aerobic (oxygen-requiring) cellular respiration is not, most biologists consider it to be the most fundamental and primitive pathway for making ATP.

Splitting Glucose

The word glycolysis means &ldquoglucose splitting,&rdquo which is exactly what happens in this stage.Enzymes split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as shown in Figure below. You can watch an animation of the steps of glycolysis at this link:

In glycolysis, glucose (C6) is split into two 3-carbon (C3) pyruvate molecules. This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?

Results of Glycolysis

Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules. These two molecules go on to stage II of cellular respiration. The energy to split glucose is provided by two molecules of ATP. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP. As a result, there is a net gain of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD + to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.

The Lactic Acid System

» This form of glycolysis is the main source of energy in some plants and organisms. It is an important source of ATP during vigorous exercise when there isn’t an enough supply of oxygen.

» This pathway is active in bacteria involved in souring milk and formation of yogurt. It also exists in yeasts where pyruvate is first converted to acetaldehyde and carbon dioxide and then to ethanol in the absence of oxygen.

» There are two types of anaerobic fermentation processes that can occur in the absence of oxygen. They are lactic acid fermentation and alcoholic fermentation. Let us get some more information about these processes from the upcoming passages.

Lactic Acid Fermentation

➜ Lactic acid fermentation pathway is commonly seen in animal cells and in lactic acid bacteria. Animal tissues produce energy through this pathway.

➜ During this process, breakdown of glucose takes place in the absence of oxygen. Carbohydrate break down occurs in the cells and results in the formation of pyruvic acid and hydronium ions.

➜ The pyruvate further undergoes oxidation forming lactic acid, which then dissociates into lactate and H+. NADH gets oxidized in this whole process, which is the source of energy for the cells.

The reaction involved in the conversion of pyruvate into lactate can be represented as follows:

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Puruvate + NADH + H+ → Lactate + NAD+

➜ The lactate produced diffuses out of the cell and passes into the liver. It is then converted to glucose which is capable of passing back into the peripheral cells to re-enter glycolysis. This forms a continuous cycle.

➜ The red blood cells obtain most of their energy through this process. However, excess lactic acid production can lead to lactic acidosis.

Alcoholic Fermentation

► This pathway generally takes place in organisms, like yeast and many plants. It involves the conversion of pyruvate into acetaldeyde and carbon dioxide which are further converted into ethanol.

► NADH is converted back to NAD+ and ethanol is the end product of this pathway. This process is employed in the manufacturing of alcoholic beverages and also in the biotechnology industry to generate carbon dioxide that is necessary for bread making.

The main difference between anaerobic and aerobic glycolysis is that the sugar is not broken down completely in the latter. Instead, it is converted to lactic acid or ethyl alcohol. However, a lot of animals and plants use the anaerobic pathway for ATP production.

Related Posts

Anaerobic respiration is a process in which organisms produce energy in absence of oxygen. This BiologyWise article tells you about all the steps of anaerobic respiration in detail.

Anaerobic fermentation is a complicated process that is 100% natural and is carried out on microorganisms. Read this BiologyWise article to know what anaerobic fermentation is and some interesting facts&hellip

Glycolysis is the breakdown of glucose at the cellular level for energy-generating metabolic reactions. This article discusses the products of this process, which play an important part in body metabolism.

Fate of Pyruvate: Acetyl CoA, Lactate, Alcohol Formation.

After the formation of Pyruvate through the glycolysis pathway, it may enter into different pathways such as lactate fermentation in muscles and.

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Last updated onJanuary 18th, 2021

In this article, we will discuss the Fate of Pyruvate or the Utilization of Pyruvate after the formation via glycolysis.

After the formation of Pyruvate through the glycolysis pathway, it may enter into different pathways such as lactate fermentation in muscles and rbc, alcohol fermentation, and acetyl CoA fermentation (acetyl CoA then enters into TCA cycle to generate energy), or it may enter in biosynthetic pathways such as gluconeogenesis and fatty acid synthesis.

Carbohydrate Metabolism

Antonio Blanco , Gustavo Blanco , in Medical Biochemistry , 2017

General Considerations About Gluconeogenesis

Lactate formed during anaerobic glycolysis enters the gluconeogenic pathway after oxidation to pyruvate by lactate dehydrogenase. After intense exercise, the lactate produced diffuses from the muscle into the blood and is taken up by the liver to be converted into glucose and glycogen.

Oxaloacetate is a common intermediary in the first reactions of gluconeogenesis and the citric acid cycle. All cycle intermediates and any compound producing it may become a glucose precursor. The carbon chains of some amino acids originate α-ketoglutarate, others produce succinate, fumarate, oxaloacetate, or pyruvate (p. 383) and can contribute to glucose formation.

Acetyl-CoA is not glucogenic. Practically, each acetate moiety entering the citric acid cycle is completely oxidized. Therefore, fatty acids degraded to acetyl-CoA in the organism are nonglucogenic. However, glycerol, another lipid component, is glucogenic. In liver tissue, for example, glycerol can be phosphorylated to glycerol-3-phosphate, which is subsequently oxidized to DHAP, and then oxidized. The triose-phosphate has two metabolic choices: (1) to follow the gluconeogenesis pathway by binding to glyceraldehyde-3-phosphate to yield fructose-1,6-bisphosphate or (2) to enter glycolysis to become glyceraldehyde-3-phosphate and 1,3-bisphosphoglycerate. The final destination is determined by the cell needs.