Are humans capable of both anaerobic respiration, and lactic acid fermentation?

Are humans capable of both anaerobic respiration, and lactic acid fermentation?

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Are humans capable of both anaerobic respiration, and lactic acid fermentation?

And if so, when do they do each?

I understand that the difference between respiration and fermentation is that respiration takes place in the electron transport chain. I understand that aerobic respiration uses oxygen in the electron transport chain, and anaerobic respiration uses some other molecule in the electron transport chain instead of oxygen, like nitrate. Whereas fermentation doesn't use the electron transport chain at all.

I know humans have an anaerobic metabolic process that produces lactic acid, but I'm not clear whether it's respiration or fermentation, or whether it could be either, in which case when it is which?

some further discussion w roland at the chat link / / and on this q at chat link here / / and example of conflicting definitions available /

Humans have no anaerobic respiration, if we define this as oxidation of a substrate with an external electron acceptor other than oxygen. In humans, the terminal electron acceptor in respiration is always oxygen, which is reduced at complex IV in the respiratory chain. Alternative electron acceptors are mostly found in bacteria and archaea.

I would call the anaerobic metabolism of glucose to lactate in humans a fermentation process. It consists of glycolysis, which converts glucose to pyruvate, which in turn is converted to lactate by lactate dehydrogenase. There is no terminal electron acceptor in this case; instead, energy is extracted from glucose by rearranging the molecular structure of the sugar into a more favorable ("low energy") configuration, without any net donation of electrons.

Note: this is how I use the terms, and I believe this is the most common usage in biochemistry today. But I am sure you can find other sources that define them differently. This is rather common in biology and biochemistry, and these terms are very old and fraught with all sorts of historical connotations. But names are not crucial; understanding the biochemical processes is. The important distinction here is that conversion of glucose to lactate does not oxidize the substrate and therefore needs no external electron acceptor; in this way it is fundamentally different from oxidative metabolism.

Muscle tissue is a good example of anaerobic fermentation. Lactic Acid is fermented and builds up in this tissue when we do large amounts of exercise. We use the Pyruvate molecule and LDH to produce Lactate when required but it is only for short bursts of energy in specific tissues.

Try a google image search for the glycolytic pathway. Usually Aerobic and Anaerobic paths are listed after the Pyruvate molecule is formed at the end of this pathway.

I mentioned in comment quite early "apparently humans don't do anaerobic respiration at all. They only do a)aerobic respiration and b)fermentation. Humans can't use nitrate or sulfite as acceptors in the electron transport chain"

I'll expand on that a bit…

"Anaerobic cellular respiration is similar to aerobic cellular respiration in that electrons extracted from a fuel molecule are passed through an electron transport chain, driving ATP synthesis. Some organisms use sulfate as the final electron acceptor at the end of the transport chain, while others use nitrate, sulfur, or one of a variety of other molecules… What kinds of organisms use anaerobic cellular respiration? Some prokaryotes-bacteria and archaea-that live in low-oxygen environments rely on anaerobic respiration to break down fuels." (i.e. not humans)

and "Fermentation is a widespread pathway, but it is not the only way to get energy from fuels anaerobically (in the absence of oxygen). Some living systems instead use an inorganic molecule other than O2, such as sulfate, as a final electron acceptor for an electron transport chain. This process, called anaerobic cellular respiration, is performed by some bacteria and archaea."

Humans aren't able to use sulfate or nitrate(or any inorganic molecule other than o2) in the electron transport chain. So they can't do anaerobic respiration.

That is certainly the point made by khanacademy, and made by another microbiologist I spoke to, and echoed by Roland's answer.

Note- as of writing, Roland has undermined his answer somewhat… Since the first sentence of his answer says "Humans have no anaerobic respiration" But a comment he made, says "I think these terms are defined somewhat differently depending on who you ask, and I agree with @David that it's not very interesting to argue over definitions. But my own preference would be to use the term "anaerobic respiration" only for oxidation of substrates that delivers electrons to a terminal electron acceptor other than O2, and "fermentation" to mean a process that does not result in any net oxidation. This is a major biochemical difference, so it motivates different terms, imho. With these definitions, muscle does not do anaerobic respiration"

i.e. he's saying the definition is only his preference. And that while it has good reason behind it, it's just still his preference. And so it's a question of some peoples' preferred definitions, over others.

I have spoken to a microbiologist who says all the advanced scientific texts use that definition (introductory texts aside, and human biology tests aside), i.e. it's not just some peoples' preference. I have added further re that - And what I have found strong evidence/proof that supports that there is a strict distinction made, in the non-introductory microbiology texts. He is probably correct that scientific journals such as PNAS, Nature and Science, make the distinction too.


The microbiologist that I spoke to said that often biology 101 texts get it wrong so e.g. that boundless biology textbook, even the title gives away that it's a very intro level book. So no surprise that it gets it wrong. He said often introductory undergraduate books get it wrong. And books on human biology, since they don't need to distinguish between types of anaerobic metabolising, can get it wrong sometimes, because they don't need as much clarity as they aren't needing to distinguish between anaerobic processes. But the science journals such as PNAS, Science, Nature. And the advanced microbiology texts get it right. Sure it's possible for a person to use a very general definition of respiration and then to ask whether the electron transport chain is used, but the microbiology texts have no need for that question to even be asked, since they define their terms to make that distinction.

If one goes to google books and searches microbiology, I see he is correct.

For example, googling in google books for microbiology fermentation respiration

The first result is introductory so I won't look at it.

The second result is Microbiology & Plant Pathology by Dr. P.D. Sharma

"heterotrophs exhibit two basic strategies the fermentation and respiration"

"In contrast to fermentation, respiration requires an external electron acceptor"


The third result distinguishes but is by the same author so i'll skip it.


The fourth result - Principles of Microbiology - Page 530

"A comparison of aerobic respiration, anaerobic respiration, and fermentation"

And it distinguishes


The fifth result "Microbiology: A Clinical Approach, Second Edition: - Page 39" here

has a diagram that distinguishes


The sixth result Alcamo's Fundamentals of Microbiology - Page 183

"a different inorganic molecule as a final electron acceptor"

note- can't be pyruvate as final one, as pyruvate contains carbon so is organic.

He then gives the examples of a species of bacteria that uses nitrate and a species that uses sulfate


It is indeed the case that fermentation doesn't involve an electron transport chain, but also, fermentation does involve a final electron acceptor, just it's pyruvate or a pyruvate derivative.

Microbiology By Cynthia Nau Cornelissen, Richard A. Harvey, Bruce D. Fisher here "the terminal electron acceptor in fermentation, is pyruvate or a pyruvate derivative"

And as the microbiologist I spoke to said, it's more a question of whether the final electron acceptor is internal/endogenous (i.e. produced by the organism itself), or whether it's external. (rather than whether it's organic/inorganic)

One thing boundless biology says (using the strict definition of fermentation and respiration), and consistent with that.

" Both inorganic and organic compounds may be used as electron acceptors in anaerobic respiration. Inorganic compounds include sulfate (SO42-), nitrate (NO3-), and ferric iron (Fe3+). Organic compounds include DMSO."
(DMSO for example is organic/carbon based, but it's an external electron acceptor, thus, still respiration when it's used)


That and all those definitions clearly distinguish respiration and fermentation, with the same strict definition, that confirms that humans do not do anaerobic respiration, they do fermentation, which is not a form of respiration.

Nevertheless, despite that, it's clearly the case that outside of advanced microbiology texts… (And I suppose outside of scientific journals), so in introductory biology texts, and in human biology texts, and high school texts… and among many on this website who clearly have a deep interest in biology, that general definition of respiration is still around. And in such a situation, it makes sense to pay less attention to the term but to ask regarding specific features e.g. "Does it involve the electron transport chain" / Does it use an inorganic compound such as nitrate or sulfate as an alternative to O2? if one wants to know if it's respiration, Or if one doesn't care what the term is and just wants to know what the features are so may ask that and further questions like "Does it use oxygen", or features that aren't even relevant to the term, such as what interested in commenter in how they think of the terms - "Is it being done by an animal, or not(so directly from the environment to the cell)".

Further conclusion

further clarification on the distinction…

When I asked this question to the microbiologist-

I understand respiration is usually done with an inorganic substance (O2) as terminal electron acceptor, but that it can be done with an organic substance DMSO. So, is there still an enormous chemical difference between respiration and fermentation, when respiration is done with DMSO? I don't see how the final electron acceptor being internal vs being external, would cause a major different in the chemistry.

I got a very good answer-
He said - Fermentation doesn't use a proton gradient and chemiosmotic force to generate ATP. Oxidative phosphorylation vs substrate-level phosphorylation

Looking further… I see Glycolysis produces ATP via substrate-level phosphorylation. Looking at the distinct parts of respiration and fermentation i.e. after glycolysis. (some define respiration and fermentation as excluding glycolysis anyway). Then, (after glycolysis), Respiration uses oxidative phosphorylation. Fermentation, if it produces ATP (which it usually does), then it is done via substrate-level phosphorylation.

(Physiology and Biochemstiry of Prokaryotes">Do acetic acid bacteria use the electron transport chain when converting ethanol to acetic acid?

Fermentation / Anaerobic Respiration

Some examples of anaerobic respiration include alcohol fermentation, lactic acid fermentation.


Some examples of anaerobic respiration include alcohol fermentation, lactic acid fermentation (which can result in yogurt and in sore muscles), and in decomposition of organic matter. The equation is: glucose + enzymes = carbon dioxide + ethanol/lactic acid.


Fermentation is an anaerobic metabolic process in which an organism converts a carbohydrate to an alcohol or an acid.


The first step in all fermentation processes is glycolysis, the conversion of glucose to pyruvate:

#"C"_6"H"_12"O"_6 → "2CH"_3"COCOO"^(−) + "2H"_2"O" + 2"H"^+#

There are two main types of fermentation one converts pyruvate into lactate (lactic acid) and the other into ethanol.


In lactic acid fermentation, pyruvate is converted into lactic acid.

#underbrace("CH"_3"COCOO"^-)_color(red)("pyruvate") stackrelcolor (blue)("enzymes")(→) underbrace("2CH"_3"CH(OH)COOH")_color(red)("lactic acid") #

In alcohol fermentation,the pyruvate is decarboxylated to acetaldehyde, and then into ethanol.

#"CH"_3"COCOO"^(-)+ "H"^+ stackrelcolor (blue)("pyruvate decarboxylase")(→) "CH"_3"CHO" + "CO"_2#

#"CH"_3"CHO" stackrelcolor (blue)("alcohol dehydrogenase")(⇌) "CH"_3"CH"_2"OH"#

In an aerobic process, the pyruvate is converted by respiration to carbon dioxide and water.

Here is a summary of the three possible fates of pyruvate:


What Are the Two Main Types of Anaerobic Respiration?

The two main types of anaerobic respiration are alcoholic fermentation and lactic acid fermentation. These methods of respiration occur when the amount of oxygen available is too low to support aerobic respiration.

Alcoholic fermentation converts glucose into ethanol. In alcoholic fermentation, glucose is broken down by glycolysis, and two ATP molecules are released in the process. The pyruvic acid molecules produced during glycolysis break down into ethanol and carbon dioxide. In animals, the process of lactic acid fermentation, similarly, occurs after the glycolysis process. Pyruvic acid is changed into lactic acid, and muscle tissue is broken down by lactic acid. Lactic acid fermentation is the reason that muscles burn during an intense or long workout. Lactic acid breakdown of muscles results in muscle tissue rebuilding itself to become stronger.

How does this Apply to Sports?

Many athletes take advantage of this type of respiration as it helps with short bursts of energy. They can train to lengthen the time anaerobic respiration goes on in their cells, and increase the amount of lactic acid they can build up. Training usually consists of high intensity exercises such as jumping or sprinting for a short period of time repeatedly. Over time, a person can lengthen the amount time their body spends on anaerobic metabolism. This can be useful in sports such as baseball, or football, where plays are short and intense.

Anaerobic Respiration and Its Application

Anaerobic respiration is the process by which incomplete oxidation of respiratory substrate takes place. In this case, it occurs in the absence of oxygen resulting the end products of ethyl alcohol and CO2 in plants and lactic acid (in animals) with very slight energy.

Anaerobic respiration is observed in certain bacteria, yeast and other fungi, endoparasites and animal muscles cells. It is also known as fermentation. The common reaction of anaerobic respiration is:

Features of Anaerobic Respiration

Phase of Anaerobic Respiration

There are two definite phases of anaerobic respiration:

1. Glycolysis: The first phase of anaerobic respiration is glycolysis in which 2 molecules of pyruvic acid and 4H + are formed from a molecule of glucose from the same reaction of glycolysis(EMP pathway) found in aerobic respiration.

2. Fermentation: The second phase of anaerobic respiration is fermentation which consists of decarboxylation and reduction reactions converting the pyruvic acid into either ethyl alcohol with the evolution of carbon dioxide (CO2).

Ethanol Fermentation: Two steps involve in this process. At first pyruvic acid undergo carboxylation in the presence of pyruvic carboxylase enzyme and produce acetaldehyde and CO2. Then the acetaldehyde dehydrogenated by NADH2 into ethanol in presence of dehydrogenase enzyme.

  • Lactic acid Fermentation: The pyruvic acid dehydrogenated by NADH2 into lactic acid in anaerobic condition of cell and in presence of dehydogenase enzyme. Beside anaerobes lactic acid is formed in muscle cell of higher animal. Higher plants do not produce any lactic acid.

Energetic of Anaerobic Respiration

Two molecules of NADH2 and two molecules of ATP are formed in glycolysis. During fermentation two molecules of NADH2 are used. Only two molecules of ATP are used to produce slight energy of 20 Kcal.

Application of Anaerobic Respiration

Various microorganisms take part in the fermentation process and produce highly useful end products. These useful end products make benefit to the mankind in many ways. Some notable fermentation activities in the industrial sectors are given below:

Alcoholic Fermentation: A "New" Source of Energy?

Have you fueled your car with corn? You have, if you bought gas within the city of Portland, Oregon. Portland was the first city to require that all gasoline sold within the city limits contain at least 10% ethanol. By mid-2006, nearly 6 million “flex-fuel” vehicles – which can use gasoline blends up to 85% ethanol (E85 – Figure 5) were traveling US roads. This “new” industry employs an “old” crew of yeast and bacteria to make ethanol by an even older biochemical pathway – alcoholic fermentation. Many people consider “renewable” biofuels such as ethanol a partial solution to the declining availability of “nonrenewable” fossil fuels. Although controversy still surrounds the true efficiency of producing fuel from corn, ethanol is creeping into the world fuel resource picture (Figure 6).

Figure 5: Ethanol provides up to 85% of the energy needs of new “fuel-flex” cars. Although its energy efficiency is still controversial, ethanol from corn or cellulose appears to be more “renewable” than fossil fuels.

Figure 6: One of the newest kids on the block, ethanol from corn or cellulose is produced by yeasts through alcoholic fermentation – an anaerobic type of respiration.

You are probably most familiar with the term ”fermentation” in terms of alcoholic beverages. You may not have considered that the process is actually a chemical reaction certain bacteria and yeasts use to make ATP. Like lactic acid fermentation, alcoholic fermentation processes pyruvate one step further in order to regenerate NAD+ so that glycolysis can continue to make ATP. In this form of anaerobic respiration, pyruvate is broken down into ethyl alcohol and carbon dioxide:

$C_3 H_3 O_3 ( ext) + NADH longrightarrow C_2 H_5 OH ( ext) + CO_2 + NAD^+$

We have domesticated yeast (Figures 7 and Figure 8) to carry out this type of anaerobic respiration for many commercial purposes. When you make bread, you employ the yeast to make the bread “rise” by producing bubbles of carbon dioxide gas. Why do you suppose that eating bread does not intoxicate you?

Figure 7: Yeasts are facultative anaerobes, which means that in the absence of oxygen, they use alcoholic fermentation to produce ethyl alcohol and carbon dioxide. Both products are important commercially.

Figure 8: We employ yeasts to use their anaerobic talents to help bread rise (via bubbles of (CO_2)) and grapes ferment (adding ethanol).

Brewers of beer and wine use yeast to add alcohol to beverages. Traditional varieties of yeast not only make but also limit the quantity of alcohol in these beverages, because above 18% by volume, alcohol becomes toxic to the yeast itself! We have recently developed new strains of yeast which can tolerate up to 25% alcohol by volume. These are used primarily in the production of ethanol fuel.

Human use of alcoholic fermentation depends on the chemical energy remaining in pyruvate after glycolysis. Transforming pyruvate does not add ATP to that produced in glycolysis, and for anaerobic organisms, this is the end of the ATP-producing line. All types of anaerobic respiration yield only 2 ATP per glucose.

Aerobic vs. Anaerobic Respiration: A Comparison

As aerobes in a world of aerobic organisms, we tend to consider aerobic respiration “better” than fermentation. In some ways, it is. However, anaerobic respiration has persisted far longer on this planet, through major changes in atmosphere and life. There must be value in this alternative way of making ATP. We will compare the advantages and disadvantages of these two types of respiration.

A major argument in favor of aerobic over anaerobic respiration is overall energy production. Without oxygen, organisms can only break 6-carbon glucose into two 3-carbon molecules. As we saw earlier, glycolysis releases only enough energy to produce two (net) ATP per molecule of glucose. In contrast, aerobic respiration breaks glucose all the way down to (CO_2), producing up to 38 ATP. Membrane transport costs can reduce this theoretical yield, but aerobic respiration consistently produces at least 15 times as much ATP as anaerobic respiration. This vast increase in energy production probably explains why aerobic organisms have come to dominate life on earth. It may also explain how organisms were able to increase in size, adding multicellularity and great diversity.

However, anaerobic pathways persist, and a few obligate anaerobes have survived over 2 billion years beyond the evolution of aerobic respiration. What are the advantages of fermentation?

One advantage is available to organisms occupying the few anoxic (lacking oxygen) niches remaining on earth. Oxygen remains the highly reactive, toxic gas which caused the “Oxygen Catastrophe.” Aerobic organisms have merely learned a few tricks – enzymes and antioxidants - to protect themselves. Organisms living in anoxic niches do not run the risk of oxygen exposure, so they do not need to spend energy to build these elaborate chemicals.

Individual cells which experience anoxic conditions face greater challenges. We mentioned earlier that muscle cells “still remember” anaerobic respiration, using lactic acid fermentation to make ATP in low-oxygen conditions. Brain cells do not “remember”, and consequently cannot make any ATP without oxygen. This explains why death follows for most humans who endure more than four minutes without oxygen.

Variation in muscle cells gives further insight into some benefits of anaerobic respiration. In vertebrate muscles, lactic acid fermentation allows muscles to produce ATP quickly during short bursts of strenuous activity. Muscle cells specialized for this type of activity show differences in structure as well as chemistry. Red muscle fibers are “dark” because they have a rich blood supply for a steady supply of oxygen, and a protein, myoglobin, which holds extra oxygen. They also contain more mitochondria, the organelle in which the Krebs cycle and electron transport chain conclude aerobic respiration. White muscle cells are “light” because they lack the rich blood supply, have fewer mitochondria, and store glycogen rather than oxygen. When you eat dark meat, you are eating endurance muscle. When you eat white meat, you are eating muscle built for sprinting.

Each type of muscle fiber has advantages and disadvantages, which reflect their differing biochemical pathways. Aerobic respiration in red muscles produces a great deal of ATP from far less glucose - but slowly, over a long time. Anaerobic respiration in white muscles produces ATP rapidly for quick bursts of speed, but a predator who continues pursuit may eventually catch a white-muscled prey.

In summary, aerobic and anaerobic respiration each have advantages under specific conditions. Aerobic respiration produces far more ATP, but risks exposure to oxygen toxicity. Anaerobic respiration is less energy-efficient, but allows survival in habitats which lack oxygen. Within the human body, both are important to muscle function. Muscle cells specialized for aerobic respiration provide endurance, and those specialized for lactic acid fermentation support short but intense energy expenditures. Both ways of making ATP play critical roles in life on earth.


Lactic acid fermentation | Cellular respiration | Biology

Exploring how the oxidation of co-enzymes like NADH to NAD+ can eventually lead to the production of ATP through oxidative phosphorylation and the electron transport chain.

Source: YouTube, Khan Academy, 2015, Duration 11:21, URL:

The fermentation process in yogurt

Jake describes the chemical changes that occur as growing bacteria transform milk into yogurt.

Productive Chain of Biofuels and Industrial Biocatalysis

Ayla Sant’Ana da Silva , . Viridiana S. Ferreira-Leitão , in Biotechnology of Microbial Enzymes , 2017

20.4 Biogas/Biomethane Production

The process of anaerobic fermentation (AF) has been considered as a viable technology for the treatment of organic waste materials and for bioenergy production. This process generates intertwined agricultural and environmental benefits, such as renewable energy production, environmentally friendly organic waste treatment, greenhouse gas emission reduction, pathogen reduction through sanitation, and improved fertilization efficiency ( Holm-Nielsen et al., 2009 Mao et al., 2015 ). For these reasons, the AF of organic waste has received great attention worldwide in recent years.

AF is a degradation process of organic material into biogas by microorganisms, in the absence of oxygen. AF offers significant advantages, such as low power consumption, low nutrient requirements, low sludge production, and high efficiency in the reduction of organic load and generation of biogas ( Khalid et al., 2011 Rajagopal et al., 2013 ). A variety of organic materials have been used as feedstocks for AF, for example, lignocellulosic biomass ( Arreola-Vargas et al., 2015 Li et al., 2015 Sambusiti et al., 2013 ), municipal solid waste ( Beevi et al., 2015 Luo et al., 2014 ), animal manure ( Babaee et al., 2013 Dareioti and Kornaros, 2015 ), and food processing waste ( Browne and Murphy, 2013 Zhang et al., 2014, 2015 ), among others. These feedstocks are usually available in small-scale biogas plants, which avoid additional transportation costs and thereby render biogas production economically feasible ( Naik et al., 2014 Yang et al., 2014 ). In addition to biogas production, AF also makes it possible to obtain an effluent called digestate, which can be used as a biofertilizer. The quality of the digestate is essential for its acceptance as a replacement for mineral fertilizers in crop production. Parameters that allow high-quality digestate are appropriate pH, nutrient and chemical content, with no inorganic impurities and no pathological contamination ( Hamawand, 2015 ).

Several factors can affect the productivity and stability of the anaerobic fermentative system for biogas production, such as temperature, pH, carbon-to-nitrogen mass ratio (C:N ratio), redox potential, organic loading rate (OLR), and retention time. Temperature is one of the main factors affecting AF, as it directly influences the CH4 yield. In general, CH4 production increases with increasing temperature ( Zhang et al., 2014 ). The growth rate of microorganisms is significantly affected by pH. For example, the growth rate of methanogenic archaea is greatly reduced at pH below 6.0 and above 8.0 ( Mao et al., 2015 ). The C:N ratio affects the performance of the AF, as the anaerobic bacteria require a balanced nutritional medium for their growth and maintenance of a stable environment. According to the literature, a C:N range of 20–30 was considered to be the optimum condition for AF ( Puyuelo et al., 2011 Zhang et al., 2014 ). The redox potential can be used as an indicator of the AF, as the growth of methanogenic archaea requires a low redox potential. This redox potential has been reported to range from −200 to −400 mV ( Naik et al., 2014 ). The stability of the AF is dependent on the OLR and hydraulic retention time (HRT). When the OLR is high, the fermentative system may become unbalanced due to excessive production of volatile acids, leading to inhibition of the process. The same behavior is observed at short HRT. Thus, a low OLR and a long HRT provide the best strategy for achieving constant and maximal methane yields ( Naik et al., 2014 Mao et al., 2015 ).

The conversion of organic material into biogas is conducted by a consortium of microorganisms through a series of metabolic phases, namely hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The first phase involves the hydrolysis of complex organic materials into simple organic materials such as sugars, amino acids, and fatty acids. In the acidogenic phase, the soluble products from the previous step are converted into volatile organic acids, alcohols, CO2, H2, and new bacterial cells. The acetogenic bacteria are responsible for oxidation of the products generated in the acidogenic phase into suitable substrates (H2 and acetic acid) for methanogenic archaea. In the last step, the methanogenic archaea convert the H2 and acetic acid into CH4 and CO2 ( Sá et al., 2014 Christy et al., 2014 ).

The composition of biogas varies with the type of feedstock used in the process and the operating conditions of the digester. In general, biogas consists of 50–75% methane (CH4) and 25–50% carbon dioxide (CO2) with small amounts of water vapor (H2O), hydrogen sulfide (H2S), and ammonia (NH3), among others ( Surendra et al., 2014 ). CH4 is the component of biogas responsible for heating value. In general, 1 m 3 of biogas containing 60% CH4 has a heating value of 21.5 MJ, which corresponds to approximately 6 kWh of electricity ( Hamawand, 2015 Surendra et al., 2014 ). In addition to electricity generation, biogas also allows heat generation in a combined heat and power unit ( Yang et al., 2014 ).

20.4.1 Generation and Use of Biogas/Biomethane in Brazil

Biogas produced from the AF process has been presented as an efficient alternative in the production of bioenergy. Biogas production in the IEA Bioenergy Task 37 member countries is clearly dominated by Germany, with more than 10,000 biogas plants. None of the other member countries (Austria, Brazil, Denmark, Finland, France, Norway, Ireland, Korea, Sweden, Switzerland, Netherlands, and the United Kingdom) have more than 1000 biogas plants apiece. The annual biogas production is approximately 80 TWh in Germany, 20 TWh in the United Kingdom, 4 TWh in both the Netherlands and France, and between 0.5 and 2 TWh in remaining countries. In countries such as the United Kingdom, Brazil, and South Korea, biogas produced in landfills is the largest source, while landfill gas is only a minor contributor in countries such as Germany, Switzerland, and Denmark. The biogas produced is mainly used for the generation of heat and electricity in most countries, with the exception of Sweden, where approximately half of the produced biogas is used as vehicle fuel. Many countries, such as Denmark, Germany, and South Korea, among others, show initiatives and interest in increasing the share of biogas to be used as a vehicle fuel in the near future ( IEA, 2015 ).

The Brazilian potential for biogas production is great because of the amount of wastewater and organic waste generated, as well as the disposal of waste in landfills. According to Applied Economic Research Institute (IPEA) data, in 2009, Brazil produced 291 million tons of wastes from the agroindustry. If all these residues can be used for energy production, it could represent an energy potential of up to 23 GW/year, which is equivalent to 191,398 GWh/year. The wastes with the highest potential for energy production, approximately 69% of the total estimated for the sector, are bagasse and filter cake from sugarcane, generated mostly in Brazil’s Southeast region. In addition, this region has a high amount of waste generated by dairy cattle (106 million tons/year) and laying hens (4.3 million tons/year). The South Brazil region already generates a large amount of waste through the creation of broilers (7.5 million tons/year—not considering the poultry litter) and pig manure (9.8 million tons/year) ( IPEA, 2012 ). The projection of the biogas/biomethane production potential is approximately 12 billion cubic meters per year in the sugar and alcohol sector and 8 billion cubic meters per year in the food agroindustry sector ( Bley Jr., 2015 ). However, the energy use of biogas for electrical systems in Brazil remains insufficient. The majority of the biogas plants are located on agricultural properties to process residues and on landfills ( IEA, 2015 ). According to the Bank of Generation Information of the Brazilian Electricity Regulatory Agency (ANEEL), there are 403 thermoelectric plants fueled by biomass in operation in the country. Of the total related plants, only 24 are driven by biogas, totaling approximately 66.1 MW of installed capacity, which represents little more than 0.5% of the electricity production by biomass ( ANEEL, 2015 ).

The purification of biogas, through the removal of CO2, H2O, H2S, NH3 and other impurities, makes it possible to obtain biomethane, which can be used as a substitute for natural gas and as a transportation fuel ( Holm-Nielsen et al., 2009 Pöschl et al., 2010 ). This approach allows an efficient integration of biogas into the energy sector, and it is also observed that the industries are strongly interested in this product, not only in Brazil but in Africa, Europe and throughout the Americas ( Bley Jr., 2015 ). Applications of gaseous fuels developed from shale gas in the United States have been tendered competitively around the world. In Brazil, impacts are already observed on the use of engines relying on 100% natural gas (perfectly replaceable by biomethane), including heavy loads, trucks and buses. Shale gas has accelerated the arrival of the “Age of gas” in the world energy matrix and in Brazil ( Bley Jr., 2015 ).

Currently, the use of biogas as a vehicle fuel is rare. However, one project developed by ITAIPU Binacional, the Itaipu Technology Park Foundation, Scania, Haacke Farm, and the International Center on Renewable Energy-Biogas/CIBiogás-ER has demonstrated the viability to use biomethane as a vehicle fuel ( IEA, 2015 ). A recent initiative is the creation of legislation (Resolution No. 8, Jan 30, 2015) that will allow the development of the biomethane market in Brazil. This legislation was developed by the government’s National Agency of Petroleum, Natural Gas and Biofuels (ANP) and applied to biomethane produced from biodegradable materials originating from agroforestry and organic waste and intended for nationwide use as a fuel for vehicles, in commercial shipping and for residential use. The standard includes obligations regarding quality control to be met by the various economic agents who trade biomethane throughout Brazil ( IEA, 2015 ).

20.4.2 Biohydrogen Production via AF

Another strategy that has been extensively studied is H2 production from the AF process. In particular, hydrogen has attracted great interest due to its high energy content (143 kJ·g −1 ) and clean burning. H2 production via AF can be performed by mixed microbial cultures derived from natural environments or pure cultures selected from H2-producing bacteria. The use of mixed cultures for large-scale processes is considered favorable due to the control and operation of the process being facilitated by the use of nonsterile media, reducing the overall cost. In this approach, H2-consuming microorganisms are inhibited and/or eliminated, allowing the selection of H2-producing microorganisms. This effect is obtained by pretreatment of the inoculum. During pretreatment, the H2-producing-bacteria, such as Clostridium, can form endospores as a result of bacterial stress when in hostile environmental conditions (high temperature, nutrient limitation, extreme acidity and alkalinity), while methanogenic archaea (H2-consuming microorganisms) cannot resist these conditions ( Sá et al., 2013 ). Different chemical (acid, alkali, or organic compounds) and physical (heat, aeration, ultraviolet, ultrasonic, and freezing/thawing) methods of inoculum pretreatment have been reported in the literature to favor H2 production ( Cui and Shen, 2012 Dong et al., 2010 Wang and Wan, 2008 Wang et al., 2011 ).

Studies have shown that microorganisms of the genus Clostridium are primarily responsible for H2 production in inocula with different methods of pretreatment ( Lee et al., 2009 Liu et al., 2009 Ren et al., 2008 ). These microorganisms produce hydrogenase enzymes that catalyze the reversible reaction of hydrogen oxidation (2H + +2e − ↔H2) ( Kirtay, 2011 ). Sá et al. (2013, 2011) have used the level of hydrogenase gene (hyd) expression as an indicator of H2 production in different systems.

A wide variety of materials rich in carbohydrates, lipids, and/or proteins can be used as substrates in the production of H2 by AF. Carbohydrates, such as sucrose and glucose, are easily assimilated by fermentative bacteria. However, sources of pure carbohydrates represent expensive substrates for H2 production in large scale. In this context, the use of waste materials as potential substrates for H2 production has attracted great interest ( Lin et al., 2012 ). Different waste materials have been used for H2 production, such as food waste ( Yong et al., 2015 Gadhe et al., 2014 ), sugarcane vinasse from ethanol production ( Fernandes et al., 2010 Santos et al., 2014 ), dairy industry wastewater ( Karadag et al., 2014 ), lignocellulose hydrolysates ( Chen et al., 2013 Nissila et al., 2014 ), glycerin from biodiesel processing ( Fernandes et al., 2010 ), and palm oil mill effluent ( Chong et al., 2009a,b ), among others.

20.4.3 Sequential Production of H2 and CH4

The use of waste materials for H2 production has been gaining importance to support environmental sustainability. However, most of the organic fraction remains soluble at the end of the fermentation process ( Peixoto et al., 2012 ). A two-stage process for sequential production of H2 and CH4 has been considered as an alternative to improve the viability of soluble organic fraction treatment. This system includes the separation of acidogenic and methanogenic processes for the production of H2 and CH4, respectively. In the first stage (acidogenic process), organic matter is degraded to organic acids and H2, and in the second stage (methanogenic process), organic acids are metabolized to CH4 and CO2 ( Kiran et al., 2014 ).

The purpose of using a two-stage system for the production of H2 and CH4 is to optimize each process separately. In addition, previous studies showed that the two-stage process for the production of H2 and CH4 allows the production of energy with higher efficiency than a single-stage process for CH4 production ( Luo et al., 2011 Liu et al., 2006 ). A recent review showed that the sequential production of H2 and CH4 has higher energy potential than the production of CH4 in a single-stage process. The authors have shown through calculations of the theoretical energy production that the two-stage process using sucrose, glucose or fructose as a substrate presented approximately 9% more energy than the CH4 production process in a single stage. Values of approximately 14% and 11% were obtained for xylose and glycerol, respectively ( Sá et al., 2014 ).

12 Pros and Cons of Anaerobic Respiration

Respiration takes place in two different ways in cells: either aerobically or anaerobically. The amount of energy that is produced will then be distributed to a number of different needs throughout the body. What makes anaerobic respiration different is that it doesn’t need to have oxygen present for it to begin. There are certain advantages to anaerobic respiration that aerobic respiration cannot provide, but there are certain disadvantages that must also be considered.

Here is a look at some of the key points to consider when evaluating anaerobic respiration today.

What Are the Pros of Anaerobic Respiration?

1. Muscles can respire even when they don’t have oxygen available.
When you’re working out, the amount of oxygen the body needs to aerobically respire increases. Inevitably there won’t be enough oxygen available within the body for every muscle group to be able to have what it needs. Through the process of anaerobic respiration, the muscles can still get the energy it needs to continue working so that the body doesn’t just shut down.

2. It assists aerobic respiration.
A unique component of anaerobic respiration is the fact that it can metabolize pyruvic acid. This acid is used to regenerate the enzymes that the body needs for a process called glycolysis, which starts the respiration process in the first place. If oxygen becomes available, then the body can transition from anaerobic respiration to aerobic respiration, thus completing an energy cycle that can keep the body moving.

3. The body can adapt the energy more quickly.
Anaerobic respiration is an essential part of the human fight or flight reaction. The energy it produces is more readily absorbed when compared to aerobic energy, allowing the cells to start the respiration process for replenishment. This gives people bursts of energy when necessary to respond to any given situation so that reactions can be appropriate.

4. It can create a renewable source of energy.
Certain organisms that are 100% anaerobically inclined will produce gas as a byproduct of their respiration process. This happens most frequently when they are breaking down waste products of some sort. The gas that is produced is often combustible, which means it could potentially be harnessed to create a source of energy that is renewable.

5. People can raise their threshold levels for lactic acid.
With regular high-intensity exercise training sessions, it becomes possible to raise a personal threshold level for lactic acid. Although there are eventually caps on the threshold that exist, people are able to extend their ability to exercise by up to 50% with frequent high-intensity sessions.

6. Only a few minutes of anaerobic exercises are typically necessary to improve respiration rates.
Compared to aerobic exercises, it only takes about 50% of the time to use strength training for maximum benefit. Even resistance machines can help to create these exercises to build up threshold levels. When first getting started, this often means only a few minutes per day need to be dedicated to this process.

What Are the Cons of Anaerobic Respiration?

1. It produces lactic acid as a side effect.
Because there isn’t always oxygen involved in the respiration process, the amount of glucose that is broken down is reduced greatly. According to some estimates, anaerobic respiration only provides 5% of the energy potential from glucose that aerobic respiration is able to provide. Because every action as an equal and opposite reaction, a waste product is created and in this instance it is lactic acid. There is only so much lactic acid that can be stored, which means eventually the body shuts down because it just doesn’t have any room to leave waste products from energy production.

2. Too much of it can cause pain and cramping to occur.
Anaerobic respiration may help to allow muscles to receive the energy that they need, but too much of it causes pain and cramping within the muscle. This is because the lactic acid has built up so much that the muscle affected cannot properly contract any more. It requires rest periods where a person’s circulatory system can filter out the lactic acid for the muscle to recover and that can be a lengthy amount of time. Although stretching out muscles can help filter out some of the lactic acid, it won’t increase the muscle capacity.

3. It is a temporary process for many forms of life.
Although anaerobic respiration is a bit of evolution that may trace back as far as 3.5 billion years, the fact remains that for most forms of life, it is a temporary solution to a lack of oxygen problem. Some living microbes, such as yeast, thrive in oxygen-free environments because they are naturally geared toward the anaerobic process. For humans, without oxygen, eventually the body shuts down. It means this form of respiration is a temporary process that can only provide a limited benefit.

4. It does not provide endurance energy.
Anaerobic energy might be excellent for the fight or flight response, but it doesn’t have the power to endure. Once muscle groups reach their threshold of lactic acid, they stop functioning. That’s the feeling of fatigue that sets in rapidly. Even in well conditioned athletes, the maximum level of energy output from anaerobic respiration that can be produced is about 4 minutes worth of maximum effort.

5. Metabolism rates increase for calorie burning, but not for fat burning.
The problem with anaerobic respiration is where it tends to originate: within the body’s muscle fibers. The cells of the muscle are stimulated by this form of respiration, but it also requires energy for it to happen. This often results in muscle tissues being consumed by the body instead of fat tissues. There is no guarantee that the targeted muscle groups are going to see the consumption either, which means the toning process can become inconsistent without aerobic respiration added from time to time.

6. Working to muscle fatigue increases the chances of an injury.
Even with proper stretching and conditioning, working until the muscles refuse to contract creates more risk for strains, sprains, and tears to occur. People are often encouraged to push out “one more rep” to increase their threshold levels, but that extra rep might take the muscles beyond their capacity. Only listen to your body and you’ll be able to avoid this disadvantage.

The pros and cons of anaerobic respiration show that it is an essential component of life. 3.5 billion years of potential evolution have created a system where a person can max out their energy output for up to 4 minutes. It takes time to develop threshold levels, so be sure to include weight training with running or other forms of aerobic exercise for a maximum level of benefit.

Lesson 4.4 - Anaerobic Pathways: Life Without Oxygen Flashcards Preview

Define Alcohol Fermentation

A process in which pyruvate is decarboxylated, producing a molecule of CO2, ethanol, and an NAD + .

What is a process in which pyruvate is decarboxylated, producing a molecule of CO2, ethanol, and an NAD +​ known as?

Define Lactate Fermentation

A process in which pyruvate reacts with NADH and is converted directly into lactate and regenerates NAD + .

What is a process in which pyruvate reacts with NADH and is converted directly into lactate and regenerates NAD + known as?

Fermentation pathways enable organisms to use _____ as a source of ATP, without an _____ transport chain.

Fermentation pathways enable organisms to use glycolysis as a source of ATP, without an electron transport chain.

Alcohol fermentation is performed by _____ and has significant commercial value.

Alcohol fermentation is performed by yeast and has significant commercial value.

Lactate fermentation in muscles provides a supplementary source of _____ when energy demands are very high.

Lactate fermentation in muscles provides a supplementary source of ATP when energy demands are very high.

_____ respiration uses inorganic substances other than O2 as terminal electron acceptors in an electron transport chain.

Anaerobic respiration uses inorganic substances other than O2 as terminal electron acceptors in an electron transport chain.

Compare aerobic respiration and fermentation in terms of the amount of ATP that can be generated from a single glucose molecule.

Aerobic respiration and fermentation can generate quite different numbers of molecules of ATP from a single glucose molecule. Cells that rely on fermentation to generate ATP using only glycolysis generate 2 ATP per molecule of glucose. Conversely aerobic respiration uses an electron transport chain and produces approximately 36–38 ATP per molecule of glucose.

Why do cells rely on fermentation rather than glycolysis alone?

If cells relied on glycolysis alone, they would quickly run out of NAD+, a necessary reactant in glycolysis. They rely on fermentation to regenerate the NAD+.

Explain the anaerobic pathway that is used to create a loaf of bread. How does this pathway work?

The alcoholic fermentation pathway is used by bakers to create a loaf of leavened bread. Yeast is mixed with a small amount of sugar and is blended into dough, where oxygen levels are low. As the yeast cells convert the sugar into ethanol and CO2, the gaseous CO2 expands and creates gas bubbles that cause the dough to rise. In the oven, thermal energy evaporates the alcohol and causes further expansion of the bubbles, producing leavened bread.

Do our muscle cells produce alcohol? Given that alcohol and lactate fermentation both yield two ATP molecules for every glucose molecule, do you think it would make any difference which pathway was used? Explain.

No, our muscle cells do not produce alcohol instead they undergo lactate fermentation under anaerobic conditions. Even though they produce the same number of ATP per molecule of glucose, alcohol is toxic. Producing it in large amounts during strenuous exercise would cause a variety of problems for the cell and for the organism as a whole.

Using what you know about lactic acid fermentation, explain why a person could not perform sternuous exercise indefinitely.

Muscle tissue has a lactate threshold, which is the point of lactic acid buildup at which the acid cannot be carried away from muscle tissue as quickly as it is produced. After reaching this threshold, muscle damage would result and a person would eventually be unable to continue exercising.

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