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Anaerobic Respiration

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Biology

In this article, we discover anaerobic respiration, its definition, formula, and the difference between aerobic and anaerobic respiration. Hopefully, by now, you have learned something about aerobic respiration, the process by which oxygen and ATP break down glucose. But what happens when an organism does not have access to oxygen but still needs energy for its metabolic processes? That is where anaerobic respiration comes into play.

Anaerobic respiration describes how ATP breaks down glucose to form either lactate (in animals) or ethanol (in plants and microorganisms).

Anaerobic respiration occurs in the cytoplasm (a thick fluid surrounding organelles) of the cell and involves two stages: glycolysis and fermentation. It is a distinct process from aerobic respiration.

Have you ever done an intense workout and woken up the next day with sore muscles? Until recently, the lactic acid produced during anaerobic respiration was to blame for this muscle soreness! It is true that the body switches to anaerobic respiration during intense exercise, but this theory was disproved in the 1980s.

Recent research suggests that stiff muscles are due to various physiological effects in response to the trauma suffered by muscles during exercise. Nowadays, the theory is that lactic acid is a valuable fuel for your muscles, not an inhibitor!

Anaerobic Respiration The cytoplasm of plant and animal cells StudySmarter

The cytoplasm of plant and animal cells, sciencefacts.net

What is the difference between aerobic and anaerobic respiration?

We cover the differences between aerobic and anaerobic respiration in more detail in our article on respiration. However, if you are short on time, we have helpfully summarised them below:

  • Aerobic respiration occurs in the cytoplasm and mitochondria, while anaerobic respiration occurs only in the cytoplasm.
  • Aerobic respiration requires oxygen, whereas anaerobic respiration does not.
  • Anaerobic respiration produces less ATP overall than aerobic respiration.
  • Anaerobic respiration produces carbon dioxide and ethanol (in plants and microorganisms) or lactate (in animals), whilst the main products of aerobic respiration are carbon dioxide and water.

However, it is also important to remember that both processes have some things in common, including:

  • Both produce ATP to power important metabolic processes.
  • Both involve the breakdown of glucose through oxidation, occurring during glycolysis.

What are the stages of anaerobic respiration?

Anaerobic respiration has only two stages, and both occur in the cell's cytoplasm.

Table 1 should help you recognise the symbols used in the chemical formulas. You might notice some formulas contain numbers before the substance. The numbers balance chemical equations (no atoms are lost during the process).

Table 1. Summary of the chemical symbols.

Chemical Symbol
Name
Glucose
Pi
Inorganic phosphate
Pyruvate
Pyruvic acid
Lactic acid
Ethanol
Acetaldehyde

Glycolysis

The process of glycolysis is the same whether respiration is aerobic or anaerobic. Glycolysis occurs in the cytoplasm and involves splitting a single, 6-carbon glucose molecule into two 3-carbon pyruvate molecules. During glycolysis, several smaller, enzyme-controlled reactions occur in four stages:

  1. Phosphorylation – Before breaking down into two 3-carbon pyruvate molecules, the glucose must be made more reactive by adding two phosphate molecules. Therefore, we refer to this step as phosphorylation. We obtain the two phosphate molecules by splitting two ATP molecules into two ADP molecules and two inorganic phosphate molecules (Pi). We get this via hydrolysis, which uses water to split ATP. This process provides the energy needed to activate the glucose and lowers the activation energy for the following enzyme-controlled reaction.
  2. Creation of triose phosphate – In this stage, each glucose molecule (with the two Pi groups added) splits in two to form two triose phosphate molecules, a 3-carbon molecule.
  3. Oxidation – Once these two triose phosphate molecules form, we need to remove hydrogen from them. These hydrogen groups then transfer to NAD+, a hydrogen-carrier molecule, producing reduced NAD (NADH).
  4. ATP production – The two newly oxidised triose phosphate molecules convert into another 3-carbon molecule known as pyruvate. This process also regenerates two ATP molecules from two molecules of ADP.

The overall equation for glycolysis is:

Anaerobic Respiration Glycolysis Equation StudySmarter

Fermentation

As mentioned earlier, fermentation can produce two different products depending on which organism respires anaerobically. We will first examine the fermentation process in humans and animals that produces lactic acid.

Lactic acid fermentation

The process of lactic acid fermentation is as follows:

  1. Pyruvate donates an electron from an NADH molecule.
  2. NADH is thus oxidised and converted to NAD +. The molecule of NAD + is then used in glycolysis, allowing the whole process of anaerobic respiration to continue.
  3. Lactic acid forms as a by-product.

The overall equation for this is:

Anaerobic Respiration Lactic acid fermentation StudySmarter

Lactic dehydrogenase helps speed up (catalyse) the reaction!

The following diagram illustrates the entire process of anaerobic respiration in animals:

Anaerobic Respiration Lactic acid fermentation steps StudySmarter

The steps of anaerobic respiration in animals, khanacademy.org

Lactate is a deprotonated form of lactic acid (i.e., a lactic acid molecule missing a proton and with a negative charge). So when you read about fermentation, you often hear that lactate is produced instead of lactic acid. There is no material difference between these two molecules for A-level purposes, but it is important to bear this in mind!

Ethanol fermentation

Ethanol fermentation occurs when bacteria and other microorganisms (e.g., fungi) respire anaerobically. The process of ethanol fermentation is as follows:

  1. A carboxyl group (COOH) is removed from pyruvate. Carbon dioxide () is released.
  2. A 2-carbon molecule called acetaldehyde forms.
  3. NADH is reduced and donates an electron to acetaldehyde, forming NAD+. The molecule of NAD+ is then used in glycolysis, allowing the entire process of anaerobic respiration to continue.
  4. The donated electron and H+ ion allow the formation of ethanol from acetaldehyde.

Overall, the equation for this is:

Pyruvate decarboxylate and aldehyde dehydrogenase are the two enzymes that help catalyse ethanol fermentation!

The following diagram summarises the entire process of anaerobic respiration in bacteria and microorganisms:

Anaerobic Respiration Ethanol fermentation steps StudySmarter

The steps of anaerobic respiration in bacteria and microorganisms, khanacademy.org

What is the anaerobic respiration equation?

The overall equation for anaerobic respiration in animals is as follows:

The overall equation for anaerobic respiration in plants or fungi is:

Anaerobic Respiration - Key takeaways

  • Anaerobic respiration is a form of respiration that does not require oxygen and can occur in animals, plants and other microorganisms. It only occurs in the cytoplasm of the cell.
  • Anaerobic respiration has two stages: glycolysis and fermentation.
  • Glycolysis in anaerobic respiration is similar to that in aerobic respiration. A 6-carbon glucose molecule of glucose still splits into two 3-carbon pyruvate molecules.
  • Fermentation then occurs following glycolysis. Pyruvate is converted into either lactate (in animals) or ethanol and carbon dioxide (in plants or fungi). A tiny amount of ATP forms as a by-product.
  • In animals: ; in bacteria and microorganisms:

Anaerobic Respiration

Only aerobic respiration requires oxygen, whilst anaerobic respiration does not. Anaerobic respiration can only occur without oxygen, altering how glucose breaks down into energy.

Anaerobic respiration does not require oxygen but only occurs when oxygen is absent. It only takes place in the cytoplasm. The products of anaerobic respiration differ in animals and plants. Anaerobic respiration in animals produces lactate, whereas ethanol and carbon dioxide in plants or fungi. Only a small amount of ATP forms during anaerobic respiration.


Anaerobic respiration only has two stages:


  1. Glycolysis in anaerobic respiration is similar to that in aerobic respiration. A 6-carbon glucose molecule of the glucose still splits into two 3-carbon pyruvate molecules.
  2. Fermentation then occurs following glycolysis. Pyruvate is converted into either lactate (in animals) or ethanol and carbon dioxide (in plants or fungi). A tiny amount of ATP forms as a by-product. 


Anaerobic respiration is how glucose breaks down in the absence of oxygen. When organisms respire anaerobically, they produce ATP molecules through fermentation, which can produce lactate in animals, or ethanol and carbon dioxide in plants and microorganisms.

The main differences between aerobic and anaerobic respiration are listed below:


  • Aerobic respiration occurs in the cytoplasm and the mitochondria, whilst anaerobic respiration only occurs in the cytoplasm.
  • Aerobic respiration requires oxygen to take place, whilst anaerobic respiration does not. 
  • Anaerobic respiration produces less ATP overall than aerobic respiration.
  • Anaerobic respiration produces carbon dioxide and ethanol (in plants and microorganisms) or lactate (in animals), whilst the main products of aerobic respiration are carbon dioxide and water.

The products of anaerobic respiration vary depending on what kind of organism is respiring. The products are ethanol and carbon dioxide (in plants and microorganisms) or lactate (in animals).

Final Anaerobic Respiration Quiz

Question

Anaerobic respiration requires oxygen. True or False?

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Answer

False.

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Question

Where in the cell does anaerobic respiration occur?

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Answer

Cytoplasm.

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Question

Does anaerobic respiration produce more or less ATP than aerobic respiration?


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Answer

Aerobic respiration produces more ATP.

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Question

What molecule is glucose converted into by the end of glycolysis?


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Answer

Pyruvate.

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Question

How many ATP molecules are produced overall during glycolysis (per glucose molecule)?


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Answer

2 molecules of ATP for every glucose molecule.

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Question

What is the word we use to describe the splitting of ATP into ADP and Pi using water?


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Answer

Hydrolysis.

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Question

What two 3-carbon molecules form after we add two phosphate groups to glucose?

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Answer

Triose phosphate.

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Question

What is the end product in the fermentation of pyruvate in humans and animals?


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Answer

Lactic acid.

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Question

What are the end products in pyruvate fermentation in bacteria/other microorganisms?


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Answer

Ethanol and carbon dioxide.

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Question

What are the name and chemical symbol of the group removed from pyruvate during ethanol fermentation?


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Answer

Carboxyl group (-COOH).

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Question

What happens to NAD+ after fermentation?


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Answer

NAD+ is used again in glycolysis.

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Question

What is the name of the 2-carbon molecule produced before the reduction of NADH during ethanol fermentation?


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Answer

Acetaldehyde.

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