Anaerobic Respiration

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 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.

The cytoplasm of plant and animal cells

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.

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:

$$\begin{gathered} {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+2\mathrm{ADP}+2\mathrm{Pi}+2{\mathrm{NAD}}^{+}\to 2{\mathrm{CH}}_3\mathrm{COCOOH}+2\mathrm{ATP}+2\mathrm{NADH} \\ \mathrm{Glucose}\mathrm{Pyruvate} \end{gathered}$$

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:

$$\begin{gathered} {\mathrm{C}}_3{\mathrm{H}}_4{\mathrm{O}}_3+2\mathrm{NADH}\stackrel{\mathrm{Lactic}\mathrm{dehydrogenase}}{\to }{\mathrm{C}}_3{\mathrm{H}}_6{\mathrm{O}}_3+2{\mathrm{NAD}}^{+} \\ \mathrm{Pyruvate}\mathrm{Lactic}\mathrm{acid} \end{gathered}$$

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

The steps of anaerobic respiration in animals

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 (${\mathrm{CO}}_2$) 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:

$$\begin{gathered} {\mathrm{CH}}_3\mathrm{COCOOH}\stackrel{\mathrm{Pyruvate}\mathrm{decarboxylase}}{\to }{\mathrm{C}}_2{\mathrm{H}}_4\mathrm{O}+{\mathrm{CO}}_2 \\ \mathrm{Pyruvate}\mathrm{Acetaldehyde} \\ {\mathrm{C}}_2{\mathrm{H}}_4\mathrm{O}+2\mathrm{NADH}\stackrel{\mathrm{Aldehyde}\mathrm{dehydrogenase}}{\to }{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}+2{\mathrm{NAD}}^{+} \\ \mathrm{Acetaldehyde}\mathrm{Ethanol} \end{gathered}$$

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

The steps of anaerobic respiration in bacteria and microorganisms

What is the anaerobic respiration equation?

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

$$\begin{gathered} {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\to 2{\mathrm{C}}_3{\mathrm{H}}_6{\mathrm{O}}_3 \\ \mathrm{Glucose}\mathrm{Lactic}\mathrm{acid} \end{gathered}$$

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

$$\begin{gathered} {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\to 2{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}+2{\mathrm{CO}}_2 \\ \mathrm{Glucose}\mathrm{Ethanol} \end{gathered}$$