Aerobic vs. Anaerobic Respiration: 11 Differences, Examples

Aerobic Respiration Definition

Aerobic respiration is a set of metabolic reactions that take place in the presence of oxygen, occurring in a cell to convert chemical energy into ATPs.

• Aerobic respiration takes place in all plants, animals, birds, and humans, except for some primitive prokaryotes.
• In aerobic respiration, oxygen acts as an electron acceptor which helps produce ATPs more effectively and more quickly.
• The double bond in the oxygen has higher energy than other bonds which aids to produce more ATPs.
• It is the preferred method of degradation of pyruvate after glycolysis where the pyruvate then enters the mitochondria to be fully oxidized during the Kreb’s cycle.
• The process of aerobic respiration is utilized for the oxidation of carbohydrates, but products from fats and proteins are also used as reactants.
• Carbon dioxide gas and water are the two products of aerobic respiration along with the energy that is used to add a third phosphate group to ADP and form ATP.
• Other energy-rich molecules like NADH and FADH₂ are converted into ATP via electron transport chain with oxygen and protons.
• During aerobic respiration, most ATPs are produced during oxidative phosphorylation where the energy of oxygen molecule is used to pump protons out of the membrane.
• The passage of protons creates a potential that is then used to initiate ATP synthase and produce ATP from ADP and a phosphate group.
• Ideally, a total of 38 ATPs are produced at the end of the aerobic respiration. However, some energy is lost due to leaking of the membrane or the cost of moving pyruvate through the cell, as a result of which about 29-30 ATPs are only produced.
• Aerobic respiration results in complete oxidation of carbohydrate molecules which take place in the mitochondria of eukaryotic cells as the enzymes for the process are present there.

Anaerobic Respiration Definition

Anaerobic respiration is a process of cellular respiration where the high energy electron acceptor is neither oxygen nor pyruvate derivatives.

• In anaerobic respiration, the electron acceptor can be sulfate ion (SO₄^–) or nitrate ion (NO₃^–) or a variety of other molecules.
• Some archaea, called methanogens, are known to use carbon dioxide as the electron acceptor, producing methane as a by-product.
• Similarly, another group of purple sulfur bacteria uses sulfate as an electron acceptor, thus producing hydrogen sulfide as a by-product.
• These organisms reside in low-oxygen environments and thus opt for anaerobic pathways to break down the chemical fuels.
• Anaerobic respiration is similar to aerobic respiration in that the molecules enter the electron transport chain to pass the electrons to the final electron acceptor.
• The final electron acceptors involved in anaerobic respiration have a smaller reduction potential than oxygen molecules which results in less energy production.
• Anaerobic respiration, however, is essential for biogeochemical cycles of carbon, nitrogen, and sulfur.
• The nitrate that acts as an electron acceptor in anaerobic respiration produces nitrogen gas as a by-product, and this process is the only route for fixed nitrogen to reach the atmosphere.
• Fermentation is another pathway for anaerobic respiration, where the only energy extraction pathway is glycolysis, and the pyruvate is not further oxidized via the citric acid cycle.
• The energy-rich molecule, NADH, is also not utilized during fermentation.
• Anaerobic respiration takes place in many environments like freshwater, soil, deep-sea surfaces. Some microbes in oxygenated environments also utilize anaerobic respiration because oxygen cannot readily diffuse through their surface.
• Anaerobic respiration and fermentation, both take place in the cytoplasm of the prokaryotic cell.
• Anaerobic respiration and fermentation processes take place in the muscle cells during immediate contraction and relaxation.
• Fermentation results in a total gain of only two ATPs per glucose molecule.

Key differences (Aerobic Respiration vs Anaerobic Respiration)

Examples of Aerobic Respiration

Respiration in humans

• The process of cellular respiration in humans is aerobic respiration, where complete oxidation of glucose yields the energy required for the body.
• It begins in the cytoplasm of the cell, and the products are then moved into the mitochondria, where further reactions take place.
• The oxygen is absorbed by the lungs and is stored in the red blood cells. The oxygen is then passed to the cells that require energy.
• The glucose is then oxidized to produce energy while releasing carbon dioxide gas.
• Cellular respiration in humans includes the major metabolic pathways for the oxidation of carbohydrates to release energy.

Examples of Anaerobic Respiration

Lactic acid production in muscles

• During intense exercise, the muscles in our body cannot get enough oxygen and thus perform more glycolysis than the body can transfer oxygen to the electron transport chain.
• This results in anaerobic respiration due to insufficient oxygen in our muscles.
• Thus, instead of aerobic respiration, anaerobic respiration takes place which results in the formation of lactic acid.
• This type of anaerobic respiration is called lactic acid fermentation that produces just 2 ATPs per glucose molecules.
• The equation of lactic acid fermentation can be written as:

C₆H₁₂O₆    →    C₃H₆O₃ + energy

• Lactic acid fermentation in muscles results in the accumulation of lactic acid in the tissues, which leads to sore muscles.
• Because less energy is produced per glucose molecule during anaerobic respiration than aerobic respiration, this results in weakness and shortness of breath.

Alcoholic fermentation by yeasts

• Fermentation is another type of anaerobic respiration that takes in anaerobic organisms like yeasts.
• When carbohydrate-rich substances are bottled with yeasts, ensuring minimum oxygen content in the bottle, yeasts undergo anaerobic respiration.
• As a result, fermentation takes place where the yeast converts carbohydrates into ethyl alcohol.
• The alcohol produced in the bottle, however, is toxic to the yeasts, which is why they start to dies as the alcohol concentration increases.
• Only about 30% of alcohol can be brewed with yeasts while the higher concentrations are obtained through the distillation process.
• As in lactic acid fermentation, fermentation also results in just 2 ATPs as energy.
• The overall reaction of fermentation can be written as:

C₆H₁₂O₆   →    C₂H₅OH + CO₂ + energy

Fermentation in methanogens

• Methanogens are prokaryotes that belong in archaea.
• These organisms are named methanogens because they produce methane as a by-product by oxidizing carbohydrates in the absence of oxygen. This process is called methanogenesis.
• This is also a type of fermentation that results in the formation of different alcohol, methanol. This process is also called methanol poisoning.
• Methanogens (e.g. Methanosarcina barkeri) oxidize cellulose from plants to produce methanol instead of ethyl alcohol as in the case of yeasts.
• Methanol poisoning might result in nerve damage or even death in some people.
• The overall reaction of methanol production is:

C₆H₁₂O₆    →   CH₃OH + CO₂ + energy

Propionic acid fermentation in cheese

• Propionic acid fermentation occurs when some bacteria (e.g. Propionibacterium shermanii) utilize carbohydrates like lactose and glucose to produce propionic acid and carbon dioxide.
• The most common application of this process can be observed in Swiss cheese.
• The carbon dioxide gas produced during this process results in the formation of bubbles in the cheese along with the distinct flavor due to the carboxylic acid.
• This process, like all other anaerobic respiration processes, occurs during the absence or low concentration of oxygen.
• The overall reaction of this process is:

C₁₂H₂₂O₁₁    →    C₃H₆O₂ + CO₂ + energy

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