What is Aerobic Respiration?

• Aerobic respiration refers to a series of metabolic reactions that occur in the presence oxygen in a cell. It is used to convert chemical energy into ATPs.
• All plants, animals, birds and humans can experience aerobic respiration, with the exception of some primitive prokaryotes.
• Aerobic respiration is a process where oxygen acts as an electron acceptor, which allows for more efficient and faster production of ATPs.
• The double bond of oxygen has a higher energy than other bonds, which helps to produce more ATPs.
• This is the preferred method for pyruvate degradation after glycolysis, where the pyruvate enters the mitochondria and is fully oxidized in the Kreb’s Cycle.
• Aerobic respiration is used to oxidize carbohydrates. However, products made from fats or proteins can also be used as reactants.
• The two products of aerobic respiration are carbon dioxide gas and water. They also contain the energy used to add a third phosphate to ADP or form ATP.
• Other energy-rich molecules, such as NADH2 and FADH2, can be converted to ATP through an electron transport chain that includes oxygen and protons.
• ATPs produced in aerobic respiration are mainly formed during oxidative phosphorylation. This is where the energy from an oxygen molecule is used for pumping protons out of membranes.
• Protons passing through create a potential which is used to start ATP synthase, and then produce ATP from ADP or a phosphate group.
• Ideally, 38 ATPs are made at the end of aerobic respiration. Some energy is lost through leaking or cost of moving pyruvate though the cell. This results in only 29-30 ATPs being produced.
• Complete oxidation of carbohydrate molecules by aerobic respiration occurs in the mitochondria in eukaryotic cells, as these enzymes are necessary for this process.

Examples of Aerobic Respiration

Respiration in humans

Aerobic respiration is the process by which cells respire in humans. Complete oxidation yields the energy needed for the body. It starts in the cell’s cytoplasm, and then the products are moved to the mitochondria for further reactions. The lungs absorb the oxygen and store it in the red blood cell. The oxygen is then transferred to the cells that need energy. The glucose is then oxidized and released as carbon dioxide gas. The major metabolic pathways that allow for the oxidation and release of energy in cells are part of cellular respiration.

What is Anaerobic Respiration?

• Anaerobic respiration refers to cellular respiration in which the high-energy electron acceptor is not oxygen nor pyruvate derivatives.
• Anaerobic respiration can accept a variety molecules, such as sulfate (SO4-), nitrate (NO3-), or any other molecule.
• Some archaea are called methanogens and use carbon dioxide to accept electrons. This results in methane being produced as a byproduct.
• Another group of purple sulfur bacteria uses the sulfate electron acceptor to produce hydrogen sulfide.
• These organisms live in low oxygen environments, so they choose anaerobic pathways for the breakdown of chemical fuels.
• Anaerobic respiration, which is analogous to aerobic respiration, involves the entry of molecules into the electron transport chain in order to pass electrons to the final electron accepting agent.
• Anaerobic respiration’s final electron acceptors have a lower reduction potential than oxygen molecules, which means less energy production.
• However, anaerobic respiration is crucial for biogeochemical cycles containing carbon, nitrogen and sulfur.
• Anaerobic respiration uses nitrate as an electron acceptor to produce nitrogen gas. This is the only way for nitrogen fixed to the atmosphere.
• Another pathway for anaerobic respiration is fermentation, in which the only way to extract energy is through glycolysis and the pyruvate cannot be further oxidized by the citric acid cycle.
• It is not used during fermentation to produce NADH, an energy-rich molecule.
• Anaerobic respiration can be found in many environments, including freshwater, soil, and deep-sea surface. Anaerobic respiration is also used by some microbes living in oxygenated environments. This is because oxygen cannot easily diffuse through their surfaces.
• Both anaerobic respiration (and fermentation) take place within the cytoplasm of a prokaryotic cell.
• During immediate contraction and relaxation, anaerobic respiration takes place and fermentation occurs in muscle cells.
• Fermentation yields a total gain only of two ATPs per glucose molecule.

Examples of Anaerobic Respiration

Lactic acid production in muscles

The muscles cannot receive enough oxygen during intense exercise and so perform more glycolysis than can be transferred to the electron transport chain. Anaerobic respiration occurs when there is not enough oxygen in the muscles. Anaerobic respiration is a form of aerobic respiration that results in the formation lactic acid. This is an anaerobic type of respiration, which produces only 2 ATPs for every glucosemolecule. You can write the equation for lactic acid fermentation as:

C6H12O6    →    C3H6O3 + energy

The accumulation of lactic acid in tissues results from the fermentation of lactic acid in muscles. This causes sore muscles. Anaerobic respiration produces less energy per glucose molecule than aerobic respiration. This causes weakness and shortness in breath.

Alcoholic fermentation by yeasts

Another type of anaerobic respiratory process that includes yeasts is fermentation. Anaerobic respiration occurs when carbohydrate-rich substances, such as sugars, are bottled with yeasts to ensure that there is minimal oxygen in the bottles. The yeast converts carbohydrates to ethyl alcohol. However, the alcohol in the bottles is toxic to yeasts. This is why they begin to die as the alcohol concentration rises. The yeasts can only make about 30% of the alcohol, while higher levels can be obtained by distillation. Just 2 ATPs are produced by fermentation as energy, just like in lactic acid fermentation. You can summarize the fermentation process as follows:

C6H12O6   →    C2H5OH + CO2 + energy

Fermentation in methanogens

Prokaryotes classified as methanogens belong to archaea. Because they produce methane by oxidizing carbohydrates without oxygen, these organisms are called methanogens. This is methanogenesis. This type of fermentation also results in the formation methanol, another alcohol. This is known as methanol poisoning. Methanogens (e.g. Methanosarcina barkeri oxidizes cellulose to make methanol, which is not as strong as ethyl alcohol like yeasts. Methanol poisoning can cause nerve damage and even death for some people. Methanol production has the following effects:

C6H12O6    →   CH3OH + CO2 + energy

Propionic acid fermentation in cheese

Propionic acid fermentation is when bacteria such as Propionibacterium shermanii (e.g. Propionibacterium shermanii uses carbohydrates such as lactose or glucose to make propionic acid. This process is most commonly used in Swiss cheese. This process produces carbon dioxide gas, which results in bubbles and a distinct flavor due to carboxylic acid. Like all anaerobic respiration processes it occurs in the absence of or low level oxygen. This is the overall reaction.

C12H22O11    →    C3H6O2 + CO2 + energy

Difference between Aerobic and Anaerobic Respiration – Aerobic vs Anaerobic Respiration

Base for comparison	Aerobic respiration	Anaerobic respiration
Definition	Aerobic respiration refers to a series of metabolic reactions that occur in the presence oxygen in a cell. It is used to convert chemical energy into ATPs.	Anaerobic respiration refers to cellular respiration in which the high-energy electron acceptor is not oxygen nor pyruvate derivatives.
Overall equation	The equation for aerobic respiration is as follows:C6H12O6 + 6O2 + 6CO2 + 6H2O+ energy	Anaerobic respiration can be described as:C6H12O6 + C2H5OH + C2H2O2 + energy
Oxygen Presence	Aerobic respiration occurs when oxygen is present.	Anaerobic respiration occurs in an environment with low oxygen levels.
Gas exchange	Aerobic respiration involves the exchange of gases. Carbon dioxide is absorbed and carbon dioxide is expelled.	Anaerobic respiration doesn’t allow for the exchange of gases. Some organisms can release certain gases, such as sulfur and nitrogen.
Location	After glycolysis, aerobic respiration occurs in the mitochondria and cytoplasm prokaryotes.	Anaerobic respiration is only found in the cytoplasm.
End products	End products of aerobic respiration include carbon dioxide, water and energy.	Acids, alcohols and gases are the end products of anaerobic respiratory respiration.
Energy produced	Aerobic respiration produces 38 ATPs, with some being lost.	Anaerobic respiration produces only 2 ATPs.
Reactants	Aerobic respiration is dependent on carbohydrate and oxygen.	Along with the carbohydrates, other electron acceptors such as sulfur and nitrogen will be required.
Oxidation	Aerobic respiration is the best way to complete the oxidation process of carbohydrates.	Anaerobic respiration causes incomplete oxidation of carbohydrates.
The nature of the process	Aerobic respiration takes a lot longer than anaerobic.	Anaerobic respiration lasts for a shorter time than aerobic respiration.
Occurs in	Most higher organisms, such as plants and animals, experience aerobic respiration.	Primitive prokaryotes are susceptible to anaerobic respiration. Anaerobic respiration occurs in muscle cells of humans when they are performing extreme movements.