What is the difference between fermentation and respiration?

Question

The other day our teacher was asking about how fermentation differs from respiration, and it got me thinking. I remembered reading how microbes use these processes depending on whether oxygen is available or not. This answer helped me really understand how cells manage energy under different conditions and why their end products vary so much.

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Anttilampinen 2025-06-08T08:02:04+00:00 1 Answer 2 views
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  1. fleming
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    2025-06-08T08:03:31+00:00
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    Respiration

    Respiration is a metabolic process that involves the complete oxidation of organic fuel molecules (like glucose) to carbon dioxide and water, utilizing an electron transport chain (ETC) and an external final electron acceptor.

    Types:

    1. Aerobic Respiration: Uses oxygen (O2) as the final electron acceptor. This is the most efficient form of respiration.
    2. Anaerobic Respiration: Uses an inorganic molecule other than oxygen as the final electron acceptor (e.g., nitrate (NO3–), sulfate (SO42-), ferric iron (Fe3+), carbonate (CO32-)). This occurs in some bacteria and archaea, typically in oxygen-deprived environments.

    Key Stages (Aerobic Respiration):

    1. Glycolysis: Glucose (6 carbons) is broken down into two molecules of pyruvate (3 carbons) in the cytoplasm, producing a small amount of ATP (net 2 ATP) and NADH.
    2. Pyruvate Oxidation: Pyruvate enters the mitochondria (in eukaryotes) or remains in the cytoplasm (in prokaryotes) and is converted to acetyl-CoA, releasing CO2 and producing NADH.
    3. Krebs Cycle (Citric Acid Cycle): Acetyl-CoA enters the cycle, where it is completely oxidized to CO2. This generates more ATP (or GTP), NADH, and FADH2.
    4. Electron Transport Chain (ETC) and Oxidative Phosphorylation: NADH and FADH2 donate electrons to the ETC, a series of protein complexes located in the inner mitochondrial membrane (eukaryotes) or plasma membrane (prokaryotes). As electrons move down the chain, energy is released and used to pump protons (H+) across the membrane, creating a proton gradient. Oxygen (in aerobic respiration) acts as the final electron acceptor, combining with electrons and protons to form water. The flow of protons back across the membrane through ATP synthase drives the synthesis of a large amount of ATP (oxidative phosphorylation).

    Final Electron Acceptor:

    • Aerobic: Oxygen (O2)
    • Anaerobic: Inorganic molecules other than O2 (e.g., NO3–, SO42-)

    ATP Yield:

    • High: Aerobic respiration yields the most ATP, typically around 30–32 ATP molecules per glucose molecule.
    • Anaerobic respiration yields less ATP than aerobic respiration but more than fermentation.

    End Products:

    • Aerobic: CO2, H2O, ATP
    • Anaerobic: CO2, reduced inorganic compounds (e.g., N2, H2S), ATP

    Location:

    • Eukaryotes: Cytoplasm (glycolysis), Mitochondria (other stages)
    • Prokaryotes: Cytoplasm (glycolysis, pyruvate oxidation, Krebs cycle), Plasma membrane (ETC)

    Requirement:

    Requires an external final electron acceptor and often involves an ETC.

    Fermentation

    Fermentation is an anaerobic metabolic process that occurs after glycolysis when an external electron acceptor is not available. It involves the partial oxidation of organic compounds, using an organic molecule (usually derived from the initial substrate, like pyruvate) as the final electron acceptor.

    Key Purpose:

    The primary purpose of fermentation is not to generate additional ATP beyond glycolysis but to regenerate NAD+ from the NADH produced during glycolysis. NAD+ is essential for glycolysis to continue.

    Key Stages:

    1. Glycolysis: Glucose is broken down into pyruvate, producing 2 ATP (net) and 2 NADH.
    2. Fermentation Pathway: Pyruvate (or a derivative) accepts electrons from NADH, oxidizing it back to NAD+. This step does not produce ATP.

    Final Electron Acceptor:

    An organic molecule (e.g., pyruvate, acetaldehyde).

    ATP Yield:

    • Low: Only the net 2 ATP produced during glycolysis are generated per glucose molecule.

    End Products:

    • Lactic Acid Fermentation: Pyruvate is reduced to lactate (e.g., muscle cells, Lactobacillus in yogurt).
    • Alcohol Fermentation: Pyruvate is converted to acetaldehyde, then to ethanol with CO2 released (e.g., yeast).
    • Other types may produce acids, alcohols, and gases like propionic acid, acetone, or hydrogen.

    Location:

    Occurs entirely in the cytoplasm.

    Requirement:

    Does not require oxygen or an ETC. Uses an internal organic molecule as the final electron acceptor.

    Summary of Key Differences

    Feature Respiration (Aerobic/Anaerobic) Fermentation
    Oxygen Requirement Aerobic requires O2; Anaerobic does not Does not require O2
    Final Electron Acceptor Inorganic (O2, NO3–, SO42-, etc.) Organic (e.g., Pyruvate, Acetaldehyde)
    ATP Production Method Substrate-level & Oxidative Phosphorylation Substrate-level Phosphorylation
    ATP Yield per Glucose High (~30–32 in aerobic, variable in anaerobic) Low (Net 2 ATP)
    Extent of Oxidation Complete Incomplete
    Primary Purpose Maximize ATP production Regenerate NAD+
    Electron Transport Chain Yes No
    End Products CO2, H2O, or reduced inorganics Organic acids, alcohols, gases
    Location (Eukaryotes) Cytoplasm & Mitochondria Cytoplasm only

    Source: Brock Biology of Microorganisms; Campbell Biology.

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