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Cellular Respiration and Fermentation - General Biology

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  • Catabolic pathway
    A metabolic pathway that releases stored energy by breaking down complex molecules; fermentation and cellular respiration harvest energy from organic fuels.
  • Fermentation
    Partial degradation of sugars generating ATP without oxygen or an electron transport chain; relies on glycolysis and regenerates NAD⁺.
  • Cellular respiration
    Catabolic pathways consuming oxygen and organic fuel to generate ATP using an electron transport chain; includes aerobic and anaerobic forms.
  • Aerobic respiration
    Cellular respiration using oxygen as the final electron acceptor in the electron transport chain.
  • Anaerobic respiration
    Cellular respiration using an electron transport chain with a final electron acceptor other than oxygen, such as sulfate.
  • Summary equation for cellular respiration
    C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy (ATP + heat); glucose degradation is exergonic with ΔG = −686 kcal/mol.
  • Redox reaction
    A chemical reaction involving electron transfer from one reactant to another; short for oxidation-reduction reaction.
  • Oxidation
    Loss of electrons from a substance; in organic molecules, often the loss of hydrogen atoms.
  • Reduction
    Gain of electrons by a substance; often adds hydrogen atoms to an organic molecule.
  • Reducing agent
    The electron donor in a redox reaction; it becomes oxidized.
  • Oxidizing agent
    The electron acceptor in a redox reaction; it becomes reduced. Oxygen is a strong oxidizing agent.
  • NAD⁺ (nicotinamide adenine dinucleotide)
    A coenzyme that acts as an oxidizing agent by accepting electrons and is reduced to NADH.
  • NADH
    The reduced form of NAD⁺; stores energy as high-energy electrons delivered to the electron transport chain.
  • Electron transport chain
    Series of electron carriers in the inner mitochondrial membrane that pass electrons in redox steps, releasing energy gradually.
  • Oxidative phosphorylation
    Production of ATP using energy from redox reactions of the electron transport chain; accounts for most ATP in respiration.
  • Substrate-level phosphorylation
    Enzymatic transfer of a phosphate group from a substrate directly to ADP, forming ATP; occurs in glycolysis and citric acid cycle.
  • Glycolysis
    Pathway splitting glucose into two pyruvate molecules; occurs in cytosol, does not require oxygen, starts fermentation and respiration.
  • Pyruvate oxidation
    Conversion of pyruvate to acetyl CoA in mitochondria, releasing CO₂ and reducing NAD⁺ to NADH.
  • Citric acid cycle (Krebs cycle)
    Mitochondrial cycle oxidizing acetyl CoA to CO₂, producing NADH, FADH₂, and ATP; regenerates oxaloacetate each turn.
  • ATP synthase
    Enzyme complex in inner mitochondrial membrane that synthesizes ATP from ADP and phosphate using H⁺ flow down its gradient.
  • Proton gradient (H⁺ gradient)
    Difference in proton concentration across inner mitochondrial membrane; stores potential energy used to make ATP.
  • Chemiosmosis
    Energy-coupling mechanism using H⁺ gradient energy to drive ATP synthesis during oxidative phosphorylation.
  • Alcohol fermentation
    Fermentation converting pyruvate to ethanol via acetaldehyde, releasing CO₂ and regenerating NAD⁺.
  • Lactic acid fermentation
    Fermentation reducing pyruvate directly to lactate, regenerating NAD⁺ without releasing CO₂; occurs in some bacteria and muscle cells.
  • Facultative anaerobe
    Organism that can switch between aerobic respiration and fermentation depending on oxygen availability.
  • Beta oxidation
    Metabolic process breaking fatty acids into two-carbon fragments that enter the citric acid cycle as acetyl CoA.
  • Phosphofructokinase (PFK)
    Allosteric enzyme catalyzing a key early step in glycolysis; regulated by ATP, AMP, and citrate to control respiration rate.
  • Efficiency of respiration
    About 34% of glucose energy is transferred to ATP; the rest is lost as heat.