Cellular Respiration: A Comprehensive Guide to Energy Production

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Cellular Respiration

Glycolysis

  • Enzymes break down a 6-carbon glucose molecule into two 3-carbon pyruvate molecules.
  • A small amount of ATP is synthesized through substrate-level phosphorylation, an enzyme-catalyzed reaction transferring a phosphate group from a substrate to ADP.

Oxidative Phosphorylation

  • High-energy electrons are delivered to oxygen through a series of electron carriers in the electron transport system.
  • The free energy released by electron flow generates an H+ gradient via chemiosmosis.
  • ATP synthase utilizes the H+ gradient as the energy source for ATP production.

Reaction Locations

  • Glycolysis: Occurs in the cytosol.
  • Pyruvate oxidation and citric acid cycle: Take place in the mitochondrial matrix.
  • Electron transfer system and ATP synthase enzymes: Located in the inner mitochondrial membrane.

Glycolysis: Energy Investment and Yield

  • The initial steps of glycolysis require energy, consuming 2 ATP molecules.
  • 4 ATP molecules are generated through substrate-level phosphorylation, resulting in a net gain of 2 ATP.
  • The electron carrier NAD+ is reduced to NADH, carrying 2 electrons and a proton (H+) removed from fuel molecules.

Pyruvate Oxidation and the Citric Acid Cycle

  • Active transport moves pyruvate into the mitochondrial matrix, where pyruvate oxidation and the citric acid cycle occur.
  • Pyruvate oxidation produces CO2, acetyl-coenzyme A (acetyl-CoA), and NADH.
  • The acetyl group of acetyl-CoA enters the citric acid cycle.
  • In the citric acid cycle, carbon products from pyruvate oxidation are oxidized to CO2.
  • All available electrons are transferred to 3 NAD+ (forming NADH) and 1 FAD (forming FADH2).
  • Each turn of the citric acid cycle generates 1 ATP through substrate-level phosphorylation.

Fermentation: An Alternative Pathway

  • When oxygen is absent or limited, the 2 NADH molecules produced during glycolysis can be used in fermentation.

Types of Fermentation

  • Lactate Fermentation
    • Converts pyruvate into lactate.
    • Occurs in some bacteria, plant tissues, and skeletal muscle.
    • Used in the production of buttermilk, yogurt, and dill pickles.
  • Alcoholic Fermentation
    • Converts pyruvate into ethyl alcohol and CO2.
    • Occurs in some plant tissues, invertebrates, protists, bacteria, and single-celled fungi like yeasts.
    • Used in bread making and alcoholic beverage production.

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