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Overview of Cellular Respiration

  • Cellular Respiration: The process of releasing energy from molecules such as glucose accompanied by ATP synthesis.
    • Types:
    • Aerobic: Requires O extsubscript{2}, produces CO extsubscript{2}.

Glucose Breakdown and ATP Production

  • Glucose is a high-energy molecule that releases energy when broken down.
  • The breakdown of one glucose molecule results in 36 or 38 ATP molecules, depending on the efficiency of the process.
  • Pathways of cellular respiration allow for a slow release of energy for ATP synthesis.

Involvement of Coenzymes in Cellular Respiration

  • Cellular respiration consists of many reactions, each requiring specific enzymes.
  • Certain enzymes utilize coenzymes:
    • NAD extsuperscript{+} (Nicotinamide adenine dinucleotide)
    • FAD (Flavin adenine dinucleotide)
  • Both NAD extsuperscript{+} and FAD carry two electrons and two hydrogen atoms.
    • They selectively pick up electrons during specific enzymatic reactions and transport these electrons to the electron transport chain.

Phases of Cellular Respiration

  • Glycolysis
  • Preparatory Reaction
  • Citric Acid Cycle (Krebs Cycle)
  • Electron Transport Chain

Glycolysis

  • Glycolysis occurs in the cytoplasm and is anaerobic (does not require oxygen).
    • It is a series of reactions that break down glucose into smaller units (pyruvate).
    • Outputs:
    • Produces 2 NADH (Nicotinamide adenine dinucleotide hydrogen) and 2 net ATP.

Preparatory Reaction

  • Pyruvate is oxidized to form acetyl CoA (Coenzyme A) with carbon dioxide removed:
    • One 3-carbon pyruvate molecule is converted into one 2-carbon acetyl CoA molecule.
    • This reaction happens twice because each glucose produces two pyruvates.

Citric Acid Cycle

  • A cyclical series of oxidation reactions that occur in the mitochondria:
    • Acetyl CoA is transformed into citric acid before entering the cycle.
    • Each turn of the cycle produces:
    • 1 ATP
    • 2 CO extsubscript{2}
    • The cycle turns twice per glucose because two acetyl CoAs are produced.

Electron Transport Chain

  • A series of electron carrier molecules located in the cristae of mitochondria:
    • Electrons are transferred from one carrier to another, releasing energy to synthesize ATP.
    • Under aerobic conditions, the process can yield 32 to 34 ATP per glucose molecule.

Pyruvate and Fermentation

  • Pyruvate is pivotal; without O extsubscript{2}, it is reduced to:
    • Lactate (in animals)
    • Ethanol and CO extsubscript{2} (in other organisms)
  • Fermentation yields only 2 ATP per glucose through substrate-level ATP synthesis, substantially lower than the yield in cellular respiration.

Inputs and Outputs of Glycolysis

  • Inputs:

    • 6C glucose
    • 2 NAD extsuperscript{+}
    • 2 ATP
    • 2 ADP + 2 P

  • Outputs:

    • 2 (3C) pyruvate
    • 2 NADH
    • 4 ATP

  • Net Gain:

    • 2 ATP after accounting for the initial investment.

Fermentation

Types of Fermentation:

  • Lactic Acid Fermentation

    • In animals, pyruvate is reduced to lactate, generating 2 ATP per glucose during glycolysis.
    • Enables the continuation of glycolysis under low oxygen.
    • Lactate can be toxic to cells and can lead to a decrease in pH, causing muscle fatigue.

  • Alcohol Fermentation

    • In yeasts, pyruvate is reduced to ethyl alcohol (ethanol) and CO extsubscript{2}.
    • In this process, NADH supplies electrons and CO extsubscript{2} is released.

Energy Yield From Fermentation

  • Fermentation only yields 2 ATP, compared to the 36 to 38 ATP produced by cellular respiration; thus, most of the energy in glucose remains unutilized.

Mitochondrial Reactions

Preparatory Reaction in Mitochondria

  • Occurs in the matrix of mitochondria, where:
    • Pyruvate converts to a 2C acetyl group, producing CO extsubscript{2} and forming NADH from hydrogen removed from pyruvate.
    • The entire reaction occurs twice per glucose, generating 2 NADH.

Citric Acid Cycle Mechanism

  • Occurs in the mitochondrial matrix:
    • Input: 2 acetyl CoA
    • Output: 4 CO extsubscript{2}, 6 NADH + H extsuperscript{+}, 2 FADH extsubscript{2}, 2 ATP
    • Each acetyl group combines with oxaloacetate to generate citrate, a 6C molecule.
    • The cycle runs twice for each glucose molecule.

Electron Transport Chain Details

  • Located in the cristae of mitochondria:
    • Electrons from NADH and FADH extsubscript{2} are passed through a series of carriers, forming a proton gradient.
    • Oxygen combines with protons to produce water, and energy from moving electrons facilitates ATP synthesis.

Mechanism of ATP Generation

  • The flow of H extsuperscript{+} ions through ATP synthase back into the mitochondrial matrix leads to ATP production via chemiosmosis.

Summary of Energy Yield for Cellular Respiration

  • Total of 4 ATP produced directly by substrate-level ATP synthesis:

    • 2 ATP from glycolysis
    • 2 ATP from the citric acid cycle

  • 32 to 34 ATP produced via the electron transport chain and chemiosmosis.

  • Overall, cellular respiration can yield between 36 to 38 ATP per glucose molecule, signifying a highly efficient energy conversion process.