A&P chapter 26

Overview of Cellular Respiration

• Cellular respiration is the biochemical process that converts glucose into ATP, the energy currency of the cell.

• It comprises three main stages: glycolysis, the Krebs cycle (also known as the citric acid cycle), and the electron transport chain (ETC).

Glycolysis

• Definition: The process of breaking down glucose into two molecules of pyruvate.

• Location: Occurs in the cytoplasm.

• Key Points:

  • Inputs: 1 glucose molecule and 2 ATP molecules.

  • Outputs: 2 pyruvate, a net gain of 2 ATP (4 produced, 2 consumed), and 2 NADH.

  • Glycolysis is an anaerobic process, meaning it does not require oxygen.

Transition Step

• Definition: The process that converts pyruvate into acetyl-CoA before entering the Krebs cycle.

• Location: Occurs in the mitochondrial matrix.

• Key Points:

  • Each pyruvate loses a carbon, producing carbon dioxide, and becomes acetyl-CoA.

  • Each transformation produces 2 NADH and releases 2 CO2 (one from each pyruvate).

Krebs Cycle (Citric Acid Cycle)

• Definition: The metabolic pathway that breaks down acetyl-CoA to produce energy carriers.

• Location: Takes place in the mitochondrial matrix.

• Key Points:

  • Each acetyl-CoA combines with a four-carbon molecule (oxaloacetate) to form citric acid.

  • Each turn of the cycle produces:

    • 3 NADH

    • 1 FADH2

    • 1 ATP (or GTP)

    • 2 CO2

  • Two acetyl-CoA enter the cycle from one glucose, so total outputs are doubled:

    • 6 NADH, 2 FADH2, 2 ATP, and 4 CO2 per glucose.

Electron Transport Chain (ETC)

• Definition: A series of protein complexes that transfer electrons through redox reactions.

• Location: Inner membrane of the mitochondria.

• Key Points:

  • Electrons from NADH and FADH2 are transferred through three main complexes (I, II, and III) and ATP synthase.

  • NADH donates electrons at complex I, contributing to the generation of a proton gradient:

    • Each NADH can generate up to 3 ATP through this process.

  • FADH2 donates electrons at complex II, leading to the production of 2 ATP:

    • Only passes electrons to complex III, not generating protons from complex I.

  • Protons pumped into the intermembrane space create a gradient, which ATP synthase uses to synthesize ATP as protons flow back into the matrix.

  • Oxygen serves as the final electron acceptor, forming water from the combined protons and electrons.

ATP Yield from Cellular Respiration

• Total ATP yield is influenced by the efficiency of the electron transport chain:

  • Each NADH produces ~3 ATP.

  • Each FADH2 produces ~2 ATP.

  • Overall: From one glucose molecule, up to 38 ATP can be produced from complete cellular respiration:

    • 2 ATP from glycolysis

    • 2 ATP from Krebs cycle

    • 34 ATP from oxidative phosphorylation (electron transport chain).

Summary of Products

• Glycolysis: 2 ATP, 2 NADH, 2 pyruvates

• Transition step: 2 NADH, 2 CO2, 2 acetyl-CoA

• Krebs cycle: 6 NADH, 2 FADH2, 2 ATP, 4 CO2

• Final totals per glucose: 38 ATP, 6 CO2, 6 H2O from cellular respiration.

Important Notes

• The process of cellular respiration is vital for energy generation in all aerobic organisms.

• Glycolysis can occur with or without oxygen, whereas the Krebs cycle and the electron transport chain require oxygen to function efficiently.

• The metabolic pathways discussed are interconnected; for instance, fats and proteins can also enter the Krebs cycle after appropriate conversion.