3.6 - Cellular Respiration

Cellular Respiration and Fermentation

• Cellular Respiration: This is a catabolic process by which cells generate ATP through the breakdown of biological macromolecules. It is a fundamental energy-producing pathway in many organisms.

• Types of Cellular Respiration:

  • Aerobic Respiration:

    • Definition: This form of respiration requires oxygen to convert organic fuel into ATP, carbon dioxide, and water.

    • Equation: C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP

    • Efficiency: Aerobic respiration is more efficient at generating ATP compared to anaerobic processes.

    • Phases:

      • Glycolysis: Occurs in the cytosol, breaking glucose into two pyruvate molecules, yielding 2 ATP and 2 NADH.

      • Krebs Cycle (Citric Acid Cycle): Pyruvate is converted to acetyl CoA, which enters the Krebs cycle within the mitochondria, producing additional ATP, NADH, and FADH2.

      • Electron Transport Chain (ETC): Electrons from NADH and FADH2 are transferred through a series of proteins, generating a proton gradient that drives the synthesis of a large amount of ATP through oxidative phosphorylation.

  • Fermentation:

    • Definition: This is an anaerobic process where organic fuels are degraded without the use of oxygen.

    • Characteristics: It is less efficient than aerobic respiration, generally utilized by most prokaryotes and some eukaryotes under anaerobic conditions.

    • Pathway: In fermentation, glycolysis is followed by the conversion of pyruvate into other molecules like lactate or ethanol, allowing NAD+ to be regenerated and glycolysis to continue producing a small amount of ATP.

• Phosphorylation:

  • Definition: This is the process of adding a phosphate group to ADP to form ATP, a key step in energy transfer within cells.

  • Types:

    • Substrate-Level Phosphorylation: Direct transfer of a phosphate group to ADP during glycolysis and the Krebs cycle.

    • Oxidative Phosphorylation: Occurs in the mitochondria during the electron transport chain, where energy from electrons is used to pump protons across the mitochondrial membrane, creating a gradient used by ATP synthase to produce ATP.

Krebs/Citric Acid Cycle

• Location: The Krebs cycle takes place in the mitochondrial matrix, the innermost compartment of mitochondria.

• Process:

  • This cycle is a key component of cellular respiration where pyruvate, derived from glucose during glycolysis, is fully oxidized to produce energy carriers.

  • CO2 Release: During the cycle, carbon dioxide is released as a byproduct from the decarboxylation of organic intermediates.

  • Energy Production:

    • The cycle generates 1 ATP per turn through substrate-level phosphorylation.

    • It also produces 3 molecules of NADH and 1 molecule of FADH2. These are energy-rich carriers that store potential energy to be used in the electron transport chain.

Mitochondrial Electron Transport Chain (ETC)

• Location: The electron transport chain is located along the inner mitochondrial membrane, which provides the necessary surface for complex protein structures involved in electron transfer.

• Function:

  • Energy Release and Transfer: Electrons from NADH and FADH2 are transferred through a series of protein complexes and electron carriers within the inner mitochondrial membrane.

  • As electrons move through the ETC, the energy they release is used to pump protons (H+) from the mitochondrial matrix into the intermembrane space, creating a proton gradient.

• Oxidative Phosphorylation:

  • ATP Synthesis: The proton gradient drives protons back into the matrix through ATP synthase, a process known as chemiosmosis. This movement powers the synthesis of ATP.

  • This phase of respiration is highly efficient, producing the majority of ATP during cellular respiration.