4.5 Cellular Respiration

3 min read•august 22, 2024

Cellular respiration is the powerhouse of energy production in cells. It breaks down glucose to create ATP, the cell's energy currency. This process involves three main stages: glycolysis, the Krebs cycle, and the electron transport chain.

Understanding cellular respiration is key to grasping how organisms obtain and use energy. It's a complex process that showcases the intricate workings of cells and their ability to convert food into usable energy for life's functions.

Stages of Cellular Respiration

Overview of Cellular Respiration

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  • Cellular respiration is a catabolic process that breaks down organic molecules to release energy in the form of ATP
  • Occurs in three main stages: glycolysis, the Krebs cycle, and the electron transport chain

Glycolysis

  • First stage of cellular respiration that takes place in the cytoplasm
  • Breaks down glucose into two pyruvate molecules
  • Produces a net gain of 2 ATP and 2 NADH molecules

Krebs Cycle (Citric Acid Cycle)

  • Occurs in the mitochondrial matrix
  • Pyruvate is oxidized to form CO2
  • Generates 2 ATP, 6 NADH, and 2 FADH2 molecules per glucose molecule

Electron Transport Chain

  • Final stage of cellular respiration, located in the inner mitochondrial membrane
  • Involves a series of redox reactions that transfer electrons from NADH and FADH2 to oxygen
  • Creates a proton gradient used to drive ATP synthesis

Aerobic vs Anaerobic Respiration

Aerobic Respiration

  • Requires the presence of oxygen
  • Yields a high amount of ATP (around 30-32 ATP per glucose molecule)
    • Occurs in most eukaryotic cells (animals, plants, fungi) and some prokaryotes (bacteria)
  • Final electron acceptor is oxygen
  • End products are carbon dioxide and water

Anaerobic Respiration

  • Occurs in the absence of oxygen
  • Produces fewer ATP molecules (around 2 ATP per glucose molecule)
    • Common in certain prokaryotes (bacteria) and eukaryotic cells under oxygen-deprived conditions (muscle cells during intense exercise)
  • Final electron acceptor is an inorganic molecule other than oxygen (sulfate, nitrate)
  • End products vary depending on the organism (ethanol, lactic acid, other fermentation products)

Electron Transport Chain in ATP Production

Electron Transport Chain Components

  • Consists of a series of protein complexes (Complex I, II, III, and IV) and mobile electron carriers (ubiquinone and cytochrome c)
  • Transfers electrons from NADH and FADH2 to the final electron acceptor, oxygen
  • Pumps protons (H+) from the mitochondrial matrix into the intermembrane space, creating an electrochemical gradient (proton motive force)

Chemiosmosis and ATP Synthesis

  • Chemiosmosis is the process by which the proton gradient is used to drive ATP synthesis
  • Protons flow back into the mitochondrial matrix through ATP synthase, a protein complex that acts as a molecular machine
  • ATP synthase generates ATP from ADP and inorganic phosphate (Pi)
    • Typically requires around 3-4 protons per ATP molecule, varying between species
  • Coupling of the electron transport chain and chemiosmosis is known as oxidative phosphorylation, the primary source of ATP production in aerobic respiration

Factors Affecting Respiration Efficiency

Substrate Availability

  • Efficiency of cellular respiration can be influenced by the availability of substrates (glucose, oxygen)
  • Limited supply of these substrates can reduce the rate and efficiency of ATP production

Mitochondrial Function

  • Integrity and function of mitochondria play a crucial role in the efficiency of cellular respiration
  • Mitochondrial damage or dysfunction can impair the electron transport chain and ATP synthesis

Uncoupling Proteins (UCPs)

  • UCPs in the inner mitochondrial membrane can decrease the efficiency of ATP production
  • Allow protons to leak back into the matrix without passing through ATP synthase

Environmental Factors

  • Temperature and pH can affect enzyme activity and the stability of cellular components involved in cellular respiration
    • Extreme temperatures (very high or low) can denature enzymes and disrupt cellular processes
    • Optimal pH range for most enzymes is around 6-8; significant deviations can reduce enzyme efficiency

Inhibitors and Toxins

  • Presence of inhibitors or toxins can disrupt the electron transport chain or other key processes
    • Cyanide inhibits Complex IV, preventing electron transfer to oxygen
    • Oligomycin inhibits ATP synthase, blocking ATP production
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