3.2

Chapter 9: Cellular Respiration and Fermentation

Introduction to Cellular Respiration

  • Living Cells Require Energy:

    • To perform work such as assembling polymers, membrane transport, movement, reproduction, etc.

    • Energy is sourced externally through feeding on other organisms (animals feeding on photosynthetic organisms).

Energy Flow in Ecosystems

  • Energy Input and Output:

    • Energy flows into ecosystems as sunlight and leaves as heat.

    • Essential chemical elements for life are recycled through biological cycles.

  • Photosynthesis:

    • Generates organic molecules that are subsequently utilized in cellular respiration to extract chemical energy.

  • ATP Generation:

    • Cells utilize chemical energy stored in organic compounds to generate ATP, which powers biological processes.

Concept 9.1: Catabolic Pathways

  • Catabolic Pathways:

    • Break down complex molecules and release stored energy.

    • Involve electron transfer, key in cellular respiration.

    • These catabolic pathways yield energy by oxidizing organic fuels.

ATP Production through Catabolic Processes

  • Energy Release:

    • Breakdown of organic molecules is exergonic (releases energy).

  • Aerobic Respiration:

    • Consumes organic molecules and oxygen (O₂) producing ATP efficiently.

  • Anaerobic Respiration:

    • Similar to aerobic but uses electron acceptors other than oxygen.

  • Fermentation:

    • Partial degradation of sugars occurring without oxygen and generating ATP directly through substrate-level phosphorylation.

Concept 9.2: Glycolysis

  • Glycolysis Overview:

    • Breaks down glucose into two molecules of pyruvate.

    • Takes place in the cytoplasm and consists of two major phases:

    1. Energy Investment Phase

    2. Energy Payoff Phase

    • Occurs regardless of the presence of oxygen.

Energy Input and Output of Glycolysis

  • Energy Investment Phase:

    • 2 ATP utilized to phosphorylate glucose.

  • Energy Payoff Phase (Net Gain):

    • Produces 4 ATP (net gain of 2 ATP), 2 NADH, and 2 pyruvate.

Pyruvate Oxidation and the Citric Acid Cycle

  • Pyruvate Conversion:

    • In the presence of oxygen, pyruvate is converted to acetyl CoA.

    • This is a crucial link between glycolysis and the citric acid cycle.

  • Citric Acid Cycle (Krebs Cycle):

    • Completely oxidizes acetyl CoA, generating:

    • 1 ATP

    • 3 NADH

    • 1 FADH₂

    • 2 CO₂ (per turn).

    • Eight distinct steps, each catalyzed by specific enzymes.

    • Cycle regenerates oxaloacetate to continue the process.

Redox Reactions

  • Definition:

    • Chemical reactions involving the transfer of electrons are called oxidation-reduction (redox) reactions.

    • Oxidation: loss of electrons; Reduction: gain of electrons.

    • Reducing Agent: The substance that donates electrons (is oxidized).

    • Oxidizing Agent: The substance that accepts electrons (is reduced).

Oxidative Phosphorylation and Chemiosmosis

  • NADH as Electron Carrier:

    • NADH donates electrons to the electron transport chain (ETC) in mitochondria.

    • Each NADH represents stored energy that contributes to ATP generation.

  • Electron Transport Chain:

    • Series of proteins embedded in the inner mitochondrial membrane that facilitate the transfer of electrons.

    • Energy derived from these reactions pumps protons (H⁺) from the mitochondrial matrix to the intermembrane space, creating a proton gradient.

  • Chemiosmosis:

    • H⁺ flow back into the mitochondrial matrix via ATP synthase catalyzes the conversion of ADP to ATP.

    • This process highlights the coupling of electron transport to ATP synthesis by utilizing the established proton gradient.

ATP Yield from Cellular Respiration

  • Efficiency of Energy Transfer:

    • Approximately 34% of the energy in glucose is converted to chemical energy in ATP, which typically yields around 30 to 32 ATP molecules per glucose molecule.

  • Factors affecting ATP production:

    1. Differences in ATP yield due to NADH and FADH₂ contributions.

    2. Photophosphorylation/Oxidative phosphorylation coupling.

Fermentation and Anaerobic Respiration

  • Definition:

    • Allows ATP production without oxygen.

    • Anaerobic respiration uses alternative electron acceptors (e.g., sulfate).

    • Fermentation relies on substrate-level phosphorylation for ATP.

  • Types of Fermentation:

    • Alcohol Fermentation: Converts pyruvate to ethanol + CO₂. (Used in brewing & baking).

    • Lactic Acid Fermentation: Converts pyruvate to lactate. (Used by human muscle cells when oxygen is low).

Comparing Pathways of Energy Production

  • Similarities and Differences:

    • All rely on glycolysis as the initial step to oxidize glucose.

    • Different end products and ATP yield: 32 ATP cellular respiration vs. 2 ATP fermentation.

  • Evolutionary Significance:

    • Glycolysis is ancient, believed to be utilized by early anaerobic life forms.

    • It is conserved across different organisms demonstrating its fundamental role in energy metabolism.

Regulation of Cellular Respiration

  • Feedback Inhibition:

    • Mechanism to maintain steady energy supplies. If ATP decreases, respiration increases to compensate.

    • Key points of control include regulating the activity of enzymes throughout the pathways involved in catabolism.