Catabolism: Energy Release and Conservation

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Overview of Catabolism

• All organisms require three main products from their metabolic reactions:

  • ATP: Used to conserve energy from an energy source.

  • Reducing power: Molecules that provide a readily available supply of electrons for chemical reactions.

  • Precursor metabolites: Serve as carbon skeletons for the biosynthesis of monomers.

Nutritional Types of Organisms

• Organisms can be classified based on their sources of energy, electrons, and carbon.

Major Nutritional Types

• Several critical processes are available for energy generation:

  • Aerobic respiration: Utilizes oxygen.

  • Anaerobic respiration: Utilizes other electron acceptors.

  • Fermentation: Anaerobic process without an electron transport chain.

Aerobic Respiration

• Overview of Aerobic Respiration: Glucose (or similar substrates) is broken down to harvest energy.

  • Key Components:

  • High energy electrons.

  • Proton motive force (PMF).

  • ATP production.

Phases of Aerobic Respiration

1. Glycolysis: Occurs in the cytoplasm and produces 2 ATP, 2 NADH.

2. Preparatory reaction: Conversion of pyruvate to Acetyl CoA; releases CO2.

3. Citric Acid Cycle: Also known as Krebs Cycle; produces NADH, FADH2, and ATP.

4. Electron Transport Chain (ETC): Converts NADH and FADH2 into ATP using oxygen as the final electron acceptor.

Complete Aerobic Respiration

• Four Major Processes:

  • Glycolysis:

  • Produces pyruvate from glucose.

  • Forms 2 ATP via substrate-level phosphorylation without oxygen.

  • Preparatory Reaction:

  • Pyruvate enters mitochondria, yielding NADH and releasing CO2.

  • Occurs in mitochondria of eukaryotes and cytoplasm of prokaryotes.

  • Citric Acid Cycle:

  • Situated in the mitochondrial matrix; 6 NADH, 2 FADH2 produced per glucose molecule.

  • Releases 4 CO2, turns twice for each glucose.

  • Electron Transport Chain:

  • Found in the inner mitochondrial membrane; involves multiple membrane-bound enzymes.

  • Electrons pass through and generate PMF, driving ATP synthesis via ATP synthase.

Mitochondrion Structure

• Mitochondrial Anatomy:

  • Double membrane structure:

  • Outer membrane

  • Inner membrane: Contains cristae where ETC occurs.

  • Matrix: Site of the citric acid cycle and preparatory reaction.

Glycolysis Detail

• Phase 1:

  • Occurs in the cytoplasm (both in eukaryotes and prokaryotes).

  • 2 ATP are produced via substrate-level phosphorylation (anaerobic process).

Citric Acid Cycle (Krebs Cycle)

• Phase 3:

  • Takes place in mitochondrial matrix of eukaryotes and cytoplasm of prokaryotes.

  • Produces NADH, FADH2, and releases 4 CO2.

  • Each turn generates 2 ATP and 6 NADH, 2 FADH2 from one glucose.

Electron Transport Chain Dynamics

• Phase 4:

  • Enzymes in the ETC arrange in the inner membrane.

  • Transfers electrons down the chain to pump protons, creating PMF.

  • ATP synthase uses this force to catalyze ATP formation.

Eukaryotic vs Prokaryotic ETC

• Eukaryotic:

  • Carries occur in the inner mitochondrial membrane.

• Prokaryotic:

  • Located in the plasma membrane, more flexible and shorter.

  • Can utilize different electron carriers entirely.

ATP Production Methods

• Oxidative phosphorylation: Synthesis of ATP during electron transport driven by the oxidation of chemical sources.

  • Chemiosmotic Hypothesis: Protons diffuse back through ATP synthase creating ATP from ADP and inorganic phosphate ($PO_4^{3-}$).

Anaerobic Processes

• Anaerobic Respiration: Utilizes electron acceptors like $NO<em>3^-$, $SO</em>4^{2-}$, or $CO_2$ instead of oxygen, yielding less energy than aerobic processes.

Theoretical vs Actual Yield of ATP

• Theoretical maximum ATP yield during aerobic respiration: 38.

• Actual yield is closer to 30 due to variability in electron transport chains and growth conditions.

  • Under anaerobic conditions, glycolysis yields only 2 ATP.

Fermentation

• Occurs in the absence of a final electron acceptor. Glucose is partially metabolized.

  • End Products: Can be lactic acid or alcohol.

  • NADH must be recycled to NAD+ through fermentation reactions, permitting glycolysis to continue.

  • ATP production is by substrate-level phosphorylation, not via oxidative phosphorylation.

Types of Fermentation

• Alcoholic Fermentation: Produces ethanol, used in beverages and bread.

• Lactic Acid Fermentation: Produces lactic acid, used in yogurt and food preservation.

Chemolithotrophy

• Energy derived by oxidizing inorganic molecules.

  • Examples include: Hydrogen, nitrogen (nitrification), and sulfur oxidizers.

Summary of Chemolithotrophs

• Chemolithotrophs have specific electron donor/acceptor preferences.

• Requires substantial quantities of inorganic materials to produce enough ATP for growth.