AEROBIC RESPIRATION AND MITOCHONDRIA

Chapter 5: Aerobic Respiration and the Mitochondrion

5.1 Mitochondrial Structure and Function

Overview of Anaerobes and Aerobes

  • Anaerobes: Organisms that capture and utilize energy through oxygen-independent metabolism, such as glycolysis and fermentation.

  • Aerobes: Organisms that utilize oxygen to extract more energy from organic molecules.

  • Mitochondrion: Specialized organelle in eukaryotes where aerobic energy extraction occurs.

Diversity of Mitochondrial Structure

  • Mitochondria vary in shape (typically bean-shaped but can be round or threadlike) depending on the cell type.

  • Size and quantity are reflective of the cell's energy needs.

Mitochondrial Dynamics

  • Mitochondrial Fusion: Mitochondria can join together to form larger organelles.

  • Mitochondrial Fission: Mitochondria can divide into two, which is induced by thin tubules from the ER.

    • The ER tubules constrict the mitochondrion, completing fission with soluble proteins from the cytosol.

Mitochondrial Roles

  • ATP Production: Mitochondria oxidize fatty acids from oil droplets to produce ATP.

  • Biosynthesis: Sites of synthesis for certain amino acids and heme groups.

  • Calcium Regulation: Involved in calcium ion uptake and release, crucial for cellular activities.

  • Cell Death Regulation: Influential in signaling pathways that lead to programmed cell death.

Mitochondrial Membranes

Structure and Composition

  • Outer Membrane: Serves as the outer boundary.

  • Inner Membrane: Divided into two domains:

    • Inner Boundary Membrane: Rich in proteins for mitochondrial protein import.

    • Cristae: Invaginated sheets where aerobic respiration machinery and ATP formation take place.

Compartmentalization

  • Mitochondria contain two aqueous compartments:

    • Matrix: Interior filled with enzymes, substrates, and mitochondrial DNA.

    • Intermembrane Space: Area between the inner and outer membranes.

Mitochondrial Matrix and DNA

  • Contents: Contains ribosomes and circular DNA (mtDNA), allowing mitochondria to synthesize their own RNAs and proteins.

  • mtDNA is thought to originate from an ancestral aerobic bacterium.

5.2 Aerobic Metabolism in the Mitochondrion

Glycolysis

  • Initial steps in oxidative metabolism occur in glycolysis.

  • Produces:

    • Pyruvate: Transported across the inner membrane and converted into acetyl CoA.

    • NADH and ATP (two molecules).

Tricarboxylic Acid (TCA) Cycle

  • Acetyl-CoA enters a stepwise cycle for substrate oxidation and energy conservation:

    • Acetyl group condenses with oxaloacetate to form citrate.

    • Carbon atoms are oxidized to CO2, regenerating oxaloacetate.

  • Four reactions generate NADH or FADH2 from the cycle.

ATP Formation

  • The electrons from NADH and FADH2 feed into the electron-transport chain in the mitochondrion, leading to ATP formation via chemiosmosis:

    • Three ATP are formed from NADH and two from FADH2 per electron pair.

5.3 The Role of Mitochondria in the Formation of ATP

Energy Storage and Utilization

  • Mitochondria create an ionic gradient across the inner mitochondrial membrane to synthesize ATP during oxidative phosphorylation.

Oxidation Reduction Potentials

  • Measurement of redox potential occurs as electrons transfer between couples (e.g., NAD+/NADH).

Electron Transport Chain (ETC)

Transport Mechanics

  • Electrons from NADH and FADH2 transfer through electron carriers of the ETC, embedded in the inner membrane.

  • Electrons are transferred down a potential gradient:

    • Each carrier reduces and oxidizes subsequent carriers.

    • The final acceptor is O2, producing water.

Types of Electron Carriers

  • Flavoproteins: Associated with FAD or FMN for electron transport.

  • Cytochromes: Iron-containing proteins that undergo oxidation-reduction.

  • Ubiquinone (Coenzyme Q): Lipid-soluble and participates in electron transport through the membrane.

  • Iron-sulfur Proteins: Involved in redox reactions without heme groups.

Electron Transport Complexes

  • Composed of four major complexes (I-IV), which facilitate electron transport and accompany proton translocation, generating free energy.

  • Each complex harnesses energy to pump protons, creating a proton gradient used in ATP synthesis.

5.4 Engineering Linkage: Measuring Blood Oxygen

  • Cells require consistent oxygen supply for respiration, measured via:

    • Clark Electrode: Measures voltage related to oxidation-reduction reactions.

    • Pulse Oximeter: Continuous measurement based on colorimetric analysis.

5.6 The Machinery for ATP Formation

Structure of ATP Synthase

  • Comprised of:F1 particle (catalytic subunit) with three catalytic sites, and F0 particle embedded in the inner membrane, allowing proton flow.

The Binding Change Mechanism

  • Theory: Proton movements change the binding affinities of active sites for ATP, synthesizing ATP through conformational changes and rotational catalysis.

  • Evidence includes tracking of rotation through attached fluorescent markers.

Other Roles for the Proton-Motive Force

  • Beyond ATP synthesis, the proton-motive force aids in transporting ADP, inorganic phosphate, and calcium ions into the mitochondrion and drives mitochondrial fusion events.

5.7 Peroxisomes

  • Membrane-bound vesicles housing oxidative enzymes:

    • Oxidizes long-chain fatty acids and synthesizes plasmalogens.

    • Engage in oxidative metabolism, breaking down hazardous hydrogen peroxide.

5.8 Green Cells: Glyoxysomes

  • Specialized peroxisomes in plant seedlings:

    • Convert stored fatty acids into energy and materials for growth via the glyoxylate cycle.