Lecture_12_Mitochondria

Mitochondria Overview

• Mitochondria: Key players in cellular energy production through oxidative phosphorylation.

• Semiautonomous Organelles: Can reproduce independently within the cell, sustaining cellular energy needs.

• Structure & Function: Complex structures that facilitate energy conversion; distinct from chloroplasts in function and location.

Objectives

• Compare mitochondria and chloroplasts.

• Describe mitochondrial structure and its relation to the respiratory chain.

• Understand the functions of each complex in the Electron Transport Chain (ETC).

• Know how ATP Synthase generates ATP and the role of the proton motive force.

• Explain how redox potential affects proton transport directionality in various ETC complexes.

• Detail how the H+ gradient and membrane potential aid in transporting molecules across the inner mitochondrial membrane.

• List electrons transferred and ATP generated in TCA/ETC.

Energy Production Mechanisms

• Oxidative phosphorylation: Occurs in mitochondria during cellular respiration.

• Photophosphorylation: Takes place in chloroplasts during photosynthesis.

• Both rely on a proton gradient established across a membrane.

ATP Generation Process

• Two stages of membrane-based ATP generation:

  • Stage 1: High-energy electrons drive the ETC; energy released is used to pump protons across the membrane, creating a gradient.

  • Stage 2: Protons flow back through ATP synthase, converting ADP and inorganic phosphate into ATP. This process is known as chemiosmotic coupling.

Mitochondrial Characteristics

• Membrane Structure: Double membrane organization: an outer membrane and an inner membrane with a highly folded structure (cristae).

• Energy Conversion Sites:

  • Mitochondria: ATP synthesis from respiration.

  • Chloroplasts: ATP synthesis from photosynthesis.

• Genetic Material:

  • Contain their own circular DNA genomes, independent of nuclear DNA.

  • Replication occurs autonomously.

• Protein Synthesis Machinery: Participate in their own protein synthesis with ribosomes.

• Division Method: Mitochondria replicate by fission.

Human Mitochondrial Genome

• Small circular DNA (16,569 base pairs).

• Contains 37 genes: 13 proteins, 22 tRNAs, and 2 rRNAs.

• Maternal inheritance, with mutations linked to various mitochondrial diseases.

Energy Metabolism Differences Between Mitochondria and Chloroplasts

• Key differences include:

  • Mitochondria do not utilize carbon dioxide; instead, they focus on oxidative phosphorylation.

  • Chloroplasts convert light energy into chemical energy, storing it as glucose.

Mitochondrial Functions

• Table 14-1: Mitochondrial functions include:

  • Production of ATP through oxidative phosphorylation.

  • Regeneration of NAD+ for glycolysis support.

  • Provision of biosynthetic precursors for macromolecules.

  • Participation in synthesis of heme and iron-sulfur clusters critical for electron transport.

  • Regulation of calcium ion concentration impacting signaling.

  • Generation of reactive oxygen species and participation in apoptosis regulation.

Proton Transport and the Inner Mitochondrial Membrane

• H+ ions use gradients to transport essential molecules:

  • Cotransport: Pyruvate and inorganic phosphate enter with H+.

  • Antiport Mechanism: ADP/ATP exchange driven by membrane potential.

Energy Accounting

• Total H+ ions pumped per NADH through respiratory complexes: 10 protons, yielding:

  • 4 H+ produces 1 ATP (3 for synthesis + 1 for export).

  • 2.5 ATP per NADH; 1.5 ATP per FADH2; ~30 ATP per glucose molecule.

ATP Synthase Mechanism

• Composed of two main parts:

  • F1 Head: Site of ATP synthesis.

  • F0 Rotor: Driven by the proton gradient, powers ATP generation through conformational changes in F1.

Redox Potential and Directionality in ETC

• Redox potential indicates a molecule's electron affinity; increases along the ETC, allowing electron transfer from carriers with lower to higher potentials, driving proton pumps in complexes I, III, IV.

Complex Specific Functions

• Complex I: NADH dehydrogenase; oxidizes NADH and pumps protons.

• Complex II: Succinate dehydrogenase; oxidizes succinate but does not pump protons.

• Complex III: Cytochrome c reductase; transfers electrons and pumps protons.

• Complex IV: Cytochrome c oxidase; transfers electrons to O2 and pumps protons, reducing O2 to H2O.

Summary of Electron Transport Chain and Properties

• Components include various electron carriers vital for the electron transfer process, driving ATP synthesis through the complexes. ATP production is efficiently coupled with the electrochemical gradients created during the transport processes.