Mitochondria Overview

Mitochondria are organelles known for their role in energy production.
Dimensions: Approximately 0.5 μm in size.
Components:
  - Outer Membrane: Acts as a barrier separating the mitochondrion from the cytoplasm.
  - Inner Membrane: Contains proteins essential for the transport and oxidation processes.
    - Divided into two major domains:
      - Inner Boundary Membrane Domain: Rich in proteins responsible for importing mitochondrial proteins.
      - Cristae: Highly folded structures increasing surface area and containing the machinery for aerobic respiration.
  - Intermembrane Space: The space between the outer and inner membranes.
  - Matrix: Enclosed by the inner membrane, containing enzymes, mitochondrial DNA, and ribosomes.

Molecules < •1000 daltons:
  - Outer Membrane: Contains porins to allow the passage of ions and small molecules.
  - Inner Membrane:
    - Contains oxidative phosphorylation enzymes, citric acid cycle enzymes, and fatty acid oxidation enzymes.
    - Also contains cardiolipin, a major lipid in the inner mitochondrial membrane.
Specifically, the composition includes:
  - Oxidative phosphorylation enzymes: 850
  - Mitochondrial genome: 39 genes
  - Citric acid cycle enzymes: Number not specified
  - Fatty acid oxidation enzymes: Number not specified
  - Cardiolipin: 880

The outer boundary of mitochondria consists of two membranes:
  - Outer mitochondrial membrane: Structures the outer boundary.
  - Inner mitochondrial membrane: Divided into two major domains.
    - Inner Boundary Membrane Domain: Protein-rich area crucial for mitochondrial function.
    - Outer Domain: Contains cristae, invaginated structures housing respiratory machinery.
The cristae are critical for aerobic respiration and ATP synthesis.
The membranes separate the organelle into two aqueous compartments:
- Matrix: Contains enzymes for metabolic processes and mitochondrial DNA.
- Intermembrane space: Space that plays a role in chemiosmosis during ATP synthesis.

Contains ribosomes (mitoribosomes) and circular DNA, allowing mitochondria to manufacture RNAs and proteins.
Mitochondrial DNA (mtDNA): A remnant from an ancestral aerobic bacterium which integrated into eukaryotic cells, supporting the endosymbiont hypothesis.

Explains how eukaryotic cells may have evolved:
  - Suggests that an ancestral cell engulfed a bacterium capable of oxidative phosphorylation, forming mitochondria.
  - Bacterial plasma membrane became the inner membrane of mitochondria.
  - Mitochondrial genes closely resemble bacterial genomes.

Mitochondria possess:
- Outer membrane: Similar to gram-negative bacteria.
- Inner membrane: Contains the electron transport chain and ATP synthase.
Mitochondrial DNA: Includes structures that share characteristics with bacterial DNA.

Nuclear Genes: Most (>1000 mitochondrial proteins) encoded in nuclear DNA.
Mitochondrial Genes: Limited to 13 mitochondrial membrane proteins, 22 tRNAs, and 2 rRNAs.

Key processes include:
  - Energy Production: ATP synthesis from oxidative phosphorylation.
  - Metabolism: Involvement in lipid, nucleotide, amino acid metabolism, and heme synthesis.
  - Regulation of Calcium Ions: Critical for cellular signaling and processes.
  - Apoptosis Regulation: Mitochondria play a vital role in programmed cell death.

Differences noted in various organisms:
- UGA: Codon for tryptophan in mitochondria vs. stop codon in standard code.
- AGA, AGG: Arginine codons in standard code are stop codons in mammalian mitochondria.

Mitochondria can undergo:
- Fusion: Combining into one larger organelle.
- Fission: Splitting into smaller mitochondria, aiding in distribution and quality control.
Proteins involved in fission: Drp1, fusion regulatory proteins such as Mfn1 and Opa1.

Aerobic Metabolism includes:
  - Glycolysis: Breakdown of glucose in the cytosol, producing pyruvate.
  - Citric Acid Cycle (TCA Cycle): Acetyl CoA enters the cycle producing NADH, FADH2, and ATP.
  - Electron Transport Chain: Utilizes high-energy electrons from NADH and FADH2 to produce ATP.

Key stages include:
1. Generation of a proton gradient by electron transport chain reaction.
2. ATP synthesis via proton flow through ATP synthase.
Chemiosmosis: The process coupling the movement of protons to ATP synthesis.
Energy production summary:
- From glucose metabolism through glycolysis, TCA, to oxidative phosphorylation can yield approximately 36 ATP per glucose molecule.

Mutations in mtDNA or nuclear DNA encoding mitochondrial proteins can lead to various disorders:
- Leigh Syndrome, Leukodystrophy, and Sporadic Myopathies.

Mitochondria are crucial for cellular metabolism and energy production, play roles in apoptosis, and hold genetic material indicative of their symbiotic origins. Their structural features and dynamic functions are essential for maintaining cellular homeostasis and physiological functions in higher organisms.