Mitochondria

Overview

Mitochondria: double-membraned organelles in eukaryotic cells. Function: ATP synthesis via oxidative phosphorylation. Roles: energy metabolism, apoptosis regulation, calcium homeostasis, reactive oxygen species (ROS) generation. Size: 0.5–10 μm, number varies by cell type and metabolic demand. Present in almost all eukaryotic cells except mature erythrocytes.

"The mitochondrion is the powerhouse of the cell." -- Lewis J. Kleinsmith

Structure

Double Membrane

Outer membrane: permeable to ions and small molecules via porins. Inner membrane: impermeable, contains respiratory chain complexes and ATP synthase. Highly folded into cristae to increase surface area.

Compartmentalization

Intermembrane space: between outer and inner membranes, involved in proton gradient formation. Matrix: contains enzymes of the citric acid cycle, mitochondrial DNA, ribosomes.

Membrane Composition

Inner membrane rich in cardiolipin, critical for membrane integrity and function. Outer membrane contains proteins for transport and signaling.

Cristae Morphology

Shape varies: lamellar, tubular, or vesicular depending on tissue type. Correlates with metabolic activity.

Mitochondrial DNA

Genomic Characteristics

mtDNA: circular, double-stranded, 16.5 kb in humans. Encodes 13 proteins, 22 tRNAs, 2 rRNAs. Inherited maternally.

Replication and Transcription

Replication independent of nuclear DNA cycle. Transcription produces polycistronic RNA processed into functional units.

Genetic Code Variations

mtDNA uses a slightly different genetic code than nuclear DNA, e.g., UGA codes for tryptophan instead of stop.

mtDNA Genes:- 13 protein-coding: components of respiratory complexes I, III, IV, V- 22 tRNAs: mitochondrial translation- 2 rRNAs: 12S and 16S rRNA subunits

Heteroplasmy

Presence of mixed mtDNA populations within a cell. Influences mitochondrial disease expression and inheritance.

Energy Production

ATP Synthesis

Primary function: convert energy from nutrients into ATP. ATP: cellular energy currency used for biochemical reactions.

Electron Transport Chain (ETC)

Series of protein complexes (I-IV) embedded in inner membrane. Transfer electrons from NADH/FADH2 to oxygen, generating proton gradient.

Proton Motive Force

Electrochemical gradient drives ATP synthase. Proton flow from intermembrane space to matrix powers ATP synthesis.

Substrate Utilization

Oxidizes carbohydrates, fats, proteins via citric acid cycle and ETC.

Oxidative Phosphorylation

Complexes I-IV

Complex I (NADH dehydrogenase): transfers electrons from NADH to ubiquinone. Complex II (succinate dehydrogenase): transfers electrons from FADH2. Complex III (cytochrome bc1): transfers electrons to cytochrome c. Complex IV (cytochrome c oxidase): reduces oxygen to water.

ATP Synthase (Complex V)

Enzyme complex synthesizing ATP from ADP and Pi using proton gradient energy. Rotary mechanism coupling proton flow to catalytic activity.

Proton Gradient Formation

Electron transfer drives proton pumping from matrix to intermembrane space. Gradient components: membrane potential (Δψ) and pH gradient (ΔpH).

Δp = Δψ - (2.303RT/F)ΔpHWhere:Δp = proton motive forceΔψ = membrane potentialΔpH = pH differenceR = gas constantT = temperature (K)F = Faraday constant

Uncoupling

Process where proton gradient dissipates without ATP synthesis. Results in heat production, e.g., brown adipose tissue thermogenesis.

Metabolic Functions

Citric Acid Cycle

Matrix-located enzymatic pathway oxidizing acetyl-CoA to CO2. Generates NADH, FADH2 for ETC.

Fatty Acid β-Oxidation

Sequential removal of two-carbon units from fatty acids, producing acetyl-CoA, NADH, and FADH2.

Amino Acid Metabolism

Deamination, transamination, and catabolism of amino acids feeding into TCA intermediates.

Calcium Homeostasis

Mitochondria buffer cytosolic calcium, modulating signaling pathways and metabolism.

Reactive Oxygen Species (ROS) Generation

By-products of electron leakage in ETC. Roles: signaling and oxidative damage.

Apoptosis

Mitochondrial Outer Membrane Permeabilization (MOMP)

Initiated by pro-apoptotic signals. Release of cytochrome c into cytosol triggers caspase cascade.

Cytochrome c Role

Forms apoptosome with Apaf-1, activates caspase-9, leading to executioner caspases activation.

Bcl-2 Family Proteins

Regulate MOMP: pro-apoptotic (Bax, Bak), anti-apoptotic (Bcl-2, Bcl-xL).

Apoptosis Regulation

Balance between survival and death signals determines cell fate.

Biogenesis and Dynamics

Mitochondrial Biogenesis

Coordination of nuclear and mitochondrial gene expression. Regulated by PGC-1α, NRF1, TFAM transcription factors.

Fission and Fusion

Dynamic processes maintaining mitochondrial morphology, distribution, and quality control. Key proteins: Drp1 (fission), Mfn1/2, OPA1 (fusion).

Mitophagy

Selective autophagy of damaged mitochondria. Maintains cellular health. Mediated by PINK1 and Parkin pathways.

Protein Import

Mitochondrial proteins mostly nuclear-encoded, synthesized in cytosol, imported via TOM/TIM complexes.

Mitochondrial Diseases

Genetic Basis

Caused by mutations in mtDNA or nuclear genes encoding mitochondrial proteins. Inheritance: maternal (mtDNA) or Mendelian (nuclear).

Common Disorders

Examples: Leber’s hereditary optic neuropathy (LHON), mitochondrial myopathy, MELAS, Leigh syndrome.

Symptoms

Multisystemic: muscle weakness, neurological deficits, cardiomyopathy, metabolic abnormalities.

Diagnosis and Treatment

Diagnostic tools: biopsy, genetic testing, biochemical assays. Treatment: symptomatic, supportive; experimental gene therapies in development.

Evolutionary Origin

Endosymbiotic Theory

Origin: aerobic α-proteobacterium engulfed by ancestral eukaryotic cell ~1.5–2 billion years ago. Evidence: double membranes, mtDNA, bacterial-type ribosomes.

Genome Reduction

Loss and transfer of many genes to nucleus. Retention of genes essential for oxidative phosphorylation.

Co-evolution with Host

Integration of mitochondrial and nuclear genomes for cellular function.

Research Techniques

Microscopy

Electron microscopy: ultrastructure visualization. Fluorescence microscopy: mitochondrial membrane potential, ROS detection.

Biochemical Assays

Oxygen consumption rate (OCR) measurements, ATP quantification, enzyme activity assays.

Genetic Manipulation

mtDNA sequencing, gene editing (CRISPR/Cas9 targeting nuclear-encoded mitochondrial genes), transgenic models.

Proteomics and Metabolomics

Identification of mitochondrial proteins, metabolic flux analysis.

References

• Alberts B., Johnson A., Lewis J., et al. Molecular Biology of the Cell. 6th ed. Garland Science; 2014.
• Wallace DC. Mitochondrial DNA mutations in disease and aging. Environ Mol Mutagen. 2010;51(5):440-450.
• Friedman JR, Nunnari J. Mitochondrial form and function. Nature. 2014;505(7483):335-343.
• Spinelli JB, Haigis MC. The multifaceted contributions of mitochondria to cellular metabolism. Nat Cell Biol. 2018;20(7):745-754.
• Scorrano L. Keeping mitochondria in shape: A matter of life and death. Eur J Clin Invest. 2013;43(8):886-893.