Mitochondria Structure and Function

Mitochondria

• Outer Membrane: Contains porins for passive transport (<10kDa), TOM complex (protein import), and enzymes for lipid metabolism and apoptosis.
• Inner Membrane: Rich in cardiolipin, houses respiratory chain complexes, ATP synthase, and transport proteins (e.g., TIM), forms cristae increasing surface area.
• Matrix: Contains mtDNA, RNA, ribosomes, and enzymes for pyruvate oxidation, fatty acid oxidation, and the citric acid cycle.
• Intermembrane Space: Contains H+ ions, cytochrome C, and enzymes. A narrow space for transient molecule passage.
• Oxidative Metabolism: Three stages: Citric Acid Cycle, Electron Transport Chain, and Oxidative Phosphorylation.
• Oxidative Phosphorylation: ATP synthesis using a proton gradient.
• Mitochondrial DNA (mtDNA): Circular, multiple copies (4-5 nucleoids) with 16,569 base pairs. Inherited maternally.
• Mitochondrial Biogenesis: Process where cells increase mitochondrial number to meet energy needs.
• Protein and Lipid Import: Proteins targeted to TOM complex by presequences, directed to TIM complex, and assisted by chaperones in the matrix. Phospholipids acquired from the ER.
• Mitochondrial Division: Divides by fission. Defective mitochondria are removed by autophagy.

Nucleus

• Structure: Double membrane (inner and outer membrane) separating nuclear content from the cytoplasm.
• Perinuclear Space: Located between the inner and outer membranes.
• Nuclear Envelope: The double-membrane structure.
• Nuclear Matrix: Insoluble materials supporting nuclear organization and includes the nuclear lamina.
• Nuclear Pores: Channels for communication between nucleoplasm and cytoplasm. Composed of nucleoporins arranged in octagonal symmetry.
• Nuclear Localization Signal (NLS): Necessary for proteins to enter the nucleus, containing charged amino acids like lysine.
• Nuclear Export Signal (NES): Necessary for protein export from the nucleus, often hydrophobic sequences.
• Nuclear Lamina: Protein framework inside the nucleus.
• Laminopathies: Genetic disorders from mutations in lamina proteins, affecting the envelope's structure.

Nucleus (Continued)

• Chromosomal Territories: Each chromosome occupies a specific territory in the nucleus, minimizing intermixing.
• Gene Activity: Genes near the surface of territories are actively transcribed.
• Karyopherins (Importins/Exportins): Proteins responsible for transporting molecules across the nuclear envelope.
• Ran Protein: Regulates directionality of transport in the nucleus.
• Ran-GTP: Ran bound to GTP.
• Ran-GDP: Ran bound to GDP.
• Ran-GEF: Located inside the nucleus and converts Ran-GDP to Ran-GTP.
• Ran-GAP: Located in the cytoplasm and converts Ran-GTP to Ran-GDP.
• RNA Transport: mRNA transport does not require Ran-GTP gradient, while transport of other RNA types (tRNA, rRNA, snRNA, miRNA) does. Exportins/Importins and Ran gradient facilitate this transport.