3.1 - Mitochondrial Structure,

1. Mitochondria:

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United by: MiToChOnDrIa iS tHe PoWeRhOuSe oF tHe CeLL

Overview:

Mitochondria are membrane-bound organelles crucial for energy production in aerobic cells. They are the primary site of ATP synthesis through oxidative phosphorylation.

Structure:

Mitochondria have a double-membrane structure with four main regions:

1. Outer Membrane:

   • Smooth and permeable to small molecules (up to 5 kDa).
   • Contains porin proteins forming large channels.
   • Key Functions:
     • Fatty acid elongation
     • Membrane phospholipid synthesis
     • Hydroxylation reactions

2. Intermembrane Space:

   • Located between the outer and inner membranes.
   • Proton accumulation during electron transport occurs here.
   • Similar ionic composition to the cytosol.

3. Inner Membrane:

   • Impermeable to ions and most molecules.
   • Contains embedded protein complexes responsible for:
     • Electron transport (ETC)
     • ATP synthesis (Complex V, ATP synthase)
     • Transport proteins for ions and metabolites
   • Highly folded into cristae to increase surface area for metabolic processes.

4. Matrix:

   • Innermost compartment filled with a gel-like substance.
   • Contains enzymes for:
     • Pyruvate oxidation (Pyruvate Dehydrogenase Complex)
     • Citric Acid Cycle (Krebs Cycle)
     • Fatty acid β-oxidation
     • Amino acid oxidation
   • Contains mitochondrial DNA, ribosomes, and tRNA.

Functional Significance:

• Mitochondria are often called the “powerhouses” of the cell due to their role in ATP production.
• Most energy for ATP synthesis comes from oxidative phosphorylation.
• Mitochondria house enzymes for metabolic processes that generate NADH and FADH₂, which feed into the electron transport chain.

Key Points for Exams:

• Membrane Composition:
  • Outer membrane: Permeable, porin channels.
  • Inner membrane: Impermeable, contains ETC and ATP synthase.
• Compartmentalization:
  • Specific metabolic pathways are localized to distinct regions (e.g., Krebs cycle in the matrix).
• ATP Production:
  • Proton gradient generated across the inner membrane.
  • ATP synthase uses this gradient to produce ATP during oxidative phosphorylation.
• Unique Features:
  • Mitochondrial DNA and ribosomes allow for some degree of autonomous protein synthesis.