Mitochondria and Intracellular Transport - Study Notes

Mitochondria comprise the outer lipid bilayer, inner lipid bilayer, intermembrane space, and matrix.
Internal components include mitochondrial DNA (mtDNA) and mitochondrial ribosomes.
Structural features named in the material: outer membrane, inner membrane, intermembrane space, matrix, cristae, DNA, ribosomes.
Cristae: folds of the inner membrane that increase surface area for processes such as ATP synthesis.

Function 1: Synthesize ATP in the presence of oxygen (oxidative phosphorylation).
- ATP synthase particles are located on the inner membrane and use the proton gradient across the inner membrane to generate ATP.
- Relevant components shown: inner membrane, outer membrane, matrix, cristae, ATP synthase particles.
Function 2: Use its own DNA and ribosomes to synthesize some of its own proteins; most mitochondrial proteins are encoded by the nuclear genome and imported via the endomembrane system.
Function 3: Stress alert when mitochondrial DNA leaks into the cytoplasm, signaling host cell stress.
- Locations referenced: DNA present in mitochondria; intermembrane space; cristae; matrix; inner membrane; outer membrane.

Inheritance pattern: mitochondria are inherited from the mother.
Concept: mitochondrial Eve — a single common female ancestor from whom all current human mitochondria are descended.
- Timeframe: approximately $160{,}000\ \text{years ago}$ , in Africa.
- Supporting data come from molecular clock analyses which measure mutation rates in mitochondrial DNA to estimate time.
- Note: mitochondrial Eve was not the first or only female alive at the time; her mtDNA lineage is the one that survived in modern humans.
Terminology: Most recent common female ancestor is called mitochondrial Eve.

Intracellular transport: movement of molecules, vesicles, and organelles within the cell.
Key terms visible: lumen (interior of organelles/vesicles), cytosol (fluid outside organelles), budding (vesicle formation), vesicular transport, fusion.

Formation (budding): packages of proteins and lipids leave a departure organelle by pinching off a vesicle.
Destination targeting: vesicle arrives at its target destination and fuses with the target membrane.

Endocytosis: inward budding of the vesicle at the cell membrane; also described as "cell eating".
- Context: vesicle forms from the cell membrane and moves into the cell.
Exocytosis: vesicle fuses with the plasma membrane to secrete contents; described as "cell exiting".
- Examples: secretion of neurotransmitters or peptide hormones.

Vesicles are not passively diffused; they are actively transported along cytoskeletal tracks.
Transport along tracks requires motor proteins:
- Microtubule-associated motors: two motors, kinesin and dynein.
- Actin filament motor: myosin.
Structural components mentioned: MTOC (microtubule organizing center), microtubule tracks, actin filaments, Dynactin complex (a dynein cofactor).
Directionality overview:
- Kinesin: anterograde transport (toward the plus end, away from the MTOC).
- Dynein: retrograde transport (toward the minus end, toward the MTOC).
- Myosin: moves along actin filaments; can operate in multiple directions since actin filaments are oriented in various directions.
The general arrangement: MTs radiate from the MTOC like spokes on a wheel; kinesin carries vesicles toward the plasma membrane; dynein moves vesicles toward the cell interior.

Specialized example: in neurons, neurotransmitter (NT) synthesis occurs in the endomembrane system.
NTs are transported in vesicles by kinesin along microtubules (MT) toward the axon terminal.
At the synapse, NT-containing vesicles wait for an appropriate action potential to trigger release.
Following release, NTs are reabsorbed/recycled by endocytosis (reuptake and recycling of NT).

The motor mechanism of motor proteins requires one molecule of ATP for each step.
Chemical basis (conceptual): ATP hydrolysis provides the energy for stepping.
- General reaction: \text{ATP} + \text{H}2\text{O} \rightarrow \text{ADP} + \text{P}i + \Delta G}, with a negative ΔG indicating energy release for movement.

Mitochondria are multifunctional organelles with their own genome and ribosomes, capable of ATP production, protein synthesis, and signaling when compromised.
Mitochondria are inherited maternally, and their evolutionary history is traced via mtDNA and molecular clocks (mitochondrial Eve as a point of reference).
The abundance of mitochondria varies by tissue, reflecting metabolic demand (e.g., higher mitochondrial content in heart and liver relative to their resting cell mass).
Intracellular transport relies on coordinated motor proteins along cytoskeletal tracks to move vesicles and organelles efficiently, rather than relying on diffusion alone.
Vesicle formation (budding) and fusion control cargo delivery between organelles and the plasma membrane, with endocytosis and exocytosis describing vesicle interactions with the cell surface.
In neurons, vesicular transport is specialized for rapid and directional NT delivery to the synapse, illustrating the integration of organelle biology with cellular signaling and physiology.
ATP is the energy currency driving motor protein function, with each step consuming one ATP molecule, linking chemistry to mechanical movement within the cell.