Friday slides feb14pdf

Cell Compartments and Membrane Transport

• Types of Membrane Channels:

• Voltage-gated: Channels that open in response to voltage changes across the membrane.

• Ligand-gated (extracellular ligand): Channels that open when a ligand binds from outside the cell.

• Ligand-gated (intracellular ligand): Channels that open when a ligand binds from inside the cell.

• Mechanically-gated: Channels that open in response to physical pressure or stretching.

Hair Cells and Hearing

• Components Involved in Hearing:

  • Auditory Hair Cells: Specialized sensory cells in the inner ear.

  • Tectorial Membrane: A membrane where hair cells are embedded.

  • Supporting Cells: Cells that provide structural support and protection to hair cells.

  • Basilar Membrane: Movement of this membrane triggers the hair cells' response to sound.

  • Auditory Nerve Fibers: Transmit auditory signals to the brain.

  • Stereocilia: Hair-like projections on hair cells crucial for mechanotransduction.
    Mechanotransduction is the process through which cells convert mechanical stimuli into biochemical signals. In the context of the note on hair cells involved in hearing, mechanotransduction occurs when sound waves cause the stereocilia on auditory hair cells to move, leading to the opening of ion channels. This process is crucial in translating sound vibrations into electrical signals that can be interpreted by the brain.

  • Ion Channel Behavior: Illustrates how channels open or close in response to mechanical displacement of stereocilia.

Spectrin and Disease

• Role of Spectrin:

  • Structural Component: A cytoskeletal protein involved in maintaining the cell shape.

  • Defects Associated with Diseases:

    • Inherited anemias due to spectrin or ankyrin defects lead to abnormal RBC shapes and fragility.

    • Implications in neurodegenerative diseases like spinocerebellar ataxia.

    • Defects in dystrophin (similar to spectrin) cause Duchenne muscular dystrophy.

Energetics of Transport

• Solute Movement Across Membranes:

  • Movement is largely driven by equilibrium.

  • For uncharged solutes, the energetics are based on concentration differences.

  • The change in free energy (ΔG) for inward movement is given by:

    • ( ΔG_{inward} = +RT\ln \left(\frac{[S]{inside}}{[S]{outside}}\right) )

Charged Solutes

• The energetics of charged solutes are dependent on both concentration and membrane potential:

  • The equation for free energy change incorporates both factors:

    • ( ΔG_{inward} = +RT\ln \left(\frac{[S]{inside}}{[S]{outside}}\right) + zFVm )

  • Variables Explained:

    • z: Charge of the ion. (e.g. +1 for Na+)

    • F: Faraday’s constant (23,062 cal/mol V)

    • Vm: Membrane potential (in volts)

  • Ions can move against charge gradients if concentration differences are sufficiently large.

Na+/K+ ATPase Activity

• Ion Transport Mechanism: Na+/K+ ATPase pumps 3 Na+ out and 2 K+ into the cell for every ATP molecule hydrolyzed.

• Membrane Potential Impact:

  • Na+ ions move against a negative membrane potential (attract K+) while K+ moves with it (repel Na+).

• Free Energy Calculations: Include charge of ions and membrane potential to compute changes for transport.

Constants for Calculation

• Gas Constant (R): 1.98 x 10^-3 kcal/oK mole

• Temperature (T): 37°C

• Faraday's Constant (F): 23 kcal/V mole

Concentration Values for Ions

• Intracellular Na+: 10 mM

• Extracellular Na+: 145 mM

• Intracellular K+: 140 mM

• Extracellular K+: 5 mM

• ATP: 5 mM

• ADP: 1 mM

• Inorganic Phosphate (Pi): 10 mM