Specialized Membranes and Membrane Potential

Specialized Membranes

  • Most organelle membranes resemble the cell membrane in composition and characteristics.
  • Some membranes are specialized for specific functions.
    • Example: Sarcolemma of muscle cells maintains membrane potential for muscle contraction.
  • Membrane composition can vary, especially in mitochondria.

Membrane Potential

  • Impermeability of the cell membrane to ions plus selectivity of ion channels create an electrochemical gradient.
  • Membrane potential (V_m) is the difference in electrical potential across a cell membrane.
  • Resting potential: -40 to -80 mV for most cells, but can reach +35 mV during depolarization.
  • Maintaining membrane potential requires energy to counter passive ion diffusion through leak channels.
  • Ion transporters (e.g., Na+/K+ ATPase) regulate intracellular and extracellular ion concentrations.
  • Chloride ions also contribute to establishing membrane potential.

Nernst Equation

  • Used to determine membrane potential based on intra- and extracellular ion concentrations.

    E = \frac{RT}{zF} \ln \frac{[ion]{outside}}{[ion]{inside}} = \frac{61.5}{z} \log \frac{[ion]{outside}}{[ion]{inside}}

    • R = ideal gas constant
    • T = temperature in Kelvins
    • z = ion charge
    • F = Faraday constant (96485 C/mole e-)
    • 61.5 mV simplification assumes body temperature (310 K)

Goldman-Hodgkin-Katz Voltage Equation

  • Extends the Nernst equation, considering the contribution of each major ion.

    Vm = 61.5 \log \frac{P{Na^+} [Na^+]{outside} + P{K^+} [K^+]{outside} + P{Cl^-} [Cl^-]{inside}}{P{Na^+} [Na^+]{inside} + P{K^+} [K^+]{inside} + P{Cl^-} [Cl^-]_{outside}}

    • P = permeability for the ion
    • Note: Chloride (Cl-) is inverted due to its negative charge.

Sodium-Potassium Pump

  • Na+/K+ ATPase establishes a steady-state relationship between ion diffusion.
  • The Na+/K+ ATPase maintains low intracellular sodium and high intracellular potassium concentrations.
    • Pumps 3 Na+ ions out for every 2 K+ ions pumped in.
    • Removes one positive charge from the intracellular space, maintaining the negative resting potential.
  • Leak channels allow passive ion diffusion (Na+ and K+) down concentration gradients.
  • Membranes are more permeable to K+ at rest due to more K+ leak channels than Na+ leak channels.
  • Na+/K+ ATPase activity and leak channels together maintain stable resting membrane potential.

Mitochondrial Membranes

  • Mitochondria produce ATP via oxidative respiration.
  • Two membranes: inner and outer mitochondrial membranes.

Outer Mitochondrial Membrane

  • Highly permeable due to large pores, allowing passage of ions and small proteins.
  • Surrounds the inner mitochondrial membrane, separated by the intermembrane space.

Inner Mitochondrial Membrane

  • Restricted permeability compared to the outer membrane.
  • Cristae: Numerous enfoldings that increase surface area for membrane proteins.
  • Proteins involved in the electron transport chain and ATP synthesis.
  • Encloses the mitochondrial matrix where the citric acid cycle produces high-energy electron carriers.
  • High cardiolipin content, no cholesterol.

Conclusion

  • Understanding biological membranes is essential.
  • Key Concepts Reviewed:
    • Fluid mosaic model
    • Membrane components (lipids, phospholipid bilayer)
    • Cell junctions
    • Membrane transport (passive, active, endocytosis, exocytosis)
    • Specialized membranes
  • Foundation for future medical studies (metabolic pathways related to biomolecules).