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Membrane Permeability

Membrane Permeability

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CELL MEMBRANE
  • Separates intracellular and extracellular environments
  • Regulates import and export of molecules
  • Lipid bilayer
SIMPLE DIFFUSION Molecules diffuse across the cell membrane
Rate of diffusion is determined by:
  • Size: smaller molecules diffuse across faster than large ones
  • Polarity: the less polar, the faster it diffuses across the membrane
  • Charged molecules: do not diffuse across hydrophobic interior
Types of molecules
  • Nonpolar molecules: hydrophobic, diffuse rapidly
i.e. oxygen, carbon dioxide, nitrogen and steroid hormones
  • Small, uncharged polar molecules: diffuse across
i.e. water, glycerol and ethanol
  • Large, uncharged polar molecules: do NOT diffuse across
i.e. amino acids, glucose and nucleosides
  • ions (charged molecules): cannot diffuse across
ie. Hydrogen, sodium, potassium, calcium and chloride ions
MEMBRANE TRANSPORT PROTEINS Allow molecules to cross the membrane faster and more efficiently than simple diffusion i. Channels: form open pores in the membrane ii. Transporters: undergo conformational changes during transport
  • Solute binds solute binding site
  • Binding produces conformational change in protein
  • Solute leaves transporter (release facilitated by conformational change)
TWO TYPES OF TRANSPORT ACROSS MEMBRANES
Passive transport
  • Molecules move down their electrochemical gradient
  • No energy is required
Active transport
  • Molecules move against their concentration gradient
  • Energy is required
ELECTROCHEMICAL GRADIENT Accounts for voltage across the membrane (electro-) and concentration gradient (chemical)

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