Membrane Permeability

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)

Membrane Permeability

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