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)