Membranes

• • cell membrane is not an inert bag holding the cell together
  • composed of phospholipid bilayer
  • lipid soluble molecules and gases diffuse easily
  • water soluble molecules cannot cross without help
  • impermeable to organic anions (proteins)
• permeability depends on molecular size, lipid solubility and charge
• if a substance can cross the membrane = permeable

  • gases diffuse across membrane
  • polar molecules and ions need proteins to cross
• Simple diffusion

  • small lipid-soluble molecules and gases pass directly through the phospholipid bilayer
  • moves down concentration gradient, high to low
  • the rate of diffusion is roughly proportional to the concentration gradient across the membrane
  • passive, no ATPase
• Facilitated diffusion

  • diffuse across membrane with help from carrier proteins
  • aids in the movement of polar molecules
  • down the concentration gradient
  • energy comes from the concentration gradient of the solute
  • passive
• Active Transport

  • allows for molecules to move against concentration gradient
  • there is a conformation change in carrier protein to move it across the membrane
  • requires ATP to hydrolyze to ADP
  • e.g. Na+/K+ pump
• Secondary Active Transport

  • when a substance moves against concentration gradient without ATP catabolism
  • kinetic energy of movement for one substance down its concentration gradient powers the simultaneous transport of another up its concentration gradient
  • sequential binding of a substance and ions to specific sites in the transporter protein induces a conformational change of protein
• Channels

  • membrane spanning protein forms a 'pore' though membrane
  • still has specificity
  • 4-5 protein subunits fit together such that a central pore is created thought the membrane through which specified ions can diffuse
  • physical properties of the pore loops create a selectivity filter
  • pores =membrane channels
• Gated Channels

  • channels are not always open, can be closed are gated
  • still selective
  • factors determining channels protein shape: a) ligand gated channels: binding of chemical agent, b) voltage gated channels: voltage across the membrane
• Ligand Gated Channels

  • cell membrane receptors are part of the body's chemical signaling system
  • the binding of a receptor with its ligand triggers events at the membrane
  • e.g. activation of an enzyme