Chapter 11: Membrane Transport of Small Molecules and the Electrical Properties of
Membranes
Because of its hydrophobic interior, the lipid bilayer of
cell membranes restricts the passage of most polar
molecules
o barrier function allows the cell to maintain
concentrations of solutes in tis cytosol that
differ from those in the extracellular fluid and in
each of the intracellular membrane-enclosed
compartments
o membrane transport proteins
o two classes of membrane proteins:
transporters and channels
Protein-free lipid bilayers are impermeable to ions
Transporters / carriers / permeases bind the
specific solute to be transported and undergo a series
of conformational changes that alternately expose
solute-binding sites on one side of the membrane and
then on the other to transfer the solute across it
Channels – interact with the solute to be transported
much more weakly
o much faster rate
o water channels /
aquaporins
passive transport –
requires no energy
active transport –
requires energy against electrochemical gradient (ions) / concentration
gradient
o coupled to a source of
metabolic energy
Vmax when transport is maximal; all
solute-binding sites are occupied
Km equal to the concentration of
solute when the transport rate is half
its maximum
Binding sites can be blocked by
competitive inhibitors or non-
competitive inhibitors
Three ways to carry out active
transport:
o 1) coupled transporters
o 2) ATP-driven pumps
o 3) light- or redox-driven pumps
Membranes
Because of its hydrophobic interior, the lipid bilayer of
cell membranes restricts the passage of most polar
molecules
o barrier function allows the cell to maintain
concentrations of solutes in tis cytosol that
differ from those in the extracellular fluid and in
each of the intracellular membrane-enclosed
compartments
o membrane transport proteins
o two classes of membrane proteins:
transporters and channels
Protein-free lipid bilayers are impermeable to ions
Transporters / carriers / permeases bind the
specific solute to be transported and undergo a series
of conformational changes that alternately expose
solute-binding sites on one side of the membrane and
then on the other to transfer the solute across it
Channels – interact with the solute to be transported
much more weakly
o much faster rate
o water channels /
aquaporins
passive transport –
requires no energy
active transport –
requires energy against electrochemical gradient (ions) / concentration
gradient
o coupled to a source of
metabolic energy
Vmax when transport is maximal; all
solute-binding sites are occupied
Km equal to the concentration of
solute when the transport rate is half
its maximum
Binding sites can be blocked by
competitive inhibitors or non-
competitive inhibitors
Three ways to carry out active
transport:
o 1) coupled transporters
o 2) ATP-driven pumps
o 3) light- or redox-driven pumps
