Electrical Properties of Membranes
Distribution of ions in gradients
Lipid bilayers are impermeable to solutes and ions – due to
the hydrophobic interior
Given enough time, virtually any molecule will diffuse
across a lipid bilayer – the rate at which it diffuses, varies
enormously depending on the size of the molecule and its
solubility properties
1. Small nonpolar molecules: molecular oxygen, carbon
dioxide readily dissolve in lipid bilayers and diffuse across them
2. Uncharged polar molecules diffuse rapidly across bilayer if small enough – water,
ethanol (18, 46 daltons) diffuse rapidly, glycerol (92 daltons) less rapidly,
glucose (180 daltons) not at all
3. Highly impermeable to all ions and charged molecules – charge and strong
electrical attraction to water molecules inhibit them from entering hydrocarbon
phase of bilayer
Membrane Transport Proteins – 2 Classes
Each protein provides a passageway across the membrane for a particular class of
molecule – ions, sugars or amino acids, can be highly selective
2 classes differ in how they discriminate between solutes
Transporters Channels
Allows passage only to those Aqueous pore which discriminates
molecules/ions that fit into a binding site mainly on the basis of size and
on the protein electric charge
Molecules are then transferred across If channel is open, an ion/molecule
the membrane one at a time by changing small enough carrying the
its own conformation appropriate charge can pass through
Tranporters bind their solutes with
great specificity, similar to an enzyme
(specific binding gives selectivity)
, Solutes cross membrane by passive or
active transport
Ion channels are highly selective –
dependent on protein structure
Directionality of ion gradients are assumed
to have been inherited from the parent cell,
though some cells are able to establish from scratch
Passive ion transport is determined by the
electrochemical gradient
For some ions, voltage and concentration
gradient work in the same direction, creating a
relatively steep electrochemical gradient (vice
versa – opposing gradients creates a small gradient)
Cellular ion gradients are generated by active transport
Na+-K+ Pump – P-type ATPase
Pump transports ions in a cyclic manner (each
cycle ~10ms)
Each step in the cycle depends on the one before, - tight
coupling ensures the pump operates only when appropriate ions are
available – avoiding useless ATP hydrolysis
Charge and concentration separation achieved = electrogenic pump
1. Na+ binds to pump at sites exposed inside the
cell, activating ATPase activity
2. ATP is split,with release of ADP and transfer
of phosphate group into a high-energy linkage
to the pump
3. Pump phosphorylates itself causes pump to
switch its conformation, releasing Na+ at the
exterior surface of the cell and simultaneously
expose a binding site for K+ at the same surface
, 4. Binding of extracellular K+ triggers the removal of the phosphate group causing
the pump to switch back to its original conformation, discharging the K + into the
cell interior
Pump helps maintain the osmotic balance of animal cells – maintain ostmotic
pressure
Ca2+ Pumps – P-Type ATPases
Intracellular [Ca2+] kept low by pumps
Ca2+ like Na+, kept a low concentrations in
cytosol compared to extracellular fluid – is
much less plentiful than Na+
Ca2+ crucially important as it can bind
tightly to a variety of proteins in cell,
altering their activities, influx of Ca 2+ into
cytosol through channels is often used as a
signal to trigger other intracellular events
(secretion of signal molecules, contraction
of muscle cells)
SERCA: generate Ca2+ gradients across
intracellular stores (eg. ER)
ATP binding pocket, use ATP to
phosphorylate an aspartic residue –
transient binding, not very stable
Similar structure and cyclic function as Na+-K+ pump suggesting a common
evolutionary origin
PMCA: generate Ca2+ gradients across plasma membrane
Membrane spanning regions + Phosphoryation domain, ATP binding site, CaM
binding site to regulate function
H+ pump – V (vacuolar) –type ATPase
Generate H+ gradients across endosomes and lysosomes
Region on cytoplasmic side which splits ATP, resulting in an
ejection of a proton on the other side
Hydrolytic enzymes have a low pH optimum
How Ion Gradients are used
Distribution of ions in gradients
Lipid bilayers are impermeable to solutes and ions – due to
the hydrophobic interior
Given enough time, virtually any molecule will diffuse
across a lipid bilayer – the rate at which it diffuses, varies
enormously depending on the size of the molecule and its
solubility properties
1. Small nonpolar molecules: molecular oxygen, carbon
dioxide readily dissolve in lipid bilayers and diffuse across them
2. Uncharged polar molecules diffuse rapidly across bilayer if small enough – water,
ethanol (18, 46 daltons) diffuse rapidly, glycerol (92 daltons) less rapidly,
glucose (180 daltons) not at all
3. Highly impermeable to all ions and charged molecules – charge and strong
electrical attraction to water molecules inhibit them from entering hydrocarbon
phase of bilayer
Membrane Transport Proteins – 2 Classes
Each protein provides a passageway across the membrane for a particular class of
molecule – ions, sugars or amino acids, can be highly selective
2 classes differ in how they discriminate between solutes
Transporters Channels
Allows passage only to those Aqueous pore which discriminates
molecules/ions that fit into a binding site mainly on the basis of size and
on the protein electric charge
Molecules are then transferred across If channel is open, an ion/molecule
the membrane one at a time by changing small enough carrying the
its own conformation appropriate charge can pass through
Tranporters bind their solutes with
great specificity, similar to an enzyme
(specific binding gives selectivity)
, Solutes cross membrane by passive or
active transport
Ion channels are highly selective –
dependent on protein structure
Directionality of ion gradients are assumed
to have been inherited from the parent cell,
though some cells are able to establish from scratch
Passive ion transport is determined by the
electrochemical gradient
For some ions, voltage and concentration
gradient work in the same direction, creating a
relatively steep electrochemical gradient (vice
versa – opposing gradients creates a small gradient)
Cellular ion gradients are generated by active transport
Na+-K+ Pump – P-type ATPase
Pump transports ions in a cyclic manner (each
cycle ~10ms)
Each step in the cycle depends on the one before, - tight
coupling ensures the pump operates only when appropriate ions are
available – avoiding useless ATP hydrolysis
Charge and concentration separation achieved = electrogenic pump
1. Na+ binds to pump at sites exposed inside the
cell, activating ATPase activity
2. ATP is split,with release of ADP and transfer
of phosphate group into a high-energy linkage
to the pump
3. Pump phosphorylates itself causes pump to
switch its conformation, releasing Na+ at the
exterior surface of the cell and simultaneously
expose a binding site for K+ at the same surface
, 4. Binding of extracellular K+ triggers the removal of the phosphate group causing
the pump to switch back to its original conformation, discharging the K + into the
cell interior
Pump helps maintain the osmotic balance of animal cells – maintain ostmotic
pressure
Ca2+ Pumps – P-Type ATPases
Intracellular [Ca2+] kept low by pumps
Ca2+ like Na+, kept a low concentrations in
cytosol compared to extracellular fluid – is
much less plentiful than Na+
Ca2+ crucially important as it can bind
tightly to a variety of proteins in cell,
altering their activities, influx of Ca 2+ into
cytosol through channels is often used as a
signal to trigger other intracellular events
(secretion of signal molecules, contraction
of muscle cells)
SERCA: generate Ca2+ gradients across
intracellular stores (eg. ER)
ATP binding pocket, use ATP to
phosphorylate an aspartic residue –
transient binding, not very stable
Similar structure and cyclic function as Na+-K+ pump suggesting a common
evolutionary origin
PMCA: generate Ca2+ gradients across plasma membrane
Membrane spanning regions + Phosphoryation domain, ATP binding site, CaM
binding site to regulate function
H+ pump – V (vacuolar) –type ATPase
Generate H+ gradients across endosomes and lysosomes
Region on cytoplasmic side which splits ATP, resulting in an
ejection of a proton on the other side
Hydrolytic enzymes have a low pH optimum
How Ion Gradients are used
