Chapter 2
Transport of ions and small molecules
across membranes
By
Stephan E. Lehnart & Andrew R. Marks
2.1 Introduction
• Cell membranes define compartments of
different compositions.
• The lipid bilayer of biological membranes has
a very low permeability for most biological
molecules and ions.
2.1 Introduction
• Most solutes cross cell membranes through
transport proteins.
• The transport of ions and other solutes
across cellular membranes controls:
– electrical functions
– metabolic functions
2.2 Channels and carriers are the main
types of membrane transport proteins
• There are two principal types of membrane
transport proteins:
– Channels
– Carriers
2.2 Channels and carriers are the main types of membrane transport proteins
• Ion channels catalyze the rapid and selective
transport of ions down their electrochemical
gradients.
• Transporters and pumps are carrier proteins.
– They use energy to transport solutes against their
electrochemical gradients.
• In a given cell, several different membrane
transport proteins work as an integrated
system.
2.3 Hydration of ions influences their flux
through transmembrane pores
• Salts dissolved in water form hydrated ions.
• The hydrophobicity of lipid bilayers is a
barrier to movement of hydrated ions across
cell membranes.
2.3 Hydration of ions influences their flux through transmembrane pores
• By catalyzing the partial dehydration of ions,
ion channels allow for the rapid and selective
transport of ions across membranes.
• Dehydration of ions costs energy, whereas
hydration of ions frees energy.
2.4 Electrochemical gradients across the
cell membrane generate the membrane
potential
• The membrane potential across a cell
membrane is due to:
– an electrochemical gradient across a membrane
– a membrane that is selectively permeable to ions
2.4 Electrochemical gradients across the cell membrane generate the membrane potential
• The Nernst equation is used to
calculate the membrane potential as a
function of ion concentrations.
• E: equilibrium potential (volts)
• R: the gas constant (2 cal mol–1 K–1)
• T: absolute temperature (K; 37°C = 307.5 °K)
• z: the ion’s valence (electric charge)
• F: Faraday’s constant (2.3 104 cal volt–1 mol–1)
• [X]A: concentration of free ion X in compartment A
• [X]B: concentration of free ion X in compartment B
2.4 Electrochemical gradients across the cell membrane generate the membrane potential
• Cells maintain a negative resting membrane
potential with the inside of the cell slightly
more negative than the outside.
• The membrane potential is a prerequisite for
electrical signals and for directed ion
movement across cellular membranes.
2.5 K+ channels catalyze selective and rapid
ion permeation
• K+ channels function as water-filled pores that
catalyze the selective and rapid transport of
K+ ions.
• A K+ channel is a complex of four identical
subunits, each of which contributes to the
pore.
2.5 K+ channels catalyze selective and rapid ion permeation
• The selectivity filter of K+ channels is an
evolutionarily conserved structure.
• The K+ channel selectivity filter catalyzes
dehydration of ions, which:
– confers specificity
– speeds up ion permeation
2.6 Different K+ channels use a similar gate
coupled to different activating or inactivating
mechanisms
• Gating is an essential property of ion
channels.
• Different gating mechanisms define functional
classes of K+ channels.
2.6 Different K+ channels use a similar gate coupled to different activating or inactivating mechanisms.
• The K+ channel gate is distinct from the
selectivity filter.
• K+ channels are regulated by the membrane
potential.
2.7 Voltage-dependent Na+ channels are
activated by membrane depolarization and
translate electrical signals
• The inwardly directed Na+ gradient maintained by the
Na+/K+-ATPase is required for the function of Na+
channels.
2.7 Voltage-dependent Na+ channels are activated by membrane depolarization and translate electrical signals
• Electrical signals at the cell membrane activate
voltage-dependent Na+ channels.
• The pore of voltage-dependent Na+ channels is
formed by one subunit, but its overall architecture is
similar to that of 6TM/1P K+ channels.
• Voltage-dependent Na+ channels are inactivated by
specific hydrophobic residues that block the pore.
2.8 Epithelial Na+ channels regulate Na+
homeostasis
• The epithelial Na+ channel/degenerin family of
ion channels is diverse.
• The epithelial Na+ channels and Na+/K+-ATPase
function together to direct Na+ transport through
epithelial cell layers.
• The ENaC selectivity filter is similar to the K+
channel selectivity filter.
2.9 Plasma membrane Ca2+ channels
activate intracellular functions
• Cell surface Ca2+ channels translate membrane
signals into intracellular Ca2+ signals.
2.9 Plasma membrane Ca2+ channels activate intracellular functions
• Voltage-dependent Ca2+ channels are asymmetric
protein complexes of five different subunits.
• The α1 subunit of voltage-dependent Ca2+ channels
forms the pore and contains pore loop structures
similar to K+ channels.
2.9 Plasma membrane Ca2+ channels activate intracellular functions
• The Ca2+ channel selectivity filter forms an
electrostatic trap.
• Ca2+ channels are stabilized in the closed
state by channel blockers.
2.10 Cl– channels serve diverse biological
functions
• Cl– channels are anion channels that serve a
variety of physiological functions.
• Cl– channels use an antiparallel subunit
architecture to establish selectivity.
2.10 Cl– channels serve diverse biological functions
• Selective conduction and gating are structurally
coupled in Cl– channels.
• K+ channels and Cl– channels use different
mechanisms of gating and selectivity.
2.11 Selective water transport occurs
through aquaporin channels
• Aquaporins allow rapid and selective water
transport across cell membranes.
• Aquaporins are tetramers of four identical
subunits, with each subunit forming a pore.
2.11 Selective water transport occurs through aquaporin channels
• The aquaporin selectivity filter has three
major features that confer a high degree of
selectivity for water:
– size restriction
– electrostatic repulsion
– water dipole orientation
2.12 Action potentials are electrical signals
that depend on several types of ion
channels
• Action potentials enable rapid communication
between cells.
• Na+, K+, and Ca2+ currents are key elements
of action potentials.
• Membrane depolarization is mediated by the
flow of Na+ ions into cells through voltagedependent Na+ channels.
2.12 Action potentials are electrical signals that depend on several types of ion channels
• Repolarization is shaped by transport of K+
ions through several different types of K+
channels.
• The electrical activity of organs can be
measured as the sum of action potential
vectors.
• Alterations of the action potential can
predispose for arrhythmias or epilepsy.
2.13 Cardiac and skeletal muscles are
activated by excitation-contraction coupling
• The process of excitation-contraction
coupling, which is initiated by membrane
depolarization, controls muscle contraction.
• Ryanodine receptors and inositol 1,4,5trisphosphate receptors are Ca2+ channels.
– Ca2+ ions are released from intracellular stores
into the cytosol through them.
2.13 Cardiac and skeletal muscles are activated by excitation-contraction coupling
• Intracellular Ca2+ release through ryanodine
receptors in the sarcoplasmic reticulum
membrane stimulates contraction of the
myofilaments.
• Several different types of Ca2+ transport
proteins, including the Na+/Ca2+-exchanger
and Ca2+-ATPase are important for
– decreasing the cytosolic Ca2+ concentration
– controlling muscle relaxation
2.14 Some glucose transporters are
uniporters
• To cross the blood-brain barrier, glucose is transported
across endothelial cells of small blood vessels into
astrocytes.
2.14 Some glucose transporters are uniporters
• Glucose transporters are uniporters that transport
glucose down its concentration gradient.
• Glucose transporters undergo conformational
changes that result in a reorientation of their
substrate binding sites across membranes.
2.15 Symporters and antiporters mediate
coupled transport
• Bacterial lactose permease functions as a symporter.
– It couples lactose and proton transport across the
cytoplasmic membrane.
• Lactose permease uses the electrochemical H+
gradient to drive lactose accumulation inside cells.
• Lactose permease can also use lactose gradients to
create proton gradients across the cytoplasmic
membrane.
2.15 Symporters and antiporters mediate coupled transport
• The mechanism of transport by lactose
permease likely involves inward and outward
configurations.
– They allow substrates to:
• bind on one side of the membrane and to
• be released on the other side
• The bacterial glycerol-3-phosphate
transporter is an antiporter that is structurally
related to lactose permease.
2.16 The transmembrane Na+ gradient is
essential for the function of many
transporters
• The plasma membrane Na+ gradient is
maintained by the action of the Na+/K+-ATPase.
• The energy released by movement of Na+ down
its electrochemical gradient is coupled to the
transport of a variety of substrates.
• The Na+/Ca2+-exchanger is the major transport
mechanism for removal of Ca2+ from the cytosol of
excitable cells.
2.16 The transmembrane Na+ gradient is essential for the function of many transporters
• The gastrointestinal tract absorbs sugar
through the Na+/glucose transporter.
• The Na+/K+/Cl–-cotransporter regulates
intracellular Cl– concentrations.
• Na+/Mg2+-exchangers transport Mg2+ out of
cells.
2.17 Some Na+ transporters regulate
cytosolic or extracellular pH
• Na+/H+ exchange controls intracellular acid and cell
volume homeostasis.
• The net effect of Na+/HCO3–-cotransporters is to remove
acid by directed transport of HCO3–.
2.18 The Ca2+-ATPase pumps Ca2+ into
intracellular storage compartments
• Ca2+-ATPases undergo a reaction cycle involving
two major conformations, similar to that of Na+/K+ATPases.
• Phosphorylation of Ca2+-ATPase subunits drives:
– conformational changes
– translocation of Ca2+ ions across the membrane
2.19 The Na+/K+-ATPase maintains the
plasma membrane Na+ and K+ gradients
• The Na+/K+-ATPase is a P-type ATPase that is similar
to the Ca2+-ATPase and the H+-ATPase.
• The Na+/K+-ATPase maintains the Na+ and K+
gradients across the plasma membrane.
• The plasma membrane Na+/K+-ATPase is
electrogenic:
– it transports three Na+ ions out of the cell for every two K+
ions it transports into the cell.
2.19 The Na+/K+-ATPase maintains the plasma membrane Na+ and K+ gradients
• The reaction cycle for Na+/K+-ATPase is
described by the Post-Albers scheme.
– It proposes that the enzyme cycles between two
fundamental conformations.
2.20 The F1Fo-ATP synthase couples H+
movement to ATP synthesis or hydrolysis
• The F1Fo-ATP synthase is a key enzyme in
oxidative phosphorylation.
• The F1Fo-ATP synthase is a multisubunit
molecular motor.
– It couples the energy released by movement of protons
down their electrochemical gradient to ATP synthesis.
2.21 H+-ATPases transport protons out of
the cytosol
• Proton concentrations affect many cellular
functions.
• Intracellular compartments are acidified by the
action of V-ATPases.
• V-ATPases are proton pumps that consist of
multiple subunits, with a structure similar to F1FoATP synthases.
2.21 H+-ATPases transport protons out of the cytosol
• V-ATPases in the plasma membrane serve
specialized functions in:
– acidification of extracellular fluids
– regulation of cytosolic pH
Supplement: Most K+ channels undergo
rectification
• Inward rectification occurs through voltagedependent blocking of the pore.
Supplement: Mutations in an anion channel
cause cystic fibrosis
• Cystic fibrosis is caused by mutations in the
gene encoding the CFTR channel.
• CFTR is an anion channel that can transport
either Cl– or HCO3–.
• Defective secretory function in cystic fibrosis
affects numerous organs.