Harvey Lodish • Arnold Berk • Paul Matsudaira •
Chris A. Kaiser • Monty Krieger • Matthew P. Scott •
Lawrence Zipursky • James Darnell
Molecular Cell Biology
Fifth Edition
Chapter 7:
Transport of Ions and Small Molecules
Across Cell Membranes
Copyright © 2004 by W. H. Freeman & Company
Cell membrane
Barrier to the passage of most polar
molecule
Maintain concentration of solute
Aquaporin, the water channel, consists of four
identical transmembrane polypeptides
Relative permeability pf synthetic lipid bilayer to
different classes of molecule
Diffusion rate depends on :
1. Concentration gradient or electrochemical
gradient
2. Hydrophobicity
i.e. higher partition coefficient
3. Particle size
Three main class of membrane protein
1.ATP- power pump( carrier, permease)
 couple with energy source for active transport
 binding of specific solute to transporter which
undergo conformation change
2. Channel protein
 formation of hydrophilic pore
 allow passive movement of small inorganic
molecule
3. Transporters
 uniport
 symport
 antiport
1. All transmembrane
proteins
2. ATP binding sites
3. Move molecules
uphill against its
gradient
Kinetics of simple diffusion and carrier mediated
diffusion
Unique features for Uniport transport:
1. Higher diffusion rate for uniport
2. Irrelevant to the partition coefficient
3. Transport rate reach Vmax when each uniport
working at its maximal rate
4. Each uniport transports only a single
species of molecules or single or closely
related molecules
Liposome containing a single type of ytransport protein are
useful in studying functional properties of transport protein
Families of GLUT proteins( 1-12)
GLUT1
GLUT2: express in liver cell ( glucose storage)
and ß cell( glucose uptake) pancrease
GLUT4: found in intracellular membrane,
increase expression by insulin, lowers
the blood glucose
ATP powered pump
1. P- class
2, 2 subunit
i.e. Na+-K+ ATP ase, Ca+ATP ase, H+pump
2. F-class
 locate on bacterial membrane , chloroplast
and mitochondria
 pump proton from exoplasmic space to
cytosolic for ATP synthesis
3. V-class
maintain low pH in plant vacuole
Operational model of the Ca+-ATP ase in the SR
membrane of skeletal muscle cells
Higher Ca+2
Lower Ca+2
Structure of the catalytic  subunit of the muscle Ca+2 ATP ase
-helix
Phosphorylation site
Operational model of the Na+/K+ ATP ase in the plasma
membrane
Higher affinity
for Na+
V-class H+ ATP ase pump protons
across lysosomal and vacuolar
membrane
Effect of proton pumping by V-class ion pumps on H+ concentration
gradients and electric potential gradients across cellular membrane
Generation of electrochemical gradient
Electrochemical gradient combines the
membrane potential and concentration
gradient which work additively to increase
the driving force
ABC transporter
2 T ( transmembrane ) domain
6 - helix
form pathways for transported substance
2A ( ATP- binding domain)
30-40% homology for membranes
i.e. bacterial permease
 use ATP hydrolysis
 transport a.a ,sugars, vitamines, or peptides
 inducible, depend on the environmental condition
i.e. mammalian ABC transporter ( Multi Drug Resistant)
 export drug from cytosol to extracellular medium
 mdr gene amplified by drus stimulation
 mostly hydrophobic for MDR proteins
Structural model for E.coli flippase
6 - helix
Flippase model of transport by MDR1 and similar ABC
proteins
Diseases linked with ABC proteins
1. ALD( X-link adrenoleukodestrophy)
defect in ABC transport protein( ABCD1)
located on peroxisome, used for transport for very
long fatty acid
2. Tangiers disease
Dificiency in plasma ABCA1 proteins, which is used for
transport of phospholipis and cholesterol
3. Cystic fibrosis
mutation of CTFR( cyctic fibrosis transmenbrane
regulator; a Cl- transporter in the apical membrane of
lung, sweat gland and pancrease)
Ion Channel
Generation of electrochemical gradient across
plasma membrane
i.e. Ca+ gradient
regulation of signal transduction , muscle
contraction and triggers secretion of digestive
enzyme in to exocrine pancreastic cells
i.e. Na+ gradient
uptake of a.a , symport, antiport
Q: how does the electrochemical gradient
formed?
Selective movement of Ions Create a
transmembrane electric potential difference
Measuring the electrochemical gradient
Structure of resting K+channel from the bacterium
Streptomyces lividans
Important for selection
Smaller Na+ does
not fit perfectly
Replacement of carboxyl backbone from P segment
Oocyte expression assay is useful in comparing the function of
normal and mutant forms of channel proteins
Cotransport:
Use the energy stored in Na+ or H+
electrochemical gradient to power the transport
of another subatance
Symport: the transportd molecules and cotransported
ion move in the same direction
Antiport: the transported molecules move in opposited
direction
Operation Model for the two-Na+/one glucose symport
Glucose transport against its gradient in the epithelial cells of intestine
1 glucose in
2 Na+ in
G=0
Na+ linked antiport Exports Ca+2 from cardiac Muscle
Cells
3Na+ out+ Ca+2 in
3Na+ in+ Ca+2out
maintenance of low cytosolic Ca+2 concentration
i.e. inhibition of Na+/K+ ATPase by Quabain and Digoxin
raises cytosolic Na+
lowers the efficiency of Na+/Ca+2 antiport
increases cytosolic Ca+2
( used in cogestive heart failure)
Cotransporters that regulate cytosolic pH
H2CO3
H+
H+ + HCO-
can be neutrolized by
1.Na+/HCO3-/Cl- antiport
2. Cabonic anhydrase
HCO3-
3. Na+/H+ antiport
CO2+OH-
The activity of membrane transport proteins that
regulate the cytosolic pH of mammalian cells changes
with pH
Plant vacuole membrane
 pH 3—6
 Low acidity maintained by
 V-class ATP-powered pump
 PPi -powered pump
Concentration of ions and sucrose by the plant vacuole
Movement of water
Osmosis: movement of water across semipermeable
membrane
Osmotic pressure: hydrostatic pressure uses to stop
the net flow of water
Osmotic pressure
=RT( C -C )
B
A
Expression of aquaporin by frog oocytes increases
their permeability
Aquaporin 1 erythrocyte
Aquaporin2 kidney cells
Water channel pprotein( aquaporin)
tetrameric
6 -helices for
each subunit
2-nm-long water
selective gate
 0.28nm gate width
Highly conserved
arginine and histidine
in the gate
 H2O for HO bonding
with cystein
Transepithelial transport
Import of molecules on the lumen side of
intestinal epithelial cells and their export on
the blood facing sides
Transcellular transport of glucose from the intestinal lumen into the
blood




Cholera toxin
activated by Cl-
Acidification of the stomach lumen by parietal cells in the gastric
lining



Typical morphology of two types of mammalian neurons
100m/sec
Neurotransmitters Receptors
1. Ligand gated ion channels
2. G-protein coupled receptors
Synaptic vesicle:
Storage of neurotransmitter.
Low pH of vesicle lumen powers entry of
neuritransmitter into lumen by H+/protein antipoter
Structures of small molecules function as
neurotransmitters
Exocytosis of synaptic vesicle
1. Action potential
2. Influx of Ca+2 triggers release of
neurotransmitter
Cycling of nuerotransmitters and of synaptic vesicles in axon
terminals
H+/protein antiport
Signaling at synapse id terminated by degradation or
reuptake of neurotransmitter
1. degradation
i.e. acetyocholine
hydrolyzed by acetyocholineaterase
2. reuptake
i.e.transport into axon terminals by Na+/linked
symport transporters for GABA, norepinephrine,
dopamine, and serotonin
Synaptic vesicles in the axon terminal near the region
where neurotransmitter release
Sequential activation of gated ion channels at a neurotransmuscular
junction
Incoming signals must reach the threshold potential to trigger an
action potential in post synaptic cells