Chapter 9 (part 3)
Membranes
Membrane transport
• Membranes are selectively permeable
barriers
• Hydrophobic uncharged small molecules
can freely diffuse across membranes.
• Membranes are impermeable to polar and
charged molecules.
• Polar and charged molecules require
transport proteins to cross membranes
(translocators, permeases, carriers)
Transport of non-polar
molecules
• Non-polar gases, lipids, drugs etc…
• Enter and leave cells through diffusion.
• Move from side with high concentration
to side of lower concentration.
• Diffusion depends on concentration
gradient.
• Diffusion down concentration gradient is
spontaneous process (-DG).
Transport of polar or charged
compounds
Involves three different types of integral
membrane proteins
1. Channels and Pores
2. Passive transporters
3. Active transporters
Transporters differ in kinetic and energy
requirements
Channels and Pores
• Have central passage that
allows molecules cross the
membrane.
• Can cross in either direction
by diffusing down
concentration gradient.
• Solutes of appropriate size
and charge can use same
pore.
• Rate of diffusion is not
saturable.
• No energy input required
Porins
• Present in bacteria plasma membrane and
outer membrane of mitochondria
• Weakly selective, act as sieves
• Permanently open
• 30-50 kD in size
• exclusion limits 600-6000
• Most arrange in membrane as trimers
Passive Transport (Facilitated Diffusion)
• Solutes only move in the
thermodynamically favored direction
• But proteins may "facilitate"
transport, increasing the rates of
transport
• Two important distinguishing features:
– solute flows only in the favored
direction
– transport displays saturation kinetics
Three types of transporters
• Uniporter – carries single molecule across
membrane
• Symport – cotransports two different
molecules in same direction across
membrane
• Antiport – cotransports two different
molecules in opposite directions across
membrane.
Saturation Kinetics of transport
•Rate of diffusion is
saturable.
•Ktr = [S] when rate of
transport is ½ maximun rate.
•Similar to M-M kinetics
•The lower the Ktr the higher
the affinity for substrate.
• Transporters undergo
conformational change
upon substrate binding
• Allows substrate to
transverse membrane
• Once substrate is
released, transported
returns to origninal
conformation.
Active Transport Systems
• Some transport occur such that
solutes flow against
thermodynamic potential
• Energy input drives transport
• Energy source and transport
machinery are "coupled"
• Like passive transport systems
active transporters are saturable
Primary active transport
• Powered by direct source of energy(ATP,
Light, concentration gradient)
Secondary active transport
• Powered by ion concentration gradient.
• Transport of solute “A” is couple with
the downhill transport of solute “B”.
• Solute “B” is concnetrated by primary
active transport.
Na+-K+ ATPase
• Maintains intracellular
Na low and K high
• Crucial for all organs,
but especially for neural
tissue and the brain
• ATP hydrolysis drives
Na out and K in
+
+
Na -K
ATPase
• Na+ & K+ concentration gradients are maintained
by Na+-K+ ATPase
• ATP driven antiportsystem.
• imports two K+ and exports three Na+ for every
ATP hydrolyzed
• Each Na+-K+ ATPase can hydrolyze 100 ATPs
per minute (~1/3 of total energy consumption of
cell)
• Na+ & K+ concentration gradients used for 2o
active transport of glucose in the intestines
1o active transport of Na+
2o active transport of
glucose
Transduction of
extracellular signals
• Cell Membranes have specific receptors
that allow cell to respond to external
chemical stimuli.
• Hormone – molecules that are active at a
distance. Produced in one cell, active in
another.
• Neurotransmitters – substances involved
in the transmission of nerve impulse at
synapses.
• Growth factors – proteins that regulate
cell proliferation and differentiation.
• External stimuli(first messenger) – (hormone,
etc…)
• Membrane receptor – binds external stimuli
• Transducer – membrane protein that passes
signal to effector enzyme
• Effector enzyme – generates an intracellular
second messenger
• Second messenger – small diffusible molecule
that carrier signal to ultimate destination
G-Proteins
• Signal transducers.
• Three subunits, (a,b, g) a and g anchored to
membrane via fatty acid and prenyl group
• Catalyze hydrolysis of GTP to GDP.
• GDP bound form is inactive/GTP bound form active
• When hormone bound receptor complex interacts
with G-protein, GDP leaves and GTP binds.
• Once GTP -> GDP G-protein inactive
• GTP hydrolysis occurs slowly (kcat= 3min-1) good
timing mechanism
Epinephrine signaling pathway
• Epinephrine regulation of glycogen
degradation
• Fight or Flight response
• Ephinephrine primary messenger
• G-protein mediated response.
• G-protein activates Adenyl-cyclase to
produce cAMP
• cAMP is the second messenger
• Activates protein kinase
• Activates glycogen phosphorylase
Effect of Caffeine
• Caffeine inhibits cAMP
phosphodiesterase , prevents
breakdown of cAMP.
• Prolongs and intensifies Epinephrine
effect.
Phosphatidylinositol (PI) Signaling
Pathway
• G-protein mediated
• G-protein activates phospholipase C (PLC)
• PLC cleaves PI to form inositoltriphosphate (IP3) and diacylglycerol
(DAG) both act as 2nd messengers
• IP3 stimulates Ca2+ releases from ER
• DAG stimulates Protein kinase C