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 (-G).

Transport of polar or charged compounds

Involves three different types of integral membrane proteins

1. Channels and Pores2. Passive transporters3. 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, () and 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 inositol-

triphosphate (IP3) and diacylglycerol (DAG) both act as 2nd messengers

• IP3 stimulates Ca2+ releases from ER

• DAG stimulates Protein kinase C