Transcript Membrane Transport

Membrane Transport
3 Types of transport
Passive Transport
Simple diffusion
– Small non-polar molecules
• No ions
– Examples:
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Fatty acids
Steroids
CO2
O2
H2O (?)—osmosis
– Only move DOWN concentration gradient
Osmosis
Osmosis, con't.
Mediated Transport
• Passive mediated transport
– Facilitated diffusion
• Carrier proteins
• Channel proteins
– DOWN a conc. gradient
• Active transport
– Primary active transport—uses ATP
– Secondary active transport—uses a
different energy source
– Pumps things UP a conc. gradient
How to tell mediated transport vs.
simple diffusion
• Saturation kinetics
• Competition kinetics
• Specificity
Some more terms
GluT1 (RBCs)—a carrier protein
Channel proteins
• Ion channels
– Ions
– Selective
– Generally gated
• Porins
– Larger
– Less specific
• Aquaporins
– water
Porins
The Pore-Forming Toxins
• Lethal molecules produced by many
organisms
• Insert themselves into the host cell
plasma membrane
• Kill by
– collapsing ion gradients
– facilitating entry by toxic agents
– introducing a harmful catalytic activity
Colicins
• Produced by E. coli
• Inhibit growth of other bacteria (even
other strains of E. coli)
• Single colicin molecule can kill a host!
Other Pore-Forming Toxins
• Hemolysin from
– Staphylococcus aureus
– a symmetrical pore
• Aerolysin
– Aeromona hydrophila
• Anthrax toxin protective antigen
– Bacillus anthracis
Hemolysin
Ionophore Antibiotics
Valinomycin
• Cyclic peptide
• Valinomycinpotassium
complex
diffuses freely
and rapid
across
membranes
Gramicidin—pore forming
Active Transport
Direct and Indirect AT
The Na/K ATPase
Consequences of Na/K ATPase
• Two ion gradients
– Used as energy source
– Electrical signaling
• Charge difference across membrane
– Membrane potential difference
– Negative on inside
• -60 to –90 mV in animal cells
• ~ -150 mV in bacteria
• -200 to –300 mV in plants
– Not just due to these ions
• Phosphatidylserine on inside of PM
• Other ions
Indirect (secondary) AT
• Na/glucose symporter
– Intestinal absorptive cells
Energetics of Transport
Ain
Aout
GA = RT ln ([A]in/[A]out)
if [A]out>[A]in, then G<0 for inward movement
Thermodynamics of Transport,
charged
GA = RT ln ([A]in/[A]out) + ZAF
Z= charge on A
F = Faraday's constant, the charge in a mole of electrons
 = membrane potential, difference in charge
between in and out, generally negative