Membrane Transport

3 Types of transport

Passive Transport

Simple diffusion– Small non-polar molecules

• No ions

– Examples:• 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• Valinomycin-

potassium 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 AF = Faraday's constant, the charge in a mole of electrons = membrane potential, difference in charge between in and out, generally negative