MCB 2000 Chapter 12

selective barrier

lets some things through blocks other

flexible structure

can bend without breaking

self-assembly

forms spontaneously in water

asymmetric

inside does not equal outside

dynamic

components can move around

fluid mosaic model

many different types of molecules all come together to make the whole

hydrophilic head

loves water

hydrophobic tail

fears water

Why do membranes form spontaneously?

Water molecules are highly ordered around hydrophobic substances. Entropy increases when hydrophobic tails cluster together, no covalent bonds needed

What can cross easily?

small, nonpolar molecules (O2, CO2)

small, uncharged polar molecules (H2O)

What is blocked?

Ions (Na+, K+, Cl-) - high energy cost to remove water shell

large polar molecules (glucose, amino acids)

charged molecules

too fluid = problems

membrane loses integrity, proteins can’t function properly

too rigid = problems

membrane can crack, transport processes shut down

just right = functional

flexible enough for protein function, stable enough ofr barrier function

saturated fats

more rigid

straight chains pack tightly

higher melthing temperature

unsaturated fats

kinks from double bonds prevent tight packing, lower melting point

chain length

longer = more rigid

shorter = more fluid

cholesterol

acts as a fluidity buffer in animal cells

cholesterol at high temperatures

restrains phospholipid movement, decreasing fluidity

cholesterol at low temperatures

prevents membranes from becoming too rigid (crystalline), increasing fluidity

cholesterol in saturated membranes

tends to disrupt packing and introduce spacing, increasing fluidity

cholesterol in unsaturated membranes

tends to fill gaps created by kinks in chains, decreasing fluidity

bacteria

hopanoids

plants

sterols

intergral proteins

embedded in or spanning the membrane

have hydrophobic regions that interact with lipid tails

ex: ion channels, transporters

peripheral proteins

associated with membrane surface

easily removed from membrane

usually interact with polar head groups

passive transport

no energy needed; move down gradient

simple diffusion, facilitated diffusion

active transport

energy required; move against gradient

primary active transport; secondary active transport

primary active transport; Na+/K+ pump

pumps 3 Na+ out and 2 K+ in per ATP

maintains membrane potential

drives secondary transport processes

uses ~30% of cell’s total energy 

essential for nerve function

target of important drugs

symporter

same direction

 ex: Na+ glucose cotransporter in intestines

Na+ gradient powers glucose uptake

anitporter

ex: Na+/Ca2+ exchanger

Na+ gradient drives Ca2+ removal

highly selective

specific ion types only

extremely fast

millions of ions per second

regulated

can open and close (gated)

K+ Channel

selectively filter perfectly fits K+ ions

charge repulsion creates rapid transport

demonstrates structure-function relationship

What is tetrodotoxin (TTX)?

potent neurotoxin produced by pufferfish

also found in blue-ringed octopus, some frogs, and bacteria

kethal dose: at little as 1-2 mg for humans

How does TTX work?

TTX selectively blocks voltage-gated Na+ channels

binds to channel opening and physically plugs the pore

prevents Na+ influx - blocks nerve depolarization and signal propagation

results in paralysis and respiratory failure