Membrane Structure and Transport
January, 21, 2013
The membrane is a phospholipid bilayer
o It has a hydrophilic head and hydrophobic tail
Phospholipid bilayer can be formed spontaneous, no energy required
The membrane are more than 50% protein
Saturated vs. unsaturated
o Unsaturated have lots of kinks in them, lots of movement
Physiological importance of unsaturation
o Saturated ->(desaturase) -> Unsaturated
o When you regulate desaturase, you can regulate unsaturation
o Maintaining proper fluidity is very important, can’t be too lucid and can’t be too rigid
Membrane Permeability
o The tail of the fatty acid is very hydrophobic
o CO2, N2, O2 can move through the membrane, because they are very small
o As size and charge increases, will be denied
o So how to get other stuff across?
We use proteins to transport others across the membrane, the membrane
protein
Membrane proteins
o The protein interact with the membrane
o They can form some kind of channels
o For membrane protein, all one need is the primary sequence
Some sequences are rich in polar amino acids sequences and some sequences
are non-polar amino acid, it takes about 17 to 20 amino acids made of
predominantly of non-polar amino acid, then you’ll know it’s a membrane
protein
Alpha helical structures
o How can proteins interact with hydrophobic core?
Membrane transport
o Diffusion: if the molecules are small and non-charged
Driven by energy change (change in free energy), concentration gradient
o Facilitated: they leak from a high concentration to low concentration, there is a channel
just for one specific molecule/ion, without it they can’t move
o Active transport: (ABC transporter): moving molecules from low concentration to high
concentration, endergonic process
Trans-membrane domain is going to be different, not the same (need 1000s of
them)
They have a ATP binding site, ATP is the energy given to the transport Modeling a Neuron
o Ions diffuse along electrochemical (charged) gradients
o In a neuron, there is a selective permeable membrane only permeable to potassium (K)
One side will have more K, so it will diffuse across from high to low
concentration, but it the other side has a more
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