Membrane Structures.docx

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14 Apr 2012
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Readings: pg. 617-624.
- Membrane Structure
- Plasma membrane: encloses cell, defines boundaries, maintains differences b/w cytosol and extracellular
environment
o Contains protein sensors (receptors) allows cell to change behaviour according to environment/ transfer
info, also accepts signals from surrounding cells.
- Inside eukaryotic cells, membranes of endoplasmic reticulum, Golgi apparatus, mitochondria, and other
membrane enclosed organelles maintain the characteristic differences b/w contents of each organelle and
cytosol.
- Ion gradients drive transmembrane movement of solutes, or transmit electrical signals in nerve and muscle cells
- All membranes have thin lipid layer and protein molecules which are held together by noncovalent interactions
- The lipid bilayer (5nm thick) Provides basic structure for all cell membranes. All lipids are amphiphilic
- Major lipids in cell membrane:
o Phospholipids: have polar head, 2 hydrophobic HC tails (fatty acids). One tail has one or more cis-
double bonds (unsaturated) other has single bond (saturated)
Cis double bonds create a kink in tail
o phsophoglycerides: have 3 carbon glycerol backbone. 2 long chain fatty acids are linked together
through ester bonds to adjacent carbon atoms of glycerol, and third C atom attached to phosphate
group. By combining diff fatty acids and head grps = phosphoglycerides
phosphatidylethanolamine, phosphatidylserine, phosphatidylcholine
o Sphingomyelin: built from sphingosine (long acyl chain with an amino grp (NH₂) and 2 hydroxyl grps
(OH) at one end of molecule) A fatty acid tail is attached to NH₂ and phosphocholine attached to
terminal hydroxyl grp, leaving one hydroxyl grp free which contributes to polar properties of adjacent
head grp as it can form hydrogen bonds with head grp of neighbouring lipid, with a water molecule, or
with a membrane protein
Spphingomyelin, sphingosine
o Cholesterol (polar head grp, rigid steroid ring structure, nonplar HC tail) and glycolipids also in lipid
bilayer
- Formation of hydrophobic or hydrophilic grps increases free energy
o Lipid molecules have hydrophobic in interior and hydrophilic exterior… can do this in 2 ways:
Can form spherical micelles (cone shaped) with tails inward
Form double layered sheets (bilayers) with hydrophobic tails sandwiched between them
hydrophilic head grps (cylinder)
- Forces that drive phospholipids to form bilayers also provide a self healing property. Can rearrange to fix tear in
bilayer. Closed structure is stable b/c it avoids exposure of hydrophobic HC tails to water which would be
energetically unfavourable
- Bilayers made in form of spherical vesicles called liposome’s
- Planar bilayers called black membranes formed across a hole in a partition b/w two aqueous compartments.
black membranes are used to measure the permeability properties of synthetic membranes
- Techniques used to measure motion of individual lipid molecule:
o Use fluorescent dye or small gold particle attached to polar head and follow diffusion of individual
molecules in membrane
o Modify head grp to carry spin label such as nitroxyl grp (contains unpaired electron whose spin creates
signal that can be detected by electron spin resonance ESR)
Shows that phospholipids molecules in synthetic bilayers very rarely migrate from monolayer
(leaflet) on one side to that on the other. Called flip flop process
Lipid molecules exchange places w/ neighbours within monolayer (rapid lateral diffusion): an
avg lipid molecule diffuses length of large bacterial cell in 1 sec.
Lipid molecules also rotate rapidly along axis and have flexible HC chains
- Fluidity of lipid bilayer depends on its composition and temp:
o Synthetic bilayer made up from single type of phospholipids changes form liquid state to a 2-D rigid
crystalline state at freezing point. Change = phase transition and temp lowers if HC chain is short or has
double bonds.
o Membranes remain fluid at lower temp if has short HC chain b/c that reduces the tendency of HC tail to
interact with one another and cis double bonds produce kinks in HC chain which makes it difficult to
pack together
o Cholesterol modulates properties of lipid bilayers b/c it enhances permeability barrier properties of lipid
Inserts into bilayer with OH grp close to polar head grps so its rigid, platelike steroid rings
interact. However, at high [ ] it prevents HC chains form coming together and crystallizing
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