BIO130H1 Study Guide - Final Guide: Drug Resistance, Symporter, Osmotic Concentration

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17 Apr 2012
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BIO130 Exam Notes
Chapter 10
Pgs 617-624
-The plasma membrane encloses cells and maintains the difference between cytosol and extracellular
environment.
-membrane-enclosed organelles maintain the characteristic differences between contents of organelles
in the cytosol
-Cells are fluid
-The Lipid Bilayer provides basic structure for cell membranes and can be easily found
-the lipid molecules spontaneously form a bilayer, they are fluid individual lipid molecules can diffuse
rapidly within their own monolayer.
-All the lipid molecules in a cell membrane are amphiphillic meaning that they have hydrophilic and
hydrophobic ends
-Mostly phospholipids in membrane lipids, 1 polar head and 2 hydrophobic tails
-Main phospholipids in animal cells are Phosphoglycerides composed of 3 carbon glycerol backbone,
linked via ester bonds
-Another phospholipid sphyngomyelin composed of sphyngose(long acyl chain with one amino group
and two hydroxyl groups, one free hydroxyl group forms the polar properties for the head)
-Lipid bilayers in many cells contain cholesterol and glycolipids. Chlosterol is found in large amount in
eukaryotic plasma membranes and is sterol with hydroxyl group oriented near the polar head
-The lipid bilayer is a two dimensional fluid
-liposomes are bilayers that are made in the form of spherical vesicles.
-black membranes or planar bilayers form across a hole in partition between two aquous
compartments.
-to study motion of an individual lipid molecule, you can stain it, or you can modify it to carry a spin
label, whose spin creates a paramagnetic signal which can picked up by ESR (electron spin resonance)
Process called flip flop
-The fluidity of a lipid bilayer depends on its composition and temperature
-for example bacteria and other organisms whose temperature fluctuates with their environment, adjust
the fatty acid composition of their membrane lipids meaning when temperature drops they syntheisze
more cis-double bonds between their fatty acids potentially avoiding decrease in bilayer fluidity.
-Chloseterol modulates the properties of lipid bilayers; when mixed with phospholipids it enhances
permeability properties. It tightens the packing of lipid bilayers but does not make membranes any less
fluid
-Despite their fluidity bilayers can form domains of different compositions.
Pgs629-640
-Membrane Proteins perform membrane’s specific tasks, and thus gives each cell membrane its
characteristic functional properties.
-Transmembrane Proteins are amphiphillic
-while still in the endoplasmic reticulum the transmembranesegment of the protein is cleaved off and a
GPI(glycosolphos...) anchor is added.
-Transport vesicles eventually deliver the protein to the plasma membrane, Those attached by GPI
anchor can be cut off releasing them from the Membrane
-Only transmembrane proteins can function on both sides of the bilayer or transport molecules across it.
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-Cell-Surface receptors which are transmembrane proteins bind signal molecules in the extra-cellular
space and generate different intracellular signals on the opposite side of the plasma membrane
-To allow for the transport of small hydrophilic molecules across the membrane the multipass
membrane is ideally suitable. This is because they create a pathway for molecules to cross the
membrane solely based on their molecular architecture.
-Proteins that function on only one side of the bilayer are either linked with lipid monolayer or protein
domain on that side
-For example some intracellular signalling proteins which convert extracellular signals into intracellular
ones are bound to the cytosolic half of the plasma membrane
-Membrane attachment through a single lipid anchor is not very strong, thus a second lipid group is
often added to anchor proteins more strongly to the membrane.
-transmembrane protein has unique orientation in the membrane
-in most transmembrane proteins the polypeptide chsin crosses the lipid bilayer in a-helical
conformation
-A transmembrane protein is composed of a cytosolic and noncytosolic domains. Theses domains are
seperated by segments of polypeptide chains which contact the hydrophobic part of the lipid bilayer
-Since the prptide bonds are polar, water is absent, all peptide bonds are forced to form hydrogen bonds
with one another. The H bonding is maximized if the polypeptide chain forms a regular helix as it crosses
the bilayer and this is how most membrane spanning segments of polypetide chains transverse bilayer
-Single -pass transmembrane Proteins the polypetide chains cross only once.
-Multi-pass transmembrane proteins the polypetide chains cross more than once
-Another way for the peptide bonds to satisfy the hydrogen bonding is for multiple transmembrane
strands to be arranged as a B-sheet rolled up in a close barrel.
-Strong urge for H-bonfding in absence of water means a polypetide chain that enters bilayer is likely to
pass entirely through it before switching direction(since chain bending requires loss of H-bond
interactions) Multipass membrane proteins however do not need to maximize H-bonding and can
instead use secondary structures.
-transmembrane a-helices of many single-pass proteins do not contribute to folding of the protein
domain on either side of the membrane
-A transmembrane a-helix even in a single pass protein does not only anchor protein to the lipid bilayer
but also form homodimers. The sequence of hydrophobic amino acids contain information which directs
protein-protein interactions.
-In multipass membrane proteins, neighbouring transmembrane helices in the folded structure of the
protein shield many of the transmembrane helices form the membrane lipids.
-Why are shielded helices not composed of hydrophobic amino acids?
----a-helices are inserted into lipid bilayer sequentially by a protein translocator. after leaving
translocator each helix is surrounded by lipids in the bilayer which requires the helix to be hydrophobic.
Only as protein folds up into final structure that contacts are made between adjacent helices and
protein-protein contacts replace some of the protein-lipid contacts
-some B Barrels form large transmembrane channels
-Multipass transmembrane proteins that have their transmembrane segments arranged as B-barrel
rather than an a-helix are comparatively rigid and tend to crystallize readily. Barrel can have from 8
strands to 22.
-B-barrel proteins are abundant in outer membrane of mitochondria, chloroplasts and many bacteria.
Some are pore-forming proteins which create water filled channels allowing for small hydrophilic
molecules to cross the lipid bilayer.
-Not all B-barrel proteins are transport proteins. Some smaller barrels filled with amino acid side chains
function as receptors or enzymes and the barrel serves as rigid anchor.
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