01:694:301 Lecture Notes - Lecture 12: Lipid Bilayer, Oleic Acid, Hydrophile

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Chapter 12 – ‘Lipids and Cell Membranes
Cell Membranes and Their Properties
This chapter represents the structures of several membrane lipids.
o The boundaries of cells are defined by ‘
biological membranes’
! These barriers prevent molecules generated inside the cell from leaking out and
unwanted molecules from diffusing in
! All have protein components that act as transport systems (channels and pumps)
allowing cells to accept specific molecules and reject others
This property of membranes is known as
selective permeability
! Eukaryotes unlike prokaryotes also have membrane bounded organelles
You should be generally familiar with the list of membrane properties on (pg. 346)
o 1)
Sheet like structures
; only 2 molecules thick, that form closed boundaries b/w
different compartments (Thickness 60Å to 100Å)
o 2) Consist
mainly of lipids and proteins
; mass ratio (4:1 or 1:4); also consist carbs linked
to lipids and proteins
o 3) Contain both
hydrophobic and hydrophilic moieties
; this forms a closed bimolecular
sheet in aq media; lipid bilayer allows flow polar molecules
o 4) Specific
proteins mediate distinctive functions
of membrane; protein can act as pumps,
channels, enzymes, receptors, etc.; embedded in bilipid layer
o 5)
Non-covalent assemblies
; protein and lipids are held by non-covalent interactions;
cooperative structures
o 6) They are
asymmetric
; two faces of membrane always differ
o 7) They are
fluid structures
; lipid and protein molecules diffuse quickly in plane of
membrane; but do not easily rotate across membranes;
two dimensional solution of
oriented protein and lipids
o 8) Most are
electrically polarized
; inside typically negative (-60 mV)
! Dependent on temperature (speed); amount of cholesterol, # unsaturated fatty acids
Fatty Acids & Lipids
Hydrophobic properties of membranes is due to their fatty acids
o Fatty acids are long hydrocarbons chain of various lengths and degrees of unsaturation
terminated –COOH groups
! Some examples:
C16 unsaturated fatty acid "
hexadecanoate
(palmitate)
C18 unsaturated fatty acid "
octadecanoate
(stearate)
C18 saturated fatty acid "
cis
Δ9
octadecanoate
(oleate)
! 18:0 denotes fatty acid w/ no = bond; 18:2 denotes 2 = bonds;
! Numbered starting at –COOH terminus; carbon 2 & 3 referred to as α and β
! Position = bond is represented by Δ
Ex:
cis
Δ9 means cis = b/w C9 and C10
! Ionized at physiological pH; referred in carboxylate form
o Typically have even # C atoms (b/w 14–24); 16C and 18C chains are most common
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o Chain length and degree unsaturation affect properties of fatty acids (i.e. melting temp)
! Saturated lower –mp; longer chains – higher mp
o
Short chain length and unsaturation enhance the fluidity of fatty acids and of their
derivatives.
! Two saturated fatty acids can easily stack on top of each other
! One saturated and unsaturated fatty acid are not able to stack uniformly " ‘form kink’
making it difficult for proteins to diffuse
Common Membranes Lipids
Lipids
by definition are water insoluble biomolecules that are highly soluble in organic solvents
o There major forms of membrane lipids are:
phospholipids
,
glycolipids
,
and
cholesterol
Phospholipids constructed from 1 or more fatty acids, platform that fatty acid is attached to,
a phosphate, and –OH attached to phosphate
o Fatty acid is hydrophobic components; remaining are hydrophilic
o Platform can be either
glycerol
(3 C alcohol) or
sphingosine
(more complex –OH)
! Glycerol phospholipids "
phosphoglycerides
Phosphatides (diacylglycerol-3 phosphate) are the simplest phosphoglycerides
Major phosphoglycerides are derived from formation of ester bond b/w
phosphate group of phosphatides and –OH group of different alcohols
You should be able to draw phosphatidates with the head groups shown on pg. 349 (Fig.
12.5)
Glycolipids – sugar containing lipids; in animal cells derived from sphingosine; differ from
sphingomyelin by identity of unit linked to primary –OH;
o One or more sugars are attached to primary –OH; simplest glycolipids " cerebroside
o More complex glycolipids "
gangliosides
contain branched chain 7 or less sugar residues
! Asymmetrical orientation; gangliosides are usually on the outside of the membrane
KNOW the structure of sphingosine, sphinomyelin, and cerebroside (pg. 349-350)
*Phosphatidylglycerol
is a major lipid
constituent in bacteria
and plants
*Cardiolipin is found in
bacteria
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