13 lipids and biological membranes
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Lipids and
Membranes
A Angeles
Chem 40
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Lipids are the fourth major group of
biomolecules found in cells.
Lipids exhibit greater structural variety than
proteins, nucleic acids and carbohydrates. Lipids are largely hydrophobic.
Sparingly soluble in water
Soluble in organic solvents (e.g., CHCl3 and CH3OH)
Three major biological functions:
1. They serve as energy stores.
2. These molecules form lipid bilayers essential in cellularmembranes.
3. They participate in many intra- and intercellular
signalling pathways.
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Many lipids are ampiphatic. Although lipids do not form polymers, they
associate with each other non-covalently resulting
in supramolecular structures that correspond totheir function.
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Lipids as energy stores. Fats and oils used as stored forms of energy are
fatty acid derivatives. (At physiological pH, what's thepredominant form?)
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Lipids as energy stores.
Fatty acids contain carbon atoms in their most reduced
form (as opposed to carbohydrates, why?).
The hydrophobic quality of FAs result in efficient
compartmentalization in aqueous systems.
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Fatty Acid Naming System
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Packing of Saturated and Unsaturated FAs
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Packing of Saturated and Unsaturated FAs
Saturated FAs contain single bonds that are free to
rotate. Most stable conformation: extended
Unsaturated FAs contain double bonds that are cis.
Introduces a rigid 30o bend in the hydrocarbon chain
PUFAs contain more than one isolated double bonds
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The benefits of polyunsaturated fatty acids.
-linolenic acid (ALA; 18:3(9,12,15) is an essential
FA it must be sourced from the diet
ALA is the precursor of EPA and DHA
Imbalance in -6 and -3 in the diet is implicated
to an increased risk of cardiovasular diseases.
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The heart of oils and fats.
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Esterification of glycerol and FAs result in
triacylglycerols (or triglycerides, fats, neutral fats)
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Esterification of glycerol and FAs result in
triacylglycerols (or triglycerides, fats, neutral fats)
The ic suffix of FAs becomes oyl in FA ester
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Esterification of glycerol and FAs result in
triacylglycerols (or triglycerides, fats, neutral fats)
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Triacylglycerols proved stored energy and
insulation.
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Triacylglycerols proved stored energy and
insulation.
Fat tissues in mammals are located under the skin,
the abdominal cavity and mammary glands. Moderately obese people with 15-20 kg stored TAG
could live off of fat stores for 2- 3 months.
In contrast, the human body can store less than adays supply of glycogen.
Ratio of energy derivation per weight basis
(TAG:glycogen) 6:1
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Triacylglycerols proved stored energy and
insulation.
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Saturated and Unsaturated Fats in Food Lipids
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Saturated and Unsaturated Fats in Food Lipids
Foods rich in lipids turn
rancid when exposed forlong periods to air (whichcontains O2) due to
ox a ve c eavage adouble bonds. Commercial vegetable oils are
subjected to partialhydrogenation.
Some cis double bonds aretransformed into trans doublebonds.
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Waxes serve as energy stores and water
repellents.
Biological waxes are esters of long chain (C14-
C36) FAs with long chain (C16-C30) alcohols. Planktons use waxes as primary metabolic fuel.
skin, hair, feathers; plants use them to prevent
excessive water loss and defense against
parasites.
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Waxes serve as energy stores and water
repellents.
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Lipids as membrane structural molecules.
Biological membranes are made from lipid bilayers.
Control of entry of ions and polar molecules Membrane lipids are ampiphatic.
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Lipids as membrane structural molecules.
There are five major types of
membrane lipids:1. Glycerophospholipids
2. Galactoli ids and Sulfoli ids
3. Tetraether lipids4. Sphingolipids
5. Sterols
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Glyerol 3-phosphate is the backbone of
glycerophospholipids.
Two fatty acids are esterified to C1 and C2 of
glycerol; a polar group is attached through a
phosphodiester linkage at C3.
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Glycerophospholipids are derivatives of
phosphatidic acid.
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Some glycerophospholipids have ether-linked
FAs
Found in large
quantities in hearttissue
Important in
inflammation andallergic reactions
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Chloroplasts contain galactolipids and
sulfolipids.
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Chloroplasts contain galactolipids and
sulfolipids.
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Tetraether lipids are found in archaeabacteria.
Long-chain branched hydrocarbons (C32) are linked
to two glycerol molecules
Ether bonds are more stable to at low pH and high
temperatures
Can span the entire membrane
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Sphingolipids are derivatives of sphingosine.
Sphingosine is structurally similar to glycerol.
Sphingolipids have a fatty acid linked by an amide
linkage to the amino group of sphingosine.
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Sphingomyelins and phosphatidylcholines are
structurally similar.
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The two other subclasses of sphingolipids
With the exception of
sphingomyelins, sphingolipidsDO NOT contain phosphate
groups.
Glycosphingolipids cerebrosidesand globosides
Gangliosides contains sialic acidresidues
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Glycosphingolipids as blood group
determinants.
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Membrane lipids are constantly synthesized
and degraded.
Phospholipid and sphingolipid degradation is
facilitated by enzymes in the lysosome.
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Sterols have four fused carbon rings.
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Sterols have four fused carbon rings.
Cholesterol is the most abundant steroid in animals. It is a major component of the plasma membrane (30-40
mol %) Modulates membrane fluidity
synthesize other sterols (e.g. stigmasterol,ergosterol respectively).
Prokaryotes cannot synthesize sterols but can
integrate exogenous sterols into their membranes.
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Lipids as Signals, Cofactors and Pigments
Storage and structural lipids play passive roles
within the cell. Some lipids have active roles in metabolic
processes.
1. Lipid hormones: intra- and extracellular messengers2. Enzyme cofactors
3. Emulsifying agents
Another function of lipids highly conjugated double bonds act as pigments
make up essential oils in plants that give distinctive
aromas
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Eicosanoids as paracrine hormones
Eicosanoids serve as potent signals in relation toreproductive function; inflammation, fever, pain;
formation of blood clots; regulation of blood pressure;gastric juice secretion.
Eicosanoids are derived from arachidonic acid (20:4-
6). Prostaglandins body temperature elevation, inflammationand pain, blood flow to specific organs, the circadianrhythm
Thromboxanes produced by platelets; aids in bloodclotting by reducing blood flow to the indicated site
Leukotrienes first found in leukocytes; implicated inasthmatic attacks and allergic reactions
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Eicosanoids as paracrine hormones
Steroid hormones carry messages between
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Steroid hormones carry messages between
tissues.
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Steroids derived from cholesterol in animalsinclude the following families:
Androgens Mediate development of sexualcharacteristics and function (e.g., testosterone,estradiol)
Steroid hormones carry messages between
tissues.
Progestins Regulate menstrual cycle (e.g.,progesterone) Glucocorticoids Regulate carbohydrate, protein
and lipid metabolism (e.g., cortisol)
Mineralocorticoids Regulate salt balance intissues (e.g., aldosterone)
Steroid hormones and transcription
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p
M di l li i f id
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Medical applications of steroid-receptor
interactions
Bil id D d b ll
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Bile acids Detergents secreted by gall
bladder that help solubilize lipids in diet
T li id
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Terpene lipids
This type of lipid commonly occurs in oils that give plants
their fragrance.
Monomer of terpenes: isoprene (C5H8)
Terpenes usually occur in multiples of 5 carbon atoms Monoterpene: C10
Sesquiterpene: C15
Diterpene: C20 Triterpene: C30 Precursor of cholesterol and other steroids
Terpene lipids
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Terpene lipids
Many monoterpenes are readily recognized by their characteristic
flavors or odors (limonene in lemons; citronellal in roses and
perfumes; menthol used in cough drops.
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Terpene lipids
Diterpenes include retinal
(the visual pigment in
rhodo sin and h tol
(found in chlorophyll.Gibberellic acid is a plant
hormone.
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Terpene lipids
The triterpene
lanosterol is a
constituent of woolfat and is also a
precursor to
c o es ero an e
other steroids.Lycopene is a
carotenoid found in
ripe fruit,especially
tomatoes.
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Terpene lipids as pigments
T li id th t l t i
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Terpene lipids that serve as electron carriers
Vitamins D A E and K are lipids that play
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Vitamins D, A, E and K are lipids that play
essential roles in animal physiology.
Vitamin D3 (cholecalciferol) regulation of Ca2+
uptake
Vitamins D A E and K are lipids that play
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Vitamins D, A, E and K are lipids that play
essential roles in animal physiology.
Vitamin D3 deficiency causes rickets (stuntedgrowth and deformed bones due to insufficient
bone mineralization)
Vitamins D, A, E and K are lipids that play
i i i i
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Vitamin A (retinol) visual pigment of vertebrate
eye -carotene is the precursor of retinol
essential roles in animal physiology.
Vitamins D, A, E and K are lipids that play
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Vitamins D, A, E and K are lipids that play
essential roles in animal physiology.
Vitamin E (tocopherol) an antioxidant; free-
radical scavengers, thus protects unsaturated fattyacids
Vitamins D, A, E and K are lipids that play
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Vitamins D, A, E and K are lipids that play
essential roles in animal physiology.
Vitamin K (Koagulation) cofactor for
posttranslational modification of blood clottingproteins
Th Bi l i l M b
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The Biological Membrane
The Biological Membrane
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The Biological Membrane
Th Bi l i l M b
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The Biological Membrane
defines the external boundaries of cells andregulates the passage of molecules in and out of the
cell for eukaryotes, it compartmentalizes the different
components essential in energy conservation and cell to cell
signalling
Membranes are flexible, self-sealing and selectivelypermeable to solutes.
Th Bi l i l M b
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The Biological Membrane
Cell membrane flexibility permits:
shape changes cell growth
Because membranes are self-sealing, membranescan undergo fusion and fission without leaking
cellular components.
Th Bi l i l M b
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The Biological Membrane
http://www.csc.mrc.ac.uk/microscopy/
The Biological Membrane
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The Biological Membrane
The Biological Membrane
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The Biological Membrane
The Biological Membrane
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The Biological Membrane
The Biological Membrane
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The Biological Membrane
The Biological Membrane
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Lipid composition of organelle membranes vary
probably due to specialized function
The Biological Membrane
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All biological membranes share fundamental
properties Selective permeability
5-8 nm thick
The Biological Membrane
Trilaminar phospholipid bilayer Asymmetric functionality protein and lipid composition
FLUIDITY!
The Biological Membrane
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The Biological Membrane
The Biological Membrane
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The Biological Membrane
The Biological Membrane
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Plasma membrane lipids are distributed
asymmetrically.
The Biological Membrane
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Proteins have various associations with the
membrane Integral/intrinsic membrane proteins highly
associated with the membrane
The Biological Membrane
Peripheral membrane proteins associated with themembrane through electrostatic interactions with
hydrophilic moieties
Amphitropic proteins can be found in the cytosol and
in association with membranes
The Biological Membrane
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The Biological Membrane
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The fluidity of the membrane is temperature-
dependent.
Transition temperature
The Biological Membrane
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Transition temperature of the bilayer increases with
chain length and degree of saturation of FAs
Bacteria and cold-blooded animals (e.g., fish) can
modif the FA com osition of their membrane li ids
g
to suit ambient temperature Cholesterol can decrease membrane fluidity; at the
same time broadens transition temperature
A membrane plasticizer
The Biological Membrane
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The Biological Membrane
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The Biological Membrane
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The Biological Membrane
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Lipids and proteins diffuse laterally in the bilayer.
Membrane Transport
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Nonmediated transport: diffusion of substance acrossan electrochemical gradient (e.g. O2, steroidal
hormones) Mediated transport: transport proteins enable ions
+ + + - . ., , , , . .,
amino acids, sugars, nucleotides) to traverse the cellmembrane Passive-mediated transport or facilitated diffusion
Transport through an electrochemical gradient
Active transport Transport against a gradient
Must be coupled to a sufficiently exergonic process (i.e., a reactionwith G
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Membrane Transport
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Membrane Transport
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Membrane Transport
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Membrane Transport
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Membrane Transport
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