Chapter 9 - Lipids and Membranes
• Lipids are essential components of all living
organisms
• Lipids are water insoluble organic compounds
• They are hydrophobic (nonpolar) or
amphipathic (containing both nonpolar and
polar regions)
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Fig 9.1 Structural relationships
of major lipid classes
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Fatty Acids
• Fatty acids - R-COOH (R=hydrocarbon chain)
are components of triacylglycerols,
glycerophospholipids, sphingolipids
• Fatty acids differ from one another in:
(1) Length of the hydrocarbon tails
(2) Degree of unsaturation (double bond)
(3) Position of the double bonds in the chain
•
•
•
•
•
Nomenclature of fatty acids
Most fatty acids have 12 to 20 carbons
Most chains have an even number of carbons
IUPAC nomenclature: carboxyl carbon is C-1
Common nomenclature: a,b,g,d,e etc. from C-1
Carbon
farthest from carboxyl
is w
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Fig 9.2
Table 9.1
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Structure and nomenclature of fatty acids
• Saturated - no C-C double bonds
• Unsaturated - at least one C-C double bond
• Monounsaturated - only one C-C double bond
• Polyunsaturated - two or more C-C double bonds
Double bonds in fatty acids
• Double bonds are generally cis
• Position of double bonds indicated by Dn, where n indicates
lower numbered carbon of each pair
• Shorthand notation example: 20:4D5,8,11,14
(total # carbons : # double bonds, D double bond positions)
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Fig. 9.3 Structures of three C18 fatty acids
(a) Stearate (octadecanoate)
(b) Oleate (cis-D9-octadecenoate)
(c) Linolenate (all-cis-D9,12,15octadecatrienoate)
• The cis double bonds produce
kinks in the tails of unsaturated
fatty acids
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Fig 9.4 Structures of stearate, oleate, linolenate
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Triacylglycerols
• Fatty acids are stored as neutral
lipids, triaclyglycerols (TGs)
Fats have 2-3 times the energy of
proteins or carbohydrates
• TGs are 3 fatty acyl residues
esterified to glycerol
• TGs are hydrophobic, stored in fat
cells (adipocytes)
Fig 9.5
Structure of a
triacylglycerol
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Fig. 9.8a Structure of
glycerophospholipids
Phosphatidylethanolamine (PE)
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Fig. 9.8b Structures of
glycerophospholipids
Phosphatidylserine (PS)
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Fig. 9.8c Structures of
glycerophospholipids
Phosphatidylcholine (PC)
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Fig 9.9 Phospholipases hydrolyze phospholipids
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Fig 9.10 Structure of an
ethanolamine plasmalogen
• Plasmalogens - C-1
hydrocarbon substituent
attached by a vinyl ether
linkage (not ester linkage)
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Sphingolipids
• Sphingolipids - sphingosine is the backbone
abundant in central nervous system tissues
• Ceramides - fatty acyl group linked to C-2 of sphingosine by an
amide bond
• Sphingomyelins - phosphocholine attached to C-1 of ceramide
• Cerebrosides - glycosphingolipids with one monosaccharide
residue attached via a glycosidic linkage to
C-1 of ceramide
• Galactosylcerebrosides - a single b-D-galactose as a polar
head group
• Gangliosides - contain oligosaccharide chains with
N-acetyl-neuraminic acid (NeuNAc)
attached to aChapter
ceramide
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Fig 9.11
(a) Sphingosine
(b) Ceramides
(c) Sphingomyelin
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Fig 9.12
• Structure of a
galactocerebroside
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Fig 9.13 Ganglioside GM2
(NeuNAc in blue)
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Steroids
• Classified as isoprenoids - related to 5-carbon isoprene
(found in membranes of eukaryotes)
• Steroids contain four fused ring systems: 3-six carbon rings
(A,B,C) and a 5-carbon D ring
• Ring system is nearly planar
• Substituents point either down (a) or up (b)
Fig 9.14
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Isoprene
Chapter 9
Fig 9.15
18
Fig 9.15
Structures of
several steroids
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Fig 9.15
Structures of
several steroids
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Cholesterol
• Cholesterol modulates the fluidity of mammalian cell
membranes
• It is also a precursor of the steroid hormones and bile salts
• It is a sterol (has hydroxyl group at C-3)
• The fused ring system makes cholesterol less flexible than
most other lipids
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Cholesterol esters
• Cholesterol is converted to cholesteryl esters for cell storage
or transport in blood
• Fatty acid is esterified to C-3 OH of cholesterol
• Cholesterol esters are very water insoluble and must be
complexed with phospholipids or amphipathic proteins for
transport
Fig 9.17 Cholesteryl ester
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Waxes
• Waxes are nonpolar esters of long-chain fatty acids and
long chain monohydroxylic alcohols
• Waxes are very water insoluble and high melting
• They are widely distributed in nature as protective
waterproof coatings on leaves, fruits, animal skin, fur,
feathers and exoskeletons
Fig 9.18 Myricyl palmitate, a wax
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Eicosanoids
• Eicosanoids are oxygenated derivatives of C20
polyunsaturated fatty acids (e.g. arachidonic acid)
• Prostaglandin E2 - can cause constriction of blood vessels
• Thromboxane A2 - involved in blood clot formation
• Leukotriene D4 - mediator of smooth-muscle contraction
and bronchial constriction seen in asthmatics
• Aspirin alleviates pain, fever, and inflammation by inhibiting
cyclooxygenase (COX), an enzyme critical for the synthesis
of prostaglandins. (NSAID family of compounds)
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Fig 9.19 Arachidonic acid and eicosanoid derivatives
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Lipid vitamins
• Vitamins A,D,E,
and K are
isoprenoid
derivatives
Vitamin E
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Biological Membranes Are Composed
of Lipid Bilayers and Proteins
• Biological membranes define the external
boundaries of cells and separate cellular
compartments
• A biological membrane consists of proteins
embedded in or associated with a lipid bilayer
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Several important functions of membranes
• Some membranes contain protein pumps for ions or small
molecules
• Some membranes generate proton gradients for ATP
production
• Membrane receptors respond to extracellular signals and
communicate them to the cell interior
Lipid Bilayers
• Lipid bilayers are the structural basis for all biological
membranes
• Noncovalent interactions among lipid molecules make them
flexible and self-sealing
• Polar head groups contact aqueous medium
• Nonpolar
tails point toward the
interior
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Fig 9.21 Membrane lipid and bilayer
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Fluid Mosaic Model of Biological Membranes
• Fluid mosaic model - membrane proteins and
lipids can rapidly diffuse laterally or rotate within
the bilayer (proteins “float” in a lipid-bilayer sea)
• Membranes: ~25-50% lipid and 50-75% proteins
• Lipids include phospholipids, glycosphingolipids,
cholesterol (in some eukaryotes)
• Compositions of biological membranes vary
considerably among species and cell types
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Fig 9.22 Structure of a typical
eukaryotic plasma membrane
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Lipid Bilayers and Membranes Are Dynamic Structures
Fig 9.23 (a) Lateral diffusion is very rapid
(b) Transverse diffusion (flip-flop) is very slow
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Fig 9.25 Freeze-fracture electron microscopy,
distribution of membrane proteins
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Fig 9.22 Structure of a typical
eukaryotic plasma membrane
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Three Classes of Membrane Proteins
(1) Integral membrane proteins
Contain hydrophobic regions embedded in the lipid bilayer
• Usually span the bilayer completely
(2) Peripheral membrane proteins
• Associated with membrane through charge-charge or hydrogen
bonding interactions to integral proteins or membrane lipids
• More readily dissociated from membranes than covalently
bound proteins
• Change in pH or ionic strength often releases these proteins
(3) Lipid-anchored membrane proteins
• Tethered to membrane through a covalent bond to a lipid
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Fig 9.22 Structure of a typical
eukaryotic plasma membrane
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Membrane Transport
• Three types of integral membrane protein
transport:
(1) Channels and pores
(2) Passive transporters
(3) Active transporters
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Table 9.3 Characteristics of membrane transport
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Pores and Channels
• Pores and channels are transmembrane proteins with a
central passage for ions and small molecules
• Solutes of appropriate size, charge, and molecular structure
can diffuse down a concentration gradient
• Process requires no energy
• Central passage allows
molecules and ions of
certain size, charge and
geometry to transverse
the membrane.
• Figure
9.30
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Passive Transport
• Passive transport (facilitated diffusion) does not require an
energy source
• Protein binds solutes and transports them down a
concentration gradient
Types of passive transport systems
• Uniport - transporter carries only a single type of solute
• Some transporters carry out cotransport of two solutes,
either in the same direction (symport) or in opposite
directions (antiport)
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Fig 9.31
• Types of passive transport
(a) Uniport
(b) Symport
(c) Antiport
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Fig 9.32 Kinetics of passive transport
• Initial rate of
transport
increases until a
maximum is
reached
(site is saturated)
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Active Transport
• Transport requires energy to move a solute up its
concentration gradient
• Transport of charged molecules or ions may result
in a charge gradient across the membrane
Types of active transport
• Primary active transport is powered by a
direct source of energy as ATP, light or
electron transport
• Secondary active transport is driven by an
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ion concentration gradient
Fig 9.34 Secondary active transport in E. coli
• Oxidation of Sred
generates a
transmembrane
proton gradient
• Movement of H+
down its gradient
drives lactose
transport (lactose
permease)
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Fig 9.35 Secondary active transport
in animals: Na+-K+ ATPase
• Na+ gradient (Na+-K+ATPase) drives glucose transport
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Endocytosis and Exocytosis
• Cells import/export molecules too large to be
transported via pores, channels or proteins by:
• Endocytosis - macromolecules are engulfed by
plasma membrane and brought into the cell
inside a lipid vesicle
• Exocytosis - materials to be excreted from the
cell are enclosed in vesicles that fuse with the
plasma membrane
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Transduction of Extracellular Signals
• Specific receptors in plasma membranes
respond to external chemicals (ligands) that
cannot cross the membrane: hormones,
neurotransmitters, growth factors
• Signal is passed through membrane protein
transducer to a membrane-bound effector
enzyme
• Effector enzyme generates a second
messenger which diffuses to intracellular target
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Fig 9.37 General mechanism of signal
transduction across a membrane
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OUT
IN
Fig 9.43
• Summary of
the adenyl
cyclase
signaling
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pathway
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