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)
Structural and Functional
Diversity of Lipids
• Fatty acids - R-COOH (R=hydrocarbon chain)
are components of triacylglycerols,
glycerophospholipids, sphingolipids
• Phospholipids - contain phosphate moieties
• Glycosphingolipids - contain both sphingosine
and carbohydrate groups
• Isoprenoids - (related to the 5 carbon isoprene)
include steroids, lipid vitamins and terpenes
Structural relationships
of major lipid classes
Fatty Acids
• Fatty acids (FA) 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
(synthesized from two-carbon units)
• IUPAC nomenclature: carboxyl carbon is C-1
• Common nomenclature: a,b,g,d,e etc. from C-1
• Carbon farthest from carboxyl is w
Structure and nomenclature of fatty acids
• Saturated FA - no C-C double bonds
• Unsaturated FA - at least one C-C double bond
• Monounsaturated FA - only one C-C double
bond
• Polyunsaturated FA - 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)
Structure and nomenclature
of fatty acids
Fatty acids are stored as triglycerols
(triglycerides)
Glycerophospholipids
• The most abundant lipids in membranes
• Possess a glycerol backbone
• A phosphate is esterified to both glycerol and
another compound bearing an -OH group
• Phosphatidates are glycerophospholipids with
two fatty acid groups esterified to C-1 and C-2
of glycerol 3-phosphate
(a) Glycerol 3-P and (b) phosphatidate
O
O P O
O
(a)
H2 C
CH
CH2
O
O P O
O
H2 C
OH OH
O
(b)
CH2
CH
O
O
O
Glycerol 3-phosphate
phosphatidate
(R1) (R2)
Structures of glycerophospholipids
(a) Phosphatidyl ethanolamine
(b) Phosphatidyl serine
(c) Phosphatidylcholine
NH3
CH2
NH3
CH COO
CH2
O
O P O
O
H2C
O
CH2
CH
CH2
O
O P O
O
H2 C
O
O
O
R1 R2
phosphatidylethanolamine
CH3
H3C CH3
N
CH2
O
CH2
CH
CH2
O
O P O
O
H2 C
O
O
O
R1 R2
phosphatidylserine
O
CH2
CH
O
O
O
R1 R2
phosphatidylcholine
Phospholipases hydrolyze phospholipids
Structure of an
ethanolamine
plasmalogen
• Plasmalogens - C-1
hydrocarbon substituent
attached by a vinyl ether
linkage (not ester linkage)
Sphingolipids
• Sphingolipids - sphingosine (trans-4-sphingenine)
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
• Structure of a
galactocerebroside
Steroids
• Classified as isoprenoids - related to 5carbon isoprene (found in membranes of
eukaryotes)
• Steroids contain four fused ring systems: 3six carbon rings (A,B,C) and a 5-carbon D
ring
• Ring system is nearly planar
• Substituents point either down (a) or up (b)
Cholesteryl ester
Other Biologically Important Lipids
• 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
Myricyl palmitate, a wax
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
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
Membrane lipid and bilayer
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
Structure of a typical eukaryotic
plasma membrane
Lipid Bilayers and Membranes
Are Dynamic Structures
(a)Lateral diffusion is very rapid
(b) Transverse diffusion (flip-flop) is very slow
Phase transition of a lipid bilayer
• Fluid properties of bilayers depend upon
the flexibility of their fatty acid chains
Three Classes of Membrane Proteins
(1) Integral membrane proteins (or intrinsic
proteins or trans-membrane proteins)
(2) Peripheral membrane proteins
(3) Lipid-anchored membrane proteins
Lipid-anchored membrane proteins
Membrane Transport
• Three types of integral membrane protein
transport:
(1) Channels and pores
(2) Passive transporters
(3) Active transporters
Table 9.3 Characteristics of
membrane transport
A. 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
• Rate may approach diffusion-controlled limit
Membrane transport
through a pore or channel
• Central passage
allows molecules and
ions of certain size,
charge and geometry
to transverse the
membrane
B. 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)
Kinetics of passive transport
• Initial rate of
transport
increases until a
maximum is
reached (site is
saturated)
C. 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
ion concentration gradient
• Primary active transport
protein function
• Protein binds specific
substrate, conformational
change allows molecule or
ion to be released on the
other side of the membrane
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)
Secondary active transport
in animals: Na+-K+ ATPase
• Na+ gradient (Na+-K+ATPase) drives glucose transport
VALINOMYCIN