Lipids and Proteins Are Associated
in Biological Membranes
Chapter 8
What is a lipid?
 Lipids: consists of nonpolar groups
- Classified on the basis of common solubility
properties
- insoluble in water
- soluble in organic solvents like chloroform and
acetone
- Amphipathic in nature
What are the two groups of lipids?
 Lipids include:
 Open Chain forms
 fatty acids, triacylglycerols, sphingolipids,
phosphoacylglycerols, and glycolipids,
 lipid-soluble vitamins
 prostaglandins, leukotrienes, and thromboxanes
 Cyclic forms
 cholesterol, steroid hormones, and bile acids
What are Fatty Acids?
 - Carboxyl group at polar end and hydrocarbon chain at
non polar tail
 - Length of fatty acid plays a role in its chemical
character
- Usually contain even numbers of carbons (can contain
odd, depending on how they are biosynthesized)
What are saturated fatty acids?
-
Contain only C-C bonds - saturated
What are unsaturated fatty acids?
 In most unsaturated fatty acids, the cis isomer
predominates; the trans isomer is rare
 FA that contain C=C, are unsaturated
 Unsaturated fatty acids have lower melting points
than their saturated counterparts
What are triacylglycerols?
 Triacylglycerol (triglyceride): an ester of glycerol with three
fatty acids
 natural soaps are prepared by boiling triglycerides (animal
fats or vegetable oils) with NaOH, in a reaction called
saponification (Latin, sapo, soap)
What are soaps?
 Soaps form waterinsoluble salts when
used in water containing
Ca(II), Mg(II), and
Fe(III) ions (hard
water)
 Results in characterisitc
scum left on insides of
bathtubs and sinks
What are phosphoacylglycerols?
 When one alcohol group of glycerol is esterified by a phosphoric
acid rather than by a carboxylic acid, phosphatidic acid
produced
 Phosphoacylglycerols (phosphoglycerides) are the second most
abundant group of naturally occurring lipids
Other important members
 Phosphatidyl ethanolamine (cephalin),
phosphatidyl serine, phosphatidyl choline
(lecithin), phosphatidyl inositol, phosphatidyl
glycerol and diphosphatidyl glycerol
(cardiolipin)
What are Waxes?
 A complex mixture of esters of long-chain carboxylic
acids and long-chain alcohols
 Found as protective coatings for plants and animals
What are Sphingolipids?
 Contain sphingosine, a longchain amino alcohol
sphingosine
 Found in plants and animals
 Abundant in nervous system
 Bares structural similarity to
phospholipids
What are Glycolipids?
 a compound in which a
carbohydrate is bound
to an -OH of the lipid
 many glycolipids are
derived from
ceramides
 Gangliosides are
Glycolipids with
complex carbohydrate
moiety that contains
more than 3 sugars
What are Steroids?
 Steroids: a group of
lipids that have fusedring structure
- of 3 six-membered rings
- and 1 five-membered
ring
Sex Hormones
 Androgens: male sex hormones
 synthesized in the testes
 responsible for the development of male secondary
sex characteristics
 Testosterone
 Estrogens: female sex hormones
 synthesized in the ovaries
 responsible for the development of female secondary
sex characteristics and control of the menstrual cycle
What is Cholesterol?
 Present in biological
membrane
 Does not occur in
prokaryotes
 Precursor of vitamin D3
 Harmful - when present
in excess Atherosclerosis
What is the structure of lipid bilayers?
 The polar surface of the
bilayer contains charged
groups
 The hydrophobic tails
lie in the interior of the
bilayer
How does the composition of bilayer affect
its properties?
 Both inner and outer
layers of bilayer contain
mixtures of lipids
 Compositions on inside
and outside of lipid
bilayer can be different
How does the composition of bilayer affect
its properties?
 In saturated fatty acids,
a linear arrangement of
hydrocarbon chains
leads to close packing of
molecules in bilayer
 Provides rigidity
How does the composition of bilayer affect
its properties? (Unsaturated)
 Kink in hydrocarbon chain
 Causes disorder in packing
against other chains
 This disorder causes greater
fluidity in membranes with
cis-double bonds vs......
saturated FA chains
Biological Membranes
 Plant membranes have a higher percentage of unsaturated
fatty acids than animal membranes
 The presence of cholesterol is characteristic of animal
rather than plant membranes
 Animal membranes are less fluid (more rigid) than plant
membranes
 The membranes of prokaryotes are fluid - contain no
steroids
Temperature Transition in Lipid Bilayer
• With heat, the transition temperature is higher for
more rigid membranes
- it is lower for less rigid membranes
Membrane Proteins
 Functions: transport substances across membranes; act as receptor
sites, and sites of enzyme catalysis
 Peripheral proteins
 bound by electrostatic interactions
 can be removed by raising the ionic strength
Membrane Proteins
 Integral proteins
 bound tightly to the interior of the membrane
 can be removed by treatment with detergents or
ultrasonification
 removal generally denatures them
Fluid Mosaic Model
 Fluid: there is lateral motion of components in the
membrane
 Proteins float in the membrane and can move along its
plane of membrane
 Mosaic: components in the membrane exist side-by-side as
separate entities
 Forms a lipid bilayer with proteins, glycolipids, and
steroids such as cholesterol embedded in it
 no lipid-protein complexes, are formed
Fluid Mosaic Model of Membrane
Structure
How does transport through membranes take
place?
Passive transport
 driven by a
concentration gradient
(from higher to lower)
 Simple diffusion: a
molecule or ion moves
through an opening
 Facilitated diffusion: a
molecule or ion is
carried across a
membrane by a
carrier/channel protein
How does transport through membranes take
place?
 Active transport
 a substance is moved against a concentration
gradient
 Primary active transport: transport is linked to the
hydrolysis of ATP or other high-energy molecule;
for example, the Na+/K+ ion pump (Figure 8.24)
 Secondary active transport: driven by H+
gradient
How do membrane receptors work?
 Membrane receptors
 oligomeric proteins
 binding of a biologically
active substance to a
receptor initiates an
action within the cell
Lipid-Soluble Vitamins
 Vitamins are divided into two classes: lipid-soluble and watersoluble
How are prostaglandins related to lipids?
 Prostaglandins: a family of compounds that
have the 20-carbon skeleton of prostanoic acid
 First detected in seminal fluid…from prostate
 The metabolic precursor is arachidonic acid (20
carbon atoms: 4 double bonds)
How are leukotrienes related to lipids?
 Compounds also derived from arachidonic acid
 Found in white blood cells (leukocytes)
 Consists of 3 conjugated double bonds
 Constriction of smooth muscles, especially in
the lungs
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