Lipids and Proteins Are Associated in Biological
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Transcript Lipids and Proteins Are Associated in Biological
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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