Chapter 19
Lipids
Chapter 19
Table of Contents
19.1 Structure and Classification of Lipids
19.2 Types of Fatty Acids
19.3 Physical Properties of Fatty Acids
19.4 Energy-Storage Lipids: Triacylglycerols
19.5 Dietary Considerations and Triacylglycerols
19.6 Chemical Reactions of Triacylglycerols
19.7 Membrane Lipids: Phospholipids
19.8 Membrane Lipids: Sphingoglycolipids
19.9 Membrane Lipids: Cholesterol
19.10 Cell Membranes
19.11 Emulsification Lipids: Bile Acids
19.12 Messenger Lipids: Steroid Hormones
19.13 Messenger Lipids: Eicosanoids
19.14 Protective-Coating Lipids: Biological Waxes
19.15 Saponifiable and Nonsaponifiable lipids
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Section 19.1
Structure and Classification of Lipids
Lipids
Lipid: An organic compound found in living organisms
that is insoluble (or only sparingly soluble) in water but
soluble in non-polar organic solvents.
Unlike other biomolecules, lipids do not have a common
structural features that serves as the basis for defining
such compounds.
Classification: Based on two methods
Biochemical function
Saponification (hydrolysis under basic conditions)
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Section 19.1
Structure and Classification of Lipids
Classification based on Biochemical Function
For purposes of simplicity of study lipids are divided into
five categories based on their biochemical function:
Energy-storage lipids - triacylglycerols
Membrane lipids - phospholipids, sphingoglycolipids,
and cholesterol
Emulsification lipids - bile acids
Chemical messenger lipids - steroid hormones and
eicosanoids)
Protective-coating lipids - biological waxes
Transport lipids - lipoproteins
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Section 19.1
Structure and Classification of Lipids
Classification Based on Saponification
Saponification reaction: Hydrolysis reaction that occurs
in a basic solution.
Based on saponification reactions lipids are divided into
two categories :
Saponifiable lipids triacylglycerols phospholipids,
sphingoglycolipids, cholesterol and biological waxes
Nonsaponifiable lipids - bile acids, steroid hormones
and eicosanoids)
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Section 19.1
Structure and Classification of Lipids
Structure
Lipids exhibit
structural diversity
Some are esters,
some are amides,
and some are
alcohols (acyclic
and cyclic) and
some are
polycyclic.
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Section 19.2
Types of Fatty Acids
Saturated and Unsaturated Fatty Acids
Carboxylic acids with linear (unbranched) carbon chain - Fatty acids
are naturally occurring monocarboxylic acids
Even # of Carbon atoms:
Long chain fatty acids: C12 - C26
Medium chain fatty acids: C6 - C11
Short-chain fatty acids: C4 - C5
Two Types:
Saturated - all C-C bonds are single bonds
Unsaturated
Monounsaturated: one C=C bond
Polyunsaturated: 2 or more C=C bonds present - up to six double
bonds are present in fatty acids
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Section 19.2
Types of Fatty Acids
Saturated Fatty Acids
Fatty acid with a carbon chain in which all C-C bonds
are single bonds
Numbering starts from the end of -COOH group
Structural notation: it indicates number of C atoms
Example - Lauric acid has 12 C atoms and no double
bonds so it is (12:0)
O
H 3C
(CH) 10
C
OH
O
or
12
11
9
10
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8
7
6
5
4
C
3
2
1
OH
8
Section 19.2
Types of Fatty Acids
Unsaturated Fatty Acids
A monounsaturated
fatty acid is a fatty
acid with a carbon
chain in which one
carbon carbon
double bond is
present.
Different ways of
depicting the
structure
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Section 19.2
Types of Fatty Acids
Polyunsaturated Fatty Acids (PUFAs)
A polyunsaturated fatty acid is a fatty acid with a carbon
chain in which two or more carbon carbon double bonds
are present.
Up to six double bonds are found in biochemically
important PUFAs.
Two types of unsaturated fatty acids.
Omega ( )-3 fatty acids - An unsaturated fatty acid with its
endmost double bond three carbon atoms away from its methyl
end.
Omega ( )-6 fatty acid is an unsaturated fatty acid with its
endmost double bond six carbon atoms away from its methyl
end.
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Section 19.2
Types of Fatty Acids
Selected Unsaturated Fatty Acids of Biological Importance
Numbering starts from the other end of -COOH
Structural notation: it indicates number of C atoms
E.g., 18:2 18 carbons, 2 double bonds
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Section 19.2
Types of Fatty Acids
Omega Acids
Essential Fatty Acids: Must be part of diet
Nutritionally important Omega-3 and Omega-6 fatty acids
Linolenic acid Omega-3
Linoleic acid Omega-6
Linoleic Acid Deficiency:
Skin redness - becomes irritated
Infections and dehydration
Liver abnormalities
Children need it the most
Human milk has more than cow s milk
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Section 19.2
Types of Fatty Acids
American Diet
Sufficient in omega 6 fatty acids
Deficient in omega 3 fatty acids
Fish - good source for omega 3 fatty acids
High rate of heart disease may be due to imbalance in
omega 3 and 6 fatty acids
Ideal ratio: Omega 6 : Omega 3 (4 - 10 g: 1g)
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Section 19.3
Physical Properties of Fatty Acids
Water solubility: Short chain fatty acids have some
solubility whereas long chain fatty acids are insoluble
Short chain fatty acids are sparingly soluble because
of carboxylic acid polar group
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Section 19.3
Physical Properties of Fatty Acids
The Melting Point
Melting Point
Depends Upon:
Length of
carbon chain
Degree of
unsaturation
(number of
double bonds
in a molecule)
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Section 19.3
Physical Properties of Fatty Acids
Space-Filling Molecules
The number of bends in a fatty acid chain increase as
the number of double bonds increase
Less packing occurs
Melting point is lower
Tend to be liquids at room temperature
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Section 19.4
Energy-Storage Lipids: Triacylglycerols
Energy-Storage Materials
With the notable exception of nerve cells, human cells
store small amounts of energy providing materials:
The most widespread energy storage material carbohydrate glycogen
Present in small amounts
Major Energy Storage material is triacylglycerols:
Triacylglycerols are concentrated primarily in special
cells (adipocytes)
Adipocytes are nearly filled with triacylglycerols.
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Section 19.4
Energy-Storage Lipids: Triacylglycerols
p661
Section 19.4
Energy-Storage Lipids: Triacylglycerols
p663
Section 19.4
Energy-Storage Lipids: Triacylglycerols
Two Types of Triacylglycerols
Simple Triacylglycerols: Three identical fatty acids are
esterified
Naturally occurring simple triacylglycerols are rare
Mixed Triacylglycerols: A triester formed from the
esterification of glycerol with more than one kind of fatty
acid
In nature mostly mixed triacylglycerols are found and
are different even from the same source depending
on the feed, e.g., corn, peanut and wheat -fed cows
have different triacylglycerols
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Section 19.4
Energy-Storage Lipids: Triacylglycerols
Figure 19-5 p661
Section 19.4
Energy-Storage Lipids: Triacylglycerols
Figure 19-6 p662
Section 19.4
Energy-Storage Lipids: Triacylglycerols
Fats and Oils
Physical State:
Fats
Predominantly Saturated
Solids or semisolids at room
temperature
Oils:
Predominantly unsaturated
Liquids at room temperature
Source:
Fats: Animal source and tasteless
Oils: Plants and fish oil
Pure oils and fats are colorless,
odorless
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Section 19.4
Energy-Storage Lipids: Triacylglycerols
Figure 19-8 p664
Section 19.5
Dietary Considerations and Triacylglycerols
Good Fats Versus Bad Fats
Studies indicate that type of dietary fat and amount of dietary fat are
important for a balanced diet:
Current recommended amounts are: total fat intake in calories:
15% - Monounsaturated fat
10% - Polyunsaturated
<10% - Saturated fats
Studies also indicate that:
Saturated fats are considered bad fats
Monounsaturated fats are considered good fats
Trans-monounsaturated fats are considered bad fats
Polyunsaturated fats can be both good fats and bad fats
Omega 3 and 6 are important good fats
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Section 19.5
Dietary Considerations and Triacylglycerols
Fat and Fatty Acid Composition of Nuts
Numerous studies now indicate that eating nuts can have a
strong protective effect against coronary heart disease:
Low amounts of saturated fatty acids
Nuts also contain valuable antioxidant vitamins, minerals,
and plant fiber protein
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Section 19.5
Dietary Considerations and Triacylglycerols
Essential Fatty Acids
Fatty acids that must be obtained from dietary sources are not
synthesized within the body
Two most important essential fatty acids are:
Linoleic acid (18:2) - omega 6
Linolenic acid (18:3) - omega 3
Both are needed for:
Proper membrane structure
Serve as starting materials for the production of several
nutritionally important longer-chain omega-6 and
omega-3 fatty acids
Deficiencies of above two acids may result in skin redness,
infections and dehydration likely and liver abnormalities may
develop
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Section 19.5
Dietary Considerations and Triacylglycerols
Artificial Fat Substitute : Olestra
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Section 19.5
Dietary Considerations and Triacylglycerols
Artificial Fat Substitute : Olestra
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Section 19.6
Chemical Reactions of Triacylglycerols
Partial Hydrolysis
Chemical Properties due to two functional groups: esters
and alkenes
Hydrolysis: Partial hydrolysis of triacylglycerols
Breaking of 1-2 ester bonds to give rise to mono- or
diacylglycerol and fatty acid(s)
Carried out by enzymes produced by the pancreas
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Section 19.6
Chemical Reactions of Triacylglycerols
Figure 19-10 p671
Section 19.6
Chemical Reactions of Triacylglycerols
Saponification
Hydrolysis in basic solution: Produce salt of fatty acid
and glycerol
O
H2C O
O
R
C
O
CH
H2C O
C
O
C
H 2C OH
R
+
R
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3NaOH
HC OH
H 2C OH
+
3RCOONa
Soap
32
Section 19.6
Chemical Reactions of Triacylglycerols
p672
Section 19.6
Chemical Reactions of Triacylglycerols
Hydrogenation
Addition of hydrogen across double (=) bond - increases degree
of saturation
O
O
H2C O C
H2C O C
O
+ 2H2
HC O C
O
HC O C
O
O
H2C O C
H2C O C
Oil
Solid
Many food products are produced by partial hydrogenation of
oils and fats
Peanut oil + H2
Peanut Butter
Vegetable oil + H2
Margarine
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Section 19.6
Chemical Reactions of Triacylglycerols
Figure 19-11 p673
Section 19.6
Chemical Reactions of Triacylglycerols
p675
Section 19.6
Chemical Reactions of Triacylglycerols
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Section 19.6
Chemical Reactions of Triacylglycerols
Halogenation
Addition of halogen across double (=) bond increases degree of saturation
a test for unsaturation
the amount of halogen absorbed by a lipid can be
used as an index of the degree of unsaturation; the
index value is called iodine number , the number of
grams of iodine that will add to 100 g of fat or oil
the rule is: high I2 number indicates a high degree of
unsaturation
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Section 19.6
Chemical Reactions of Triacylglycerols
Oxidation
Double bonds in triacylglycerols are subject to oxidation with oxygen
in air (an oxidizing agent )-Leads to C=C breakage
Remember that oxidation of alkenes may result into two short chain
molecules an aldehydes or a carboxylic acid:
The aldehydes and/or carboxylic acids so produced often have
objectionable odors - fats and oils are said to be rancid
To avoid this unwanted oxidation process antioxidants are
added as preservatives, e.g., Vitamin C and vitamin E are good
antioxidant preservatives.
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Section 19.6
Chemical Reactions of Triacylglycerols
p673
Section 19.7
Membrane Lipids: Phospholipids
All cells are surrounded by a membrane that confines
their contents.
Up to 80% of the mass of a cell membrane can be lipid
materials -- dominated by phospholipids.
Phospholipid: contains one or more fatty acids, a
phosphate group, a platform molecule (glycerol or
sphingosine) to which the fatty acid(s) and the phosphate
group are attached, and an alcohol that is attached to the
phosphate group.
G
l
y
c
e
r
o
l
Fatty acid
Sphingosine
Fatty acid
Fatty acid
Phosphate
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Alcohol
Phosphate
Alcohol
41
Section 19.7
Membrane Lipids: Phospholipids
p676
Section 19.7
Membrane Lipids: Phospholipids
Glycerophospholipids
A glycerophospholipid is a lipid that contains two fatty
acids and a phosphate group esterified to a glycerol
molecule and an alcohol esterified to the phosphate
group.
All attachments (bonds) between groups in a
glycerophospholipid are ester linkages
Glycerophospholipids have four ester linkages as
contrasted to three ester linkages in triacylglycerols.
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Section 19.7
Membrane Lipids: Phospholipids
p677
Section 19.7
Membrane Lipids: Phospholipids
Glycerophospholipids
Glycerophospholipids undergo hydrolysis and
saponification reactions in a manner similar to that for
triacylglycerols
The alcohol attached to the phosphate group in a
glycophospholipid is usually one of three amino alcohols:
choline, ethanolamine, or serine - respectively known as
phosphatidylcholines, phosphatidylethanolamines, and
phosphatidylserines.
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Section 19.7
Membrane Lipids: Phospholipids
p678
Section 19.7
Membrane Lipids: Phospholipids
p677
Section 19.7
Membrane Lipids: Phospholipids
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Section 19.7
Membrane Lipids: Phospholipids
Figure 19-13b p679
Section 19.7
Membrane Lipids: Phospholipids
Glycerophospholipids
Structurally glycerophospholipids are although similar to
triacylglycerols, they have different biochemical
functions.
Triacylglycerols serve as energy storage molecules
Glycerophospholipids function as components of cell
membranes
A major structural difference between the two types of
lipids is that of their polarity
Responsible for the their
differing biochemical functions.
Triacylglycerols are a non-polar
Glycerophospholipids are polar.
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Section 19.7
Membrane Lipids: Phospholipids
Sphingophospholipids
Structures based on the 18-carbon monounsaturated
aminodialcohol sphingosine
Contains one fatty acid and one phosphate group
attached to a sphingosine molecule and an alcohol
attached to the phosphate group
Sphingosine
Fatty acid
Phosphate
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Alcohol
51
Section 19.7
Membrane Lipids: Phospholipids
p680
Section 19.7
Membrane Lipids: Phospholipids
Saponifiable lipids
Sphingophospholipids in which the alcohol esterified to
the phosphate group is choline are called
sphingomyelins.
Sphingomyelins are found in all cell membranes and are
important structural components of the myelin sheath of
neurons
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Section 19.8
Membrane Lipids: Sphingoglycolipids
Sphingoglycolipids: Contains both a fatty acid and carbohydrate
Simple sphingoglycolipids are called cerebrosides: contains a
single monosaccharide unit - either glucose or galactose
They occur primarily in brain (7% of dry mass)
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Section 19.8
Membrane Lipids: Sphingoglycolipids
Gangliosides
Complex sphingoglycolipids are called Gangliosides:
contain a branched chain of up to seven
monosaccharide residues.
Occur in the gray matter of the brain as well as in the
myelin sheath.
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Section 19.8
Membrane Lipids: Sphingoglycolipids
p682
Section 19.9
Membrane Lipids: Cholesterol
Cholesterol-Third major type of membrane lipid
A steroid is a lipid whose structure is
based on a fused ring system of
three 6 carbon rings and one 5
carbon ring.
Cholesterol: C27 steroid molecule
The principal constituent of
gallstones from which it can be
isolated as white crystalline solid;
name derived from this source
(Greek, chole bile; steros solid)
Important in human cell membranes,
nerve tissue and brain tissue
Important in chemical synthesis
of various hormones and
vitamins essential for life
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Section 19.9
Membrane Lipids: Cholesterol
Cholesterol in Food
Liver synthesizes cholesterol: ~ 1g everyday; so it is not
necessary to consume in the form of diet
Cholesterol synthesis decrease if it is ingested but reduction
is not sufficient: Leads to cardiovascular disease
Animal Food: Lot of cholesterol
Plant Food: No cholesterol
Medical science now considers high blood cholesterol, along
with high blood pressure and smoking, as the major risk
factors for cardiovascular disease (CVD). High blood
cholesterol contributes to atherosclerosis, which is
characterized by the buildup of plaque along the inner walls of
the arteries.
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Section 19.9
Membrane Lipids: Cholesterol
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Section 19.10
Cell Membranes
Cell Membrane (Plasma Membrane)
Cells are surrounded by plasma membranes:
Separates aqueous interior of a cell from the aqueous
environment surrounding the cell
Up to 80% of plasma membrane is lipid material
The membranes are lipid bilayer made up of phospholipids
Bilayer: Nonpolar tails of phospholipids in the middle and polar
heads are on the surface
6 - 9 billionths of a meter thick or 6-9 nanometer thick
The membrane is a liquid like structure due to unsaturation in
lipid tails
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Section 19.10
Cell Membranes
Figure 19-17 p685
Section 19.10
Cell Membranes
Cholesterol and Cell Membrane
Cholesterol molecules are also components of plasma membranes:
Cholesterol helps regulate membrane fluidity The fused ring
system does nor allow rotation of fatty acid tails in the vicinity
Fits between fatty acid chains of the lipid bilayer: Make it rigid
Cholesterol thus acts a membrane plasticizer
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Section 19.10
Cell Membranes
Membrane Proteins
The membranes also contain proteins:
Responsible for moving substances such as nutrients and electrolytes
across the membrane
Receptors for hormones and neurotransmitters
The membrane proteins and some lipids are further reacted with
carbohydrates molecules:
Act as markers: process by which different cells recognize each other
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Section 19.10
Cell Membranes
Transport Across Cell Membranes:
To maintain cellular
processes various
molecules are
transported across the
cell membranes.
Three types of
transport.
Passive transport
Facilitated transport
Active transport
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Section 19.10
Cell Membranes
Passive Transport
Passive transport - a substance moves
across a cell membrane by diffusion
from a region of higher concentration to
a region of lower concentration.
Only a few types of molecules,
including O2, N2, H2O, urea, and
ethanol, can cross membranes by
passive transport
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Section 19.10
Cell Membranes
Facilitated Transport
Facilitated transport - a substance
moves across a cell membrane with
the aid of a membrane protein from a
region of higher concentration to a
region of lower concentration.
The specific protein carriers or
transporters are involved in the
process
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Section 19.10
Cell Membranes
Active Transport
Active transport - a
substance moves across a
cell membrane, with the aid
of membrane proteins,
against a concentration
gradient with the expenditure
of cellular energy.
Proteins involved in
active transport are called
pumps. The needed
energy is supplied by
molecules such as ATP.
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Section 19.11
Emulsification Lipids: Bile Acids
An emulsifier is a substance that can disperse and
stabilize water-insoluble substances as colloidal
particles in an aqueous solution.
Bile Acids: Cholesterol derivatives that functions as
emulsifying agents that make dietary lipids soluble in
aqueous environment of the digestive tract:
Approximately one third of cholesterol produced by
liver is converted to bile acids.
Action similar to soap in washing
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Section 19.11
Emulsification Lipids: Bile Acids
Bile Acids
Bile acids are tri- or dihydroxy
A large percentage of gallstones, the
cholesterol derivatives
The carbon 17 side chain of
cholesterol that has precipitated from
cholesterol has been oxidized to a
bile solution.
carboxylic acid
The oxidized acid side chain is
bonded to an amino acid (either
glycine or taurine) through an amide
linkage
Bile: A fluid containing emulsifying
agents (Bile acids) secreted by the
liver, stored in the gallbladder, and
released into the small intestine
during digestion
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Section 19.11
Emulsification Lipids: Bile Acids
Figure 19-24 p688
Section 19.12
Messenger Lipids: Steroid Hormones
Hormones
A hormone is a biochemical substance produced by a ductless
gland that has a messenger function.
Hormones serve as a means of communication between various
tissues.
Some hormones are lipids.
The lipids that play the role of chemical messengers include:
Steroid hormones derivatives of cholesterol
Eicosanoids- derivatives of arachidonic acid
There are two major classes of steroid hormones:
Sex hormones - control reproduction and secondary sex
characteristics
Adrenocorticoid hormones control numerous biochemical
processes in the body
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Section 19.12
Messenger Lipids: Steroid Hormones
Sex Hormones
Classified into three major groups:
Estrogens - the female sex hormones
Androgens - the male sex hormones
Progestins - the pregnancy hormones
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Section 19.12
Messenger Lipids: Steroid Hormones
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Section 19.12
Messenger Lipids: Steroid Hormones
Adrenocorticoid Hormones
Produced by the adrenal glands
- small organs located on top of
each kidney
28 Different hormones have
been isolated from the adrenal
cortex
Two types of adrenocorticoid
hormones:
Mineralocorticoids - control
the balance of Na and K
ions in cells
Glucocorticoids - control
glucose metabolism and
counteract inflammation
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Section 19.13
Messenger Lipids: Eicosanoids
Eicosanoids are arachidonic acid (20:4) derivatives:
Have profound physiological effects at extremely low
concentrations.
Eicosanoids are hormone-like molecules
Exert their effects in the tissues where they are synthesized.
Eicosanoids usually have a very short life.
Physiological effects of eicosanoids:
Inflammatory response
Production of pain and fever
Regulation of blood pressure
Induction of blood clotting
Control of reproductive functions, such as induction of labor
Regulation of the sleep/wake cycle
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Section 19.13
Messenger Lipids: Eicosanoids
Figure 19-28 p693
Section 19.13
Messenger Lipids: Eicosanoids
Principal Types of Eicosanoids
1. Prostoglandins: C20-fatty-acid
derivative containing cyclopentane
ring and oxygen-containing
functional groups
Involved in raising body
temperature,
Inhibiting the secretion of gastric
juices,
Increasing the secretion of a
protective mucus layer into the
stomach,
Relaxing and contracting smooth
muscle, directing water and
electrolyte balance, intensifying
pain, and enhancing
inflammation responses.
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Section 19.13
Messenger Lipids: Eicosanoids
Principal Types of Eicosanoids
2. Thromboxanes: C20-fatty-acid
derivative containing a cyclic ether ring
and oxygen-containing functional
groups
Promote platelet aggregation.
3. Leukotrienes: C20-fatty-acid derivative
containing three conjugated double
bonds and hydroxyl groups
Promote inflammatory and
hypersensitivity (allergy) responses
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Section 19.14
Protective-Coating Lipids: Biological Waxes
A biological wax: a
monoester of a long-chain
fatty acid and a long-chain
alcohol.
The fatty acids found in
biological waxes:
Generally are saturated
fatty acids
Contain 14 to 36 carbon
atoms.
The alcohols found in
biological waxes:
May be saturated or
unsaturated
May contain 16 to 30
carbon atoms.
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Section 19.14
Protective-Coating Lipids: Biological Waxes
Figure 19-29 p695
Section 19.1
Structure and Classification of Lipids
Transport Lipids : Lipoproteins
responsible for the transport of
other lipids in the body; lipids are
only sparingly soluble in water,
and the movement of lipids from
one organ to another through the
blood stream requires a transport
system that operates via plasma
lipoproteins
lipoprotein particles consist of a
core of hydrophobic molecules
such as triglycerides or
cholesterol esters (cholesterol
esterified to
). The shell around
the core consists of polar lipids
and proteins
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Section 19.1
Structure and Classification of Lipids
Transport Lipids : Lipoproteins
1.
2.
3.
4.
Four major classes:
Chylomicrons transport dietary
TAG from the intestine to the liver
and to adipose tissue
Very-low-density lipoprotein
(VLDL) transport TAG synthesized
in the liver to adipose tissue
Low-density lipoprotein (LDL)
transport cholesterol synthesized in
the liver to cells throughout the body
High-density lipoprotein (HDL)
collect excess cholesterol from body
tissues and transport it back to the
liver for degradation to bile acids
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Section 15.15
Saponifiable and Nonsaponifiable Lipids
LIPIDS
SAPONIFIABLE
NONSAPONIFIABLE
Triacylglycerols
Glycerophospholipids
Sphingophospholipids
Sphingoglycolipids
Biological waxes
Cholesterol
Bile acids,
Steroid hormones
Eicosanoids
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Section 15.15
Saponifiable and Nonsaponifiable Lipids
Saponifiable Lipid: A lipid that undergoes hydrolysis
in a basic solution to yield 2 or more small molecules.
Saponification is possible in molecules that contain
the following linkages (bonds):
Ester
Amide
Glycosidic
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Section 15.15
Saponifiable and Nonsaponifiable Lipids
Saponifiable Lipids and Linkages:
Triacylglycerols 3 ester bonds
Glycerophospholipids 4 ester bonds
Sphingophospholipids 1 amide and 2 ester
bonds
Sphingoglycolipids: 1 amide, 1 ester and 1
glycosidic bond
Biological waxes 1 ester bond
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