chapter iii. lipids-3b (5)

8/11/2019 Chapter III. Lipids-3b (5)

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LIPIDS are heterogeneous class of naturally occurring bioorganic compounds that have the common

property of being soluble in non-polar solvents. They are grouped together not by the presence of a

distinguishing functional group or structural feature, but rather on the basis of common solubility properties:

they are insoluble in water*but highly soluble in one or more organic solvents.

-

*although certain lipids contain ionized groups (e.g., phosphate or choline), the bulk of any lipid

molecule is nonpolar

- widely distributed in biological world and play a wide variety of roles in both plant and animal tissues:

o as an energy source, lipids provide 9 kcal of energy per gram

o triglycerides provide energy storage in adipocytes

o phosphoglycerides, sphingolipids, and steroids are structural components of cell membranes

o steroid hormones are critical intercellular messengers

o lipid-soluble vitamins (A, E, D, K)

o dietary fat acts as a carrier of lipid-soluble vitamins into cells of small intestine

o provide shock absorption and insulation

- the primary form of energy storage; stored fat allows certain animals to hibernate during winter

- has a very high energy value, twice as much as an equal weight of carbohydrates-

the amount of fat in the body is very much related to the carbohydrate level in the body: when

carbohydrate levels become low, fats are degraded to provide energy; when high, fats are storedin cells of adipose tissues (under the skin, around the kidneys, etc.)

- structurally, the lipids are quite diverse; there is no common subunit in their structure.- the primary building blocks in human lipids are fatty acids, glycerol, sphingosine, and sterols.

CLASSIFICATION OF LIPIDS

3.1 Fatty Acids 3.4 Complex lipids

3.2 Glycerides 3.4.1 Phospholipids

3.2.1 Neutral glycerides (TAG) 3.4.2 Glycolipids

3.2.2 Phosphoglycerides (lecithins, cephalins) 3.4.3 Lipoproteins

3.3 Nonglycerides 3.5 Miscellaneous Lipids

3.3.1 Waxes 3.5.1 Terpenes (carotenes)

3.3.2 Sphingolipids 3.5.2 Fat-soluble vitamins

a. sphingomyelin

b. glycolipid (cerebrosides)

c. ganglioside

3.3.3 Steroids

a. sterols c. sex hormones

b. bile acids/bile salts d. adrenocortical hormones

Classification of lipids on the basis of polarity and type of structural subunits present

A. Nonpolar, fatty-acid containing lipids (TAG: fats, oils)

B. Polar, fatty-acid containing lipids

a) Phosphoacylglycerols (lecithins, cephalins)

b) Sphingolipids (sphingomyelins, cerebrosides)

C. Non-fatty-acid containing lipidsSteroids: cholesterol, bile salts, steroid hormones (sex hormones, adrenocortical hormones)

3.1 FATTY ACIDS

-

are long straight-chain (no branching) naturally occurring monocarboxylic acids; rarely found free in nature

but mostly in esterified form in the lipids

- as a consequence of their biosynthesis, fatty acids typically have an even # of C atoms, long-chain fas (12 -

26 C) are found in meats and fish; medium-chain (6 - 10 C) & short-chain fas (< 6C) primarily in dairy

products

-

can be either saturated or unsaturated; saturated fatty acids have no double bonds, unsaturated fatty

acids contain one or more carbon-carbon double bonds, the first is usually at the 9th

carbon


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- the double bonds are not conjugated and are usually in a cis configuration; cisdouble bonds result in abent chain which doesnt allow fatty acids to pack as close together and consequently lower the melting

point

saturated fatty acids (CnH2n + 1):

4:0 butyric acid n-butanoic C3H7COOH butter

6:0 caproic n-hexanoic C5H11COOH goats milk

8:0 caprylic n-octanoic C7H15COOH

10:0 capric n-decanoic C9H19COOH

12:0 lauric n-dodecanoic C11H23COOH coconut

14:0 myristic n-tetradecanoic C13H27COOH nutmeg

16:0 palmitic n-hexadecanoic C15H31COOH palm

18:0 stearic n-octadecanoic C17H35COOH animal fat20:0 arachidic n-eicosanoic C19H39COOH peanut oil

unsaturated fatty acids : (up to six double bonds are found in biochemically important fatty acids)

16:19

palmitoleic cis-9-hexadecenoic acid C15H29COOH

18:19

oleic cis-9-octadecenoic C17H33COOH olive, corn

18:29,12

linoleic cis,cis-9,12-octadecadienoic C17H31COOH soybean,

safflower, sunflower

18:39,12,15

linolenic all cis-9,12,15-octadecatrienoic C17H29COOH herring, linseed

20:45,8,11,14

arachidonic all cis-5,8,11,14-eicosatetraenoic C19H31COOH

20:55,8,11,14,17

EPA

22:64,7,10,13,16,19

DHA

essential fatty acid is a PUFA that is needed by the human body and that must be obtained from dietarysources because it cannot be synthesized within the body from other substances. Linoleicand linolenicacids

are called essential fatty acids that must be supplied in the diet.

omega () fatty acids

- a fatty acid has two ends, the methyl (CH 3) end and the carboxyl (COOH) end; the omega classification

system for fatty acid is based on numbering the carbon chain beginning at the methyl end, thus omega-3(3)

fatty acid is a PUFA with its endmost double bond three carbons away from its methyl end; omega-6 (6)fatty

acidis a PUFA with its endmost double bond six carbons away from its methyl end.

C

O

OCH

2CH2

C C

CH2

CH2

CH2

CH2

CH2

CH2

HH

CH2

CH2

CH2

CH2

CH3


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- Linolenic acidis the primary member of the 3family of fatty acids. From dietary linolenic acid, the body canmake the 20- and 22- carbon members of the 3 fatty acid series. Omega-3 fatty acids are important for

the structure and function of cell membranes, particularly in the retina of the eye and the CNS. Because

fish are a good 3 fatty acid source, nutritionists recommend adding more fish to the diet; cold-water fish

tend to have higher 3fatty acid concentrations than do warm-water fish.

- Linoleic acid is the primary member of the 6 family of fatty acids. Given dietary linoleic acid, the body can

make necessary longer carbon-chain members of the 6fatty acid series such as arachidonic acid(20:4).

Normally, vegetable oils and meats supply enough linoleic acid to meet the bodys needs for it. Omega-6

fatty acids are important for growth, skin integrity, fertility, and maintaining red blood cell structure. Lackof linoleic acid causes the skin to redden and become irritated. Infants have especial need for linoleic acid

for their growth. Human breast milk has a much higher percentage of it then cows milk.

- Arachidonic acid, a 20:4 fatty acid, is an 6 fatty acid synthesized from linoleic acid. In the body, it serves as the

precursor for a family of molecules called eicosanoids, which are oxygenated derivatives of this acid.

Eicosanoids are present in all cells except the red blood cells. Eicosanoids regulate a wide range of body

functions including blood pressure, blood clotting, blood lipid levels, the sleep/wake cycle, and the

inflammation response to injury and infection

- Eicosanoidsare hormone-like molecules; they are not transported in the bloodstream to their site of action but

rather exert their effects in the tissues where they are synthesized. The name eikosanoid is derived from

the Greek word eikos, meaning twenty, because they are all derivatives of 20 carbon fatty acids. The

eicosanoids include three groups of structurally related compounds: the prostaglandins, theleukotrienes,

and the thromboxanes


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Biological processes regulated by eicosanoids

1.

Blood clotting - Thromboxane A2stimulates constriction of blood vessels and platelet aggregation;

Prostacyclin dilates blood vessels and inhibits platelet aggregation

2. Inflammatory response - Prostaglandins mediate aspects of inflammatory response

3. Reproductive system - Stimulation of smooth muscle by PGE24. Gastrointestinal tract - Prostaglandins inhibit gastric secretion; Prostaglandins increase

secretion of protective mucus; Inhibition of hormone-sensitive lipases

5. Kidneys - Prostaglandins dilate renal blood vessels; Results in increased water and electrolyte

excretion6. Respiratory tract - Leukotrienes promote the constriction of bronchi; Prostaglandins promote

bronchodilation

3.2 GLYCERIDES - these are glycerol-containing lipids

3.2.1. Neutral glycerides

- so named because they are nonionic and nonpolar; produced from the esterification of glycerol with one or

more fatty acids: monoglycerides; diglycerides ; triglycerides

Triglycerides (or triacylglycerol, TAG)

- serve as energy storage in adipose cells; esters of glycerol and fatty acids

a)

simple triglycerides

- if all three OH groups of glycerol molecule are esterified with the same fatty acid; have been

synthesized in the laboratory but rarely occur in nature

e.g., Glyceryl tripalmitate (Tripalmitin)

b) mixed triglyceride

- contain 2 or 3 different fatty acid components; most fats and oils are mixed TAG

e.g., CH2-O-COC17H33

CHO-COC11H23

CH2-O-COC15H31Glyceryloleolauropalmitate

Fats vs. Oils

- fats contain a greater proportion of saturated fatty acids than unsaturated fatty acids; so solid or semi-solid at room temperature; beef tallow and pork lard are fats

- oils contain a greater proportion of unsaturated fatty acids than saturated fatty acids; so liquid at same

temperature; soybean oil, canola oil, peanut oil


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- coconut oil, which is highly saturated, is an exception: this oil is a liquid not because it contains moredouble bonds within the fatty acids but because it is rich in shorter-chainfatty acids, particularly lauric

acid (12:0)

- lipids obtained from animal sources are usually solids whereas oils are generally of plant origin. Hence,

we commonly speak of animal fats and vegetable oils .

A comparison of saturated and unsaturated fatty acids in some foods

Physical Properties of Lipids

-

either liquid or noncrystalline solids at room temperature

-

pure fats and oils are colorless, tasteless, odorless, lighter than water, poor conductors of heat and

electricity (therefore, serves as excellent insulators for the body)- the characteristic color, odor, flavor associated with lipids are imparted to them by foreign substancesthat have been absorbed by the lipid and are soluble in them. For example, the yellow color of butter is

due to the presence of carotene, taste is due to 2 compounds: diacetyl, CH3CO-COCH3, and 3-hydroxy-2-

butanone, that are produced by bacteria in the ripening of the cream.

- the melting points of fatty acids depend on both the length of their hydrocarbon chains and the degreeof unsaturation; typical saturated fatty acids are tightly packed together so they have higher melting

points than unsaturated acids with the same number of carbon atoms; cisdouble bonds prevent good

alignment of molecules in unsaturated fatty acids leading to poor packing and lower mp relative to

saturated or transacid.

- the greater the degree of unsaturation the greater the reduction in melting point; this is explained bydecreased molecular attractions between carbon chains with increasing degree of unsaturation.

Chemical Properties of Lipids1) Alkaline hydrolysis

-

lipids may be hydrolyzed by alkali or by enzymes called lipases; reaction is termed saponification

because one of the products of hydrolysis is a soap, generally Na- or K- salts of fatty acids

- provides a useful analytical method for the determination of a constant, saponification number, which

is characteristic of the simple lipids

- saponification number is the number of mg of KOH required to saponify 1 g of a fat or oil; high for lipidscontaining short-chain fatty acids (contains more acid molecules); low for lipids containing long-chain

fatty acids (contains fewer acid molecules) and indicates high molar mass- experimentally,a weighed sample of fat is saponified with a standard solution of alc. KOH. Following

saponification, the excess alkali is determined by titration with standard acid

2) Halogenation

-

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 I2number indicates a high degree of unsaturation

- experimentally, a weighed sample of lipid is treated with an excess of the iodine reagent. After reactionis complete, the unused iodine is determined by titration with a standard solution of Na2S2O3

3) Hydrogenation

- converts oils to fats (hardening); hydrogen is bubbled through hot oil in the presence of a nickelcatalyst; control of the degree of hydrogenation gives the various types of partially hydrogenated

vegetable products on the market today soft margarine, solid stick margarine, and shortenings

- partial hydrogenation of unsaturated fats generates trans fats; if hydrogenation is allowed to continue

for a long period of time, glycerol and long-chain alcohols are formed (reduction of esters)


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4) Rancidity

- the development of disagreeable odor

- a fat or oil becomes rancid when its double bonds are oxidized by oxygen and lipases furnished bymicroorganisms in the air forming short-chain fatty acids and aldehydes as products which have

disagreeable odor; also responsible for the odors associated with workouts and heavy perspiration

- oxidation also occurs in the oils that accumulate on the surface of the skin during heavy exercise. The

relatively high body temperature and the presence of microorganisms on the skin promote rapid

oxidation of these oils as they are exposed to oxygen and water

a) hydrolytic rancidity

* under moist and warm conditions, microorganisms in the air furnish the lipases that catalyze the

process; hydrolysis of ester linkages occurs, liberating the volatile, low mol. wt. acids (e.g., butyricacidfrom rancidity of butter)

* prevented by storing butter covered in a refrigerator

b) oxidative rancidity

* occurs in triglycerides containing unsaturated fatty acids

* highly unsaturated oils react with oxygen forming aldehydes and acids

* antioxidants may be used like Vit. E, ascorbic acid, BHA, BHT

[O] O | [O] | |

-CH=CH- -C-H + H-C=O HO-C=O + HO-C=O

Fat Substitute

Olestra (also known by its brand name Olean) is a fat substitute that adds no fat, calories, or

cholesterol to products. It has a sucrose base instead of the alcohol base of fat. Compared with conventionalfats, which have up to three fatty acids attached to the base, olestra has between six and eight fatty acids

attached to alcohol groups. These groups hang from a ring of sucrose molecules. The ring is completely

impenetrable to fat-removing enzymes and therefore remains indigestible, contributing zero calories. It has

been used in the preparation of traditionally high-fat foods such as potato chips, thereby lowering or

eliminating their fat content. In the late 1990s, Olestra lost its popularity due to side effects (cramping,

flatulence, loose bowels, diarrhea), but products containing the ingredient can still be purchased at grocery

stores.


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Soaps and Detergents

Soaps are sodium or potassium salt of long-chain fatty acids; potassium soaps are more expensive but

produce a softer lather and are more soluble, used in liquid soaps and shaving creams.

Soap molecule structure, CH3(CH2)16COO-Na

+, is composed of a large nonpolar hydrocarbon portion

(hydrophobic) and a carboxylate salt end (hydrophilic)

The cleansing action of soap: (emulsification and lowering of the surface tension of water)

When soap is added to water, the hydrophilic ends of the molecules are dissolved, but the

hydrophobic ends are not and consequently form a thin film (suds) on the surface of water. When this soap

solution is brought into contact with grease or oil (most of the dirt is held to clothes by a thin film of grease or

oil), soap molecules become reoriented. The hydrophobic portions dissolve in the grease or oil and the

hydrophilic ends remain dissolved in water. Mechanical action, such as scrubbing, causes the oil or grease to

disperse into tiny droplets, and soap molecules arrange themselves around the surface of the globules formingmicelles which dont coalesce because of repulsions between their surrounding carboxylate groups. The entire

micelle becomes water-soluble which may be washed away by a stream of water.

Detergents dont form precipitates with ions of hard water

1. Anionic detergents sulfates of fatty acids or sulfonate salts of hydrocarbons

2. Cationic detergents referred to as invert soap because their water-soluble end carries a positive,rather than negative charge; widely used in hospitals, good cleansing agents, have germicidal

properties

3. Nonionic detergents contain polar covalent structures that provide the required water solubility;

used extensively in dishwashing liquid and on all occasions that call for absence of inorganic ions.


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3.2 2 Phosphoglycerides (glycerophospholipids, or phosphatides)

phospholipid is a more general term

- a group of lipids containing a phosphate group; found in all living organisms; abundant in the liver,

brain, & spinal tissue and are found in the outer membranes of most cells

- contain acyl groups derived from long-chain fatty acids esterified at C1 and C2 of glycerol-3-P-

the simplest phosphoglyceride contains a free phosphoryl group and is known as phosphatidate

-

when the phosphoryl group is attached to another hydrophilic group, a more complex phospho-glyceride is formed.

e.g. phosphatidylcholine (lecithin) & phosphatidylethanolamine (cephalin)

- two of the common PL found in membranes; primary function is to act as emulsifying agent at cell

membrane surfaces since it contain both a polar and nonpolar component

- their bipolar nature is central to the structure and function of cell membranes

a) lecithin (phosphatidylcholine)

- contains choline, a 4oammonium salt, HOCH2CH2

+N(CH3)3, joined to a H3PO4residue by means of an

ester linkage

- the N in choline carries a formal positive charge and the phosphate a negative charge so that insolution, at most pH values, lecithin exists as an internal salt or ZWITTERION

-

pure lecithin is especially abundant in eggyolk and soybeans, wheat germ and yeast; also found in

brain and nervous tissue

- excellent emulsifiers and for this reason eggyolk is an excellent emulsifier to hold olive oil and watertogether as mayonnaise; emulsifying agent (aids in the suspension of fats in water) in ice cream

-

claims arise that lecithins should be taken as a nutritive supplement; some claims indicate it will

improve memory. There is no evidence that these supplement s are useful. The enzyme lecithinase in

the intestine hydrolyzes most of the phosphatidylcholine taken orally before it passes into body fluids,

so it does not reach body tissues. The phosphatidylcholine present in membranes is made by the liver;

thus phophatidylcholines are not essential nutrients.


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b) cephalin (phosphatidylethanolamine or phosphatidylserine)

-

the term is derived from its chief occurrence in the body, namely the head, & spinal tissue (Greek,

Kephalikos, head); found in heart and liver tissue and in high concentrations in the brain.

- in cephalins, the choline is replaced by ethanolamine, H2N-CH2CH2OH, (phophatidyl ethanolamine) or

by the amino acid serine, H2N-CH(CH2OH)-COOH, (phosphatidyl serine)

- play an important role in the process of blood clotting

3.3 NONGLYCERIDES (these are lipids not derived from glycerol)

3.3.1 Waxes

- easily melted solids, widely distributed in nature and are found in both plants and animals

- not as easily hydrolyzed as the triglyerides and therefore are useful as protective coatings

- plant waxes are found on surface of leaves and stems and protect the plant from dehydration andinvasion by harmful organisms; animal waxes are found on surface of feathers, skin, and hair.

Some common waxes Source Application

a) Beeswax honeycomb shoe polishes, candles, lipstick,(myricyl palmitate) wax paper

C15H31COOC30H61

b) Carnauba wax Carnauba palm polishes, floor wax, automobile wax

(myricyl cerotate) (in Brazil) wax

C25H51COOC30H61

c) Spermaceti sperm whale ointments, soaps, cosmetics

(cetyl palmitate) manufacture of candles

C15H31COOC16H33d) Lanolin wool skin ointments & lotions to aid

mixture of cholesterol retention of water which softens

& esters of several fatty acids the skin

-

paraffin wax is different because it is merely a mixture of hydrocarbons and is not an ester.

3.3.2 sphingolipids

- derived from sphingosine, an 18-carbon unsaturated amino dialcohol-

can be regarded as derivatives of ceramide(core of each type of sphingolipid), a compound consisting

of sphingosine and a fatty acid

CH3(CH2)12CH = CH CH OH|

H2N CH|

Sphingosine CH2OH

a) sphingomyelin

-

the white matter of the myelin sheath, a coating surrounding the nerve cells that increases the speed

of nerve impulses and insulates & protects the nerve cells

- located throughout the body but function principally in brain and nerve tissue; found in all cellmembranes and are important structural components of the myelin sheath that surrounds and

insulates cells of the central nervous system. Their role is essential to proper cerebral function and

nerve transmission.

- may also be classified as phospholipids since they contain phosphate group


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b) glycolipids (cerebrosides)

- occur primarily in the brain (7% of the solid matter) and in the myelin sheath of nerves

- composed of sphingosine, a fatty acid, and a sugar moiety; galactocerebrosides are almost entirely

found in the cell membranes of brain; various members of the class differ only with respect to their

constituent fatty acid

c) gangliosides

- similar to cerebrosides but contain oligosaccharide groups with one or more sialic acid residues.-

important in cell membranes as receptors for hormones, viruses, and several drugs

Diseases originating from abnormal metabolism

and accumulation of sphingoilipids

Disease Symptom Sphingolipid Enzyme

Tay-Sachs Blindness,muscles weak Ganglioside -hexose-aminidase A

Gauchers Liver & spleen enlarge,

MR

Glucocerebroside -glucosidase

Krabbes demyelation,MR Galactocerebroside -galactosidase

Nieman-Pick MR Sphingomyelin Sphingomyelinase


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3.3.3 steroids

-

complex derivatives of triterpenes; occur in plant and animal tissues, yeasts, and molds, but not in

bacteria, and may exist free or combined with fatty acids or carbohydrates

- for many years a great deal of controversy has surrounded various steroids. We worry about the

amount of cholesterol in the diet and the possible health effects. We are concerned about the anabolic

steroids by athletes who wish to build muscle mass and improve their performance. However,

members of this family of molecules derived from cholesterol have many important functions in the

body. The bile salts that aid in the emulsification and thus digestion of lipids are steroid molecules, as

are the sex hormones testosterone and estrone.- all steroids have cyclopentanoperhydrophenanthrene nucleus which consists of a completely

saturated phenanthrene moiety fused to a cyclopentane ring

a) sterols

-

steroids containing one, two, or three double bonds and one or more hydroxyl groups- differ from other lipids in that they do not undergo saponificatione.g., cholesterol

-

best known and most abundant (about 240 kg) steroid in the body; high occurrence in the brain and

nervous tissue; the principal constituent of gallstones from which it can be isolated as a white

crystalline solid. Its name is derived from this source (Greek, chole bile; steros solid)

- found in the membrane of most animal cells; readily soluble in the hydrophobic region of membranesand is the principal membrane lipid involved in regulation of the fluidity of the membrane.

- Cholesterol plays a vital biochemical role in chemical synthesis within the human body; it is the

starting material for the synthesis of numerous steroid hormones, vit. D, and bile salts; its presence in

the body is essential to life

-

Cholesterol is not necessary in the diet. The human body, mainly within the liver, synthesizes about 1

gram of cholesterol each day, an amount sufficient to meet the bodys biosynthetic needs.; when it isingested, the amount synthesized by the body is reduced; however, the reduction is less than the

amount ingested so that the body cholesterol level increases with dietary cholesterol level.

-

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.

Plaque is a mound of lipid material mixed with smooth muscle cells and calcium; much of the lipid

material in plaque is cholesterol. Cholesterol, in combination with other substances, appears to coat

the arteries, resulting in a narrowing. As this narrowing increases, more and more pressure is

necessary to ensure adequate blood flow. The pressure in the blood vessels increases, and high blood

pressure (hypertension) develops. Hypertension is also linked to heart disease.


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b) bile acids/ bile salts

-

salts of bile acids are the most important constituents of human bile; bile is produced by the liver,

stored in the gall bladder, and secreted into the intestine

- a bile salt is an emulsifying agent that makes dietary lipids soluble in the aqueous environment of the

digestive tract; main function is to facilitate the absorption of fats through the wall of the intestine

- cholic acidis the most abundant bile acid

- bile salts are cholesterol oxidation products where cholesterol is oxidized to carboxylic acid which is

then bonded to an amino acid by amide linkage; the two principal bile salts are sodium glyocholate

(glycine is the amino acid) and sodium taurocholate(taurine is the amino acid).

c) steroid hormones

- a hormone is a chemical messenger secreted by specific glands and carried by the blood to a targettissue, where it triggers a particular response. Hormones serve as a means of communication between

various tissues; hormones, together with the central nervous system(CNS), are the regulators of body

reactions like metabolism, growth and development, etc.adrenocortical hormones

- produced by the outer part, or cortex, of the adrenal glands, small organs on top of each kidney- regulate numerous biochemical processes in the body; effect the metabolism of foodstuffs and control

inflammation and allergies (glucocorticoids); maintain the proper balance of electrolytes Na+and K

+

ions in cells (mineralocorticoids)

- adrenal hormones are widely used in the treatment of rheumatic fever and rheumatoid arthritis.

` Aldosterone Cortisol Cortisone Prednisolone(a mineralocorticoid) (a glucocorticoid) (an anti-inflammatory drug) (an anti-inflammmatory drug)

sex hormones

a. androgens - the male sex hormones, the most important is testosterone; synthesized in the testes and

adrenal cortex; regulate the development of 1oand 2

omale sex characteristics

b.

estrogens - the female sex hormones; synthesized in the ovaries and adrenal cortex; regulate the

development of 1o and 2

o female sex characteristics and for regulation of the menstrual cycle; also

stimulate the development of the mammary glands during pregnancy and induce estrus (heat) in animals.

c. progestins - the pregnancy hormones; for normal pregnancy; produced in the ovaries and in the placenta,

it is responsible for both the successful initiation and completion of pregnancy;

it prepares the lining of the uterus (endometrium) for implantation of the ovum. Once the egg is attached,

progesterone is involved in the development of the fetus and also plays a role in the suppression of further

ovulation during pregnancy

Estradiol Testosterone(the principal estrogen; (the principal androgen; (the principal progestin;

for female sex characteristics) for male sex characteristics) for normal pregnancy)


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- increased knowledge of the structures and functions of sex hormones has led to the development of anumber of synthetic steroids; among the best known are oral contraceptives and anabolic agents.

- anabolic steroid abuse can result in a wide range of harmful side effects including some that are

physically unattractive such as acne and breast development in men; the side effects can also be life-

threatening, such as liver cancer and heart attacks.

Norethynodrel RU-486 Methandrostenolone(a synthetic progestin) (mifepristone; a synthetic abortion drug) (a synthetic tissue-building steroid)

3.4 COMPLEX LIPIDS (lipids that are bonded to other types of molecules)

3.4.1 phospholipids

- see phosphoglycerides, & sphingomyelin;

- fatty acids, glycerol, H3PO4and a nitrogenous base on hydrolysis

3.4.2 glycolipids

- see cerebrosides, & gangliosides; fatty acids, sphingosine, and a carbohydrate on hydrolysis

3.4.3 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 fas). The shell around the core consists of polar lipids and proteins

Four major classes:

a) chylomicron

-

transports dietary TAG, cholesterol, etc. from the intestines to other tissues (adipose tissue and liver),

except kidney; 90% triglyceride + 1% protein

b) very low density lipoprotein (VLDL)

-

bind triglycerides synthesized in the liver and carry them to adipose tissues and other tissues forstorage; 50% triglyceride + 10% protein

c) low density lipoprotein (LDL)

- carry cholesterol to peripheral (adipose) tissues and help regulate cholesterol levels in those tissues;

regulates de novosynthesis of cholesterol; 10% triglyceride + 20% proteins

- liver LDL receptors enable large amounts of cholesterol to be removed from the blood, thus ensuringlow concentrations of cholesterol in plasma. Other factors being equal, the person with the most

lipoprotein receptors will be the least vulnerable to a high-cholesterol diet and will have the least

likelihood of developing atherosclerosis.

d) high density lipoprotein (HDL)

-

transports cholesterol from peripheral tissues to the liver; 5% triglyceride + 50% protein


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- there is evidence that high levels of HDL in the blood help to reduce the incidence of atherosclerosis.This may be due to the fact that HDL carries cholesterol from the peripheral tissues back to the liver. In

the liver, some of the cholesterol is used for bile synthesis and secreted into the intestine, from which

it is secreted.

* High VLDL and low HDL are risk factors in atherosclerosis and predisposes toward strokes

and coronary infarction

* When the level of LDL is high in relation to the HDL there is a high risk of coronary disease

(LDL/HDL above 4.0)

Structure of Cell Membranes

Living cells contain ~10,000 kinds of molecules in an aqueous environment confined by a cell

membraneA cell membrane is a structure that separates a cells aqueous-based contents from the aqueous

environment surrounding the cell. Besides its separation function, a cell membrane also controls the

movement of substances into and out of the cell. Up to 80% of the mass of a cell membrane is lipid material;

hence the consideration of cell membranes in the chapter on lipids. Cell membranes contain variousphosphoacylglycerols (phospholipids) and sphingolipids. The head and two tail structure of these lipids is of

key importance to an understanding of the lipid bilayer structural feature of the cell membranes. A lipid

bilayer is ~6-9 nanometers, two-layer thick structure of lipid molecules aligned so that the nonpolar tails of the

lipids form the interior of the structure and the polar heads form the outside surfaces.

Cholesterol molecules are also components of cell membranes. They regulate membrane fluidity.

Because of their compact shape cholesterol molecules fit between the fatty acid side chains of the lipid bilayer,

restricting movement of the fatty acid side chains and making the bilayer more rigid.

Proteins are also components of the lipid bilayers. They are responsible for moving substances such as

nutrients and electrolytes across the membrane, and they also act as receptors that bind enzymes, hormones,

and neurotransmitters.


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Small carbohydrate molecules are also components of cell membranes. They are found on the outer

membrane surface covalently bonded to protein molecules (a glycoprotein) or lipid molecules (a glycolipid).

They function as markers, substances that play key roles in the process by which different cells recognize each

other.

Characteristics of cell membrane

1. Fluidity - increasing percentage of unsaturated fats leads to more fluidity of the membrane. Lateral

movement of phospholipids is rapid. Flip-flop, from one side to the other is rare.

2. Selective permeability - the hydrophobic nature of the membrane makes it impenetrable to the transport of

ionic and polar substances. Membrane proteins regulate passage of ionic and polar substances by

binding to the polar compound or by providing a channel.

3. Self-sealing capacity - a break in the membrane immediately and spontaneously seals.

4. Asymmetry - bulkier molecules occur more often in the inner side of the membrane.

5. Strong and rigid

Membrane Transport Mechanisms1. Passive transport there is no direct energy input

a) simple diffusion molecules move through a membrane down a concentration gradient (toward

lower concentration)

b) facilitated diffusion molecules move through protein channels in membrane; permease, a

membrane protein assists in diffusion

2. Active transport require energy

a) primary energy is provided by ATP (e.g., Na+- K

+pump system)

b) secondary concentration gradients generated by the primary active transport are used to move

substances across membranes (e.g., Na+gradient from Na

+- K

+pump system is used to transport

glucose in kidney tubules.


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Saponifiable and Nonsaponifiable Lipids

- saponifiable lipids produce fatty acids upon treatment with NaOH

- include fats/oils, waxes, phospholipids (glycerophospholipid, sphingolipid), and glycolipids

- nonsaponifiable lipids include steroids and terpenes, cant be hydrolyzed by NaOH

3.5 MISCELLANEOUS LIPIDS

- Vitamin = vital amine, from an early belief that vitamins might all be amines

- the body cannot make vitamins; they must be in the diet

-

the term vitamin applies to any compound or a closely related group of compounds satisfying thefollowing criteria:

it is organic rather than inorganic or an element

it cannot be synthesized at all (or at least in sufficient amounts) by the body

its absence causes a specific vitamin deficiency disease

its presence is essential to normal growth and health

it is present in foods in small concentrations, and it is not a carbohydrate, a saponifiable lipid, an

amino acid, or a protein

-

function in the body either as precursors for coenzymes or as coenzymes themselves. Most of the

nutritionally required minerals function as cofactors to enzymes

3.5.1 Terpenes or isoprenoids

-

are polyenes, alkenes with several double bonds, are found in nature; built from one ormore five-carbon units called isoprene, CH2=CCH3CH=CH3

- steroids, chlorophyll & carotenoid pigments, fat-soluble vitamins.

3.5.2 Fat-soluble vitamins

- fat-soluble vitamins are largely hydrocarbon-like, and water-soluble vitamins are polar or ionic-

fat-soluble vitamins occur in the lipid fractions of their sources

- their molecules have double bonds or phenol rings, so oxidizing agents readily attack them; hence,

these vitamins are destroyed by prolonged exposures to air or to the organic peroxides that develop in

fats and oils turning rancid. These are actually good properties; because the fat-soluble vitamins are

easily oxidized, they destroy oxidizing agents (which are involved in the development of coronary

heart disease, genetic mutations, and cancer)

a) Vitamin A (retinol)

-

a primary alcohol of molecular formula C20H30O; occur only in the animal world, where the best

sources are cod-liver oil and other fish-liver oils, animal liver and dairy products

- provitamin A is found in the plant world in the form of carotenes. Provitamins have no vitamin

activity; however, after ingestion in the diet, -carotene is cleaved at the central carbon-carbon double

bond to give 2 molecules of Vit. A.

- the major action of Vit. A is probably on epithelial cells, particularly those of the mucous membranesof the eye, oral cavity, digestive, respiratory, reproductive and genitourinary tracts. Without adequate

supplies of vitamin A these membranes become hard and dry (keratinized)


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- though harmless, excessive -carotene ingestion makes the skin yellow or orange. In distinction to

observation in cases of jaundice, the sclera remains white

- Vit. A (retinol) is oxidized to retinal, or vitamin A-aldehyde, which combines with opsin, a protein, toform rhodopsin, the light-seeing pigment in the retina.

- most obvious effects of vit. A deficiency is on the eye. The cells of the tear glands become keratinized

and stop secreting tears, & the external surface of the eye becomes dry, dull, and often scaly. Without

tears to remove bacteria, the eye is susceptible to serious infection, which if not treated on time,

blindness resultsxerophthalmia

-

a less serious condition is night blindness, the inability to see dim light or to adapt to subdued light

b) Vitamin D (solar vitamin)

-

a 20alcohol; C28H44O; the antirachitic vitamin; primary effect is on Ca

2+metabolism, it increases the

absorption of Ca2+

from the intestinal tract; necessary for the normal calcification of bone tissue

- compounds with antirachitic activity:

* Vitamin D2or ergocalciferol (vegetable origin)

* Vitamin D3or cholecalciferol (animal origin); fish-liver oils richest source

Ergocalciferol (D2) Cholecalciferol (D3)

- pigment in the skin, 7-dehydrocholesterol, is a provitamin D; when irradiated by the sunlight becomes

converted to Vit. D3

- humans exposed to sunlight year-round do not require dietary Vit. D

c) Vitamin E

-

a group of about seven compounds of similar structure; of these, -tocopherol has the greatest

potency; tocopherolGreek, promoter of childbirth; the antisterility vitamin

- functions in the body as an antioxidant in that it inhibits the oxidation of unsatd fatty acids by O2


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-

occurs in fish oil, cottonseed and peanut oil, green leafy vegetables; the richest source is wheat germoil

-tocopherol

d) Vitamin K

- essential for the synthesis of prothrombin in the liver; the antihemorrhagic vitamin

Vitamin K2

(n may be 5, 6, or 8)

- synthesized by intestinal bacteria-

deficiency may occur during the first few days after birth, because newborns lack the intestinal

bacteria that produce Vit. K and because they have no store of Vit. K (it does not cross the placenta)

- hence, all newborns are given vitamin K injection to prevent hemorrhagic disease- deficiency may also occur following antibiotic therapy that sterilizes the gut


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chapter iii. lipids-3b (5)

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