LIPIDS

1. Lipids – Lipids are structurally heterogenous group of compounds of biochemical origin that are soluble in

nonpolar organic solvents and insoluble in water. Lipids are divided into five major types on the basis of biochemical

function: energy-storage lipids, emulsification lipids, membrane lipids, messenger lipids, and protective-coating

lipids.

2. Types of fatty acids – Fatty acids are monocarboxylic acids that contain long, unbranched carbon chains. The

carbon chain may be saturated, monounsaturated, or polyunsaturated. Length of carbon chain, degree of

unsaturation, and location of the unsaturation influence the properties of fatty acids. Omega-3 and omega-6 fatty

acids are unsaturated fatty acids with the endmost double bond three and six carbons, respectively, away from the

methyl end of the carbon chain.

3. Triacylglycerols – Triacylglycerols are energy-storage lipids that contain one or more fatty acids, a phosphate

group, a platform molecule to which the fatty acid(s) and phosphate group are attached, and an alcohol attached to

the phosphate group. The platform molecule is either glycerol (glycerophospholipids) or sphingosine

(sphingophospholipids). Phospholipids has a “head and two tails” structure. Lecithins, cephalins, and sphingomyelins

are types of phospholipids.

4. Sphingoglycolipids – Sphingoglycolipids are membrane-lipids in which a fatty acid and a mono- or oligosaccharide

are attached to the platform molecule sphingosine. Cerebrosides and gangliosides are types of sphingoglycolipids.

5. Cholesterol – Cholesterol is a membrane-lipid whose structure contains a steroid nucleus. It is the most abundant

type of steroid. Besides its membrane functions, it also serves as a precursor for several other types of lipids.

6. Lipid bilayer – A lipid bilayer is the fundamental structure associated with a cell membrane. It is a two-layer

structure of lipid molecules (mostly phospholipids and glycolipids) in which the nonpolar tails of the lipids are in the interior and the polar heads are on the outside surfaces.

7. Membrane transport mechanism – The transport mechanisms by which molecules enter and leave the cells include passive transport, facilitated transport, and active transport. Passive and facilitated transport follow a concentration gradient and do not involve cellular energy expenditure. Active transport involves movement against

a concentration gradient and requires the expenditure of cellular energy.

8. Bile acids – Bile acids are cholesterol derivatives that functions as emulsification lipids. They cause dietary lipids to be soluble in the aqueous environment of the digestive tract. Cholic acid and deoxycholic acids are the major types of bile acids.

9. Steroid hormones – Steroid hormones are cholesterol derivatives that function as messenger lipids. The two

major types steroid hormones are sex hormones and adrenocorticoid hormones.

10. Eicosanoids – Eicosanoids are fatty acid derivatives that function as messenger lipids. The major classes of

eicosanoids are prostaglandins, thromboxanes, and leukotrienes.

Family of Lipids

• Lipids: family of biochemicals that are soluble in non-polar organic solvents (hexane, ether, benzene) but

not in water.

• Classes:

– Waxes: fatty acid + a long-chain alcohol

– Triglycerides (fats & oils): glycerol + 3 fatty acids

– Phospholipids: glycerol + 2 fatty acids + phosphate + amino alcohol

– Glycolipids: glycerol or sphingosine + fatty acid + monosaccharide

– Steroids: 3 cyclohexane + 1 cyclopentane fused together

CLASSIFICATION

A. Based on Biochemical Function:

1. Energy-storage lipids (triacylglycerols)

2. Membrane lipids (phospholipids, sphingoglycolipids, and cholesterol)

3. Emulsification lipids (bile acids)

4. Messenger lipids (steroid hormones and eicosanoids)

5. Protective-coating lipids (biological waxes)

B. Based Upon Whether or Not Saponification Occurs When Lipid is Placed in Basic Aqueous Solution:

1. Saponifiable lipids (triacylglycerols, phospholipids, sphingoglycolipids, and biological waxes)

2. Nonsaponifiable lipids (cholesterol, steroid hormones, bile acids, and eicosanoids)

1. ENERGY STORAGE LIPIDS

• Triacylglycerol/Triglycerides – a lipid formed by esterification of three fatty acids to a glycerol molecule.

• Concentrated in adipocytes (cells)

• Adipose tissues – under the skin, abdominal cavity, mammary glands, various organs.

• Glycerol + 3 fatty acids

Fatty Acids

• Fatty acids: monocarboxylic acids

• Form fats and oils (usually have an even number of carbons)

• Saturated fatty acid: all single bonds

• Unsaturated fatty acid: at least 1 double bond (monounsaturated)

– Polyunsaturated: two or more double bonds

– Most double bonds are cis- bonds that cause a bend in the chain

• Short-chain fatty acids: C4 and C6

• Medium-chain fatty acids: C8 and C10

• Long-chain fatty acids: C12 and C24

Saturated Fatty Acids (BAD)

• 2C – acetic acid (water soluble)

• 4C – butyric acid (water soluble)

• 6C – caproic acid

• 8C – caprylic acid

• 10C – capric acid

• 12C – lauric acid

• 14C – myristic acid

• 16C – palmitic acid (most abundant in humans)

• 18C – stearic acid

• 20C – arachidic acid

Monounsaturated Fatty Acids (GOOD)

• Palmitoleic acid (16:1 9)

• Oleic acid (18:1 9)

Polyunsaturated Fatty Acids (BOTH)

• Linoleic acid (18:2 9,12) 6 (essential)

• Linolenic acid (18:3 9,12,15) 3 (essential)

• Arachidonic acid (20:4 5,8,11,14) 6

• EPA (eicosapentaenoic) (20:5 5,8,11,14,17) 3

• DHA (docosahexaenoic) (22:6 4,7,10,13,16,19) 3

4,7,10,13,16,19) 3

Properties

• Saturated fatty acids are solid at room temperature and have a high melting point.

• Unsaturated fatty acids are liquid at room temperature and have a low melting point.

Fatty Acids in the Human Body

• Essential fatty acids: polyunsaturated fatty acids that the body cannot make.

– Important: linoleic and linolenic acids

– Lack of in infants cause skin dermatitis

• Arachidonic acid is converted into prostaglandins which serve as local chemical messengers

Fats: more saturated fatty acids; solid at room temperature (cod liver oil)

• Oils: more unsaturated fatty acids; liquid at room temperature (Theobroma oil)

Properties

• Hydrogenation (adding H2)

– Converts a double bond to a single bond

– Changes liquid oil to a soft semisolid fat

• Oxidation to carboxylic acid

– Fat/oil becomes “rancid” with a disagreeable odor

o Some oils have antioxidants to slow oxidation

o Refrigeration in an air-tight container slows oxidation

– Microorganisms oxidize body oils during exercise

• Saponification (basic) with heat and strong base produces a glycerol and fatty acid salts

• Hydrolysis (acid)

– Requires a strong acid catalyst or lipase enzyme

– Produces 1 glycerol + 3 fatty acids

Phospholipids

alcohol + phosphate + fatty acid

1. Glycerophospholipid: glycerol + 2 fatty acids + phosphate/alcohol

• Lecithins and cephalins abundant in brain & nerve tissue, egg yolks, wheat germ, and yeast.

• Forms cell membranes.

2. Sphingophospholipid: sphingosine + 1 fatty acid + phosphate/ alcohol

• A phospholipid with sphingosine instead of the glycerol.

• Important in the myelin sheath that surrounds most nerve fibers.

Sphingoglycolipids

sphingosine + fatty acid + carbohydrates

• Cerebrosides – contain a single monosaccharide unit (glucose/galactose). Present in brain and myelin sheath

of nerves.

• Gangliosides – branched chain of 7 monosaccharides. Present in brain and myelin sheath of nerves.

Cholesterol

Steroids – a lipid whose structure is based on a fused-ring system that involves three 6-membered rings and one 5-

membered ring.

• cyclopentanoperhydrophenanthrene ring

• egg yolks and organ meats

Cholesterol

• Synthesized in the liver

• Gives strength to cell membranes

• In myelin sheath, bile salts, vitamin D produced by the skin

• Excess in body leads to gall stones and plaque in the arteries

3. EMULSIFICATION LIPIDS

• Bile acids

Bile acids – cholesterol derivative that functions as a lipid-emulsifying agent in the aqueous environment of the

digestive tract.

• Synthesized from cholesterol in the liver.

• Stored in gallbladder.

• Sent to the digestive tract to emulsify fats during digestion.

• Promotes absorption of cholesterol in the digestive tract.

• TYPES:

1. Cholic acid

2. 12- Deoxycholic acid

3. 7- Deoxycholic acid

4. MESSENGER LIPIDS

• Steroid hormones

• Eicosanoids

Steroid Hormones

• Hormone – biochemical substance, produced by a ductless gland, that has a messenger function. Serves as

communication between various tissues.

• Steroid hormone – hormone that is a cholesterol derivative.

– Two types: Sex hormones and Adrenocorticoid hormones

Sex Hormones

1. Estrogens (Estradiol) – female sex hormone

2. Androgens (Testosterone) – male sex hormones

3. Progestins (Progesterone) – pregnancy hormones

– Anabolic steroid, derivative of testosterone

o Prevents breakdown of worn out muscle cells.

o Has dangerous side effects.

Adrenocorticoid Hormones

1. Mineralocorticoids – control the balance of Na+ and K+ ions in cells and body fluids

2. Glucocorticoids – control glucose metabolism and counteract inflammation

Eicosanoids

Eicosanoids – an oxygenated C20-fatty acid derivative that functions as messenger lipid

• Eikos – “twenty”

• Arachidonic acid (20:4 fatty acid)

• 3 Types: Prostaglandins, Thromboxanes, Leukotrienes

Prostaglandins

• 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

• Intensifying pain

• Enhancing inflammation responses

Thromboxanes

• Promote the formation of blood clots

Leukotrienes (leukocytes)

• Inflammatory and hypersensitivity (allergy) responses

PROTECTIVE-COATING LIPIDS

• Biological waxes

Biological Waxes – lipid that is a monoester of a long-chain fatty acid and a long-chain alcohol.

Long chain fatty acid + Long chain alcohol

• Protects hair and skin to keep it pliable and lubricated

• Animal fur/feathers – water repellant

• Examples

– Carnauba Wax from palm tree (automobile wax, boat wax, floor wax, shoe wax)

– Lanolin from sheep wool (used in creams/ointments to enhance retention of water)

– Candles

Lipids in the Body

• Lipoproteins

– Water insoluble lipids must be delivered to all parts of the body by the water-based blood

– Body packages lipids in membrane with proteins to make the lipids water soluble

• Types: VLDL, LDL, HDL, chylomicrons

– HDL on the way to the liver is “good cholesterol”

– LDL on way to the cells is prone to deposit in blood vessels

– Enzyme in blood capillaries releases lipids from the protein carrier