LIPIDS CHEMICAL COMPOSITION OF LIPIDS
Lipids are a group of substances made up of carbon, hydrogen and oxygen but in different ratios from those of carbohydrates. Lipids have more carbohydrates and hydrogen in proportion to their oxygen. The lower the amount of oxygen in relation to carbon and hydrogen results in fats being a more concentrated source of energy than carbohydrates.
Most of the energy from fats is provided by a class of lipids called triglycerides. These are lipids that are made up of two components i.e. Glycerol (a trihydric alcohol and fatty acid). Fatty Acids
Almost all naturally occurring lipids yield fatty acids on hydrolysis. Some yield only one fatty acid per molecule while others may yield as many as three fatty acids. In other words, fatty acids are the building blocks of lipids.
Fatty acids are compounds that are composed of an even numbered carbon chain of from about 4-20 carbon atoms in the length. They have a single carboxyl group and a long hydro-carbon chain which is the one responsible for the oily nature of the lipids. The long hydrocarbon chain is said to be hydrophobic because of its non-polar characteristics while the carboxyl end is said to be hydrophilic because it is polar (negatively charged).
Fatty acids with no double bonds in their structural are called Saturated fatty acids and they include:
 Butryric acid (Butatonoic acid C-4)
 Caproic acid (Hexanoic C-6)
 Capric acid (Octanoic acid C-8)
Fatty acids are named on the basis of hydrocarbon from which they are derived. The saturated fatty acids end with a suffix anoic (e.g. octanoic – 8c). fatty acids with double bonds are called unsaturated fatty acids and they have the suffix –enoic e.g. octadenoic.
Saturated Fatty Acids
The general formula of saturated fatty acids is CnH2n+1COOH. These fatty acids have two hydrogen atoms attached to each carbon atom in the chain i.e. they have the maximum number possible. They can be considered to be based on acetic acid which is the first member of the series. Examples of saturated fatty acids are:

Name Formula no. of C atoms where found
Acetic acid CH3COOH 2
Butyric acid C3H7COOH 4 butter fat
Caproic acid C5H11COOH 6 butter fat
Caprylic acid C7H15COOH 8 butter & coconut fat
Lauric acid C11H23COOH 12 palm & coconut oil
Myristic C13H27COOH 14 animal & vegetable fat
Palmitic C15H31COOH 16 “ & coconut oil
Stearic C17H35COOH 18 plant & animal fat
Arachidic C19H39COOH 20 peanut oil
A few branched chain fatty acids have been isolated in both plants & animal sources. Unsaturated Fatty Acids
These fatty acids have fewer than maximum number of hydrogen atoms attached to the carbon chain. They can further be subclassified into either mono-unsaturated (MUFA’s) or poly-unsaturated (PUFA’s).
A. Mono-unsaturated Fatty Acids
These are fatty acids that lack one hydrogen atom from each of two adjacent carbon atoms resulting in a double bond between two carbon atoms. To cope with the loss of hydrogen atoms, the two carbons form an additional carbon bond between themselves making a double bond C=C in the carbon chain. Their general formula is CnH2n-1. Examples are oleic and palmitoteic acids found in nearly all cells.
B. Poly-unsaturated Fatty Acids
These are fatty acids in which double bonds between carbon atoms appear in 2 or more places. e.g. linoleic acid (18 carbons 2 double bonds), linolenic acid (18 carbons 3 double bonds) and Arachidonic acid (18 carbons 4 double bonds).

Unsaturated FA No. of carbon atoms No. of bonds Sources
Palmitoleic
Olleic acid
Linoleic
Linolenic acid
Arachidonic 16
18
18
18
20 1
1
2
3
4 Butter & most fats & oils
Most fats & oils
Many seed oils e.g peanut, corn
Cotton seed, soya bean oils
Animal fats & peanut oil LIPIDS
Fats are a combination of one to three fatty acids with a glycerol molecule. Glycerol is an alcohol with 3-OH groups otherwise known as trihydric alcohol. If a fat contains only one fatty acid, it is known as a monoglyceride. Diglycerides are fats with two fatty acids attached to any two positions out of three positions on the glycerol molecule. Triglycerides are the fats which have fatty acids at all possible spots on the glycerol. Glycerol molecule structure

Fatty Acids
These are long chained hydrocarbons with the group –COOH on one end and a methyl group (CH3) on the other end. i.e. CH3………….. COOH
Saturated: CH3CH2CH2CH2COOH ESSENTIAL FATTY ACIDS
Essential fatty acids refer to two long-chain fatty acids namely Linoleic and Linolenic. They cannot be synthesized by the body and must be provided in the diet. They serve as precursors of hormone-like substances called Prostaglands, which are known to cure many ailments and are important in both human and veterinary medicine. The two fatty acids are known to restore growth of young animals that have been fed on a diet very low in fat. PROSTAGLANDINS
They are a group of compounds found in seminal plasma and other tissues. They are of interest because of their pharmacological and biochemical activity on the smooth muscles, blood vessels and adipose tissues. Prostaglandins are synthesized from arachidonic acid in the lab e.g. Prostaglandin E2 or PG E2
Essential fatty acids are imprecursors in the biosynthesis of these fatty acid derivatives called Prostaglandins. They are hormone-like compounds which affect a number of physiological activities even in traces/ very small amounts. Biological Action of Prostaglandins
PGs act as local hormones in their functions. They perform different functions in different tissues. Sometimes they bring about opposite effect on the same tissues. Excess production of prostaglandins may cause many symptoms such as pain, fever, nausea, vomiting and inflammation.
Prostaglandins are involved in a number of biological functions e.g.
i. Regulation of blood pressure. They are vasodilators so they act as agents in treatment of high blood pressure/ hypertension. ii. Inflammation: PGE1 and PGE2 can induce inflammation. They are natural mediators of inflammatory reactions of rheumatoid arthritis (joints), psoriasis, and conjuctives. Corticosteroids are frequently used to treat these inflammations e.g. Calvasion/ Dexamethasone. These drugs inhibit prostaglandins synthesis. iii. Reproduction: PGE2 & PGE3 are used for medical termination of pregnancy and induction of labor. They are also administered to cattle to induce oestrus. iv. Pain & fever: PGE2 along with histamine and bradykin cause pain. Migranes are also due to PGE2. Aspirins and other non-steroidals are PGE2 inhibitors and are therefore used to control fever and relieve pain.
v. They are used to treat gastric ulcers. vi. PGE1 & PGE2 are used to treat Asthma. vii. PGE promotes urine output by increasing glomerular filtration rate (GFR). This action also increases excretion of Na+ & K+. viii. Macrophages secrete PGE which decreases the immunological functions of B- and T- Lymphocytes.
BIOMEDICAL APPLICATION OF PROSTAGLANDINS
Prostaglandins are widely used as therapeutics.
1. They are useful in the treatment of: ulcers, hypertension and thrombosis.
2. They are used in medical termination of pregnancy, prevention of conception and induction of labor.
3. In veterinary field, they are used in:
i. Embryo transfer ii. Synchronization of oestrus
Aspirins and Ibuprofen are used in controlling fever, pain, migranes and inflammation. Prostaglandins are responsible for causing these ailments and the two drugs are prostaglandin inhibitors.
PHYSICAL & CHEMICAL PROPERTIES OF FATTY ACIDS
1. Fatty acids from butyric acid (C4) to Caproic acids (C10) are liquids at ordinary temperature while higher members have higher members above C10 are solids.
2. The melting point of fatty acids increases with chain length.
NB. Fatty acids are classified on the basis of the number of carbons in their chain.
i. Short-chain fatty acids have 2-6 carbons (these are rare) ii. Medium chain fatty acids have 8-12 carbon atoms. They make up 4 – 19% of fatty acids in foods. iii. Long chain fatty acids have more than 16 carbon atoms. They are the most common
3. Most short chain fatty acids are steam volatile. This property decreases with chain length.
4. Most fatty acids are only soluble in hot alcohol.
5. Saturated and unsaturated fatty acids have quite different conformations.
6. Solubility in water decreases with chain length. Butyric acid & lower acids are soluble in water but as the chain length increases, this property decreases or is not there at all.
7. Fatty acids form salts with sodium and potassium to form products that are commonly called SOAPS. Soaps are excellent cleansing and emulsifying agents. The sodium salts of oleic, palmitic and stearic acids form commercial soaps which are soluble in water & alcohol but insoluble in ether and benzene. The salts of calcium and magnesium are quite insoluble in water and precipitate from soluble soap solutions in hard water that contains these elements.
8. Fatty acids can be reduced by hydrogen to their corresponding alcohol at very high temperatures.
9. Unsaturated fatty acids can be oxidized at the double bond by alkalines to give dihydroxy fatty acids which can be later broken down to simpler fatty acids.
10. Atmospheric oxygen can oxidize fats and oils which contain highly unsaturated fatty acids to form products called resins which give the fats the characteristic of odour and flavor. This condition is known as rancidity.
11. Halogens e.g. iodine, bromine can react with unsaturated fatty acids to give halogenated derivatives producing saturated halogenated glycerides.
12. Hydrolysis of a lipid such as triglycerides may be accomplished through action of the enzymes lipases to yield fatty acids and glycerol.
Lipids are esters of fatty acids and glycerol.

Simple fats can be converted to cpd by adding other groups. If a phosphate group is added to the reaction, then we will produce other types of fats called phospholipids. If carbohydrates are added, then we get Glycolipids.
Lipids can be either fats or liquids. Fats are solids at room temperature while oils are liquid. These differences are brought about by unsaturation or presence of double bonds. Molecules with only saturated fatty acids are more closely packed and together are therefore solids while molecules with double bonds gives a far apart packing resulting in a liquid state.
In most cases, fats are from animal origin while oils are from vegetable origin. e.g. palm oil (palmitic acid) is liquid and yellowish in colour and is from plant source. These oils can be hardened into fats by adding hydrogen, a process called hydrogenation (kasuku kimbo e.t.c) the oils then become fats which are easy to handle and transport.
Fat as a source of energy Energy is stored in the body mainly as saturated fatty acids in the adipose tissue. The liver stores energy in the form of glycogen but these supplies of glycogen can be exhausted after a few hours of tasting.
Fat stores are exhausted at a very slow pace and are particular useful in times of fasting (man) or drought (animals). One gram of fat can produce 9.5K Cal/joules while one gram of glycogen produce about 3.8 Kcal. Hence the amount of energy from that is greater than that from carbohydrate. It is also economical in terms of space/bulk because fats can be stored in a much more compact form than the glycogen. Classification of Lipids
They are classified mainly on their backbone structure namely simple compound and derived lipids. Simple Lipids
These are esters of fatty acids with various alcohols such as glycerol some do not contain fatty acids hence are said to be non-saponifiable e.g terpees, steroids and prostaglandins. They are also called neutral fats or triglycerides. They are colorless, odorless, tasteless substances. They are called fats when solids and oils when liquids.
Fats have well defined melting and solidifying points. They are characterized by their saponification number and iodine number. Common neutral fats include; beef fat, butter fat, linseed and coconut oils.
On hydrolysis with water at high temperatures and pressure, in an autoclave, in the presence of acids, triglycerides may yield three molecules of fatty acids and one molecule of glycerol waxes. Ester of fatty acids with high molecular weight with higher monohydric alcohols (not glycerols) are called waxes. They are found in insect secretive protective coatings on animal furs and plant leaves.
In human body, the common waxes found are cholesterol esters. Waxes are complex mixtures that are solid at room temperatures and are not easily hydrolysed as fats. Examples are:
i. Bees wax- are esters of palmitic acid & myricyl alcohol (C30H61OH) ii. Lanoline/ wool fat: are esters of cholesterol with stearic acid, palmitic acid and oleic acids. Lanoline is useful in manufacture of cosmetic creams and ointments. iii. Spermaceli: Esters of palmitic acid with cetyl alcohol (C16H33OH). It is an oil from the head of the sperm of whales and is useful in the manfacture of polishes, ointments and candles.
Compound Lipids
Esters of fatty acids with alcohols and contain other additional groups. Fats thus combine with other non fatty prosthetic groups. Examples are phospholipids & Glycolipids Phospholipids
Lipids containing in addition to fatty acids & alcohol, a phosphoric acid residue, nitrogenous base and other substituents. They are present in large amounts in the brain, nerve tissue, kidney, liver, pancrease and heart. They are also called glycerol phosphatides. Examples are:
 Lecithin (phosphatidyl / choline)
 Cephalin (phosphatidyl / Ethanolamine)
 Phosphatidyl serine
 Sphongolipids
 Inosital phospholipids ( or lipositol)
 Plasmalogens
Phospholipids are found in every cellular organism where they form major lipid components in the cell membrane. Egg yolk, some seeds and brain tissues are very rich in phospholipids (more in the brain).

In formation of phospholipids, the phosphate group first attaches itself to the glycerol to form Glycerol phosphate which then reacts with the lipid to form phospholipids.
BIOLOGICAL FUNCTIONS OF PHOSPHOLIPIDS
1. They act as carriers of inorganic ions across the cell membranes,
2. They help in blood clotting.
3. They act as prosthetic groups to certain enzymes.
4. They form the structure of membranes of cell wall, myelin sheath and mitochondria.
5. They increase the rate of fatty acid oxidation during fat/lipid metabolism.
CHOLESTEROL
This is animal sterol. It is found in all animal tissues but not in plant tissue. The highest concentration occurs in the brain & nervous tissue, the corpus luteum, adrenal cortex, testis & egg yolk. Small amounts are present in the liver, kidney & spleen. It has a ring structure consisting of 19 carbon atoms & a side chain with 8 carbon atoms. The formula is C27H46O. Role
Cholesterol serves as an insulation cover in the nerve tissue therefore it is abundant in the spinal cord and nerves (play a key role in transmission of messages & impulses). It is a poor conductor of heat thus has good insulating properties & even insulates on electrical changes.
Ergosterol
It is the principal sterol of fungi & yeast (called mycosterols). It yields vitamin D2 and other derivatives when irradiated by U-V rays/ sunlight.
BILE ACIDS & BILE SALTS
Bile acids are composed of four steroids. The most abundant is cholic acid & chenoide oxycholic acids. These salts are formed in the liver from cholesterol through several intermediate steps.
Bile salts are useful forms of sterols (steroids) and are surface active agents which act by lowering the surface tension and thus tending to emulsify or breakdown fats.
They serve as detergents to emulsify triglycerides/ fats in the intestines. Bile contains a large amount of alkali ions such as Na & K and has a high pH. The bile salts play a major role in digestion and absorption of fats.
BIOLOGICAL FUNCTIONS OF LIPIDS
Lipids together with carbohydrates are important dietary constituents of many animals because of the fat soluble vitamins and also because of their high energy value. They have several important biological functions that the serve.
1. They serve as structural components of membranes of the cell e.g. lipoproteins.
2. They serve as chief storage form of energy.
3. They serve as insulating materials in subcutaneous tissues & around certain organs e.g. kidney fat. They act as protective cushions for these organs.
4. They are important components concerned with cell to cell recognition.
5. Phospholipids and cholesterol are constituents of membrane structure and they regulate membrane permeability.
6. Lipids serve as a source of fat soluble vitamins namely vits A, D, E & K. (Lipids act as vehicles of natural fat soluble vits A, D, E & K.
7. Lipids such as steroids and prostaglandins are important cellular metabolic regulators.
8. Lipids participate in metabolism in the electron transport chain in that they form component of inner mitochondrial membrane where these reactions take place.
REACTION AT DOUBLE BONDS
Double bonds are more reactive than single bonds and several reactions occur at the double bonds of fatty acids. These are:
1. Iodination
This is the addition of iodine to a double bond and is used as an indicator of the degree of saturation. One molecule of iodine (two atoms) adds to each bonds. Iodine numbers indicate the number of grams of iodine absorbed by 100g of fat and provide an index to the relative saturation of the predominant fatty acids. Iodine number of some common fats and oils are: Butter – 26-38 Lard – 50-65 Olive oil – 80-90 Coconut used oil - 105-115 Corn oil – 115-124 Soy bean oil – 130-138
2. Hydrogenation
This is a chemical process by which hydrogenation is added to unsaturated or polyunsaturated fats to reduce the number of double bonds, making them more saturated (solids) and more resistant to oxidation (rancidity). For example, when hydrogen is bubbled through a vegetable oil in the presence of a nickel catalyst, the hydrogen adds onto the double bonds converting the unsaturated oils to saturated. This process is used commercially to saturate and harden oils into margarine and shortening that resemble in consistency to butter and land for which they are substitutes.
3. Oxidation
This is the addition of oxygen at the double bonds. Double bonds make unsaturated fatty acids spoil easily when exposed to oxygen. The double bonds react with atmospheric oxygen in the process called oxidation which produces peroxides that are responsible for the rancidity, spoilage and unpleased flavours in some fats. It is therefore necessary to add antioxidants (substances that oppose oxidation) to unsaturated oils if they are to be kept for a long time without getting spoilt. Vitamins C and E, butylated hydroxylotulence (BHT) and butylated hydroxyanisole (BHA) are most commonly used anti-oxidants.
4. Saponification Number
This is the number of milligrams of KOH/NaOH required to change one gram of fat or oil into soap. (this is what is referred to as saponification). It is an index of the average molecular size of the fatty acid present.
5. Acid Number
This is also the amount of KOH required to neutralize the free fatty acid of 1gram of lipids. It is important in determining the degree of rancidity due to free fatty acids. Others are:
 Acetyl Number
 Poleske Number
 Volatile fatty acid number
Rancidity of Fats
This is a chemical change that results in unpleasant odours or taste in fat. It develops when certain fats or oils are exposed to heat, air, light moisture and bacterial action. Rancid fats and oils contain certain short chain dicarboxylic acids, aldehydes and ketones which are formed from unsaturated fatty acids.
The oxygen of air is believed to attack the double bonds in fatty acids to form a peroxide linkage. The iodine number therefore becomes reduced. Lead & copper catalyses rancidity. The peroxide attacks another molecule of fat/oil & breaks it down to short chain acids all of which have offensive smell.
Rancidity may also be due to hydrolysis of fat components into free fatty acids and glycerol. This process is often enhanced by presence of lipophytic enzymes in fat which in the presence of moisture & warm temperature bring about hydrolysis. Exclusion of O2 or addition of anti-oxidants delays this process e.g. vitamin E
Free radicals are produced during formation of peroxide and these can damage living tissue
Anti-oxidants
These are compounds which prevent oxidation & rancidity of lipids. e.g. vitamin E (tocopherol) & Hydroquinones. They are often used to protect fats.