LIPIDSThese are organic compounds containing carbon, hydrogen and oxygen but the proportion of oxygen issmaller than in carbohydrates hence they are more reduced than the carbohydrates.Lipids are insoluble in water.They are of two types i.e. fats and oils. Fats are solid at room temperature while oils are liquids at roomtemperature.Lipids are made of fatty acids and glycerol. Glycerol has 3 OH groups and each combines with a separatefatty acid to form a lipid chemically known as a triglyceride. This is a condensation reaction that leads toliberation of 3 water molecules.Formation of a triglycerideQuestion: using the structural formula:For glycerol, and molecularformula CH3(CH2)nCOOH for afatty acid, show the formation of atriglyceride from fatty acids andglycerol.(Leave 8 lines for the answer)FATTY ACIDSAll occurring lipids have glycerol and therefore it is the nature of the fatty acids which determines thecharacteristics of any particular lipid. All fatty acids have a carboxyl group (COOH), the reminder of themolecule being a hydro carbon chain of varying length.The chain may possess one or more double bonds in which case it is said to be unsaturated. If itpossesses no double bonds, it is said to be [email protected][Type text]Page 1

Nature of fatty acid1. Butyric acid2. Linoleic acid3. Oleic acid4. Palmitic acid5. Serotic acid6. Arachidic acidGeneral enceButter fatSeed oilAll fatsAnimal & veg fatWood oilP.nut oilQuestion: explain why lipids are insoluble in waterThe hydrocarbon chains of the fatty acid may be very long forming long tails which extend from the glycerolmolecules. These tails are hydrophobic (water repelling) hence making lipids insoluble in water.PHOSPHOLIPIDSThese are lipids in which one of the fatty acid groups is replaced by phosphoric acid.CH2O-OC-fatty acidCHO-OC-fatty acidCH2O- phosphoric acidPhosphoric acid is hydrophilic in contrast to the remainder of the molecule, having an end attracting waterwhile the other end repelling it, ie hydrophobic. This gives the phospholipid its characteristics as one thecomponents of the cell membrane.Explain the suitability of phospholipids as a component of the cell membrane.WAXESThese are formed by combination of fatty acids with an alcohol other than glycerol. Their major function iswater proofing in plants and animals. They are also storage compounds in seeds e.g. castor oil seeds.STEROIDSThese are related to lipids ie they are lipid derrivatives. e.g. cholesterol in animals used to synthesize sexhormones. Other steroids include vitamin B and bile acids.FUNCTIONS OF LIPIDSStructural:i)They are components of the plasma/cell [email protected][Type text]Page 2

ii) They form subcutaneous fat in the dermis of the skin hence insulating the body since they are poorconductors of heat.iii) They are components of the waxy cuticle in plants and insects there by preventing water loss(desiccation).iv) They form a component of the myelin sheath of nerves hence playing a role in the transmission ofimpulses.v) They protect delicate organs e.g. the heart and kidney from They coat on fur of animals enabling it to repel water which would otherwise wet the organism.vii) They are components of adipose tissue.Physiological:i) They provide energy through oxidation.ii) They are solvents for fat soluble vitamins (ADEK).iii) They are a good source of metabolic water to desert animals, young birds and reptiles while still in theirshells.iv) They are a constituent of the brown adipose tissue which provides heat for temperature regulation(thermogenesis).Other functions:i) Some lipids provide a scent in plants which attracts insects for pollination.ii) Wax is used by bees to construct honey combs.iii) Wax from bees is used in the manufacture of candles.QUESTION: WHAT PROPERTIES DO LIPIDS POSSES AS STORAGE COMPOUNDS?i)ii)iii)iv)v)They are compact taking up little space.They are insoluble in water hence cannot be lost in solution.They are light to keep the weight to a minimum and allow buoyancy.They have a high calorific energy value.They have a high hydrogen-oxygen content hence can yield a lot of water on oxidation.TESTS FOR LIPIDSThey are tested for using the emulsion test or the grease spot (translucent spot) test.a) The emulsion test:The reagents used are ethanol and water.ProcedureObservationDeductionTo 1 cc of food solution, add 1 A turbid solution turns to a Lipids of ethanol followed by 5 cream emulsiondrops of water and shake.Turbid or colourless solution Lipids absent.remains a turbid or [email protected][Type text]Page 3

solution.b) Translucent spot test:ProcedureAdd 2 drops of test solution ona piece of filter paper.Allow to dry and observe underlight.ObservationConclusionA translucent spot or patch is Lipids presentleft on the paper.No translucent spot is formed Lipids absent.on the paper.PROTEINSThese are organic compounds of large molecular mass and insoluble in water. In addition to carbon,hydrogen and Oxygen, they always contain Nitrogen, usually Sulphur and sometimes Phosphorus.Whereas there are few carbohydrates and fats, the number of proteins is limitless e.g. a single bacteriummay have around 800 types of proteins while man has 10,000 types. This is because there are severalamino acids which may join in different patterns hence forming the various types of proteins.Proteins are specific to each species hence determine the character of the species.Proteins are not stored in the organism except in eggs and seeds where they are used to form new tissues.Their building blocks are the amino acids.AMINO ACIDSThese are about 20 amino acids occuring in proteins. They contain an amino group (NH2) and a carboxylgroup (COOH). Most amino acids have one of each and are therefore neutral but a few have more aminogroups than carboxyl making them alkaline or may have more carboxyl than amino groups making themacidic.Structure of an amino [email protected][Type text]Page 4

Where R is a variableAmino acids are soluble in water and ionize to form ions.The carboxyl end (COOH) of the amino acid is acidic in nature. It ionizes in water to give the hydrogen ionmaking one end negatively charged.The amino end (NH2) is basic in nature. It attracts the hydrogen ion in solution making this end positivelycharged. The ion is now dipolar i.e. having a negative and a positive pole. Such ions are called zwitterionsi.e. the negative and positive charges exactly balance and the amino acid ion has no overall charge i.e.Zwitterion (no overall charge)Therefore in acidic solutions, an amino acid acts like a base and in alkaline solutions, it acts as an acid. Inneutral conditions found in the cytoplasm of most living organisms, the amino acid acts as both showingboth basic and acidic properties, hence it is amphoteric.The overall charge of the amino acid depends on the pH of the solution.At some characteristic pH, the amino acid has no overall electric charge i.e. it exists as a zwitterion. ThispH is called the isoelectric point of an amino acid.(leave 8 lines)If the pH falls below the isoelectric point i.e. the solution becomes more acidic, hydrogen ions are taken upby the carboxyl ion. This reduces the concentration of hydrogen ions in solution making the solution lessacidic and the amino acid gains an overall positive charge.If the pH rises above the isoelectric point i.e. it becomes less acidic or more alkaline, hydrogen ions are lostby the amino group. This increases the concentration of free hydrogen ions in the solution making it moreacidic and the amino acid gains an overall negative charge. Therefore being amphoteric, amino acids arebuffers.NOTE: A buffer solution is one which resists the tendency to alter its pH even when small amounts of acidor base are added to it.Questions:Hhow do amino acids act as buffer solutions?TYPES OF AMINO [email protected][Type text]Page 5

1. Essential Amino acidsThese are amino acids that cannot be synthesized by the body and therefore got from the diet that theorganism feeds on. They include: neArgininelysineMethionineTryptophan2. Non-Essential amino acidsThese are amino acids that are synthesized by the body through a process called transamination. Theyinclude: Aspartic acidGlutamic acidAsparagineProteins can be classified into: first class proteins which contain all the essential amino acids e.g. from beansand second class proteins which are deficient of one or more essential amino acids.FORMATION OF POLYPEPTIDESThey are formed as a result of condensation reaction between the amino group of one amino acid and thecarboxyl group of another amino acid to form a [email protected][Type text]Page 6

Further combinations of this type extend the length of the chain to form a polypeptide which usuallycontains many amino acids.The shape of the polypeptide molecule is due to four types of bonds which occur between the variousamino acids in the chain. These bonds include:1. Disulphide bonds formed between sulphur containing groups on any two cysteine molecules.2. Ionic bonds formed between the amino ions (NH3 ) and carboxyl ions (COO-).3. Hydrophobic interactions which are interactions between non polar R groups which cause the proteinsto fold as hydrophobic side groups.The polypeptide chain showing 3 types of bonding(Leave 8 lines)PROTEIN STRUCTURESThere are four main protein structures i.e. primary structure, secondary structure, tertiary structure andquaternary structure.Primary structureIt is a sequence of amino acids in a polypeptide chain. The sequence dictates the biological role of theprotein. It is strictly determined by the sequence of bases in the DNA molecule.Secondary structureIt refers to the helical coiling of the polypeptide chains. It is maintained by many hydrogen bonds formedbetween the oxygen and hydrogen atoms of the neighbouring COO - and NH3 groups.Such structures are found in hair, wool, vertebrate skins, horns, nails, claws, beaks and feathers.Its hardness and stretch ability vary with a degree of cross linkage between di sulphide bridges andneighbouring chains. They are joined together by hydrogen bonds between adjacent chains.Tertiary structuresIt refers to the way in which the helically coiled chains are folded. This is determined by interactionsbetween R groups, rather than interactions between backbone constituents. The polypeptide chains bendand fold extensively forming a precise, compact globular shape. These interactions between R groupsinclude hydrogen bonds, ionic bonds, hydrophobic interactions, and van der Waals interactions. Strongcovalent bonds called disulfide bridges may reinforce the protein’s [email protected][Type text]Page 7

The hydrophobic interactions are quantitatively the most important and occur when a protein folds so as toshield the hydrophobic side groups from the aqueous surrounding and at the same time exposinghydrophobic side chains.Quaternary structureIt is a combination of several polypeptide chains held together and associate with non-protein parts to formcomplex conjugated proteins e.g. in haemoglobin.TYPES OF PROTEINSa) Classification according to structure1. Fibrous proteinsThese form long parallel unbranched polypeptide chains crosslinked at intervals forming long fibres or sheets.They are physically tough and insoluble in water.The secondary structure is the most important with little or no tertiary structure.They perform structural functions in cells of organisms eg collagen (in tendons, bone, connective tissues), myosin(in muscles), keratin (in hair, nails, feathers, horns) and silk (in spider webs)2. Globular proteinsTheir polypeptide chains are tightly folded to form a spherical shape.They have a highly irregular sequence of amino acids in their polypeptide chainsThey are easily soluble in water and compact.The tertiary structure is very important.They have metabolic roles ie they form enzymes eg maltase, antibodies and some hormons like insulin.3. Intermediate proteinsThey are fibrous but soluble eg fibrinogen that forms insoluble fibrin during blood clotting.Comparison of globular and fibrous proteinsGlobular proteinsSoluble in waterHave a tertiary structureNo cross linkagesFibrous proteinsInsoluble in waterHave a secondary structureHave cross linkages between adjuscent polypeptidechainsPolypeptide chains are folded in to a spherical Polypeptide chains form long parallel sheets /shapestrandsHave metabolic rolesHave structural [email protected][Type text]Page 8

Have irregular amino acid sequencesHave repetitive regular sequences of amino acidsRelatively unstable structureStable structureLength of the chain is always identical in two Length of the chain may vary in two examples of theexamples of the same proteinsame proteinb) Classification according to composition1. Simple proteinsThese are proteins with only amino acids in their structure.2. Conjugated proteinsThese are complex compounds consisting of globular proteins and tightly bound to non protein material,called prosthetic group which plays a virtual role in the functioning of the proteins e.g.Name of proteinHaemoglobinMucinCaseinCytochrome oxidaseNucleoproteinLipo proteinWhere it is foundBloodSalivaMilkElectron carrier path wayRibosomesMembranesProsthetic groupHaem (iron)CarbohydratePhosphoric acidCopperNucleic acidLipidsQUESTION: HOW DOES THE MOLECULAR STRUCTURE OF PROTEINS RELATE TO THEIR ROLES?i)ii)iii)iv)v)vi)vii)Some proteins have structural functions. These are fibrous proteins with a secondary structureinsoluble in water and physically tough e.g. collagen in connective tissues, bone, tendons and cartilage.Other structural proteins include keratin in feathers, nails, hair, horns, beaks and skin.Some proteins function as enzymes. These have a globul