Biology - An Illustrated Guide to Science (794127), страница 2
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An OH groupfrom each monosaccharide unit reactstogether to make water (H2O) andform an oxygen bridge between thesugar rings.● Maltose (C12H22O11) is a disaccharidethat is a product of starch digestionand is also found in some germinatingseeds.
It is formed by two glucosemolecules joined together by aglycosidic (C-O-C) bond.● OH groups at the end of a disaccharidemolecule can link with more rings tomake longer chains. However, mostsugars have three rings or fewer.single monosaccharide unit(* n = usually 3 to 6)Molecular structure of glucose© Diagram Visual Information Ltd.●hydrogenoxygencarbon9Complex carbohydratesUNITYKey wordsComplex carbohydratescelluloseglycogenpolysacchariderespirationstarchTwo glucose moleculesHHCHOHCHHOHCOHOHHHCOHPolysaccharidesOCH+CHCOHHCOHCCCHOHHCCarbohydrates with large numbers ofrings in their molecules are calledpolysaccharides.● Polysaccharides are used in livingthings for energy storage and to buildstructures (see page 10).OHEnergy storage●Hcondensation reaction(synthesis)hydrolysis(breakdown)H2OStarches are large polysaccharidesformed (synthesized) by joining longchains of monosaccharide units (suchas glucose) together.
Since starchesare insoluble, they form granuleswithin a cell and do not upset thewater balance of the cell in the waythat the same amount of soluble sugarwould.● When energy is needed, a reaction,called hydrolysis breaks the starchdown into its sugar molecules. Thesesugar molecules can then be used toprovide energy by respiration.● Animals use the polysaccharideglycogen as a carbohydrate energystorage molecule.●OHOHsugarH2OBuilding structuresCell walls in plants are made of apolysaccharide called cellulose. Acellulose molecule may containthousands of monosaccharide unitsbonded together.● The links between themonosaccharide units in cellulose arearranged to produce a flat moleculethat is stronger than a steel fiber.These molecules run through the cellwalls of plants like the steel rods inreinforced concrete.Disaccharide: MaltoseHHHCOHCHOHHCOOHHCCHOHHHCCOglycosidic bondCOHCOOHHCCHOHHCOH© Diagram Visual Information Ltd.●10UNITYKey wordscelluloseexoskeletonglucoseglycogengutpolysaccharideImportantpolysaccharidesUses of polysaccharidespolysaccharidesα glucosePolysaccharides in animalsβ glucoseIn animals polysaccharides are mainlyused for energy storage.
In humans upto 10 percent of the weight of the livercan be glycogen—an instant store ofenergy that is easier to mobilize thanfat, which is used for long-term energystorage.● A typical glycogen molecule maycontain 300 to 400 glucose units in abranching molecule.● Glycogen also occurs in yeasts andbacteria.● Chitin is made of acetylglucosamine,glucose units with an amino groupattached. It is common in shellfish(the edible crab can be 70 percentchitin) where it is an important part ofthe shell.● Chitin is also found in the exoskeletonof insects.● Chitin is a structural polysaccharideand is not used as an energy store.acetylglucosamine●Polysaccharides in plantsPlants store starch as granules insidetheir cells. Roots such as potatoes andcarrots are often rich in starch, whichprovides the energy needed for thenext generation to develop before itcan produce its own food byphotosynthesis.● Cellulose is a structural polysaccharideand gives the cell wall its strength.Animals cannot digest cellulose, andso it passes through the gut largelyuntouched as roughage.© Diagram Visual Information Ltd.●structural polysaccharidescellulose(plant cell walls)chitin(arthropodexoskeletonsand fungi)storagepolysaccharidesstarch(plant cells)glycogen(liver andmuscles)11Amino acidsGeneralized aminoacid structureKey wordsvariable group (R)RHNamino group(basic)CHamino acidpeptide bondpolypeptideOCOHHcarboxyl group(acidic)non-variable part(Chemical structureGlycine (Gly) Alanine (Ala) Valine (Val) Serine (Ser) Threonine (Thr) Aspartic acid (Asp)HCH3 CH3 CH3CHOHCH3CH2H C OHOOHCCH2Asparagine (Asn) Cysteine (Cys)ONH2CLeucine (Leu)Isoleucine (Ile)Methionine (Met)SHCH3 CH3CH3CH3CH2CHCH2SCH2CH CH3CH2CH2C NHNHJoining amino acidsCH2●Glutamic acid (Glu) Glutamine (Gln) Phenylalanine (Phe) Tyrosine (Tyr) Arginine (Arg)OH ONH2CCCH2CH2CH2CH2OHNH2CH2CH2CH2CH2Lysine (Lys)CH2Tryptophan (Trp)NH2Histidine (His)HC NCH2CH CH2CH2CH2Proline (Pro)CH2NH C NHCCHCH2NHNatural amino acidsThere are about 20 naturally occurringamino acids.● The simplest amino acid is glycine.The R group here is a single hydrogenatom.● More complicated amino acids, suchas proline, have R groups containingmany atoms, complex rings, andsometimes elements such as sulfur orphosphorus.CH2OAmino acid molecules are made offour groups bonded with a singlecarbon atom.
Three of these groupsare non-variable.● The amino group NH2 is a basic group,which means it behaves as an alkali insolution.● At the other end of the molecule is acarboxyl group (COOH), which acts asan organic acid.● The third group is a hydrogen atom.● The fourth group is variable. It is oftenshown in diagrams by the letter R.Different amino acids have different Rgroups.●Natural amino acidsCH2●Amino acids can join to make chainscalled polypeptides when the acidgroup from one amino acid reacts withthe carboxyl group of another. Thereaction releases water and produces alink called a peptide bond.● More amino acids can be added ateach end of the new molecule (seepage 12).CH C OHOCH2non-variable partof amino acid molecule© Diagram Visual Information Ltd.(UNITY12UNITYKey wordsamino acidhemoglobininsulinpeptide bondproteinSmall moleculesAll proteins are made of small aminoacid molecules linked by peptidebonds in long chains resembling astring of beads.● The number and order of amino acidsin the chain decides how the proteinwill behave.● Some proteins have more than onechain of amino acids and some haveextra groups of atoms added.
Forexample, hemoglobin, whichtransports oxygen from the lungs tocells throughout the body, is a proteinwith four amino acid chains wrappedaround a central group containingiron.Protein structureExample of protein structureInsulin(produced in pancreas)B-polypeptidechainA-polypeptidechainPheGlyValIleAsnValGlnGluHisGln●Protein sizeInsulin (right) is a small proteinmolecule with only 51 amino acids ontwo chains tethered together by 3disulfide bridges.● Some of the large immunity proteinshave thousands of amino acids and arebigger than some simple livingorganisms!●Twisting and turning●LeuCysCysdisulfidebridgeSSSerdisulfidebridgeSSCysGlyAlaCysTyrValGlnSerLeuSerGluHisAsnLeuTyrValThe amino acid chain twists as itgrows. The twisted chain then forms aspiral. The spiral shape is heldtogether by links along its length.LeuValLeuGluCysAladisulfidebridgeSSGlyThrProLysAlaTyrPhePheGlyTyrCysAsnGluArg© Diagram Visual Information Ltd.Leupeptidebond13Classification of proteinsUNITYKey wordsTypes of proteinproteinscollagenenzymehemoglobinhormonepeptide bondfibrousTypes of proteinThere are two main groups ofproteins: fibrous and globular.● Both groups have the same basicstructure—they are long chains ofamino acids joined by peptide bonds.● The difference between the twogroups depends on the way theprotein chains are arranged.●structural(e.g., collagen)contractile(e.g., myosin)Fibrous proteinsFibrous proteins have chains twistedinto spiral shapes held together bystrong bonds to make the moleculelook like a spring.● Fibrous proteins can be divided intostructural and contractile proteins.Structural proteins form the structureof an organism.
For example, they canbe found in skin and hair. Collagenfibers in the skin give it elasticity andkeep it smooth. Contractile proteinssuch as myosin help muscles contract.●globularGlobular proteinsGlobular proteins have chains thatwind in and out of each other, twistinginto complex shapes that look like aball of wood. Their chains are heldtogether with a mixture of strong andweak bonds.● Globular proteins often have morethan one chain and can contain extranon-protein groups.
For example,hemoglobin contain iron ions.● Globular proteins are often delicateand easily damaged by heat orchemicals. If their molecular shape ischanged by heat they cannot workproperly.● There are various types of globularproteins. Some transport smallermolecules. Some act as enzymes,controlling the rate of chemicalreactions. Some have a protectivefunction.
Still others are hormones,the chemical messengers of the body.enzymestransport(e.g., hemoglobin)protective(e.g., antibodies)hormones(e.g., insulin)© Diagram Visual Information Ltd.●14UNITYKey wordsactive siteenzymesubstrateEnzymes: mechanismLock-and-key hypothesisEnzyme + substrateenzymeactive siteEnzymes and reactionsEnzymes are proteins that control therate of reactions in living things.
Sugarreacts easily with oxygen to givecarbon dioxide and water—butoutside organisms it needs to beheated to well above 300°F (150°C)to start the reaction. Inside livingorganisms, enzymes make the samereaction work at temperatures as lowas the freezing point.● Each reaction has its own enzyme—ifthe enzyme is missing the reactiondoes not take place. An enzyme forone reaction will not work on anotherreaction.● Most reactions in living things arebroken down into many steps—andeach step needs its own enzyme.● There are two hypotheses of enzymeaction: lock and key and induced fit.substratemolecules●active siteunchangedenzymeused againEnzyme + productproductmoleculeEnzyme-substratecomplexLock-and-key hypothesisIn this hypothesis, when the chemicalsinvolved in a reaction (the substrates)get near an enzyme molecule, they“fit” into a part of the molecule calledthe active site, like a key in a lock.
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