Biology - An Illustrated Guide to Science (794127), страница 3
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Theenzyme is shaped so that theimportant parts of each chemical areclose enough to each other to reacttogether.● When the reaction has occurred, thenew chemicals (the products) do notfit in the lock and are released. Thisleaves the enzyme free to catalyzeanother reaction.●Induced-fit hypothesis© Diagram Visual Information Ltd.●This hypothesis suggests that thesubstrate helps the enzyme to formthe correct shape to receive it.Induced-fit hypothesis15Enzymes and coenzymesUNITYKey wordsThe coenzyme mechanismactive sitecoenzymeenzymeenzymecoenzymecomplexEnzyme+ coenzymecoenzymeenzymeactive siteEnzyme+ coenzyme+ substratemoleculessubstratemoleculessubstrateCoenzymesCoenzymes are usually smallmolecules that are needed in someenzyme reactions to help the reactionwork properly.● As with enzymes, many coenzymesonly work with particular enzymereactions.
If the coenzyme is missing,the reaction will not work properly.The coenzyme from another reactionwill not do the job.● Vitamins and minerals are ofteninvolved in reactions as coenzymes.●The coenzyme mechanismMost enzymes will not react with anychemical other than their substrate.This is known as specificity—theenzyme is specific for a particularsubstrate.● Some enzymes can only react in thepresence of a coenzyme. Thecoenzyme binds to the enzyme andchanges its shape. The active site isnow ready to receive its normalsubstrate.● The substrate bonds to the enzymeand reacts to produce the requiredproduct.●Unchangedenzyme +coenzymeare used again.EnzymesubstratecomplexReusing the coenzymeWhen the enzyme-catalyzed reactionhas occurred, the product is releasedfrom the enzyme-coenzyme complex.● The coenzyme is also released andbecomes available for anotherreaction.● Respiration in cells is a good exampleof a complex enzyme pathway thatdepends on a collection of coenzymes.productmoleculeEnzyme+ coenzyme+ product© Diagram Visual Information Ltd.●16UNITYKey wordsactive siteenzymeinhibitorsubstrateInhibitorsInhibitors reduce or destroy theactivity of an enzyme—sometimes todangerous levels.● There are two types of inhibitors:competitive inhibitors and noncompetitive inhibitors.●Enzymes and inhibitorsInhibitorsA noncompetitive inhibitor binds to anotherpart of the enzyme and blocks the active site.noncompetitiveinhibitorA competitive inhibitor bindsto the active site and blocks it.enzymeactivesitecompetitiveinhibitorCompetitive inhibitorsenzymeCompetitive inhibitors bind with theactive site of an enzyme.
In effect,they “compete” with the normalsubstrate for this site and block it.● Many competitive inhibitors arereleased from the active site so theenzyme can be regenerated. Thehigher the concentration of the“normal” substrate compared with theinhibitor, the less effect the inhibitorhas.●substratemoleculesNon-competitive inhibitorsA non-competitive inhibitor does notbind to the active site. It binds with adifferent part of the enzyme molecule.● This distorts the shape of the enzymeso it cannot function properly.● Non-competitive inhibitors are notreleased from the enzyme molecule sothe enzyme cannot be regenerated.● Even a low concentration of a noncompetitive inhibitor can be verydangerous.● Cyanide is a non-competitive inhibitorthat completely blocks an essentialenzyme in the respiration pathway. Itis therefore a very powerful poison.© Diagram Visual Information Ltd.●substratemoleculesThe inhibitor isnot displaced byexcess substratemolecules.The inhibitoris displacedby excesssubstratemolecules.17Fatty acids and glycerolUNITYKey wordsStearic acid (saturated): modelGlycerol: molecularstructureOHCOHHCOHCHOGlycerol●HCOHOleic acid (unsaturated): modelHdouble bondOCFatty acidsHOH2OHCOThree fatty acid moleculesHOOHH2OHCH2OHCTristearin (triglyceride)OHCOCOHCOCOHCOCHstearicacidCstearicacidHHstearicacidCOHOOHCOHOOHFatty acids are long chains of carbonatoms (sometimes up to 30 or 40) witha COOH group at one end.
TheCOOH group means that they behaveas acids in solution.● Fatty acids may be saturated (havingonly a single carbon-tocarbon bond [see stearicacid], or unsaturated (one ormore double or triplecarbon-to-carbon bonds [seeoleic acid]). The number andlocation of double bondsvaries.● Fatty acids are the buildingblocks of fat.● Fatty acids react with glycerolto bond their long chains tothe OH group in glycerol.When three fatty acids joinon all three of the OH groupsin glycerol, a triglyceride (fat)is formed.● Some triglycerides are simple and haveonly one type of fatty acid joined tothe glycerol molecule.
Othertriglycerides are mixed: they havethree different fatty acids joined ontoone glycerol molecule.●Glycerol moleculeHGlycerol is a small molecule with threeOH groups emerging from a shortcarbon chain. It is important in theformation of lipids, substancesinsoluble in water that include fats andoils.TriglyceridesThe fat on meats such as baconconsists of a variety of mixedtriglycerides.● Different fats have different mixturesof these triglycerides.●© Diagram Visual Information Ltd.Hfatty acidglycerolsingle bond18Light microscopeUNITYKey wordsLight microscopeobjective lensocular lenseyepiece containingocular lensTwo lensesA light microscope uses twosets of lenses, objective andocular lenses, to createmagnifications of up to 1000X.● The lens near the specimen iscalled the objective lens. Thiscannot produce an image byitself.● The lens in the eyepiece atthe end of the viewing tube iscalled the ocular lens.
Thishelps to focus the beams oflight to produce the image.● To calculate the magnificationof the microscope, you haveto multiply the magnificationof the objective lens by themagnification of the ocularlens.coarseadjustment knob●body tubefineadjustment knobrevolvingnose piecelow powerobjective lenshigh powerobjectivelensarmeyestageeyepiece(ocular lens)diaphrammirrorTwo focusing devicesLenses in microscopes arevery delicate.
To prevent themfrom being damaged byscratching them against thesample, the light microscopeuses two-stage focusing.● The coarse adjustment knobmoves the low powerobjective lens through a largedistance. When the area youwish to observe is in thecenter of the field of view andin sharp focus, you may clickthe high power objective lensinto place. The image shouldalready be nearly in focus. Ifany adjustment is needed, useonly the fine adjustmentknob.© Diagram Visual Information Ltd.●baseImage formationobjectivelensspecimendiaphramA clear light●The diaphragm regulates theamount of light reaching theobject.mirrorlamp19Cells: light microscopeUNITYKey wordsGeneralized animal cellcellulosechloroplastcytoplasmnucleusphotosynthesisplasmamembranevacuolenucleusnuclear envelopenucleoplasmnucleolusplasma membranecytoplasmfood granulesCell sizeTypical cells are anything between.005 and .025 mm (.0002 and .001 in).This is about ten times smaller thanthe diameter of a human hair.● Light microscopes can only seerelatively large structures in a cellbecause they can only magnify up to1,000X.●Animal cellsThe cell contains a large nucleus,which helps to control the cell.
Thenucleus is separated from thecytoplasm by the nuclear envelope(membrane). Inside the nucleus, thenucleoplasm, the liquid matrix of thenucleus, surrounds the nucleolus,where proteins are synthesized.● The area outside the nucleus butwithin the outer membrane is calledthe cytoplasm. It often contains acollection of smaller bodies such asfood or secretory granules, andsometimes small vacuoles (small sacsenveloped in a membrane).
These areoften very difficult to see with a lightmicroscope.●secretory granulesGeneralized plant cellcell wall ofneighboring cellcell wallplasma envelopePlant cellsPlant cells are surrounded by a thickcell wall made of cellulose.● Immediately inside the cell wall is theplasma membrane of the plant cell.This is identical to the plasmamembrane of animal cells.● Plant cells have a large central vacuolethat occupies much of the cell volume.It stores salts, water, water solublepigments, and potentially toxicmolecules in the form of crystals.● The cytoplasm contains many of theinclusions (globules, granules, etc.)found in animal cells and a largevacuole.
Sometimes large green discshaped bodies called chloroplasts arepresent: these carry outphotosynthesis.vacuolechloroplastcytoplasmnucleusnucleolusnucleoplasmnuclearenvelope© Diagram Visual Information Ltd.●20Electron microscopeUNITYKey wordsElectron microscopespecimenSimplified section through a simpletransmission electron microscopeHT cableElectrons not lightAn electron microscope (EM)uses electrons rather thanbeams of light. Magnetic andelectric fields are used tofocus the electrons instead ofglass lenses.● The use of electrons allowsmagnifications up to 10,000Xand beyond.illuminatingsysteminsulator●Function●Electron microscopes functionjust like light microscopesexcept that they use a beamof electrons instead of light toimage the specimen. Througha series of magnetic lensesand apertures, themicroscope focuses a beam ofelectrons on a specimen.
Thebeam interacts with thesample, and the microscoperecords the results of theinteraction as an image.Types of information●Electron microscopes canexamine the tomography(surface features) of an object,the morphology (size andshape of the particles) of anobject, the composition of theobject, and the arrangementof the atoms in the object.shield and filamentanodecondenser lenselectron gunspecimendoorspecimenairlock© Diagram Visual Information Ltd.Specimens need verycomplicated preparationbefore they can be used in theEM. This treatment cansometimes produce artefacts,objects that have nothing todo with the sample.condenserlensstageobjectivelensintermediate lensprojector lenseyepiecespecimenobjectivelensobjectivelensapertureintermediatelensprojectionchamberwindowDisadvantages●Image formationprojectorlensfluorescentscreencameradoorimagingsystemplate camerato vacuum pumpfluorescent screen21Animal cell:electron microscopeUNITYKey wordscentrioleendopasmicreticulumGolgi bodylysosomeAnimal cellsmoothendoplasmicreticulumplasma membranepinocytoticvesiclemitochondrionplasmamembraneribosomeSmaller sizes●cytoplasmThe electron microscope can seemuch smaller objects than the lightmicroscope is able to see.Membrane structuresThe cell uses a double-layeredmembrane to build many structures:the plasma membrane, Golgi body,lysosomes, and the endoplasmicreticulum.● The plasma membrane covers thewhole of the outside of the cell.● The endoplasmic reticulum is ameshwork of the same membrane thatruns throughout the cell.
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