Biology - An Illustrated Guide to Science (794127), страница 25
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The tail thencontracts and pierces the surface ofthe bacterium.● Nucleic acid from the bacteriophagepasses into the bacterium. Here itstarts to replicate while the normalbacterial DNA is switched off.● Over a period of time, the bacterialcell is full of bacteriophage DNA.Protein coats for the virus are thenmanufactured by the bacteriumfollowing instructions stored in theviral DNA.● The viral DNA is inserted into the viralprotein coats, and the bacterial cellbursts to release the completedbacteriophages.Lysis: a bacterialcell burstsreleasingviruses.●© Diagram Visual Information Ltd.base plateprotein coatphage DNAbacterial DNAbacterial cell wallNew protein coats aresynthesized; newviruses are assembled.virus(bacteriophage)bacterium(host)attachmentViral DNA is injectedinto the bacterium;bacterial DNA isinactivated.penetrationViral DNAreplicationoccurs.147Reproduction: butterflyMAINTENANCEKey wordsButterfly life cycleabdomenlarvalife cyclemetamorphosispredatorcompound eyeAdultantennaDorsal viewpupathoraxforewingComplete life cycleheadButterflies undergo a completem etam orphosis during the four stagesof their life cycle: egg, larva, pu pa,butterfly.
The female lays eggs, whichfirst develop into larva calledcaterpillars. This juvenile form has nowings and uses its mandible to feed onleaves. As the larva grows, it molts(sheds its external skeleton) multipletimes. When the larva reaches acertain minimum weight, it transformsinto a pupa. After it emerges from thepupal stage, it is a winged butterfly,which feeds on nectar from flowers.● Although the adult and larval stageslook very different, they possess thesame basic body parts common to allinsects: head, thorax, and abdom en .●thoraxhindwingabdomenLateral viewproboscislegpupaSurvival ratesAdult butterflies lay eggs. To increasethe chances of successful hatching andlarval growth, the eggs are typicallylaid in areas where food will beplentiful, e.g., on the underside ofleaves.● Larvae that survive the first few dayshave to eat large amounts of plantfood to amass enough energy for thenext stage of development.
Manylarvae are lost to predators at thisstage.● Pupae are also susceptible to selectionpressures. They rely on camouflage toavoid predators.● The hatched butterfly is a primesource of food to many insect-eatingbirds. Only a minority will survive longenough to produce eggs and start thecycle again. For all of these reasons,butterflies produce large numbers ofeggs.Larval stages (caterpillar)abdomenthoraxheadmandiblet rue legsecdysis (molting)© Diagram Visual Information Ltd.●egg148MAINTENANCEKey wordscloacafertilizationgametelife cyclemetamorphosisReproduction: frogFrog urino-genital systems(right ovary removed)fat bodyComplete life cycleFrogs undergo a completem etam orphosis during their life cycle.Their juvenile form is a tadpole thathas no lungs or legs and lives entirelyin water.
The adult frog has lungs andlegs and can survive out of water forextended periods.● The change from tadpole to frog is acontinuous process—there is no pupalstage as there is in metamorphosinginsects.● Tadpoles live for three to four months,with the exact time depending oncertain environmental conditions.Adult frogs can survive the winter andlive for many years, provided thetemperature does not drop too low.●Fertilization anddevelopmentFertilization in frogs is primarilyinternal with mating pairs of frogsexchanging gam etes by bringing theircloacas close together.
The cloaca isan opening that connects the bladderand the reproductive systems with theoutside world.● Mating frogs do not just mate in pairs.If sufficient numbers of compatiblemales and females are available in anarea, they will mate in larger groups,implying that some fertilization takesplace externally.● The eggs are covered in an outer shellof protective jelly, which swells incontact with water. The female lays hereggs in a sheltered pond or creek. Theeggs hatch into tadpoles, whichgradually develop into froglets—resembling adults but retaining avestigial tail—and then mature frogs.● Tadpole development is stronglyaffected by temperature and oxygenavailability.
The presence of meat or asource of iodine encourages earlychange into a frog and produces verysmall adults.oviductfat bodyovaryt estiskidneykidneyureterureterovisacseminalvesiclecloacabladderbladdercloacaFrog life cycleAdults mating© Diagram Visual Information Ltd.●Metamorphosisfrom tadpoleto frogEggszygote and yolkprotective jellyremainsof tailTadpolesone day oldmouthwidensexternal gillsmucous glandthree months oldforelimbthree weeks oldmouthhindlimbone month oldanusexternal gillseyetwo months oldspiracle (opening of gill chamber)149Growth and development:plants: monocotyledonsMAINTENANCEKey wordscotyledonendospermgerminationmonocotyledonphotosynthesisGermination of the corn seed (monocotyledon)Seed: external viewSeed: longitudinal sectionposition of cotyledonendospermMonocotyledonsCotyledon s are swollen leaves that actas a source of energy while the seed isgerminating.
During the first stages ofgerm in ation , photosyn thesis cannotoccur, so only stored food is availableto the plant for development.● Mon ocotyledon s like corn have asingle cotyledon (unlike the beanfamily and many other vegetableswhich are dicotyledonous and havetwo) with a large supply ofen dosperm —specialized storage tissuethat nourishes the embryo.●cotyledoncoleoptile (plumule sheath)plumulepericarp (fusedovary wall and testa)coleorhiza (radicle sheath)position of embryoradiclepoint ofattachmentpoint of attachmentGerminationGermination is a multistage processthat mobilizes the stored food reservesof the seed and prepares the plant forthe active production of food byphotosynthesis.● The root is the first structure todevelop to allow the intake of water.This develops from the radicle, theroot of the embryo plant.
The radicleis protected by a sheath called thecoleorhiza. As water is absorbed, thecoleoptile can develop.● The coleoptile is a photosyntheticallyactive organ rather like a sheathprotecting the growing stem orplumule. It pushes above the groundand the stem grows out of it.Adventitious roots then grow fromthe stem.● Prop roots develop to support thestem, and the first true leaves arethen produced from the stem.●Germinationleafsplitcoleoptilecoleoptile(plumule sheath)Fruit wallsplits;radicleappears.Plumule growsin coleoptile.Coleoptileappearsabove soil;adventitiousroots develop.prop rootFirst leavesappear.radiclecoleorhiza(radicle sheath)root hairscoleoptile (plumule sheath)adventitious rootcoleorhiza (radicle sheath)adventitious rootadventitious root© Diagram Visual Information Ltd.radicle150MAINTENANCEKey wordscotyledondicotyledongerminationphotosynthesisGrowth and development:plants: dicotyledonsGermination of the bean seed (dicotyledon)Seed: external viewSeed: longitudinal sectiontestaDicotyledonsDicotyledon s are plants that have twocotyledon s, leaf-like parts of theembryo that act as is a food reservoirwhile the seed is germ in atin g.
Duringthe first stages of germination,photosyn thesis cannot occur, so theplant must use stored food fordevelopment.● Within each seed are the cotyledons,the radicle (the root of the embryo),and the plumule (the embryonicleaves of the plant). The completeseed is covered by the testa (seedcoat) which protects the seed againstmechanical damage.●plumuleposition of radicleradicletestacotyledon(one of two)Germination (epigeal)Hypocotylstraightens;tr ue leavesappear.GerminationThe radicle is the first structure todevelop to allow the intake of water.The hypocotyls, the part of theseedling stem below the cotyledonsand above the radicle, emerges fromthe testa and pushes its way upthrough the soil.
It is bent in an archas it grows. Once the hypocotyl archemerges from the soil, it straightensout in response to light. Thecotyledons spread apart, and theepicotyl forms a young stem.● The germination process differsamong plants. Plants like peasexperience hypogeal germination—thecotyledons remain below ground asthe plumule develops. Plants likebeans experience epigealgermination—the growth of thehypocotyl raises the cotyledons aboveground. The cotyledons often thenbecome photosyn thetically active andform the first leaves of the new plant.© Diagram Visual Information Ltd.●Hypocotyl growsthrough soilsurface.Testa splits;radicleemerges.radiclet estaHypocotylstarts togrow.hypocotylroothairscotyledon(one of two)hypocotylt rueleafCotyledonsemergefrom soil.cotyledon(one of two)epicotylhypocotylcotyledon(one of two)hypocotyl151Growth and development:plants: tropismsMAINTENANCEKey wordsauxintropismGrowth responses to light (phototropism) of oat coleoptileExposed to light from all directionsThe plant grows upward.TropismsTropism s are directional growthresponses to environmental stimuli.So hydrotropism is directional growthrelated to water.
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