Biology - An Illustrated Guide to Science (794127), страница 26
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Phototropism isrelated to light.● Roots are positively hydrotropic,which means they grow towardsources of water. They are negativelyphototropic and grow away fromsources of light.● Stems are positively phototropic andnegatively hydrotropic.●coleoptilerootseedExposed to light from one directionThe plant grows toward light (positive phototropism).The tip is removed: no growth.Mechanism of tropismsTropisms are growth responses. Thehormone au xin has an effect on thegrowth and development of cells. Inthe stem, an increase in the level ofauxin increases growth of cells byallowing them to enlarge more easily.● In the root, auxin increases lead to areduction in cell growth.● Light tends to inhibit the productionof auxin.
So, if a shoot is illuminatedfrom one side, the side in the dark willgrow more rapidly, and the shoot willbend toward the light.● Auxin is only produced by the growingtip of the plant. If this is removed,growth ceases. If the tip is covered bya light-proof cap, the stem does notexhibit phototropism. The auxindiffuses downward through the stemand can be collected in an agar block.The block then has the ability to act asa source of auxin to a decapitatedstem and so produce growth.Tip is covered by lightproof cap: grows upward.lightproof capZone of elongation covered by lightproofcollar: grows toward light.lightproofcollarTip is removed and placed on agar block. The block is replaced on the right side of anotherdecapitated coleoptile.
Auxin diffuses into zone of elongation, causing growth to the left.agar blocklight© Diagram Visual Information Ltd.●152HUMAN BIOLOGYKey wordsabsorptionassimilationdigestiongutNutrition: digestivesystemHuman digestive systemDigestion to assimilationFood particles need to be brokendown into smaller molecules beforethey can pass into the bloodstream.Digestion is the process of breakinglarge particles into smaller molecules.It involves both mechanicalmanipulation and chemical action.● Absorption is the process that takesfood molecules into the body.
Thistakes place in the gu t, mainly in thesmall intestine.● Assim ilation is the process of usingabsorbed materials to build newtissues. This occurs throughout thebody.●buccal cavity (mouth)tongueesophagusdiaphragmMechanical digestionMechanical digestion is the breakdownof large lumps into smaller particles.This begins in the mouth.● Teeth such as the incisors tear offlumps of food, while molars crushthese lumps into smaller particles.● Food leaving the mouth has beenreduced to a small particle size toincrease its surface area and soincrease the rate of enzyme activitylower in the gut.●Chemical digestionEnzymes act on large, insolublemolecules to break them down intosmaller, soluble molecules that passthrough the gut wall into thebloodstream.● The gut produces a series of enzymesto break down food molecules.
Thehigher part of the gut is acidic, thelower part is neutral.bile ductstomachgall bladderpyloric sphincterliverduodenumpancreas© Diagram Visual Information Ltd.●colon(large intestine)ileum(small intestine)cecumappendixrectumanus153Nutrition: teethHUMAN BIOLOGYKey wordsHuman teethcanineincisormolarpremolarFrom the age of about 6 years, the 20 deciduous (“ baby” ) teeth of a child are graduallyreplaced by 32 permanent teeth.● The third molars, or wisdom teeth, are usually the last to appear, generally in early adulthood.● The chisel-shaped incisors are adapted for biting and cutting food, while the broaderpremolars and molars are responsible for grinding and chewing.● Each tooth has a crown, covered in hard-wearing enamel to resist abrasion, and a root thatis held in its own socket by cement and a fibrous lining.●Tooth typesSkullThere are a number of different typesof teeth.
Each type has a particularfunction in the mechanical breakdownof food.● In cisors at the front of the mouth cutfood into smaller lumps.● Can in es tear off lumps of food. Theseare not particularly well developed inhumans compared with othercarnivores like tigers and dogs.● Prem olars and m olars are large teethat the back of the mouth withflattened top surfaces. These crushfood lumps to reduce them to a fineparticles.●Side viewmaxilla(upperjaw)canineIncisorincisorsmandible(lowerjaw)CaninePremolarMolarTop viewsmolarspremolarsTooth structurecrownThe portion of the tooth below thegumline is the root. The part that risesabove is called the crown.● All teeth have the same internalstructure, with a layer of hard enamelon the outside supported by softerdentine inside, packed around acentral pulp cavity.
Enamel is nonliving material like hair and nails,dentine is living material.● The pulp cavity contains a bloodsupply and a nerve.●rootMolarVertical sectionVertical sectionenamelenameldentinedentinepulpcavitycrownIncisorpulpcavitycrownSide viewsgumgumcementfibrouslining© Diagram Visual Information Ltd.bloodvesselrootfibrousliningrootnervecement154HUMAN BIOLOGYKey wordsbilechymeenzymeguthepatic portalveininsulinliverNutrition: liver, stomach,and pancreasLiver , stomach, and pancreasesophagusLiverThe liver is a complex organ thatcarries out a range of metabolicfunctions, not all of them concerningdigestion. Blood from the gu t drainsinto the liver through the hepaticportal vein , and all digested foodchemicals pass through the liver forsorting before they pass on to the restof the body.● The liver also produces bile, which isan alkaline solution that helps with thedigestion of fat.
It is stored in the gallbladder and expelled into theduodenum when fatty food is present.section ofstomach wallliverstomach●gall bladderbile ductpancreaticductpyloricsphincterpancreasduodenumsection of pancreasSection of stomach wallgastric glandopening of gastric glandmucosaStomachThe stomach is a large muscular sacthat acts as a storage vessel for food,passing it out through the pyloricsphincter to the duodenum. Themuscles contract and relax to mix thefood and stomach secretions into aslurry called chym e.● It also starts protein digestion using anen zym e called pepsin.
The stomachcontents are strongly acidic, whichhelps pepsin to work and kills bacteriain the food.● The stomach protects its wall from theacid and enzymes by secreting a layerof mucus that covers the inner surface.●submucosaoblique musclecircular musclelongitudinal musclesmooth musclelayersgastric glandSection of pancreasthin muscle layerGastric glandmucus-secretingcellsPancreasThe pancreas produces a package ofdigestive enzymes.
These pass to thegut in pancreatic juices along thepancreatic duct.● Cells in the pancreas called islets ofLangerhans also produce the hormoneinsulin, which regulates the level ofsugar in the blood.© Diagram Visual Information Ltd.●oxyntic cells(secretehydrochloricacid)branch ofpancreatic ductblood capillaryislet of Langerhans(secretes insulin)zymogen cell (secretespancreatic enzymes)chief zymogen cells(secrete pepsin)155Nutrition: small intestineHUMAN BIOLOGYKey wordsIleumabsorptionguthepatic portalveinlymphatic systemridge coveredwith villilivervillusMorphologyThe small intestine, or ileum, is a longmuscular tube running from theduodenum to the large intestine. Inhumans it is over 18 feet (6 m) inlength.● The inner surface has many ridges thatincrease the surface area. This isessential for the absorption ofdigested foods.● The small intestine is well-suppliedwith blood vessels.
These supplyoxygen to the cells of the intestine,which are metabolically quite active,producing a constant stream ofenzymes to digest food in the gu t.Blood is drained away from the smallintestine by the hepatic portal vein ,which carries the food-rich blood tothe liver.● As with all of the gut, the ileum hastwo layers of muscle in its wall: one setrunning around the intestine (thecircular muscles) and one set runningthe length of it (the longitudinalmuscles).●part of ileum wall showing villivillusmucus-secreting celllacteal (carrying fattyacids and glycerol)mucosasubmucosacircularmusclelongitudinalmusclethin muscle layerepitheliumVillus (vertical section)venule (carrying aminoacids and monosaccharides)The inner wall of the ileum is coveredwith small fingerlike projections calledvilli.
These further massively increasethe surface area available forabsorption.● A villus has a network of capillaries init, and dissolved food materials passinto this through the outer wall of thevillus. A central space called the lactealprovides a transport route for fattysubstances that do not dissolve easilyin blood. The lacteals are filled with afatty fluid and drain into the lym phaticsystem .●network ofblood capillariesthin muscle layerMicroscopic structurearteriole© Diagram Visual Information Ltd.Part of ileum wallshowing villi156HUMAN BIOLOGYKey wordsabsorptionenzymegutSequence of processesDigestion is the breaking down oflarge, insoluble molecules into smaller,soluble molecules.● The first part of digestion is thebreaking down of large lumps of foodinto smaller particles.
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