1 ean and standard deviation There is almost alwaysvariati oni n biological data. The amount of variation can be shown using a graph calleda frequency di stribution . Most var iation gives a bell -shaped frequency distribution cal led the normaldistribution. The mean val ueis inthe middle of the di stributi on. The mean ofa set of values is calculatedby di vid ing the sumofthe values by the number of values. For example, the sum of the values 7, 9, 11 and 17 is 44 and as here are four values, the mean is 44 divided by 4, which is 11. The standard deviation is used toassesshowfar the values are spreadabove andbelowthe mean. It is calculatedby entering data int oa graphicdisplayor scient if iccalculator and pressing the standard devi ationfunct ion key. Ahi gh standard devi ati onshowsthat the data are widelyspread, whereas a lowstandarddevi ationshows that the data are cl ustered closelyaroundthe mean. The standarddevi ationcan be used tohelpdecid e whether the difference between two means is l ikel y tobe signi ficant. Two examples are describ ed below. LEFT ANDRIGHTHANDLENGTHS Thirty teenage boys measuredthe length of theirleft andright hands, tofi ndout whether theyare di fferent.Indivi dualboys' left andright handlengthvaried byas muchas 10mm. The normal distribution mean ~- 2,. 0-1 .0+1.0+2.0 ,-- -----1-- ----- ~ --- --- -j----~ - o ~ than 95% of the 68% of the area isarea is betwee n - 2.0 between - 1.0 andand+2.0 standa rd +1.0standard deviations.deviations. Auseful rule is that 68% of the values li e within one standard deviat ionofthe meanina normal di stributi onand approximate ly95% of the values lie withi n twostandard devi ati ons ofthe mean (above). The results are shown in the frequency di stributionbelow. ERRORBARS HandMean lengthStandard Bars on graphs extending above and belowthe mean value deviation are used toshowthe variabi l ityof the data. They may show the range of the data, orthe standard devi ati on.left188.6mm11.0 mm ri ght188.4mm10.9mm 12 left ~10 .--G 8 c CJ'lo "0 ..c 0'0 Q.) ~~EffectofCO2 concentrationonphotosynthesis At very highCO2 concentrationsthe rate reaches a plateau. AthighIight intensities the rate reaches a plateau. I Atlowtomediumlight intensities the rate is directly proportionalto Iightintensity. Light intensity- ~Effectoflightintensity onphotosynthesis I Nophotosynthesis at verylow CO2 concentrations. CJ'l'Vi Q.) ..cAtJow to fairly high C CO2 concentrations>CJ'lthe rate is positively .8 o correlatedwithCO2 ..c Q.. concentration. '0 Q.) ~~Effect oftemperature onphotosynthesis As temperature increasesthe rate increases more and more steeply. Optimum temperature Abovethe optimum temperature the rate fallssteeply. CJ'l'Vi Q.) - c >CJ'l.8 o ..c 0'0 Q.) ~~Temperature ~ CO2 concentration~The chemistry of life21 EXAMQUESTIONS ON TOPIC3 1Thetablebelow shows thebasecompositionofgeneticmaterialfromtensources. Source ofgeneticBase composition(%) material Cattle thymus gland Cattle spleen Cattlesperm Pig thymus gland Salmon Wheat Yeast E.coli (bacteri a) human sperm influenza virus Adenine 28.2 27.9 28.7 30.0 29.7 27.3 31.3 26.0 31.0 23.0 Guanine 21.5 22.7 22.2 20.4 20.8 22.7 18.7 24.9 19.1 20.0 Thymine 27.8 27.3 27.2 28.9 29.1 27.1 32.9 23.9 31.5 0.0 Cytosine 22.5 22.1 22.0 20.7 20.4 22.8 17.1 25.2 18.4 24.5 Uracil 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 32.5 a)Deduce thetype ofgeneticmaterialusedby (i)cattle[1] (ii)E.coli[1] (iii) influenza viruses.[1] b)Suggest areasonforthedifference betweencattle thymusgland,spleenandsperminthemeasurements of their base composition.[1] c)(i)Explainthereasonsforthetotalamount ofadenineplusguaninebeing closeto50% ofmany ofthespeciesinthetable. inthegeneticmaterial [3] (ii)Identify two other trendsinthebasecompositionofthespeciesthat have50%adenine andguanine.[2] d)(i)Identify a speciesshowninthetable that doesnotfollow thetrendsinbase compositiondescribedin(c).[1] (ii)Explainthereasonsforthebase compositionofthisspeciesbeing different.[2] 2Thegraph(right)shows theresultsofa datalogging experiment. Chlorell,a type ofalgathat is oftenusedinphotosynthesis experiments,wasculturedinwaterinalargeglass vessel.Light intensity,temperatureandthepHofthewater weremonitoredover a three-day period.ThechangesinpH were due tochangesin carbondioxide concentration. AnincreaseinCO 2 concentration causesa decreaseinpH. I 0...7.5 7.0 6.5 a)State therelationship showninthegraphbetween (i)Iight intensity andCO2 concentration[1] 6.0 (ii)temperature andCO2 concentration.[1] b) c) Deduce, fromthedatainthegraph,whether theeffect oflight intensity ortemperature oncarbondioxide concentrationis greater.[2] Thegraphshowsbothrises andfallsinCO2 concentration. Explainthecausesof (i)risesinCO2 concentration (ii)fallsinCO2 concentration. [2] [2] x ~-0200 /"'"'0 ..c KEY , 0...30DCand 0.15%CO2 '0

CJ.) 20DCand 0.15%CO 100 4 2

x x x x x30DCand 0.035%CO2 o00 0 020DCand 0.035%CO2 0'iii iIi Iii Iio 1 234 56 78 9 10 11 12 Lightintensity/arbitrary units Summary ofcyclic photophosphorylation excited electrons PhotosystemPhotosystem IIj r'\

I ./ADPATP CYCLICPHOTOPHOSPHORYLATION Whenlight is not thelimiting factor,NADPH tendsto accumulateinthestroma andthereis a shortageofNADP+. Thenormalflow ofelectronsinthethylakoidmembranesis inhibitedbecauseNADP+is neededas a finalacceptor of electrons.Analternativeroutecanbeused that allows ATP production whenNADP+is notavailable. Thispathway is calledcyclic photophosphorylation. PhotosystemI absorbsIight andis photoactivated. Excited electrons arepassed fromphotosystemI toa carrier inthechai nbetweenphotosystemII andphotosystemI. Theelectronspass along thechainof carriersback to photosystemI. As theelectrons flow along thechainofcarriers they cause pumping ofprotons across thethylakoidmembrane. Aprotongradient is formedandthisallows production of ATPbyATPsynthase. Thefigure(left)showsthepathway usedincyclic photophosphoryl ation. Cellrespirationand photosynthesis81 x EXAMQUESTIONS ON TOPIC8 The elect ron mi crographbelowshows partofa pl antrootcell , including mitochondri a. [Source: Dr S, E. Juniper, Dept. ofPlantSciences, University of Oxford] a)Exp lainbrief ly twofeatu res that al low t hemitochondri a i n the micrograph tobei dentif ied. b)Redr awthe st ructure of the mitochondr ionmarkedX. c)Ann ot ate the micrograph(not your draw ing) toshow one exam ple of (i)a regi onw here the Krebs cycle takes place (i i)a locationof ATP synt hetase (i i i) a regionw here glycolysis takes pl ace. [2] [2 ] [3J 2a)Dr awa cur ve of t he act ionspectrumforphoto synthes is on the axis below.[2] 3 400500600700 violetbluegreenyelloworangered Wavelength/ nm b)Expl ai n th e rel ation shi p b etw een theactionspectrum andth e absorptio nspectraofphot osyntheti c pigments. a)State tw oprocesses thatinvol ve chemiosmosis. b)Expl aint heneedfora membrane inchem iosmos is. c)Suggest a locat ion w here chemiosmosis couldocc ur inprokaryotes. [2j [3] [1] 8218Questions Cell respirationand photosynthesis 9L ...Leaf structure and function LEAVESANDPHOTOSYNTHESIS The funct ion of leaves is to produce foodfor the plant by photosynthesis. The leafis adapted byits structureto carryout photosynthesis eff iciently. On page 5 is a scanni ng electron micrograph of a leaf. The figure (below)is a plan diagram of tissues inpart ofa leaf ofa dicotyledonous pl ant to show the adaptations forphotosynthesis. Tissues of theleaf and theirfun cti ons Pali sade mesophyll - consists of densely packed cylindrical cells wi th many chloroplasts. This is LEAVESAND TRANSPIRATION Photosynt hesis depends ongas exchange over a moi st surface. Spongy mesophyl l cel l wa lls provid e thi s surface. Water often evaporates from the surface and is lost,in a process cal led transpirat ion.Transpiration is the 10 55of water vapour from the leaves and stems of p lants. The fi gure (below)shows adaptat ions tominimize the amount of transpiration. Upper epidermis- a continuous layer of cells covered by a thick waxy cuticle. Prevents water lossfromthe upper surface the main photosynthetic tissue and is positionedeven when heated by sunlight. Lower epidermis in a cooler near the upper surfacewhere the light intensityposition has a thinner waxy cuticle The main part of the leaf is the leaf blade orlamina. It has a large surface area to absorb sunlight butis very-volum . ~ e - "", ~ 2.5 0 X "'0/ / ';:; 3 /2.25 / // 50=t X / / / 402.00 / / /x -- - -< / 1.7530 - - - ~ venti lation rate 1.50 --'-----r-----,----,-- ,------,------r- -..--'- 20 7911 13 15 1719 speed /km h:' 2. Psychological preparation - if a specific warm-uproutine is used every ti me befor e an event,itmayhelp toget the bodyment allyready forphysical act ivityand for competition, byadrenalin secretio n orother means. 3. Preventing injuries- muscles that have been war medup and tendons andligaments that have been gent ly stretched maybe less vul nerable toinjuri es. The evidence forthe effectiveness of warm -uproutines is rather thinand is based mostly onsmal l numbers of individu al cases (anecdotal evidence) rather than on controlled tri als wi th large numbe rs. Athletes are understandabl yreluctant to compete without warmi ng up, for research purposes. Some anecdotal evidence suggeststhat war ming upmaynotbe essential - reserves often compete successfullyin matches, after l ittl e or no warmi ng up! 118Physiology of exercise Trainingand thecardiovascular system MEASURING HEARTFUNCTION The cardiovascular systemconsists of theblood, the heart and thebl oodvessels. Heart functioncan be assessed usingthese measures: Heart rate is the number of contractions of the heart per minute. Stroke volumeis the volumeof blood pumped out with each contraction of the heart. Cardiac output is thevolumeof blood pumpedout bythe heart per minute. Venous return is the volume of bloodreturning to the heart via the veins per minute. EFFECTSOFEXERCISE ON THE CARDIOVASCULAR SYSTEM 1. Venous returnincreases during exerci se. Wh en muscl es i n the legs andarms contract, the muscles become shorter and wider andso exert pressure on adj acent vei ns. There are valves in these veins, ensuring that bloodf lows towards the heart.Pressure therefore causes bloodtobe squeezed along veins tothe heart,increasing venous return . Thi s al lowscardiac output tobe increased. 2. Cardiacoutput i ncreases as a result of incr eases inheart rate and stroke volume. Exercise involves a rise in carbon dioxide productionbymuscles. Absorpti onof this extra carbon di oxide intothe blood causesa decrease in blood pH. The brain detects the pHdecrease and sends impu lses to the heart' s pacemaker, causing the increase in cardiac output. 3. The di stributionof blood changes when exerci se starts. Art eriol es supplyin g the organs of the body can narrowor widen, decreasing or i ncreasi ng the flow of bl ood. The li sts belowshow which organs receive more bloodduring exercise than at rest, wh ichreceive less blood and which receives the same volume. Mor e dur ingl ess duringSame exerciseexercisevolume skeletalmuscleskidneysbrain heart wallstomach ski nintestines In summary, dur ing exercise, bloodreturns to the heart andis pumped out at a greater rate. Much of thi s bloodfl ows to the muscles,increasing the supply of oxygen, allow i ng an i ncrease in the rate of aerobicrespiration and ATP supply for muscle contraction. Ankle sprai ns torn li gament swel ling movement causing ankle sprain EFFECTSOF TRAINING ON THEHEART Trai ning can increase the thickness of theheart wa l l and the volume ofthe ventricles. The stroke volumeis therefore larger, bothatrest and during exercise. The body does notneed a larger cardiacoutput at rest, so the heart rate can be lower. Training canreduce the heart rate at rest to50 beats per minute. Atanylevel ofintensity of exercise, the heartrate is lower after training, because of the larger stroke volume. The maximumheartrate is notgreatlyaffected bytraini ng, but because of the greater stroke volume, cardiac outputis much greater at maximum heart rate after training . Thi s al lowsthe trained athlete toperform a much greater int ensity of exercise. RISKSANDBENEFITSOF EPO Ath letes someti mes increase the amount ofred bloodcells, as a proporti on of the volume of theirbl ood. This is cal led the packedcel l volume (PCV). At sea levela normalPCVis 0.4-0.5. There are several ways ofincreasing PCV above 0.5, inclu di ng the fo llowing: 1. Inj ecti ons ofEPO(erythropoietin), a naturall yproduced hormone that sti mulates red bloodcellprodu cti on. 2. Bloodtransfusions, shortlybefor e an event. Oftenthe transfused bloodwas removed fromthe at hlete's bodylong enough beforethe event forthe bloodcell s tohave been replaced. There are clear benefits interms of performance ofincreasing PCV. As these cell s transportoxygen, thelarger the numbers of them, the greater the rate at whi ch oxyge n can be carried aroundthe body by the blood.Withmore oxygen, skeletal muscl es cancontract more vigorously. There are also some ri sks. Hi gh levels ofPCV increase the chanceof bl oodclot format ion (thrombosis). Bloodcl ot s cause heart attacks andstrokes. There have been deaths among cyc lists and ot her athletes, whohad usedone or other of the methods abovetoincrease PCV. INJURIES TOMUSCLES AND JOINTS Vi gorou s exercise someti mes causes injuri es tomuscles and j oints. Tornmuscles - excessive stretching causes muscl e fibres, or mor e rarely an entir e muscle, totear, forexample the quadri ceps or hamstrings. Sprai ns - abnormal movement at a j oint causesstretching or minor tearing of ligaments, for example jo int s inthe fingers or the ankle (see fi gure,left). Tornli gament s - large abnormal movements cause l igaments to tear completely, forexample the cruci ate l igaments inthe knee. Di slocati on- abnormalmovementata joint causesthe bones tomove out ofalignment.Usuall yli gament s wi llbe tornat the same tim e. Intervertebr al di sc damage - abnormal movements or heavyloads cause the soft centre of a di sc to bulge out , thr ough a teari n the di sc wa ll(see question3 on page 120). Physiology of exercise119 EXAMQUESTIONS ON OPTIONB - PHYSIOLOGY OFEXERCISE B1Humans and other mammals can store oxygen in the lungs, in muscles and in the blood . The pie charts below show the volume of oxygen (cm3)per ki logram ofbody mass stored in these tissues inhumans andin a marine mammal, the Weddel l seal. Weddell sealHuman 2.9 3.6 14.2 o blood o muscle Dlung a)Compare the tot alamo unt ofoxyge n storedper ki logram ofbodymass inseals w it h thati n humans. [1] b)Compare the pro po rtio ns of oxygen storedinblood, muscleandlung ofseals with those i n hum ans. (Nocalculat ions arerequired). [3] c)Suggest t hreefactors w hichaffecthowmuch oxygen canbe sto redin musclein the bodyofa mammal. [3] B2a)Dr awa di agramtoshow the structur e ofa sarcomere. [3] b)O utli ne the role ofATP inmuscle co ntractio n. [3] c)Compare cardiacoutput at rest andw hen vi goro us musclecontractions are bein g performed. [1] B3The scan (right) shows damage to intervertebr al discs in theneck ofa person. Gr ey andwhit e matter inthe spi nal cordcanbe di stingui shed. a)State the number ofdiscs that are damaged.[1] b)Descri be the damage to these di scs.[2] c)State the other part of the person' s body that is affectedbythe disc damage.[1] d)Suggest howdamage toi ntervertebraldiscs may be caused.[2] 12018Quest ions - Physiology of exercise 14 lAW'"FE""OIIlr? EXAMQUESTIONS ONOPTIONC - CELLSANDENERGY Topics i n Option C are covered onpages 66-81. C1The rate ofphotosynt hesis i n plants can be influenced by many factors . Experiment s wer e carried out to i nvestigate the effect of hi gh and lowlight intensiti es on photosynthesis at different temperatur es. Allother factors were kept constant. A summary ofthe results is presentedin the graph below. .~ 10 c ::>>~ 8 :0 ~ 6 'V; o 0o 0 . ~1000 Homo habilis o ao 0 J:J oo ~Uao0 0 o.8 Australopithecus - - - ~ . 8 E "" 00:C,\\ ......i Nerve tracts contai ning axons of neurosecretory cells Network of capillari esthat release hypothalamic hormones and absorb anterior pi tuitary hormones HYPOTHALAMUS ANDPITUITARY GLAND The hypoth alamus is a smallpart ofthebrainthatlinks the nervou s andendocr ine systems. It controls hormone secretio n by the pituitary glandlocatedbelowit (shownin the figure,aboveright ). The anterior andposterior lobes of the pitu itary glandare cont rol ledina diff erentwayby the hypoth alamus: Anterior pituitary - neurosecretory cel ls in the hypoth alamus secrete hormones, cal ledreleasing hormones,intocapil laries in the hypothalamu s. These capi l laries join toforma blood vesselthat lead tothe capi l laries in the anterior pitui tary. Thi s vesselis a port al vei n - an unusualtype of bl oodvessel that carries blood directl yfromone capi l larynetworktoanother. The releasing hormones st imulate the anterior pituitary to secretehormones. For example, GnRHsti mulates the release ofFSHandLH. Posterior pituitary - neurosecretory cellsi n the posterior pi tuitary synthesizehormones, pass themvia axons to nerve endings inthe posterior pituitary andcontrol their secretion. The secretionof ADHis control ledin thi s way (see right ). POSTERIORLOBEOFANTERIOR LOBEOF PITUITARY GLANDPITUITARYGLAND CONTROL OF ADHSECRETION Neurosecretory cellsin the supra-opticnucl eus ofthe hypothalamus synthesize ADH,transportit downtheiraxons and storeit in nerve endi ngs i n the posterior pituitary gland. Osmoreceptor cells in the hypoth alamus monitorthe concentrat ion of the bl oodpl asma. If the pl asma becomes too concentrated,impulsesare passed tothe ADH-secretin g neurosecretory cel ls, whic h convey the impul ses to theirnerve endi ngs in the posteriorpituitary. The impul ses sti mulate release of ADH into the blood fromthe stores in the nerve endings. ADHcauses a reductioninthe concent ration ofthe blood pl asma,by stimulati ng the kidn ey toprodu ce hypertoni c urin e (see page 102). If the osmoreceptor cel ls detect that the concentration ofblood pl asma is toolow, the neurosecretory cells are not stimul ated to rel ease ADHandthe bloodADHlevelrapidl y drops, allowing larger volumes ofdilute hypotonic urine tobe exc reted. 162Further human physiology Secretion of digestive juices SUMMARY OFDIGESTION Foodis digested as it passes along the ali mentary canal, from the mouth tothe anus. Longit udi nal and ci rcular muscl e fibr es inthe wal l of the alimentarycanal contract andrel ax, squeezi ng the food andbreaking uplarge solidlumps. Digestive j uices, contai ni ng enzymes, are mixed with the food. The enzymes di gest proteins,nucl ei c aci ds, starch and other macromolecules. Di gestive j ui ces are secreted by the saliva ry glands, by glands in the wal l of the stomach andby t he pancreas. These are al l examples ofexocri ne glands. Some macromolecules cannot be digested byhumans, for example cel lulose. The enzyme cel lulase digests cel lulose, buthumans lack the gene that codes for this enzyme, and so cannotmake it. Undigested cell ulose is an important part of dietary fibre, which has beneficial effects on the digestive system. EXOCRINE GLANDS The secretorycells i n an exocr i ne gland are i n a layer that is onlyone cel l thi ck. The total area of the layer of secretory cel ls can be very large because ofi nvagi natio n and branching. The digestive jui ce is released fromthe cel ls by exocy tosis. It is then discharged from the gland by travell i ng along ducts. One group ofsecretory cel ls, cl ustered around the end of a duct,is calledan acinus . The ducts and aciniin part of the pancreas that secretes pancreaticj uice are shown below. Structure of exocrine glandtissue inthepancreas basement membrane wall of duct one acinus EXOCRINEGLANDCELLS Exocrine gland cell s have disti nctive features. One or two prominent nucl eoliinside the nucl eus, for producti on ofribosome subunits. An extensive "area of rough endop lasmic reticul um, for protein synthesis. Golgiapparatuses forprocessing proteins. Manylarge vesicles, sometimes cal led secretory granules, for storage of the substances being secreted and transport of them to the plasma membrane. The vesicl es are usual ly densely stainedbecause of the concentration of proteins. Mitochondria, to provide ATP forprotei n synthesis and other cellact ivities. The figure (right) is an electron micrograph of a pancreas cel l and shows these disti nct ive features. CONTROL OF GASTRIC JUICESECRETION The controlof di gestive j uicesecretion invol ves bothnerves andhormones. The control of gastricj uice secreti on is describedhere as an example. Before foodreaches the stomach, gastricj ui ce is already being secreted, as a resultofa ref lex action. The sight or smel l of foodstimulates the brain to send nerve impul ses to exocri ne gland cells i n the wal lof the stomach. The gland cells start to secrete gast ricjui ce i n response. Mu chmore gastric jui ce is secreted when food enters the stomach. The foodis detectedby touchreceptors and chemoreceptors i n the lining of the stomach andby stretch receptors in the stomach wal l.Impul ses are sent fromthese receptors to the brain, which sends more nerve impulsesto the exocrine gland cell s. W hen foodis in the stomach,impulses are also sent to endoc rine gland cells in the stomach lining that secrete a hormone called gastri n. Gastrin is carried to the exocri ne gland cell s i n the stomach wall , where it stimulatesthem toincrease the secretion ofhydrochl oric acid. Thi s causesthe pH of the food that has entered the stomach to fall to about pH3.0. MEMBRANE-BOUND DIGESTIVE ENZYMES Enzymes secreted by exocrine gland cells become mixed with the foodi n the alimentary canal and carry out all the init ial stages of digestion. However, some of the enzymes that complete the processof di gestion wo rk in a different way. They are produced by the wal l of the smallintestine, but are not secreted. Instead, these enzymes remaini n the plasma membranes of cell s on the surface of t he vil li (epithelium cells). The active sites of the enzymes are exposed to the food in the smallintestine. They can di gest their substrates and the products of digestioncan then immediately be absorbed. Epi theliumcel ls tend to be lost from the tips of vil li by abrasion, but t he membrane-bound enzymes conti nue to wo rk as they become mixed into the foodin the smal l intestine. Electronmicrographof an exocrine gl and cellinthe pancreas(X6000). The cent ral region of one cellis shown incl udi ng the nucl eus. Further human physiology163 Digestive enzymes SOURCES OF DIGESTIVEENZYMES Foodcontai ns many different types of substance that have to be digested before they can be absorbed.Digestiontherefore involves many different enzymes, secretedby exocrine glands. The table allowsthe contents of sal iva,gastric j uice and pancreati c ju ice to be compared - there are both sim ilar ities and differences. Di gesti ve juiceSourceCont ent sal ivasal ivary- sal ivary amylase glands- mucus gastric jui ceglands i n- pepsinogen stomach- hydrochloric aci d wall- mucus pancreati c jui cepancreas- pancreaticamylase - pancreaticlipase - phospholipase - trypsi nogen - carboxy peptidase - HC0 3ions (alkaline) Pepsin and trypsi n are potenti all y veryharmful to the exocrine gland cells that secrete them. They are therefore secreted as inactive precur sors, cal ledpepsinogen and tr ypsinogen. Pepsinogen is activated byhydrochlori c aci d, whi ch converts it into pepsin.Di fferent cells in the wal l of the stomach secrete pepsinogen andhydrochl oric acid (below) . Pepsinogenis therefore only activated after ithas been secreted. Anenzyme, enterokinase, which is secreted by t he lini ng of the smal lintestine, activates trypsi nogen. Acti vati on therefore onlyhappens when trypsinogen enters the smallintesti ne. Structure of the stomach wall pits epitheli um cells in neck - - 1 - - - - l - - - l ~of gastric gland (secrete mucus) oxyntic cells - - + - - - + - - - 4 ~(secrete hydrochloric acid) pepti c cells--- -+----J,;:1DJ (secrete pepsinogen) DIGESTION OFLIPIDS The di gesti onoflipids poses special problems,because they are insoluble inwater. Foods and the di gestive juices added to them are mai nly composed of water. In the ali mentary canal,li pids infoods melt and formliquid droplets. Because of their insol ubil ity, these drop lets tendto coalesce toform larger droplets. Lipase is water-soluble so it does not enter the lipid droplets, butits activesiteis hydrophobic (shownon page 68) and hydro lyses lipid s on the surface ofdroplets. The dropl ets graduallydecrease in sizeas the li pids on thei r surface are digested.However, fooddoes notremainin the ali mentary canal long enough forlarge droplets tobe digested completely. Bi le helps to overcome thi s probl em. It contai ns substances cal led bilesalts, which are natural detergents. Bi le salt molecul es have a hydrophobic endand a hydroph ili c end. They are therefore attracted to bothwater and lipids and coat l ipid droplets, causi ng them tobreakupinto smal ler dropl ets. This process is cal led emulsificat ion. Bi leis secreted by the l iver and storedin the gal l bladder. Whenitis di schargedinto the smal l intestine it emulsifies l ipids, whi ch speeds up their di gestion, because many small drop lets have a larger total surface area, accessible tolipase, than one large droplet of the same volume. With the hel p of bi le,lipids can be completely di gestedin the smal l i ntestine. THEEFFECTSOFHELICOBACTER PYLORI Heli cobacter pylori is an acid-tolerant bacteriumthat infects the l ining of the stomach. There is evi dence thatit causes several diseases of the stomac h. 1. Stomachulcers These are areas of damage to the l ini ng of the stomach. Old medical textbooks state that they are causedby excessive secretion of gastric j uice, containi ng acid. There is nowstrong evidence t hatinfect ion of the stomach withH. pyloriis a more signif icant factor than gastricaci d. Ant acidtreatments may relieve the symptoms of ulcers for a while,butnotpermanentl y. Antimicro bial treatments that el imi nateH. pylori i nfection cure ulcers on a long-term basis. H.pylorii nfecti onis strongly associated with the presence of stomach ul cers. Volunt ary infect ion with the bacterium has shown that it can cause gastrit is, which often leads toul cerati on. About half ofthe H. pylori strains isol ated from patients wi th stomach disease produce toxins that cause i nflammation - andpatients i nfected withthese strai ns tend tohave the most severe ulcerati on. Proteases and ot her enzymes that are rel eased byH. pylori damage the stomach lin i ng. 2. Stomach cancer Stomachcanceris the growth of tumour s i n the wal l of the stomach. As with stomach ulcers, a far higher percentage of pati ents with stomach cancer are infected with H. pyl ori than the general popu lation. H. pylor i i nfect ion is associated wit h reduced vitami n C concentration ingastric j uice. This wi ll i ncreasethe chance of a tumourformi ng, but further research is needed to establish a causal l ink between H. pylori infecti onand stomach cancer. 164Further human physiology Absorptionof digested foods STRUCTURE OF THEILEUM Digested foods are absorbedin the small intestine, mainly in the latter part, ca lled the ileum. The tissue layers of the wall of theileum are shown in thetransversesection below(left). Thesetissuelayers are visiblein thelight micrographof theileumbelow(right). villi Tran sversesect ionofileumMicro graph ofileuminlongitudinal section(x40) longitudinal musclelayer circular musclelayer VILLUSEPITHELIUMCELLS Digestedfoods areabsorbedby villi In theileum. The st ructure of a villus is shown o n page 47. Theouter layer of ce lls whereabso rption oc cursis thee pithe lium. Thefigure (right) is elect ronmicrograp h of epithe lium ce lls, showing the structura l featu res t hat are typical of this ce ll type . Theplasma membranes of adjacent ce lls are firmly linked togeth ernear thefreesurfaceby structuresca lledtight junctions. These structuresprevent molecu les fromleakingbetween the epithe lium ce lls. Tobe abso rbed, di gestedfood s have topass throughtheplasmamembraneof theepit hel iumce lls,and abso rpt ion ca n thereforebe ca refully co ntrolled. Thetable be low describes the mech an isms usedto absorb foods and the struct ura l features usedin these mechan isms. Somematerials are not absorbed,inclu din g ce llulose, ligni n, bile pigments, bacter ia and abrade dintest inal ce lls. They are the reforeegestedinthefeces . Micrograph ofvillusepitheliumcell s(x2500) __ JIo:O;W.;-.y.

Rel ationship sbetw een st ructur eandfunctioninvillus epitheliumcell s Structuralfeature Microvilli - prot rusio ns of thefreesurfaceof the plasmame mbraneinto thelume n of theileum; about1urnlo ng and Oil urnwide . Mitochondri a - ther eare man ymitochondria sca tte redthrough the cyto plasm. Pinocyt ic ves icles - ther e areman y sma ll ves icles, es pecially nearthe microvi lli. Function Microvi lli great lyincr easethesurfaceareaofplasmamembran eexposedtothe digestedfood in the ileum. This increases therate of abso rption of foods by diffusion . Lipids, and othe r foods that ca n passeas ily through thehyd ro pho bic ce ntre of theplasma membrane of the epithel ium ce lls, areabso rbed by simpl e diffusion . Fructose and so me ot her hydrophilic foodsubsta nces at a low co nce nt ration inside bod y ce lls areabsorbedby facilitateddiffu sion . Ther eis a steep enough co nce ntration gradient for abso rpt ion of these substances by diffusion , but theyneed ass ista nce to pass thr ough the plasma membrane. Channe l pro teins hel p themto cross the hydrophobi c ce ntre of the me mb rane. MitochondriaproducetheATP thatisneed edforabso rption of substa ncesbyactivetransport. Pumpprot eins in theplasmamembrane of themicrovi lli ca rry out theactive transport. Glucose, amino acids and miner al ions incl uding sodium, ca lcium andiron are abso rbed in th is way. Pinocyt ic vesicles are formedby endocytosis. Each ves icle co nta ins a sma ll droplet of fluid fromthelumen of the ileum. The membranes of these ves icles are formed from the plasma membran e and so co nta in cha nnels for faci litated diffusionand pumps for active transport . Digested food s ca n be abso rbedfrom the vesicle s into the cytoplasm. Furtherhuman physiology165 Liver BLOODFLOW THROUGH THELIVER The li veris the largest organ in the human abdomen. It contai ns huge numbers of cel ls call edhepatocytes, which carry out many vi tal processes. The liver is suppliedwit h bloodby two vessels - the hepaticportal vein and the hepatic art ery. One vessel, the hepatic vein, carries blood away. The bloodbrought by the hepati c portal vein is deoxygenated, because it has alreadyflowed through the wal l of the stomach or the i ntestines. The level of nutr ients in this blood varies considerably, depending on the amount of di gested foodthatis being absorbed. One of the main funct ions of the li veris toregul ate level s ofnutrients before the blood f lowsonto the rest of the body. Excessively high level s of glucose and othernutrients wo uld cause damage to the organs of the body, especial ly the brai n. Inside the liver, the hepaticport al vei n divides upinto vessels cal led sinusoids. These vessels are wider than normal capi l laries, with wal ls that are more porous. The wal ls consist of a si nglelayer of very thincel ls. There are many pores or gaps between the cells but nobasement membr ane. Blood f lowi ng along the sinusoidsis therefore in cl ose contact with the surrounding hepatocytes. The hepatic artery supplies the l iver wi th oxygenatedblood fromthe left side of the heart via the aorta. Branches of the hepatic artery jo in the sinusoids at various points alongtheir length, provi di ng the hepatocytes with the oxygen that they need foraerobiccel l respi ration. The si nusoids draininto wider vessels that are branches of the hepati c vein.Blood fromthe li ver is carriedby the hepatic vei n to the right side of the heart viathe infer ior vena cava. The figure (below)showsthe relationshi ps between blood vessels in li ver ti ssue. St ructur e ofa si nusoidinthe li ver hepatocytes lumen of sinusoid Kupffer cell branch of hepati c vein f 166Furtherhuman physiology ROLESOF THELIVER Nutrient storage and regulati on When certain nutri ents are i n excess i n the blood, hepatocytes absorb andstore them,releasing them when they are at tooIowa level.For example, when the blood glucose level is toohigh,i nsulinstimulates hepatocytes to absorb glucose and convert it to glycogen forstorage. When the blood glucose levelis toolow, glucagon stimulates hepatocytes tobreak downglycogen and releaseglucose into the blood. Iron,retinol (vitamin A) and calciferol (vitamin D) are also storedin the li ver when they are in surplus andreleased when there is a deficitin the blood. Breakdownof erythrocyt es Erythrocytes, also calledred bloodcel ls, have a fai rly short lifespan of about 120 days. The plasma membrane becomes fragi le and eventuallyruptures, releasing the hemoglobin into the blood plasma. The hemoglobin is absorbed by phagocytosis, chiefly in the liver. Some of the cells in the walls of the sinusoids are phagocyti c, called Kupff er cells. Inside these cells the hemoglobinis splitintoheme groups and globins. The globi ns are hydrolysed toamino acids, whichare releasedinto the blood. Ironis removed from the heme groups, toleave a yellow coloured substance called bil e pigment or bi li rubin . The iron and the bile pigment are released into the blood. Mu ch of the ironis carried tobone marrow, to be used in production of hemoglobi n for newred blood cell s. The bi le pigment is used for bil e producti onin the liver . Globi ns~Ami no acids Hemoglobi n/