lntrod uctionImmunity is based on rccognition oT self anddcstmction of ftrreign material. The roles of thelnuscukrskeletal systern are movement, sLtpportand protection. All animals excrete nitrogenous

waste products and some animals also balanccwater and solute concentratittns. Sexttalreproduction involves the dcvelopment andfusion oI haploid gametcs.

Antigens on the surface of red blood cellsstimulate antibodg production in a person witha different blood group.

Smallpox was the f irst infectious diseaseof humans to have been eradicated bU

vaccination.Monoclonal antibodies to hCG are used inpregnancU test kits.

Understandingà Everg organism has unique molecules on the

surface of their cells.

à B lgmphocgtes are activated bg T lgmphocatesin mammals.

à Plasma cells secrete antibodies.) Activated B cells multiplg to form a clone of

plasma cells and memorg cells.t Antibodies aid the destruction of pathogens.

à lmmunitg depends upon the persistence ofmemorg cells.

Ð Vaccines contain antigens that trigger immunitgbut do not cause the disease.

à Pathogens can be species-specif ic althoughothers can cross species barriers.

+ White cells release histamine in response toallergens.

t Histamines cause allergic sUmptoms.

à Fusion of a tumour cell with an antibodg-producing plasma cell creates a hgbridoma cell.

-) Monoclonal antibodies are produced bghgbridoma cells.

@ srirrà Analgsis of epidemiological data related to

vaccrnatron program mes.

' Consider ethical implications of research:Jenner tested his vaccine for smallpox ona child.

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ANTMAL PHYSt0L0GY (AHLJ

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Antigens in blood transfusionEverg organism has unique molecules on the surface oftheir cells.Any foreign molecule that can trigger an immune response is referredto as an antigen. The most common antigens are proteins and very largepolysaccharides. such molecules are found on the surface of cancer cells,parasites and bacteria, on pollen grains and on the envelopes of viruses.As an example, flgure I shows a representation of an influenza virus.Hemagglutinin and neuraminidase are two antigens found on thesurface of the virus. Hemagglutinin allows the virus to stick to host cells.Neuraminidase helps with the release of newly-formed virus pafticles.The surface of our own cells contains proteins and polypeptides.Intrlune systenìs luncLion basecl on recognizing rhe distincrion berween"foreign" antigens and "self". Figure 2 shows a mixture of pollen grainsfrom several species. The antigens on the surface of these grains areresponsible for triggering immune responses that are called "allergies" or"hay fever" in common language,

hemagglutinin

r Figure 2 Pollen grainslipid membrane

other protein

genetic material IRNA]

neuraminidase

r, Figure 1 lnfluenza virus

@ mtigens in blood trensfus¡onAntigens on the surface of red blood cells stimulate antibodg product¡on in aperson with a different blood group.Blood groups are based on the presence orabsence of certain types of antigens on the surfaceof red blood cells. I(nowledge of this is importantin the medical procedure called transfusion wherea patient is given blood from a donor. The ABOblood group and the Rhesus (Rh) blood group arethe two most important antigen systems in bloodtransfusions as mismatches between donor andrecipient can lead to an immune response.

In figure 3, the differences between the three A, Band O phenotypes are displayed. All three allelesinvolve a basic antigen sequence called antigen H.In blood type A and B, this antigen H is modifiedby the addition of an additional molecule. If theadditional molecule is galactose, antigen B results.If the additional molecule is N-acetylgalactosamine,

antigen A results. Blood type AB involves thepresence of both tlpes of antigens.

Keg

f redbloodcell I Nacetgl-galacrosamine f fucosef Nacetgl-glucosamine fgalactose

À Figure 3

466

11.1 ANTIBOOY PRODUCTION AND VACCINATION

If a recipient is given a transfusion involving thewrong type of blood, the result is an immuneresponse called agglutination followed byhemolysis where red blood cells are destroyed andblood may coagulate in the vessels (figure 4).

+

red blood cells with antibodies from agglutination hemolgsissurface antigens from recipient Iclumping]an incompatable donor

^r Figure 4

Blood typing involves mixing samples of bloodwith antibodies. Figure 5 shows the result of ablood group test showing reactions between bloodtypes (rows) and antibody serums (columns). Thefi.rst column shows the blood's appearance priorto the tests. There are four human blood types: A,B, AB and O. Type A blood has type A antigens(surface proteins) on its blood cells. \pe B bloodhas type B antigens. Mixing type A blood withanti-A+B serum causes an agglutination reaction,producing dense red dots that are different fromthe control in the flrst column. TYpe B bloodundergoes the same reaction with anti-B serumand anti A-|B serum. AB blood agglutinates in allthree anti-serums. Type O blood has neither the Aor B antigen, so it does not react to the serums.

'l¡i1

r Figure 5

The specific immune responseB lgmphocgtes are activated bg T lgmphocgtes in mammalsThe principle of "challenge and response" has been used to explainhow the immune system produces the large amounts of the specificantibodies that are needed to fight an infection, and avoid producingany of the hundreds of thousands of other types of antibodies thatcould be produced. Antigens on the surface of pathogens that haveinvaded the body are the "challenge". The "response" involves thefollowing stages.

Pathogens are ingested by macrophages, and antigens from them aredisplayed in the plasma membrane oI the macrophages. Lymphocytescalled helper T cells each have an antibody-like receptor protein intheir plasma membranes, which can bind to antigens displayed bymacrophages. Of the many types of helper T cell, only a few havereceptor proteins that flt the antigen. These helper T cells bind and areactivated by the macrophage.

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ANTMAL PHYSt0L0GY IAHL)

v@ Antibodies produced bg the clone of

plasma cells are specific to antigenson the pathogen and help to destrog it

r Figure 6 The stages in anlibodg production

Å Figure 7 A plasma cell

The activated helper T cells then bind to lymphocytes called B cells.Again, only B cells that have a receptor protein to which the antigenbinds are selected and undergo the binding process. The helper T cellactivates the selected B cells, both by means of the binding and byrelease of a signalling protein.

The role of plasma cellsPlasma cells secrete antibodies.Plasma cells are mature B lymphocytes (white blood cells) that produceand secrete large number of antibodies during an immune response.Figure 7 shows a plasma cell. The cell's cytoplasm (orange) contains anunusually extensive network of rough endopÌasmic reticulum (rER).rER manufactures, modifles and transports proteins, in this case, theantibodies. The cell produccs a lot of the same type of protein meaningthat the range of genes expressed is lower than a typical cell. Thisexplains the staining pattern of the nucleus where dark staining indicatesunexpressed genes.

Clonal select¡on and memorg cell formationActivated B cells mult¡plg to form a clone of plasma cellsand memoru cells.The activated B cells divide many times by mitosis, generating a clone ofplasma cells that all produce the same antibody type. The generation oflarge numbers of plasma cells that produce one speciflc antibody type isknown as clonal selection.

The antibodies are secreted and help to destroy the pathogen in waysdescribed below. These antibodies only persist in the body for a fewweeks or months and the plasma cells that produce them are alsogradually lost after the infection has been overcome and the antigensassociated with it are no longer present.

Although most of the clone of B cells become active plasma cells, asmaller number become memory cells, which remain long after theinfection. These memory cells remain inactive unless the same pathogeninfects the body again, in which case they become active and respondvery rapidly. Immunity to an infectious disease involves either havingantibodies against the pathogen, or memory cells that allow rapidproduction of the antibody.

The role of antibodiesAntibodies a¡d the destruct¡on of pathogens.Antibodies aid in the destruction of pathogens in a number of ways.o Opsonization: They make a pathogen more recognizable to

phagocytes so they are more readily engulfed. Once bound, they canlink the pathogen to phagocytes.

@ Macrophage ingests pathogenand displags antigens from it

@ Helper T cell specific to th. \antigen is activated bg the \macro

@B cell also dividestomemorg cel

OB cell specific to the antigen is actlvatedbg proteins from the helper T cell

¡'{

B cell divides repeatedlgto produce antibodg.secreting plasma cells

v7{ 7v

468

11.1 ANTIBODY PRODUCTION AND VACCINATION

o Neutralization of viruses and bacteria: Antibodies can preventviruses from docking to host cells so that they cannot enter the cells.

o Neutralization of toxins: Some antibodies can bind to the toxinsproduced by pathogens, preventing them from affecting susceptible cells.

o Activation of complement: The complement system is a collectionof proteins which ultimately lead to the perforation of the membranesof pathogens. Antibodies bound to the surface of a pathogen activatea complement cascade which leads to the formation of a "membraneattack complex" that forms a pore in the membrane of the pathogenallowing water and ions to enter into the cell ultimately causing thecell to lyse.

o Agglutination: Antibodies can cause sticking together or"agglutination" of pathogens so they are prevented from entering cellsand are easier for phagocytes to ingest. The large agglutinated masscan be flltered by the lymphatic system and then phagocytized. Theagglutination process can be dangerous if it occurs as a result of anincorrect blood transfusion.

Figure 8 summarizes some of the modes of action of antibodies.

functlon of antibodiesact¡vation of complement

complement

bacterium lgslx

What can game theorg tellsus about the pers¡stence ofsmallpox stockpiles?

0nce wild smallpox hadbeen eradicated thereremained the challenge ofwhat to do with samples ofsmallpox still in the handsof researchers and themilitarg. Despite calls forthe remaining stockpiles tobe eradicated bg the WH0,both the US and Russia havedelaged complging with thisdirective.

Game theorg is a branch ofmathematics that makespredictions about humanbehaviour when negotiationsare being undertaken. lnterms of pagoff, if one sidereneges and the otherproceeds on the basis oftrust, the gain to the dealbreaker is maximized. lnthis case, theg are no longerthreatened bg the adversargbut retain the abilitg tothreaten. lf both partiesrenege, the risk remainsthat the virus will be usedas a weapon in both the firstattack and in retaliation.Maximum net gain for allwould involve both partiescomplging with the directivebut this involves trust andrisktaking.

*n Figure I

Immunitglmmunitg depends upon the persistence of memorg cells.Immunity to a disease is due either to the presence of antibodies thatrecognize antigens associated with the disease, or to memory cellsthat allow production of these antibodies. Immunity develops whenthe immune system is challenged by a specific antigen and producesantibodies and memory cells in response. Figure 9 distinguishes aprimary immune response (launched the first time the pathogen infects

TOK

aggluti nation

reduces number of

neutralization

blocks adhesion of bacteria and docking

loxrn

of viruses to cellsblocksactivltgof toxi ns

phagocgte

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469

ANTMAL PHYSt0L0GY (AHL)

the body) and the secondary immune response which ìs ìaunchecr thesecond time the pathogen infects the body. Memory cells ensure thatthe second time an antigen is encountered, the body is ready to respondrapidly by producing more antibodies at a faster rate.

secondarg response

pnmarg response

10 ?0 3 40

1

50 60time/dags

second encourìterwith antigen

r Figure 9 The secondarg immune response

Vaccines lead to immunitgVaccines conta¡n antigens that trigger immunitg but donot cause the disease.A vaccine is introduced into the body, usually by injection. The vaccinemay contain a live attenuated (weakened) version of the pathogen, orsome derivative of it that contains antigens from the pathogen. Thisstimulates a primary immune response. If the actual microorganismenters the body as a result of infection, it will be destroyed by theantibodies in a secondary immune response.

Figure 10 shows a phagocyte engulflng a Mycobacterium bovis bacterium(orange). This is the strain of the bacterium used in the vaccination fortuberculosis (TB), The bacteria are live but attenuated (weakened) andnot as pathogenic as their relative Mycobacterium tuberculos¡s. The vaccineprimes the immune system to produce antibodies that act on bothspecies of bacteria, without causing the disease, so that it responds morerapidly if infected with Mycobacterium tuberculosis (TB) bacteria.

@ eÜrical considerations of Jenner's vaccine exper¡mentsConsider ethical implications of rêsêârch: Jenner tested his vaccine for smallpoxon a ch¡ld.Edward Jenner was an lSth century scientistwho noted that a milkmaid claimed thatbecause she had caught the disease cowpox

ÐEU3Co

Eo.EoCoCo

f¡rst encounterwith antigen

L Figure 10

she would never develop smallpox. He infectedan eight-year-old boy with cowpox. After abrief illness, the boy recovered. Jenner then

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470

purposely infected the boy with smallpox toconflrm that he had the ability to resist thedisease.

He was the first person to use human beingsas research subjects in testing a vaccine. He did notdo any preliminary laboratory research nor anypreliminary animal studies before experimentingwith human beings, his subject was a small childwell below the age of consent, and he deliberatelyinfected him with an extremely virulent, oftenfatal, disease-causing agent.

11.1 ANTIBODY PRODUCTION AND VACCINATION

Jenner's experiments were performed well before theformulation of any statements of ethical principlesfor the protection of human research subjects. TheNuremberg Tfials condemned medical experimentson children. These trials that followed the SecondWorld War resulted in the Nuremberg Code forthe protection of research subjects, and later theWorld Health Organization's Intemational EthicalGuidelines for Biomedical Research Involving HumanSubjects (19931. Jenner's experiments would not beapproved by a modem ethical review committee.

The eradication of smallPoxSmallpox was the f irst infectious disease of humans to have been eradlcatedbg vaccinat¡on.The efforts to eradicate smallpox are an exampleof the contributions that intergovernmentalorganizations can make to address issues of globalconcern. The first such effort was launched in I950by the Pan American Health Organization. TheWorld Health Assembly passed a resolution in 1959to undertake a global initiative to eradicate smallpox.It met with mixed success until a well-fundedSmallpox Eradication Unit was established in 1967.

The last known case of wild smallpox was in 1977in Somalia, though there were two accidentalinfections after this. The campaign was successfulfor several reasons:

. Only humans can catch and transmitsmallpox. There is no animal reservoir where

@ Vaccines and epidemiologgAnalgsis of epidemiological data related to vacc¡nat¡on programmes.

a

a

the disease could be maintained andre-emerge. This is the reason a yellowfever eradication effort failed in theearly 1900s.

Symptoms of infection emerge quite quicklyand are readily visible allowing teams to"ring vaccinate" all of the people who mighthave come in contact with the afflictedperson. In contrast, efforts to eradicatepolio have been hampered because infectedpersons do not always present readilyrecognized symptoms.

Immunity to smallpox is long-lasting unlikesuch conditions as malaria where reinfection ismore common.

Epidemiology is the study of the distribution,patterns and causes of disease in a population. Thespread of disease is monitored in order to predictand minimize the harm caused by outbreaks as

well as to determine the factors contributing to theoutbreak. Epidemiologists would be involved inplanning and evaluating vaccination programmes.

An effort to achieve the global eradication of poliowas begun in 1988, as a combined effort between

the World Health Organization (WHO), UNICEFand the Rotary Foundation. Similarly, UNICEF isleading a worldwide initiative to prevent tetanusthrough vaccination.

A small number of polio cases are the result ofa failure in vaccination programmes. Figure I Ishows the incidence of "wild" rather than vaccine-induced polio cases in India over a seven-yealperiod. Epidemiologists would investigate to

47t

ANTMAL PHYSt0L0GY fAHL)

determine the causes of the two peaks in numbers.Figure 12 shows the geographic distributionof polio cases over a l3-year period in India.Epidemiologists would use information aboutgeographic distribution to determine origins ofoutbreaks so they could focus resources on thoseareas. They could track incidence to determinethe effectiveness of reduction campaigns. lt isheartening to know that by 2012, India had beendeclared polio-free.

The concem is that polio-free countries can still seesome polio cases if infected individuals cross borders.

2000 2001 2002 2003 2004 2005 2006 2007gear

ØoØoooo¡E=c

18001600140012001000800600400200

0

À Figure 11

AJJÒ

1,934 cases

2002

1,600 casesJ

2006

255 cases

2007

lYVY 2 nnn

265 cases

2004

268 cases

2005

tt I1,126 cases

2003

134 cases 66 cases

2009

lI t I

Isescâ676 74 cases

2008

559 cases,

20IT

Ttt

20to

III

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66

26s268 255 134

873

576

472

À Figure 12

11.1 ANTIBODY PRODUCTION AND VACCINATION

Data-based questions: Polio incidence in?0t2Figure 13 provides data aboul" polio incidencein the three countries where wild polio was stillendemic as of mid-20I2.I Define the term "endemic" (l)2 Identify the three countries where polio

was still endemic as of mid-2012. (l)3 Identify the strain of polio virus which is

the most prevalent. (I )

4 Idcntify one country where the situation appearsto have improved between 2}ll and2}l2' (2)

5 Given that in 1988 there were an estimated350,000 cases of polio globally, discuss thesuccess of the polio eradication programme. (5)

ó Suggest sorne of the challenges an epidemiologistmight face in gathering reliable data. (5)

7 Research to find the status of polio eradicationin these countries.

o

n

total in 2011!lear-to-date 2011!learto-date 2012

¡4

=,

BO

date of mostrecent case

30 June 20 12WPVl WPV2 W1W3 totaltt00tlWPVl WPV2 W1W3 total130013

20Llto-date 2012 date of mostrecent case

22 JuneZOLZ

total in 2011

198WPVl WPV3 W1W3 total581059WPVl WPV3 W1W3 total20?L23

total in 2011qear-to-date 2011Ltearto-date 2012 date of mostrecent case

22 JuneZOIZ

total cases

WPVl WPV3 W1W3 total4213055

total in 20 1 1YTD 2012 YTD 2011274 6s095globallt.l

WPV1 WPV3 W1W3 total146020 62

34r309

915

91183

in endemic counlriesin non-endemic countries

Global

Nigeri

Figure 13

Zoonosis are a grow¡ng global health concernPathogens can be species-spec¡f¡c although others cancross species barriers.Pathogens are often highly specialized with a nalrow range of hosts. Thereare viruses that are speciflc to birds, pigs and bacteria for example. There are

bacteriaÌ pathogens that only cause disease in humans. Humans are the onÌyknown organism susceptible to such pathogens as slphilis, polio and measles,

but we are resistant to canine distemper virus, for example. The bacteriumMycobacterit.ttn htberculosis does not cause disease in lrogs because frogs rarelyreach the 37 'C temperature necessary to support the proliferation of thebacterium. Rats injected with the diphtheria toxin do not become ill becausetheir cells lack the receptor that would bring the toxin into the cell.

473

ANTMAL PHYSt0L0GY (AHLJ

Figure 14 A thermal scanning camera is beingused to monitor the skin temperature ofpassengers arriving at Nizhng Novgorod airport,in Russia. Raised skin temperature can be anindicator of fever from illnesses. Such camerashave been used widelg to screen for possiblecarriers of various possible zoonotic epidemicinfluenzas such as bird flu and swine flu

Figure 15 The rash across the bodg of thismale patient is due to the release of excessivehistamines in response to taking Amoxicillin(penicillin J antibiotic

A zoonosis is a pathogen which can cross a species barrier. This is anemerging global health concern. Bubonic plague, Rocky Mountain spottedfever, Lyme disease, bird flu and west Nile virus are all zoonotic diseases. Themajor factor contributing to the increased appearance of zoonotic diseasesis the growth of contact between animals and humans by such means ashumans living in close contact with livestock or disruption of habitats.For example, in the late t990s in Malaysia, intensive pig farming in thehabitat of bats infected with the Nipah virus eventually saw the virusmove from the bats to the pigs to the humans and resulted in over100 human deaths.

The immune sgstem produces histaminesWhite cells release histamine ín response to allergens.Masr ceiis are immune ceiis founcl rn connective tissue that secretehistamine in response to infection. Histamine is aÌso released bybasophils which circulate in the blood. Histamine causes the dilation ofthe small blood vessels in the infected area causing the vessels to becomeleaky. This increases the flow of fluid containing immune components tothe infected area and it allows some of the immune components to leavethe blood vessel resulting in both speciflc and non-specific responses.

Efiects of histam¡nesHistamines cause allergic sgmptoms.Histamine is a contributor to a number of symptoms of allergic reactions.cells in a variety of tissues have membrane-bound histamine receptors.Histamine plays a role in bringing on the symptoms of allergy in the nose(itching, fluid build-up, sneezing, mucus secretion and inflammation).Histamine aÌso plays a role in the formation of allergic rashes and in thedangerous swelling known as anaphylaxis.To lessen the eifects of allergic responses, anti-histamines can be taken.

The process for creat¡ng hgbridoma cellsFusion of a tumour cell with an antibodg-producingplasma cell creates a hgbridoma cell.Monoclonal antibodies are highly speciflc, purifled antibodies thar areproduced by a cÌone of cells, derived from a single cell. They recognizeonly one antigen.

______+isolatespleen B

cells

plasma cells

lmmuntze mouse

antigen and dgeused to screen to find

desired hgbridoma

cell culture

Figure 16

rc-i

t

474

------+

mgeloma cells

__>

hgbridomas

--+

11.1 ANTIBODY PRODUCTION AND VACCINATION

To produce the clone of cells that will manufacture a monoclonalantibody, the antigen recognized by the antibody is injected into amouse, or other mammal. In response to this challenge, the mouse'simmune system makes plasma B cells that are capable of producingthe desired antibody. Plasma cells are removed from the spleen of themouse. They will be of many different types with only some producingthe desired antibody.

The B cells are fused with cancer cells called myeloma cells. Thecells formed by fusion of plasma B cells and myeloma cells are calledhybridoma ceìls.

Production of monoclonel ant¡bodiesMonoclonal antibodies are produced bg hgbridoma cells.Because the full diversity of B cells are fused with the myeloma cells,many different hybridomas are produced and they are individuallytested to flnd one that produces the required antibody'

Once identified, the desired hybridoma cell is allowed to divide andform a clone. These cells can be cultured in a fermenter where theywill secrete huge amounts of monoclonal antibody. Figure 17 shows a2000-litre fermenter used in the commercial production of monoclonalantibodies. The hybridoma cell is multiplied in the fermenter to producelarge numbers of genetically identical copies, each secreting the antibodyproduced by the original lymphocyte.

Monoclonal antibodies are used both for treatment and diagnosis ofdiseases. Examples include the test for malaria that can be used toidentify whether either humans or mosquitoes are infected with themalarial parasite, the test for the HIV pathogen or the creation ofantibodies for injection into rabies victims.

@

Pregnancg tests ernploU monoe lonalantibodies

Monoclonal antibodies to hCG are used in pregnane g

test kits.Monoclonal antibodies are used in a broad range of diagnostic tests,including tests for HIV antibodies and for an enzyme released duringheart attacks. Pregnancy test kits are available that use monoclonalantibodies to detect hCG (human chorionic gonadotrophin). hCG isuniquely produced during pregnancy by the developing embryo andlater the placenta. The urine of a pregnant woman contains detectablelevels of hCG.

Figure l8 shows how the pregnancy test strip works. At point C,there are antibodies to hCG immobilized in the strip' At point Bthere are free antibodies to hCG attached to a dye. At point D thereare immobilized antibodies that bind to the dye-bearing antibodies'Urine applied to the end of a test strip washes antibodies downthe strip.

o

@

,* Figure 18

Activitg1 Explain how a blue band appears

at point C if the woman ispregnant. t3l

2 Explain whg a blue band doesnot appear at point C ifthewoman is not pregnant. t3l

3 Explain the reasons for the useof immobilized monoclonalantibodies at point D, eventhough theg do not indicatewhether a woman is pregnantor not. t3l

+ +

475

ANTMAL PHYSt0L0GY (AHL)

II.2 Movement

Understandingà Bones and exoskeletons provide anchorage for

muscles and act as levers.à Movement of the bodg requires muscles to

work in antagonistic pairs.à Sgnovialjoints allow certain movements but

not others.\ Cl---l--1^l ----- -t- artz Jt\trturdt iltusLtg ilute5 dtc iltulUfìuulgalg ano

contain specialized endoplasmic reticulum.Ð Muscle fibres contain mang mgofibrils.t Each mgofibril is made up of contractile

sarcomeres.) The contraction of the skeletal muscle is

achieved bg the sliding of actin and mgosinfilaments.

) Calcium ions and the proteins tropomgosin andtroponin control muscle contractions.

9 ATP hgdrolgsis and cross-bridge formation arenecessarU for the f ilaments to slide.

Antagonistic pairs of muscles in an insect leg.

@ srirrÐ Annotation of a diagram of the human elbow.à Drawing labelled diagrams of the structure of a

sarcomere.à Analgsis of electron micrographs to find the

state of contraction of muscle f ibres.

@ uature of sc¡enceà Fluorescence was used to studU the cAclic

interactions in muscle contraction.

Figure 1

Bones and exoskeletons enchor musclesBones and exoskeletons prov¡de anchorage for musclesand act as levers.ExosÌ<eletons are external skeletons that surround and protect most ofthe body surface of animals such as crustaceans and insects. Figure Ishows a scanning electron micrograph of a spider next to exoskeletonsthat have been moulted.Bones and exoskeletons facilitate movement by providing an anchoragefor muscles and by acting as levers. Levers change the size and directionof forces. In a lever, there is an effort force, a pivot point caÌled a fulcrumand a resultant force. The relative positions oT these three determine theclass of lever.

In figure 2, Íhe diagram shows that when a person nods their headbackward, the spine acts as a flrst-class lever, with the fulcrum (F) beingfound between the effort force (E) provided by the splenius capitis muscleand the resultant force (R) causing the chin to be extended.The grasshopper leg acts as a third-class lever as the fulcrum is at the bodyend and the effort force is between the fulcrum and the resultant force.

476

Muscles are attached to the insides of exoskeletons but to the outsideof bones.

A

11.2 MOVEMENT

bicepscontracted

l rice psrelaxed

elbow extended scapula

humerusbl ce psrelaxed

radiustricepscontracted

ulna

'r, Figure 3 The biceps and triceps areantagonistic muscles

E

¡E

IaJ First-class lever

,,r Figure 2

Ib] Second-class lever Ic] Third-class lever

Skeletal muscles ere antagonisticMovement of the bodg requ¡res muscles to work inantagon¡stic pairs.Skeletal muscles occur in pairs that are antagonistic. This means thatwhen one contracts, the other relaxes. Antagonistic muscles produceopposite movements at a joint. For example, in the elbow, the tricepsextends the forearm while the biceps flex the forearm.

Data-based questions: Flight musclesIn one research project, pigeons (Columba livia)were trained to take off, fly 35 metres and landon a perch. During the flight the activity oftwo muscles, the sternobrachialis (SB) and thethoracobrachialis (TB), was monitored usingelectromyography. The traces are shown in figure 4.The spikes show electrical activity in contractingmuscles. Contraction of the sternobrachialis causesa downward movement of the wing.

take off fast flight landing

I Deduce the number of downstrokes ofthe wing during the whole flight.

2 Compare the activity of thesternobrachialis muscle during thethree phases of the flight.

3 Deduce from the data in theelectromyograph how thethoracobrachialis is used.

4 Another muscle, the supracoracoideus, isantagonistic to the sternobrachialis. Statethe movement produced by a contractionof the supracoracoideus.

5 Predict the pattern of the electromyographtrace for the supracoracoideus muscleduring the 35-metre flight.

I

13

I

ttl

TB

400 ms

r, Figure 4 Electrical activitg in the sternobrachialis ISBJ and the thoracobrachialis (TBJ muscles during flight of a pigeon

l2l

477

ANTMAL PHYSt0L0GY IAHLJ

h!i

'\.

@ m insect leg has antagonistic musclesAntagonistic pairs of muscles in an insect leg.The grasshopper, like all insects, has three pairs of appendages. Thehindlimb of a grasshopper is specialized for jumping. It is a jointedappendage with three main parts. Below the joint is referred to asthe tibia and at the base of the tibia is another ioint below which isfound the tarsus. Above the joint is referred to as the femur. Relativelymassive muscles are found on the femur.When the grasshopper prepares to jump, the flexor muscles willcontract bringing the tibia and tarsus into a position where theyresemble the letter "2" and the femur and tibia are brought closertogether. This is referred to as flexing. The extensor muscles relaxduring ihis pirase. Thc exlenso¡ muscles wiii then contract exrendingthe tibia and producing a powerful propelling force.

exte ns0 r

muscle relaxes extensormuscle contractstibia

flexes

flexormuscle flexor muscle

relaxesconlra cts

I Figure 6 Composite high-speed photographof a grasshopper ( 0rder 0rthoptera)jumping from the head of a nail

humerus bone - to which thebiceps and tricePs are attached

trlcops - extends blceps - flexesthe joinl the

ulne bone - to whichthe triceps is attached

cartllage - covers thebones and prevents friction

A Figure 7 The elbow joint

tibiaextends

The point where bones meet is called a joint. Mostjoints allow the bones to move in relation to eachother - this is called articulation. Most articulatedjoints have a similar structure, including cartilage,synovial fluid and joint capsule.

o Cartilage is tough, smooth tissue that coversthe regions of bone in the joint. It preventscontact between regions of bone that mightotherwise rub together and so helps to preventfriction. It also absorbs shocks that might causebones to fracture.

o Synovial fluid fills a cavity in the jointbetween the cartilages on the ends of thebones. It lubricates the joint and so helps toprevent the friction that would occur if thecartilages were dry and touching.

o The joint capsule is a tough ligamentous coveringto the joint. It seals the joint and holds in thesynovial fluid and it helps to prevent dislocation.

r Figure 5

@ ffre human elbow is an example of a sgnovial jointAnnotation of a diagram of the human elbow.

joint-capeule - sealsthe joint and helps toprevent dislocation

sgnovlal fluld -lubricates the joinrand prevents friction

redlus bone - to wh¡chthe biceps is attached

478

11.2 MOVEMENT

Difrerent joints allow difiercnt ranges of movementSgnovialjoints allow certain movements but not others.The structure of a joint, including the joint capsule and theligaments, determines the movements that are possible. The kneejoint can act as a hinge joint, which allows only two movements:flexion (bending) and extension (straightening).It can also act as apivot joint when flexed. The knee has a greater range of movementwhen it is flexed than when it is extended. The hip joint, betweenthe pelvis and the femur, is a ball and socket joint. It has a greaterrange of movement than the knee joint in that it can flex andextend, rotate, and move sideways and back. This latter type ofmovement is called abduction and adduction.

flexion

abduction

adduction

h gperextension extension

r Figure 8 Range of motion at the shoulder

flexion

abduction

IIIII

outward rotation

inward rotation

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It

(

II

outward rotation

,l,ri

exte nsr on

r Figure 9 Range of motion at the hip

adduction inward rotation

479

ANTMAL PHYST0L0GY IAHLJ

0ne sarcomere

light band Z-line dark band

a Figure 11 The ultrastructure ofthemuscle fibre

Structure of muscle fibresSkeletal muscle fibres are mult¡nucleate and containspecialized endoplasmic reticulum.The muscles that are used to move the body are attached to bones, sothey are called skeletal muscles. When their structure is viewed usinga microscope, stripes are visible. They are therefore also calÌed striatedmuscle. The two other types of muscle are smooth and cardiac.Striated muscle is composed of bundles of muscle cells known as muscleflbres. Although a single plasma membrane called the sarcolemmasurrounds each muscle fibre, there are many nuclei present and musclefibres are much longer than typical cells. These features are due to thefact that embryonic muscle cells fuse together to form muscle fibres.Figure l0 shows a muscle fibre.

sarcolemma

nucleus

mgof ibril

sarcoplasmicreticu lum

r Figure 10

A modifled version of the endoplasmic reticulum, called the sarcoplasmicreticulum, extends throughout the muscle fibre. It wraps around everymyoflbril, conveying the signal to contract to all parts of the muscle flbre atonce. The sarcoplasmic reticulum stores calcium. Between the myofibrils areIarge numbers of mitochondria, which provide AIP needed for contractions.

MgofibrilsMuscle fibres conta¡n manU mgofibrils.Within each muscle fibre there are many parallel, elongated structurescalled myoflbrils, These have alternating light and dark bands, whichgive striated muscle its stripes. In the centre of each light band is a disc-shaped structure, referred to as the Z-line.

nr ;n H;

480

Structure of mgofibrilsEach mgof ibril is made up of contractile sarcomeres.The micrograph in figure l3 shows a longitudinal section through amyoflbril. A number of repeating units that alternate between light anddark bands are visible. Through the centre of each ìight area is a linecalled the Z-line. The part of a myoflbril between one Z-line and thenext is called a sarcomere. It is the functional unit of the myofibril.

The pattern of light and dark bands in sarcomeres is due to a preciseand regular arrangement of two types of protein fllament - thin actinfllaments and thick myosin filaments. Actin fllaments are attachedto a Z-line at one end. Myosin filaments are interdigitated with actinfllaments at both ends and occupy the centre of the sarcomere. Eachmyosin fllament is surrounded by six actin fllaments and forms cross-bridges with them during muscle contraction.

@ rrre sarcomereDrawing labelled diagrams of the structure of a sarcomere,

dark band

thick mgosinfilaments

thin actinf ilaments

Z-line sarcomere Z'line

t Figure 14 The structure of a sarcomere

When constructing diagrams of a sarcomere, ensure to demonstrateunderstanding that it is between two Z-lines. Myosin filaments shouldbe shown with heads. Actin filaments should be shown connected toZ-lines. Light bands should be labelled around the Z-line. The extentof the dark band should also be indicated.

TT.2 MOVEMENT

, Figure 12 A transverse section through a

skeletal muscle f ibre showing numerousmgofibrils. A nucleus is shown in the bottom left

lightband

|'Ìçt'.

Data-based questions: Transverse sect¡ons of str¡ated muscleThe drawings in figure 15 show myoflbrils in I Explain the difference between a transversetransverse section. and a longitudinal section of muscle. I2l

2 Deduce what part of the myofibril isrepresented by the drawings as small dots. [2

3 Compare the pattern of dots in the threediagrams. Í3

4 Explain the differences between thediagrams in the pattern of dots.

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ANTMAL PHYSt0L0GY fAHL)

Mechanism of skeletal muscle conttact¡onThe contraction of the skeletal muscle is achieved bg thesliding of actin and mgosin f ilaments.During muscle contraction, the myosin filaments pull the actin filamentsinwards towards the centre of the sarcomere. This shortens each sarcomereand therefore the overalÌ length of the muscle flbre (see figure 16).

The contraction of skeletal muscle occurs by the sliding of actin andmyosin fllaments. Myosin filaments cause this sliding. They have headsthat can bind to special sites on actin filaments, creating cross-bridges,through which they can exert a force, using energy from AIP. The headsare regularly spaced along myosin filaments and the binding sites areregularly spaced along the actin filaments, so many cross-bridges canform at once (see flgure t7).

Ia)relaxed muscle

Z-line Z-line

muosrnfilament

Figure 1Z

cross-bridged etac h es

actin

mgosin head

cross-sence

mu0srn

light band shortens,ìndicating actinslides along mgosin

light band light banddark band

act n

dark band remainsthe same length

cross-bridgemoves actin along

st head sarcomere contracts

Ib] contracted muscle

I Figure 16 Diagram ofrelaxed and contracted sarcomeres

@ Oetermin¡ngthe stete of skeletal muscle contrastionAnalgsis of electron m¡crographs to find the state of contraction of muscle f ibres.

relaxed sarcomere In a relaxed sarcomere, the Z-lines are fartherapart, the light bands are wider and overallthe sarcomere is longer. In the centre of thesarcomere, there is another line called theM-line. In a relaxed sarcomere, there is a morevisible light band on either side of the M-line.

contracted sarcomere

A Figure 18 Electron micrograph of relaxed and contractedsarc0meres

Relaxedmuscle

Contractedmuscle

482

The control of skeletal musclecontractionCalcium ions and the proteinstropomgosin and troponin controlmuscle contract¡ons.In relaxed muscle, a regulatory protein calledtropomyosin blocks the binding sites on actin. Whena motor neuron sends a signal to a muscle fibre tomake it contract, the sarcoplasmic reticulum releasescalcium ions. These calcium ions bind to a proteincalled troponin which causes tropomyosin to move,exposing actin's binding sites. Myosin heads thenbind and swivel towards the centre of the sarcomere/moving the actin filament a small distance.

The role of ATP in the sliding offilamentsATP hgdrolgsis and cross-bridgeformation are necessarU for thefilaments to slide.For significant contraction of the muscle,the myosin heads must carry out this actionrepeatedly. This occurs by a sequence of stages

@ mgosin f ilaments have heads whichform cross-bridges when theg are

attached to binding sites on actlnfilaments.

LL.2 }4OVEMENT

o ATP causes the breaking of the cross-bridges byattaching to the myosin heads, causing them todetach from the binding sites on actin.

o Hydrolysis of the AIP, to ADP and phosphate,provides energy for the myosin heads toswivel outwards away from the centre of thesarcomere - this is sometimes called the cockingof the myosin head.

o New cross-bridges are formed by the bindingof myosin heads to actin at binding sitesadjacent to the ones previously occupied(each head binds to a site one position furtherfrom the centre of the sarcomere)'

o Energy stored in the myosin head when it wascocked causes it to swivel inwards towardsthe centre of the sarcomere, moving theactin filament a small distance. This sequenceof stages continues until the motor neuronstops sending signals to the muscle flbre.Calcium ions are then pumped back into thesarcoplasmic reticulum, so the regulatoryprotein moves and covers the binding sites onactin. The muscle fibre therefore relaxes.

@ ATP binds to the mgosin headsand causes them to break thecross-bridges bg detachingfrom the binding sites.

ATP

@ nfP ls hgdrolgsed to ADP andphosphate, causing the mgosin

ADP+Pheads to change their angle.The lreads are said to be'cocked'in their new position as theg arestorlng potential energg from ATP

movement

@ tne nOe and phosphate arereleased and the heads push theactin filament inwards towardsthe centre olthe sarcomere-this is called the power stroke.

@ the heads attach to binding sites on

actin that are further from the centre ofthe sarcomere than the previous slles.

-ADP+P

ADP+P

483

ANTMAL PHYSt0L0GY (AHL)

@ fne use of fluorescence to studg contractionFluorescence has been used to studg the cgclic interactions in muscle contraction.Fluoresence is the emission of electromagneticradiation, often visible light, by a substance after ithas been illuminated by electromagnetic radiationof a different wavelength. The fluorescencecan often be detected in a light microscope andcaptured on fllm for later analysis.

Some of the classic experiments in the history ofmuscle research have depended on fluorescence. Thecoelenterate Aequorea victoria (flgure 20) produces acalcium-sensitive bioluminescent protein, aequorin.Scientists studied the contraction of giant singlemuscle fibres of the acom bamacle Balanus nubitusbyinjecting samples of the muscle with aequorin. Whenmuscles were stimulated to contract in the study,initially there was strong bioluminescence coincidingwith the release of Ca2+ from the sarcoplasmicreticulum. The light intensity began to decreaseimmediately after the cessation of the stimulus.

In another experiment, researchers cut apartNitella axillaris cells. These celÌs are unique inthat they have a network of actin filamentsunderlying thcir mcmbranes. Researchersattached fluorescent dye to myosin molecules inan effort to show that myosin can "walk along,,actin fllaments.

fluorescent dgeattached to mVosin

muosrn ATPactin filament from

ADP Nitella øxilloris

actrn

With this technique, the researchers were able todemonstrate the ATP-dependence of myosin-actininteraction.The graph in figure 2l shows the velocityof myosin molecules as a function of ATpconcentration.

5

50 100 150ATP, ¡rM

200 400 1000

À Figure 21

bead

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II.3 The k¡dneg and osmoregulat¡on

11.3 THE KIDNEY AND OSMOREGULATION

;t5i{ir,*tlt:n:;:, Consequences of dehgdration and

overhgdration."'. Treatment of kidneg failure bg hemodialgsis or

kidneg transplant.,, Blood cells, glucose, proteins and drugs are

detected in urinarg tests.

@ srittsà Drawing and labelling a diagram of the human

kidneg.Ð Annotation of diagrams of the nephron.

Different responses to changes in osmolaritgin the env¡ronmentAnimals are either osmoregulators or osmoconformers.Osmolarity refers to the solute concentration of a solution. Manyanimals are known as osmoregulators because they maintain aconstant internal solute concentration, even when living in marineenvironments with very different osmolarities. All terrestrial animals,freshwater animals and some marine organisms like bony flsh areosmoregulators. Typically these organisms maintain their soluteconcentration at about clnc third of the concentration of seawater andabout l0 times that of fresh water.

Osmoconformers are animals whose internal solute concentration tendsto be the same as the concentration of solutes in the environment.

UnderstandingI Animals are either osmoregulators or

osmoconformers.à The Malpighian tubule sustem in insects and the

kidneg carrg out osmoregulation and removal ofnitrogenous wastes.

; The composition of blood in the renal arterg isdifferent from that in the renalvein.

¿ The ultrastructure of the glomerulus andBowma n's capsu le faci litate u ltraf iltration.

à The proximal convoluted tubule selectivelgreabsorbs useful substances bg active transPort.

; The loop of Henlé maintains hgpertonicconditions in the medulla.

¿ The length of the loop of Henlé is positivelgcorrelated with the need for water conservationin animals.

à ADH controls reabsorption of water in thecollecting duct.

+ The tgpe of nitrogenous waste in animals iscorrelated with evolutionarg historg and habitat.

@ nature of sc¡enceI Curiositg about particular phenomena:

investigations were carried out to determinehow desert animals prevent water loss in theirwastes.

485

ANTMAL PHYST0L0GY fAHLJ

Data-based questionsThe striped shore crab Pachygrapsus crassipes(flgure l) is found on rocky shores over thewest coast of North and Central America aswell as in l(orea and Japan. P. crassipes is oftenexposed to dilute salinities in tide pools andfreshwater rivulets, but it only rarely encounterssalt colcellral-iuus lruch highcr Lhau tha[ oI theocean. Samples of crabs were placed in waterconcentrations of varying osmolarity and samplesof blood were taken to determine osmolarityof the blood. In this experiment, the unit ofosmolarity is measured in units based on freezingpoint depression. When solutes are added to waterthey disrupt hy<irogen bonding. Freezing requiresadditional hydrogen bonding so adding solute

r Figure 1 The striped shore crab is exposed to varging saltconcentrations in its habitat

lowers the freezing point. 2 delta is equivalent toabout 100% ocean seawater, 0.2 delta is equivalentto about l0% ocean seawater, and3.4 deltaisequivalent to about l7O7o seawater.

I Determine the solute concentration ofcrab blood at which the concentration ofsurrounding water is I delta. (t )

2 Determine the range over which P. crassipesis able to keep its blood solute concentrationfairly stable. (l)

3 Predict what the graph would look like ifP. crassipes was not able to osmorequlate. (l)

4 Discuss whether P. crassipes is anosmoconformer or an osmoregulator. (3)

3.0

1.0 2.0water delto

3.0

A Figure 2

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00

line of isosmoticirg ----7'

a

a

ocea nseawater

The Malpighian tubule sgstemThe Malpighian tubule sustem in insects and thekidneg carrg out osmoregulation end removal ofnitrogenous wastes.Arthropods have a circulating fluid, known as hemolymph, that combinesthe characteristics of tissue fluid and blood. Osmoregulation is a form ofhomeostasis whereby the concentration of hemolymph, or blood in the caseof animals with closed circulatory systems, is kept within a certain range.

When animals break down amino acids, the nitrogenous waste productis toxic and needs to be excreted. In insects, the waste product is usuallyin the form of uric acid and in mammals it is in the form of urea.Insects have tubes that branch off from their intestinal tract. These areknown as Malpighian tubules. Cells lining the tubules actively transpoftions and uric acid from the hemolymph into the lumen of the tubules.This draws water by osmosis from the hemolymph through the walls ofthe tubules into the lumen. The tubules empty their contents into the

486

11.3 THE KIDNEY AND OSMOREGULATION

gut. In the hindgut most of the water and salts are reabsorbed while thenitrogenous waste is excreted with the feces.

hindgut@ dehgdrated uric acid paste

is released with other waste

hindgut O uric acid, Na+ and K+ aretransported into the tubulesand water follows bg osmosis

Figure 3

@ On*ringthe human kidnegDrawing and labelling a diagram of thehuman kidneg.When drawing a diagram of the kidney, the shapeshould be roughly oval with a concave side towhich the renal artery and vein are attached.Drawings should clearly indicate the cortex shownat the edge of the kidney. It should be shownwith a thickness of abour ] the entire width. Themedulla should be shown inside the cortex, withpyramids. The renal pelvis should be shown onthe concave side of the kidney. The pelvis shoulddrain into the ureter. The renal artery should havea smaller diameter than the renal vein.

Naf6+ Hzo

some ions are activelg reabsorbed¡n the hindgut and some water follows

Hzo

renal a

renal vein

midgutM a lpighian

lubule

@ the tubules emptginto the gut

midgut

M a lpighiantubules

Hzo

semisolid wastes

cortex

medulla

pelvis ofkidneg

@

Na+ K+uric acid

- ureter Icarries urine from the kidnegJ

r Figure 4 Structure ofthe kidneg

Comparing the composition of blood in the renalartefg and the renalve¡nThe compos¡t¡on of blood in the renal arteru is differentfrom that in the renal ve¡n.ICdneys function in both osmoregulation and excretion. The kidneys areresponsible for removing substances from the blood that are not neededor are harmful. As a result, the composition of blood in the renal artery,

487

ANTMAL PHYSI0L0GY IAHLJ

through which bloocl enters the kicìney, is different from that in therenal vein, through which blood leaves.

Substances that are present in higher amounts in the renal artery thanthe renal vein include:

o Toxins and other substances that are ingested and absorbed but arenot fully metabolized by the body, for example betain pigments inbeets and also drugs.

o Excretory waste products including nitrogenous waste products,mainly urea.

Other things removed from the blood by the kidney that are notexcretory products include:

o Excess water, produced by cell respiration or absorbed from food inthc orrt!a¡r òu!.

o Excess salt, absorbed from food in the gut.

These are not excretory products because they are not produced by bodycells. Removal of excess water and salt is part of osmoregulation. Whileblood in the renal artery might contain a variable watel or salt content,blood in the renal vein will have a more constant concentration becauseosmoregulation has occurred.

The kidneys filter off about one fifth of the volume of plasma from the bloodflowing through them. This filtrate contains all of the substances in plasmaapart from large protein molecules. The kidneys then actively reabsorbthe speciflc substances in the filtrate that the body needs. The result of thisprocess is that unwanted substances pass out of the body in the urine. Thesesubstances are present in the renal artery but not the renal vein.

Data-based questions: Blood suPPlg to the kidnegTable I shows the flow rate of blood to the kidneyand other organs, the rate of oxygen delivery andoxygen consumption. All of the values are givenper l0O g of tissue or organ. The rates are for aperson in a warm environment.

I Compare the rate of blood flow to thekidney with flow to the other organs. 12)

Blood flowrate

Imlmin-1100 g_1)

0xggendeliverg(mlmin-l100 g-1)

0xggenconsumPt¡on

(mlmin-l100 g-1)

Brain s4.0 10.8 3.70Skin 13.0 2.6 0.38SkeletalmuscleIrestingl

2.7 0.5 0.18

Heartmuscle

87.0 17.4 11.0

Kidneq 420.0 84.0 6.80

2 Calculate the volume of oxygen deliveredto the organs per litre of blood. l2l

3 In the brain, 34 per cent of the oxygenthat is delivered is consumed. Calculatethe same percentage for the other organs. Í41

4 Discuss the reasons for the differencebetween the kidney and the other organsin the volume of blood flowing to the orgar:r,and the percentage of oxygen in the bloodthat is consumed. Í41

5 Some parts of the kidney have a highpercentage rate of oxygen consumption,for example the outer part of the medulla.This is because active processes requiringenergy are being carried out. Suggest oneprocess in the kidney that requires energy. tll

6 Predict, with a reason, one change inblood flow that would occur if the personwere moved to a cold environment. Í21

488

¡ Table 1

11.3 THE KIDNEY AND OSMOREGULATION

A final set of differences between the composition of blood in the renalartery and the renal vein is due to the metabolic activity of the kidneyitsell. Blood leaving the kidney through the renal vein is deoxygenatedrelative to the renal artery because kidney metaboÌism requires oxygen.It also has a higher partial pressure of carbon dioxide because this is awaste product of metabolism. Even though glucose is normally filteredand then entirely reabsorbed, some glucose is used by the metabolism ofthe kidney and therefore the concentration is slightly ìower in the renalvein compared to the renal artery.Plasma proteins are not filtered by the kidney so should be present in thesame concentration in both blood vessels. Presence in the urine indicatesabnormal function. This is Ìooked for during clinical examination of aurine sample.

Are there criteria that cen be developedto justifg the use of animals in research?

Figure 5 shows some of the techniquesthat have been used to investigatekidneg function. The animals usedinclude rats, mice, cats, dogs and pigs.

1 What are the reasons for carrging outkidneg research?

2 What criteria should be used todecide if a research technique isethicallg acceptable or not?

3 Applg gour criteria to the rhreetechniques outl¡ned in figure 5 todetermine whether theg are ethicallgacceptable.

4 Who should make the decisions aboutthe ethics of scientific research?

Living animal is anaesthetized and its kidnegis exposed bg surgerg. Fluid is sampled fromnephrons using micropipettes. Animal is thensacrificed so that the position ofthe sample pointin the kidneg can be located.

Animalis killed and kidnegs are removed andfrozen. Samples oftissue are cut from regionsofkidneg that can be identified. Temperature atwhich thawing occurs is found, to give a measureof solute concentration.

nephron\

-

-E-external fluid

Animal is killed and kidnegs are dissected toobtain samples ofnephron. Fluids are perfusedthrough nephron tissue, using experimentalexternal fluids to investigate the action ofthe wallofthe nephron.

A Figure 5

The ultrastructure of the glomerulusThe ultrastructure of the glomerulus and Bowman'scapsule facilitate u ltrafiltration.Blood in capillaries is at high pressure in many of the tissues of the body,and the pressure forces some of the plasma out through the capillarywall, to form tissue fluid.In the glomerulus of the kidney, the pressure in the capilÌaries is particularlyhigh and the capillary wall is particularly permeable, so the volume of fluidforced out is about t00 times greater than in other tissues. The fluid forcedout is called glomerular filtrate. The composition of blood plasma and flltrateis shown in table 2. The data in the table shows that most solutes are fllteredout freely from the blood plasma, but almost all proteins are retained in thecapillaries of the glomerulus. This is separation of particles differing in sizeby a few nanometres and so is called ultrafiltration. All particles with arelative molecular mass beÌow 65,000 atomic mass units can pass through.The permeability to larger molecules depends on their shape and charge.Almost all proteins are retained in the blood, along with all the blood cells.

Content (per dm 3 of blood plasmaJ

urea ImolJproteins Img]

Table 2

The structure of a section of the fllter unit is shown in figure ó and flgure 7.Figure 6 is a coloured transmission electron micrograph (TEM) of a sectionthrough a kidney glomerulus showing its basement membrane (brown linerunning from top right to bottom Ìeft). the basement membrane separatesthe capillaries (the white space at the left is the lumen of a capiìlary). Notethe gaps in the wall of the capillary which are referred to as fenestrations.The smaller projections from the membrane are podocyte footprocesses, which attach the podocytes (specialized epithelial cells) ro the

plasma

151

110

filtrate1.44

7r45

5

3.5

5

5

740

489

ANTMAL PHYSIoLoGY IAHL)

I Figure 6

podocAtes - strangelg shaped cellswith finger-like projections which wraparound capillaries in the glomerulusand provide support

fenestrated basement membrane -wall of the f iltercapillarg

blood red nucleus ofplasma blood cell capillarg wall cell

a Figure Z Structure of the filter unitof the kidneg

membrane. The podocytes fr,rnction as a barrier throrrgh which wasteproducts are filtered Trom the blood.

There are three parts to the ultrafiltration system.

I Fenestrations between the cells in the wall of the capillaries. Theseare about lO0 nm in diameter. They allow fluid to escape, but notblood cells.

2 The basement membrane that covers and sltpports the wall ofthe capillaries. It is made of negatively-charged glycoproteins, whichform a mesh. It prevents plasma proteins from being flltered out, dueto their size and negative charges.

3 Podocytes forming the inner wall of the Bowman's capsule'These cells have extensions that wrap around the capillaries of theglomerulus and many short side branches called foot processes.Very narrow gaps between the foot processes help prevent smallmolecules from being flltered out of blood in the glomerulus.

If particles pass through all three parts they become part of theglomerular flltrate.

Figure 8 shows the relationship between the glomerulus and theBowman's capsule.

afferent arteriole

pod ocgtes

basement membrane

fe n estratedwall of capillarg

efferent arterioleproximal

convoluted lubule

lumen ofBowman's capsule

* Figure I

Data-based questions: Ultrafiltration of charged and uncharged dextransDextrans are polymers of sucrose. Different unit of rat glomeruli. Animal experimentssizes of dextran polymer can be synthesized, like this can help us to understand how theallowing their use to investigate the effect of kidney works and can be done without causingparticle size on ultrafiltration. Neutral dextran suffering to the animals.is uncharged, dextran sulphate has many. . I state the relationship between the sizenegative charges, and DEAE is dextran with of particles and the permeability tomany positive charges. them of the filter unit of theFigure 9 shows the relationship between glomerulus. Il

490

i particle size and the permeability of the filter

11.3 THE KIDNEY AND OSMOREGULATION

2 a) Compare the permeability of the filterunit to the three types of dexrran. t3l

b) Explain these differences inpermeability. t3l

3 One of the main plasma proteins isalbumin, which is negatively chargedand has a particle size of approximateÌy4.4 nrr'. Using the data in the graph,explain the diagnosis that is made ifalbumin is detected in a rat's urine. til

1.00.90.80.70.60.50,40.30.20.1

0

DEAE

2particle size / nm

n Figure 9 Relationship between particlesize of dextrans and filtration rate

ooco@gooo

20 ?

dextre nsulphate

The role of the proximal convoluted tubuleThe proximal convoluted tubule selectivelg reabsorbsuseful Substances bg act¡ve transport.The glomeruìar filtrate flows into the proximal convoluted tubule.The volume of glomerular fiÌtrate produced per day is huge - about180 dm r. This is several times the rotal volume of fluid in the bodyand it contains nearly 1.5 kg of salt and 5.5 kg of glucose. As thevolume of urine produced per day is only about I.5 dmr and itcontains no glucose and far less than 1.5 kg of saÌt, almost all of thefiltrate must be reabsorbed into the blood. Most of this reabsorptionhappens in the first part of the nephron - the proximal convolutedtubule. Figure l0 shows this structure in transverse section. Themethods used to reabsorb substances in the proximal convolutedtubule are described in table 3. By the end of the proximal tubule allglucose and amino acids and 80 per cent of the water, sodium andother mineral ions have been absorbed.

Sodium ions: are moved bg active transport from filtrate to space outside thetubule. Theg then pass to the peritubular capillaries. Pump proteins are locatedin outer membrane of tubule cells.

chloride ions: are attracted from f iltrate to space outside the tubule because ofcharge gradient set up bU active transport of sodium ions.

Glucose: is co-transported out of filtrate and into fluid outside the tubule, bgco-transporter proteins in outer membrane of tubule cells. sodium ions movedown concentration gradient from outside tubule into tubule cells. This providesenergV for glucose to move at the same time to fluid outside the tubule. Thesame process is used to reabsorb amino acids.

Water: pumping solutes out of f iltrate and into the fluid outside the tubulecreates a solute concentration gradient, causingwaterto be reabsorbed fromfiltrate bg osmosis.

Table 3

mitochond riamicrovilli

invaginations ofouter membrane basement membrane

,¡ Figure 10 Transverse section oftheproximal convoluted tubule

ActivitgThe drawing below shows thestructure of a cell from the wall ofthe proximal convoluted tubule.Explain howthe structure oftheproximal convoluted tubule cell, asshown in the diagram, is adapted tocarrg out selective reabsorption.

I0 ¡-r,m

lumencontainingfi lt rate

491.

ANTMAL PHYSI0L0GY fAHL)

affefentarteriole

effefentarleriole

Bowman's

glomerul

@ rrre nephronAnnotation of diagrams of the nePhron.The basic functional unit of the kidney is thenephron. This is a tube with a wall consisting ofone layer of cells. This wall is the last layer of cellsthat substances cross to leave the body - it is anepithelium. There are several different parts ofthe nephron, which have different functions andstructures (see figure ll):

o Loop of Henle - a tube shaped like a hairpin,consisting of a descending limb that carries thefiltrate deep into the medulla of the kidney,and an ascending limb that brings it back out tothe cortex.

o Distal convoluted tubule - another highlytwisted section, but with fewer, shortermicrovilli and fewer mitochondria.

o Collecting duct - a wider tube that carries theflltrate back through the cortex and medulla tothe renal pelvis.

o Blood vessels - associated with the nephronare blood vessels. Blood flows though them inthe following sequence:

r Afferent arteriole - brings blood from therenal artery.

r Glomerulus - a tight, knot-Iike, high-pressure capillary bed that is the site ofblood filtration.

r Efferent arteriole - a narrow vessel thatrestricts blood flow, helping to generatehigh pressure in the glomerulus.

r Peritubular capillaries - a low-pressurecapillary bed that runs around the convolutedtubules, absorbing fluid from them.

r Vasa recta - unbranched capillaries thatare similar in shape to the loops of Henle,with a descending limb that carries blooddeep into the medulla and an ascendingIimb bringing it back to the cortex.

r Venules - carry blood to the renal vein.

proximal convoluted tubule

distal convoluted tubule

venuleperitubularcapillaries

collecting duct

vasa recta

ascending limbof loop of Henlédescending limbof loop of Henlé

l, Figure 11 The nephron and associated blood vessels. The

human kidneg contains about a million nephrons

. Bowman's capsule - a cup-shaped structurewith a highly porous inner wall, which collectsthe fluid filtered from the blood.

o Proximal convoluted tubule - a highlytwisted section of the nephron, with cells in thewall having many mitochondria and microvilliprojecting into the lumen of the tube.

The role of the loop of HenleThe loop of Henle maintains hgpertonic conditionsin the medulla.The overall effect of the loop of Henle is to create a gradient of soluteconcentration in the medulla. The energy to create the gradient isexpended by wall cells in the ascending limb. Here sodium ions arepumped out of the flltrate to the fluid between the cells in the medulla -called the interstitial fluid. The wall of the ascending limb is unusual inthat it is impermeable to water, so water is retained in the filtrate, even

\

492

11.3 THE KIDNEY AND OSMOREGULATION

though the interstitial fluid is now hypertonic relative to the filtrate; i.e.,it has a higher solute concentration.

Normal body fluids have a concentration of 100 mOsm. The pumpproteins that transfer sodium ions out of the flltrate can create a gradientof up to 200 mOsm, so an interstitial concentration of 500 mOsm isclearly achievable. The cells in the wall of the descending limb arepermeable to water, but are impermeable to sodium ions. As flltrateflows down the descending limb, the increased solute concentrationof interstitial fluid in the medulla causes water to be drawn out of theflltrate until it reaches the same solute concentration as the interstitialfluid. If this was 500 mOsm, then flltrate entering the ascending limbwould be at this concentration and the sodium pumps could raise theinterstitial fluid to 700 mOsm. Fluid passing down the descendinglimb would therefore reach 700 mOsm, and the sodium pumps in theascending limb could cause a further 200 mOsm rise. The interstitialfluid concentration can therefore rise further and further, until amaximum is reached, which in humans is 1,200 mOsm.

This system for raising solute concentration is an example of acountercurrent multiplier system. If is a countercurrent system because ofthe flows of fluid in opposite directions. It is a countercurrent multþlierbecause it causes a steeper gradient of solute concentration to develop inthe medulla than would be possible with a concurrent system. There is alsoa countercurrent system in the vasa recta. This prevents the blood flowingthrough this vessel from diluting the solute concentration of the medulla,while still allowing the vasa recta to carry away the water removed fromflltrate in the descending limb, together with some sodium ions.

from the proximal to the distalconvoluted convol

tubule tubule

1200

'' Figure 12 Solute concentrations inthe loop of Henle (in mOsm]

Some animals have relat¡velg long loops of HenleThe length of the loop of Henle is positivelg correlated w¡ththe need for water conservation in an¡mals.The longer the loop of Henle, the more water volume will be reclaimed.Animals adapted to dry habitats will often have long loops of Henle.Loops of Henle are found within the medulla. In order to accommodatelong loops of Henle, the medulla must become relatively thicker.

Data-based questions: Medulla thickness and urineconcentrat¡onTable 4 shows the relative medullary thickness (RMT) and maximumsolute concentration (MSC) of the urine in mOsm for l4 speciesof mammal. RMT is a measure of the thickness of the medulla inrelation to the overall size of the kidney. All the species in the tablethat are shown with binomials are desert rodents.

I Discuss the relationship between maximum soluteconcentration of urine and the habitat of the mammal

2 Plot a scattergraph of the data in the table, either byhand or using computer software.

[3

17l

493

ANTMAL PHYST0L0GY (AHLJ

[aJ lowADH Ib] high ADH

¡nterst¡tial

300

renal pelvis

A Figure 13 Solute concentrat¡ons in thecollecting duct

flud

3 a) Using the scattergraph that you have plotted, state therelationship between RMT and the maximum soluteconcentration of the urine. tl

b) Suggest how the thickness of the medulla could affect themaximum solute concentration of the urine. Í4

Species RMT MSC

ImOsmJbeaver 1.3 5t7Prg 1.6 7076

human 3.0 1399

dog 4.3 246scat 4.8 312?rat 5.8 24650ctomgs mimox 6.1 207LDipodomgs deserti 8.5 5597

Joculus joculus 9.3 6459

Tg mpo n o ctomgs b o rrero e 9.4 7080

Psommomgs obesus 1,0.7 4952

Eligmodontia tgpus 1.1,4 8612

Colomgs mus 1,2.3 8773

Solinomgs delicotus t4.o 7440

¡, Table 4

Function of ADHADH controls reabsorpt¡on of water in the collecting duct.When filtrate enters the distal convoluted tubule from the loop ofHenle, its solute concentration is lower than that of normal bodyfluids - it is hypotonic. This is because proportionately more solutesthan water have passed out of the filtrate as it flows through the loopof Henle in the medulla.If the solute concentration of the blood is too low, relatively littlewater is reabsorbed as the filtrate passes on through the distalconvoluted tubule and the collecting duct. The wall of these partsof the nephron can have an unusually low permeability to water.A large volume of urine is therefore produced, with a low soluteconcentration, and as a result the solute concentration of the blood isincreased (see figure l3a).If the solute concentration of the blood is too high, the hypothalamusof the brain detects this and causes the pituitary gland to secrete ahormone - antidiuretic hormone or ADH. This hormone causes thewalls of the distal convoluted tubule and collecting duct to become

494

-\

11.3 THE KIDNEY AND OSMOREGULATION

much more permeable to waler, and most of the water in the flltrate isreabsorbed. This is helped by the solute concentration gradient of themedulla. As the fìltrate passes down the collecting duct, it flows deepinto the medulla, where the solute concentration of the interstitial fluidis high. Water continues to be reabsorbed along the whole length of thecollecting duct and the kidney produces a small volume of concentratedurine (figure f 3b). As result the solute concentration of the blood isreduced. The action of the kidney therefore helps to keep the relativeamounts of water and solutes in balance at an appropriate Ìevel. This iscalled osmoregulation.

Data-based questions: ADH release and feelings of th¡rstThe plasma solute concentration, plasma b) Compare intensity of thirst and plasmaantidiuretic hormone (ADH) concentration ADH concentration.and feelings of thirst were tested iof volunteers. Figures 14 and 15 ¡ what would happen to plasma

relationship between intensity of oncentration and ADH

concentration and plasma solute , 'ration if a person were to'ater to satisfy his/her thirst.

a) Identify the plasma ADH concentration r\ ----_at a plasma solure concentrarion ot ;óö o)

:lî.1: two reasons why a person's plasma

mosmol kg-r using the line of ¡est ntl ttl solute concentration may increase'

I

2

2

10Øo,¿f8)ß).=¡6ü5È4oÐ3.:C¿o.cI

0

?0

1B

- 16IE ¡¡õ .,Et¿>" 10Oo5oo-d4

2

0280 290 300 310 320plasma solute concentration/m0smol kg-1

ø Figure 14

280 290 300 310 320plasma solute concentration/m0smol kg-1

,t, Figure 15

!.!l'

Animals varg in terms of the tgpe of nitrogenouswaste theg produceThe tgpe of nitrogenous waste in animals is correlatedwith evolutionarg historg and hab¡tat.When animals break down amino acids and nucleic acids, nitrogenouswaste in the form of ammonia is produced. Ammonia is highly basicand can alter the pH balance. It is also toxic as it is a highly reactivechemical. If the organism Ìives in a marine or freshwater habitat, such asflsh, echinoderms or coelenterates, they can release the waste directly asammonia as it can be easily diluted within that environment. Terrestrial

495

ANTMAL PHYST0L0GY (AHL)

organisms will expend energy to convert ammonia to the less toxicforms of urea or uric acid depending on their habitats and evolutionaryhistory. Marine mammals, despite their habitat, release urea because oftheir evolutionary history.

Some organisms like amphibians release the waste as ammonia when theyare larva and after metamorphosis, release the waste as urea. Convertingammonia to urea requires energy and converting it to uric acid requireseven more energy. The advantage of uric acicL is that it is not water-solubleand therefore does not require water to be released. Birds and insectsrelease their nitrogenous waste as uric acid. For birds, not having to caffywater for excretion means less energy needs to be expended on flight.

Uric acid is linked to adaptations for reproduction. Nitrogenous wastesare released by the developing organism within eggs. Uric acid is releasedas it is not soÌuble and crystallizes rather than building up to toxicconcentrations within the egg.Figure 16 The white paste in bird

droppings is uric acid

Dehgdration and overh gdration[onseq ue nces of de h gdration a nd overh gd rationDehydration is a condition that arises when morewater leaves the body than comes in. It can arisefrom a number of factors including exercise,insufficient water intake or diarrhoea. It can leadto the disruption of metabolic processes.

One sign of dehydration is darkened urine due toincreased solute concentration. Water is necessaryto remove metabolic wastes so dehydration canlead to tiredness and lethargy due to decreasedefflciency of muscle function and increased tissueexposure to metabolic wastes. Blood pressurecan fall due to low blood volume. This can lead

to increases in heart rate. Body temperatureregulation may be affected because of an inabilityto sweat.

Overhydration is less common and occurs whenthere is an over-consumption of water. The resultis a dilution of blood solutes. It might occur whenlarge amounts of water are consumed after intenseexercise without replacing the electrolytes lost atthe same time. This makes body fluids hypotonicand could result in the swelling of cells due toosmosis. If this occurs, the most notable symptomsare headache and nerve function disruption.

Treatment opt¡ons for kidneg failureIreatrnent of kidneg failure bg hemodialgsis 0r kidneU transplant.I(idney failure can occur for a number of reasonsbut most commonly occurs as a complicationfrom diabetes or chronic high blood pressure(hypertension) as a result of diabetes.

Figure I7 shows a patient undergoing renal dialysis(hemodialysis). The dialysis machine (artiflcialkidney) is on the left. Hemodialysis is requiredwhen the kidneys are no longer able to fllterwaste products from the blood properly. Duringthe procedure, a steady flow of blood passes overan artificial semi-permeable membrane in thedialysis machine. The small waste products in the

blood pass through the membrane, but the largerblood cells and proteins cannot. The purifled bloodis then returned to the patient via a vein. Thisprocedure takes several hours.

An alternative to dialysis is a kidney transplant. Inthis treatment option, a kidney from one personis placed in the body of a person whose kidneysaren't functioning. The donor can either be livingor deceased. A living donor is possible because aperson can survive with one functional kidney.This approach can result in greater independenceof movement and freedom to travel as compared

496

11.3 THE KIDNEY AND OSMOREGULATION

blood in tubing flowsthrough dialgsis fluid

blood pump+

velna rteru

shunt

air detector dialgsis machine

A Figure 17

to dialysis. Dialysis also carries with it the risk ofinfection and other complications.A drawback to a transplant is that the recipient'sbody can reject the organ. Figure l9 is of a light

Urine is a product of osmoregulation, excretionand metabolism. These processes can be disruptedby illness or drug abuse. Urinalysis is a clinicalprocedure that examines urine for any deviationfrom normal composition.Figure 20 shows a urine test strip beingcompared to the results chart on the testingkit bottle. This strip contains three test areasdesigned to change colour to indicate a positiveor negative result after being dipped in urine.

freshdialgsis compressedfluid air

micrograph through a transplanted kidney thathas been rejected by the recipient's immunesystem. Numerous lymphocytes (with small dots)have infiltrated the kidney tissue.

The colours displayed can then be comparedto a results chart on the testing kit. This testindicates the pH, protein level and glucoselevel in the urine. High levels of glucose andprotein in the urine can be an indication ofdiabetes. High protein levels can indicate damageto the kidneys as these do not get throughultrafiltration in a healthy kidney. The strip inthe picture is a normal negative result for proteinand glucose.

Iused dialgsis fluid

<-

r Figure 18 r Figure 19

@ urinalgsisBlood cells, glucose, prote¡ns and drugs are detected in urinarg tests.

497

ANTMAL PHYST0L0GY (AHL)

ç

presence of traces of banned and controlled drugsin urine. Figure 2l shows a drug test card beingdipped into a sample of urine. The card containsfive vertical strips that each test for a differentdrug. Here, the results are negative for all but theone second from left. This indicates a positive testfor opiates.

4#,

þ

Ä Figure 20

The panel drug test also uses test strips based onmonoclonal antibody technology to look for the

r Figure 21

Microscopic examination of urine is carried outto determine if cells are present, as under normalcircumstances, these cells should not be present.Figure 22 shows white blood cells. The presenceof 6-10 neutrophils (white blood cells with anucleus visible) can be a sign of urinary tractinfection. Figure 23 indicates the presence ofred blood cells (erythrocytes) in the urine of thispatient. This can be a sign that there is a kidneystone or a tumour in the urinary tract.

498

À Figure 22 I Figure 23

11.4 SEXUAL REPRODUCTION

LI.4 Sexua I reproduction

Understandingà Spermatogenesis and oogenesis both involve

mitosis, cell growth, two divisions of meiosisand differentiation.

à Processes in spermatogenesis and oogenesisresult in different numbers of gametes withdifferent amounts of cgtoplasm.

à Fertilization involves mechanisms that preventpolgspermg.

t Fertilization in animals can be internal orexternal.

à lmplantation of the blastocAst in theendometrium is essential forthe continuationof pregnancg.

à hCG stimulates the ovarg to secreteprogesterone during earlg pregnancA.

à The placenta facilitates the exchange ofmaterials between the mother and embrgo.

Ð Estrogen and progesterone are secreted bg theplacenta once it has formed.

à Birth is mediated bg positive feedbackinvolving estrogen and oxgtocin.

, The average 38-week pregnancg in humanscan be positioned on a graph showing thecorrelation between animal size and thedevelopment of the goung at b¡rth for othermammals.

@ srirrcI Annotation of diagrams of seminiferous

tubule and ovarg to show the stages ofgametogenesis.

Ð Annotation of diagrams of mature sperm andegg to indicate functions.

@ nature of sc¡enceà Assessing risks and benefits associated with

scientific research: the risks to human malefertilitg were not adequatelg assessed beforesteroids related to progesterone and estrogenwere released into the environment as a resultof the use of the female contraceptive pill.

Similarities between oogenesis andsPermatogenesisSpermatogenes¡s and oogenes¡s both involve mitosis, cellgrowth, two divisions of meiosis and different¡ation.Oogenesis is the production of egg cells in the ovaries, Oogenesis startsin the ovaries of a female fetus. Germ cells in the fetal ovary divide bymitosis and the cells formed move to distribute themselves through thecortex of the ovary. When the fetus is four or flve months old, these cellsgrow and start to divide by meiosis. By the seventh month, they are stillin the first division of meiosis and a single layer of cells, called folliclecells, has formed around them. No further development takes place untilafter puberty. The cell that has started to divide by meiosis, togetherwith the surrounding follicle cells, is called a primary follicle. Thereare about 400,000 in the ovaries at birth. No more prinary folliclesare produced, but at the start of each menstrual cycle a small batch are

499

ANTMAL PHYST0L0GY IAHLJ

r Figure 2 Coloured scanning electronmicrograph ISEM J of ovarg tissue, showingtwo secondarg follicles. A secondarg oocgteIpinkJ is seen at the centre of one follicle.Follicles are surrounded bg two tgpes of folliclecells Icoloured blue and green]. Between thefollicle cells a space develops Iat centre right,coloured brown], into which follicular fluid issecreted. The amount of fluid will increasesignif icantlg as the follicle matures

stimulated to develop by FSH. Usually only one goes on to become amature follicle, containing a secondary oocyte.

primarg follicle

maturing follicle

r Figure 1 Light micrograph of a section through tissue from an ovarU, showing a primargfollicle Ileft] and a maturing follicle (centreJ. Primarg follicles contain a central oocuteIfemale germ cell, eggJ surrounded bg a single lager of follicle cells. A mature ovarianfollicle has manU more follicle cells, outer and inner follicle cells and cavities, and theoocgte is now more fullg developed compared to the primordial and primarg stages

Spermatogenesis is the production of sperm. It happens in the testes,which are composed of a mass of narrow tubes, called seminiferoustubules, with small groups of cells fllling the gaps between the tubules.These gaps are called interstices, so the cells in them are interstitialcells. They are sometimes called Leydig cells. The seminiferous tubulesare also made of cells. The outer layer of cells is called the germinalepithelium. This is where the process of sperm production begins. Cellsin various stages of sperm production are found inside the germinalepithelium, with the most mature stages closest to the fluid-fllled centreof the seminiferous tubule. Cells that have developed tails are calledspermatozoa, though this is almost always abbreviated to sperm.Also in the wall of the tubule are large nurse cells, called Sertoli cells.Figure 3 shows a small area of testis tissue, in which the structuresdescribed above can be seen.

spermatogonium

flagella of spermatozoa

s00

r Figure 3 Transverse section through a seminiferous tubule

lumen of seminiferous tubule

11.4 SEXUAL REPRODUCTION

@ Oiagmms of a seminiferous tubule and the ovargAnnotation of diagrams of seminiferous tubule and ovarg to show the stagesof gametogenesis.

basement membrane

@ An outer lager calledsPermatogonrumgerminal epithelium cells

[2n) divide endlesslgbg mitosis to producemore diploid cells

pnmargspermatoc9te

secondargspermatocute

@ Sperm detach fromSertoli cells andeventuallg are carriedout ofthe testis bg thefluid in the centre oftheseminiferous tubule

I Figure 4

@ ln a secondarg follicle, the folliclecells proliferate, a fluid-filled cavitgdevelops and the oocgte starts thesecond division of meiosis

mature follicle

@ Diploid cells growlarger and are thencalled primargspermatocgtes (2n)

@ Each primargspermatocgte carries outthe f irst division of meiosisto produce two secondargspermatocAtes In)

@ Each secondargspermatocgte carriesout the second divisionof meiosis to producetwo spermatids In)

secondargfollicle

developingfollicles

spermat¡ds

@ Spermatids become associatedwith nurse cells, called Sertoli cellswhich help the spermatids to developinto spermatozoa InJ. This is anexample of cell differentiation

primarg follicles@ Primarg follicles cons¡st of a central

oocgte surrounded bg a single lagerof follicle cells. Everg menstrual cgcle,a few primarg follicles start to developand the oocgte completes the f¡rstdivision of meiosis

degeneratingcorpus luteum

corpus luteum

À Figure 5

ovulated ovum

developingcorpus luteum

501

ANTMAL PHYST0L0GY IAHLJ

@ Oiagnms of sperm and eggAnnotation of diagrams of meture sperm and eggto indicate functions.

two centrioles

first polar cell

plasmamembrane

lager of follicle cells(corona radiata)

A Figure 6 Structure of the female gamete

haploid nucleus

mid-piece(7 pm longJ

I

ta¡l (40 pm long, two-thirds ofit omitted from this drawing]

I

haploidnucleus

lager ofgel composedof glgcoproteins(zona pelfucidal

cutoplasm Ior golk)containing droplets of fat

Diameter of eggcell : 110 pm

cortical granules

acrosomeo¡tcoEastîtcoop=E

(f)!oos,

centriolemicrotubules in a9*2 arrangement

plasma membrane helicalmitochondria

protein fibres tostrengthen the tail

502

A Figure 7 Structure of the male gamete

11.4 SEXUAL REPRODUCTION

Oata-based questions: Sizes of spermSperm tails have a 9 I 2 arrangement ofmicrotubules in the centre, with thicker proteinfibres around. Table I shows the structure ofsperm tails of eight animals in transverse section,with the tail lengths and the cross-sectional areaof the protein fibres.

I Draw a graph of tail length and cross-sectionalarea of protein fibres in the eight species ofanimal. Í4)

a Table 1

2 Outline the relationship between tail lengthand cross-sectional area of protein fibres. Í21

3 Explain reasons for the relationship. Í214 Discuss whether there is a relationship

between the size of an animal and the sizeof its sperm. l2l

çï i:-¡

chinesehemster

let guineap¡c

hamstel bull m0use human seaurchin

cross-sectional area offibrous sheaths / pm2

0.2? 0.16 0.13 0,1 1 0.08 0.04 0.02 0

length of sperm / pm 258 L87 L07 t87 s4 t23 58 45

Difierences in the outcome of spermatogenes¡send oogenes¡sProcesses in spermatogenesis and oogenes¡s result indifferent numbers of gametes with different amountsof catoplasm.While there are similarities in spermatogenesis and oogenesis, there aredifferences that are necessary to prepare the gametes for their differentroles. Each mature sperm consists of a haploid nucleus, a system formovement and a system of enzymes and other proteins that enable thesperm to enter the egg. Each complete meiotic division results in fourspermatids. The process of sperm differentiation eliminates most of thecytoplasm, whereas the egg must increase its cytoplasm.

All of the requirements for beginning the growth and development of theearly embryo must be present in the egg. In females, the flrst division ofmeiosis produces one large cell and one very small cell (figure 8). The smallcell is the flrst polar body which eventually degenerates. The large cellgoes on to the second division of meiosis, completing it after fertilization.Again one large cell and one very small cell are produced. The small cellis the second polar body and it also degenerates and dies. Only the largecell, which is the female gamete, survives. The result is that the egg ismuch larger than the sperm cell. Figures 6 andT show the differencesin structure. Note that the scale bars indicate that the sperm and egg aredranm to different scale and that the egg is much larger than the sperm.

r Figure I The micrograph shows a primargoocgte split into two cells, known as thesecondarg oocgte (green] and the firstpolar bodg Igellow]

503

ANTMAL PHYST0L0GY fAHL)

sperm tru topush throughthe lagers offollicle cellsaround theegg

folliclecel I

zonepellucida

plasma membrane ot egg

acrosomalcap

tail andmitochondriausuallg remainoutside

cortical granules

hardenedzona pellucida

exocutoslsof contentsof corticalgfanules sperm nucleus

The process of egg formation happens once per menstrual cycle inhumans and usually only one egg cell per cycle is produced. Duringthe years from puberty to the menopause only a few hundred femalegametes are likely to be produced.

From puberty onwards, the testes produce sperm continuously. At anytime, there are millions of sperm at all stages of development.

Preventing polgspermgFerti lization i nvo lves mechan isms that preventPoluspermg.Fertilization is the union of a sperm and an egg to form a zygote.

The membranes of sperm have receptors that can detect chemicalsrcicasetl by tÌre cgg, allowing clirectionai swinrrning iowarcis rhe egg.Figure 9 illustrates that multiple sperm arrive at the egg. Once the eggis reached, a number oT events take place (see figure I0). These eventsare designed to result in the union of a single sperm with the egg. Theevents are also designed to prevent multiple sperm entering, known aspolyspermy.

r Figure 9 Micrograph of egg surrounded bg sperm

l The acrosome reactionThe zona pellucida is a coat of glycoproteins that surrounds the egg.The acrosome is a large membrane-bound sac of enzymes in the headof the sperm. In mammals, the sperm binds to the zona pellucida andthe contents of the acrosome are released. The enzymes from it digestthe zona pellucida.

two haploidnuclei from thesperm and the egg

r Figure 10 Stages in fertilization

504

11.4 SEXUAL REPRODUCTION

2 Penetration of the egg membraneThe acrosome reaction exposes an area of membrane on the tip of thesperm that has proteins that can bind to the egg membrane. The firstsperm that gets through the zona pellucida therefore binds and themembranes of sperm and egg fuse together. The sperm nucleus entersthe egg cell. This is the moment of fertilization.

3 The cortical reactionNot only does the sperm bring male genes, it also causes the activationof the egg. The flrst effect of this is on the cortical granules - vesicleslocated near the egg membrane. There are thousands of these vesiclesand when activation of the egg has taken place their contents arereleased from the egg by exocytosis. In mammals, the cortical vesicleenzymes result in the digestion of binding proteins so that no furthersperm can bind. The enzymes also result in a general hardening of thezona pellucida.

I nternal a nd externel fertilizationFertilization ¡n animals can be internal or externalAquatic animals often release their gametes directly into water in aprocess that will lead to fertilization outside of the female's body. Suchanimals often have behaviours that bring eggs into proximity withsperm (see flgure I t ). External fertilization has several risks includingpredation and the susceptibility to environmental variation such as

temperature and pH fluctuations and more recently, pollution.

Terrestrial animals are dependent on internal fertilization. Otherwise,gametes would be at risk of drying out. Internal fertilization also ensuressperm and ova are placed in prolonged close proximity to each other.Marine mammals which have reinvaded aquatic habitats still useinternal fertilization. Once the eggs are fertilized, the developing embryocan be protected inside the female.

lmplantation of the blastocgstlmplantation of the blastocgst ¡n the endometrium isessent¡al for the cont¡nuation of pregnancU.After fertilizalion in humans, the fertilized ovum divides by mitosisto form two diploid nuclei and the cytoplasm of the fertilized egg celldivides equally to form a two-cell embryo. These two cells replicate theirDNA, carry out mitosis and divide again to form a four-cell embryo' Theembryo is about 48 hours old at this point. Further cell divisions occur,but some of the divisions are unequal and there is also migration of cells,giving the embryo the shape of a hollow ball. It is called a blastocyst(figure 12). At 7 days old the blastocyst consists of about 125 cells andit has reached the uterus, having been moved down the oviduct by thecilia of cells in the oviduct wall. At this age the zona pellucida, whichhas surrounded and protected the embryo, breaks down' The blastocysthas used up the reserves of the egg cell and needs an external supplyof food. It obtains this by sinking into the endometrium or uterus

r Figure 11 Breeding pair of Anomalochromisthomosicichlids. The female Ibottom) is lagingeggs on a rock with the male in close proximitg

r Figure 12 Blastocgst

e Figure 13 lmplantation of theblastocgst

r Figure 14 Growth and differentiationof the earlg embrgo

505

ANTMAL PHYSt0L0GY IAHL)

lining in a process called implantation (flgure l3). The outer layer ofthe blastocyst develops flnger-like projections allowing the blastocystto penetrate the uterus lining. They also exchange materials with themother's blood, including absorbing foods and oxygen. The embryogrows and develops rapidly and by eight weeks has started to form bonetissue. It is then considered to be a fetus rather than an embryo. It isrecognizably human and soon visibly either male or female.

Role of hCG in earlg pregnancghCG stimulates the ovarg to secrete progesterone duringearlg pregnancg.Pregnancy depends on the maintenance of the endometrium, whichdepends on the continued production of progesterone and estrogen.in part these hormones prevent the ciegeneration oT the uterus liningwhich is required to support the developing fetus. Early in pregnancythe embryo produces human chorionic gonadotropin - hCG. Thishormone stimulates the corpus luteum in the ovary to continue tosecrete progesterone and estrogen. These hormones stimulate thecontinued development of the uterus wall, which supplies the embryowith everything that it needs.

Materials exchange bg the placentaThe placenta facilitates the exchange of materialsbetween the mother and embrgo.Humans are placental mammals. There are two other groups ofmammals: the monotremes lay eggs and the marsupials give birth torelatively undeveloped offspring that develop inside a pouch. By rhestage when a marsupial would be born, a human fetus has developed arelativeìy complex placenta and so can remain in the uterus for monthslonger. The placenta is needed because the body suriace area to volumeratio becomes smaller as the fetus grows larger.

The placenta is made of fetal tissues, in intimate contact with maternaltissues in the uterus wall. The fetus also develops membranes that formthe amniotic sac. This contains amniotic fluid, which supports andprotects the developing fetus.

The basic functional unit of the placenta is a finger-like piece of fetaltissue called a placental villus. These villi increase in number duringpregnancy to cope with the increasing demands of the fetus for theexchange of materials with the mother. Maternal blood flows in theinter-villous spaces around the villi (figure l5). This is a very unusualtype of circulation as elsewhere blood is almost always confined in bloodvessels. Fetal blood circulates in blood capillaries, cÌose to the surface ofeach villus. The distance between fetal and maternal blood is thereforevery small - as little as 5 pm. The cells that separate maternal and fetalblood form the placental barrier. This must be selectively permeable,allowing some substances to pass, but not others (flgure l6).

506

11.4 SEXUAL REPRODUCIIOII

maternalvenule

malernalarteriole

fetal blood

carbon dioxide

placental barrier maternal blood

diffusion

urea

water

r Figure 16 Exchange processes in the placenta

ìå.

umbilical cord

umbilicalveinu mbilical

arteries

fetal portion ofplacenta Ichorion]

maternal portionof placenta

a, Figure 15

Release of hormones bg the placentaEstrogen and progesterone are secreted bg the placentaonce it has formed.By about the ninth week of pregnancy, the placenta has started tosecrete estrogen and progesterone in large enough quantities to sustainthe pregnancy, and the corpus luteum is no longer needed for this role.There is a danger of miscarriage at this stage of pregnancy if this switch-over fails.

Data-based questions: Electron m¡crograph of placentaFigure 17 shows a small region at the edge of aplacental villus. The magniflcation is x 17,000.

f a) Identify the structures that are visible inthe upper part of the micrograph. tll

b) Explain the functions of thesestructures. t3l

2 In much of the area of the electron micrographthere are rounded structures, surrounded by asingle membrane. These are parts of a systemof tubules called the smooth endoplasmicreticulum (sER). Its function is the synthesis oflipids, including steroids. Suggest a function forthe sER in the placenta. t3l

3 ldentify, with reasons, the structure in the .* Figure 1Z Small region atthe edge of a placenralIower left part of the micrograph. t3l vilus

diffusion

facilitateddiffusion

endocr.ttosis

osmosts

507

ANTMAL PHYST0L0cY IAHL)

@ nstessing risks of estrogen pollutionAssessing risks and benef its of scientif ic research: the risks to human male fertilitg werenot adequatelg assessed before steroids related to progesterone and estrogen werereleased into the environment as a result of the use of the female contraceptive pill.High levels of estrogen are present in pregnant 2004 rli'ú 86% of male flsh sampled at 5l siteswomcn and inhibit |SII release. If women around the country were intelsex, [ha[ is rrralc fisLconsume pills containing estrogen, then showed signs of "feminization". However, therethis would mimic pregnancy and inhibit the is limited scientific consensus that pollution withdevelopment of mature follicles thus preventing steroids related to estrogen and progesterone is thepregnancy. Ethinyl estradiol is a synthetic causative agent behind reduced male fertility.form of estrogen that was first introduced as a .conrraceprive in r943. At the time, tittle irrîùn, 11'.':-11.'nt

European commission proposed a

was given ro the iciea that iî a iarge rrr-¡.."Ji^" ::]::: which would limit the concentrations inwomen used this form of conrracepri"".;;;^ i:l:::-la widelv nsed contraceptive drug' This

levels of esrrogen in bodies of warer mighi'-" lÏ:|1:oto intense lobbving bv the water and

be raised through sewage. It wasn'r untit ttre *|T::t"tical industries' which say that the

mid-Igg0s that the first reports of elevateä - science is uncertain and the costs too high'

contraceptive pill hormones present in water were Upgrading the technology for wastewaterreported. Since then, a number of problems have treatment could eliminate most of the pollution.been attributed to estrogen pollution. Researchers and policy expefis suggest sharing the

rn t992, a review arricle summarizing 6l different :.".::jÏ""g all responsible parties' including the

srudies concruded that human -ul" ,p"r,i;;;; :i:::Td drug industries' and that some expense

have declined by 50% over rhe past 50 o**'^^'" ::,1t1i: passed on to the public' The drugs arewidely used in livestock, so preventing animals

In one of the largest studies of the problem, the from urinating close to rivers could further reduceUI( government's Environment Agency found in the amount of drugs leaking into surface waters.

Data-based questions: Estrogen pollutionRivers vary in terms of the quantities of syntheticestrogen (Er) found. A study was conducted toinvestigate the relationship between concentrationsof synthetic estrogen in water and impacts on maleflsh from the genus Rutilus (roach) (see figure l8).a) State the relationship between synthetic

estrogen (Er) and the appearance ofoocytes in testes. Ill

b) Determine the mean percentage of male flshwith oocytes in their testes at concentrationsof estrogen greater than l0 ng/L. Í21

E2 concentration Ing/L)Figure 18

Source: Jobling eî al, Environ Health Perspect.2006 April; 114[S-1J: 32-39.

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The role of hormones in parturitionBirth is mediated bg positive feedback involving estrogenand oxgtocin.During pregnancy, progesterone inhibits secretion of oxytocin bythe pituitary gÌand and also inhibits contractions of the muscularouter wall of the uterlrs - the myometrium. At the end of pregnancy,

I OOCUIeS rn testesr¡ feminized reoroductive ducts

508

11.4 SEXUAL REPRODUCTION

hormones produced by the fetus signal to the placenta to stop secretingprogesterone, and oxytocin is therefore secreted.

Oxytocin stimulates contractions of the muscle fibres in the myometriumThese contractions are detected by stretch receptors, which signal tothe pituitary gland to increase oxytocin secretion. Increased oxytocinmakes the contractions more frequent and more vigorous, causing moreoxytocin secretion. This is an example of a positive feedback system - avery unusuaÌ control system in human physioÌogy. In this case it has theadvantage of causing a gradual increase in the myometrial contractions,allowing the baby to be born with the minimum intensity of contraction.

Relaxation of muscle flbres in the cervix causes it to dilate. Uterinecontraction then bursts the amniotic sac and the amniotic fluid passesout. Further uterine contractions, usually over hours rather thanminutes, finally push the baby out through the cervix and vagina. Theumbilical cord is broken and the baby takes its first breath and achievesphysioÌogical independence from its mother.

@ eang positions itself before birth so that ¡ts headrests close to the cervix

bladder mucus plug

Icompressed) [pusheddownuterus wall

i Data-based questions: Hormone levels during pregnancAi tn the graph (figure 20), the thickness of the arrows indicatesi relative quantities.

c0rpus luteu m

30 dags120 dags

full term

, ESTROGEN

01?3456789conception months of pregnancg

-->

deliverg

Figure 20

I Describe the changes over the course of a pregnancy in relativeamounts and source of:

a) hCG

b) estrogen

c) progesterone

2 Suggest reasons for the clrop in hCG concentration after thesecond month of the pregnancy.

3 Prcdict the consequences of the placenta failing to secreteestrogen and progesterone dr-rring a pregnancy.

ØoooçoE@r

placenta umbilical spinecord

rectu m

@ eang passes into vagina and amniotlcfluid is released

@ eaUg is pushed out of mother's bodg

@ Placenta and umbilical cord are expelledfrom bodg

placenta becomingdetached from uterus wall

umbilical cord

Figure 19 Stages in childbirth

front ofpelvis

into vaginaJ

2

2

2

2

PR(]GESTERONE

12)s09

ANTMAL PHYSt0L0GY IAHL)

@ Oertation times, mass and growth, and development strategiesThe average 38-week pregnancA in humans can be positioned on a graph showingthe correlation between animal size and the development of the Uoung at birth forother mammals.Mammals differ in their growth and developmentstratcgics. Altricial spccies give birth to relativelyhelpless, incompletely developed offspring. Theirnewly-born young are relatively immobile, lackhair and are unable to obtain food on their or¡m.At the opposite end of the spectrum are precocial

mammals in which the offspring have open eyes,hair, are immediately mobile and are somewhalable to defend themselves against predators.

Mammals with a large body size are more likelyto be precocial. This is correlated with a longgestation period.

Data-based questions: Gestation length and bodg massFigure 2l shows the relationship betweengestation period and body mass for 429 placentalmammal species subdivided into whether thespecies is described as altricial or precocial.

3

234s6logl¡ bodg mass

EoocC.ooØoo¡oo0o

2

L0

r, F gure 21

78

I The solid dots and open dots arerepresentative of two different growth anddevelopment strategies. Deduce which circlesare used to represent precocial mammals. t2l

2 Outline the relationship between adult bodymass and gestation period. ttl

3 Explain the relationship between body massand the length of gestation. I3l

4 The mean length of human gestation is 283 days(logro283 : 2.45) The mean body mass of anadult human is ó5 kg (log,o 65 : 1.8).

(i) Determine the approximate location ofhumans on the graph. tll

(ii) Suggest reasons for humans being anoutlier on this graph. I3l

r Figure 22 Laboratorg mice are altriclal. Theg havea gestation period ofabout 19 dags

r Figure 23 Elephant calves are born after a 2Z-monthgestation period and theg nurse for around three gears.Theg are categorized as precocial. The African elephant isthe largest and heaviest land animal alive todag

I

510

0 u Esïl0t{ s

0uestionsI Figure 24 shows how the surface pH of human

skin varies between different areas of thebody. It also shows differences between adultsand newborn infants (neonates). Skin pHprotects the skin from colonization by certainmicroorganisms.

soles

back

abdomen

palms

forearm

forehead

I

b) Suggest reasons for calves that haveendured a long and difflcult birth beingmore likely to suffer from infection. Í2)

c) Predict how the concentration of antibodiesmight vary in the cow's colostrum over thefirst 24 hours after birth. t2l

d) Deduce the reasons for vaccinating sheepagainst pulpy kidney and other life-threatening diseases three weeks beforelambs are due to be born. Í2)

e) Explain which method of transportacross membranes is likely to be used forabsorption of antibodies in the stomach ofnewborn mammals. t2l

3 The blood glucose concentration of a person withuntreated diabetes often rises to 300-500 mg per100 ml of blood. It can even rise to concentrationsabove 1,000 mg per 100 ml. When the bloodglucose level rises above 225 mg per 100 ml,glucose starts to appear in the urine. The volumesof urine produced become larger than normal,making the person dehydrated and thirsty.

a) Explain how glucose is completelyreabsorbed from the glomerular filtrate ofpeople who do not have diabetes. t3l

b) Explain why glucose is not all reabsorbedfrom the glomerular filtrate of diabeticpatients. [4]

c) Suggest why untreated diabetics tend topass large volumes of urine and often feelrhirsry. t3l

4 Muscles often increase in mass if the amountthat they are used increases. An experimentwas performed to examine the effect of flighton muscle mass in European starlings (Sturnusvulgaris). Study birds were randomly assignedto three groups. Over 6 weeks, each groupwas subjected to 34 l-hour study periods. Theexercise group was trained to fly for I hour byreceiving food rewards. Control group I wasallowed to feed freely but placed into cagesthat prevented flying. Control group 2 wasfed the same food rewards at the same timeas the exercise group, but was also placed intocages that prevented flying. Body mass wasmonitored before and during the experiment(see flgure 26). Af the end of the experiment,

5 6 7

pH

I neonates I adults

r Figure 24 How the surface pH of human skin varies betweendifferent areas ofthe bodg

a) Compare the skin pH of neonates andadults. I2l

b) Suggest how the adult skin pH might beestablished. tll

c) Suggest why the use of soaps (which arebasic) might have a more irritating effect onthe skin of a neonate. 12)

d) Deduce how basic soaps might underminethe skin's defensive function. Í21

2 Figure 25 shows the ability of a calf (Bos taurus)to absorb antibodies after birth.

06t21824303642calf's age at first feeding/hours

I Figure 25 The abilitg of a calf [8os tourusJ to absorba ntibodies

a) Describe how the ability of a calf to absorbantibodies changes over the initial hoursafter birth. I2l

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100

75

50

25

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ANTMAL PHYSt0L0GY IAHLJ

the mean mass of the birds'pectoralis muscleswas compared (flgure 2ó).

a) Compare the changes in body mass incontrol group 2 and the exercise group. Í21

b) Evaluate the claim that preventingexercise increases pectoralis musclemass. t3l

c) Suggest how the mass of the birds'pectoralis muscle could be determined. [2]

d) One hypothesis that might be generatedfrom this experiment would be thatreducing motion in birds might leadto greater muscle mass per bird. Suchknowledge might be used in the farmingof poultry. Greater meat production perbird would result from the motion ofthe birds being restricted. Discuss theethics of designing and carrying outexperiments to test this hypothesis. t3l

[.ì 8s

,,"¡---'----tA-

-G control 1

--l- control 2+ exerc¡segroup

before 2weeks 4weeks 6weeks

[b] ?.s

5 controll control2 exercise

A Figure 26 The effect of exercise on bodg mass and musclemass in starlings

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65

60

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