1. bio- maintaining a balance

8/9/2019 1. Bio- Maintaining a Balance

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MAINTAINING A BALANCE

Module 1 Chapter 1.indd 1l i 21/5/08 1:07:44 PM


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Most organisms are active in a limited temperature range

Temperature regulation

CHAPTER 1

IntroductionMost organisms are active within alimited temperature range, despitethe large uctuations in temperaturethat occur in the outside environment.Organisms that live in environments

where they may be subjected toextremes of temperature haveadaptations that enable them to keeptheir internal temperature within arelatively narrow range. Organismsmust also maintain a relatively constantbalance of chemicals within their bodiesif they are to remain functionally active.One of the main reasons why themaintenance of a constant temperatureand chemical balance is so importantis to ensure efcient metabolism maintaining optimum conditions forthe functioning of enzymes, the organiccatalysts that control all chemicalreactions in cells.

Enzymes function underbalanced conditions

All metabolic reactions in living cellsare controlled by enzymes. Enzymesare protein molecules, present in cells,

which act as biological catalysts ,controlling the rate of each step of thecomplex chemical reactions that takeplace in cells. Catalysts are chemicalsubstances that can accelerate (speedup) chemical reactions, but they remainunchanged at the end of the reactionand can be reused. They function veryrapidly at low temperatures, makingthem ideal for cell functioning.

Metabolism is the sum total of allchemical reactions occurring withina living organism. Each step of a

metabolic pathway in cells is catalysedby enzymes.Metabolism is divided into two:

anabolic and catabolic. Those reactionsthat involve building up large organiccompounds from simpler molecules aretermed anabolic reactions, for examplea large polysaccharide moleculesuch as starch being made fromsmall monosaccharide units such asglucose, a product of photosynthesis inplants. (You may have heard the term

anabolic used to describe steroids.Discuss the meaning of the term in thiscontext.)

Chemical reactions that involvebreaking down complex organiccompounds to simpler ones are termedcatabolic reactions. For example, in thedigestion of food, large food moleculessuch as proteins are broken down intosmall units called amino acids, whichcan then be easily absorbed from thegut into the bloodstream.

Chemical reactions may be classiedaccording to whether they use up orrelease energy. Anabolic reactions areusually endergonic reactions, requiringan energy input. Catabolic reactionsusually give out energy and so they areexergonic reactions.

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Additional informationand websites

anabolic steroids



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TEMPERATURE REGULATION

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identify the role of enzymes in metabolism, describe their chemical composition and use a simple model to describe their specicity on substrates

By understanding the chemicalcomposition, functions andcharacteristics of enzymes, wecan better understand their role incontrolling chemical reactions in cellsand therefore metabolism in livingorganisms.

The chemical composition ofenzymesEnzymes are protein molecules and aremade by living cells. They are globularproteins, meaning that they have longchains or sequences of amino acids thathave been folded into a specic shape.Their effective functioning relies ontheir shape. The molecule on which anenzyme acts is called a substrate . Anenzyme ts together with its substrate

molecules at a precise place on thesurface of the much larger enzymemolecule, called the active site (muchlike a key ts a particular lock). Theshape of this active site must not be

altered if the enzyme is to function (seeFig. 1.1).

CofactorsSome enzymes have a non-proteingroup such as a vitamin (e.g. riboavin= B2, pantothenic acid = B 5) or ametal ion (e.g. zinc, copper or iron)

that binds with the protein part andhelps to form the active site. This istermed a co-enzyme or cofactor itcan be easily separated from theprotein part of the enzyme, but itspresence is essential for the enzymereaction to occur because the enzymecannot function without the cofactor.

A functional enzyme may thereforeconsist of protein only, or it may bein the form of an enzymecofactorcomplex (where the enzyme part of

the complex is a protein). Poisons aresubstances that have harmful effectson living organisms. Some poisonsexert their toxic effect by disablingcofactors and thereby inhibiting enzyme

Figure 1.1 Thechemical structure of

the enzyme lysozyme(a) represented

in ribbon style;(b) represented asa three-dimensionalmodel; (c) showing

the formation of anenzymesubstratecomplex

active site

(b)(a)

substrate

(c)

active site

enzyme madeof protein

groove of active sitefits shape of substrate

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Enzymeco-enzymesubstrate complex

Enzymes and metabolism 1.1



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Figure 1.2 Schemeof activation energyrequired for chemicalreactions: (a) withouta catalyst, activationenergy must besupplied for achemical reaction;(b) catalysts acceleratespeci c reactions bylowering the amountof activation energyneeded to initiate thereaction

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Teaching strategyenzymes reduceactivation energy

functioning. The heavy metals mercuryand cadmium replace zinc cofactorsin some enzymes and inhibit theirfunctioning.

The role of enzymes inmetabolismThe following functions of enzymeslead to their effective role inmetabolism:

Acceleration of chemical reactions

Enzyme catalysts are able to speedup (or slow down) reactions withouta change in temperature. This isextremely important in cells, since heatdamages living tissue. For a chemicalreaction to begin, activation energy isnecessary (see Fig. 1.1). The role ofan enzyme is to lower the activationenergy needed to start a reaction, sothat the reaction can proceed quickly,

without a change in temperature.

Lowering of activation energy In chemical reactions that occur in thenon-living world, heat could providethe necessary activation energy for achemical reaction, but in the living

world, heat burns tissue. It is importantto remember that an enzyme does not

provide activation energyit reducesthe amount of activation energyneeded (by bringing specic moleculestogether, rather than relying on themcolliding randomly). For example,

oxygen and glucose may be chemicallycombined to release energy. In thelaboratory, we can activate this reactionby adding heatwe burn the glucoseand cause it to react with oxygen in theair to release energy as light and heat.In the human body, we cannot addheat to glucose and oxygen to initiate areaction and so an enzyme is necessaryto lower the required activation energy,so that glucose can react with oxygento release energy. (See Fig. 1.2.)

Action on specic substratesEnzymes are therefore substrate-specic , meaning that one particularenzyme can work on only oneparticular substrate molecule, becausethe active site is reciprocally shaped tobind with that molecule. The enzymeitself is not chemically changed inthe reaction and so it can be reusedin subsequent reactions. Enzyme-controlled reactions are alwaysreversible.

Characteristics of enzymes

Enzymes, due to their protein nature,are sensitive to temperature (heatand excessive cold) and to pH (ameasure of the acidity or alkalinity ofa substance).

Temperature-sensitive

Enzymes within cells function bestat the body temperature of the living

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Student worksheetenzymes



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organism in which they occur. Inmost living things, enzymes functionnormally at temperatures up to40C; above this temperature theirefciency (rate of reaction) decreases.

At temperatures above 60C, mostenzymes stop functioning altogether.This is because heat causes thehydrogen bonds that maintain the formof the enzyme to break and this, in

turn, alters both the structure and shapeof the moleculethe molecule is saidto denature . Any change in shapethat affects the active site will alter thefunctioning of the enzyme becausethe altered active site is no longerreciprocally shaped to the substratemolecule. Excessive cold also causesthe enzyme to change shape and itsfunctioning to slow down or stop, butthe change in shape due to extremecold is often reversible.

pH-sensitive

Each enzyme has its own narrowrange of pH within which it functionsmost efciently. Levels of alkalinity oracidity outside of the optimum pH foran enzyme have a similar effect to thatof temperature changethey alter theshape of the enzyme and slow down orstop its functioning. Within cells, mostenzymes function at or near neutral, butenzymes in the digestive tract function

in an acidic or alkaline medium. Forexample, the protein-digesting enzymespepsin and rennin, found in gastricjuice in the stomach, function best ina strong acid. The enzyme salivaryamylase, found in saliva, helps breakdown starch and it functions best in a

weak alkaline medium. The action ofamylase on starch stops when the foodpasses into the stomach, because of the

low pH of gastric juice. Extremes of pH,like temperature, cause the enzymes todenature.

Substrate-specic

Enzyme molecules are specic,acting on only one type of substrate;therefore, each enzyme catalyses oneparticular chemical reaction involvingthe substrate for which it is specic.This is due to the lock-and-key t ofthe active site to the substrate molecule

(described, overleaf in more detail inthe section How enzymes work).Examples of enzyme specicity are:

amylase acts on starch, changing itto glucose

rennin acts on the protein in milk,causing it to curdle

the enzyme catalase, present in mostliving cells (e.g. potato/meat/apple)acts on toxic hydrogen peroxide andconverts it to harmless water andoxygen gas.

Figure 1.3 (a) Graphshowing the effect of

temperature on therate of enzyme action;(b) graph showing thepH-speci city of twodigestive enzymes

optimum pHfor trypsin

optimum pHfor pepsin

optimumtemperature

30 40 50

R a

t e o

f r e a c

t i o n

Temperature of reaction (C)

1 9

R a

t e o

f r e a c

t i o n

pH of reaction

2 3 4 5 6 7 8

(a) (b)

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Teacher resource

terminology related toenzymes



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identify the pH as a way of describing the acidity of a substance

pH is a way of describing the acidityof a substance. The pH scale is usedto measure the acidity or alkalinityof a substance, as shown below (seeFig. 1.4). pH is a logarithmic value ofthe concentration of hydrogen ions(H+) in solution. Since it is a logarithmic

value, the greater the hydrogen ionconcentration, the lower the pH. The

pH scale runs from 0 to 14, where 7(the midpoint) represents a neutralsolution. The presence of hydrogenions in a solution makes it more acidicand so solutions with a pH below 7are acidic and those with a pH above7 are alkaline or basic. The furtheraway from the neutral value of 7, thestronger the respective acid or base.

Figure 1.4 pH scale

pH scale

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l e s

p H 0

p H 1

p H 2

p H 3

p H 4

p H 5

p H 6

p H 7

p H 8

p H 9

p H 1 0

p H 1 1

p H 1 2

p H 1 3

p H 1 4

b a

t t e r y a c

i d

h y d r o c

h l o r i c a c

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neutral

s a l i v a

( p H 6

. 5 )

strongacid

increasingacidity

weak acid

b l o o

d ( p H 7

. 4 )

weak base

strongbase

increasingalkalinity

PFA

H2How enzymes work: models to describe enzyme specicity onsubstratesEnzymes are large, globular proteinmolecules with one or moreindentations on their surface calledactive sites . For an enzyme tocatalyse a reaction, the small substratemolecules must temporarily bind tothese active sites. At rst a lock-and-key model was proposed: it wasthought that the active site is rigidand the small substrate molecule isreciprocally shaped and ts into theactive site, like a lock ts a key. Oncethis enzymesubstrate complex hasformed, the close proximity of the

molecules allows the reaction to berapidly catalysed and the products ofthe reaction are released. To validatethis model, predictions were made andtested. The results led to the proposalof the currently accepted amended

version of the model, known as theinduced-t model . This model isbased on the realisation that proteinsare not rigid. Evidence suggests that thebinding of a substrate to the active siteof an enzyme induces the enzyme toalter its shape slightly, to t more tightlyaround the substrate. (See Fig. 1.5.)

ti the H as a wa o describin the acidit

What is pH?

of

pH is aof a sub

ide

Wha1.2



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as the saturation point . Increasingthe substrate concentration beyond thesaturation point will not increase therate of reaction, since all enzymes are

working at their maximum turnover rateand will have to be reused to act onthe additional substrate. The only wayto increase the reaction rate would beto increase the enzyme concentration.(See Fig. 1.6.)

Figure 1.6 Graph showing the effect of substrateconcentration on enzyme activity

Substrate concentration

R a

t e o

f r e a c

t i o n

maximum

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Student worksheetgraphs related toenzyme activity

Background informationEnzymes are protein molecules that are madeby living cells and function as catalysts withinthe cells. They accelerate the rate of reactionwithout themselves being changed. A substrate is another name for a reactant in an enzyme-controlled reaction.

In each of the investigations that follow,the activity of a named enzyme will be studied.There are a variety of enzymes that aresuitable to use for this investigation. Eachhas its advantages and disadvantages (seeTable 1.1).

Investigating enzyme activity identify data sources, plan, choose equipment or

resources and perform a rst-hand investigation to test the effect of:

increased temperature change in pH change in substrate concentrations on the activity

of named enzyme(s)

FIRST-HANDINVESTIGATION

BIOLOGY SKILLS

H11.1; H11.2; H11.3

H12.1; H12.2; H12.3;H12.4

H13.1

H14.1; H14.2; H14.3

IRINV

I

Table 1.1 Advantagesand disadvantages ofenzymes

Enzyme and source SubstrateChemical reactioncatalysed

Evidence of enzymeactivity

Determining enzymeactivity

Catalase(potato or any freshplant or animal tissue)

Hydrogen peroxide Hydrogen peroxideconverted to water andoxygen

Creates a zzing effect Measure the height ofbubbles

Amylase(commercially availableor found in saliva)

Starch(available as powderedstarch that can bemixed with water, orboiled potato)

Starch converted toglucose

Starch no longerpresent

Starch can be stainedwith iodine. Timehow long until starchdisappears: enzymeactive no morestarch present

Rennin(available as junket

tablets)

Milk protein(caseinogen)

Converts solublecaseinogen proteininto an insoluble form(casein)

Milk curdles and aprecipitate forms

Time how long milk takes to curdlethisindicates rate ofenzyme activity



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TaskStudents will need to plan and conductthree separate experiments so that theycan investigate the effect of each factorindependently. That is, in each experiment onlyone variable is changed to ensure the validityof the investigation. The effect of each of thefollowing factors on enzyme activity will beinvestigated: increased temperatureExperiment 1 change in pHExperiment 2 change in substrate concentrations

Experiment 3.There are several ways in which this can be

tackled. Group work is recommended, as eachexperiment (especially the effect of temperatureon enzyme activity) is fairly labour-intensive.

Planning the scientic investigationStudents should consult the teacher and usethe information on the Student Resource CD todecide whether they will investigate the activity ofthe same enzyme and its substrate for all threeexperiments, or whether they will use a differentenzyme for one or more of the experiments. To plan the investigation, a variety of

sources should be consulted, including theinformation in the table on the previouspage, the Student Resource CD and thetext on pages 35 on the role of enzymesin metabolism.

Teachers may like to guide the class throughplanning and conducting one of the threeexperiments on enzyme activity and thenallow the students to plan and conductthe other two experiments on their own.(Teachers resource material, The ve stepsof investigation, available on the TeacherResource CD, may be useful.)

For each experiment, students need to: identify the enzyme and substrate to be

used discuss with the teacher the sour ces

from which both the enzyme and thesubstrate that you have chosen to use

can be obtained research the chemical reaction that theenzyme catalyses and write out a wordequation for this reaction

determine a method to measure theactivity of the enzyme in a laboratory.

Research and list all safety precautions tobe taken and the hazards of any chemicals thatmay be used.

Ensuring the validity of theinvestigationA valid experiment is one that actually testswhat it sets out to test. To arrive at valid

conclusions, it is necessary to use a control:remove the factor you are testing and comparethe results with the experiment when the factor

was present. The comparison should showthat if the factor is missing (the control), thesame result is not obtained, proving that it isthe presence of that factor which brings aboutthe result. Set up two sets of apparatus foreach runone with the factor being tested(experimental apparatus) and one without thefactor (control apparatus). Validity also dependson keeping variables constant and ensuringreliability and accuracy. Identify the independent and dependent

variables and plan how you will keep allother variables constant

Ensure reliability and accuracy: read the

Biology Skills on pages xxii (and, inparticular, take note of 12.4 e and f) todetermine how you will ensure:

reliability: the same method should yieldthe same results when repeated by otherpeople (this may require modicationand inter-group co-operation after a testrun) and averaging and/or comparison ofresults

accuracy: the results should complywith similar scientic information (e.g.data from other scientic sources suchas scientic journals); accuracy alsorelies on choosing precise measuringequipment and using it correctly to avoidexperimental error

Results: choose suitable format(s) torepresent your data (e.g. tables, graphsthe correct type of graph and the line of bestt).Additional information is available on the

Teacher Resource CD.

Reporting on the investigationFor each experiment, write up a practical reportunder the standard scientic headingsaim,hypothesis, materials, safety, method, results,conclusion and discussion. Results : data from results should be

measured, recorded in the form of a tableand then graphed.

Conclusions : read the aim of eachexperiment again, consider your hypothesisand then write a valid conclusion based onyour results (no inferences).

Discussion : any suggested modicationsto the method, materials or equipmentand explanations of unexpected results orexperimental error should appear under thisheading. Answer all discussion questions aswell.

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Teaching strategy forthe investigation and

teacher resourcevalid investigations

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Sample experimentson investigating

enzyme activity andpractical reports

Experiment reportinvestigating enzyme

activity

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Of all living organisms the mammalianbody has best perfected keepinginternal functioning constant, no matter

what changes occur in the externalconditions in the environment. Themodule Maintaining a balance has asits central theme the maintenance ofinternal stability, called homeostasis ,

within living organisms. In this module, we will study regulatory systems inboth plants and animals that act tomaintain a balance in their internalenvironments: temperature regulation (brought

about mainly by the skin inmammals and by leaves in plants)

control of chemical substancesavailable to cells, transportedthrough organisms (by blood vessels

in mammals and vascular tissue inplants)

the control of water and salt balance(osmoregulation) and of pH and

waste products (brought aboutmainly by the kidneys in mammalianbodies).

An organism is healthy as long ashomeostasis is maintained. When aperson visits a doctor for a medicalcheck-up, the doctor will monitor their

wellbeing by carrying out standardchecks, including measuring theirbody temperature and taking bloodsamples to compare the patients bloodcomposition with a standard set of

values that indicate the normal rangefor optimal metabolic efciency.

in or anisms the mammalian in mammals and vascular tissue in

Homeostasis and feedback mechanismsmaintaining a balance

body hainternal

what ch

Of all li

Hommain1.3

Homeostasis describe homeostasis as the process by which organisms

maintain a relatively stable internal environment

The word homeostasis comes from theGreek words homoios , meaning like orthe same and stasis , meaning state . Thisimplies a state of balance or constancy,

where conditions stay the same inthe internal environment of livingorganisms to allow them to functionefciently, despite uctuations in theexternal environment.

Homeostasis is dened as the maintenance by an organismof a constant or almost constantinternal state, regardless of externalenvironmental change.

Any organised infrastructure, whether a living organism or a non-living enterprise, needs careful controland certain constants if it is to runsmoothly and efciently, particularly

when external circumstances uctuateor change. If we consider the smoothrunning of a hospital or even ahousehold, a sudden external change,

for example a power cut, could havedrastic results if the organisation cannotcontinue to work independently of theoutside changes. The uctuations needto be monitored and counter measuresmust be put in place. For example, ifthere is a power failure and a hospitaldoes not have a back-up plan, manylives will be lost.

In order to maintain a constantinternal environment, the following twosteps are essential:1. detect the change2. counteract the change.

In a similar way, living organismsmust have mechanisms in place toenable them to function independentlyof external changes and to maintaina relatively constant internal state. Inthis chapter, we look at homeostasis and how living organisms maintain aconstant internal environment.



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When we consider our analogy inmore detail, it becomes evident thatsome organisations are better equippedto cope with change than otherspartof the back-up plan in a hospital isto have its own emergency generator,

which can be put into use in the eventof a power failure; however, most ofus do not have emergency generatorsin our homes. It is interesting (andnot unexpected) to note that certain

living organisms have a better back-up plan than others when it comesto maintaining a constant internalenvironment.

Living organisms have developedmechanisms that ensure that they areable to maintain a constant or almostconstant internal state, regardlessof changes from the stable state ofconditions in the external environment.

CLASSROOM ACTIVITY

Discuss the following analogy, which should help us to understand the importance of maintainingconstant internal conditions in an organisation such as: a hospital a home.

In order to maintain a constant internal environment in the event of a power cut, how would peoplewithin the hospital or home:1. detect the changehow will people become aware that the power supply has been cut off?2. counteract the changewhat measures could be put in place within each organisation to

temporarily overcome the problem until things return to normal?Compare the efciency of these measures and relate this to the importance of the functioning of

the organisation.

explain why the maintenance of a constant internal environment is important for optimal metabolic efciency

Living organisms are made of cells, which must function efciently tomaintain life. All chemical reactions

within cells must occur efciently

and be effectively co-ordinatedto bring about optimal metabolicefciency.

Each cell is surrounded by a smallamount of uid called intercellular orinterstitial uid and this, together withthe cytoplasm inside cells, makes uptheir internal environment. Cells areextremely sensitive to changes in theirinternal environment and any imbalanceadversely affects their functioning. Theinternal environment of an organism

must be maintained within a narrowrange of conditions, for exampletemperature, volume (the amountof cells or of uid such as blood or

cytoplasm) and chemical content inthe internal environment must be keptstable so that enzymes can functioneffectively and metabolic efciency canbe maintained. Enzymes are extremelysensitive to the temperature and pHof the environment and changes inconcentrations of these, as well asnutrients such as glucose and oxygen,affect their activity. Cells cannottolerate any build-up in levels of wasteproducts such as carbon dioxide or

The importance of a constant internal environment 1.4



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explain that homeostasis consists of two stages: detecting changes from the stable state counteracting changes from the stable state

Homeostasis involves an enormousamount of co-ordination and control ina living organism. In mammals, boththe nervous system and endocrine(hormonal) systems are involved.

Homeostasis is brought about in twostages:1. detecting change: sensory cells or

receptors present within the bodydetect change in the temperatureand/or chemical composition

within the body. This change in theenvironment is called a stimulus.

2. counteracting change:

effector organs (suchas muscles or glands)then work to reverse thechange. A response thatsuccessfully reverses thechange will return the bodyto homeostasisits relativelyconstant state.Homeostatic mechanisms

ensure that variables (suchas temperature or theconcentration of chemical

substances ) in the internal environmentof an organism are maintained within anarrow range . Within each organism,these variables have an ideal ornormal value, called the set point . Homeostasis does not maintain theexact set point, but homeostasis ismaintained as long as there is only anarrow range of uctuation (increaseand decrease) of the variable aroundthe set point . (See Figure 1.7.)

If the uctuation is large andexceeds the normal range, a negative

STUDENT ACTIVITY

An explanation involves nding a cause and effect relationship. (Refer to theverb scaffold for explain on the Teacher Resource CD.)

Analyse the above explanations of the importance of maintaining a constantinternal environment in terms of each variable, and in the form of a table: state the underlying cause(s) of the phenomenon (the change to the internal

environment) outline any intermediate effects state clearly the overall effect on metabolic efciency.

functioning (for example, carbondioxide alters the pH of uid). In either

case, enzyme functioning is inhibited

and so these wastes must be removedto ensure metabolic efciency.

Negative feedbackthe mechanism of homeostasis 1.5

Time

N o r m a

l r a n g e

upper value that triggersa response to counteractthe increase

set point (ideal value)

lower value that triggersa response to counteractthe decrease

Figure 1.7 Graphshowing homeostasisas the maintenance ofa relatively constantinternal environmentaround an ideal valueor set point. The

value of the variable uctuates within anarrow range andis maintained by anegative feedbackmechanism

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Verb scaffoldexplain



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Temperature regulation and the nervous systemTem1.6 outline the role of the nervous system in detecting and

responding to environmental changes

Any change in the externalenvironment could affect the balancein the internal environment of theorganism and so a mechanism isneeded to ensure homeostasisthemaintenance of a stable internalenvironment, despite uctuations in theexternal environment. The mechanismsthat allow this to occur are based on anegative feedback system, co-ordinatedby the nervous system.

Introduction to the nervoussystemCo-ordination

The function of the nervous system isco-ordination and this takes place inthree steps:1. It detects information about an

animals internal and externalenvironments.

2. It transmits this information to acontrol centre.

3. The information is processed in thecontrol centre, generating a response to ensure the maintenance of arelatively constant internal state.

The structures of the nervous systeminvolved in the stimulusresponsepathway of co-ordination are: receptors sensory cells, sometimes

in sense organs (for example,olfactory receptors in the nose)

a control centre the centralnervous system, which includesbrain and spinal cord

effectors (e.g. muscles and glands) nerves , which link all the other

parts, relaying messages from one

part to another in the form ofelectrochemical nerve impulses.

The stimulusresponse pathway

A stimulus is detected by a receptor,a message is carried by nerves to acontrol centre and a response istriggered (see Fig. 1.8).

For example, if you touch a hotstove with your nger, receptors in

your skin detect the heat and pain, andthe result is that you withdraw yournger rapidly. How is this co-ordinated?This rapid reaction requires a linkbetween the receptors that detect thestimulus and the effectors, the musclesFigure 1.8 Flow chart

showing the stimulusresponse pathway

controlcentre effectors responsereceptorstimulus

feedback mechanism comes intooperation in response to this change;it is termed negative because itcounteracts the change (the stimulus),returning the body to within the normalrangei.e. to a state of homeostasis.

Note: The secondary-sourceinvestigation to model a feedbacksystem (see page 20) may be done atthis point in time OR after temperatureregulation.



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(or sometimes glands) that carryout a response. The co-ordinationis carried out by the nerves andthe central nervous system (brainand spinal cord) of the body. SeeFigure 1.9, which illustrates therole of the nervous system in thestimulusresponse pathway.

The role of the nervoussystem in homeostasisThe role of the nervous systemin homeostasis is co-ordination .

A pathway exists, whereby astimulus is detected by a receptor,a message is carried by nerves toa control centre and a responseis triggered. In homeostasis, theresponse usually counteractsthe stimulus (change), reducingits effect so that a balance ismaintained. This is termed anegative feedback mechanism.

Detecting change:receiving stimuli

Sensory cells called receptors

detect stimuli (changes in theinternal or external environmentof an organism). In their mostsimple form, receptors consist ofsingle cells, scattered over thebody of an organism. In theirmore complex form, receptorshave become concentrated inparticular areas to form senseorgans such as the eye, earand tongue. In many animals(including humans), receptors

in sense organs detect stimuliin the external environment.However, there are also receptorsthat are sensitive to internal stimuli within the body. Theseinteroreceptors within thebody are important in detectingchanges related to homeostasis that is, internal stimuli such aschanges in pH, body temperature ,osmotic pressure and the chemical composition of blood.

Figure 1.9 The role of the nervous system in detectingand responding to environmental change

Co-ordination pathway

(change inenvironment)

(sensory cells insense organ)

(sensory nervecarrying nerveimpulses)

(brain and spinalcord)

(motor nervecarrying nerveimpulses)

(muscles orglands)

(reaction)

detected by

stimuli

convert stimuli to impulses

receptors

transmit impulses

messengers

process information and triggernew impulses

CNS

react

effectors

response

transmit impulses

Loud noise

hair cells in ear

auditory nerve

brain

motornerves

muscles

head jerksand looksback

III+++II

+++III++

Example

messengers



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Receptors may be named accordingto the type of energy or molecules they detect. Those receptors importantin our study of homeostasis arethermoreceptors , which detectinternal changes in temperature, andchemoreceptors , which detect theconcentration of certain chemicalsinside the body (for example, carbondioxide levels) in the blood. Otherreceptors that you may come acrossin your studies (e.g. if you study thebiology option Communication) are

photoreceptors (sensitive to light, foundin the eye) and mechanoreceptors (sensitive to movement or vibrations,found in the ear).

Co-ordination: the role of the nervoussystem in processing information

The brain and spinal cord make up thecentral nervous system (CNS). Theperipheral nervous system consists ofnerves, which carry information to andfrom the CNS. The information carriedby nerves is messages transmitted in theform of electrochemical nerve impulses.Incoming information passes fromsensory receptors via sensory nerves to the CNS, which in turn transmitsoutgoing information to effector organs

via motor nerves . The role of the CNS isto process incoming information, analyseit and then initiate an appropriateresponse. Within the CNS, informationis processed and analysed by a numberof interconnecting nerve cells (neurons)and then a message is generated andtransmitted, stimulating the effectororgans. Some actions involving thenervous system may take place

voluntarily, but all of those involvedin homeostasis take place without anyconscious thoughtthey are involuntaryand many are inborn, unconditionedreexes in response to a particularstimulus.

Counteracting change: responding

A response is a reaction in anorganism or its tissues, as a result

of receiving a stimulus. It is carriedout by structures in the body knownas effector organsthese are oftenmuscles and/or glands. The responsereaches the effectors from the CNSand causes the body to correct anydeviation from the normal balancedstate, thereby maintaining homeostasis.

The role of the nervous systemin thermoregulation in humansCauses of temperature changewithin the body

Heat gain within the body may arise asa result of: normal cell functioning

(metabolism ): the oxidation processof chemical respiration in cellsreleases heat energy

muscle contractions : a largeproportion of the energy needed forany muscle activity is converted intoheat during muscle functioning (thisexplains why we get hot when weexercise)

hot food and drinks heat (radiant energy) from external

sources such as the sun, radiatorsand heaters.

Heat loss from the body results from: radiation of heat from the body to

cooler surroundings convection : air currents (wind)

remove warm air surrounding thebody and replace it with cool air

evaporation (for example sweating): when liquid droplets on the body

surface evaporate, heat is requiredto change them from liquid(droplets) to gas (water vapour).

We are familiar with the fact that vaporisation requires heatforexample, a kettle heats water andturns it to steam. In temperatureregulation, heat from an organismsbody is used for evaporation,cooling the internal environment ofthe body down in the process.

(See Fig. 1.11.)



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Detecting change

Thermoreceptors are presentboth outside and inside the body.Peripheral receptors are located inthe skin and central thermoreceptors monitor the temperature of the blood

as it circulates throughout the brain. The central receptors are present inthe hypothalamus of the brain (seeFig. 1.11) and are sensitive to extremelysmall temperature changes (a fractionof a degree).

Figure 1.10 Humansare able to maintain arelatively constant body

temperature despite uctuations in theexternal environment

(a) (b)

Figure 1.11 Flowchart showing theregulation of body

temperature inhumansskin blood vessels dilate; bloodcarries heat to the skin surface

body temperaturedecreases: hypothalamusshuts off cooling

mechanisms

STIMULUS: increased bodytemperature (e.g. whenexercising or in hotsurroundings)

HOMEOSTASISbody temperature

sweat glands activated,increasing evaporativecooling

STIMULUS: decreasedbody temperature (e.g.due to cold surroundings)

body temperature increases:hypothalamus shuts offwarming mechanisms

skin blood vessels constrict,keeps control centre warm andreduces heat loss from skinsurface

skeletal musclesactivated; shiveringgenerates heat

hypothalamus control centredetects change and activateswarming mechanisms

or begin here

begin here

high

low

in hypothalamus control centre detects

change and activates cooling mechanisms



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Co-ordinationThe hypothalamus is also the controlcentre for temperature regulation

in the mammalian body and so thereceptors do not have to transmit theinformation very far in order to elicit aresponse. The anterior hypothalamushas a heat-loss centre , which sendsmessages to effectors to cool the bodydown, and the posterior hypothalamushas a heat-gain centre , which initiatesresponses that help the body to

warm up.

Counteracting changeThe main homeostatic organ involvedin temperature regulation in humans isthe skin. Effectors that assist the bodyto cool down when it has overheated,or to warm up if it has overcooled,include the blood vessels (arterioles) inthe skin, sweat glands and hair erectormuscles in the skin, and the musclesof the body. The thyroid gland, whichaffects overall metabolic rate, is also aneffector. (See Fig. 1.11.)

Warming the body

If the body becomes too cold, the heat-gain centre of the hypothalamus stimulates responses in the effectororgans to generate and/or retain heat

within the bodyon a cold day we getgoose bumps on our skin, becomepale and shiver: Raised hairs on the body (goose

bumps) are an attempt to trapa layer of warm air around thebody to reduce the amount of heatlost by radiation, convection and

conduction. The hypothalamusstimulates the erector muscles in theskin to contract, raising the hairs.This is more effective at trappingheat where the hair is thicker, forexample on our heads (and all overon animals with thick fur).

Vasoconstrictionconstriction(narrowing) of the arterioles to theskin : people who are very coldtend to appear pale-faced, withblue-tinged lips, ngers and toesdue to poor circulation. Heat iscarried throughout the body inthe bloodstream. To prevent too

much heat being lost from thebody surface, the muscular wallsof the small blood vessels knownas arterioles constrict so that mostblood ow is redirected to the core(centre) of the body, preventing heatloss from the cooler body surface.

Shivering is brought about by rapid,small muscle contractions, whichgenerate heat in the body.

Increased metabolism : the heat-gaincentre stimulates the activity of the

thyroid gland, causing it to speed upmetabolism.(See Fig. 1.12.)

Cooling the body

If the body becomes too hot, webecome red, sweaty and sluggish, signsthat our heat-loss mechanism has beenactivated to bring about cooling of thebody. The heat-loss centre of thehypothalamus stimulates the effectororgans to lose heat:

CLASSROOM ACTIVITY

In pairs, discuss the familiar responses that you are aware of in your own bodies on a hot day orwhen you have been exercising, as opposed to your body responses on a really cold day. Try towork out how these responses bring about heating or cooling.



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Vasodilationdilation (expansion)of the arterioles to the skin : bloodcarrying heat is directed towards thesurface of the body so that heat canbe lost by conduction, convectionand radiation to the surroundings.

Sweating : Sweat glands, the mainheat-loss structures in the body, areactivated by the heat-loss centre inthe hypothalamus. Liquid sweat issecreted through the sweat poresonto the surface of the skin andheat is removed from the body toevaporate the liquid. (If you standin the sun and the heat from thesun evaporates the sweat, you willnot cool down as quickly as in theshade, where heat is being removedfrom your body for evaporation.)

Animals that do not have sweatglands still lose heat by evaporation;for example, dogs pant, and rodentsand kangaroos lick their bodies sothat the saliva evaporates and coolsthem down. A cooling process basedon evaporation occurs in plantsas wellwater evaporates fromthe leaves, removing the heat of

vaporisation from the plant in theprocess. This loss of water from theplant is known as transpiration.

Decreased metabolism : the heat-loss centre causes the thyroid glandto lower the rate of metabolism,generating less heat. This accountsfor why we feel tired and lethargicon hot days.

Figure 1.12Temperature-regulatingresponses of the skin:(a) vasoconstrictionconserves heat;(b) vasodilation bringsabout heat loss;(c) sweating bringsabout heat loss

blood vessel constricts(vasoconstriction)

heat conservation

blood vessel dilates(vasodilation)

increased heat loss

epidermis

increasedheat lossacrossepidermis

epidermis

sweat pore

evaporation

sweatduct

sweatgland

epidermis

painreceptors

hairwater

vapoursweat

droplet heat

(a) (b)

(c)

increased heat loss

SR TR

Student worksheettherole of the nervous

system in the stimulusresponse pathway for

temperature regulation



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Background informationTo maintain homeostasis, organismsmust monitor any changes in the internalenvironment and then correct the deviations.Monitoring change and then responding toit is termed feedback . The type of responsedetermines whether this feedback is positiveor negative. If the response counteracts orcancels out the change (stimulus), this is known

as negative feedback and this mechanismensures that a constant internal environment ismaintained. Temperature regulation is a typicalfeedback mechanism. Most living systems relyon negative feedback to maintain homeostasis.

If the body implements a response thatincreases (enhances) the change (stimulus),this is termed positive feedback . Positivefeedback is very unusual in living systems andoccurs only in rare and specic instances. Forexample, during childbirth the stretching of theuterus wall causes the muscles of the uterusto contract. The contractions cause the uteruswall to stretch further; this in turn increases

the contractions, eventually resulting in thebirth of the baby. Within the body, most positivefeedback systems are part of some broaderoverall mechanism that maintains homeostasis.

There are many examples of negativefeedback in everyday life, both in living systemsand in the non-living world. For example, thethermostat control of oven temperature in thekitchen or the cooling and heating of buildingsby air-conditioning units both rely on a negativefeedback mechanism. Within biological systems,examples include the regulation of temperature in the organisms, as well as maintaining theconcentration of the many chemicals present. In

mammals, chemical balance in blood includesmaintaining the glucose (blood sugar) level,the oxygen and carbon dioxide concentration,regulating pH levels and much more. Negativefeedback loops in the human body aremeticulously co-ordinated by the nervous and/ or endocrine (hormonal) systems.

TaskStudents are required to develop a modelto demonstrate the concept of a feedbackmechanism. The model should entail ageneralised representation of a negativefeedback loop and may take the form of a ow

chart, an annotated sequence of diagrams ora combination of these, or it may be an actualworking model accompanied by a writtenexplanation. This model will then be applied toexplain the negative feedback mechanism oftemperature regulation in the human body. (SeePFA H2.)1. To develop a model to show the sequence

of steps typical of a negative feedbackmechanism:(a) Gather information from a variety of

sources, looking at several negativefeedback mechanisms in both theliving and non-living world (see therecommended websites on the StudentResource CD).

(b) Present your model in a simple andconcise format that can be applied toexplain specic examples of negativefeedback loops typical of livingorganisms.

(c) Represent each of the following on yourmodel:

(i) stimuli: stimulus increases/decreases (ii) co-ordinating (control) centre (iii) effectors (iv) responses.

2. Use your model to explain how temperatureregulation in humans is a negative feedbackmechanism.

3. Answer the questions below.

Discussion questions1. Draw a ow-chart diagram of your model of

a negative feedback mechanism.2. Use the websites listed to develop a general

model for a negative feedback mechanism

and then compare your model with negativefeedback in temperature regulation inhumans.

ModelTemperatureregulation

The stimuli

The co-ordinating (control)centre

The effectors

The negative feedback loop

Model of a feedback system

gather, process and analyse information from secondary sources and use available evidence to develop a model of a feedback mechanism

SECONDARY SOURCE

INVESTIGATION

BIOLOGY SKILLS

H12.2; H12.3; H12.4

H13.1

H14.1f; H14.3

BIO

Relevant websites andquestionsnegative

feedback model

SR

TR

Answers to studentworksheet



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3. Using the model of a negative feedbackmechanism that you have developed orthe generalised one given to you by your

teacher, use different-coloured pens toannotate the model with the various stagesof temperature regulation in humans.

4. Validating your model:(a) DescribeDescribe ways in which the application

of your model to temperature control isan accurate representation of a negativefeedback mechanism.

(b) DescribeDescribe any limitations of this model fortemperature control.

5. Complete the table below by naming theeffectors and summarisingsummarising the responsesthat occur in each when body temperatureincreases in mammals.

Heat ________ centre of the hypothalamus sends nerve impulses to effector organs:

Effectors Responses

6. Complete the table below by naming theeffectors and summarisingsummarising the responsesthat occur in each when body temperaturedecreases in mammals.

Heat ________ centre of the hypothalamus sends nerve impulses to effector organs:

Effectors Responses

Temperature limits of living organisms 1.7 identify the broad range of temperatures over which life

is found compared with the narrow limits for individual species

Temperature tolerance inliving things

Temperature is one of the manylimiting factors that can determine thepresence of life on Earth. Without theselimiting factors (such as water, nutrients,light, oxygen and a balanced pH) livingorganisms cannot survive. A reductionin the accessibility of these resourcesrestricts the metabolic processes orgrowth within an organism. Chemicalreactions that occur in cells takeplace only within a relatively narrowrange of temperatures, due to thetemperature sensitivity of enzymes.For example, tissue temperaturesgreater than 42C are lethal to mostorganisms, as important enzymes beginto denature at this temperaturethe

weak hydrogen bonds in enzymesbreak and temperature increases; thechanged shape of the enzymes (andtheir distorted active sites) results in areduced ability to function and this hasadverse effects on metabolism. Extremetemperatures (above 100C) denature

not only proteins, but also nucleicacids; this destruction of DNA results in

cell death. It is therefore not surprisingthat habitats that offer temperatureconditions that are fairly stable andthose that fall within a relatively narrowrange are highly sought after and resultin much competition. Most living thingslive at temperatures between 10 and35C. Active growth in most plantsoccurs between 5 and 40C. Somespecies of plants and animals havemoved and adapted to occupy niches

where temperatures fall outside of the

optimal temperature range, expandingthe range of temperatures over whichlife can be found.

The broad range of temperaturesover which life is found

The diverse array of living organisms onEarth are found across a broad range oftemperaturesthere are living creaturesthat can survive in temperatures as lowas 70C (at the poles) or as high as56C in deserts and 350C (in hot vents



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in the sea). However, individual species cannot survive in an environment witha temperature range this large; theyneed much narrower ranges.

There is an enormous variation in temperature over the Earth. Theaverage variation in environmentaltemperature is more prominent on land(89 to 60C) compared with ocean

water (2 to 30C), although nearsubmarine hydrothermal vents oceantemperatures can exceed 350C. This

vast range of temperatures found onEarth has been benecial in allowingdiversity of niches for species. Speciesthat occupy habitats with extremeconditions (such as very hot water,ice or extreme salt conditions) aresometimes referred to as extremophiles.

The narrow limits of temperaturefor individual species

Much like enzymes, species havean optimal range of temperatures at

which they can function. For eachliving species, this is a fairly narrowtemperature range within which theycan live comfortably. The temperaturerange in which a species can surviveis termed its tolerance range fortemperature and is usually only a fewdegrees outside of the range at whichit is comfortable. There are exceptions(e.g. the Pompeii worm describedbelow), but very few organismscan survive in a broad range oftemperatures.

Tolerance ranges for individual species

Water-holding frog ( Cyclorana platycephala ) 3 to 39C

Platypus ( Ornithorhynchusanatinus ) 8 to 34C

Sydney blue gum ( Eucalyptussaligna ) 1 to 34C

Silky oak ( Grevillea robusta )found in alpine regions 0 to 38C.(The tolerance range of an organism

is the degree to which an organismcan tolerate and survive a signicant

variation in environmental factors,

including extremes such as salinity,drought and ood.)

One of the hottest environments onEarth is in the vicinity of submarinehydrothermal vents, where temperaturecan reach 350C. These extremeenvironments support a communityof creatures including microbes suchas the hyperthermophilic microbePyrolobus fumarii , which growsoptimally at 106C but can withstandtemperatures of 113C. The mostheat-tolerant animal known is thePompeii worm ( Alvinella pompejana ),discovered by French scientists in the1980s (see Fig. 1.13). These polychaeteslive in tubes on the sea oor nearhydrothermal vents and they showextraordinary tolerance to an extremely

wide range of temperaturesthey havebeen recorded living in water withthe tail end at 80C and the head endat 22C. Scientic research into howPompeii worms can withstand suchextreme temperatures seems to suggestthat they are insulated to some degreeby a eece-like covering of bacteria

on their backs. They have a symbioticrelationship with the bacteriathe

worms secrete mucus from tiny glandson their backs to feed the bacteria

Figure 1.13 Pompeii worm



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in return (see the interactive website

on the Student Resource CD). Otherorganisms living in this communityinclude vent crabs and tubeworms.

Deserts are another environment where there are extreme temperatureconditions. In some deserts, thedifference between day and nighttemperatures is very large. The Saharadesert in North Africa is the locationof the most heat-tolerant insect the Sahara desert ant ( Cataglyphisbicolor ). It can maintain its core body

temperature at approximately 56C foran extended period of time, when thesurface temperature is 70C. Australiaalso has a large number of plants andanimals that can survive the extremetemperatures associated with deserts these will be studied in the secondary-source investigations that follow.

Some organisms can withstand

the immense temperatures of res.

Australian plants such as the banksiarely on the intense temperature ofres for seed release; and bottlebrushtrees have buds in a protected positionbeneath the barkthese buds resproutafter re.

In contrast to extreme heat, freezingenvironments also provide extremeconditions. Microbes including bacteria,lichen (a symbiotic association betweenalgae and fungi) and fungi (yeasts)have been found in environments

where the temperature range is 17Cto 20C. Some multicellular organisms,such as the Arctic fox, can withstandeven colder temperatures such as

70C, having adaptations such ascountercurrent exchange and shuntingblood vessels within their limbs. Polarbears can survive temperatures as coldas 50C.

STUDENT ACTIVITY

Source an image of hydrothermal vents and/or the organisms that can be found living in theirvicinity.

The term hypothermophilic means extremely heat loving and is derived from Greek. Try to matchthe English meaning with its Greek word roots.

Figure 1.14Animals that live in

temperature extremes:(a) arctic fox; (b) camel

Student activitytemperature and living

things

SR

(a) (b)

Module 1 Chapter 1.indd 23l i 22/5/08 8:38:41 AM


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The terms ectotherms and endothermsrelate to the ability of an animal toregulate its body temperature. ( Therm relates to temperature; ecto meansoutside and endo means within.)Ectothermic organisms depend on anexternal sourcethe environment forheat energy. Fish, amphibians, reptilesand most invertebrates fall into thiscategory. Endotherms rely on internalsources such as metabolic activity forheat energy. Birds and mammals are allendothermic.

The ambient temperature is thetemperature of the environment the air or water in the immediatesurroundings of an animal.

Ectothermic organismsUnder laboratory conditions, the bodytemperature of ectotherms tends touctuate (rise and fall) over a widerrange of temperaturesit is inuencedby the ambient temperature and theorganism has only a limited ability tocontrol its body temperature. In nature,these organisms adapt their behaviour to regulate their body temperature andso if it is measured in the wild (usinga radio telemetry device), their body

temperature does not show as wide arange of uctuations.

The eastern brown snake(Pseudonaja textilis ) is found in hot,dry areas of Australia, along the easternseaboard. Brown snakes are foundacross most of Australia, inhabiting arange of habitats from open grasslandsto desert scrub, but not in rainforestareas.

Brown snakes are usually diurnal(awake during the day), but may

become active at night if the daytimetemperature is too hot. If the ambienttemperature rises beyond the brownsnakes tolerance level, it will seekshelter in the shade during the dayand become active in the later part ofthe day when it is cooler, or even atnight. If the ambient temperature drops below the optimum, snakes bask in thesunlight to gain additional heat. In verycool weather, the snake becomes lessactive, slowing down its metabolismand using fat reserves. If the coldperiod is prolonged (e.g. in winter), thesnake will hibernate in a sheltered spot.

The central netted dragon(Ctenophorus nuchalis ) is an Australiandesert-adapted lizard that inhabitscentral Australias plains and openscrub. It is able to withstand variationsin body temperature from 13 to 44C.In low ambient temperatures the dragon

will lie in the sunlight and alter itsbody position to expose more of itsbody surface area to the suns rays,increasing its core body temperature.

Temperature regulation in ectothermic andendothermic organismsTemendo1.8 compare responses of named Australian ectothermic

and endothermic organisms to changes in the ambient temperature and explain how these responses assist temperature regulation

Figure 1.15 Central netted dragon



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south-eastern Australia. It has shortlegs, a round body and small ears

with limited circulation, which assist

in minimising heat loss. In prolongedcold during the winter months, theyhibernate and go into a state oftorporthe pygmy possums curl intoa ball, drawing all appendages (legs,nose, ears and tail) in towards the bodyto reduce the surface area exposed tothe cold. They also use a burrow toshelter from the cold in shorter periodsof low ambient temperature.

To avoid overheating, mountainpygmy possums are nocturnalmarsupialsduring the day they shelterin rock crevices and this behaviourallows them to avoid exposure toexcessive temperatures (and predators)and to keep their metabolic rate lowduring the heat of the day.

Figure 1.16Fairy penguins

Background information What is an adaptation?Have you ever experienced what it is like to spend winter outdoors in the freezing cold ofthe Snowy Mountains, or summer in the hot, drydesert regions of central Australia? Most of usare not very comfortable at these temperatures,yet indigenous Australian ora and fauna livethere year after year. These organisms areable to do so because they are well suitedto their unusual environments, as a result of

evolutionary change by natural selectionthatis, the process of adaptation . An adaptationis a characteristic that increases the survivaland reproductive chances of an organism in itsenvironment.

Note: An adaptation is not a change that anorganism makes in response to the environment,to help it survive. Adaptations usually begin asvariations that arise randomly in individuals andhave a genetic basis (i.e. they can be inherited).Natural selection acts upon these variations,so that those that suit the organism to itsenvironment are passed on within a populationsurvival of the ttest. (The genetic basis of

adaptation will be dealt with in more detail whenyou cover evolution and genetics in Module 2.)

Adaptations can be divided into three majorgroups: behavioural (the way an organismacts), structural (the physical characteristicsof the organism) or physiological (the way theorganisms body functions). Organisms willshow a combination of adaptations to deal withtemperature regulation.

Behavioural adaptationsBehavioural adaptations are displayed by

both ectotherms and endotherms. The mainbehavioural adaptation seen in animals is thatthey alter the position of the body and increaseor decrease the amount of exposure of theirsurface area to the sunlight. Many organisms willseek shade or shelter in burrows if the ambienttemperate exceeds their tolerance level. Frill-necked lizards ( Chlamydosaurus kingii ) baskin the sun until they reach an adequate corebody temperature and will then retreat into theshade. During the hottest part of the day thered kangaroo ( Macropus rufus ) will seek shadeand sit in a position where its hind legs and tailare shaded by the rest of the bodythey are

Adaptations and responses of Australian organismsfor temperature regulation

analyse information from secondary sources to describe adaptations and responses that have occurred in Australian organisms to assist temperature regulation

SECONDARY SOURCEINVESTIGATION

BIOLOGY SKILLS

H12.3; H12.4

H13.1

H14.1

V

Student activityadaptation and

responses to change

SR



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positioned at right angles to the body, with thetail pointing forward, to reduce the large surfacearea exposed to sun. The water-holding frog

(Cyclorana platycephala ) retires to a burrowin extreme temperature conditions. It surviveshot, dry conditions by living in burrows belowthe surface. In extremely arid conditions, it liveswithin a cocoon made from secreted mucus andits cast-off skin, which is shed after rain andthen dries out, forming a waterproof covering.This minimises exposure to heat as well asreducing water loss and dehydration.

Nocturnal activity is another commonbehavioural adaptation seen in animals thatlive in habitats where the daytime temperatureis very hot. Nocturnal animals remain relativelyinactive during the heat of the day, so that theydo not generate additional metabolic body heatas a result of increased activity. (Increasedactivity must be supported by greater energyproduction, which relies on a higher metabolicrate.) Nocturnal activity is seen in many reptilesand birds that inhabit hot, arid areas and thefew mammals that are able to survive desertconditions (for example, the bilby, Macrotislagotis ). Some organisms like the commonwombat ( Vombatus ursinus ) and the brownsnake are diurnal, but change their normalactive periods from daytime to night during hotweather.

Migration is another behavioural adaptationthat can assist in the regulation of bodytemperature. Migrating organisms physicallymove to a different habitat that is within theirtolerance range. The grey plover ( Pluvialissquatarola ) breeds in the Northern Hemispherebetween May and August and then migratesto Australia over August and stays until April.This migration allows the birds to avoid severe

weather during winter. (See the StudentResource CD for additional information.)

As these migratory waterbirds inhabit many

countries, there is a need for international co-operation to recognise and to conserve thesespecies. Over the past 30 years, this has comeabout through international conventions onmigratory species, and bilateral agreementswith Japan, China and more recently theRepublic of Korea have assisted withconservation of the species and their habitats.The ight path, East AsianAustralian Flyway,launched in 2006, has also been acknowledgedas one of eight major waterbird yways, whichcover 22 countries.

Structural adaptations

Structural adaptations that assist withtemperature control include insulation such asfur, hair, feathers, insect scales and coats thatenable a layer of air to be trapped to reducethe amount of heat lost. The feathers of theemu ( Dromaius novaehollandiae ) act as aninsulator to reduce heat gain or loss. Blubberis another form of insulation to reduce heatloss from organisms living in water, such asthe Australian fur seal ( Arctocephalus pusillusdoriferus ). This signicantly minimises heatloss.

The surface area to volume ratio is also animportant structural component of temperatureregulation, as larger animals have a smallersurface area to volume ratio, which means theywill not lose as much heat as smaller animals.Larger animals such as the common wombat(Vombatus ursinus ) have large, compact bodiesthat have relatively small surface areas fromwhich they can lose their internally producedheat; therefore the wombat loses very little heat Figure 1.17 (a) Red

kangaroos lying in ashaded position;(b) water-holding frog(a) (b)



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to its surroundings, which is mostly helpful inthe cooler months.

Colouration of animals also assiststemperature regulation, since dark coloursabsorb light (and associated heat) and so theseanimals can tolerate colder temperatures (e.g.the diamond-backed python, Morelia spilota ).

Physiological adaptationsPhysiological adaptations focus on theinner body functions. Metabolic activity isimportant for the functioning and the survivalof individuals, but this activity also generatesheat within the body. The rate of this activitycan be altered to ensure that an individual hasa better chance of surviving conditions belowor above their tolerance range for temperature.Hibernation and torpor are examples whereorganisms lower their metabolic rate toconserve energy and, as a result, reduce theamount of metabolic heat energy that theygenerate within their own bodies. Anotheradvantage of hibernation and torpor is thatthe organism requires very little food in thisstate because it does not need to expend largeamounts of energy trying to regulate its bodytemperature by other means (e.g. shivering orsweating).

Hibernation is an extended period ofinactivity in response to cold, where the bodytemperature does not drop below 30C, butthe heart rate and oxygen consumption dropconsiderably. (Oxygen consumption is a goodindicator of metabolic activity involved ingenerating energy.) Hibernation is a form ofmild torpor and is less intense, but may last fora longer period of time.

A state of torpor is a short-term hibernationwhere the body temperature drops much lower(below 30C) and metabolism, heart rate andrespiratory rate decrease, accompanied by areduced response to external stimuli. Torpor

may be part of a daily cycle of temperaturechange and, because the body temperaturedrops to almost the same temperature as the

air around it, brings with it the advantage of aslower metabolism, in addition to helping themto conserve energy, which is in short supply asthey do not eat and drink in this state.

In contrast, the mountain pygmy possumhibernates during cold winters to reduce theamount of energy required to keep its bodywarm.

The common wombat ( Vombatus ursinus )slows its metabolism down to a third of itsnormal metabolic rate on hot days, particularlywhen sheltering in its burrow. This is a usefulstrategy, as wombats do not have sweat glandsto assist in heat loss.

Organisms can also regulate the bloodow to increase or decrease the amount ofheat lost to the surroundings. Since bloodcarries heat and usually the body temperatureof an organism is higher than that of itssurroundings, vasodilation of capillaries nearthe skin surface increases the amount of heatreleased. This mechanism is used in the redkangaroo (along with a behavioural adaptationof licking the forearm to increase heat loss asthe saliva evaporates). Blood ow can also beincreased or decreased at extremities to controltemperature. The bilby ( Macrotis lagotis ) hasan extensive network of capillaries throughoutthe ear which aid in releasing heat to itssurroundings. Furthermore, a mechanism calledcountercurrent exchange allows the warm bloodin arteries (owing from the heart towards theextremities) to heat the cooler blood in the veinscoming back from the cold extremities, beforethis blood is returned to the hear t. This occurs inthe feet of platypus ( Ornithorhynchus anatinus )as well as the ns of the Australian fur seal, sothat the internal core temperature is not loweredby cool blood returning from limbs that have alarge surface area exposed to the cold water.

Change to colouration can occur in someorganisms in response to exposure to high orlow temperatures. As previously mentioned,

colour plays a role in temperature regulationbecause darker colouration assists in theabsorption of light to gain heat. If the colour ofan organism can change, this enables it to liveand remain active over a wider temperaturerange. For example, the male Australian alpinegrasshopper ( Kosciuscola tristis ), commonlyreferred to as the chameleon grasshopper, is adark, almost black colour at temperatures below15C (for example, during the cool parts of theday such as morning) and as it basks in the sunit becomes a paler blue colour to reect lightand avoid overheating. Its blue colouration istypically seen at temperatures above 25C.

Figure 1.18 Bilby



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In addition to this physiological changelinked to behaviour, they also show otherbehavioural methods of reducing overheating

such as seeking shade or exposing themselvesto wind. In this way the Australian alpinegrasshopper increases the amount of time thatit can be active during the day.

As is evident from the above examples,some adaptations are a combination ofstructural, behavioural and physiologicalfeatures. For example, a red kangaroo licks itspaws to cool itself down through the evaporationof water on its skin. The location of many bloodvessels near the surface of the skin in theforearms and paws is a structural adaptation;the dilation of arterioles in hot conditions todirect more blood ow through these vessels isphysiological; and the licking activity to impar tsaliva for evaporative cooling is behavioural.

Task1. Select TWO named Australian animals

that you will use for an in-depth study oftemperature regulation. One should be anectotherm and one an endotherm.

Some suggested examples are: Australian ectotherms blue-tongue

lizard, water-holding frog, brownsnake, broad-headed snake, thornydevil, Kangaroo Island tiger snake andcrocodile

Australian endotherms red kangaroo,emu, duck-billed platypus and spinifexhopping mouse.

2. Analyse information from secondarysources relating to these animals andthen answer the questions on the StudentResource CD. Read information in thetextbook (pages 2429) and on the StudentResource CD, which are secondary sources.Additional sources may be accessed,

depending on the organisms selected forstudy.

Discussion questionsSee the Student Resource CD for discussionquestions.

Adaptations and responsesof Australian organisms fortemperature regulation:

http://www.environment.gov.au/events/iydd/ pubs/fauna.pdf

Australian desert-dwelling animals and theiradaptations

http://jap.physiology.org/cgi/content/ abstract/20/6/1278Body-temperature regulation studies insome Australian Aboriginal people andinvestigating animals in extremes-polarand desert environments

Figure 1.19Australian alpinegrasshopper(Kosciuscola tristis )has blue colouring athigher temperaturesand an almostblack colour at low

temperatures

TR

Skillprocessing andanalysing information

from secondarysources

Temperature changes and responses in plants 1.9 identify some responses of plants to temperature changeChanges in temperature in the naturalenvironment of plants affect boththeir functioning and their growth.(Growth and temperature change isdealt with on the Student ResourceCD.) Maintenance of a relativelystable internal environment is just asimportant for plant metabolism as

it is for animals. Plants respond tochanges in light, water availabilityand temperature, all of which arelinked, since heat is often associated

with light (for example, the radiantenergy of sunlight) and hot areas areoften dry, compromising evaporativecoolinga plant needs to strike a ne



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balance between the risks of excess water loss during cooling versus heatbuild-up during water conservation.Low availability of water may also beassociated with very cold temperatures,since frozen water (ice and snow) isnot available for use by plants. In thischapter, we deal with responses ofplants to temperature change, and inChapter 3 we deal with adaptations ofplants to assist in water conservation,but these are closely linked.

Plant responses to hightemperaturesTemperatures above 40C may causedamage to proteins and those above75C to chlorophyll pigment within theplant. Since plants cannot move intothe shade the way animals can, plantresponses to excessive temperature aremostly structural and physiological: Evaporative cooling (transpiration) :

exposure to heat (and light)causes the stomata in plants toopen , leading to a loss of water bytranspiration (evaporation of waterfrom the stomata of leaves). Theadvantage of this water loss is thatit decreases the internal temperaturein plants by evaporative cooling.However, the plants run the risk ofdehydration due to water loss andso excessive heat in plants will causestomata to close. This poses thethreat of overheating. Plants havedeveloped adaptations to cope withthis (see Chapter 3).

Turgor responsewilting : someplants respond with changes inturgor pressure, which allowsthem to reduce the exposure oftheir surface area to the sun andits associated heat and light, forexample a wilting response. Inextreme heat, plants transpire andlose turgor in the palisade cells ofleaves; as a result the leaves wilt,reducing the surface area that isexposed to the sun. If water isavailable to the plant, this wilting is

temporary, but, if not, permanent wilting followed by death willoccur. Many exotic plants that areintroduced into Australia do nothave adaptations that are favourablefor the dry climate and so they wiltin hot temperatures. Examples arehydrangeas, peace lilies and roses(see Fig. 1.20).

Leaf orientation : to overcomethe problems of overheating andexcessive water loss, some plants,for example eucalypts, are ableto change the orientation of theirleaves so that they hang verticallydownwards in hot weather. Thisreduces the surface area that isexposed to the sun during the heatof the noonday sun. The at part ofthe leaf blade, with its large surfacearea, is exposed to the less intenserays of the early morning and lateafternoon sun, but in the middleof the day when the sun is at itshottest, the suns rays strike thethin edge near the leaf stalk of the

vertical leaves. In addition, eucalypts regulate

the times of stomata opening andclosing: during the cooler earlymorning and late afternoon, stomataare open for photosynthesis andtranspiration can also occur to keepthe internal temperature down, but

when the temperatures increase to alevel that causes water stress to theplant, the stomata will close.

Leaf fall : many trees lose their leavesduring the cold winter months, but

eucalypts are evergreen trees thatdrop some of their leaves duringthe dry season in hot climates toreduce the surface area exposed toabsorb heat. This also reduces therisk of losing too much water bytranspiration.

Reseeding and resproutingin response to extreme hightemperaturesre : in Australia,one of the extreme temperaturechanges plants have to respond to



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is caused by bushres. Plants havetwo general responses that ensuretheir survival after the retheymay resprout or release seeds.Resprouters, such as the bottlebrush, tea trees and eucalypts, haveepicormic buds underneath the barkthat are protected from damage bya re and then resprout; or theymay have lignotubers, which areunderground and sprout new growthafter the re.

Seeders release seeds intothe environment after the plant isexposed to extreme heat. Someplants (for example, banksias) haveseed pods that need to be exposedto re to release their seeds),

whereas other plants (for example,eucalypts) release their seeds fromthe top of the canopy in response tothe intense heat.

Thermogenic plants : biologists havebeen surprised to discover that thereare some owers that are able toheat up by altering their metabolicrates when the ambient temperaturedrops. An example is the bud ofthe sacred lotus, Nelumbo nucifera

(found in Asia and Australia), whichmaintains a steady temperature of32C (see the Student Resource CD).

Plant responses to coldtemperaturesPlants have several responses to coldtemperatures: Organic anti-freeze : it is often the

water between cells that freezesrst, posing the greatest risk ofdamage to plants. Plants that inhabitenvironments where the ambienttemperature is extremely cold,for example in alpine areas, havestrategies to reduce the risk of ice

forming within the cells. Someproduce organic compounds thatact as an anti-freeze substance,reducing the temperature at whichthe cytoplasm or cell sap in the

vacuole freezes. (Biologists arecurrently researching a gene in the

Antarctic hairgrass plant , which hasthe ability to inhibit the growth ofice crystals, preventing the plantfrom freezing and dying, with a viewto genetically engineering other

Figure 1.20Orientation of theleaves of a eucalypt

to the rays of thesun over a period of12 hours

small surface area ofleaves exposed to sunsrays in heat of midday

large surface area ofleaves exposed to sunsrays in cool morning

large surface area ofleaves exposed to sunsrays in cool late afternoon

sun12 noon

sun6 am

sun6 pm

6 am 6 pm12 noon



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to grow again when the warmer weather returns.

The alpine ash uses seeddormancy to allow it to withstandcolder temperatures at higheraltitudes than other species.

Vernalisation : some plants owerin response to low temperatures;for example, tulip bulbs must beexposed to between 6 weeks and3 months of intense cold before they

will ower. Australian gardenersoften mimic this effect by removingtulip bulbs from the ground in

winter and storing them in therefrigerator, before replanting themin spring, to ensure that they ower.Many responses of plants to

temperature change (such as leaffall and owering) are the result oftemperature and/or light changingthe concentration of chemical growthregulators in plants. Responding totemperature change and the regulationof internal temperatures is importantnot only for the individual plant,but also for the continuation of thespecies.

REVISION QUESTIONS

1. DescribeDescribe the importance of homeostasis in living organisms.

2. DescribeDescribe the role of receptors in homeostasis.

3. ExplainExplain, using an example, what is meant by a negative feedback mechanism and its importancein living systems.

4. ExplainExplain the relationship between metabolic rate and temperature regulation in birds and mammals.

5. DescribeDescribe the advantage to ectotherms of allowing their body temperature to uctuate with theambient temperature, especially at low temperatures.

6. Draw a graph to illustrate the differences in body temperatures recorded in an ectothermic reptileand an endothermic mammal who are subjected to environmental temperatures that increasesteadily (in 10C increments) over a period of time from 10C to 40C. What is the optimumtemperature range for an endotherm?

7. IdentifyIdentify whether each of the following is a structural, behavioural or physiological response oradaptation to assist in heat gain or heat loss and explainexplain how it assists temperature regulation inliving organisms. Give an example of an animal that exhibits each. (Answer in the form of a table.)

Additional informationon plant responses totemperature changes

SR

SR TR

Answers to revisionquestions

Type of response oradaptation

Example of animal inwhich it occurs Explanation

(a) Animal curls in a ball,limbs drawn in

(b) Large, thin ears

(c) Burrowing

(d) Basking in the sun

(e) Shivering

(f) Panting

(g) Red face

(h) Lips and nose appear blue

(i) Thick fur



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Both plants and animals require atransport system to distribute foodand oxygen to active cells and toremove carbon dioxide and anyother waste products that mayaccumulate.

Unicellular organisms and smallmulticellular organisms rely on theprocesses of diffusion, osmosis andactive transport of substances directlybetween the surface of the organismand the environment. However, in mostmulticellular organisms, transport ofsubstances in this way is not adequate,due to their large surface area:volumeratio. The distance that substances

must move between the centre of thebody of a large organism and its outersurface is too large to rely simply ondiffusion, osmosis and active transport.Therefore specialised transport systems have developed in complex plants andanimals to carry substances.

The common features of a transportsystem are:1. a suitable transport medium (uid)2. the presence of vessels in which

substances can be carried

3. a driving mechanism to ensurethat substances move in the correctdirection.Plants produce their own food in

leaves and this food must be carried,in a dissolved form, to all parts of theplant. Chemical substances that areneeded for photosynthesis (such as

water and dissolved salts) must becarried from the roots, where they enterthe plant, to the leaves where they willbe used (see Fig. 2.23 on page 65).The transport tissue in plants is knownas the vascular tissue and consists ofxylem and phloem. (The term vascularmeans composed of vessels .)

In animals, transport of chemicalsoccurs in a uid medium (such asblood) and the same uid circulates around the body. The role of thetransport system is to pick up nutrients(such as digested foods and oxygen)and distribute them to parts of the body

where they are needed, as well as toremove wastes (such as carbon dioxideand/or nitrogenous waste products)from the cells and carry the wastes toexcretory organs where they can be

Plants and animals transport dissolved nutrients and gases in a uid medium

Transportdissolved nutrients and gases

CHAPTER 2

Table 2.1 Transport systems in plants and animals

VesselsTransport medium(uid) Driving mechanism

Plants Phloem Dissolved sugars(organic nutrients)

Pressure ow

Xylem Water and dissolvedinorganic salts

Transpiration stream

Animals(mammals)

Arteries, capillariesand veins

Blood Pumping heart (and muscle contraction)



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TRANSPORTDISSOLVED NUTRIENTS AND GASES

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removed from the body. In mammals,the transport system is known as thecardiovascular system , made up of

a pump (the heart) to move the bloodin the correct direction and a series ofvessels (see Fig. 2.20 on page 57).

Blood as a medium of transport 2.1Blood is a uid transport medium thatows through the heart and blood

vessels of the transport (cardiovascular)system in all vertebrates and someinvertebrates. It is a complex uid

which consists of blood plasma andblood cells (see Fig. 2.1

1. bio- maintaining a balance

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