Maintaining a Balance ANSWERS

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Copyright © 2008 McGraw-Hill Australia. Permission is granted to reproduce for classroom use.

MAINTAINING A BALANCE

CHAPTER

1

Answers to end of chapter revision questions

1. Describe the importance of homeostasis in living organisms.Answer: Living organisms are made of cells, which must function efficiently to

maintain life. Cells are extremely sensitive to changes in their environment. They

function properly only within relatively narrow ranges of pH and temperature,

they require particular concentrations of nutrients such as glucose and oxygen

and they can tolerate very little build-up in levels of waste products. If a change

in the external environment occurs, this must not affect the balance in the

internal environment of the organism and so a mechanism is needed to ensure

homeostasis—that the internal environment is maintained, despite fluctuations in

the external environment. The mechanism that allows this to occur is a negative

feedback mechanism, co-ordinated by the nervous system.

2. DescribeDescribe the role of receptors in homeostasis.Answer: Receptors play an essential role in homeostasis by receiving information

from the environment and passing it on to the central nervous system (the brain or

spinal cord) in order to trigger an appropriate response. The response is often one

which counteracts the change in stimulus and thereby maintains a stable internal

environment by means of a negative feedback mechanism.

3. ExplainExplain, using an example, what is meant by a negative feedback mechanism andits importance in living systems.

Answer: To minimise changes in the cellular environment, homeostasis is maintained

by a negative feedback mechanism—where a change in the environment is

counteracted by a response that returns the body to the state of homeostasis. It

is termed negative because it reverses the disturbance to the body’s condition.For example, if there is an increase in a variable (such as temperature) beyond

its accepted narrow range for humans, the person’s internal environment begins

to overheat. This change is detected by thermoreceptors in the hypothalamus and

a message is sent via the nervous system to effector organs, which then produce a

response to counteract the change—it decreases the variable (temperature change).

Similarly, a decrease in environmental temperature is counteracted by a response

that brings about heating, to stabilise the system within the normal temperature

range.

A negative feedback mechanism is therefore essential for homeostasis, allowing

the body to constantly monitor itself and to correct any deviation from the stable

state.

Temperature regulation

Please note that the following answers are sample answers only. There may be many alternative answers to the same question that are also correct. These are examples of correct answers.


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

Answer: It is well known that birds and mammals regulate their body temperatures

by increasing or decreasing their metabolic rate (and therefore their rate of heat

production), despite fluctuations in ambient temperature. At high temperatures,

metabolic heat production is reduced and evaporative cooling such as sweating or

panting is initiated. As the environment gets colder, they increase their metabolic

rate and as a result metabolic heat production is increased. This may involve

shivering, but some mammals have brown adipose tissue, which burns up energy

rapidly to release heat.

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

Answer: The main advantage is that the animal will not need to use valuable

energy to try to maintain a higher body temperature (thermoregulation uses a

large amount of energy, particularly when the internal body temperature needs to

be much higher than the ambient temperature). In addition, metabolic rate slows

down at low temperatures and so the animal will use up less energy and therefore

requires less food. Since food is often scarce in winter, this is of further advantage.

6. Draw a graph to illustrate the differences in body temperatures recorded inan ectothermic reptile and an endothermic mammal who are subjected toenvironmental temperatures that increase steadily (in 10°C increments) over aperiod of time from 10°C to 40°C. What is the optimum temperature range foran endotherm?

Answer:

0 10 20

endotherm

30 40 50

B o d y t e m p e r a t u r e ( ° C )

Environmental temperature (°C)

10

20

30

40

50

ectotherm


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7. IdentifyIdentify whether each of the following is a structural, behavioural or physiologicalresponse or adaptation to assist in heat gain or heat loss and explainexplain how it assiststemperature regulation in living organisms. Give an example of an animal thatexhibits each. (Answer in the form of a table.)

Answer:

Type of response

or adaptation

Example

of animal

in which it

occurs Explanation

(a) Animal curls in a

ball, limbs drawn in

Behavioural

(heat gain)

Mountain

pygmy possum

Reduces the surface of the body

exposed to cold and so reduces

the rate of heat loss by radiation

(b) Large, thin ears Structural

(heat loss)

Bilby Large surface area to radiate heat

into the surrounding air

(c) Burrowing Behavioural

(heat loss or gain)

Brown snake;

fairy penguin

Avoids extremes in temperature by

reducing exposure of body to heator cold

(d) Basking in the sun Behavioural

(heat gain)

Frill-necked

lizard

Warms the body by exposing its

surface to heat

(e) Shivering Physiological

(heat gain)

Birds Movement of muscles increases

heat production by the body

(f) Panting Physiological

(heat loss)

Dogs Evaporation of saliva from the

tongue removes heat from the

blood to cool the body (evaporative

cooling)

(g) Red face Physiological

(heat loss)

Humans Peripheral blood vessels dilate to

increase blood supply (carrying

heat) to the body surface so thatheat can radiate out

(h) Lips and nose

appear blue

Physiological

(heat gain)

Humans Vasoconstriction of peripheral

vessels to reduce blood supply to

the extremities and retain heat in

the core of the body

(i) Thick fur Structural

(heat gain)

Polar bear To insulate the body and retain

heat (prevent heat loss from body

surface by radiation, convection or

conduction)

CHAPTER

2Transport: dissolved nutrients and gases

1. CompareCompare the role of haemoglobin in transporting oxygen and carbon dioxide inthe blood.

Answer: Haemoglobin readily binds with oxygen when the blood oxygen

concentration is low (for example, in capillaries carrying blood from the heart

to the lungs) and it combines easily with carbon dioxide when its levels in the

blood are lower than the carbon dioxide concentration of the surrounding tissue

(for example, in actively respiring cells). The oxygen molecule combines with

the iron part of the Hb molecule, whereas the carbon dioxide combines with


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an amino group on the protein (globin) part of the Hb. The presence of oxygen

increases the affinity of Hb to pick up more oxygen atoms. Once one oxygen

atom has combined with haemoglobin, it has an increased affinity for oxygen. This

continues with each subsequent atom that combines, giving it a greater affinity

for oxygen. Carbon dioxide lowers the affinity of Hb with oxygen, causing it to

release oxygen.

2. ExplainExplain the adaptive advantage of haemoglobin in terms of its being pH sensitive.Answer: When the pH drops, haemoglobin changes shape and tends to lose its

affinity for oxygen. This has an adaptive advantage because, in the body, blood

with lower pH contains carbon dioxide and this is exactly where oxygen is most

needed. Carbon dioxide combines with water to form carbonic acid, which powers

the pH. This changes the shape of the Hb and so it drops off oxygen at sites in

the body which have a high carbon dioxide (and low oxygen) concentration. If

haemoglobin retained a strong affinity for oxygen at all times, it would readily

pick up oxygen, but would not release it. This adaptation in haemoglobin shape is

therefore essential for the release of oxygen at sites that are oxygen depleted.

3. In a table, identifyidentify the forms in which carbon dioxide is transported in the bloodand the proportion of each form.

Answer:

Form in which carbon dioxide is transported in blood Percentage

Dissolved in the plasma 7%

In red blood cells as carbaminohaemoglobin 23%

In plasma as hydrogen carbonate ions 70%

4. DistinguishDistinguish between the terms oxygenated and deoxygenated blood and identifyin which blood vessels in the body one would expect to find the most highlyoxygenated blood and why.

Answer: Oxygenated blood carries oxygen and is bright red in colour due to the

presence of oxyhaemoglobin, whereas deoxygenated blood carries carbon dioxide

and is a dark red in colour.

5. CompareCompare arteries, capillaries and veins in terms of the structure of their walls, thesize of the lumen and the direction of blood flow.

Answer:

Arteries Capillaries Veins

Similarity All have an inner endothelial layer lining their lumens.

Structure of walls Three layers, with thick middle

layer with large amount of

smooth muscle and elastic fi bres

One layer of endothelial cells only;

no elastic or smooth muscle

layers

Three layers, with thinner middle

layer with less smooth muscle

than arteries and very few elastic

fi bres

Size of lumen Relatively smaller than that of

a vein, but larger than that of a

capillary

Small lumen—the size of the

diameter of a red blood cell

Large lumen

Direction of blood

fl ow

From heart to tissues of the body Within tissues, from arterioles to

venules

From tissues back to the heart


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6. ExplainExplain, in terms of their functions, why: (a) the walls of arteries need to be thicker than those of veins (b) the walls of capillaries are so thin (c) veins have valves.Answer:

(a) The thicker walls of arteries are necessary to withstand the higher pressure of

blood arriving from the heart as a result of its pumping action. More smooth

muscle strengthens the wall to withstand the pressure, and more elastic fibres

enable the wall to expand with the arrival of blood and then to propel the

blood forwards as the wall recoils.

The walls of veins must have less smooth muscle and fewer elastic fibres so

that the thinner wall can be easily compressed by the surrounding muscles to

assist with the return of blood to the heart, since blood seeps into veins from

capillaries and there is no pumping force to drive the blood towards the heart.

This lack of a pumping action also means that the veins receive blood under

less pressure and therefore they do not need thick layers of strengthening(muscle) tissue or elastic tissue for recoil.

(b) Thin walls in capillaries are essential for efficient exchange of nutrients and

wastes (largely by diffusion) between blood and body cells. Thinner walls of

cells mean there is an increased surface area over which exchange of gases

(and exchange of other substances such as nutrients into cells and wastes out

of cells) can occur.

(c) Valves are essential to prevent the backflow of blood in veins, since veins do

not receive blood that is pumped to propel it forwards. Blood seeps into veins

from capillaries, and valves are essential for the continued movement of blood

in one direction only (towards the heart), especially in the arms and legs

where venous blood must flow against the force of gravity.7. OutlineOutline the advantages of the use of blood products as opposed to whole blood.Answer: The use of various products rather than whole blood promotes more

effective treatment because patients can receive only the blood components that

they require. It also increases (up to three times) the number of patients who can

benefit from each unit of whole blood donated.

8. IdentifyIdentify the main substances that need to be transported in plants and state theimportance of these substances in the plant.

Answer: The main substances that must be transported in plants are:

■ water and inorganic mineral ions, absorbed by the roots and required for

metabolism and photosynthesis

■ food (organic nutrients, especially sugars) that are produced as a result ofphotosynthesis and need to be transported to other parts of the plant.


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9. With the aid of a labelled diagram, illustrate the forces involved in lifting waterand dissolved mineral ions up the xylem.

Answer:

phloem

xylem

phloem

xylem

transverse sectionyoung root

transverse sectiondicot stem

transverse sectiondicot leaf

1 Transpiration: as the sun warms the leaves, stomataopen and water evaporates through the openings

2 Suction force: increased evaporation at the leafsurface creates a pull at the upper end of the watercolumn

3 Cohesion–adhesision and capillarity: the pullingforce is extended to the water column and

creates a force that pulls water upwards–thetranspiration stream (depends on properties of water)

4 this creates a force that pulls water into the roots byosmosis

1

2

3

4

4

1

Xylem Phloem

Substances

carried

Water

Dissolved inorganic minerals

Organic nutrients (such as sugars)

Main direction of

fl ow

Upwards, from roots to leaves Both upwards (e.g. from leaves to

fl owers and buds near the top of

the plant) and downwards (e.g. from

leaves towards the roots)

Mechanism of

transport (current

theory)

Transpiration stream—dependent

on evaporative suction pull of

transpiration, as well as adhesion,

cohesion and capillarity

Pressure fl ow—dependent on a

difference in the osmotic pressure

gradient between the source (leaves,

where sugars are loaded) and sink

(tissues where sugars are offl oaded)

Similarity The translocation in both types of tissue relies on a pressure-fl ow mechanism

which moves liquid contents from their source in the plant to where they are

needed.

10. In a table, comparecompare the translocation of materials in xylem with translocation inphloem.

Answer:


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1. In the form of a table, summarise the features in plants that minimise water lossunder the following headings:

Include the following mechanisms (in the first column): ■ features to reduce the internal temperature of plants ■ adaptations to reduce exposure of the leaves (or stomata) to the sun: —reduced exposure of stomata —reduced surface area of leaves or leaf-like structures

■

adaptations to reduce the difference in water concentration between theplant and the outside air ■ features related to water storage: —storing water —reducing water loss in fruits.Answer:

CHAPTER

3

Excretion: wastes, water and salt balance

Include the following mechanisms (in the first column):

Mechanisms that minimise

water loss Features evident in plants

Explanation of how this

conserves water

7

Mechanisms that minimise

water loss Features evident in plants Explanation of how this conserves water

Features to reduce the internal

temperature of plants

Shiny, refl ective waxy leaves

Thick, insulating cuticle

Temperature is kept lower in the plant, requiring

less water to be lost by evaporative cooling.

Adaptations to reduce the

exposure of the leaves (stomata) to the sun

■ reduced exposure of stomata

■ reduced surface area of

leaves or leaf-like structures

(and the organs that have

the highest proportion of

stomata)

■ Leaf orientation in eucalypts

■ Leaves reduced to leafl ets

■ Leaves reduced to scales (phyllodes and

cladodes)

■ Rolled leaves

■ Complete loss of leaves/fl owers

Stomata remain closed and so less transpiration

occurs.

Fewer stomata, ensuring less water loss. (This is

usually accompanied by some other modifi cation

to compensate for the lack of photosynthetic

surface area, such as cladodes and phyllodes.)

Adaptations to reduce

the difference in water

concentration between the plant

and the outside air

■ Sunken stomata

■ Hairs on leaves

■ Rolled leaves

A decreased concentration gradient results in

less water leaving the plant tissues by osmosis

and less water diffusing out of the stomata of

leaves. This is achieved by producing a micro-

climate immediately around the leaf, where

moist water can be trapped and a barrier is

created to prevent its evaporation or removal by

currents of dry air.

Features related to water

storage:

■ storing water

■ reducing water loss in fruits

Succulent plant organs (for example leaves

or stems)

Woody fruits

In areas of inconsistent rainfall water can be

stored during wet periods, for use during times

of drought.

Less water is lost from the plant when fruits

are dispersed if the fruit is woody and stores no

water (compared with fl esh fruits).


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2. ExplainExplain why it is essential to remove carbon dioxide and the nitrogenous wasteammonia from cells.

Answer: These are wastes that become toxic if they accumulate in cells. Carbon

dioxide combines with water to lower pH, and ammonia raises pH. This reduces

the metabolic efficiency of cells since all chemical reactions in cells are controlled

by enzymes and enzymes are pH-specific, only functioning efficiently at optimal

pH. They do not function at all outside a narrow range of pH.

3. IdentifyIdentify three reasons why it is essential to maintain the water concentration inliving organisms.

Answer:

■ Water provides the medium for biochemical reactions in cells.

■ Water is essential for the removal of wastes.

■ A change in water concentration would affect the osmotic balance of the cell

and this could affect pH and enzyme activity.

4. ExplainExplain why energy is required for the reabsorption of glucose and amino acidsin nephrons.

Answer: Glucose and amino acids are required by the body and should not be lost

in urine. Therefore they are reabsorbed from the nephron into the surrounding

kidney tissue and back into the blood capillaries, despite the fact that they may be

in lower concentration in the nephron than they are in the blood. Active transport

is the type of movement required to move substances against a concentration

gradient, and this type of transport utilises energy.

5. Copy a version of Figure 3.17 and complete the figure, showing the movementof water, salts, urea, drugs and hydrogen ions. Provide a key and indicate whichmovement is by means of active transport and which by means of passive

transport (distinguish between osmosis and diffusion).

glomerulus neck

proximal

arm

intermedia

segment

(loop of Henle)

distal arm

collecting

duct

glucose

divalentions

aminoacids

Figure 3.17


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Answer:

6. nalyse Analyse the information in Figure 3.18 and then use evidence from the diagramto explain the relationship between the type of nitrogenous waste produced and

the type of environment in which the organism lives.

Answer: The diagram indicates that a tortoise produces the excretory waste uric

acid in a semi-solid form. The diagram of a nephron provides evidence that the

uric acid is a waste that is excreted. Since uric acid is less toxic than other forms of

nitrogenous wastes, it requires very little water to be flushed out of the body. This

is typical of organisms that inhabit an environment where water is a limiting factor

and there is a need to conserve water within the body. Therefore we can deduce

that the tortoise lives in a dry area (an environment where water is limited).

filtrate

H2O (water)

salts (NaCI, etc)

HCO3–

(bicarbonate ions)

H+ (hydrogen ions)

urea

glucose; amino acids

some drugs

active transport

passive transport

NaCINaCI

nutrientsHCO3

– HCO3–

H2O

H2O

H2O

K +

K + H+H+ NH3

NaCI

H2O

urea

NaCI

NaCI

cortex

outer

medulla

innermedulla

1 proximal tubule

2 descending

limb of loop

of Henle

3 thick segment

of ascending

limb

3 thin segment

of ascending

limb

4 distal tubule

5 collectingduct

Figure 3.18

reptile

semi-solid uric acid


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7. In renal dialysis, blood is taken from a vein and run past a dialysate fluid,separated by a selectively permeable membrane. DescribeDescribe what would happen ifthe concentration of glucose in the dialysate was lower than the concentrationof the patient’s blood.

Answer: Glucose would diffuse out of the patient’s blood into the dialysate since

molecules move passively from a high to a low concentration. This would lead to

a shortage of glucose in the patient.

8. CompareCompare the chemical composition of blood arriving at the glomerulus with thecomposition of glomerular filtrate.

Answer:

9. Identify the hormone absent from people who suffer from Addison’s disease andexplain the main role of this hormone in kidney functioning.

Answer: These people lack the hormone, aldosterone. Its main function is to

increase the permeability of the membranes of the ascending limb of the loop of

Henle to salts. This results in an increase in salt reabsorption in the nephron and

so salt is conserved in the body, blood volume increases and blood pressure is

maintained.

10. Name one Australian insect and one Australian plant that are adapted tominimise water loss and describedescribe this adaptation in each.

Answer: The blowfly is an Australian insect that excretes uric acid, which requires

very little water to flush it out, and the blowfly is able to reabsorb a large amount

of water from its digestive tract, via rectal pads, into the kidney tubules, which lie

in close proximity to the rectal pads.

The she-oak is an Australian plant that reduces water loss by having a

decreased surface area of leaves and therefore fewer stomata exposed to the air.

The leaves of the she-oak are reduced to scales at the nodes of the stems and the

stem has taken over the photosynthetic function of leaves (which appear as thin,

needle-like branches). Stems have fewer stomata than leaves and therefore lose

less water by transpiration.

9. I enti y the hormone absent from people who suffer from Addison’s disease and

Blood arriving at the glomerulus Glomerular filtrate

Both contain a large amount of water and dissolved substances.

Plasma proteins, red blood cells and white blood

cells are present in the blood.

Plasma proteins and blood cells are too large to

fi lter through and so they are absent from thefi ltrate in a kidney that is functioning normally.

End products of digestion are present in the

blood, including amino acids.

Glucose and amino acids are present in the

fi ltrate as they are small enough to move out of

the capillary and into the Bowman’s capsule by

fi ltration.

Maintaining a Balance ANSWERS

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