Bio5.Maintaining a Balance

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keep it simple science TM

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HSC Biology Topic 1

Baulkham Hills High School SL#802445

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What is this topic about?

To keep it as simple as possible, (K.I.S.S.) this topic involves the study of:

1. THE FUNCTION OF ENZYMES & HOMEOSTASIS

2. TEMPERATURE REGULATION IN ORGANISMS

3. INTERNAL TRANSPORT SYSTEMS IN ORGANISMS

4. EXCRETION & WATER BALANCE

but first, an introduction...

Living Things are Made of Cells

All living things are composed of microscopic units called cells. You learned in a previous topic about the structure of a cell and the functions of the organelles.

Each cell is “alive” in its own right, and capable of all the life

functions:-• growth • reproduction

• movement • assimilation

• response to changes in its environment

Metabolism is Chemistry

Controlled by Enzymes

What goes on inside a living cell is mainly a matter of chemical reactions... new molecules are built, others are torn apart. Special reactions release the energy needed to make all this chemistry happen. In this topic you will learn about the importance of Enzymes... the special molecules that control the chemistry of each cell.

Homeostasis

The enzymes that control all the chemical reactions in every living cell are very sensitive to the temperature and the pH (acidity) of the surroundings. It is vital that the “internal environment” of any organism is kept as constant as possible so that the enzymes and the chemistry of each cell keep operating normally.

The process of “keeping everything the same” is called homeostasis, and is one of the most important and vital processes in every organism. In this topic you will study some of the basic mechanisms of homeostasis, and how certain body systems are involved by absorbing, transporting, regulating and excreting the vital chemicals of life.

As well as the homeostatic processes in mammals and some other animals, you will study some regulatory processes in plants.

HSC Biology Topic 1

MAINTAINING A BALANCE

N Neerrvvoouuss SSyysstteemm Regulates body temperature CCiirrccuullaattoorryy SSyysstteemm transports gases,

nutrients & wastes RReessppiirraattoorryy SSyysstteemGas exchange m

EExxccrreettoorryy SSyysstteemm Regulates water balance and excretes metabolic wastes

GENERALIZED DIAGRAM OF A LIVING CELL

“Membrane” on the outside contains the

cell , and controls what goes in or out

Organelles

Cytoplasm

jelly-like liquid fills the cell

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CONCEPT DIAGRAM (“Mind Map”) OF TOPIC

Some students find that memorizing the OUTLINE of a topic helps them learn and remember the concepts and important facts. As you proceed through the topic, come back to this page regularly to see how each bit fits the whole. At the end of the notes you will find a blank version of this “Mind Map” to practise on.

Functions & characteristics of Enzymes Shape & specificity of Enzymes Concept of Negative Feedback Effects of Temp, pH

& substrate conc. on enzyme activity

Receptor, Control Centre

Effectors Hypothalamus_{& Effector} Organs Processesses of heating & cooling Temperature range of life Ectotherms Endotherms Plants Substances carried in blood. Where from, where to? Artificial

blood? Blood products

Oxygen saturation Water Balance in Aust. insects & mammals Dialysis & HRT Excretion

Filtration & Reabsorption Homeostasis

ADH & Aldosterone Enantiostasis

Coping with salt

Importance of water & Water Balance Transpiration in Xylem Translocation in Phloem Blood & Blood Vessels

M

MAAIIN

NTTAAIIN

NIIN

NGG

AA

BBAALLAAN

NCCEE

Enzymes & Homeostasis Excretion & Water Balance Internal Transport Systems Temperature Regulation in Organisms Temperature regulation in...

How the gases are carried Importance of Haemoglobin Transport in Plants Water conservation in Aust. Plants

Kidney & Nephron Structure & Function

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Metabolism is Chemistry

Everything that happens inside a living thing is really a matter of cell chemistry... “metabolism”. For example... • In order to move, protein fibres inside muscle cells must be made to slide past each other. This is achieved by chemical reactions occurring along the muscle fibres. • For your body to grow, cells must divide and add more membranes, cytoplasm and organelles to increase the cell size. This involves the chemical construction of new DNA molecules, new phospholipids for membranes and so on. • All these chemical reactions require energy. Energy is delivered by the ATP molecule, itself the product of a series of chemical reactions in the mitochondria... cellular respiration.

All of these reactions, and more, add up to “metabolism”: the sum total of all the thousands of chemical reactions going on constantly in all the billions of cells in your body.

Enzymes

Every one of these reactions requires a catalyst... a chemical which speeds the reaction up and makes it happen, without being changed in the process.

In living cells there is a catalyst for every reaction type. Biological catalysts are called enzymes, and:

• are protein molecules

(made of folded chains of amino acids)

• have a particular 3-dimensional shape, which fits the “substrate” molecule(s) of the reaction

• are highly “substrate-specific”. This means that each enzyme will only catalyse one particular reaction, and no other.

• will only work effectively in a relatively narrow range of temperature and pH (acidity).

The Importance of Shape

Many of the properties of enzymes are related to their precise 3-dimensional shape.

The shape of the enzyme fits the “substrate” molecule(s) as closely as a key fits a lock.

This is why enzymes are “substrate-specific”... only one particular enzyme can fit each substrate molecule. Each chemical reaction requires a different enzyme.

Changes in temperature and pH (acidity) can cause the shape of the enzyme to change. If it changes its shape even slightly, it might not fit the substrate properly any more, so the reaction cannot run as quickly and efficiently. This is why enzymes are found to work best at particular “optimum” temperature and pH values.

1. THE FUNCTION OF ENZYMES & HOMEOSTASIS

Enzyme Various Different Substrate Molecules Only this one fits

EEnnzzyymmee shape at optimum pH and temperature

Shape changes slightly at different pH or temp. Substrate... ...no longer fits enzyme PPoollyymmeerriizzaattiioonn PPoollyyppeeppttiiddee cchhaaiinn PPrroodduucctt rreelleeaasseedd ffrroomm eennzzyymmee SSuubbssttrraattee

m

moolleeccuulleess aarree cchheemmiiccaallllyy aattttrraacctteedd ttoo tthhee eennzzyymmee’’ss

aaccttiivvee ssiittee

PPrrootteeiinn,, wwiitthh pprreecciissee 33-DD sshhaappee...

SSuubbssttrraattee mmoolleeccuulleess bbrroouugghhtt ttooggeetthheerr aanndd rreeaacctt wwiitthh eeaacchh ootthheerr AAmmiinnoo aacciidd mmoolleeccuulleess

TTwwiissttss && ffoollddss ...EEN_m_{moolleeccuullee}NZZYYMMEE

Enzyme’s “Active Site” has a shape to fit the substrate(s)

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The pH Scale

The acidity or alkalinity of any solution or environment is measured on a numerical scale known as “pH”.

On the pH scale, anything which is neutral (neither acid nor alkaline) has a pH = 7.

The inside environment of a cell, and most parts of an organism’s body, is always very close to pH 7... i.e. neutral. An exception is in the digestive system where conditions are usually quite strongly acidic in the stomach (approx. pH 2).

Enzyme Activity Graphs

You will have carried out experimental work to measure the “activity” of an enzyme under different conditions of temperature, pH and the concentration of the substrate chemical.

You may have measured the rate of a chemical reaction being catalysed by an enzyme, such as:

• the rate of milk clotting by rennin (junket tablets) • the rate of digestion of some starch by amylase • the rate of decomposition of hydrogen peroxide by

“catalase” enzyme.

A common way to measure the rate of a reaction is to measure the time taken for a reaction to reach completion... the shorter the time taken, the faster the reaction. This why the reciprocal of time taken (1/time) is used as the measure of rate of reaction.

The Effect of Temperature

When enzyme activity is measured over a range of temperatures, the results produce a graph as below.

Optimum Temperature

Not all enzymes will “peak” at the same temperature, or have exactly the same shape graph. In mammals, most enzymes will peak at around the animal’s normal body temperature, and often work only within a narrow range of temperatures.

An enzyme from a plant may show a much broader graph, indicating that it will work, at least partly, at a wider range of temperatures.

An enzyme from a thermophilic bacteria from a hot volcanic spring will show a totally different “peak” temperature, indicating that its metabolism will perform most efficiently at temperatures that would kill other organisms.

The graph of reaction rate (or “enzyme activity”) against temperature is usually not symmetrical. It tends to rise gradually at temperatures below the optimum, but often falls more steeply at temperatures above optimum, because the denaturation of the enzyme can lead to a rapid decline in activity. 77 66 88 55 44 33 99 1100 1111 Neutral increasing

acidity increasingalkalinity

Explanation: As temperature rises the rate increases because the molecules move faster and are more likely to collide and react. All chemical reactions show this response.

However, beyond a certain “peak” temperature, the enzyme’s intricate shape begins to be distorted. The substrate no longer fits the active site so well, and the reaction slows. If the temperature was lowered again, the enzyme shape, and reaction rate could be restored.

If the temperature reaches an extreme level, the distortion of the enzyme’s shape may result in total shut-down of the reaction. The enzyme may be permanently distorted out of shape, and its activity cannot be restored. We say the enzyme has been “denatured”.

Temperature 1/ tim e ta ke n fo r re ac tio n (r at e) 0 20 40 60 80 100 Temperature (oo_C) Re ac tio n Ra te Mammal Enzyme Plant Enzyme Thermophilic bacteria enzyme Experimental Points

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HSC Biology Topic 1

5 The Effect of pH

When the temperature is kept constant and the enzyme tested at various pH levels, the results will produce a graph as shown.

Generally, all intra-cellular enzymes (i.e. those from within a cell) will show peak activity at a pH close to neutrality... their optimum pH is close to 7.

The digestive enzyme “pepsin” from the stomach shows an optimum pH about 2 or 3, allowing it to work best in the acidic environment.

The shape of the pH graph is usually symmetrical on either side of the “peak”... optimum pH.

The explanation for the shape is as follows:

• at the optimum pH the enzyme’s 3-D shape is ideal for attracting the substrate, so reaction rate is maximum • at any pH higher or lower than optimum, the enzyme’s

shape begins to distort, and reaction rate declines as the substrate no longer fits so perfectly.

• at extremes of pH, the enzyme can be irreversibly denatured and shows no activity at all.

The Effect of Substrate Concentration

Generally in any chemical reaction occurring in solution the rate of the reaction increases if the concentration of the reacting chemical(s) is increased. The explanation is simply that if the molecules are more concentrated, then it becomes more likely that they will collide and react with each other.

When an enzyme is involved, the situation is a little more complicated:

Initially the rate of the reaction increases as the substrate concentration goes up, just as it does with any reaction. Soon though, the graph begins to flatten out and level off because the enzyme molecules are “saturated” with substrate and cannot work any faster.

If, at this point, you were to add more enzyme then the reaction rate would once again go up. It would level off again as the enzyme molecules were once again swamped and saturated with the substrate.

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2 3 4 5 6 77 8 9 10 11 12 ppHH 1/ tim e (r at e) E nz ym e Ac tiv ity En zy m e Ac tiv ity Substrate Concentration Re ac tio n Ra te Re ac tio n Ra te Substrate Concentration 1 2 3 4 5 6 77 8 9 10 11 12 ppHH Intra-cellular enzyme Pepsin. (Stomach enzyme) Extra enzyme added

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Homeostasis

Since...

• an organism’s metabolism is largely a matter of chemical reactions, and

• each reaction is catalysed by an enzyme, and • each enzyme is quite sensitive to temperature and pH ... it follows that the interior environment of the organism’s body and cells must be maintained at stable levels of temperature and pH close to the optimum for the enzymes. The process of maintaining a stable, internal environment is called “Homeostasis”.

As well as regulation of temperature and pH, homeostasis involves the regulation of many other factors such as: • water and salt balance in body fluids

• blood sugar levels

• oxygen and carbon dioxide levels. Feedback Mechanisms

The mechanism of Homeostasis involves “feedback”... a situation where the result of some action feeds back into the system to cause the next change to the system. In a “Positive Feedback” system any change re-reinforces itself by causing more change in the same direction. For example, a fire growing bigger...

Homeostasis always involves “Negative Feedback”. This is when any change in a system causes a shift in the opposite direction.

For example, a thermostat control of an oven:

The result is that the temperature of the oven remains fairly stable. It oscillates up and down a little, but always stays close to the temperature the oven was set at.

The key parts of a feedback system are:

• a receptor, to measure and monitor the conditions • a control centre, which “decides” how to respond, and • effectors, which carry out the commands of the control

centre and make the necessary adjustments to the system. In animals, it is the Nervous System which is largely responsible for carrying out the receptor and control centre functions necessary for many aspects of homeostasis. In mammals, which maintain fairly constant body temperatures, it is the Hypothalamus at the base of the brain which monitors blood temperature and sends out command messages for negative feedback, rather like the oven thermostat system.

ssmmaallll ffiirree pprroodduucceess

hheeaatt HHeeaatt iiggnniitteess_m_{moorree ffuueell}

H Heeaatt iiggnniitteess m moorree ffuueell FFiirree ggrroowwss llaarrggeerr FFiirree ggrroowwss llaarrggeerr PPrroodduucceess m moorree hheeaatt Positive Feedback always causes a system to grow out of control, or shrink away to nothing It never results in stability. Negative Feedback causes a system to maintain stability. Temperature Sensor (detector) Turn heater

OFF Turn heaterON

If temperature

is too high If temperatureis too low

Oven cools Oven heats up N EG AT IV E FE ED BA CK A CT IO N N EG AT IV E FE ED BA CK A CT IO N Cerebrum Hypothalamus Cerebellum Spinal chord Pituitary Gland

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Worksheet 1

Part A Fill in the blanks. Check your answers at the back. The sum total of all the chemical reactions in an organism’s body is called a)... Each reaction requires a catalyst, which is a chemical which b)... the reaction, without being c)... itself.

Biological catalysts are called d)... These have the following properties:

• They are molecules of e)..., which are polymers of f)... ...

• Each one has its own unique g)..., which perfectly fits the molecule(s) of the reaction. These molecules are referred to as the h)... • Because each enzyme only fits its own particular h)..., they are said to be h)... i)...

• Enzymes will only work effectively in a narrow range of j)... and k)... This is because their l)... changes so that they no longer fit their substrate.

The pH scale is a numerical measurement of m)... and n)... Things that are neutral have a pH= o)... Acids have pH values p)... 7, while alkalis (bases) have pH q)... The pH inside living cells, and in most parts of an organism’s body is about r)..., but an exception is the s)... which is quite strongly t)...

Part B Enzyme Graphs

1. Sketch the shape of a graph of Enzyme Activity against Temperature.

2. Explain the shape of the graph; a) at temperatures below the “optimum”

b) at temperatures above the optimum.

3. Sketch a graph of Enzyme activity against pH.

4. Explain why the graph shows a “peak” of optimum activity at a certain pH.

5. Why does activity decline at pH values higher or lower than the optimum?

6. Sketch a graph of enzyme activity against substrate concentration.

7. Explain

a) why the graph rises

b) why the graph levels off

Part C Fill in the blanks

Homeostasis is the process of keeping an organism’s internal environment a)... The factors that need to be maintained include b)... and c)... as well as d)... and salt balance, e)... ... levels and oxygen and carbon dioxide levels.

Homeostasis involves f)... feedback. The 3 parts of any feedback system are the g)..., which measures or monitors conditions, the h)... which decides how to respond and issues commands, and the i)... which carry out the commands. In animals generally it is the j)... system which is largely responsible for monitoring and control. In mammals, homeostasis of body temperature is controlled by the k)... at the base of the l)...

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Temperature Control in Mammals

In a healthy human the internal “core” temperature of the body is about 37o_{C and is maintained within about 0.5}o_{C at}

all times. If the body temperature goes up, or down, by more than about 4o_{C, this is a life-threatening situation.}

Control of body temperature is achieved as shown in this schematic diagram:

Main Parts of the System

Receptor and Control Centre is the Hypothalamus at the base of the brain. Special cells constantly monitor the temperature of blood flowing by. If blood temperature varies by even a fraction of a degree, nerve messages are sent to the effectors.

The Effectors include blood vessels, sweat glands, endocrine (hormone) glands, muscles and body hairs.

BODY TEMPERATURE

TOO HIGH BODY TEMPERATURETOO LOW

COOLING MECHANISMS Blood vessels dilate Sweat glands activated

Hair lowered Metabolic rate reduced

WARMING MECHANISMS Blood vessels constricted Muscles begin “shivering” Hairs erected (goose bumps)

Metabolic rate increased BODY TEMPERATURE REDUCES

BLOOD COOLS

BODY TEMPERATURE INCREASES BLOOD WARMS

Nerve Command to Effectors

Nerve Command

to Effectors

HYPOTHALAMUS monitors blood temperature

How the Effectors Make a Difference

Blood Vessels

Dilation (widening) of veins, arteries and capillaries near the skin allows more blood to flow out near the skin surface. This allows more body heat to escape from the skin, thus cooling the body.

Constriction (narrowing) of blood vessels causes less blood to flow near skin. Less heat flows out to skin to be lost. Body heat is retained more.

Muscles

Nerve signals can cause the skeletal muscles to begin “shivering”. This extra muscle activity generates more heat to warm the body.

Sweat Glands

When activated, the sweat glands secrete perspiration. The water evaporates from the skin, carrying away body heat... this has a powerful cooling effect.

Body Hairs

Each hair on your body has a tiny muscle at its base which can cause the hair to stand up erect and give you “goose bumps”. This traps a layer of still air against the skin and helps insulate and prevent heat loss.

If the hair follicle muscle is relaxed the hair lies flat and allows more heat loss.

Hormones

are chemicals which control various body functions, including the rate of metabolism and heat production.

The hormone thyroxine (produced by the thyroid gland in the neck) does exactly that and is under the control of the hypothalamus, via another hormone from the pituitary gland.

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The Temperature Range of Life

Homeostatic control of body temperature allows an organism to maintain its cells at a temperature close to the optimum for its enzymes. This allows metabolism to run efficiently, despite changes in the surrounding temperature of the environment.

However, homeostasis has its limits, and no organism can remain active and thriving under the full range of temperatures of the biosphere of the Earth. Different organisms have adapted to survive in extreme cold environments, or in extremely hot conditions, but never both extremes.

Extreme Heat

There are thermophilic bacteria (members of the Archaea) which live and thrive in volcanic hot springs at temperatures up to 120o_C.

In terrestrial environments such as hot deserts, the temperature can often reach 40o_{C and sometimes as high}

as 60o_{C. Many plants and animals are adapted to survive}

these extremes, but few remain active in this heat. Generally in deserts the animals seek shelter and become inactive, while plants shut down their metabolism and merely survive.

Extreme Cold

Once again, there are many organisms which can survive extreme cold, but few that remain active. Certain types of algae and photosynthetic bacteria are found to live within the snow and ice near the poles and are still metabolically active at temperatures as low as -10o_{C. Below this, the cells}

become inactive, but survive and re-activate when it warms up again.

Generally however, plants and animals cannot tolerate their body temperature going below 0o_{C, since ice crystals forming}

in cell cytoplasm can destroy membranes and kill cells. Also, the chemical reactions of metabolism run so slowly at low temperature, that life functions are not possible.

Of course, many animals do live and survive in the cold because they can produce their own body heat (mammals and birds) and are equipped with body insulation and homeostatic mechanisms to maintain their core temperature despite the cold environment. Perhaps the world champions in this regard are the Emperor Penguins which maintain core body temperatures around +33o_C

throughout the Antarctic winter in air temperatures as low as -50o_{C... an amazing difference of over 80}o_C!

Cold Water Environments

Even when ice forms on the surface, water environments rarely fall below +4o_{C, and are remarkably stable in}

temperature. Life-forms do not need to cope with change, although they may need serious insulation to stay warm. It is the terrestrial environment that is more of a challenge.

Temperature Control in Ectotherms

Ectotherms are the “cold-blooded” animals, such as reptiles, amphibians, insects, fish and worms. “Cold-blooded” is a misleading term and is best avoided, since these animals are NOT always cold, but rather they rely on the outside environment for their body heat... they do not generate heat internally like a mammal or bird.

Ectotherms have a variety of adaptations, many of them behavioural, to regulate their body temperature and keep it within the range in which they can be active; generally between 10-30o_C.

For example, the Blue-Tongue Lizard will lie in a sunny spot with its body flattened and turned side-on to the Sun on a cool morning. This way it absorbs heat more quickly to get its body temperature high enough to become active. As the day becomes hotter, the lizard will turn facing the Sun to absorb less heat, and seek shade to avoid over-heating.

In prolonged periods of cold weather, such as winter in the Australian Alps, ectotherms cannot be active because the environment cannot supply them with the body heat they need. Animals such as the Copperhead Snake and the Corroboree Frog seek shelter underground and become dormant throughout the winter.

In a process similar to the hibernation of bears, the animal’s heartbeat and breathing slow down, their metabolism almost stops and their body temperature chills to only just above freezing. As long as they are more than about 50 centimetres underground, the ground will not freeze even though buried in snow for several months. If they haven’t burrowed deeply enough they will freeze to death!

Ectotherms seek, or avoid the heat of the Sun

Reptiles sun-bake when too cool...

... and seek shelter when too hot

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Temperature Control in Endotherms

Endotherms are the animals which produce their own internal body heat and maintain a relatively constant body temperature... the birds and mammals.

All endotherms rely heavily on having bodily insulation... fur, feathers or blubber (fat). Humans are endotherms too, but we rely mostly on our technology to provide heaters, air-con, jackets, wetsuits, gloves, etc, to protect our fragile bodies from extreme temperatures. What do other endothermic animals in the wild do?

Firstly, they have all the responses for homeostasis described earlier... dilation or constriction of blood vessels, shivering and sweating etc. As well as these, they may have extra adaptations to help regulate their temperature. In hot environments such as the Australian deserts, many mammals such as the Red Kangaroo or the Bilby, have many adaptations to help them cool their bodies:

• large ears, with good blood supply, increases the surface area for heat loss

• like the reptiles, they seek shade in the heat of the day • panting evaporates water from the mouth and throat, and

cools the oral membranes which have a rich blood supply. • they may lick their forearms. The evaporation of saliva

cools their body in the same way as sweating.

(Note: many desert animals lack sweat glands because they cannot afford the water loss of perspiration.)

In the cold, endotherms go for thick fur coats (Wallaroo) or layers of fat (Australian Fur Seal) to limit the loss of body heat.

Penguins, such as the Fairy Penguins along Australia’s southern coast, have a special “blood shunt” in their legs. In warm conditions the shunt is closed and blood flows normally to the feet. Since the feet are about the only part of their body not well insulated, in cold water they could lose a lot of body heat.

So in cold water the flow of blood from body toward the feet is “shunted” via a special vein with a valve in it, back into the body. The feet receive virtually no blood, so conserving body heat.

Responses of Plants to Temperature Change

Plants cannot respond to temperature change by moving away or hiding. To cope with temperature extremes they must have structural or physiological adaptations.

To cope with seasonal cold weather, many plants (especially in the northern hemisphere) are deciduous... they shed their leaves and basically shut down their metabolism for the winter, rather like an animal hibernating. Their leaves cannot be protected from freezing, so the strategy is to lose the vulnerable parts, survive until next spring, and grow new leaves then.

Coping with heat is another story. If there is plenty of water available, such as in a tropical jungle, then the plants cool themselves by allowing maximum evaporative cooling. The leaves open their stomates and allow transpiration to occur. The evaporation has a cooling effect, in the same way that sweating cools an animal.

When it is hot and DRY as well, they have a problem. Desert plants tend to have very small leaves and thick, “stocky” shaped stems. This reduces the surface area being hit by heat radiation from the Sun, and helps prevent over-heating. The cacti plant group have taken the strategy to the limit... their leaves are spines, and stems are “fat” and rounded. They are also light coloured to reflect a lot of the radiant heat away.

The sclerophyll plants of Australia (gum trees for example) also have small narrow leaves to reduce heat absorption from the Sun. Their other “trick” is to allow the leaves to droop downward. This allows them to catch light for photosynthesis in the cooler mornings when the Sun is low, but avoid absorbing heat when the Sun is overhead in the heat of midday.

In the desert, big ears are cool!

Spikes for leaves = lower surface

area

Pale colour reflects

radiation Low surfacearea stem

Narrow,

drooping

gum tree

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Worksheet 2

Fill in the blanks

Check your answers at the back.

Temperature regulation in mammals is

controlled by the a)...

at the base of the brain. If body temperature is

too high it sends commands to the

b)... organs to cool the body.

Cooling mechanisms include c)...

of blood vessels to allow d)...

(more/less) blood to flow near skin. Also, the

e)... glands may be activated,

allowing f)... to flow. As it

g)... from the skin, it carries

heat away. Metabolic rate may be reduced, to

reduce heat production. This is achieved by

h)...

which are control

chemicals. An example is Thyroxine, produced

by the i)... gland.

If the body is too cool, then the hypothalamus

commands various warming mechanisms.

Blood vessels can be j)...

to reduce blood flow to k)...

Body hairs are l)... to trap a

layer of

still air,

which acts to

m)... better. Nerve commands

to muscles can cause them to

n)... which produces extra

heat. The metabolic rate can be raised by

hormones too.

Animals which rely on the environment to

supply their body heat are called

o)... Examples are

p)..., amphibians, fish etc.

In terrestrial environments they often seek or

avoid the heat of the q)... in order to

regulate temperature. An Australian example is

the r)..., which often

s)... in the morning to warm

up, and t)... when

too hot. In cold winters, ectotherms cannot get

any heat from the environment and many, such

as the u)... survive

by v)... for the

winter.

Animals which can regulate their body

temperature are called w)...

Examples are the x)... and

y)...

They use all the

homeostasis techniques above, and rely on

body insulation with fur, z)... or

aa)... as well.

In extreme environments endotherms may

have extra adaptations as well. In Australian

deserts many animals have large ab)...

to radiate heat away. They don’t have sweat

glands because they can’t afford to

ac)...

but may lick their ad)... or pant

to achieve some evaporative cooling.

In cold environments, thick fur or blubber

gives ae)...to

retain body heat. The penguins have a special

adaptation in the blood vessels to their legs. In

cold water, the blood flow to the feet is

af)...

so that less heat is lost through the uninsulated

feet.

Plants also have many adaptations to cope with

temperature extremes. In cold climates many

plants are ag)... which

means they ah)...

in winter.

In hot climates with plenty of water, plants

open their ai)...

allowing evaporation to cool them. In dry

climates, plants cannot afford the water loss, so

they have other ways to stay cool without

losing water. For example, cacti have

aj)...-shaped leaves to reduce the

surface area absorbing heat from direct

sunlight. They are often

ak)...-coloured to reflect heat radiation.

The Australian al)...

plants mostly have am)...

(shape) leaves to reduce surface area, and

often allow the leaves to

an)... (orientation) to

avoid the Sun’s heat at midday.

WHEN COMPLETED,

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Internal Transport in Mammals

As is the case with most animals, mammals rely mainly on their Circulatory System for internal transport of substances... their blood, heart and blood vessels; veins, arteries and capillaries. A basic knowledge of how the system operates was covered in Preliminary Topic 2.

Blood and Blood Vessels

You will have examined blood under a microscope and seen something like this:

You should be able to sketch diagrams of blood cells, and have an idea of their sizes.

Red Blood Cells

contain the red pigment haemoglobin, which carries oxygen. This is covered in more detail later.

White Blood Cells

come in a huge variety of types, but all are involved with defence against disease. This is covered in a later topic.

Arteries

carry blood from the heart out to the body tissues. The walls of an artery are relatively thick and muscular to withstand the high pressure in the blood when the heart pumps.

Artery walls are very elastic, and when a pulse of high pressure blood passes through, they expand outwards and then contract again, helping to push the blood along. This rhythmic expanding and contracting is what you can feel as your “pulse” wherever an artery is close to the skin, such as in your wrist or throat.

Veins

carry blood back from the body tissues to the heart. The blood here is under lower pressure and the walls of a vein are relatively thin. With little pressure to push blood forward, it is the contraction of the surrounding muscles which helps push the blood along.

Veins may contain valves to prevent back-flow of the blood.

Capillaries

are the tiny blood vessels which form a network throughout the tissues so that every living cell is close to the blood supply. The walls of a capillary are only 1 cell thick, so diffusion of substances from blood to cells (or cells to blood) is easily achieved.

The inside of a capillary is so small that red blood cells often travel through it in single file.

3. INTERNAL TRANSPORT SYSTEMS IN ORGANISMS

Sketch of Blood Cells

RReedd CCeellllss

Shaped like a donut with the

hole closed over no nucleus W Whhiittee CCeellll much larger than

red cells

large, irregular nucleus. Ratio: about 600 red

cells to 1 white cell

ARTERY

Cross-SSection Cross-SSectionVEIN

Connective Tissue

Layers of muscle

blood _blood

Side view of VEIN showing a valve.

Blood can flow one way, but not back the other.

blood flow

Wall just 1 cell thick for easy diffusion

CAPILLARY Cross-SSection RED BLOOD CELLS

LLiigghhtt mmiiccrroossccooppee vviieeww

Size = 7 μμm

EElleeccttrroonn m

miiccrroossccooppee vviieeww

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Changes to the Blood as it Circulates

As the blood circulates around the body its chemical composition undergoes a number of changes...

Nutrients & Nitrogenous Wastes

As the blood flows through capillaries surrounding the digestive system it picks up increased quantities of sugars, amino acids, salts, water, vitamins, etc that have been absorbed from the gut. (However, lipids are first absorbed into the lymphatic “drains” and enter the blood much later) This blood from the gut is collected in a vein which takes it directly to the liver. Here some of the nutrients may be absorbed from the blood for storage or chemical processing (e.g. glucose is extracted from the blood and polymerized to form glycogen and stored in the liver). Also in the liver, large amounts of the nitrogenous waste urea is added to the blood to be carried away and later excreted. Later, as blood flows through capillaries in body tissues such as muscle or bone, nutrients are absorbed from the blood into the cells which need energy (glucose) and new chemical building blocks (amino acids, lipids).

Sooner or later, every bit of blood flows through the kidneys which extract the nitrogenous wastes and excess salts and water for excretion as urine.

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Substances Carried in the Blood

Oxygen O₂

is carried in the red blood cells by haemoglobin. Carbon Dioxide CO₂

is partly carried by the haemoglobin in red blood cells, but most of it is carried in the blood plasma, in the form of bicarbonate ions (HCO₃-₎

Water

is carried as the liquid solvent of blood plasma. Salts & Products of Digestion

such as sugars and amino acids, are generally water soluble and are carried dissolved in the blood plasma.

Lipids (Fats)

absorbed from the digestive system are “packaged” in a protein coat which makes the fat molecule miscible in water. This means that, while not fully dissolved, the molecules can be dispersed in water and carried without joining together into droplets of fat and separating from the water.

In this form they are carried dispersed in the blood plasma. Nitrogenous Wastes

such as urea, are water soluble and carried dissolved in the blood plasma.

You will have carried out an experiment to see the effect of dissolved CO₂on the pH of water.

You might have chemically produced some CO₂ and bubbled it through water. Using a pH meter, or perhaps Universal Indicator, you will have measured any change in the pH of the water.

You would have found that the pH went down... i.e. the water became more acidic.

Explanation and Chemistry:

Carbon dioxide reacts with water to form carbonic acid CO₂ + H₂O H₂CO₃

Carbonic acid is a weak acid which partly ionizes

H₂CO₃ H+ _{+ HCO}

3

-Hydrogen ion

makes water more acidic Bicarbonate ion. This is how CO2is carried

in blood

CHANGES IN NUTRIENTS, WATER & WASTES AS THE BLOOD CIRCULATES

Heart W Waasstteess into blood Liver Gut Arteries Body tissues Veins N Nuuttrriieennttss from blood to cells Digested N Nuuttrriieennttss into blood Some NNuuttrriieennttss to storage W

Waasstteess and excess water, salts excreted in urine

Lungs

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HSC Biology Topic 1

14 Respiratory Gases O₂& CO₂

As blood passes through capillaries in body tissues, oxygen is released from the haemoglobin molecules and diffuses along the concentration gradient into the body cells. There is always a concentration gradient favouring this because the cells are constantly using up oxygen for cellular respiration.

Revision

Meanwhile, the concentration of carbon dioxide is high because of its constant production by cellular respiration, so it diffuses from the cells into the blood.

When the blood gets to the lungs the opposite occurs. Inside the alveoli (air sacs of the lungs) the air has a very high concentration of oxygen and is very low in CO₂. Therefore, oxygen diffuses into the blood, while carbon dioxide diffuses from the blood into the air.

This gas exchange and transport is essential for delivering oxygen to every cell for cellular respiration...

... but why must CO₂be removed?

The Need to Remove Carbon Dioxide

As already discussed, carbon dioxide doesn’t just dissolve in water, it reacts to form a weak acid.

It’s the hydrogen ions that create problems. Hydrogen ions are acids and can lower the pH of cell cytoplasm.

At the concentrations produced by a typical cell, the hydrogen ions could easily lower the pH of the cytoplasm by 0.5 pH unit or more. Remember that enzymes are very sensitive to pH changes and quickly change shape and lose their catalytic activity. This would be disastrous for cell metabolism. To avoid this problem, CO₂is carried away by the blood as rapidly as it is produced in the cells.

The Importance of Haemoglobin

Blood is red because of the many red cells, and red cells are red because they are packed with the red-coloured, iron-containing protein haemoglobin.

In the lungs, where the oxygen concentration is very high, some oxygen dissolves in the moisture lining the alveoli then diffuses into the blood flowing in the surrounding capillaries. Oxygen is not very soluble in water, however, and if that’s all there was to the story, then our blood could never carry enough oxygen to supply our cells with what they need. Haemoglobin molecules have a great attraction for oxygen molecules and quickly pick up 4 O₂ molecules each. Because of this, our blood can carry thousands of times more oxygen than would be possible by simply dissolving oxygen in the blood plasma.

When the blood gets to the body tissues with its load of oxygen, something very “clever” happens...

The high concentration of dissolved CO₂lowers the pH of the blood slightly. This causes the haemoglobin proteins to change shape slightly and release the oxygen molecules.

HbO₂₂ Hb + O₂₂

The oxygen diffuses into the cells, and the freed haemoglobin molecules can pick up some of the CO₂molecules and carry them back to the lungs.

Of course, this isn’t really “clever” in any sense of intelligence among haemoglobin molecules. It is the result of Natural Selection and Evolution... it gave a huge survival advantage to some primitive ancestor millions of years ago, so all mammals (and many others) have inherited this quite amazing substance. www.keepitsimplescience.com.au keep it simple science TM

C₆H₁₂O₆ + 6O₂ 6CO₂+ 6H₂O + ATP

Glucose and Oxygen delivered to cells by the

blood stream

Chemical wastes

the important product. ATP is the energy

supplier in cells O O2 O O2 CCOO₂ CCOO₂ CO₂ + H₂O H₂CO₃ H+ _{+ HCO} 3 -carbonic

acid hydrogen_ion bicarbonate_ion

Hb + O₂₂ HbO₂₂ abbreviation for Haemoglobin “Oxyhaemoglobin” Oxygen Air Blood Oxygen Blood Cells Carbon dioxide Blood Air Carbon dioxide Cells Blood Heart Arteries Body tissues Veins Lungs CHANGES IN

OXYGEN AND CARBON DIOXIDE AS THE BLOOD CIRCULATES

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Oxygen Saturation & Its Measurement

The concentration of O₂and CO₂in the blood is of great interest to doctors monitoring a patient, or an athlete in training, or even to a pilot or mountain-climber at high altitude. The most important measurement is “percentage oxygen saturation” (%SpO₂). A reading of 100 would mean that 100% of all haemoglobin in an artery is totally saturated with oxygen. Readings between 95-100% indicate good health, fitness and adequate oxygen supplies.

Lower readings (e.g. 80%) could indicate: • respiratory or circulatory problems in a patient • lack of fitness, or excessive exertion in an athlete • need for supplementary oxygen for a pilot or climber. In years gone by, %SpO₂ was measured by taking blood samples and carrying out complex chemical testing. With modern technology, however, the readings are done instantly and non-invasively by a small, portable instrument clipped onto the end of the finger or ear lobe.

The “Oximeter” works by sending red light and infra-red beams through the flesh. The amount of each light absorbed by the haemoglobin gives a direct measurement of %SpO₂, because haemoglobin carrying oxygen, or without, or with carbon dioxide, all absorb these light beams differently.

Products of Blood Donation

The Australian Red Cross Blood Service collects about a million blood donations per year. Most of this blood is used for people who need regular treatment with blood products for conditions such as leukemia.

Only a very small amount is kept as whole blood for emergency transfusions. Most donated blood is separated into about 20 different fractions or products, so each donation can treat many different patients.

The main blood products are:

Red Cell Concentrate which contains about twice as many red cells as normal, is used to boost the oxygen-carrying capacity of patients with anaemia or after blood loss.

Platelet Concentrate is given to patients who need extra blood-clotting capability, such as leukemia sufferers, or following severe blood loss.

White Cell Concentrate is given to patients needing a boost to their immune system, perhaps following a severe infection.

Plasma is the liquid part of the blood and is often given in emergency to boost the volume of blood following severe blood loss.

Cryoprecipitate is a fraction collected from plasma and contains blood-clotting factors. It is used to treat severe haemorrhaging.

Factor VIII and Monofix are extracts from plasma used to treat people who have haemophilia... an inherited, incurable disorder in which the blood will not clot properly. These blood products allow patients to lead a relatively normal life.

Finger-cclamp Oximeter measures %SpO₂₂

Light source sends red light and infra-red

Receiver measures absorption of light by haemoglobin

Why Is It Needed?

• Fresh blood cannot be stored for long, and many parts of the world lack the necessary storage facilities.

• Many blood products can set off immune-responses in long-term patients, even after correct blood-typing. (Similar to “rejection” of a transplanted organ)

• Donated blood can carry diseases, such as hepatitis or HIV.

Perfluorocarbon-Based Substitutes

Another area of research aims to develop a truly artificial blood substitute. The most promising base chemicals are the “perfluorocarbon” compounds.

These can carry up to 5 times more oxygen than blood can, can be stored indefinitely at room temperature. They can be made totally sterile and disease free.

At least 5 different products are being tested and trialled (USA), but none are yet approved for medical use. Haemoglobin-BBased Oxygen Carriers

are one of the areas of current research. Haemoglobin extracted from animal blood can be purified and treated so that it is disease-free and cannot cause any allergic or “rejection” responses in patients. The products can be stored for years at room temperature, and is highly effective at carrying oxygen and releasing it into the tissues.

Currently undergoing clinical trials, but not yet approved for medical use.

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Transport Systems in Plants

In Preliminary Topic 2 you were introduced to the transport systems in plants...

Xylem Tubes Carry Water

Xylem tubes are dead, hollow cells, joined end-to-end forming a continuous tube from root to leaf. The xylem tubes transport water (and dissolved minerals). How do they work to lift water from roots to leaves, against the force of gravity?

“Transpiration” is the evaporation of water from the leaves. When the stomates are open, water can constantly evaporate, creating a tension, or “pull” in the remaining water in the leaves.

Water molecules are quite strongly attracted to each other and tend to cling tightly together. This force is called “cohesion” and is the reason that water tends to form droplets... little blobs of water that cling together.

So, when water evaporates from leaves and creates a “pull” force, each water molecule pulls on those behind it because of the cohesion. Each molecule pulls others upward and so the entire column of water in a xylem tube moves upwards to replace the water lost by transpiration. So water is pulled upwards by a combination of transpiration and cohesion. This flow is called the “transpiration stream”.

Another factor which helps the process is called “capillarity” or the “capillary effect”. This is the way that water can “climb up” the walls of a container forming a meniscus in a test tube, for example. This happens because water molecules are not only attracted to each other (“cohesion”) but also to some other substances such as glass. This attraction is called “adhesion”.

In very narrow tubes (“capillaries”) the water will climb upwards against gravity because of adhesion, and drag more molecules along by cohesion. This happens in xylem and helps lift water upwards.

Higher Pressure DESTINATION Lower Pressure PHLOEM TUBES Translocation

Sugar is removed by active transport, requiring energy. Water flows out due to osmosis, lowering pressure

Sugar is carried in by active transport, requiring energy.

Water flows in due to osmosis, raising pressure

Sugar solution flows due to pressure differential Translocation... how it works Hollow, dead cells, joined end

to-end forming a tube

Cell walls re-inforced with rings and spirals of lignin PHLOEM CELL

alive and filled with cytoplasm. Circulation of cytoplasm carries sugars through each cell Sieve plate between cells “Companion cell” has many mitochondria to provide ATP to the phloem cell

sugars actively transported in the cytoplasm of the cells sugars diffuse from one cell into the next

Active & Passive Transport

Note that the flow of water in the xylem costs the plant nothing in energy to run the system... it is “passive” transport.

In contrast, the other transport system in plants is an “active transport” system... the plant must constantly supply energy to make it happen.

Phloem Tubes Carry Food Nutrients

While the xylem tubes are formed from dead cells, the phloem are living cells joined end-to-end. The ends of each cell are perforated (“sieve plates”) so each cell is open into the next so they form a continuous tube.

While the xylem is a one-way flow system, the phloem system can carry food (especially sugars) in either direction. If a lot of photosynthesis is occurring, the phloem will carry sugar to storage sites in roots or stem. If photosynthesis is not possible for an extended time, then the phloem will carry sugars back from the storage sites to feed the leaf cells, or supply a growing flower or fruit.

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Worksheet 3

Part A Fill in the blanks

Blood is made up mainly of a liquid called a)... and many blood cells. The most numerous blood cells are the b)... which contain the protein c)... responsible for carrying d)... gas. Most of the carbon dioxide in blood is carried in the form of e)... ions. These are made when carbon dioxide reacts with f)... forming g)... acid. Most other substances carried in blood are dissolved in the h)... This includes nutrients such as i)... and j)..., water and salts, and the nitrogenous waste k)...

Lipids (fats) are first wrapped in a coating of l)... so they can be dispersed without separating.

There are 3 types of blood vessels: the m)... have thick muscular walls to withstand the high n)... of the blood being pumped from the o)... p)... have thinner walls, and have q)... along their length to prevent blood r)... Capillaries have walls which are s)...thick and form a network throughout the body’s t)... As the blood flows around the intestines it picks up u)... It then flows straight to the v)..., where some nutrients are removed for w)...&..., and wastes such as x)...are added. These wastes are later removed from the blood by the y)... and excreted with any excess z)... & ... as urine.

Meanwhile, when blood flows through the capillaries of the lungs, aa)... gas is absorbed into the blood and ab)... gas is released from blood into lungs. When blood flows through the body tissues, nutrients move from ac)... to ad)... as does ae)... gas, while af)... gas moves the other way.

Part B Questions

1. Write 2 chemical equations to summarize how carbon dioxide reacts with water. In what form is CO₂ carried in blood?

2. With reference to a chemical equation, explain why it is essential to remove carbon dioxide from body tissues.

3. With reference to a chemical equation, explain how transfere of oxygen from blood to cells is facilitated.

Part C Fill in the blanks Check your answers at the back. Oxygen is carried by the a)...-coloured, b)...-containing protein called c)... It has a great affinity for oxygen molecules, and each molecule can absorb d)... (number) oxygen molecules, in which form it is called e)...-... In the body tissues, the presence of f)... gas lowers the pH slightly, which causes haemoglobin to change shape slightly and g)... the oxygen, which then h)... into the cells.

The “%SpO2” is a measure of the

i)... in a person’s blood. Good health, fitness and adequate oxygen supply are indicated by readings above j)...% This can be easily measured by a k)... which sends beams of l)... and ... through a finger or ear-lobe. Oxygen saturation is measured according to how much of each type of light is m)... by the blood.

Most blood donated to the “Blood Bank” is separated into different fractions for different uses. Some of the main blood products are:

• n)... Cell Concentrate, to boost O₂-carrying capacity. •White Cell Concentrate, to boost o)...

p)... Concentrate, to help blood clotting q)..., which is the liquid part of the

blood, used in emergency to increase

r)...

Research is going on into developing artificial blood. This is needed because fresh blood cannot be s)... for long, and can cause t)... in some patients, and there is a danger that donated blood might carry u)...

Two of the areas of research for artificial blood are v)...-Based w)... Carriers, made from animal blood, and completely artificial substitutes based on the chemicals called x)...

Part D

Transport in plants is carried out by 2 separate systems. The a)... tubes carry water and dissolved minerals from the b)... to c)... These tubes are d)... (dead or living) cells. The transport is e)... (active or passive) and the movement of water is called f)... Basically the process works because, as water g)... from the leaves, this “pulls” water up from above because water molecules are h)... and tend to cling together.

Meanwhile, the i)... vessels carry out j)... (name of process) which moves k)... around the plant to wherever it is needed. The cells are l)... (dead or living) and the transport is m)...(active/passive) requiring the plant to n)... in order to make the process happen.

W

WHHEENN CCOOMMPPLLEETTEEDD,, W

WOORRKKSSHHEEEETTSS BBEECCOOMMEE SSEECCTTIIOONN SSUUMMMMAARRIIEESS

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The Importance of Water

Life cannot exist without water. All living cells are about 75% water. The functions of water in living things include: Water is the solvent of life

All the chemical reactions of metabolism take place in water solution, and the transport of materials in cytoplasm, blood or phloem takes place mainly in water solution. Water is involved in life chemistry

Water is a reactant or product of many metabolic reactions. The reactions of photosynthesis and cellular respiration are just two of the many examples.

Water is vital in temperature regulation

Water has a very high specific heat capacity. This means it can absorb (or lose) relatively large quantities of energy with minimal temperature change. This helps stabilize the temperature of all living things.

Water also has a very high heat of vaporization. This means that when it evaporates it absorbs huge amounts of heat. This is why evaporation of perspiration from the skin has such a cooling effect.

Water supports and cushions cells and organs

Many plants and animals rely on water for body support. Non-woody plants pump their cell vacuoles full of water to make cells “tight” and keep stems and leaves upright. Animals such as worms rely on the hydraulic pressure of water in their tissues to support their body and maintain its shape.

In vertebrate animals the water solutions in the tissues helps to cushion organs against bumps and impacts. (eg cerebrospinal fluid around the brain)

Homeostasis of Water & Salts

It’s not just the water that is important, but its concentration, and the concentration of substances dissolved in it, such as salts.

If the concentrations are not kept at the correct levels, then osmosis may cause problems. Cells could lose water and dehydrate, or gain too much water and be damaged.

Kidneys Also Excrete Metabolic Wastes

What Are the Metabolic Wastes?

The many chemical reactions of metabolism sometimes produce chemicals which are toxic to cells, often because the chemical, when dissolved in water, can change the pH and reduce enzyme activity.

Therefore, it is essential that these wastes are removed (“excreted”) as soon as possible. The major wastes are: • Carbon dioxide, produced by cellular respiration.

As covered previously, it will lower the pH (acidic). It is carried in the blood and excreted by the lungs. • Nitrogenous wastes, (contain nitrogen) are produced

mainly from the metabolism of proteins.

There are 3 main compounds that can be produced: • Ammonia in fish and aquatic invertebrates • Uric acid in birds, reptiles and insects • Urea in mammals and amphibians

Excretion & Water Balance in Fish

Fish produce the waste ammonia which is very alkaline and toxic. Luckily it is very soluble in water. Since they live surrounded by water, fish simply excrete ammonia from their gills by simple diffusion.

Their kidneys are used not so much for excretion, but for maintaining their water balance. Freshwater fish and saltwater fish have opposite problems with water balance.

4. EXCRETION & WATER BALANCE

THE CONCENTRATION OF WATER & DISSOLVED SALTS MUST BE MAINTAINED THIS IS ANOTHER EXAMPLE OF HOMEOSTASIS

IN MOST ANIMALS WATER BALANCE IS REGULATED

BY THE KIDNEYS

SALTWATER FISH

FRESHWATER FISH

Water loss from tissues by osmosis (mainly through gills)

Tissues gain water by osmosis (mainly through gills) Gills excrete AAmmmmoonniiaa,

Carbon Dioxide and excess salt Constantly drink to replace water (but get salt, too) Kidneys produce small amounts of urine to save water Kidneys produce a lot of dilute urine to

remove water from body

Do not drink Gills excrete AAmmmmoonniiaa & Carbon

Dioxide, and actively absorb salts

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Excretion in Terrestrial Environments

The fish can get away with production of highly toxic ammonia. They can rely on constant diffusion of ammonia from the blood in their gills into the water environment which surrounds them.

In terrestrial environments, waste gases can do exactly the same; that’s how carbon dioxide is excreted... by simple diffusion from the blood to the air in the lungs. However, nitrogenous wastes are not gaseous and need to be excreted in water solution. This means that:

• nitrogenous wastes are produced not as ammonia, but the less toxic compounds urea (mammals) or uric acid (birds, reptiles, insects)

• excretion is via the kidneys, and the simple processes of diffusion and osmosis are not adequate to achieve this. For simple diffusion to achieve excretion it would require huge amounts of water to be excreted too, and no terrestrial animal can afford to do this, especially in a desert.

How the Kidneys Work in Mammals

Each kidney contains about 1 million nephrons. Each nephron is a complicated tangle of blood vessels and renal tubules (=small tubes), but what happens in a nephron can be summarized in a very simple way... K.I.S.S.

Filtration

removes some of the water and many small dissolved molecules (including the waste urea) from the blood into the renal tubules. This occurs because the walls of the glomerulus are “leaky” and the blood is under high pressure. Reabsorption

then occurs to move useful substances back into the blood. This is achieved by:

•Active Transport of sugar, amino acids & salts from the renal tubules back into the blood. This requires energy to be used to transport these chemicals across the cell membranes, against a concentration gradient.

•Osmosis then occurs, which causes water to flow from the tubules back into the blood. This is Passive Transport and costs the body no energy.

BBlloooodd iinn ffrroomm aarrtteerryy

BBlloooodd oouutt ttoo vveeiinn This blood contains urea

This blood has had wastes removed, and water balance adjusted for

Homeostasis

Glomerulus

a coiled blood vessel

Bowman’s Capsule

a “receiving cup” to collect the filtrate liquid

from the blood

Blood Capillary Network

shown in simplified form

Renal Tubules Urine flows to collecting duct then via Ureter to Bladder, for excretion

THE NEPHRON

of the KIDNEY

Filttrrattionn occurrs herre

Filtration

is the process in which some water and many dissolved substances (including sugar, salts &

urea, BUT NOT any cells or blood proteins) leave the blood and flow into the renal tubules.

Reabsorption

is the process in which any useful substances (such as sugars & amino acids) are absorbed back into the blood. Water & salts are also

reabsorbed, but in varying quantities... the body is adjusting water balance for Homeostasis Urea is not reabsorbed back into the blood.

Urea and some water continue along the tubule. This liquid is URINE. Urine flows into the Ureter and is carried to the Bladder for storage. When the bladder becomes full, the urine is excreted via the Urethra.

Reabsorrpttionn occurrs

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The Kidneys & Homeostasis

The kidneys are not just used for excretion. As well, the kidneys can adjust the “water balance” of the body by allowing more, or less, urine to be produced. In this way the kidneys are a vital part of homeostasis.

Once again, the Hypothalamus is involved, but the control mechanism is by hormones... chemicals which are released into the blood and exert a control function on some “target organ”. In this case the hormone is called “Anti-Diuretic Hormone” (ADH) and the target organ is the kidney, specifically the nephron tubules.

WATER LEVEL IN

BODY TOO LOW BODY TOO HIGHWATER LEVEL IN

Pituitary Gland releases more ADH

(Also nerve signals to brain cause “thirsty” feeling so

you will want to drink)

Pituitary Gland releases less ADH

(Also nerve signals to brain cause feeling that you do

NOT want to drink) BODY RETAINS MORE WATER,

excretes less urine. Urine is more concentrated

BODY PASSES MORE WATER, excretes more urine. Urine is more dilute.

Nerve Command to Pituitary Gland

Nerve Commands

HYPOTHALAMUS &

PITUITARY GLAND ADH causes more reabsorption of water from kidney tubules

Less ADH causes less reabsorption of water from kidney tubules.

Note the typical pattern of a negative feedback system

How the Hormones Work

The hypothalamus monitors the blood flowing through it for the “osmotic balance” of water and dissolved salt. If the body is even slightly dehydrated, more ADH is released by the pituitary gland and circulates in the blood stream. The effect of ADH is to alter the permeability of the membranes lining the tubules of the kidney nephrons. Increased ADH levels make the membranes more permeable to water, so more water is reabsorbed back into the blood. This means that less urine is produced.

If the body is over-hydrated, the production of ADH is reduced. This causes the tubules to become less permeable to water so less is reabsorbed into the blood. The result is more urine being produced.

ADH is the hormone controlling the water levels, but this is only part of the “osmotic balance” story... the salt levels can be controlled too. Read on...

Control of Salt Levels by Aldosterone

Sitting on top of the kidneys are the “Adrenal Glands” which produce a variety of steroid hormones controlling a number of body functions. One of the adrenal hormones is Aldosterone which controls reabsorption of salt from the nephron tubules.

If salt levels are too low, special cells in the adrenal glands detect this and increase the production of aldosterone into the bloodstream. This causes the cells lining the nephron tubules to actively transport more sodium ions back into the blood. Chloride ions follow the sodium, and so more salt is reabsorbed.

If salt levels are too high, the adrenal glands produce less aldosterone so less salt is reabsorbed, and the excess salt will be excreted in the urine.

Between ADH and aldosterone the body maintains a constant “osmotic balance” of water and dissolved salt... Homeostasis.

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Addison’s Disease & HRT

Addison’s Disease occurs when a person’s adrenal glands do not produce enough aldosterone, even when their salt levels are too low. Their nephrons constantly fail to reabsorb salt and so the “osmotic balance” of the body is chronically out of order.

This leads to a variety of problems and malfunctions throughout the body involving the heart, intestines and liver, and may cause psychological disorders as well. This is a disease that can be sucessfuly treated by “Hormone Replacement Therapy” (HRT).

A person with Addison’s Disease can be treated with appropriate doses of steroid hormones (usually cortisone) and although they cannot be totally cured, they can lead a normal, symptom-free life on HRT.

Renal Dialysis

If a person’s kidneys cease functioning properly he/she can no longer remove toxic wastes such as urea from the blood, nor maintain homeostasis of “water balance”. In the case of complete kidney failure, this condition is fatal within about 3 days without treatment.

Over the past 40 years or so, many people have been successfully treated by receiving a kidney transplant. However, they may have to wait months or years to find a suitable organ donor. In the meanwhile, they need to be treated by Renal Dialysis... the use of medical technology to remove wastes from the blood artificially. In effect, a renal dialysis machine is an “artificial kidney”.

The simplified diagram explains how this works.

The dialysis fluid contains water, salts, sugars, minerals etc exactly as in healthy blood plasma. Since there is no concentration gradient for these chemicals they do not diffuse in or out of the blood. However, the wastes such as urea are in higher concentration in the blood, and so they diffuse from the blood into the dialysis liquid, which is later disposed of.

Kidneys

Ureters

Bladder Urethra GENERAL STRUCTURE OF THE URINARY SYSTEM

Kidney Structure & Nephrons

You may have dissected a kidney in your laboratory work in class. You should be able to relate the gross structure of the kidney and urinary system to the structure and functioning of the nephrons. This is summarized by these diagrams.

Renal Cortex

Dark red in colour due to the many blood

capillaries of the nephrons

Artery & Vein

Medulla

Lighter in colour... less blood vessels. Here many collecting ducts carry urine to the ureter Ureter carries urine to bladder DISSECTED KIDNEY Patient’s blood from an artery Pump

Blood flows through “dialysis tube” with semi-permeable membrane walls

FLUID IN OUT D Diiaallyyssiiss fflluuiidd fflloowwss ppaasstt tthhee ttuubbeess ccaarrrryyiinngg tthhee bblloooodd Blood returns to patient’s vein

Position of an AAddrreennaall GGllaanndd (not usually present in school specimens)

Comparison of Renal Dialysis

with Natural Kidney Function Similarities

•Both processes remove urea and other wastes from the blood. •Both rely on movement of dissolved substances through semi-permeable membranes.

Differences

• Kidney function involves the 2 steps of filtration and reabsorption; dialysis involves only 1 step of diffusion of wastes from blood.

• In a kidney, movement across membranes is

achieved by both active transport and by passive osmosis and diffusion; dialysis involves only passive diffusion.

wastes such as urea diffuse

out of the blood