MAINTAINING A BALANCE

KEY WORDS AND TERMS IN THIS TOPIC

As you study this topic you should write the definitions for the following syllabus terms.

Term Definitionenzymes pH acidity amino acidslipids bicarbonate ionsmetabolismactive transportpassive transport chemoreceptorshomeostasishypothalamus feedback systemectothermendothermdormancynocturnalhaemoglobinplateletsarteriescapillariesveinsplasma phloemxylem adhesioncohesion transpirationtranslocationrenal dialysisnitrogenous wastesurea aldosteroneADH(antidiuretic hormone)hormone replacement therapyosmosisnephronenantiostasisestuarine

MAINTAINING A BALANCE

SUMMARY OF THIS TOPIC

The internal environments of plants and animals need to be kept in balance for a number of reasons. Firstly, enzymes, which regulate the metabolism of all living things, will only operate under particular conditions of temperature, solute concentration and acidity. Secondly, cellular waste products such as carbon dioxide and urea need to be constantly excreted to avoid them building up to toxic levels within organisms. This in turn necessitates a mechanism for maintaining internal solute concentrations at relatively constant levels. The process whereby organisms maintain a stable environment is called homeostasis.Internal temperatures are managed in a number of ways. In mammals and other endotherms the central and peripheral nervous systems detect and respond to any external changes. Ectothermic

animals often respond to temperature and other environmental changes by changing their behaviour, and plants have developed many structural and physiological changes to adapt in conditions of extreme temperature.Internal solute concentrations are also controlled using nervous and hormonal feedback systems in vertebrates, with the excretory role of the kidney playing an important part in most cases. Even insects have developed a method of excreting excess nitrogenous wastes while conserving optimal body water. Other techniques used to maintain internal solute concentrations include plant adaptations to reduce water loss and the ability of estuarine organisms to deal with high salt levels.

MAINTAINING A BALANCE

MAJOR OBJECTIVES OF THIS TOPIC

As indicated in the HSC Biology syllabus, the major outcomes of this topic include the ability to:

recognise the specificity of enzymes and their role in metabolism, the composition of enzymes and how enzyme activity is affected by temperature, acidity and substrate concentration.

define homeostasis and recognise the importance of maintaining a stable internal environment in terms of enzyme activity and metabolism.

recognise the role of the nervous system in responding to temperature changes and understand the processes involved in a typical feedback mechanism.

define the terms ‘endotherm’ and ‘ectotherm’ and outline the ways these organisms cope with temperature regulation.

describe some of the ways in which plants respond to temperature change, using information from secondary sources.

perform an experiment to observe the factors affecting enzyme activity.

identify the forms in which different substances are carried in the blood.

describe the adaptive advantage of haemoglobin as an oxygen carrier in the blood.

investigate the structure of blood cells and compare and relate the structure of arteries, veins and capillaries to their function.

recognise the changes that occur in the blood’s composition as it passes through major organs.

discuss why the removal of excess carbon dioxide from the blood is necessary and describe current technologies available for the measurement of oxygen and carbon dioxide levels in the blood.

investigate the structure of xylem and phloem and compare the ways in which materials move through each.

describe the uses for the different fractions of donated blood and discuss the work being carried out to develop artificial blood.

understand the role of the kidney in removing wastes and thus maintaining constant solute concentrations.

perform a first hand investigation of the kidney.

describe the different methods used in animals and plants to achieve a balance between excreting wastes and conserving water.

recognise the role of the hormones aldosterone and ADH in regulating water and salt levels in the blood.

identify the particular challenges experienced by estuarine organisms in maintaining constant internal conditions.

investigate structures in plants that assist in the conservation of water.

+ H2O

glucose

+

fructose

sucrose

sucrase ready to be re-used

sucrase

1) Most organisms are active within a limited temperature range

The nature of enzymes and their role in metabolism

Metabolism is a term used to describe all the chemical processes occurring within cells. Special biological catalysts called enzymes help to bring about specific metabolic reactions in controlled steps. Enzymes are large protein molecules which are capable of initiating reactions without being altered themselves. Each enzyme is specific for a specific ‘substrate’ molecule. For instance, the enzyme sucrase will only recognise and interact with the sucrose molecule to facilitate its decomposition into glucose and fructose, as shown below.

Enzymes combine with substrate molecules at an area called the ‘active site’. They are affected by changes in temperature and acidity; an example of this is shown in the graph below, which shows the relationship between enzyme activity and temperature.

Fig.1-2 The relationship between enzyme activity and temperature

The term ‘pH’ is used to describe the acidity of a solution; a pH below 7 indicates an acidic solution and a pH above 7 indicates a basic solution.

As a requirement of this course, you are expected to perform a first hand investigation to test the effect of increased temperature, change in pH and change in substrate concentrations on the activity of named enzymes.

Suggestions for practical enzyme investigations:

i) Test the effect of pH on the action of the enzyme pepsin on cooked egg white;ii) Test the effect of different temperatures on the ability of the enzyme rennin in junket tablets to curdle milk;iii) Test the effect of hydrogen peroxide concentration on the ability of the enzyme catalase (present in chopped liver) to produce bubbles of oxygen gas.Homeostasis - Maintaining a stable internal environment

Temperature ◦C

Relative enzyme activity

Fig.1-1 The specificity of an enzyme

The composition of fluids in the body must be maintained at a constant level in order to achieve optimal metabolic efficiency; in addition, enzymes, responsible for controlling all metabolic activities within the cell, will only operate within a limited range of temperature and acidity. Homeostasis, the process by which organisms maintain a relatively stable environment, is thus of paramount importance to the normal functioning of organisms. Homeostasis consists of two stages: i) detecting changes from the stable state - this is made possible by the presence of receptors in living organisms. Receptors in animals consist of nerve cells that detect stimuli (a stimulus is any information that causes a response). Receptors in plants usually include the shoot and root tips working together with plant hormones.ii) counteracting changes from the stable state - this is brought about in living organisms by effectors. In mammals, effectors are usually muscles or glands and in plants hormones such as auxins and cytokinins act as effectors. Changes in an organism’s surroundings are thus detected by receptors, processed by a control centre (usually the brain), and counteracted by effectors. Table 1-1, below, shows some examples of stimuli and the receptors that detect them.

Table 1-1 Stimuli and receptors

Table 1-2 shows some examples of effectors and the responses they carry out.

Effector ResponseMuscle Examples include

contraction to produce movement, blood

vessel constriction and shivering

Gland Secretion of hormones or other chemicals-examples of their

action include control of solute levels in the blood, diverting blood

to the muscles, secondary sexual characteristics.

Plant hormones Responses include control of flowering

and fruiting, growth of buds, stem elongation.

Table 1-2 Effectors and their responses

Feedback loops

In a ‘feedback loop’ the receptors detect the response and send messages back to the control centre to stop further adjustment. This is often referred to as ‘negative feedback’ because it results in a negative response by the effector; for example, the accumulation of a hormone in the blood automatically cuts down its production.

As a requirement of this course, you will need to use information from secondary sources to develop a model of a feedback mechanism.

An example of a feedback mechanism can be seen in the control of blood glucose levels, as illustrated in Fig. 1-3. High blood sugar stimulates the release of the hormone insulin in the pancreas. The insulin in turn triggers the uptake of glucose by the liver and its conversion to glycogen. This lowers the level of glucose in the blood and the pancreas responds by producing less insulin. Low insulin levels stimulate the liver to convert some of the stored glycogen back into glucose and blood sugar levels are again raised.

Stimulus ReceptorChanges in the concentration of blood chemicals such as glucose, amino acids, CO2, oxygen and dissolved ions

chemoreceptors

Temperature changes thermoreceptors Light photoreceptors Pressure, gravity, sound

mechanoreceptors

The role of the nervous system in detecting and responding to environmental change

In endotherms (animals which can maintain their own internal temperature) a constant internal temperature is controlled by an inbuilt ‘thermostat’, the hypothalamus, which is located in the brain. The hypothalamus is a region of neurones and secretory cells and it thus performs a variety of roles involving nervous and hormonal messages. In its role as a temperature regulator, it acts by first detecting and then responding to changes in the surrounding temperature.

It contains sensory receptors which receive stimuli from thermoreceptors in the skin and temperature changes in the blood. The hypothalamus then sends information via the peripheral nervous system to produce heating responses such as shivering and vasoconstriction (blood vessels constrict, so reducing heat loss to the surroundings), and cooling responses such as sweating, panting and vasodilation (blood vessels dilate and move closer to the skin’s surface, so allowing heat to escape). This process is illustrated in the diagram below.

pancreasliver converts some of the

glycogen back to glucose

uptake of glucose by liver and conversion

to glycogen

low blood sugar high blood sugar

less insulin produced

insulin produced

blood vessels dilate,

panting, sweating

peripheral nervous system

hypothalamus

shivering, blood vessels constrict, hair, fur or feathersstand up

thermoreceptors in skin detect heat

thermoreceptors in skin detect cold

cooler bloodwarmer blood

Fig.1-3 The control of blood glucose levels

Fig. 1-4 The nervous system and temperature regulation

The broad range of temperatures over which life is found compared with the narrow limits for individual species

The temperature ranges in which life on earth can occur are dictated by the heat tolerance of enzymes and other proteins within cells and by the need for a liquid medium in which molecules can react together. At temperatures of only a few degrees below 0ºC the cytoplasmic fluid of cells freezes. As a result, molecules are either left at a concentration that is too high for satisfactory metabolic activity or immobilised completely. Most proteins begin to denature at temperatures around 50ºC; their 3-dimensional structure is altered and enzymes can no longer function. The occurrence of life is consequently limited to external environments within the narrow range of just below freezing to 45-50ºC. Exceptions include some forms of cyanobacteria found in hot springs which have adapted so that metabolism can occur at temperatures of about 100ºC. Certain species of lichen and moss have been known to withstand temperatures below -200ºC and the seeds of some plants are capable of surviving temperatures of over 100ºC during bushfires.

Temperature adaptations of Australian endotherms and ectotherms

As previously mentioned, an endotherm is an animal that can maintain a constant internal temperature. Ectotherms such as reptiles cannot do this. They usually respond to environmental temperature fluctuations by adjusting their behaviour in some way; burrowing to avoid desert heat is an example of this. Endotherms respond to temperature changes both physiologically and behaviourally. Examples of animal responses to hot conditions include the evaporative cooling of water from the body by sweating or panting in endotherms such as the red kangaroo. Kangaroos may also lick their forearms to increase evaporative cooling. Vasodilation, in which blood vessels dilate and

are brought nearer to the skin’s surface for cooling, can also be observed in the ears of the kangaroo, the rat bandicoot and other desert marsupials. In addition, the ears of these mammals, as shown in Fig. 1-5, have a large surface area to increase this cooling effect.Many small desert mammals also have a large surface area to volume ratio to increase heat loss. Mitchell’s hopping mouse and the brown snake are examples of animals that avoid the heat of the day by being nocturnal. The desert lizard avoids high temperatures by burrowing under the ground and sheltering under rocks. Other lizards are light in colour to reflect heat. Some desert animals hibernate during the warmer part of the year. This slows down their metabolism and reduces heat production.Examples of responses to cold temperatures include shivering, a physiological adaptation in endotherms which produces heat, and the constriction of blood vessels to avoid heat loss. Aquatic endotherms such as whales and seals possess an insulating layer of fat, and platypuses have a thick layer of fur to reduce heat loss to the surrounding water. Counter-current exchange, in which heat passes between arteries and veins, occurs in the extremities of animals such as the fins of seals, the feet of the platypus and the legs of arctic birds. Some reptiles keep warm by flattening out their bodies to increase their surface area when the sun comes out. A smaller surface area to volume ratio is more beneficial in cold environments because heat loss to the surroundings is reduced.

Fig. 1-5 The ears of the dusky hopping mouse are large to facilitate heat loss.

As a requirement of this topic, you are expected to analyse information from secondary sources to describe some adaptations in Australian organisms to temperature change.

Explain how each of the listed features in table 1-3, below allows each animal to tolerate high temperatures:

Table 1-3 Temperature adaptations of Australian animals

Plant responses to temperature change

Plants respond to hot conditions in a number of ways. Desert plants may have waxy, shiny, hairy or light coloured leaf surfaces to help reflect radiation. The surface area of the leaves of many of these plants may be reduced to reduce heat absorption, or, as in Eucalypts, they may hang vertically to reduce exposure to the heat. Some Australian plants respond to the heat of bushfires by opening their seed pods or germinating; in this case, then, heat is actually a requirement. In cold conditions, plants may lose leaves to avoid damage, or respond positively by flowering (this process is called vernalisation).

2. Plants and animals transport dissolved nutrients and gases in a fluid medium

The forms in which substances are carried in the blood

Blood plays a major role in maintaining a constant internal environment in humans and other mammals. This is because it acts as a transport system for the cells of the body, helping to remove wastes such as CO2 and deliver needed substances such as food and oxygen. About 55% of the blood is composed of a straw coloured liquid called plasma which carries substances to and from the cells in a dissolved form. Some of the main substances in the blood are carried by the plasma in the following forms.

Substance Form in which it is carried in bloodCarbon dioxide Bicarbonate ions

Water Dissolved in plasmaOxygen Carried on haemoglobin molecule

Mineral salts Dissolved on plasma in the form of ionsNitrogenous wastes Urea dissolved in plasma

Lipids Fatty acids and glycerol in plasma or as fat droplets

Other products of digestion Sugars in the form of glucose, proteins in the form of amino acids

Vitamins B & C vitamins soluble in plasma, A & E vitamins carried in lipid droplets

Table 1-4 The forms in which substances are carried in the blood

Kangaroo rat Desert goanna Kangaroo

Small in size, nocturnal, burrows,has large ears with a

dense network of blood vessels close to the surface, posseses a large number of

sweat glands.

Small, pale in colour, burrows, lies flat on the

ground with its back to the sun in the morning, raises its body above the ground in the

middle of the day.

Light coloured fur, forelimbs have dense network of blood vessels close to the surface,

pants and sweats, licks forelimbs

As a requirement of this course you need to perform a first hand investigation to demonstrate the effect of dissolved carbon dioxide on the pH of water.

THINK!! Assuming that a constant blood pH of 7.4 needs to be maintained in mammals at all times, suggest why dissolved carbon dioxide in the blood needs to be regularly removed.

The adaptive advantages of haemoglobin

Haemoglobin is a molecule found in red blood cells in humans and other vertebrates which possesses the important property of being able to combine loosely with oxygen molecules. As a result of this property, haemoglobin is able to collect oxygen in the lungs and release it in the tissues when it is needed.

Haemoglobin + oxygen → oxyhaemoglobin Hb + 4O2 → Hb(O2)4

Oxygen is relatively insoluble in blood plasma and the presence of haemoglobin helps to raise the

oxygen carrying capacity of the blood by more than sixty times its normal ability. Besides its high oxygen carrying capacity, haemoglobin is also capable of adapting to a certain extent to the individual oxygen demands of organisms. A build up in blood acidity due to a build-up of carbon dioxide from respiration, for instance, triggers a decrease in haemoglobin’s affinity for oxygen. As a result, oxygen is released to the cells and used for further respiration. The presence of oxyhaemoglobin in the lungs also triggers the release of carbon dioxide to the lungs to be excreted. These features enable individuals to respond to changes in surrounding oxygen levels such as those experienced on aeroplanes. Inhabitants of areas of high altitude have also been able to adapt to the low oxygen levels by

phenol red solution

Carbon dioxide is produced by all cells of the body in respiration. It leaves the cells and dissolves in the blood plasma to form bicarbonate ions as shown below.

Carbon dioxide + water→ carbonic acid→ hydrogen ions + bicarbonate ionsCO2 + H2O → H2CO3 → H+ + HCO3

-

Dissolved hydrogen ions result in an acidic solution (pH lower than 7). Phenol red solution is an indicator which changes from red to yellow in the presence of hydrogen ions over a pH range of 8.4-6.8. Apparatus appropriate for investigating the effect of dissolved CO2 on the pH of water is shown below:

marble chips + dilute HCl

phenol red solution

manufacturing more red blood cells and developing a faster breathing rate and heart rate. In some cases the actual size of the heart muscle can increase. All these adaptations result in more haemoglobin molecules carrying more oxygen.

As a requirement of this topic, you are expected to perform a first hand investigation using the light microscope and prepared slides toestimate the size of red and whiteblood cells, drawing scaled diagrams of each.

Blood cellsApproximately 60% of blood is composed of plasma, a straw-like substance responsible for carrying dissolved nutrients, wastes and other substances around the body. The remaining 40% is made up of red blood cells (erythrocytes), white blood cells (leukocytes) and platelets.There are many more red blood cells than white blood cells; about 1000 for every one or two white blood cells. The main function of red blood cells is to transport oxygen around the body; haemoglobin molecules on the surface of the blood cells loosely bind to oxygen in the lungs and release it to the body cells where it is constantly needed. Mature red blood cells do not possess a nucleus, thus allowing haemoglobin to fill almost the entire volume of the cell.They are red in colour because each haemoglobin molecule in the cell contains an atom of iron; when the haemoglobin combines with oxygen they become an even brighter red. White blood cells, although fewer in number, are larger than red blood cells. Unlike red blood cells, they contain a nucleus and are capable of moving from blood vessels into other tissue. White blood cells perform a major role in defending the body against disease- causing pathogens.Platelets are small cell fragments involved in blood clotting.

The structure and function of arteries, capillaries and veins

Fig. 1-6 shows cross sections through an artery, a vein and a capillary. The table that follows

describes the structure and function of each of these types of blood vessel.

Fig 1-6 Sections through a vein, an artery and a capillary

Blood Vessel

Structure Function

Arteries Thick, muscular,

elastic walls

Carry oxygenated blood under high pressure from the heart to the rest of the body. Walls

expand and contract with each

surge of bloodVeins Thin-walled

Possess valves to prevent backflow. Wide diameter

Return deoxygenated blood to the heart. Blood in these vessels is under low pressure, and is kept moving by muscles pressing against them

Capillaries One cell thick. Their thin walls provide a large surface area to volume ratio. Narrow diameter of between 7-10µm

Allow dissolved substances to pass through their walls readily, into and out of body cells. Form a fine network that is small enough to be in close contact with all body cells. Connect arteries to veins

Table 1-4 The structure and function of the three types of blood vessel

ARTERY VEINCAPILLARY

thick walls

large diameter tube

small diameter tube

muscle and elastic fibres

layer of non-elastic fibres

valves

THINK!!! In people suffering from varicose veins, the veins in the legs become swollen and coiled. Which part of the vein is failing to operate here?

Changes in blood composition as it moves around the body

The concentration of solutes in the blood is constantly maintained as they are carried around the body; various organs act to either remove excess solutes or replenish the blood with solutes that are in short supply. In the liver, excess amino acids from the breakdown of proteins are converted to urea. The liver also stores excess glucose in the form of glycogen. Alcohol, old red blood cells and vitamins are also broken down by the liver, and it is therefore to be expected that there are less of these in the blood leaving the liver. Urea leaves the blood via the kidney because it is toxic. Excess salts and water alsoleave the blood through the kidney. Glucose and amino acids are reabsorbed back into the blood from the kidney as they are useful substances needed by the body cells.Carbon dioxide, in the form of dissolved bicarbonate ions, leaves the blood through the lungs because it would otherwise make the blood too toxic. Blood entering the right side of the heart is high in carbon dioxide and low in oxygen. It is sent to the lungs where it picks up oxygen and deposits carbon dioxide. Blood leaving the left side of the heart is therefore high in oxygen and low in carbon dioxide.

As a requirement of this topic, you need to describe current technologies that allow measurement of oxygen saturation and carbon dioxide concentrations in blood.

A pulse oximeter can be used to determine the oxygen saturation of blood. It consists of a peg placed on the finger which detects the transmission of light through the skin. Blood gas analysers can also be used to establish the bicarbonate and

oxygen concentrations in the blood. These instruments are used to assess patients suffering from lung conditions such as emphysema and mesothelioma.

The movement of materials through xylem and phloem

Xylem tissue consists of a system of narrow tubes which carry water and mineral ions in an upward direction throughout the plant. Movement of materials in the xylem is passive because no energy input from the plant is required. The major driving force behing this movement is the evaporation of water from the stomates in the leaves during transpiration. As water leaves the plant in a gaseous form, more water in the xylem vessels moves up to replace it. A continuous transpiration ‘stream’ is maintained with the help of the following: a) adhesive forces between the water molecules and the lignin walls of the xylem vessels; b) cohesive forces between adjacent water molecules and c) ‘capillarity’ - the tendency for water to move unassisted to a certain height up narrow tubes.Organic materials move through phloem tissue in the sieve tubes which, unlike xylem vessels, are living. Two current theories exist regarding the method in which materials first move into the sieve tube cells; i) the ‘symplastic loading’ theory suggests that materials move from mesophyll cells and companion cells to the sieve cells through fine connecting tubes called plasmodesmata; ii) the ‘apoplastic loading theory’ proposes that materials follow a route across cell walls into the sieve cells.The actual movement of organic compounds through the phloem is known as translocation and follows a ‘source-to-sink’ pathway. This process, in contrast to movement in the xylem, requires energy. At the ‘source’ (photosynthetic leaves or storage tissues), sugars and other photosynthetic products are accumulated in the phloem by active transport. Water then moves into the phloem by osmosis. The result is a higher turgor pressure at the source than at the ‘sinks’ (the areas where the nutrients are sent to). As a result, the material moves along a pressure gradient until it reaches a ‘sink’. Here, sugars and other nutrients are removed by cells using active transport. The water remaining in the sieve tube moves into the surrounding cells by osmosis. The driving force behind movement of nutrients in the phloem is

therefore the pressure gradient between ‘sources’ and ‘sinks. This type of movement has been referred to as the ‘bulk flow’ theory.

As a requirement of this topic, you are expected to perform a first hand investigation involving drawing transverse and longitudinal sections of phloem and xylem tissue.

THINK!!! The two longitudinal sections below depict xylem and phloem tissue. Which is which?

Fig. 1-7 Xylem and phloem tissue

Notice the position of xylem and phloem tissue in the transverse section of a stem, below. The xylem is always located in the middle of the vascular bundles.

Fig. 1-8 Transverse section of a dicot stem

As a requirement of this topic, you need to use secondary sources to identify the products extracted from donated blood and discuss the uses of these products.

Donated blood is used for trauma victims, patients undergoing surgery and organ transplants, and people being treated for other ailments such as sickle cell anaemia and haemophilia. Before being used it is screened for diseases such as hepatitis, HIV and syphilis, and tested for blood type.Blood can be separated into its major components using a centrifuge. The major blood components and their uses are shown in table 1-5, below.

Blood component UseRed blood cells chronic anaemia,

massive blood lossGranulocytes used where patients

don’t respond to antibiotics

Fresh frozen plasma bleeding disordersCryoprecipitated AHF haemophilia and von

Willebrand’s diseasePlasma

derivatives( albumin, immunoglobulin, factors

VIII and IX)

aid in blood clotting

Platelets treating thrombocytopaenia

Table 1-5 Major blood components and their uses

As a requirement of this topic, you also need to present information to report on progress in the production of artificialblood and explain why such research is necessary.

Two types of artificial blood are currently being developed; i) Haemoglobin based ‘blood’, which is intended for use as an oxygen carrier only. This is obtained by extracting haemoglobin from real blood. More research is needed to reduce the extent of oxidation of the haemoglobin once it is administered; ii) Synthetic blood, made from perflurochemicals, which has a high oxygen carrying capacity. More research is needed to achieve a fluid that can mix easily with other substances in the bloodstream.

phloem

xylem

cortexpith

epidermis

sieve tube

companion cell

sieve platenarrow, pitted tubes

vessels are dead

3. Plants and animals regulate the concentration of gases, water and waste products of metabolism in cells and interstitial fluid

Regulation of water concentration

Internal water concentrations in organisms need to be kept constant. Water is needed by the body as a solvent in which chemical reactions take place and as a transport medium to carry dissolved minerals, wastes, food and vitamins around the body. It is also needed for evaporative cooling processes such as sweating and panting, and to moisten cell membranes to allow for diffusion of materials into and out of cells. Water concentrations need to be kept constant so that cells maintain a constant osmotic pressure and so that water loss through processes such as urination is compensated for.

The importance of waste removal for continued metabolic activity

The major waste products that build up in the body are carbon dioxide, mineral salts, excess water and urea. Urea is formed when proteins are broken down in the liver in the process of deamination. The urea then combines with water in the kidneys to form urine, and is then passed out of the body. Table 1-6 outlines the various wastes produced in the body, the reasons why they need to be excreted and the organs responsible for their excretion.

THINK!!! Can you think of another way in which water is constantly lost from the body apart from through urination, sweating and panting?

Waste product

Reason for its need to be removed

Organ of excretion

Carbon dioxide

Dissolved bicarbonate ions accumulate in the blood, resulting in a low pH. This acidity can result in the denaturing of enzymes and subsequent disruption of metabolic activities

The lungs

Excess mineral salts

High levels of these ions can affect the osmotic pressure of cells. Metabolic processes are more efficient when cells and their surroundings are isotonic (i.e. they have the same solute concentration)

The kidney and sweat glands

Urea Although not as toxic as ammonia, the initial product of deamination, this can soon build up to toxic levels in the blood, poisoning the cells and retarding metabolism

The kidney

Table1-6 Wastes produced in the body and organs of excretion

The process in which proteins are broken down in the liver to form ammonia is known as deamination. In humans, this toxic molecule is immediately converted to urea, which combines with water in the kidneys to form urine and is then passed out of the body. Reptiles, insects and birds convert ammonia to uric acid, which is insoluble and non-toxic. This leaves the body in a concentrated form to avoid water loss. Marine organisms, aquatic snails and tadpoles can

release ammonia directly into the surrounding water.

The role of the kidney in the excretory system of fish and mammals

In mammals, the kidney has two major roles: a) Filtration, in which the toxic nitrogenous compound urea, formed by the deamination of proteins in the liver, is removed; b) Osmoregulation, which involves maintaining a constant solute concentration in the body by excreting or reabsorbing water and salts. These two processes occur in millions of microscopic structures called nephrons.In fish, highly toxic ammonia, the initial product of deamination, is released directly into the surrounding water through the gills. There is consequently no need for the kidney to act as a filter here. The kidney, however, still performs an osmoregulatory role by altering salt and water levels so that constant internal solute concentrations are maintained

Cartilaginous fish such as sharks do not need to regulate their internal solute concentrations because they retain enough urea inside their bodies to keep their internal environment isotonic (i.e. at the same concentration) with the sea water outside them.

As a requirement of this topic you need to compare and explain the differences in urine concentration of marine fish, freshwater fish and terrestrial mammals.

In saltwater fish, there is a tendency for water to move out of the fish by osmosis. Fish respond to this by drinking large volumes of water and excreting excess salts through their kidneys and gills. The kidneys also reabsorb water, resulting in concentrated urine. In freshwater fish, there is a tendency for water to move into the fish by osmosis because the surroundings are more dilute than the fluid inside the fish. For the same reason, salts tend to move out of the fish. The kidneys of the fish respond to

this by excreting large volumes of water in the urine and reabsorbing mineral salts into the blood. Fig 1-9 illustrates how both types of fish regulate their internal environments.

Fig 1-9 Differences in urine concentration of marine fish

Mammals need to conserve more water than fish. They therefore produce a more concentrated urine. By converting ammonia to the less toxic urea, they don’t need to dilute it as much and can store it in the body for longer.

As a requirement of this topic, you need to determine the relationship between water conservation and urine

salts tend to move out

water tends to move in by osmosis

kidneys reabsorb salts

large amounts of urine produced

freshwater fish

large amounts of water taken in through mouth

gills and kidneys excrete excess salts

water tends to be lost by osmosis

concentrated urine produced

saltwater fish

concentration in a range of Australian insects and terrestrial mammals.

The diagram below shows the forms in which nitrogenous wastes are excreted in different animals. Uric acid, excreted by birds and insects, is insoluble, and so does not need to be excreted in the presence of water. This is therefore the best way to excrete nitrogenous wastes in dry conditions as it minimises water loss. Mammals need to excrete urea with water, as it is a soluble compound, but they can reduce the amounts of water they lose. Animals adapted to desert environments tend to have a long loop of Henle, as this is where most water reabsorption occurs.

Fig. 1-10 The forms in which nitrogenous wastes are excreted

THINK!!! Ammonia is very toxic, and needs to be excreted in the presence of large amounts of water. Explain why fish and other aquatic animals do not have to convert their nitrogenous wastes to urea or uric acid in order to excrete them.

Filtration and reabsorption in the nephron

The nephron is the functional unit of the kidney. Each nephron helps to regulate body fluid composition by filtration of the blood and selective reabsorption of useful substances back into the blood. The main structures of the nephron are shown in Fig. 1-11.

Fig. 1-11 The Nephron

Filtration and reabsorption help to regulate body fluid composition in the following ways; a) Filtration of the blood in the glomerulus results in urea, water, hormones, toxins, amino acids, vitamins and glucose passing into the nephron; b) Reabsorption now takes place because many of the above substances are useful to the body. In the proximal kidney tubule glucose, amino acids and other nutrients are reabsorbed back into the network of blood vessels surrounding the nephron. This process uses active transport because the molecules move into the blood against a concentration gradient. If fluid composition and pH in the surrounding blood needs adjustment, bicarbonate ions are also reabsorbed here.Hydrogen ions may also move into the tubule by active transport. In the descending part of the loop of Henle, water is reabsorbed into the blood by osmosis. In the ascending section of the loop, salts are reabsorbed if the surrounding blood concentrations of this solute are low. More selective reabsorption of water and mineral ions occurs in the distal kidney tubule as a means of controlling fluid concentrations in the blood. Ions may also move into the tubule to help regulate pH

birds, insects, many

reptiles, land snails

mammals, amphibians

(adult), sharks,

some bony fishes

most aquatic animals and many fish, amphibians( tadpoles)

AMMONIA EXCRETED

UREA EXCRETED

URIC ACID EXCRETED

branch of renal artery

loop of Henle network of

blood vessels

collecting duct

branch of renal vein

distal convoluted

tubule

proximal convoluted

tubuleglomerulus

Bowman’s capsule

-NH2- amino groups

and solute composition. The walls of the collecting duct are selectively permeable to water. If water levels in the surrounding blood are low, the walls of the duct become permeable to water, and water

moves out by osmosis. This results in a relatively concentrated solution of urea and water known as urine.

Diffusion, osmosis, active transport and the kidney

Diffusion involves the random movement of molecules from where there is a high concentration of them to where there is a low concentration of them. Osmosis is also a type of diffusion - the diffusion of water across a semi-permeable membrane (a membrane that allows water through it but not other solutes). Water will move by osmosis from a more dilute solution to a more concentrated solution because it is moving from an area where there is more of it to where there is less of it.

THINK!!! In this diagram of two salt solutions, which way do you think water will move? Why?

Diffusion and osmosis are both forms of passive transport, as no energy is required. As much as 80% of water moves passively back into the blood from the kidney by osmosis after initial filtration by the glomerulus. This occurs largely in the lower region of the loop of Henle, where salt concentrations in the surrounding blood are high. Water also moves out of the collecting duct by osmosis when this structure becomes permeable to water.Movement of other substances in the kidney, however, can at times require energy because these substances may need to be moved against a concentration gradient or particular molecules may be too large to move passively through a

membrane. An example of this occurs when glucose, amino acids, vitamins and useful salts are returned to the blood in the proximal kidney tubule. Active transport is involved here, as they are moving back into the blood against a concentration gradient. In addition, sodium, potassium, chloride, hydrogen and bicarbonate ions are at times actively pumped into the tubule when the kidney needs to regulate solute concentrations in the surrounding blood. Certain drugs such as aspirin and other toxins are also actively eliminated from the blood here. In the ascending part of the loop of Henle, sodium ions move both actively and passively back into the blood.

As a requirement of this topic, you are expected to perform a first hand investigation of a mammalian kidney and to identify the regions involved in the excretion of waste products.

Fig. 1-12 shows the main structures of the kidney and the relative positions of the parts of the nephron.

Fig.1-12 The kidney

The role of hormones in the regulation of water and salt levels in the blood.

20% sodium chloride solution

60% sodium chloride

solution

semipermeable membrane

cortex - glomerulus and Bowman’s capsulemedulla - convoluted tubulerenal arteryrenal vein

pelvis - collecting ducts

ureter

The nephron’s role in regulating internal solute concentrations is brought about by two hormones; ADH (antidiuretic hormone) and aldosterone. ADH is released by the pituitary gland and acts to make the walls of the collecting ducts in the nephron more permeable to water. In periods of water stress, the hypothalamus responds by directing the pituitary to release ADH. The walls of the collecting ducts in turn become more permeable to water, and water moves into the blood by diffusion. When water levels return to normal, a process of negative feedback instructs the pituitary to stop making ADH. In addition to water levels in the blood, other stimuli may trigger or suppress ADH release. Alcohol, for instance, suppresses ADH secretion, resulting in larger amounts of water being retained in the urine.Aldosterone is produced in the adrenal glands, which are found near the kidneys in the adrenal cortex. The release of aldosterone makes the walls of the distal convoluted tubule more permeable to sodium ions. When these ions move into the blood, water follows by osmosis. In situations of high blood salt concentration (and therefore high blood pressure), negative feedback results in the release of less aldosterone and more water and salts are retained in the urine. Aldosterone also stimulates the secretion of potassium ions into the nephron.

Addison’s disease is a condition in which the functioning of the adrenal glands is impaired. Symptoms include weakness and eventually death. As a result, in the blood of an affected person, levels of sodium ions and water are low, and potassium levels are high.

As a requirement of this topic, you need to outline the general use of hormone replacement therapy in people who cannot secrete aldosterone.

Addison’s disease, in which insufficient amounts of the hormones cortisol and aldosterone are produced, can be treated by replacing these hormones with similar ones. Cortisol is generally replaced with cortisone acetate or hydrocortisone tablets, which are taken twice daily. Aldosterone is replaced with a synthetic steroid known as fludrocortisone (marketed as Florinef).

As a requirement of this topic, you are expected to compare the process of renal dialysis with the function of the kidney.

Patients who have lost or damaged their kidneys can benefit from renal dialysis. Dialysis produces similar results to the processes which occur in the kidney; wastes are filtered out of the body and blood solutes are maintained at constant levels. In Haemodialysis, blood is circulated outside the body and cleaned inside a machine until it is returned to the patient. Wastes move out of the blood and into a ‘dialysate’ solution by diffusion across a membrane. Red blood cells do not pass through because they are too large to fit through the small pores in the membrane. In Peritoneal dialysis the patient’s own peritoneal membrane is used as a filter and the dialysate fluid is passed into the abdomen through a catheter.Renal dialysis relies solely on passive diffusion and cannot actively reabsorb substances into the blood as the kidney does.

Water conserving adaptations of Australian plants

Many Australian plants possess water conserving structures which have helped them to adapt to arid conditions.Plants such as these are known as xerophytes, and their common purpose is to reduce water loss through stomates and to make the best possible use of the water that is available to them. Typical examples are acacias, eucalypts, mulgas, desert grasses and bottlebrushes. All of these plants tend to have leaves with a reduced surface area and/or a limited numbers of stomates to reduce unnecessary water loss through evaporation. In some cases leaves are reduced to spines and the stem takes over the role of photosynthesis, as in the phyllodes and cladodes ofsome wattles. Xerophytes must strike a balance between the need to open their stomates for the movement of gases during photosynthesis and the need to close them to avoid transpirational water loss.Extensive root systems are also a common feature; this allows the plant to obtain as much water as possible from the soil. Sometimes the actual shape of a xerophytic tree or shrub may also help in the maximum uptake of rainwater. Stems may also be

adapted for water storage, as in the Baobab tree and the cactus.Behavioural adaptations such as leaf rolling in marram grass or the shedding of leaves in dry conditions are also means by which some xerophytes reduce the surface area available for evaporation of water.

As a requirement of this topic, you need to perform a first hand investigation of structures in plants that assist in the conservation of water.

A table such as the one below may be useful in your investigation. leaves of eucalyptus, grevillea. bottlebrush, wattle, casuarina and hakea plants are among the most suitable for studying.

Enantiostasis and its importance to estuarine organisms

Enantiostasis is defined as the maintenance of metabolic and physiological functions in response to fluctuations in the environment. Organisms living in estuaries need to possess this ability, as they constantly encounter wide variations in salinity. Some organisms, including marine invertebrates and algae, adapt to these conditions by changing their internal solute concentration to match that of the surrounding water. Many fish cope with changing salinity levels by excreting or absorbing salts through their gills or kidneys to produce either dilute or concentrated urine.

As a requirement of this topic, you need to discuss processes used by different plants for salt regulation in saline environments.

Other plants that cope with high salt conditions include the saltbush plant (Atriplex), which concentrates sodium ions in salt glands within the leaf and then pumps them into bladders which eventually expand and burst, thus releasing the excess salt. In the salt marsh plant, Sarcocornia quinqueflora, salt is accumulated in swollen leaf bases which are then shed from the rest of the plant.

Useful websites to refer to in this topic

i) Artificial blood: www.med.unipi.it/patchir/blood/bmr/artif.htm

ii) Addison’s disease:www.niddk.nih.gov/health/endo/pubs/addison/addison.htm

iii) Renal dialysis: www.kidneydirections.com/us/patients/ choices/dialysis/ext-renal-dialysis.htm

iv) Transport of materials in the phloem:courses.forestry.ubc.ca/frst200/lectures/Phloem.htm

v) Uses for donated blood: www.arcbs.redcross.org.au/Donor/guide/story.asp

Name of

plant

Leaf size and

shape

Surface of leaf - e.g. shiny, hairy, waxy, dull etc.

Is the stem

modified in any way?

How does this

adaptation conserve water?

REVIEW QUESTIONS

a) MULTIPLE CHOICE

1) For which of these diseases would treatment with fludrocortisone be most appropriate?a) hepatitisb) Addison’s diseasec) haemophiliad) syphilis

2) Removal of wastes such as urea and carbon dioxide is essential because: a) Metabolic reactions cannot take place

because the wastes increase cellular solute concentrations, causing water to move out of cells.

b) Enzyme-controlled processes cannot be carried out properly because a build up of wastes increases cellular solute concentrations and alters other properties such as pH.

c) Enzymes would not function properly because they would combine with the waste molecules instead of substrate molecules.

d) Waste materials are used in respiration, producing too much heat inside cells.

3) Circle the passage below which best describes movement of materials in the phloem.a) Organic materials are actively loaded

into the phloem sieve tube cell at the ‘source’ and then move passively along a pressure gradient to the ‘sink’ cells.

b) Water and mineral ions move from roots to shoots passively.

c) Organic material moves passively from roots to shoots in a process called translocation.

d) Water and mineral ions move in all directions in the sieve tube cells, using both active and passive transport.

4) White blood cells are:a) smaller than red blood cells but more

numerous.b) smaller than red blood cells and fewer in

number.c) larger than red blood cells and fewer in

number.d) larger than red blood cells and more

numerous.

5) Which of the following lists adaptations to temperature change only?a) Sweating and panting, salt glands on

leaves, dilation of blood vessels.b) Extensive root systems in plants,

constriction of blood vessels, layer of blubber in seals.

c) Large ears in African elephant, salt bladders in Atriplex bushes, swollen stems in some desert plants.

d) Dilation of blood vessels, layer of blubber in seals, counter-current exchange in salmon.

6) Which of the following lists the process involved in temperature control by the hypothalamus in the most correct order?a) Thermoreceptors in skin detect heat,

hypothalamus, peripheral nervous system, dilation of blood vessels and panting.

b) Hypothalamus, thermoreceptors in skin detect heat, peripheral nervous system, dilation of blood vessels and panting.

c) Dilation of blood vessels and panting, peripheral nervous system, thermoreceptors in skin detect heat, hypothalamus.

d) Thermoreceptors in skin detect heat, hypothalamus, dilation of blood vessels and panting, peripheral nervous system.

7) Which of the following statements about blood vessels is correct?

a) veins have thicker, more elastic walls than arteries

b) arteries carry deoxygenated blood around the body

c) capillaries are small enough to be in close contact with all body cells

d) blood is under higher pressure in veins compared to arteries

8) Circle the incorrect statement about the excretory system, below.a) The main nitrogenous waste in fish is

ammonia.b) The mammalian kidney helps to maintain

a constant blood solute concentration because it can reabsorb or excrete water and salts when required.

c) The walls of the collecting duct in the nephron are selectively permeable to water.

d) Reptiles, insects and birds excrete nitrogenous wastes as urea.

9) Which of the following gives a correct description of the adaptations of saltwater

and freshwater fish to their aquatic environments?

a)

b)

c)

d)

10) The graph below shows the relationship between internal body temperature and external temperature for four different organisms.

Freshwater fish Saltwater fishProduces concentrated urine, takes up ions through gills and salts are reabsorbed by kidneys.

Produces dilute urine, excretes excess salts through gills and kidneys, drinks large amounts of water.

Produces dilute urine, excretes salts through gills and kidneys, drinks large amounts of water.

Produces concentrated urine, takes up salts through gills and salts are reabsorbed by kidneys.

Produces dilute urine, takes up ions through gills and salts are reabsorbed by kidneys.

Produces concentrated urine, excretes excess salts through gills and kidneys, drinks large amounts of water.

Produces concentrated urine, excretes salts through gills and kidneys, does not drink water.

Produces dilute urine, excretes excess salts through gills and kidneys, does not drink water.

Which of the alternatives below is a list of endotherms only?a) X onlyb) Y onlyc) W and Xd) Y, Z and X

SHORT ANSWER AND LONGER RESPONSE QUESTIONS

11) a) Label A, B, C, D, E and F on the diagram of a kidney nephron, below.

.

b) In which areas, A, B, C, D ,E or F do the following occur?i) filtration

ii) Reabsorption of glucose and amino acidsiii) Reabsorption of wateriv) Formation of urine

c) Mark with the letter ‘X’ the region where ADH acts on the nephron.

d) Mark with the letter ‘Z’ the region where the hormone aldosterone acts on the nephron.

e) Compare the salt concentration in the blood vessels at ‘C’ with the salt concentration in the blood vessels at ‘D’ in the diagram. Give a brief explanation for this difference.

12) What is the main driving force behind movement of materials in:a) phloem sieve tubes?b) xylem vessels?

13) List the major uses for the following blood components:a) red blood cells b) granulocytes c) cryoprecipitated AHF d) plasma derivativese) platelets f) fresh frozen plasma

14) Give three reasons why the concentration of water in the mammalian body needs to be maintained at a constant level.

15) Briefly explain why estuarine organisms need to possess adaptations to extreme environmental fluctuations.

16) Explain in terms of the solute concentrations of the surrounding water why marine fish produce a much more concentrated urine than freshwater fish.

17) Define the following:a) ectotherm b) endotherm

18) Briefly describe two behavioural ways ectotherms such as desert lizards regulate their body temperatures.

19) Describe one way each of the following endotherms help to regulate their body temperatures:a) Mitchell’s hopping mouse b) Seals c) Red kangaroo

W

X

Y

Z

A

B

CE

F

D

F

20) Describe the forms in which each of the following is carried in the human blood:a) carbon dioxideb) waterc) saltsd) lipidse) oxygenf) carbohydratesg) nitrogenous wastesh) proteins

ANSWERS

a) MULTIPLE CHOICE

1. b); In Addison’s disease, insufficient amounts of cortisone and aldosterone are produced. Aldosterone can be replaced with the synthetic steroid, fludrocortisone.

2. b); A build up of dissolved carbon dioxide lowers the pH of the blood, thus affecting enzyme activity. A build up of urea in the blood alters solute concentrations and can be toxic.

3. a); This most accurately describes the movement of materials from ‘sources’(usually leaves, where photosynthetic compounds are produced) to ‘sinks’(areas that need organic materials such as sugars). This movement is not necessarily always in an upward direction, as suggested in alternative c).

4. c); The ratio of red to white blood cells is about 1000:1.

5. d); The other alternatives include adaptations to factors such as water shortage - e.g. swollen stems on desert plants, or high salinity - e.g. salt glands on leaves.

6. a)7. c)

8. d); These animals excrete nitrogenous wastes as uric acid.

9. c); Freshwater fish need to constantly get rid of water, as water tends to move into their bodies by osmosis. The opposite situation applies to saltwater fish.

10. c); Endotherms are capable of maintaining a constant internal temperature despite changes in the external temperature.

b) SHORT ANSWER AND LONGER RESPONSE QUESTIONS

11. a) A - glomerulus; B - Bowman’s capsule; C - proximal convoluted tubule; D -Loop of Henle; E - distal convoluted tubule; F - collecting duct

b) i) glomerulus; ii) proximal convoluted tubule; iii) loop of Henle, distal tubule, collecting duct; iv) collecting duct

c) and d):

e) Salt concentrations at ‘C’ are lower than at ‘D’ because salt is reabsorbed into the blood in the loop of Henle.

12. a) The main driving forces are differences in osmotic pressure between the ‘sources’ and the ‘sinks’.

b) The main driving force is transpirational pull.

13. a) These are used in patients with chronic anaemia and in cases where the patient has experienced massive blood loss.

b) Granulocytes are used where patients do not respond to antibiotics.

c) This is used to control bleeding in sufferers of haemophilia and von Willebrand’s disease.

X

Z

d) Plasma derivatives are used to control specific cases where blood clotting is needed.

e) Used to treat thrombocytopaenis.f) This is used to treat bleeding disorders.

14. i) Water is needed as a solvent for chemical

reactions; enzymes will not function properly if substrate concentrations and pH are not optimal.

ii) Water is constantly needed to transport dissolved nutrients and wastes around the body; this transport and blood pressure will therefore be affected if water concentrations in the plasma are not maintained at a constant level.

iii) If water concentrations within cells become too low, there is a risk of water entering the cells rapidly by osmosis and bursting them.

15. Estuarine organisms must be able to tolerate fluctuating salt and tidal conditions.

16. Water tends to move out of marine fish by osmosis because the solute concentration in the surrounding water is higher than it is inside their bodies. As a result, they try to retain as much water as possible by producing concentrated urine. Some marine fish are in fact ‘aglomerular’; their nephrons do not possess glomeruli. This reduces the amount of water that is lost from the blood to the kidney. Freshwater fish tend to gain water by osmosis because the solute concentration in their bodies is higher than in the surrounding water. It is therefore beneficial to them to produce dilute urine so that they can lose as much water as possible.

17. a ) An ectotherm is an organism that cannot maintain its own internal temperature.

b) An endotherm is an organism that can maintain its own internal temperature.

18. i) During the morning when it is cooler the lizard flattens its body on warm surfaces such as rocks and exposes a large surface area to the sun in order to warm up.

ii) During the hotter parts of the day the lizard seeks shelter from the heat under rocks.

19. a) Mitchell’s Hopping Mouse is nocturnal and as a result avoids the extremely high temperatures that occur during the day.

b) Seals have a thick layer of fur and blubber that provide insulation in cold conditions.

c) The Red kangaroo has a large network of blood vessels on its forearms which it licks to enable evaporative cooling to occur.

20. a) In the form of dissolved bicarbonate ions.b) Mostly moves in the plasma in the form of

water molecules. c) As ions dissolved in the plasma.d) In the form of soluble fatty acids and

glycerol or as fat droplets in the plasma.e) On haemoglobin molecules in red blood

cells, forming ‘oxyhaemoglobin’.f) As soluble glucose molecules.g) As soluble urea.h) As soluble amino acids.