HSC – Biology – Maintaining a Balance

1) Most organisms are active in a limited temperature range

Identify the role of enzymes in metabolism, describe their chemical composition and use a simple model to describe their specificity on substrates.

Enzymes are biological catalysts, present in living systems. A catalysts is a substance that increases the rate of a chemical reaction without being changed itself. Enzymes like inorganic catalysts, function by providing an alternative pathway for a chemical reaction, thus allowing for metabolic reactions to occur faster which suffix the high-energy and changing conditions faced by an organism in day-to-day life.

The molecules that are flying around need to have a certain minimum amount of energy so that when they collide with other reacting particles, a reaction will occur. This minimum energy is known as the activation energy (EA).

In an enzyme-catalysed reaction, the molecules require a lower activation energy as shown in the figure below.

Because the reactants can reach lower activation energy values faster, enzymes are very effective at increasing reaction rates by 10^9 times – 10^20 times compared to non-catalysed reactions.

Chemical Nature of Enzymes

Most enzymes are proteins. There are a small number of RNA molecules that can also act as enzymes in certain cases. A protein is a long chain of amino acids joined by peptide bonds (called a polypeptide) folded into a 3-D shape. The folding affect brings distant parts of the polypeptide chain close together in order to form an ‘active’ site. A certain area on the protein molecule is able to attach to the reactant molecules. The reactant in an enzyme-catalysed reaction is called the substrate. The area of the enzyme that binds to the substrate is called the active site.

More than often, the enzyme molecule is a large protein and is much bigger than the substrate (which allows for the entire reactant to react – thus increasing efficiency of the enzyme).

Naming and Classification of Enzymes

- Named for the substrates eg. Lipids – Lipase. (however this is not always the case)- They have the suffix ‘-ase’

Enzymes can be classified on the basis of the type of reaction that they catalyse. There are six groups:- Oxido-reductases - catalyse oxidation and reduction reactions- Transferases – Transfer chemical groups from molecule to molecule- Hyrdolases – Catalyse hydrolysis reactions (breaking down of molecules + addition of water)- Lyases – Catalyse reactions in which a double bond is removed or added- Isomerases – Catalyse reactions in which isomers are converted into another shape- Ligases or (synthetases) – Catalyse the connection of two molecules

Specificity of Enzymes

Scientists have found that enzymes are highly specific, which means one enzyme catalyses one substrate and only that substrate. This is because the shape of the active site of the enzyme exactly matches that of the substrate.

An enzyme acts by combining with the substrate and forming an intermediate complex of the enzyme and substrate. The reaction takes place at the binding site of the enzyme and substrate called the active site. When the products are formed, they leave the enzyme surface. This leaves the enzyme free to catalyse more substrate. Thus a small amount of enzyme can catalyse a large amount of substrate. This model is called the lock and key model, since the enzyme and substrate fit together like a key in a lock. See below.

The lock and key model assumes that an enzyme has a rigid unchangeable shape. Other models assume that the active site is more flexible and can be changed by the substrate binding to it. It is probable that many enzymes operate to the lock and key model while others operate according to the induced fit model.

Whichever model explains the action of a particular enzyme, the specificity of all enzymes is due to the matching of the shape of the active site of the enzyme with the substrate. This means that there is a specific enzyme for every reaction and that an organism has thousands of enzymes.

Cofactors and Coenzymes

Cofactors are small molecules that help enzymes to act. If they are inorganic such as zinc or calcium ions they are known as cofactors. If they are organic molecules such as vitamins, they as known as coenzymes. Many enzymes have cofactors or coenzymes without which they cannot catalyse the reaction. The coenzyme usually binds to the active site and the cofactor is either bound to the enzyme (or coenzyme if applicable).

The essential role of enzymes in metabolism

Most reactions that occur in the body are not in sufficient temperature or pH conditions in order to take place, this is where enzymes catalyse those reactions in order to keep organisms functioning.

For example; Respiration: In respiration the glucose is oxidized and the energy stored in its bonds is released and trapped into ATP. Without enzymes the reaction has a very high activation energy and will only occur at extreme temperatures. But conducting these reactions at these high temperatures not only damages the cell, but also the energy is released at once, thus it cannot be trapped and used by the cell. Enzymes however, reduce the activation energy allowing the reaction to occur at moderate temperatures.

Furthermore: A large amount of the food that is consumes provides compounds that cells cannot use directly. The compounds the cell needs are synthesized in a series if steps called metabolic pathway. Every step in this pathway it catalysed by different enzymes. Every enzyme in the pathway contributes to the synthesis and is specific for its own substrate. If one enzyme is missing or defective then the entire pathway is affected. Enzymes work in teams in metabolic pathways to produce end-products.

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

Acidity is caused by the presence of hydrogen ions (H+) in a solution. In biological systems, the concentration of hydrogen ions is low but changes in the concentrations can affect living systems; including enzymes.

pH is a way of describing the acidity of a substance. A pH of 7 is neutral. This means it is neither acidic nor alkaline. High acidity is shown by a low pH level. Enzymes each have an optimum pH for activity. For example, digestive enzymes in the stomach work best under conditions with a pH of 6. Changing pH from the optimum reduces the enzyme’s activity.

pH is defined as the negative log, to the base 10, of the H+ concentration. For example, if the H+ concentration is 0.01 or 10^(-2) molar, then the pH is: -log1010-2 = 2

This enables the pH scale to be set up for dilute solutions of hydrogen ions. It ranges from 1-14 and allows for whole numbers rather than very small numbers or numbers with negative indices.

Note: The higher the hydrogen ion concentration, the lower pH and vice versa.

identify data sources, plan, choose equipment or resources and perform a first-hand investigation to test the effect of:

o increased temperature o change in pH o change in substrate concentrations on the activity of named enzyme(s)

Effect of Temperature

At low temperatures the activity of the enzyme is low. This is because most of the molecules, both enzyme and substrate have low energy. As the temperature rises, more molecules reach the activation energy required and the reaction rate increases.

At a particular temperature (approximately 40C for most mammals), the enzyme is most active. This is known as the optimum temperature. All the enzymes molecules are acting at the maximum capacity. As the temperature increases further, the activity of the enzyme falls rapidly. This is because the heat energy has been able to break the bonds that cause the proteins to fold, thus destroying the active site. This means that the substrate no longer has an active site with which to bind, so there is little or no activity. This process is known as Denaturation of the protein (enzyme). At high temperatures almost all molecules become denaturated at the same time hence the rapid decrease in activity (see graph below). Denaturation by heat is usually irreversible and the active enzyme cannot be restored.

Effect of pH

There are charges on the surface of an enzyme molecule. This charge pattern is essential in keeping the molecule folded so that the active site is functioning properly. The charge pattern is determined by the charges of the ions (including the hydrogen ions) in the solution in which the enzyme is dissolved. An enzyme cal tolerate a small range of hydrogen ion concentrations within which it is active. Within this range it is most active at the optimum pH because this is the pH that maintains the active site most efficiently. If the pH varies too much on either side of this optimum, then the shape of the enzymes is distorted and they lose activity.

Note: Changing the pH slightly on either side of the optimum pH will change enzyme activity but it is reversible. But extreme variations cause irreversible changes.Substrate Concentration.

In most enzymes reactions the enzyme concentration is much lower than that of the substrate; there are many more molecules of the substrate present than the molecules of the enzyme. So if the enzyme concentration is increased then the rate of the reaction increases linearly.

However it is a different picture when the substrate concentration is increased and the enzyme concentration remains constant. Initially increasing the substrate concentration increases the rate but a point is reached after which the rate remains constant even if the substrate concentration is increased further. At this saturation point (called the Vmax or the maximum velocity) the active sites on the enzyme molecules are all occupied by the substrate and the reaction is proceeding at its maximum rate. Further increase in the substrate cannot increase the rate because there are no more active sites available to which the substrate can bond.

Enzyme catalysed reactions in living systems proceed under conditions in which the substrate concentration is greater than the enzyme concentration, but they don’t exceed the maximum velocity.

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

If the internal environment (for example the temperature, pH and substrate concentration) does not remain stable then the rate of enzyme catalysed reactions decreases. This decreased rate could affect an entire pathway that might produce an essential compound such as haemoglobin.

Substrate Concentration: As previously mentioned above, when the substrate concentration becomes very high, the rate of reaction does not increase as the enzyme is working to its full potential and there are no free active sites. Thus to further maintain efficiency of the metabolic pathway, as each product is made, it is removed by the next enzyme as it becomes the substrate for the next enzyme. This prevents the accumulation of product. A process known as feedback in which the activity of key enzymes is regulated, is a major control process of metabolism (will be discussed later on). These feedback systems also exist to control temperature and buffers maintain a relativity constant pH.

The organism has mechanisms that enable an enzyme to operate at its optimal capacity by providing an environment that has a relatively constant temperature, pH and substrate concentration. These constant conditions are necessary for enzyme efficiency. Since the activity of enzymes influences the outcomes of metabolism and metabolic pathways, constant internal environmental conditions equate to optimal enzyme activity and metabolic efficiency.

describe homeostasis as the process by which organisms maintain a relatively stable internal environment

The human body is capable of maintaining a constant internal environment, regardless of the outside external environment. Blood sugars remain close to 90mg/100mL blood, body temperature approximates to 37C and the blood pH must be within 7.38 – 7.42.

The word homeostasis means staying similar or unchanging and it refers to the constant internal composition or steady state of an organism.

It also includes the processes by which this composition is maintained. The idea of homeostasis was first introduced by Claude Bernard but the final term and definition was provided by Walter Cannon. The condition of homeostasis is well researched as it enables these animals to adapt to nearly every environment on Earth.

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

Self-regulating non-living systems:

Some non-living systems resemble homeostasis in living systems, for example a laboratory water bath.

This laboratory system works as follows:

- The required temperature is set on a thermostat- Water bath is switched on- A sensitive thermometer (detection system) detects changes in the temp. of the water and

send the information (or feedback) to the thermostat (control center).- In a thermostat, the temperature is compared with the set temperature. It then activates the

water heater (regulating device) so that the required temperature is achieved.- If the temperature is less than the set temp, then the thermostat switches on the water

heater and similarly if temp is greater.- There is a constant relay of information (feedback) between control center and

detection system.

The essential parts of a non-living self regulating system are:

- A system in which a set value must be maintained- A detector device, which sends feedback to a control center- A control center, which responds to the feedback- A regulator, which is operated by the control center to any correct deviations from the set

value.

Living Systems

Just as a self-regulating system fixes or controls the set point of a variable such as the temperature, a homeostatic system controls the set point of a variable such as the blood glucose concentration or the blood pH. Just as the self-regulating system has three parts (the control center, regulator, and detector), the homeostatic system has three main parts as well – Receptor, Control Centre, Effector.

The receptor plays the same role as the detector. It is a type of sensor that monitors changes in the internal and external environment (called stimuli) and initiates a response to them. It does this by sending a message to the control center. The control centre, which is programmed to maintain the set value,

analyses the message and determines the appropriate response. The control centre causes a response to be sent out via an effector which can restore the set value. Thus an animals homeostatic control systems maintain internal conditions within a range in which metabolic processes can take place.

Note: The messages from each part to another are also known as Feedback.

Thus there are two stages in a homeostatic system:1. Detecting the changes from a stable state carried out

by a receptor.2. Responding to the changes carried out by the

effector.

Both changes involve the control centre in analyzing information. The control centre thus plays a central role in the process.

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

Maintenance of homeostasis involves the detection of a change and the response to the change. The mechanism is called feedback because information about the change has been fed to the control centre, which then feeds back a response to the system in which the set value is being maintained. The result is that the set point remains stable within narrow range. This has major implications for the reactions central to metabolism.

It is in the second stage (the response to the change) that negative feedback differs from positive feedback. If a set value deviates then negative feedback instructs the system to restore the correct value. This means that negative feedback counteracts the change. Negative feedback is the most common type in biological systems. Positive feedback causes the system to re-enforce the deviation and change it further. Positive feedback is rare but does occur.

How negative feedback system works in mammals:

Feedback can be defined as the effect of changes to a product of a process on the process itself. The end product of the process has an effect in controlling the process so that it does not get out of control.

An example is the control of blood glucose by hormones. The hormone insulin (produced b the pancreas) reduces blood sugar levels. If the blood sugar rises, the raised levels are detected by specialised receptor cells, which pass a message to cells in the pancreas. The pancreatic cells produce more insulin which is released into the blood. The insulin reduces the blood glucose level to the normal again.

Control systems for homeostasis

The importance of homeostasis is that it maintains the concentration of metabolites or physical conditions within a narrow range that enables cells to function efficiently. Control and regulation of this state is by negative feedback to the nervous or endocrine (hormonal) system.

The endocrine system is a system of endocrine (ductless) glands that produce hormones. Hormones are chemicals that travel in the blood to target organs where they will have affect.

Note:

Control is the ability to change a process within the internal environment.

Regulation is the actual manipulation of factors in the internal environment so that a set value is maintained within certain limits

For example, while the nervous and endocrine systems can control the internal environment of the body, the functioning of body organs actually regulates the values to be maintained.

Hypothalamus

The Hypothalamus is a control centre in the brain which maintains homeostasis in the body when the nervous and/or endocrine systems are involved.

The features of this control mechanism are as follows:- A change in external or internal environments are detected by the receptor cells- The receptors are directly connected to the neurons, which relay the change to the hypothalamus.- Hypothalamus directly controls the regulatory organs through the nervous system. In some cases it also

controls an endocrine gland that through its hormones controls an organ.

Some things that must by kept constant within a body include:- pH of blood and tissues - Blood glucose concentration - Blood pressure - Body temperature - Water balance in blood

Some regulatory mechanisms to do this are:- Perspiration - Urination- Heart Rate - Synthesis and breakdown of molecules- Blood flow

l outline the role of the nervous system in detecting and responding to environmental

changes

Physical factors and Body Temperature

Insensible perspiration is when a organism will perspire (i.e. lose water from their skin) without physically being aware of it.

Sensible Perspiration is when an organism can feel the moisture on their skin when they perspire.

Heat is a form of energy and tends to move from areas of high heat to areas that are low. The greater the heat difference between the two areas, the faster will be the heat flow between them.

The quantity of heat is the total amount of heat in a body. The temperature is the level of heat in a body and it is directly related to the average kinetic energy of the particles in the body.

When water leaves the body as perspiration water is converted from liquid to gas. This requires an input of energy, and this energy is the heat energy provided by the body. In this way heat energy is lost by the body and thus it cools down.

Note: Alcohol evaporates faster than water and the conversion of liquid alcohol to vapour takes heat from the body more quickly than water does, thus cooling the body faster.

Source of body heat

Humans are endotherms which means that they are able to control their internal body temperature by altering their metabolic rate. Body cells continuously produce heat energy as an end product of metabolism. In birds and mammals the heat released through respiration is controlled by a thermostat so that the temperature of the body can remain at 37C

Core temperature and Shell Temperature

Two major regions of the body have different temperatures. The core temperature is the temperature of the bodies organs. The shell temperature is the temperature of the skin and the peripheral tissues (near the skin). The core temperature is maintained around 36.2C despite major fluctuations in the external environment. Heat flows from hot areas to cold areas and blood is a major agent that transfers heat between the body core and shell.

Role of hypothalamus in regulating Body temperature

The brain and nerves regulate body temperature by negative feedback mechanism. This is called thermoregulation. The major thermoregulatory centre is in the hypothalamus of the brain. Two centers comprise the thermoregulatory centre of the hypothalamus:

- The heat loss centre towards the front (anterior) of the hypothalamus- The heat-promoting or heat-gain centre behind the hypothalamus

Effectors in thermoregulation and how they maintain homeostasis

In a case of too much heat production, receptors detect the excess heat. Nerve impulses pass this information from the receptors to the hypothalamus. The hypothalamus activates effectors by nerve impulses from the hypothalamus to the effectors which may be blood vessels, sweat glands, endocrine glands or skeletal muscles.Actions of effectors in cooling the body

Excessive temperatures can damage body cells. Core temp may rise above normal when large amounts of heat are produced in respiration or when the outside temperature is very high. Heat receptors in core tissues detect this and inhibit the heat promoting centre of the hypothalamus. At the same time the effectors are triggered.

The effectors are blood vessels and sweat glands which act in the following manner:

- Blood vessels at the periphery of the body dilate. This allows blood to flow to the skin allowing heat to radiate from the skin surface to the air. There is also a system of countercurrent heat exchange that involves the loss of heat from arteries to veins. However the blood is transferred to superficial veins where the heat needs to be lost and to the deep veins where heat must be conserved

- Sweat glands are activated by nerve impulses and they release large amounts of perspiration, thus cooling the body. This is less desirable as it might lead to dehydration. Furthermore it only works well when the air around is dry.

Behavioural activities supplement physiological activities, including the consumption of more water, less active, cooler environment or wearing lighter clothes.

Actions of effectors in heating the body

When the external temperature is low, the shell temperature and the core temperature can fall to a level at which there is insufficient energy for enzyme activity. This in turn reduces the metabolic rate so that normal bodily functions cannot be maintained.

Firstly cold receptors detect a low shell temperature. A nerve impulse from these receptors activates the heat-promoting centre in the hypothalamus, which triggers some of the following: The blood vessels near the skin, nerve fibre endings that produce nor-adrenaline, skeletal muscles or the endocrine glands.

The effectors are then stimulates by the nerves that lead from the hypothalamus to act as follows:

- The blood vessels near the skin are constricted, which confines the blood to internal areas of the body thus conserving heat and maintaining core temperature.

- Nerve ending release nor-adrenaline (nor-epinephrine) which increases metabolic rate particularly respiration. Thus generating more heat.

- Skeletal muscles begin shivering involuntarily if the above two mechanisms are not sufficient to regulate the temperature, this also produces heat.

- In cases when the environmental temperature decreases gradually such as in the change of season, specialised cells in the hypothalamus release thyroid-stimulating hormone. This travels the to the thyroid gland via the blood which releases the hormone. The hormone is responsible for increasing the metabolic rate and therefore heat production.

Besides these voluntary adjustments, humans also use behavioural modifications such as:- Extra clothing - Huddling up to expose less surface area to air- Consumption of hot drink - Increasing physical activity

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

Range of Temperatures over which living things exist and are active

It is important to distinguish between the range of temperatures over which living things can exist and the range over which they are active.

The range at which most organisms can survive and be active lies between 0C and 45C. This is because living cells are restricted to a narrow range of temperatures. If cells cool to below 0C, there is a risk that ice crystals may form within them. The upper limit of 45C is due to the Denaturation of proteins above 45C. There are exceptions to this – for example thermoacidophiles exist in temperatures around 120C and die if the temperature is less than 55C as it is too low for them.

Each species is most comfortable at a temperature range which is narrower than the range for living organisms, and this is due to the fact that its structural, behavioural and physiological adaptations allow it to thrive in this range. Within species there might be variations, especially due to size as larger bodies lose heat more slowly and thus are more suited to the colder environment.

compare responses of named Australian ectothermic and endothermic organisms to changes in the ambient temperature and explain how these responses assist temperature regulation

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

Heat gain loss may occur internally (to and from the core) or externally (to and from the external environment). The periphery of the body is the exchange region and the shell temperature may vary due to this loss and gain.

Internal heat is usually due to the heat produced by respiration in body cells and tissues. Respiration will increase due to two main factors, the increase in muscular activity or increased activity of hormones. Blood transfers from areas of heat gain to cooler areas.

External heat loss or gain occurs by four physical processes: Conduction, Convection, Radiation and Evaporation.

Conduction is the direct transfer of heat from one particle to another by direct contact.

Convection is the transfer of heat by movements of particles of a liquid or gas.

Radiation is the heat transfer mechanism by which the suns rays travel across space to warm Earth. It is the transfer of heat by rays, it occurs in gases and in a vacuum (space). Radiation can either heat or cool the body depending on the heat difference.

Evaporation is the loss of heat when a liquid (usually water with salts) is changed from liquid to gaseous state.

Cold blooded animals are referred to as ectotherms, which means their temperature is the same as the external environment. Warm-blooded animals are called endotherms because their temperature is internally controlled by the organism.

The major distinguishing factor between endotherms and ectotherms is that endotherms are able to regulate their core body temperature via a feedback system. Ectotherms do not have this system and rely on structural and behavioural adaptations to control internal temperature.

Ectotherms in Water

Water temperatures change very little. Therefore aquatic organisms are typically ectothermic. Their metabolism is suitable for functioning at the temperature of the surrounding water, and since that remains relatively constant, there is no need for adaptations to regulate body temperature

Endotherms in Water

Aquatic organisms that have evolved from land-dwelling ancestors are endotherms including penguins, whales and dolphins. Water is a much better conductor of heat than air and this makes it difficult to retain high body temperatures in water. To combat this, organisms have layers of fat, called bubbler, which acts as insulation preventing rapid heat loss through conduction. Furthermore, since bubbler is less dense than water, it provides additional buoyancy and helps to support the animals body in water.

Endotherms Ectotherms

Red KangarooPygmy PossumEmuBush RatHopping MouseDingoKookaburra

Frilled lizardRed-back SpiderAnemone fish (clown fish)StarfishJellyfishRed bellied black snakeGreen tree frog

Many birds and mammals that spend long periods of time in water (like ducks), secrete oils that coats their fur or feathers. This insulates the skin from the water and prevents excessive heat loss. The grooming behaviour distributes the insulating oils evenly over the whole body.

Being endothermic is generally a disadvantage in an aquatic environment. However these endotherms are able to survive in a range of aquatic environments, and thus can migrate in the sea. Ectotherms would need to be able to tolerate enormous variations in body temperature to embark on similar journeys.

Ectotherms on Land

Sun-baking: Ectothermic terrestrial animals rely on behavioural adaptations to regulate temperature. Lizards and insects sun-bake in the morning to warm their bodies until they begin to function efficiently.

lizards and locusts flatten their bodies and turn side on to absorb heat while basking in the morning, but narrow their body to reduce the surface area exposed when they become warm.

Shade and Burrows: Animals may simply move out of the sun. Many desert animals inhibit burrows. In burrows the animal remains cooler because they are not directly exposed to the sun’s radiation, and soil heats up much more slowly than air; this means that the temp within the burrow remains constant.

This is particularly advantageous in the desert where the temperatures range from 40C in the day to subzero at night. Burrowing also reduced water loss. The air in the burrows is humid due to the water lost by the animal, this means that less water is lost by the organism when the burrow.

Nocturnal Animals: Many animals in desert environments are active at night, usually at dusk or dawn. At these times the temperature is lower during the day but higher than at night. This way the organism avoids extreme temperatures and can forage for food in relatively mild conditions.

Dormant states of reduced metabolic rates: Many ectothermic animals are unable to continue to function normally under conditions of extreme cold. They simply close down for the winter and wait for the warmth of spring.

Reptiles go into a state called torpor, where the metabolic rate is slowed and their temperature can drop below freezing. Lizards have a substance in their blood that acts like antifreeze, which prevent blood freezing.

Insects behave in the same way, but their inactive state of slowed metabolism is called diapause. Some insects will enter diapause even at the larval stage. Others will feed heavily as adults and spend the winter in diapause. Ladybirds for example fatten themselves and spend the winter in diapause in mountains or other isolated environments.

Endotherms on Land

Endotherms may use behavioural adaptations to regulate their body temperature in the same way as ectotherms. However, they also have a variety of physiological and structural adaptations that allow them to maintain a constant body temperature despite fluctuations in their surroundings.

Surface Area to volume ratio: In hot environments, both endotherms and ectotherms tend to have a larger surface area to volume ration than animals in cold environments. This provides a large area for heat loss and the core of the body can lose heat easily. Small bodies in cold environments, favour the conservation of heat. Generally, larger animals are common in colder climates as they have a low SA:V ratioKangaroos however, have the added structural adaptation of large ears which act like radiators to lose heat.

Changing patterns of blood flow: In many animals, blood vessels at the extremities of the body are arranged to control heat loss. Desert animals are adapted to increase heat loss. When the conditions are hot, blood vessels to the extremities of the body dilate. This increases blood flow to areas where the heat can be easily lost (those parts that close to external contact points). For example the forearms of kangaroos contain a dense network of blood vessels adapted for heat loss. Kangaroos provide additional cooling by licking their forearms when temperatures are extreme.

In cold environments, the blood vessels are adapted to reduce heat loss. When the conditions are cold, the blood flow to the extremities is reduced. This reduces blood flow to the parts of the body in close contact with environment and decreases heat loss.

Cooling by evaporation of water: Many animals allow water to evaporate from their body to keep them cool. Humans release water from sweat glands. Animals often sweat but fur makes the evaporation process inefficient in reducing body temp. Panting provides an excellent alternative to sweating for such animals. It also results in less water loss since the evaporation occurs on the internal respiratory surfaces.

Desert animals often don’t prefer this method as they are in a state that lacks water. However, some animals such as red kangaroos will allow their body temperature to increase by a few degrees higher than normal instead of losing water.

Insulation – fur and feathers: They provide insulation from the surrounding air and thus reduce heat loss. When it is cold, fur tends to stand up. Air is trapped between the hairs and warmed by the body, surrounding the animal with a layer of warm air. In hot environments the fur insulates from the hot air around them. Fur can be arranged to reflect solar radiation in hot environments, whereas the fur is alternately arranged in cold environments to absorb the radiation.

Regulating Metabolic Rates: Animals frequently alter their metabolic rates to regulate body temp. Many animals shiver when they are cold. Increase increased metabolic rate and rapid vibration of the muscles produces heat which warms the body. The alternate is true in lack of activity. Kangaroos for example tend to remain crouched and still in the shade under conditions of extreme heat , and many desert animals sleep in burrows during the day.

Reduced metabolic rate can also be effective in cold environments. Some endotherms such as bears hibernate. During this, their temp drops as metabolic rate slows, this allows for survival in winter as they need little food to supply energy needs.

identify some responses of plants to temperature change

Extreme temperatures can affect the structure and metabolism of plants just like they do for animals. For example during high temperatures, plants can become dehydrated and plant tissues may be damaged.

For photosynthesis to occur, the stomates must open. This happens in conditions of light and heat. When this occurs, water leaves the plant through stomates in the leaves. Therefore the plant must possess adaptations to minimize transpiration, while maximizing photosynthesis.

Surface Area: Desert plants typically have a reduced surface area, which decreases water loss and reduces the area for the absorption of solar radiation. In addition plants may have shiny or hairy leaves which reflect solar radiation and reduce heat.

Evaporation of Water: This cools the plants but often the plant has low amounts of water and also needs to cool down. Eucalyptus are particularly well adapted to attain a balance between pressure of water loss and photosynthesis. Their leaves hang down vertically. In this way they provide a large surface area for the rising sun in the morning when it is cool and a low SA to hot midday sun. Furthermore, their stomates may open in the morning and if the plant is suffering from water stress, they may close for the rest of the day.

Plants must be able to tolerate changes in cells near the external surface of the plant in order to survive. Extra From Jacaranda – HSC – Biology

Effect of pH on enzymes

Most biological fluids have a pH between 6 and 8.

Enzymes that act with human cells generally have an optimum pH of about 7.6

Effect of Temperature on enzymes

Most human enzymes have an optimal temperature of about 37 C, which is normal body temperature. At high temperatures, enzymes are permanently denatured – their structure is permanently changed and they remain inactive when the temperature returns to normal.

Enzymes that are inactivated because of low temperatures become active again when the temperature is returned to normal. This feature is used to preserve enzymes. They can be frozen quickly at very low temperatures and stored until they are required.

Effect of Enzyme concentration on rate of reaction

Only a very small number of enzyme molecules are involved in a reaction and these produce a given amount of product per unit time. If the amount of enzyme is increased, the amount of product made per unit time increases. However, there is no change in the total amount of product produced. Enzymes molecules are not used up in the reaction and are available for reuse.

Effect of inhibition on rate of reaction

Other molecules may compete with the normal substrate for the active site of an enzyme. This other compound may combine permanently with the active site of the enzyme, interfering with the normal

substrate-enzyme reaction and inhibiting formation of the normal product. Enzyme inhibition can cause death.

Extra and Intracellular Fluids

Both tissue fluid and plasma are located outside the cells and can be labeled as extracellular fluids. In contrast, the fluid called cytosol, that is located within all cells, can be labeled as intracellular fluid.

Tissue fluid and plasma are separated from each other by the thin flat cells that form the walls of the capillaries , the smallest vessels of the blood circulatory system.

Extracellular fluids are separated from intracellular fluids by the semi permeable cell membranes .

Exchange occurs between fluids

Although the tissue fluid and plasma that make up the internal environment are separated, continuous exchange occurs between them. For example:

- Nutrients and gases (such as glucose and oxygen) pass from the blood to the tissue fluid- Waste products (CO2 and Urea) pass from tissue fluid into the blood

In addition, many substances can mover between the cells and the fluids of their environment. For example:

- Waste products pass from inside cells to the tissue fluid- Glucose, amino acids and other compounds pass from tissue fluid into cells.- Oxygen, required for cellular respiration, can pass from tissue fluid into cells.

Because two-way exchanges of many substances can occur between cells, tissue fluid and plasma, the make up of one fluid can be affected by that of another fluid.

Blood is readily accessible by body tissue. Therefore, blood tests can provide info about the states of cells in other parts of the body.

Alarm is raised if abnormally high/low levels or certain substances are detected in the blood as this means that these levels are also present in tissue fluid and other cells of the body.

Systems that contribute to homeostasis

The hormonal and nervous systems are the major systems responsible for the control and co-ordination of homeostasis. In addition, various types of behaviour of organisms contribute to the maintenance of homeostasis.

There are other systems which help maintain homeostasis. They include:

1) Nervous System

Receives and transmits information about both the external and internal environment. Transmits electrical impulses to body cells that respond in various ways.

2) Endocrine (hormonal) System

Produces hormones that are secreted directly into the bloodstream and transported throughout the body where they regulate cell activities.

3) Respiratory System

Obtains oxygen from air and eliminates carbon dioxide which is a waste product of metabolism of cells. Assists in regulation of pH through removal of carbon dioxide

4) Circulatory System

Transports O2 to cells, CO2 away from cells, and hormones, wastes and nutrients such as glucose, aminoacids and fatty acids throughout the body.

5) Digestive System

Obtains nutrients, water and salts from the food we eat. These are transferred to blood and lymph vessels in the intestine wall. Undigested residue is eliminated

6) Excretory System

Removes wastes such as urea, excess water, salt and other ions from the blood and eliminates them from the body in the form of urine. Important in regulating pH.

7) Integumentary (skin)

Barrier between body and external environment. Evaporation of sweat is important in temperature regulation. Also inhibits entry of micro-organisms

Structure of the Nervous System

The nervous control system is composed of the brain, spinal chord and all the nerve cells connecting these to other parts of the body. The brain and spinal chord form the central nervous system (CNS). All other nerve cells in the whole part, that lie outside the central nervous system form the peripheral nervous system (PNS).

Central Nervous System

The CNS include the brain and the spinal chord. Bone protects the CNS – the skull surrounds the brain and the vertebrae surround the spinal chord.

The largest part of the brain is the cerebrum, which has a folded surface called the cerebral cortex. The cerebrum is divided into two halves, called the cerebral hemispheres, which are connected.

The thalamus and hypothalamus lie deep within the brain. The thalamus receives impulses from sensory neurons and directs them to the various parts of the brain where they are interpreted. The hypothalamus regulates the release of many hormones as well as controlling many other aspects of homeostasis. The hypothalamus plays a role in temperature maintenance, water balance and blood pressure as well as sensations such as hunger and thirst.

Nerve impulses that pass from sensory detectors to the brain and impulses that pass from the brain to other arts of the body travel along the spinal chord.

Peripheral nervous system

The PNS has two parts: the sensory division and motor divisions.

The sensory (afferent) division transmits sensory information about the external and internal environment to the CNS where the information is processed.

The motor (efferent) division transmits information away from the CNS.

The sensory division:

The sensory division of the PNS monitors and informs the CNS of events happening both inside and outside the body. Two kinds of neurons carry out this function, somatic sensory neurons and visceral sensory neurons.

The motor division:

The motor division of the PNS transmits impulses away from the CNS to muscles and glands, called effector organs. The motor division has two distinct systems.

The somatic nervous system transmit messages to the skeletal muscles. It is also called the voluntary nervous system because we can control our skeletal muscles.

The automatic nervous system transmits messages to smooth muscle, heart muscle and glands. This is incontrollable – hence involuntary nervous system. The sympathetic nervous system and the parasympathetic nervous system.

Nerve Cells

Nerve cells are the basic units of the nervous system. Nerve cells are also known as neurons. A typical neuron has a cell body which contains the nucleus.

Extensions arise from the cell body of a neuron. The extension that carries information away from the cell body to another neuron or tissue is known as an axon. In the human body, axons vary in length from a few

millimeters to over a metre and they may branch. Connecting and effector neurons also have extensions known as dendrites. These are highly branched extensions of the cell body that receive information from other neurons and carry information towards the cell body.

Three basic kinds of neurons are found in nervous system.

Affector neurons may have one or more receptors that detect change in either the external or internal environment. Information detected is transmitted as an electrical impulse to the CNS by the affector neuron. Effector neurons carry impulses away from the CNS to muscle cells or glands and cause them to respond. Connecting neurons are typically located in the CNS and link sensory and effector neurons.

An impulse is transmitted across the small gap from one neuron to another or from a neuron to a gland or muscle by one of several chemicals known as transmitter substances and produced at the axons.

Hormones – The Endocrine System

Endocrine glands are also called ductless glands. This is because all hormones are secreted directly into the bloodstream.

Ways of Losing Heat

- Radiation: Heat, in the form of infra-red heat rays, radiates from the body in all directions. This accounts for more than half of the heat lost from a person

- Conduction and Convection: Direct contact with objects and the transfer of heat into them, accounts for about three percent loss of heat from the body. More heat is lost from the body into the air.

- Evaporation: About 20% of heat loss from a body occurs through the skin and lungs when water evaporates from those surfaces.

Ways of Gaining heat / Reducing heat loss

- Heat produced by shivering: Shivering is the alternate contraction and relaxation of small muscle groups and is an involuntary action. The hypothalamus contains a centre for shivering that activates somatic motor neurons that control muscles in the upper limbs and trunk. When muscles shiver, almost all of the energy of contraction is converted into heat energy

- Heat produced through metabolism: Metabolic processes in the body produce heat. Neurosecretory cells in the hypothalamus produce thyrotropin-releasing hormone (TRH). This hormone is transported to the anterior pituitary where it stimulates the secretion of thyroid-stimulating hormone (TSH). This is transported in the blood to the thyroid, which in turn increases its output of thyroxine (thyroid). Thyroxine is a hormone that increases the metabolic rate of all cells of the body, resulting in an increase in heat production.

- Constriction of blood flow in skin: During cold periods, neurons in the hypothalamus send impulses via the nervous system to the peripheral blood vessels in the skin. The impulses cause arterioles to constrict. This constriction reduces the surface area across which the heat transfer can occur, and reduces the amount of blood flow close to the skin. Hence heat lost via the skin is reduced.

- Piloerection: This means hairs standing on end. This is not important for humans, but in animals it acts an insulation layer, as air is trapped in the erect hair of the fur. This creates an aura of warm air around the organism, hence reducing the amount of heat lost through radiation/conduction.

Situations where Body Homeostatic conditions are overwhelmed

Dehydration

A fluid imbalance leads to an imbalance of other factors in the body, such as an imbalance of ions. In particular, bicarbonate ions which are normally absorbed with fluid from the gut, will be lost. As a result, the extracellular fluid becomes more acidic and pH is disrupted.

Loss of fluid also leads to a reduced blood volume. Among other adverse affects, this prevents the kidney from filtering wastes from the blood at a sufficiently high rate.

Fever

Many substances, including toxins of some bacteria, can cause the set point of the hypothalamus to rise. These substances are called pyrogens. For example, if the hypothalamus registers a normal core temp of 40C, then it registers the previous normal (37) to be less and thus begins to generate heat through shivering and vasoconstriction. This generates the core temp to 40C and if prolonged can cause significant damage to cells and tissues.

Once, the pyrogens is removed, the normal set point is returned, heat loss mechanisms will then be activated, returning the body temperature to its normal level.

Aspirin can be used to work against fever, as it lowers the increased setting of the hypothalamus by the pyrogen.

Adaptations by organisms to survive in cold climates

Processes that are essential for life include chemical reactions that take place between substances that are dissolved in liquid water – that is, in solution. These processes cannot take place in solid water (ice). If all the liquid water in a living organism were replaced by solid water, life would be destroyed. When ice forms, the solid water expands. If cells freeze, the expanding ice crystals rupture the cell membranes and kill the cells.

Pure water freezes at 0C but water with dissolved material in it has a lower freezing point than this. For example, a very concentrated salt solution starts to freeze only when the temperature falls to about -18C. One strategy used by some living things to assist their survival in a very low temperature is to produce antifreeze substances.

Some frogs and toads burrow underground to avoid freezing temperatures

Birds and mammals living in the polar ice caps convert chemical energy present in their food into heat energy. This internal supply of heat keeps the body temperatures of these organisms well above freezing point of water.

The heat is retained by an excellent insulation: mammals have insulating layers of fat under the skin and thick fur and birds have layers of feathers.

Aquatic organisms

Countercurrent systems to warm blood

Whales and dolphins maintain their body temperature by using a counter-current exchange system. There is a fine network of vascular tissue with the fins, tail flukes and other appendages. An outgoing artery is paired with an incoming vein. Blood coming from the body core to the skin is warm. Blood flowing from the skin back to the body core has been cooled. In this countercurrent exchange system, heat in the blood coming from the core flows to the blood that is returning from the skin to the body core. This warms the blood flowing in from the skin and so prevents the venous blood from cooling the internal organs and muscles. At the same time, the blood moving out to the skin is cooled and so the loss of heat across the skin is reduced.

Whales and dolphins have a relatively small surface-area to volume ratio and heat loss across the skin is further minimized.

Water is a good conductor of heat and the steady stream of water through the mouth of a baleen whale has the potential to conduct a significant amount of heat away from the whales mouth.

This does not occur, as the whale has a large network of countercurrent system in their tongue and mouth linings. Branches of the carotid artery transporting blood to the mouth and tongue are each surrounded by a number of small veins that join to form the jugular vein, transporting blood back to the heart. Thus the heat is transferred to the veins going towards the heart, minimized the heat lost into the environment.

Countercurrent systems to cool blood

Although the core body temperature of many mammals is around 37C, this temperature is too high for sperm to mature and survive. Since the testes in a human are outside the core of the body the temperature is reduced (35.5).

Dolphins have testes within the body cavity. They have countercurrent systems to cool these internal testes and normal spermatogenesis can occur.

Blood is carried from the aorta to the testes by arteries that pass close to veins returning to the body from the dorsal fin. Blood in these veins is cooler than blood from the dorsal fin. Blood in these veins is cooler than blood in the arteries and so heat moves from the arteries into the veins. These arteries then transport the cooled arterial blood into the testes, cooling the tissue and normal sperm development occurs.

Plant responses to temperature change.

Hot environment

To prevent overheating, a plant must lose much of the radiant energy it absorbs. A plant does this in the following ways:

- Radiation: A plant radiates heat to objects in its environments- Transpiration – plants are cooled when heat within them is used to evaporate water from

cell surfaces.- Convection – air surrounding the plant becomes heated and hence is less dense then air

further away from plant. The heated air rises, carrying heat away from the plant.

Other factors that affect heat loss from, or heat gain by, a plant are as follows:

- Leaf Shape: Leaves are thinnest where the two surfaces of a leaf come together and lose most heat from that region. The larger the ratio of edge length to surface area of a lead, the faster the leaf will be cooled.

- Heat shock proteins – These proteins protect enzymes so that they are not denatured when the temperature rises

- Leaf orientation – In hot weather, leaves may be turned to have minimal exposure to the sun’s rays – less radiant energy from the sun falls on the leaf.

- Leaf fall – Leaves are dropped in hot periods, thus decreasing the surface area through which heat may be gained and water vapour lost through transpiration

Cold Environment

Many plants survive in sub-zero temperatures, but they do not produce antifreeze.

Water is transported through plants in xylem vessels and is subject to a number of forces. These forces affect the way in which water behaves in plants in freezing temperatures. As the temperature surrounding the plant drops below freezing, ice forms suddenly in the spaces outside the living cells of the plant. The inside of the cells doesn’t freeze because the concentration of ions in the cytosol is greater than the concentration outside the cell.

Because ice has formed, the concentration of water inside the living cells is higher than outside, so water moves outside the cells. The ice crystals grow. The movement of water out of the cells increases the ion concentration, hence reducing the freezing point. The more concentrated cytosol then acts as an antifreeze.

The ice crystals grow between the cells and do not damage the cell membranes which are pliable and bend under the pressure of the ice.

Ultimately, if there is an excessive drop in the surrounding temperature, ice crystals will form inside the cells which will die and so the tree may die. It has been suggested that an excessive drop in temperature damages the protein molecules that form parts of the cells membranes, so that ions leak out of the cell.

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Metabolism is the sum total of all chemical reactions occurring within a living organism. Each step of ametabolic pathway in cells is catalysed by enzymes.

Metabolism is divided into two: anabolic and catabolic. Those reactions that involve building up large organic compounds from simpler molecules are termed anabolic reactions, for example a large polysaccharide molecule such as starch being made from small monosaccharide units such as glucose, a product of photosynthesis in plants.

Chemical reactions that involve breaking down complex organic compounds to simpler ones are termed catabolic reactions. For example, in the digestion of food, large food molecules such as proteins are broken down into small units called amino acids, which can then be easily absorbed from the gut into the bloodstream. Chemical reactions may be classified according to whether they use up or release energy. Anabolic reactions are usually endergonic reactions, requiring an energy input. Catabolic reactions usually give out energy and so they are exergonic reactions.

Lock and Key Model and Induced Fit Model Diagrams:

The importance of maintaining a constant level of each variable

pH and temperature (for enzyme functioning)

All chemical reactions necessary for the cell’s survival and functioning are controlled by enzymes. Enzymes only function within a narrow range of temperature and pH; outside of these ranges, narrow variations cause ma decrease in the activity of enzymes whereas greater variations cause the enzymes to denature, rendering them non-functional. This reduces metabolic efficiency. Further problems with extreme temperatures are that:

■ very low temperatures could cause the water in cells to freeze. This brings about changes in the concentration of solutes in the cytoplasm, which in turn affect the pH and osmotic balance of the cell. When water freezes it expands and this may cause the cell and/or organelles to rupture (burst).

■ very high temperatures cause both enzymes and other proteins (such as those in membranes of organelles and the cell) to denature, further disrupting cell functioning and metabolic activity.

Metabolites

For any chemical reaction to proceed, reactants must be present. Metabolites are chemicals that participate in chemical reactions in cells. Some (for example, glucose and oxygen) are taken in from the outside

environment, whereas others are products of other metabolic pathways (for example ATP, the type of energy produced by chemical respiration). Many metabolic reactions rely on the availability of ATP energy in cells. If cells cannot produce sufficient energy, there is a ripple effect and other metabolic activity will be adversely affected. The production of energy relies on chemical respiration, which in turn relies on an ample supply of metabolites such as glucose and oxygen, as well as respiratory enzymes and their cofactors. A lack of any of these metabolites may therefore slow down or stop chemical respiration, affecting overall metabolic efficiency.

Water and salt concentrations (osmotic balance)

All chemical reactions in living organisms take place in water. For chemical reactions to proceed, the reactants must be dissolved in water— therefore the water concentration of cells and their surrounding fluid is of enormous importance. Dissolved substances such as salt affect the osmotic balance of fluids and so the concentration of slats and other dissolved substances must also be maintained within a narrow range.

An absence of toxins

A build-up of carbon dioxide and/or other wastes (as a result of chemical reactions in the cells) may be toxic to cells, affecting enzymes either directly or indirectly. Some interact directly by blocking the active site of enzymes, while others act indirectly by altering the optimal conditions for enzymeTEMPERATURE REGULATION

Plant responses to cold environments

Vernalisation: some plants flower in response to low temperatures; for example, tulip bulbs must beexposed to between 6 weeks and 3 months of intense cold before they will flower. Australian gardenersoften mimic this effect by removing tulip bulbs from the ground in winter and storing them in the refrigerator, before replanting them in spring, to ensure that they flower. Many responses of plants to temperature change (such as leaf fall and flowering) are the result of temperature and/or light changing the concentration of chemical growth regulators in plants. Responding to temperature change and the regulation of internal temperatures is important not only for the individual plant, but also for the continuation of the species.

Heat Shock Proteins: Heat shock proteins are produced by plants when they are under stress from very high temperatures. These molecules are thought to stop the denaturing of the enzymes (proteins) within the cell, so normal cell reactions can continue

Behavioural adaptations

The main behavioural adaptation seen in animals is that they alter the position of the body and increase or decrease the amount of exposure of their surface area to the sunlight. Many organisms will seek shade or shelter in burrows if the ambient temperate exceeds their tolerance level. Frill-necked lizards bask in the sun until they reach an adequate core body temperature and will then retreat into the shade. During the hottest part of the day the red kangaroo will seek shade and sit in a position where its hind legs and tail are shaded by the rest of the body — they are positioned at right angles to the body, with the tail pointing forward, to reduce the large surface area exposed to sun. The water-holding frog retires to a burrow in extreme temperature conditions. It survives hot, dry conditions by living in burrows below the surface.. In extremely arid conditions, it lives within a cocoon made from secreted mucus and its cast-off skin, which is shed after rain and then dries out, forming a waterproof covering. This minimizes exposure to heat as well as reducing water loss and dehydration.

Nocturnal activity is another common behavioural adaptation seen in animals that live in habitats where the daytime temperature is very hot. Nocturnal animals remain relatively inactive during the heat of the day, so that they do not generate additional metabolic body heat as a result of increased activity. (Increased activity must be supported by greater energy production, which relies on a higher metabolic rate.) Nocturnal activity is seen in many reptiles and birds that inhabit hot, arid areas and the few mammals that are able to survive desert conditions (for example, the bilby). Some organisms like the common wombat and the brown snake are diurnal, but change their normal active periods from daytime to night during hot weather.

Migration is another behavioural adaptation that can assist in the regulation of body temperature. Migrating organisms physically move to a different habitat that is within their tolerance range. The grey plover breeds in the Northern Hemisphere between May and August and then migrates to Australia over August and stays until April. This migration allows the birds to avoid severe weather during winter.

Structural adaptations

Structural adaptations that assist with temperature control include insulation such as fur, hair, feathers, insect scales and coats that enable a layer of air to be trapped to reduce the amount of heat lost. The feathers of the emu act as an insulator to reduce heat gain or loss. Blubber is another form of insulation to reduce heat loss from organisms living in water, such as the Australian fur seal. This significantly minimizes heat loss.

The surface area to volume ratio is also an important structural component of temperature regulation, as larger animals have a smaller surface area to volume ratio, which means they will not lose as much heat as smaller animals. Larger animals such as the common wombat have large, compact bodies that have relatively small surface areas from which they can lose their internally produced heat; therefore the wombat loses very little heat to its surroundings, which is mostly helpful in the cooler months.

Colouration of animals also assists temperature regulation, since dark colours absorb light (and associated heat) and so these animals can tolerate colder temperatures (e.g. the diamond-backed python)

Physiological adaptations

Physiological adaptations focus on the inner body functions. Metabolic activity is important for the functioning and the survival of individuals, but this activity also generates heat within the body. The rate of this activity can be altered to ensure that an individual has a better chance of surviving conditions below or above their tolerance range for temperature. Hibernation and torpor are examples where organisms lower their metabolic rate to conserve energy and, as a result, reduce the amount of metabolic heat energy that they generate within their own bodies. Another advantage of hibernation and torpor is that the organism requires very little food in this state because it does not need to expend large amounts of energy trying to regulate its body temperature by other means (e.g. shivering or sweating).

Hibernation is an extended period of inactivity in response to cold, where the body temperature does not drop below 30°C, but the heart rate and oxygen consumption drop considerably. (Oxygen consumption is a good indicator of metabolic activity involved in generating energy.) Hibernation is a form of mild torpor and is less intense, but may last for a longer period of time.

A state of torpor is a short-term hibernation where the body temperature drops much lower (below 30°C) and metabolism, heart rate and respiratory rate decrease, accompanied by a reduced response to external stimuli. Torpor may be part of a daily cycle of temperature change and, because the body temperature drops to almost the same temperature as the air around it, brings with it the advantage of a slower metabolism, in addition to helping them to conserve energy, which is in short supply as they do not eat and drink in this state. In contrast, the mountain pygmy possum hibernates during cold winters to reduce the amount of energy required to keep its body warm.

The common wombat slows its metabolism down to a third of its normal metabolic rate on hot days, particularly when sheltering in its burrow. This is a useful strategy, as wombats do not have sweat glands to assist in heat loss.

Organisms can also regulate the blood flow to increase or decrease the amount of heat lost to the surroundings. Since blood carries heat and usually the body temperature of an organism is higher than that of its surroundings, vasodilation of capillaries near the skin surface increases the amount of heat released. This mechanism is used in the red kangaroo (along with a behavioural adaptation of licking the forearm to increase heat loss as the saliva evaporates). Blood flow can also be increased or decreased at extremities to control temperature. The bilby has an extensive network of capillaries throughout the ear which aid in releasing heat to its surroundings. Furthermore, a mechanism called countercurrent exchange allows the warm blood in arteries (flowing from the heart towards the extremities) to heat the cooler blood in the veins coming back from the cold extremities, before this blood is returned to the heart. This occurs in the feet of platypus as well as the fins of the Australian fur seal, so that the internal core temperature is not lowered by cool blood returning from limbs that have a large surface area exposed to the cold water.

Change to colouration can occur in some organisms in response to exposure to high or low temperatures. As previously mentioned, colour plays a role in temperature regulation because darker colouration assists in the absorption of light to gain heat. If the colour of an organism can change, this enables it to live and remain active over a wider temperature range. For example, the male Australian alpine grasshopper, commonly referred to as the chameleon grasshopper, is a dark, almost black colour at temperatures below 15°C (for example, during the cool parts of the day such as morning) and as it basks in the sun it becomes a paler blue colour to reflect light and avoid overheating. Its blue colouration is typically seen at temperatures above 25°C.

As is evident from the above examples, some adaptations are a combination of structural, behavioural and physiological features. For example, a red kangaroo licks its paws to cool itself down through the evaporation of water on its skin. The location of many blood vessels near the surface of the skin in the forearms and paws is a structural adaptation; the dilation of arterioles in hot conditions to direct more blood flow through these vessels is physiological; and the licking activity to impart saliva for evaporative cooling is behavioural.

Extra From NSW Biology