HSC – Biology – Maintaining a Balance



HSC – Biology – Maintaining a Balance

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

• Explain why the concentration of water in cells should be maintained with a narrow range for optimal function.

Metabolism includes all the chemical reactions that occur in an organism. It involves catabolism, the breakdown of compounds to release energy and other compounds or atoms, as well as anabolism, the synthesis of new compounds from simpler ones. These reactions occur most readily in solution. Water is the predominant solvent in the body and, as many organic compounds dissolve in water, metabolism occurs in a watery solution. Water facilitates metabolism.

Water Balance is essential

Water is the most common compound in the animal body and is a necessary medium for metabolic processes to occur. An animal cannot avoid some water loss, such as the water vapour expelled in exhaled air, but various structural and behavioural characteristics enable loss to be minimized. The kidneys are the most important organs that ensure water balance is maintained in vertebrates. If an animal drinks copious amounts of water, more water is absorbed in the gut and enters the blood stream where it will be filtered out by the kidneys. The more water an organism drinks, the less concentrated the urine will be.

If water loss from an organism is not balanced by water intake, then dehydration occurs. In humans, if our water loss exceeds six to ten percent of body water, plasma volume falls and the circulatory system may fail. The loss of water from cells reduces their ability to retain compounds in solution. The inhibits metabolic processes so that they reduce so that they decline or cease altogether. Vital functions such as the removal of wastes by excretory systems, decline, wastes accumulate in tissues and death can result.

The maintenance of an appropriate concentration of water in an organism is vital for life. Water is an essential nutrient; we can do without it for only a short time. It is vital for the functioning of every cell and organ, it is used for body temperature maintenance, as a lubricant, and for the excretion of waste products from the body. Water provides no energy for the body.

The body obtains water from beverages and food, and from metabolic water, a by-product of its own metabolic processes. Water is lost in urine, in faeces, and by evaporation from the skin and surfaces of the lungs. The body regulates the output against the input of water.

Certain conditions can affect water balance. Eg. Work conditions (excessive amounts of work also included), diarrhoea (excessive amounts of water is lost from the body via the this can lead to dehydration and thus death), foods consumed (lean meat has more water than fatty meats, fruits and vegetables contain a large amount of water)

Respiratory and Excretory Systems

The respiratory system consists of the lungs and bronchial tubes and the thoracic (chest) activity. This system ensures that oxygen can enter the blood and carbon dioxide leave so that the concentration of these gases in blood and tissues is sufficient for the organism to function properly. There are mechanisms to maintain homeostasis for these gases. These mechanisms ensure that oxygen and carbon dioxide are regulated within a narrow range.

The other substance exchanged between the body and the air in lungs is water vapour. There is some control over how much water is excreted in this way. Apart from the fact that the moist lung surfaces are internal (which decreases evaporation), the lungs are usually within the body core, which remains at a constant temperature. This keeps water loss from the lungs relatively constant also. Other factors can increase water loss from the lungs however. For example, increased rate and depth breathing during exercise makes the increased loss of water difficult to avoid.

The respiratory system is also part of the excretory system. Water and CO2 loss occurs from the lung surfaces. The other parts of the excretory system are the skin, urinary system and digestive system (large intestine).

There is less control over the amount of water lost from the skin by perspiration. The skin is on the outside surface of the body, an area that acts as a regulator for body temperature by being part of the body shell. Besides, water loss (sweating) is a mechanism for heat loss and is under the control of the autonomic nervous system. As well as water, small amounts of salts, nitrogenous wastes and vitamins may be present in sweat. Thus the skin is an excretory organ.

The urinary system is a major part of the excretory system. It consists of the kidneys, ureters, bladder and urethra. The kidneys have two main functions – excretion and osmoregulation. Excretion is the removal of wastes from the organism and osmoregulation is the regulation of the concentration of water and salts in body cells and tissues so that homeostasis is maintained.

Osmoregulation is partially under the control of hormones. The kidneys are the ultimate regulators of salt and water balance in the mammalian body. For example, if very large or very small amounts of water are excreted through the skin, the kidney compensates for this by osmoregulation.

Fibre (the undigested cellulose from plant foods), the breakdown products of haemoglobin and other wastes are excreted as faeces via the large intestine. Some water is also released with the undigested food in faeces although most of it is reabsorbed in the large intestine.

Importance of water in living systems

Water plays a number of roles, all of them essential. They are as follows:

- It is an excellent solvent. Inorganic and organic molecules are able to dissolve in water or are suspended in it. Water is a polar molecule and thus is able to dissolve ionic compounds as well as other polar molecules such as sugars. For larger molecules such as proteins, water can form a hydration layer around them, preventing them from coming in or out of the solution. This type of mixture is called a colloid.

A number of important features of cell biochemistry follow from this solvent property of water

o The solution of substances is essential to maintain osmotic balance in cells

o Some important body lubricating fluids such as mucus are colloidal mixtures with water

o Metabolic reactions only take place between chemical in solution and water as solvent

o Water is the body’s major transport medium. Plasma is composed of part-water.

- Water is an important reactant or product in many metabolic reactions. For example, the digestion of food is a hydrolysis reaction (a reaction in which a molecule is lysed [broken down] and a water molecule is added for reach bond broken. When large polymers such as starch are synthesised bonds a formed and water is released as a by product. This is called dehydration synthesis. Photosynthesis and respiration involve water as a reactant or product.

- Water has a favourable relationship with heat. Water has a relatively high specific heat capacity, that is it can absorb or release large amounts of heat without appreciable changing in temperature. Because it is so abundant in the body and because of its high specific heat, it plays a large role in preventing large fluctuations in body temperature.

Water has a high heat of vaporization, that is when it changes from liquid to gas, large amounts of heat are absorbed. This property is valuable in accelerating heat loss from the body. When we perspire, water evaporated from our skin and a large amount of heat leaves the body in evaporating the water.

- Water has a cushioning effect in the body. Water solutions in body cells/tissues form an effective cushion around many body organs. For example, the cerebrospinal fluid cushions the brain.

Because of these essential roles, water concentration in cells and extracellular fluids must be held constant.

• Explain why the removal of wastes is essential for continued metabolic activities

When nutrients enter the blood, they are involved in numerous biological reactions of metabolism. The majority of these reactions are catalysed by enzymes and a complex interaction of factors enables these reactions to proceed so that the body can function efficiently.

Sometimes metabolic products are formed that are of no use to the body. They are called wastes. Another source of wastes is the ingestion and absorption of compounds that cannot be used and that can sometimes be damaging. Examples are caffeine and alcohol.

These waste substances can affect some enzymes and disrupt metabolism and homeostasis. They may damage cellular components and certainly, at the very least, their accumulation takes up the space required by the normal structural and functional chemicals.

An example of waste that affects body function is excess hydrogen ions, which reduce pH. This can affect the activity of enzymes and the oxygen saturation of haemoglobin. The brain is especially vulnerable to some wastes such as ammonia and urea, toxins and many drugs. Hence the blood capillaries in the brain are less permeable than other capillaries. This is known as the blood-brain barrier.

Because of the importance of unwanted chemicals the body has an organ system with the specific function of removing wastes – the urinary system.

Waste products of terrestrial and aquatic organisms:

The end products of metabolism of carbohydrates and fats are water and carbon dioxide., which are relatively easy to excrete. Proteins and nucleic acids contain nitrogen in addition to carbon, hydrogen and oxygen. When they are metabolised, there is a nitrogenous waste product in addition to water and carbon dioxide. This is ammonia, which is highly toxic. Ammonia is a colourless gas, highly soluble in water. A solution of ammonia is highly alkaline. At such high alkaline levels the enzymes in the body cease to activate and can denature.

Organisms must either continuously excrete ammonia or convert it into a less toxic product and then excrete the product. The less toxic products are urea and uric acid.

Aquatic organisms do not have a problem in excreting ammonia. It diffuses into the surrounding water in which it is highly soluble. Fish have gills and there is a continuous movement of water through the gills. Ammonia is lost by diffusion from the blood across gill membranes.

Animals that excrete their nitrogenous products as ammonia are called ammonotelic organisms. As well as most fish they include aquatic invertebrates, crocodiles and tadpoles.

Terrestrial animals have more problems in excreting nitrogenous wastes since they are not surrounded by an excretory medium (water). Since ammonia cannot be removed from the blood quickly enough, it must be detoxified before removal. To achieve this, most terrestrial animals convert ammonia to urea and/or uric acid. Animals that excrete urea as their main nitrogenous waste are said to be ureotelic. Eg. Humans

Urea is synthesised in the liver and excreted in the kidneys. Ureotelic animals include mammals and amphibians. Cartilaginous fishes such as sharks and rays are an exception to most other fish. They are ureotelic.

Urea is quite soluble in water, and in excreting urea, there is a risk that excessive amounts of water will be excreted. The mammalian kidney has mechanisms that can excrete urine (a solution or mainly nitrogenous waste products) that is more concentrated than body fluids. This ensures that not too much water is lost in the excretion of urea and other wastes.

There is a third group of animals. They are mainly terrestrial, and conserve water by excreting uric acid as their principal nitrogenous waste product. They are uricotelic and include birds, reptiles and insects. Uric acid is not very soluble in water and forms a paste (bird droppings have a whitish colour due to the paste of uric acid with a small amount of water).

This classification of animals is based in the principal nitrogenous waste product. Most animals excrete more than one nitrogenous waste. Eg. Humans excrete both urea and uric acid.

The excretion of nitrogenous wastes depends upon the environment of each animal. Ammonia is more suitable for aquatic organisms, while urea or uric acid are less toxic products excreted by terrestrial animals. Animals that spend part of their lives in water and part in land, must change their excretion products accordingly.

• Identify the role of the kidney in the excretory system of the fish and mammals.

There are different types of aquatic environments – fresh water, marine (salt water) and estuarine (an environment of varying salt concentration).

Water balance in water

There are two main types of aquatic environments – fresh water and marine.

In fresh water environments, cell fluids are usually more concentrated than the surrounding water. Therefore in fresh water, cells tend to gain more water by osmosis than they lose. In the marine environment, cell fluid is often less concentrated than the surrounding water. Therefore in salt water, cells tend to lose more water by osmosis than they gain. This means that fresh water and marine organisms have different adaptations to maintain water balance.

Marine environment.

Many marine organisms have an internal salt concentration lower than that of the surrounding water. Therefore they are adapted to reduce water loss and increase water intake in order to balance the natural loss of water by osmosis. Bony fish, which have a cartilage skeleton, control and regulate their water content. These fish constantly drink salt water but they then excrete salt from the gills. These processes result in a net intake of water because the water lost in excretion is less than the water taken in by drinking. Overall, this net gain of water balances the net loss of water by osmosis and the cell concentrations remain constant.

Fresh water environment

Freshwater fish excrete masses of extremely dilute urine and actively absorb salts from the surrounding water through their gills. In this way they excrete water to balance the water taken in by osmosis.

Depending on their environment, fish can either conserve water by excreting a concentrated urine or release water by excreting a dilute urine.

A terrestrial environment is much more variable than an aquatic environment. Compared with fish, the kidneys of mammals must respond to variable water loss and water gain. Marine fish consistently excrete concentrated urine and fresh water excrete dilute urine since their environments remain relatively stable.

A mammal loses water to the environment through the lungs and skin, and it gains it through water and food intake. This loss and gain can vary depending on the external temperature, humidity, exercise pattern, availability of food and water.

While mammals usually excrete urine that is more concentrated than body fluids, the concentration can vary depending on the balance of water intake and water loss.

The role of the kidney in fish and mammals

The kidneys in fish and mammals are excretory and osmoregulatory organs and their main functions are outlined below.

■ Marine fish: their kidneys conserve water, excrete excess salts and nitrogenous wastes.

■ Freshwater fish: their kidneys excrete excess water and nitrogenous wastes (produce large amounts of dilute urine), conserve salt.

■ Mammals: their kidneys conserve water and salts when required, excrete excess water and salts and excrete nitrogenous wastes. The water potential of living cells is similar to that of sea water, but lower than that of fresh water or the surrounding air. The concentration of water in the immediate environment of an organism determines its need to conserve (retain) water or lose it.

In aquatic animals such as fish, the concentration of solutes in the surrounding aquatic environment has a direct influence on the direction of movement of water—whether it will move into or out of the body of the fish.

Freshwater fish live in rivers and lakes, where the water potential is high—these habitats contain very few dissolved salts and water is therefore freely available (not a limiting factor). Freshwater fish urinate frequently, ,as water tends to accumulate in ,their tissues as a result of passive movement by osmosis from a higher

concentration in the surroundings to a lower water concentration in the animal. These fish are faced with the problem of too much water being present in their bodies. The kidneys in freshwater fish therefore excrete excess

water (gained by osmosis from their surroundings), as well as nitrogenous wastes (as ammonia). Their kidneys are structurally suited to this role by having large glomeruli for the filtration of blood in large volumes. Their kidneys are not involved in salt balance, since these fish do not face the problem of salt accumulation from their freshwater environment. Any excess salts that they consume in their diets are excreted via the gills.

Organisms that live in marine habitats (in the sea) or terrestrial habitats (on land) tend to lose water to their surroundings, and so they have evolved mechanisms to conserve water. Marine fish urinate less. They tend to lose body water (by osmosis), across the body surface and gills, into their salty surroundings. Excess salt tends to accumulate in their bodies, moving in by diffusion from the surrounding sea water. The main function of the

kidneys in these fish is therefore to remove excess salt. Marine fish tend to drink sea water, extract the salt from it and then use the water for metabolism. They excrete the extracted salt to keep salt levels in the body to a minimum. Their kidneys tend to play a role in conserving water rather than excreting it. To meet this need, their kidneys tend to have small glomeruli as well as a mechanism for removing excess salt taken in with sea water. The kidney is also responsible for excreting nitrogenous wastes (usually in the form of urea) in marine fish.

Terrestrial mammals lose water and solutes from the body as a result of evaporation from the lung surface during respiration and as a result of excretion (for example, the production of sweat and urine). The kidneys of mammals excrete urine that is composed mainly of water and nitrogenous wastes (urea), as well as some (excess) salts. The mammalian kidney can adjust the reabsorption of nitrogenous wastes, water and salts, varying the concentration of urine produced. Mammals have a complex control mechanism to ensure that a balance is maintained between the amounts of sweat and urine excreted.

In hot weather, more water is excreted as sweat (since sweating is a form of evaporative cooling and this has the advantage of lowering body temperature) and as a result less urine produced. In cold weather, more water is lost in urine and very little in sweat. A relatively large quantity of salts is also lost during sweating and needs to be replaced to maintain a stable osmotic pressure within body fluids and cells in an organism. Any adjustment to water and salt levels in urine is brought about by the action of the hormones ADH and aldosterone on the kidney tubules. Urine produced may be more dilute (e.g. in animals that have been drinking large amounts of water) or concentrated (in those that have been sweating) and this adjustment is made depending on the needs of the body.

Structure of the Mammalian kidney and Urinary System.

Humans have two kidneys located towards the back of the abdominal cavity. A renal artery towards the back of the abdominal cavity. Each kidney releases urine it produces into a tube called the ureter.

Each ureter leads to the bladder, which stores urine. Urine is released periodically from the bladder by a sphincter muscle to the urethra and so into environment

The kidney is a bean shaped organ composed of cortex tissue on the convex side and a medulla on the concave side. Renal pyramids and the renal pelvis are areas within the medulla in which the kidney collecting tubules collect into the ureter.

Each kidney has over a million functional units called nephrons. Each nephron extends across the cortex and the medulla. Each nephrons has three parts:

1) A glomerulus : This is a ball of blood capillaries, which as resulted from branching of the renal artery into arterioles and then capillaries. The glomerulus has a large surface area and the capillaries continue out of the glomerulus and run along side the kidney tubule. They eventually collect into venules and into the renal vein.

2) A Bowman’s Capsule : This is a fist like structure surrounding the glomerulus. This is continuous with the kidney tube.

3) A Kidney Tubule: the fluid that has been filtered form the blood (called the glomerular filtrate and later called urine) flows into the tubule. Substances are removed or added to the glomerular filtrate to ensure homeostasis in the body while excreting wastes. The tubule has three parts.

i. Proximal Convoluted Tubule: Which is the first part of the Bowman’s capsule

ii. Loop of Henle: Which is a large u-shaped part of the tubule.

iii. Distal Convoluted tubule: Which empties into the collecting duct or tubule.

The tubules of a number of nephrons join to form a connecting duct and a number of collecting ducts into the ureter.

• Distinguish between active and passive transport and relate these to processes occurring in the mammalian kidney.

Movement of materials into and out of cells takes place either passively or actively. Passive movement includes the processes of diffusion and osmosis. These types of movement require no energy input from the cell, since molecules move along a concentration gradient . Active transport requires an input of cellular energy to actively move molecules against a concentration gradient.

Passive transport in the mammalian kidney

Diffusion and osmosis result from the random movement of particles, called Brownian motion, whereby particles continually collide and move randomly. When they are in a higher concentration in one region, this constant movement slowly results in an overall spreading out from the most concentrated point, finally bringing about their even distribution within a space.

Within the kidney, the movement of substances between the bloodstream and excretory fluid in the microscopic tubules (called nephrons) involves both active and passive transport. A balance in the optimal concentrations of blood chemicals is maintained by the selective excretion of wastes, as well as any excess water and salts, in urine. Therefore the ability of the kidney to alter the urine concentration plays a vital role in homeostasis. Within the kidney tubules, there is a two-way movement of substances:

■ Waste substances pass from the bloodstream into the kidney tubules, to be excreted in urine (filtration and secretion).

■ Substances required by the body are removed from the urine in the kidney tubules (before it is excreted) and returned to the bloodstream (reabsorption).

Passive transport moves water (by osmosis), and some nitrogenous wastes such as urea and ammonia (by diffusion) in the kidney of mammals. In the kidney, only excess water and salts are excreted; homeostasis requires that sometimes water and salts should be conserved to maintain the required levels within the body and at other times they should be excreted. Salt movement is via active transport and this in turn draws water by osmosis (passive movement).

Active transport in the mammalian kidney

Active transport is the movement of molecules from an area of low concentration to a region of high concentration, requiring an input of energy.

Sometimes in living organisms there is a need to move a chemical against the concentration gradient, and to do this, active transport occurs. Active transport involves a carrier protein that spans the membrane and this carrier molecule can actively move chemicals from a low to a high concentration, utilising cellular energy. Active transport moves mainly sodium ions, glucose, amino acids and hydrogen ions across the wall of the nephron:

■ All glucose and amino acids are reabsorbed by kidney cells so that they are not lost in urine and so they move against a concentration gradient.

■ Additional nitrogenous wastes (e.g. uric acid) and hydrogen ions (H+) are added to urine (from the blood capillaries) in the kidney tubules.

■ A ‘sodium pump’ mechanism operates in the tubules of the kidneys, actively transporting ions (salts) from the urine back into the kidney cells. Besides conserving salts, this process also brings about the conservation of water within the body—the active transport of salts draws water out of the urine, because water follows by osmosis (passive transport). Water is drawn by the osmotic pull of the salts in solution.

• Explain why the process of diffusion and osmosis are inadequate in removing nitrogenous wastes in some organisms

Diffusion and osmosis are both types of passive transport that require no energy input and so they are slow— the movement of molecules relies on differences in the concentration gradient between two solutions and so both diffusion and osmosis slow down as the difference in concentration gradient becomes smaller, and they stop once the concentration gradient reaches equilibrium.

Problems with relying on diffusion

The rate of movement is too slow. Nitrogenous wastes and toxins such as drugs that accumulate in the body must be dissolved in water when they are removed. If their removal by the kidney was dependent on diffusion only, wastes would be able to move only if they were more concentrated inside the cells or the bloodstream than in the fluids outside. As the concentrations begin to equalise, their movement would slow down and eventually stop. Not all wastes can be removed by diffusion. Since nitrogenous wastes are toxic, it is essential they are all removed. If concentrations within the blood and urine equalised and no further wastes were removed, their accumulation would change the pH of cells and become toxic. Active transport is therefore essential at this point to move wastes such as uric acid against the concentration gradient from blood into urine in the kidney.

Problems with relying on osmosis

Too much water may be lost in urine. If urine contains a large number of nitrogenous wastes in solution, water will be drawn into the urine by osmosis, to dilute the wastes and try to equalise the concentration of the fluid inside the urine and in the surrounding kidney tissues. However, excretion of dilute urine means the loss of large amounts of water from the body—a loss many terrestrial animals cannot afford. Movement of water may make wastes too dilute for excretion by diffusion. Organisms that live in freshwater environments have a different problem—osmosis results in water moving into the body tissues from the surrounding environment. Although this dilutes the toxic wastes in the body, it also slows down their excretion by diffusion (lowers the concentration gradient). Therefore a mechanism is essential to remove wastes against a concentration gradient.

Solution to the problems— combined active transport and osmosis

Active transport, which requires energy, is quicker and more effective than diffusion as it removes most wastes, even against a concentration gradient. It can also be used to pump salts from urine back into the kidney tissues and these in turn will draw water with them (by osmosis), ensuring in this way that the amount of water lost in urine does not affect the body’s water balance.

• Explain how the processes of filtration and reabsorption in the mammalian nephrons regulate body fluid composition.

The movement of small molecules from the blood in the glomerulus into the Bowman’s capsule is by filtration. Blood pressure forces small molecules such as urea, glucose, amino acids, salts and water across into the capsule. Blood cells and large molecules such as proteins cannot pass through the pores of the capillary and capsule membranes. So no proteins or blood cells should be found in urine. Their presence indicated kidney damage or infection.

When small molecules are filtered across, they become a fluid called the glomerular fluid. The glomerular fluid is at this stage similar to blood plasma except that it does not have blood cells or proteins.

Reabsorption is the second major process in the kidney. Because the filtrate contains many valuable solutes, the process of active transport in the proximal tubule removes them from the filtrate back into the blood. All of the glucose and amino acids are reabsorbed and some of the salts and water. Hydrogen ions and some toxic substances are also secreted (discharged by active transport) from the blood into the kidney tubule here.

Reabsorption maintains a constant concentration of essential metabolites (such as glucose) in blood and consequently body tissues. Water and salts have not been regulated and some toxins and H+ ions need further regulation so that what is not needed by the body is excreted and homeostasis is maintained. The loop of the Henle and the distal tubule act as a refinery for the filtrate so that osmoregulation can occur to produce urine at a concentration that excreted wastes but maintains homeostasis.

• Outline the role of hormones, aldosterone and ADH (anti-diuretic hormone) in the regulation of water and salt levels in the blood.

Most of the useful molecules filtered out of the blood into the kidneys are reabsorbed into the blood from the proximal tubules. Some drugs and hydrogen ions are secreted from the blood into the proximal tubes also. The filtrate (the fluid in the kidney tubules) does not yet have the composition of urine. Water and salts need to be regulated and more hydrogen ions and drug and toxin molecules need to be secreted. These events take place in the loop of the Henle and the distal tubule.

Once the filtrate empties into the collecting tubule or duct, it is called urine and its concentration is adjusted in the collecting duct. Some of these events are under the control of hormones aldosterone and antidiuretic hormone (ADH), also called vasopressin.

Control of Salt concentration and blood pressure by aldosterone.

Blood pressure levels that are adequate to maintain the glomerular filtration rate must be held constant. Otherwise the filtration slows down, with dangerous consequences for the accumulation of wastes and the disruption of homeostasis. It is equally damaging for the blood pressure to be too high. In that case, the blood flow becomes too fast so that needed substances cannot be adequately reabsorbed.

Glomerular blood pressure must therefore be carefully regulated. The body has a number of mechanisms for doing this. One of these involves aldosterone.

Aldosterone is a steroid hormone produced by the adrenal cortex, the outside part of the adrenal gland. The two adrenal glands lie on top of kidneys. The medulla (or central part) of these glands produced adrenalin, which helps the body cope with stress.

The primary function of aldosterone is to increase the reabsorption of sodium ions from the kidney tubule and so conserve sodium in the blood and body fluids. It has the opposite effect on potassium. When sodium ions enter the blood in the distal tube, water follows by osmosis. This increases the blood volume and therefore blood pressure. So aldosterone not only conserves sodium ions but also helps to maintain blood pressure in the kidneys so that further back, the glomerular filtration rate may function efficiently.

Control of urine concentration by antidiuretic hormone (ADH)

Diuresis is the loss of water in the urine. The hormone that prevents diuresis is the antidiuretic hormone (ADH), also called vasopressin. It has this effect by increasing the permeability of the membranes of the collecting ducts to water so that more water is reabsorbed into the blood and tissues. Its release is regulated by a feedback mechanism involving the hypothalamus.

Receptors in the hypothalamus detect lowered water (high solute) levels in the blood. The hypothalamus causes he ADH to be released from the posterior lobe of the pituitary gland (below the hypothalamus). The antidiuretic hormone then targets the collecting ducts of the kidneys, causing increased membrane permeability and increased water retention.

The opposite occurs in cases of decreased solute levels, when more water needs to be excreted and less to be retained. So less ADH is released.

Reabsorption in the loop of Henle and the distal tube.

Consider the filtrate entering the loop of Henle. All the essential metabolites have been reabsorbed into the blood. Some salts and water have been reabsorbed but osmoregulation is not complete. Urea and other wastes are present in the filtrate. Because of reabsorption, the interstitial (extracellular) fluid outside the tubule is more concentrated than the filtrate in the tubule.

There are two limbs in the loop of Henle – the descending limb and the ascending limb. Because of the more concentrated interstitial fluid, water moves out of the descending limb via osmosis. Water then moves into the blood and is carried away so the concentration gradient between the tubule and the interstitial fluid remains.

Just after the filtrate moves into the ascending limb, reabsorption of water stops because the membrane of the ascending limp is impermeable to water. Reabsorption of sodium and chloride ions occurs as the filtrate moves up. This mainly by active transport (influenced by aldosterone) and it concentrates the interstitial fluid further. Sodium chloride continues to be reabsorbed in the distal tube and more hydrogen ions and toxins are excreted into the tubule.

Hormonal control in the collecting duct.

When filtrate enters the collecting duct it is called urine although it is still very dilute. As it travels down the collecting duct, the concentration is regulated in order to maintain a constant composition of water and solutes in body fluids. This process is under hormonal control.

The reabsorption of NaCl in the loop of Henle and the distal tubule and the impermeability of the ascending limb to water has ensured that the filtrate is very dilute compared with the surrounding interstitial fluid and tissues. ADH is released from the pituitary gland. It travels in the blood and increased the permeability of the collecting ducts membranes to water. Because of the high concentration gradient, water is reabsorbed into the tissues and blood (by osmosis) and so is conserved in the body. The opposite occurs when excess water needs to be excreted, that is less ADH is secreted.

When urine leaves the collecting ducts and enters the ureter, its composition reflects the body’s needs in terms of waste removal and salt and water balance. The kidneys have performed their complex task of excretion and osmoregulation.

Kidneys used about a quarter of daily blood oxygen. This investment of energy ensures homeostasis.

• Define enantiostasis as the maintenance of metabolic and physiological functions in response to variations in the environment and discuss its importance to estuarine organisms in maintaining appropriate salt concentrations.

Enantiostasis is the maintenance of metabolic and physiological functions in response to variations in the environment.

It is applicable to any organism that lives in a varying environment.

Estuarine fish

Previously salt water and fresh water fish were discussed. They either conserve water and excrete salt (marine fish) or excrete water and absorb salt (fresh water fish).

Marine fish lose water to the external environment so they drink large amounts of water and excrete small amounts of concentrated urine to conserve water. They also excrete salt through gills by active transport. Fresh-water fish tend to gain water to they excrete large amounts of dilute urine and they accumulate salt across the fills by active transport.

Since both marine and fresh water fish regulate the water and salt between their external and internal environments, they are called osmoregulators. Their external environment remains relatively constant but their internal environment differs from it.

Another group of fish, the osmoconformers, have a different solution to the problem of salt and water. They maintain the concentration of their internal fluids at approximately the same level as their external environments. This is satisfactory in marine environments but not be possible in freshwater environments.

In estuarine environments, the salt and water in the environment are changing with tides and other factors, yet many of these organisms survive. Their metabolism is able to tolerate a range of salt concentrations. For example, estuarine crabs and sharks manage to tolerate fluctuations in external salt concentrations very well. They are still osmoconformers but they use small organic molecules (such as non-essential amino acids and monosaccharides) to vary the concentrations in their cells to match the environment. They can alter the concentration of these chemicals so that the osmotic pressure is much the same in the internal as in the external environment. That is, they can vary their cellular concentration to suit the environment.

Migratory Fish

A small number of fish manage to live in both freshwater and salt water environments. Some eels and salmon spend much of their time at sea, but reproduce in fresh water. These fish are osmoregulators. Their gills and kidneys function like those of a fresh water fish while in fresh water and like those of a marine fish when in the ocean.

Organisms that live on rock platforms.

Invertebrates that live in intertidal zones, such as on a rock platform, experience fluctuations in the salinity of their external environment. They have a number of physiological and behavioural adaptations that enable them to tolerate a wide range of salinity. Many animals seek refuge under a shell when the tide is out.

If sea water concentrations fall, mussels close their valves and keep a salt concentration close to that of sea water within their mantle cavity. Thus they maintain a small habitat of sea water around their bodies.

Some annelid worms burrow under rocks and in sand until the sea water returns. One of the most stressful areas is a rockpool in which salinity may fluctuate due to rain (decreasing salinity) or evaporation (increasing salinity). While a rockpool protects from wave action, it is a more difficult environment that that found under moist rock crevices. Organisms within the rock pool must be able to tolerate the variability in salinity by osmoconforming strategies.

Mangroves (Avicennia Marina)

Mangroves play an important role in the life of an estuary. Mangrove ecosystems are a habitat for a diverse group of marine animals and birds. They act as a nursery for juvenile fish. They help stop erosion of the shoreline and they produce large amounts of organic matter that is used by other organisms in the food web of the estuary.

But to do this the mangroves must be able to tolerate the variable salinity and oxygen content of the mud and water in their environment. Most plants become dehydrated due to salt, but mangroves can flourish in sea water and survive in salty mud.

The muddy water in which the mangrove roots are embedded is very low on oxygen. The roots of other trees can obtain oxygen from the soil because it is present in the air spaces or dissolved in the water between soil particles. Mangroves have little snorkels called pneumatophores that project up from the main rots. These absorb oxygen from the airs. Some mangroves have buttress and aerial roots that hang down from branches. Others have lenticels.

Many mangroves are viviparous. Seeds germinate on plants before they drop into the water. This increases a seed’s chance of survival and germination. Mangrove leaves have features such as waxy cuticles and sunken stomata that reduce the loss of the water that a plant has desalinated.

Mangroves have ingenious adaptations for maintaining a constant salt concentration in their cells despite the great variability in the salt concentration of the estuarine muddy water. Mangroves have three methods of coping with salt:

1) Exclusion: They have special tissues (endodermis) in their roots and lower stems that act as a barrier to salt uptake into xylem tissues, although allowing the passage of pure water through the xylem. All mangroves in NSW are able to exclude salt to some extent.

2) Secretion: They are able to concentrate and excrete (or release) salt through special glands on their leaves. The salt deposits crystallize when exposed to air and due to their flaky nature, they are washed away from the leaves during rain. The grey and river mangroves are examples of this adaptation.

3) Accumulation: They accumulate salt in older tissue (and leaves) which is then discarded. This occurs in the milky mangrove.

Mangroves can actually live in fresh water as they can adjust responses to suit the environment, for example the salt glands can decrease their activity.

Extra: - From Jacaranda:

The cells of marine organisms generally have an internal osmotic concentration similar to that of the external seawater. Such animals that remain in the open sea are called stenohaline and can tolerate little or no change in the salinity of their environment.

Other marine animals can tolerate changing salinity and are able to survive in salinity down to about 15 psu. They are called euryhaline species. In order to survive the fluctuating salinities they encounter, estuarine organisms must either be able to function with fluctuating internal salt concentrations (osmoconformers) or they must have mechanisms that control the salt concentrations of their bodies (osmoregulators).

Organisms unable to osmoregulate rely heavily on behavioural characteristics to survive in estuaries. Some may move with the tide. Others burrow in the mud or sand during tidal changes. Changes in the sand and mud are less than those experienced in the water. Bivalve molluscs remain closed during tidal periods to minimize contract with the changing salinity.

Human Impacts on Estuaries:

- Destroyed as they were popular areas for settlement and establishment of industry

- Seen as unattractive and smelly so they were replaced with industrial developments that dumped wastes.

- pollution from oil and fuel spills from boats, pollution from fumes

- wash from speeding power boats

- industrial / residential development and associated pollution

- visitors who leave fires / rubbish

- siltation from deposits from creeks and rivers.

• Describe adaptations of a range of terrestrial plants that assist in minimizing water loss

Apart from the limited rainforest and wet sclerophyll species, the dominant vegetation is the dry sclerophyll type.

Sclerophyll is a plant with leaves that have a leathery, hard or spiny covering and that may be reduced (have a smaller surface area). Also commonly known as sclerophyte or scleromorph.

These adaptations are present to prevent dehydration. Gum trees (eucalypts) are the most common Australian sclerophyll.

Acacias are another sclerophyllous genus that is wide spread and numerous. These are commonly called wattles and are called mulgas in semi-desert arid areas.

Eucalyptus is of the family Myrtaceae

Acacia is of the family Mimosaceae

Xerophytes are plants that are adapted to low ground-water levels and they have specific features that enable them to retain water. Some xerophytes are also sclerophytes.

Mesophytes are plants that live in areas where water is in adequate supply

Plant Adaptations to minimize water loss

|Adaptation |Advantage |Examples |

| | | |

|Needle Like leaves |Reduces surface area and hence water loss |Mulga(acacias), she-oaks(casuarinas) |

| | | |

|Photosynthetic Stems |Reduces surface area and hence water loss |She Oaks |

| | | |

|Woody Fruits |Less water loss than in fleshy fruits (often fire resistant as well) |Banksias, Hakeas |

| | | |

|Waxy Leaves |Reduce water loss as the cuticle prevents evaporation but also reflects|Salt Bush (Atriplex) |

| |infrared radiation from sun, reducing heat gain | |

| | | |

|Ephemeral growth |Plants have a very short life cycle; growing and reproducing in a very |Ephemeral plants |

| |short period in response to rain. |Eg. Paper Daisies (Helipterum) |

| | | |

|Partially deciduous |Some eucalyptus lose most of their leaves during extended dry spells |Eucalyptus |

| |reducing water loss. | |

| | | |

|Leaf Curling |Leaves roll up, forming a cylinder, which reduces surface area and |Hummock grass (triodia) |

| |traps a humid layer of air, which reduces water loss |Marrum Grass (Ammophila Arenaria) |

| | | |

| |Note: The cells may be ‘hinged’ so that when they lose turgor, they | |

| |cause the leaf to curl. Furthermore, stomata can be on one side only, | |

| |so that when the lead curls, no stomata is directly exposed to the | |

| |environment. | |

| | | |

|Sunken Stomates |Stomates lie in a cavity in the leaf, which results in humid air being |Hakeas, Figs (Ficus Carica) |

| |concentrated above the stomate, which reduces water loss. | |

| | | |

| | | |

| | | |

|Water Storage |Water is stored in trunk, leaves or roots. |Baobab tree stores water in its trunk; parakeelyas have|

| | |succulent stems and roots. |

| | | |

| | | |

| | | |

| | | |

|Hanging Leaves |Leaf hangs down, rather than being held horizontal to the ground which |Eucalyptus |

| |reduces water exposure to sun. | |

| | | |

|Hairy or shiny leaves |Hairy surfaces on under surface reduce air movement and increase |Banksias, paper flowers (Thomasia) |

| |humidity over stomates, reducing water loss; on the upper leaf, the | |

| |skinny of hair surface reflections radiation from the sun, reducing | |

| |heat gain. | |

| | | |

|Water-directing leaves and stems |Stems and leaves are shaped so that water runs down them towards the |Mulga (acacia) |

| |roots. |Xanthorrhoea (grass trees) |

| | | |

|Succulents |Some leaves have cells with very large vacuoles that act as water |Cacti (Cactaceae) |

| |stores for the plant. Some plants may have no leaves, with the stems | |

| |storing the water. | |

| | | |

|Cylindrical Leaves |Leaves are narrow and cylindrical, they have a thick epidermis with a |Hakea |

| |thick cuticle. They have sunken stomates and the palisade cells are | |

| |concentrically arranged in the palisade tissues. | |

The sclerophyll leaves so typical of many Australian native plants are covered by a very thick cuticle and when they dry out, the cells within may collapse but the outer shape of the leaf is maintained. They do not wilt like other leaves. In this way when more most conditions return, the leaves can rapidly begin to function efficiently.

Many simple plants such as moss and lichen can dry out and when water becomes available the cells take on water and the plants begin to function again. This is rare among other plant groups, but the pin-cushion plant (Boyra Nitida) has a similar adaptation. Its leaves and stems can dry out. When this occurs, the plant seems to have died, but when water becomes available the leaves take in the water and they start to photosynthesise.

Evaporation of Water

The evaporation of water from the stomates cools the plant but unfortunately often the plant is also suffering from excessive water loss when this cooling is most needed. For desert plants, it is essential that heat gain be reduced surface area since heat loss also involves water loss.

Desert plants are therefore faced with a range of conflicting pressures. They must reduce surface area to reduce heat gain, but need a large surface area to absorb light for photosynthesis. In the case of desert plants, even small surface areas receive ample light to compensate for the daily needs of the plant.

The eucalyptus are particularly well adapted to attain a balance between these pressures. Their leaves tend to hand down vertically. In this way they present a large surface area to the rising (cool) sun in the morning, and a small surface area to the hot midday sun. Furthermore, stomates will open in the morning and if the plant is suffering from water stress, may close for the rest of the day.

Due to their inability to move, they are nevertheless exposed to the extremes of their environment. They therefore must be able to tolerate changes in temperature, particularly in cells near external surfaces.

Other Adaptations

Adaptations of Leaves

Phyllodes are another adaptation to reduce water loss. Phyllodes are flattened petioles (leaf stalks) that have a tough covering and have a leaf-like appearance. Phyllodes can be reduced to spines in some cases to reduce the surface area further. Bipinnate (divided twice) leaves are tiny leaves arranged along an axis.

These both reduce the surface area of each leaf (while maintaining photosynthesis) so that water loss is reduced. They are common in members of the genus Acacia (wattles and Mulgas)

Adaptations of Stems

Some stems of Xerophytes are modified to take on the function of a leaf. These modified stems are called cladodes. The cladodes have small scale leaves on their surfaces. Since most of a plant’s water is lost through the leaves, this inefficiency reduces water loss. The cladodes have stomata located in grooves so water loss can take place through the cladodes. However, the shape of a cladode is needle-like and this feature reduces the surface area so that water loss is minimized.

Reduced flower size of flowers with few or no petals.

Acacia flowers are small and clustered in small heads or spikes. Eucalyptus flowers are enclosed in a bud cap and when this opens it reveals a flower with no petals. The presence of petals requires metabolism and therefore water. The plant minimizes its need for water by this adaptation.

Root Networks

Desert plants have wide dense root networks to capture any available water.

• Analyse information from secondary resources to compare and explain the differences in urine concentration of terrestrial mammals, marine fish and freshwater fish.

Excretion of nitrogenous wastes presents no problem for water balance in fish. They have a ready supply of water and are able to excrete the nitrogenous waste they produce without damage to their tissues, although freshwater and marine (also called saltwater) fish deal with the matter in different ways.

Because the tissues of a freshwater fish are hypertonic to its surroundings water enters the gills by osmosis and ammonia is excreted in large volumes of dilute or hypotonic urine. This removal of large quantities of water by the kidneys is important so that freshwater fish can maintain an appropriate water balance in their bodies.

Marine fish tend to lose water across their gills because the salt concentration inside their body is less than outside. To counter this, a marine fish drinks seawater. The salt that is absorbed in the intestine with the water is carried by the blood to the fills where special salt secreting cells transport it across gill membranes into the sea. Any ammonia leaves via the gills. The small amount of isotonic urine produced by marine fish carries magnesium sulfate salts and urea out of the body.

|Freshwater Fish |Marine (Saltwater) Fish |

|Tissues hypertonic to surroundings |Tissues hypotonic to surroundings |

|Concentration gradient results in a loss of salts and an uptake of water. |Concentration gradient results in a loss of water and an uptake of salts. |

|Fish must counter these changes to maintain homeostasis |Fish must counter these changes to maintain homeostasis |

|Does not drink |Drinks seawater |

|Kidney contains glomeruli and secretes copious amounts of very dilute |Minimal urine produced. Kidneys lack glomeruli. Tubules actively secrete |

|urine that contains ammonia. Tubules actively reabsorb NaCl |MgSO4 |

|Gill membranes permeable to water |Gill membranes are relatively permeable to water |

|Gill actively absorbs ions. Some ammonia leaves gills at the same time. |Gills actively secrete sodium from chloride cells, chloride ions follow. |

A terrestrial mammal’s urine concentration is dependant upon water availability. Those mammals who live in areas with plentiful water will display dilute urine that is hypotonic to their body fluids. On the other hand, those mammals who live in water scarce areas will expel urine that is hypertonic to their body fluids (ie. more concentrated urine).

From Biology In Focus:

The concentration of urine produced by different animals depends on their need to conserve water. The amount of solutes dissolved in the watery solvent of urine is a measure of how dilute or concentrated the urine solution actually is. Nitrogenous wastes and salts are the most abundant solutes in urine. If urine is high in solutes, it is said to be concentrated, but if it is very watery and contains relatively few solutes, it is termed dilute. All vertebrates are able to produce urine that is the same concentration as or more dilute than that of blood. However, only those with specialised excretory systems can produce urine that is more concentrated than blood. This requires energy and specialised mechanisms of functioning.

Mammals—a varied concentration of urine

Some organisms, such as mammals and birds, are able to vary their urine concentrations according to the changing needs of the body. When the body needs to conserve water (e.g. if large amounts of water have been lost in warm weather as a result of sweating or panting) the body will excrete concentrated urine so that water is conserved. On cooler days when the animal does not sweat or on days where large amounts of water are consumed, dilute urine is excreted by the kidneys. As a result, the water content of the blood is maintained at a relatively constant level because the kidneys are able to adjust the concentration of urine excreted and so the kidneys play a role in assisting the maintenance of a relatively constant blood volume (and therefore pressure) in the body. The kidneys of mammals also regulate the concentration of salts excreted in urine (and therefore those that remain in the blood plasma) and they keep the blood pH within a narrow range by varying the amount of hydrogen ions excreted in urine.

The concentration of urine in mammals therefore varies in terms of the concentration of water and of dissolved substances such as nitrogenous wastes, salts and hydrogen ions that it contains. Humans produce, on average, urine that is about 4.2 times more concentrated than their plasma (that is, their urine is slightly more salty than sea water). The concentration of the urine of desert mammals is greater; for example, the urine of camels is eight times as concentrated as their blood and that of the spinifex hopping mouse can be even higher. Kangaroo rats can live on a diet of plant parts that contain almost no water—they can survive on dry seeds and not drink water for prolonged periods of time. Their bodies are able to make use of metabolic water that they generate by processes such as cellular respiration and they rely on this for their survival. The urine that they produce may be up to three times as salty as sea water. The loop of Henle in their kidneys is almost three times as long as that in human kidneys, allowing them to reabsorb vast quantities of water from their urine

Fish—a set concentration of urine

The physiology of fish allows them to produce urine of one particular concentration, rather than varying the concentration as is seen in mammals. The concentration of urine that is excreted is dependent on the type of aquatic environment in which the fish lives. The exchange of water and dissolved salts occurs between the cells and body fluids of the fish and its environment, and each aquatic environment (marine and fresh water) presents its own set of problems that must be overcome by structural, physiological and behavioural adaptations in the fish.

In freshwater fish (for example, native bass), the water surrounding the fish has a lower concentration of solutes than the cells of the fish. The main problem facing the animal is that, since water moves by osmosis from a higher water concentration (fresh water) to a lower water concentration, it tends to move from the surrounding environment into body tissues of the fish. To overcome this problem, large quantities of very dilute urine are excreted.

In marine fish (for example, whiting), the sea water surrounding the fish has a higher concentration of solutes than the cells of the fish and so water tends to move out of the fish by osmosis, from a higher water concentration (in the cells of the fish) into the surrounding environment (lower water concentration). The fish therefore needs to conserve water and so small quantities of concentrated urine are excreted.

Jacaranda Extra:

Stomata:

The most accepted theory for the turgidity of the guard cells is the change in ion concentration within the guard cell. Ions, in particular potassium ions, migrate into guard cells from adjacent cells as CO2 concentration declines when photosynthesis commences as light becomes available. As the concentration of ions rises, greater water absorption by the guard cells occurs and stomata open.

When potassium ions move out of guard cells, water also moves out. Reduced light means less photosynthesis. Carbon dioxide accumulates make the cells more acidic. K+ ions move out of the guard cells and water passively follows. Consequently, the guard cell becomes flaccid and the stomatal pore closes.

• present information to outline the general use of hormone replacement therapy in people who cannot secrete aldosterone

Aldosterone belongs to a class of hormones called mineralocorticoids, also produced by the adrenal glands. Aldosterone helps maintain blood pressure and water and salt balance in the body by helping the kidneys retain sodium and excrete potassium. When aldosterone production falls too low, the kidneys are not able to regulate water and salt balance, leading to a drop in both blood volume and blood pressure.

Why some people lack aldosterone

Addison’s disease results from abnormally low levels of aldosterone in the body. The causes are not always known, but include:

■ damage to the adrenal glands that produce aldosterone, due to accident, surgery or disease (such as bacterial infections or cancer of the gland)

■ damage to the pituitary gland that controls the adrenal gland (for example, because of a tumour).

Consequences of an inability to secrete adequate aldosterone

■ Inadequate aldosterone may lead to Addison’s disease, where the adrenal cortex is unable to secrete sufficient (or, in severe cases any) aldosterone.

■ It results in low sodium levels and high potassium levels in the blood—this is a potentially dangerous situation. Initially leading to symptoms such as weakness, fatigue and weight loss.

■ In severe cases of mineral ion imbalance, blood pressure drops due to the low amounts of sodium and potassium ions, an imbalance of hydrogen ions leads to a lowering of blood pH and blood glucose imbalance may arise.

Symptoms such as these restrict the patient’s lifestyle as they cannot stand for long periods of time, may faint from low blood pressure (which brings dangers of its own, restricting activities and independence, such as not being able to drive) and are often too tired to do much.

Without treatment, this may result in the potentially lethal condition of heart failure. In a medical emergency, large amounts of salt and water must be given intravenously, as well as rapid intravenous adrenal replacement therapy.

How is adrenal insufficiency treated

Treatment of adrenal insufficiency involves replacing, or substituting, the hormones that the adrenal glands are not making. Aldosterone deficiency is replaced with oral doses of a mineralocorticoid, called fludrocortisone acetate (Florinef), taken once or twice a day. Doctors usually advise patients receiving aldosterone replacement therapy to increase their salt intake.

Symptoms of this disorder such as low blood pressure, low blood glucose, and high levels of potassium can be life threatening. Standard therapy involves intravenous injections of fludrocortisone acetate and large volumes of intravenous saline solution with dextrose, a type of sugar. This treatment usually brings rapid improvement. When the patient can take fluids and medications by mouth, the amount of fludrocortisone acetate is decreased until a maintenance dose is reached.

• use available evidence to explain the relationship between the conservation of water and the production and excretion of concentrated nitrogenous wastes in a range of Australian insects and terrestrial mammals

All animals must eliminate nitrogen-containing metabolic wastes that arise from the breakdown of protein so that they do not accumulate in toxic amounts. Excess amino acids (and nucleic acids) in the bodies of vertebrates are de-aminated and the nitrogen-containing amino group is removed and combined with carbon dioxide to produce ammonia.

The ammonia is still fairly toxic and so it must be excreted directly, diluted with large quantities of water, or it may be changed to a less toxic form of nitrogenous waste. (Just as carbon dioxide changes the pH of solutions to become more acidic, so ammonia makes the pH more alkaline—thus changing the internal environment from its optimal range and affecting enzyme functioning and metabolism.) Urea and uric acid are less toxic forms of nitrogenous wastes which can be excreted in a less dilute form. The formation of all nitrogenous wastes occurs in the liver and they are then carried to the kidneys for excretion.

Nitrogenous wastes and water conservation

The environment in which an organism lives determines how important the conservation of water is for the survival of that organism. In environments where water is scarce, for example some arid terrestrial habitats, natural selection has favoured the survival of those organisms that secrete less toxic forms of nitrogenous wastes, because they are able to conserve more water while still flushing out their wastes.

Ammonia is very toxic compared with other nitrogenous wastes. It requires no energy to be made, but must be excreted immediately and in a dilute form with a great deal of water. It is therefore most commonly secreted by aquatic invertebrates and fish that live in fresh water, where the availability of water is not a limiting factor.

Urea is the most common form of nitrogenous waste excreted by terrestrial mammals, adult amphibians and some fish. It is not as toxic as ammonia and so it can be excreted in a less dilute form, resulting in less water loss. It does, however, require more energy for its production.

Uric acid is the least toxic form of nitrogenous waste and so it is excreted (as a semi-solid, whitish paste) by animals that have a particular need to conserve as much water as possible, for example birds and most invertebrates, including insects. The synthesis of uric acid uses a large amount of energy in contrast to ammonia and urea, although it has the smallest amount of water loss in the process of excretion. The excretion of uric acid, which is not very soluble in water, allows animals such as insects to conserve water within the body, as its low toxicity means it can be excreted with minimal water loss.

Excretion of nitrogenous wastes in insects

Insects have blind-ending kidney tubules (Malpighian tubules) that open directly into the hind part of the digestive tract. Water and waste solutes are drawn into the blind end from the fluid in the body cavity of the insect. The open end of each kidney tubule empties into the hindgut of the digestive tract. In some insects (e.g. the blowfly) the blind-ending kidney tubules lie close to the end of their digestive tracts and the solutes in the tubules draw water by osmosis across the epithelium (lining) of the rectum, in this way modifying their excretory fluid so that most water is reabsorbed from their rectal contents into the body. As a result, they produce very dry faeces (which contain nitrogenous wastes as well as undigested food). Some insects such as the desert silverfish and the larval forms (meal worms) of a particular moth (Tenebrio molitor) are able to absorb water vapour from the air through the mouth or anus. Water that enters the anus of the meal worm is absorbed through the rectum and is then drawn into the adjacent kidney tubules by osmosis. These are simple forms of tubular reabsorption, more primitive versions of that in mammals that need to conserve water.

Conclusion

The challenge of regulating water content during excretion is therefore solved by varying the type of nitrogenous waste excreted, which in turn determines whether urine needs to be dilute (to safely flush out more toxic forms of waste), or if it can be more concentrated (to flush out less toxic forms). This affects the physiology of the animal: the amount of water that must be reabsorbed into the body or the amount that can be lost in urine depends on the type of nitrogenous waste excreted, as well as the concentration of salts that are being excreted. All of these factors contribute to determining the eventual concentration of urine that is excreted.

For information on Terrestrial mammals, look at previous dot points.

[pic] [pic]

• process and analyse information from secondary sources and use available evidence to discuss processes used by different plants for salt regulation in saline environments

Look At Mangroves in previous dot points.

Extra:

Salt, even in relatively small concentrations in soil water, has a damaging effect on cell ultra-structure and cellular metabolism. Plants that are adapted to saline environments are called halophytes. The plants use either salt tolerance (salt accumulation) or salt avoidance (salt exclusion) as strategies to survive in environments where they are exposed to high salt concentrations.

Salt tolerant plants (e.g. sea grass and mangroves) are able maintain metabolic functioning even though their cells accumulate sodium and chloride ions. They minimise salt toxicity by increasing their water content in large vacuoles. In contrast, salt avoidant plants (salt excluders) minimise the salt concentrations of cells through structural and physiological adaptations such as stopping salt from entering at the roots.

Examples of halophytes

Saltbush (Atriplex vesicaria) is an excluder—it actively transports excess sodium and chloride ions into bladder cells situated on the tip of hairs on the surface of leaves. When the bladder cell reaches capacity it bursts, releasing the salts into the environment.

Palmer’s grass (Distichlis palmeri) also actively secretes salts from specialised cells to avoid high salt concentration within the cells. Succulents minimise the salt toxicity through increasing water content in large vacuoles, where the accumulation of excess salt is balanced with additional water drawn into the cells. Pickleweed (Salicornia) uses this method and also actively transports salts from the cytoplasm by a sodium—potassium pump on the vacuole membrane.

Pigface (Carpobrotus glaucescens), a succulent that grows on coastal sand dunes, tolerates salt by increasing water uptake to dilute the salt. It also stores excess salt in a location away from sensitive cells.

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