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9.2 Maintaining a balance: 3. Gases, water and waste products

Syllabus reference (October 2002 version)
3. Plants and animals regulate the concentration of gases, water and waste products of metabolism in cells and in interstitial fluid

Students learn to:


Extract from Biology Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW
[Edit: 2 Jun 09]

Prior learning: Science Stages 4 - 5 syllabus: Outcomes 5.7 (content 5.7.3 e and f: compounds and reactions), Outcome 4.8 (content 4.8.5 a: humans).
Preliminary course, module 8.3 (subsections 5 and 6).

Background information: Plants and animals carry out the normal functions for living on a daily basis. To do this, they require gases such as oxygen for respiration and, in plants, carbon dioxide for photosynthesis. These metabolic reactions are chemical reactions that accumulate wastes. If these wastes aren't disposed of, they could kill the organism.

perform a first-hand investigation of the structure of a mammalian kidney by dissection, use of a model or visual resource and identify the regions involved in the excretion of waste products

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explain why the concentration of water in cells should be maintained within a narrow range for optimal function

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explain why the removal of wastes is essential for continued metabolic activity

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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


The following provides general information for the waste products, ammonia, urea and uric acid.

Ammonia is very toxic and must be removed immediately, either by diffusion or in very dilute urine. It is the waste product of most aquatic animals, including many fish and tadpoles. Ammonia is the immediate product of break down of amino acids — no energy is required to make it. It is highly soluble in water and diffuses rapidly across the cell membrane. However, it needs large quantities of water to be constantly and safely removed. Ammonia does not diffuse quickly in air.

Urea is toxic, but 10 000 times less toxic than ammonia, so it can be safely stored in the body for a limited time. It is the waste product of mammals, and some other terrestrial animals, but also of adult amphibians, sharks and some bony fish. It is made from amino acids but requires more steps and energy to make than does ammonia. It is highly soluble in water, but being less toxic than ammonia, it can be stored in a more concentrated solution and so requires less water to remove than ammonia. It is a source of water loss for these species.

Uric acid is less toxic than ammonia or urea, so can be safely stored in or on the body for extended periods of time. It is the waste product of terrestrial animals such as birds, many reptiles, insects and land snails. It is a more complex molecule than urea so it requires even more energy to produce. It is thousands of times less soluble than ammonia or urea and has low toxicity, which means that little water is expended to remove it. This is a great advantage for survival.

Terrestrial or aquatic
Waste product(s)
spinifex hopping mouse of Central Australia terrestrial urea in a concentrated form The animal lives in a very arid environment. It drinks very little water and excretes urea in a concentrated form, so that water can be conserved.
Euro, wallaroo (Macropus robustus) terrestrial concentrated urine Euros have a very efficient excretory system that recycles nitrogen and urea to make a very concentrated urine. This allows them to survive in very arid environments
Insects terrestrial uric acid Insects are covered with a cuticle impervious to water. They conserve water by producing a dry paste of uric acid.
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analyse information from secondary sources to compare and explain the differences in urine concentration of terrestrial mammals, marine fish and freshwater fish

Some summary information is provided in the box below.

Excretory system of different animals (external website) Estrella Mountain Community College, Avondale, Arizona, USA

Type of organism Excretory product and concentration Environmental reason
terrestrial mammal

marine fish

freshwater fish


Summary: Differences in urine concentration of terrestrial mammals, marine fish and freshwater fish

Terrestrial mammals

Terrestrial mammals must work to find water and they are surrounded by air into which water quickly evaporates. Water conservation is of prime concern and these animals cannot excrete large quantities of water for the removal of metabolic waste.

Marine fish

The loss of water to the external environment is a problem that all marine fish must deal with. The marine environment in which the fish lives is hyperosmotic to the internal environment, i.e. there is a higher salt concentration in the water than inside the cells. This results in an osmotic gradient in which water is lost from the fish to the environment while ions are gained by diffusion. Ions are excreted by specialised glands.

Freshwater fish

The freshwater environment is hypo-osmotic to the internal environment of fish, i.e. there is a lower salt concentration in the water than inside the cells. This results in an osmotic gradient in which water is gained by the fish from the environment without drinking and salts are lost by diffusion. Ions are absorbed in the gut and by active uptake across the gills.

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

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explain why the processes of diffusion and osmosis are inadequate in removing dissolved nitrogenous wastes

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distinguish between active and passive transport and relate these to processes occurring in the mammalian kidney

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explain how the processes of filtration and reabsorption in the mammalian nephron regulate body fluid composition

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gather, process and analyse information from secondary sources to compare the process of renal dialysis with the function of the kidney

Summary: Comparison of the process of renal dialysis with the function of the kidney

Dialysis means separation in Greek, and, like the nephrons of the kidney, the dialysis machine separates molecules from the blood removing some and returning others. The patient's blood is pumped from an artery through tubes made of selectively permeable membrane. The artificial tubing allows only water and small solute molecules to pass through it into a dialysing solution that surrounds the tube. This dialysing solution is similar to the interstitial fluid found around nephrons. As the blood circulates through the dialysis tubing, urea and excess salts diffuse out of it instead of leaving by pressure filtration, as in the nephron. Those substances needed by the body, such as bicarbonate ions (HCO3 - ) diffuse from the dialysing solution into the blood (reabsorption). The machine continually discards used dialysing solution as wastes build up in it.

Two healthy kidneys filter the blood volume about once every half-hour. Dialysis is a much slower and less efficient process than the natural processes found in a healthy kidney but it is a lifesaver for those people with damaged kidneys.

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

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


Hypoaldosteronism is a condition where people fail to secrete aldosterone. Addison's disease is the name of a disease with these symptoms which include high urine output with a resulting low blood volume. Eventually, as blood pressure falls, this can result in heart failure. A replacement hormone, fludrocortisone (Florinef), is used to treat this condition but a careful monitoring must be maintained to avoid fluid retention and high blood pressure.

Here is an Internet site to get you started in your search.

Addison's disease (external website) Better Health Channel Victorian Government

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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

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

Poster: Salinity (external website) American Society of Plant Biologists, USA

Summary: Coping with salt

Most plants cannot tolerate high salt concentrations in the root zone as it leads to water stress. The salt accumulates in the leaves and is toxic. Enzymes are inhibited by Na+ ions. Halophytes are plants that can tolerate higher levels of salt in their environment.

The grey mangrove, Avicennia marina, has special salt glands in its leaves that excrete salt. Other mangroves exclude salts at their roots through ultrafiltration and a third mechanism is to store salt in leaves and then drop the leaves.

Another mechanism involves the efficient control of transpiration. Some mangroves have small leaves hanging vertically to reduce the surface presented to the sun and thus reducing transpiration.

Salt marsh plants also have mechanisms for salt regulation. For example, Sarcocornia quinqueflora accumulates salt in the swollen leaf bases which fall off, thus removing excess salt and Sporobolus virginicus has salt glands on its leaves.

Another form of salt stress can occur in salt laden air such as in coastal environments. Some coastal plants, such as the Norfolk Island pine, have a mesh of cuticle over their stomates, which prevents small water droplets from entering the leaf.

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perform a first-hand investigation to gather information about structures in plants that assist in the conservation of water

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Describe adaptations of a range of terrestrial Australian plants that assist in minimising water loss

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