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3.14 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

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

Background

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.

Organism
Terrestrial or aquatic
Waste product(s)
Explanation
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.