Course 15.1Lesson 1 of 3
Cells make wastes all the time, and the most important of them contain nitrogen. Animals remove nitrogen as ammonia, urea or uric acid, and the choice depends on how much water the animal can spend. This lesson explains the three ways and the animals that follow each.
Try the code and the simulations beside the lesson.
Your cells make wastes all the time. When they break down food for energy, they make carbon dioxide and water. When they break down proteins and nucleic acids, they make wastes that contain nitrogen. Salts such as sodium, potassium, chloride, phosphate and sulphate may also be in excess.
Excretion is the removal of these metabolic wastes from the body. If they stay, they poison the cells. So every animal has some way to excrete.
The most important wastes contain nitrogen. They come mostly from the breakdown of amino acids, which first gives ammonia. There are three main nitrogenous wastes: ammonia, urea and uric acid.
Ammonia is the most toxic, and uric acid is the least. A toxic waste has to be kept very dilute, so it needs a lot of water to wash it out. A waste that is not toxic can be stored for a while and removed with little water.
You can see the plan of nature here. An animal that has plenty of water can use the cheapest waste to make, which is ammonia. An animal that must save water makes a waste that is harmless, even if making it costs more energy.
Animals that excrete mainly ammonia are ammonotelic. Many bony fishes, aquatic amphibians and aquatic insects are ammonotelic.
Ammonia is very soluble in water. It is removed by diffusion across the body surface or across the gill surfaces, as ammonium ions (NH). The kidneys do not play a large part. The animal can do this because it lives in water, which dilutes the ammonia at once.
On land, water is scarce, and a toxic waste such as ammonia cannot be allowed to build up. Terrestrial animals changed ammonia into less toxic wastes. Mammals, many terrestrial amphibians and marine fishes mainly excrete urea. They are ureotelic.
The ammonia that the body makes is changed into urea in the liver. The urea is released into the blood, which is filtered by the kidneys, and it leaves with the urine. In some of these animals, a part of the urea is kept in the matrix of the kidney. It helps to keep the right osmolarity there, as you will see in the next course.
A note on marine fishes. NCERT says that marine fishes mainly excrete urea. In fact the cartilaginous fishes, such as sharks, are the ones that keep urea in their bodies. Most bony fishes, marine or not, excrete ammonia. For a NEET question, follow NCERT and the words of the question.
An extra. The liver makes urea from ammonia and carbon dioxide by a cycle of reactions called the ornithine cycle. This is not in the NEET syllabus, but it is in A-Level and IB courses.
Reptiles, birds, land snails and insects excrete nitrogen as uric acid. They are uricotelic. Uric acid is almost insoluble in water. It leaves as a pellet or paste, with the least loss of water.
This is a great help to an animal that must save water. A bird or a reptile lays its eggs on land. The waste of the growing young is stored inside the egg as solid uric acid, which does no harm.
The table above counts atoms in a molecule. Uric acid holds four nitrogen atoms, so one molecule carries four times the nitrogen of an ammonia molecule. But a uric acid molecule is also much heavier. The next simulation counts the mass that carries a given mass of nitrogen.
You should find that ammonia carries the nitrogen in the least mass, and uric acid in the most. But uric acid leaves as a solid, which needs almost no water, while ammonia must be washed out in a great deal of water. What matters to an animal that lacks water is the water, not the mass.
NCERT also names the organs that other animals use. This part is not in the NEET syllabus text, but NCERT prints it, and school boards may ask it.
The program counts the nitrogen share of each waste and the mass of waste that holds 12 g of nitrogen.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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