Excretion and Homeostasis
Senior 5 · Biology
Excretion and Homeostasis
Lesson 1 of 4
20 min
Anyone who keeps chickens knows that their droppings have a white, pasty cap. That white paste is not faeces at all but uric acid, the bird's way of getting rid of the nitrogen from the protein it eats, using very little water. A cow gets rid of the same nitrogen as urea dissolved in litres of urine, and a tilapia simply lets ammonia diffuse out of its gills into the pond. Why do animals do the same job in three such different ways? This lesson looks at what excretion is, which wastes animals make, and the organs they use to remove them.
Definition
Excretion: the removal from the body of the waste products of metabolism, such as carbon dioxide and urea, and of substances in excess.
Egestion: the removal of undigested food (faeces), which was never absorbed into the body's cells. Egestion is not excretion.
Deamination: the removal of the amino group () from an amino acid, in the liver, forming ammonia and a keto acid.
Nitrogenous waste: a nitrogen-containing excretory product: ammonia, urea or uric acid.
| Waste | Where it comes from | Main excretory organ in humans |
|---|---|---|
| Carbon dioxide | Aerobic respiration | Lungs |
| Urea | Deamination of excess amino acids | Kidneys (urine); a little in sweat |
| Bile pigments, such as bilirubin | Breakdown of haemoglobin from old red blood cells | Liver, into bile and then the faeces |
| Excess water and salts | Diet and metabolism | Kidneys; skin (sweat) |
Bile pigments leave with the faeces, but they are excreted, because they were made by the body's own metabolism.
Animals cannot store amino acids. Those not needed to build proteins are deaminated in the liver (Figure 1). The keto acid that is left can be respired or turned into glucose or fat. The ammonia is far too toxic to keep, so liver cells combine it with carbon dioxide in a series of enzyme-controlled reactions, the ornithine cycle, which uses ATP. Overall:
Urea, , passes into the blood and is removed by the kidneys.
Worked example 1: how much urea do you make in a day?
A student eats 70 g of protein a day and is not growing, so all of its nitrogen must be excreted. Protein is about 16% nitrogen by mass. Assuming all the nitrogen leaves as urea, (; of N = 14), estimate the mass of urea made per day.
Check: this fits the usual figure of about 20 to 30 g of urea in an adult's urine each day. It also shows why a high-protein diet means more urea for the kidneys to remove.
| Property | Ammonia | Urea | Uric acid |
|---|---|---|---|
| Toxicity | Very high | Low | Very low |
| Solubility in water | Very soluble | Soluble | Almost insoluble |
| Water needed to excrete it | A great deal: must be diluted at once | A moderate amount | Very little: passed as a paste |
| Energy (ATP) to make it | None | Some | Most |
| Nitrogen atoms per molecule | 1 | 2 | 4 |
| Typical animals | Bony fish, tadpoles, many aquatic invertebrates | Mammals, adult amphibians, sharks | Birds, reptiles, insects, land snails |
The pattern follows the habitat. An animal surrounded by water can afford to lose water, so it gets rid of ammonia, the cheapest waste, straight away. Land animals must save water, so they spend energy on urea or uric acid. Uric acid also suits animals that develop inside a shelled egg, such as birds and reptiles: the embryo cannot get rid of its waste, and insoluble uric acid can be stored safely in the egg until it hatches.
| Organism | Structure | How it works |
|---|---|---|
| Amoeba, Paramecium | Contractile vacuole | Collects excess water that enters by osmosis and expels it; nitrogenous waste diffuses out across the cell membrane |
| Flatworms (planarians) | Flame cells | Beating cilia drive fluid into a network of tubes that open to the outside |
| Earthworm | Nephridia, one pair per segment | Coiled tubes collect fluid from the body cavity and reabsorb useful substances |
| Insects | Malpighian tubules | Tubules in the blood-filled body cavity take in uric acid, which passes into the gut and leaves with the faeces as water is reabsorbed |
| Vertebrates | Kidneys with nephrons | Filter the blood and reabsorb what is useful (lesson 4) |
Worked example 2: matching waste to way of life
A frog's tadpole excretes mainly ammonia; the adult frog, which lives partly on land, excretes mainly urea. Explain the change, using the properties in the table.
General rule: the less water an animal can spare, the less toxic and the less soluble its nitrogenous waste, and the more energy it spends making it.
Warning
Common mistakes
Khan Academy India: "Nitrogenous wastes". Watch for why ammonia must be diluted with so much water, and how animals that excrete ammonia, urea and uric acid are grouped by their habitat; compare it with the table above.
Tip
Memory hook: as you go from Ammonia to Urea to Uric acid, everything goes down except the cost: toxicity down, solubility down, water needed down, but energy cost up. Water-rich habitat: ammonia. Dry land: uric acid.
Practice: excretory products and systems
Key points
Sources and credits
Figure 1, the tables, the worked examples and the practice questions are by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
