Elimination: Nitrogenous Wastes | Biology for JAMB PDF Download

All living organisms on Earth have their unique methods of getting rid of waste materials and excess substances. Excretion is a important process that takes place in all forms of life, including plants, animals, birds, insects, and even single-celled organisms. Different organisms have different ways of excreting waste depending on their body structure and physiological processes. 

What is Excretion ?

Excretion is the natural process by which the body removes waste products that are produced during metabolism. These waste products consist of substances such as amino acids, urea, uric acid, carbon dioxide, water, and ammonia.

Importance of Excretion

• Excretion plays a role in maintaining homeostasis, which is the process of keeping the body's internal environment stable and balanced. This stability is crucial for the proper functioning and survival of organs and biological systems.
• The organisms regulate the osmotic pressure, which involves maintaining a balance between inorganic ions and water, and they also ensure the proper acid-base balance through this mechanism.

Types of Excretory Products

Types Of Excretory Products

(a) Ammonia 
Ammonia is highly toxic and highly soluble in water. Its excretion as such, therefore, requires a large amount of water. That is why, most of the aquatic arthropods, bony and freshwater fishes, amphibian tadpoles, turtles, etc, excrete ammonia.

(b) Urea
This is less toxic and less soluble in water than ammonia. Land vertebrates like adult amphibians and mammals, as well as certain aquatic animals that need to conserve water, undergo a conversion process in which ammonia is transformed into urea for the purpose of excretion.

(c) Uric Acid
Animals that inhabit dry (arid) environments, including  most insects, land reptiles like snakes and lizards, birds to conserve water ,they convert their ammonia into uric acid.

Modes of Excretion In Animals

(i) Ammonotelism

• Ammonotelism refers to the elimination of ammonia from the body, and organisms that exhibit this characteristic are called ammonotelic.  
• This type of excretion is common among fish, protozoans, echinoderms, poriferans, and crustaceans.

(ii) Ureotelism 

• Ureotelism is the process of excreting urea as a metabolic waste product, and organisms that exhibit this behavior are known as ureotelic.  
• Some Urea is retained in the kidney to help regulate osmolarity. Humans, being ureotelic, excrete urea through urine. urea is less toxic compared to ammonia.

(iii)

• Uricotelism is the process observed in uricotelic animals, where nitrogenous wastes are eliminated
  in the form of uric acid.
• Uricotelism is commonly found in reptiles, birds, and insects.

Excretory Organs In Different Organisms

• In Platyhelminthes (Flatworms), rotifers, and certain annelids, there exist specialized structures known as protonephridia or flame cells. These structures primarily regulate the balance of ions and fluids within the body, a process referred to as osmoregulation.
• Earthworms and other annelids possess tubular excretory structures called nephridia. Nephridia play a vital role in eliminating nitrogenous wastes from the body and maintaining a proper balance of fluids and ions.
• Insects, such as cockroaches, possess excretory structures called Malpighian tubules. These tubules aid in the elimination of nitrogenous wastes and contribute to osmoregulation.
• Crustaceans, like prawns, possess excretory structures known as antennal glands or green glands, which serve as waste removal organs.
• In Humans , the excretory organs are kidneys.