15. Excretion and Osmoregulation part 01 - Ammonotelism , Ureotelism ,Uricotelism and types of nephridia

 

15. Excretion and Osmoregulation part 01 - Ammonotelism , Ureotelism ,Uricotelism and types of nephridia

Metabolism :

• Array of chemical processes occur in the body of an organism. Sum total of these processes is called 'metabolism'. 
• Metabolism involves catabolic (breaking down) and anabolic (building up) processes. 
• Metabolism produces a variety of by-products, some of which need to be eliminated. Such byproducts are called metabolic 'waste products'. 
• Metabolic waste products might be fluid, gaseous, organic or inorganic. 
• Depending on the type, they are eliminated through various organs of the body of an organism.

Excretion and excretory products :

• Elimination of metabolic waste products from the body is called excretion.
• Metabolic wastes are produced inside body cells.

various excretory waste products produced in human body :

• Fluids :  water 
• Gaseous wastes :CO2
• Nitrogenous wastes : ammonia, urea and uric acid, creatinine, mineral, salts of sodium, potassium, calcium, etc. which if, are present in body in excess are excreted through urine,faeces and sweat. 
• Pigments : formed due to breakdown of haemoglobin are like bilirubin (excreted through faeces) and urochrome (eliminated through urine). 
• Also, the pigments present in foodstuff like beet root consumed by organism
•  excess of vitamins, hormones and drugs taken are eliminated. 
• Spices we consume contain volatile substances that are excreted through lungs.

Deamination :

• Body of an organism can store excess carbohydrates and fats but is unable to store excess amino acids. 
• Hence, excess amino acids are essentially broken down by a process called as deamination. 
• In this process, amino group is separated from the amino acid and ammonia is formed. 
• Toxic ammonia is either excreted as it is or further converted to less toxic forms like urea or uric acid before excretion.
• There is no clear correlation between the phylogenetic relationship of organisms and their major excretory products, but it's habitat has. 
• e.g. Tadpole of frog excretes ammonia and adult frogs mostly are ureotelic. 
• Some terrestrial turtles excrete uric acid whereas others excrete urea or ammonia.
• Ammonia is the basic product of deamination process. But it is highly toxic.Hence, it is to be diluted immediately. 
• If there is no or limited access to water, need for conversion of ammonia becomes necessary.
• Thus, availability of water plays key role in deciding mode of excretion of an organism.

Types of Animals [based on nitrogenous wastes they produce] :

1. Ammonotelism :

•  Elimination of nitrogenous wastes in the form of ammonia is called as ammonotelism. 
• It is basic in nature. Hence, it would disturb pH of body, if retained. 
• Slight increase in pH would disturb all enzyme catalyzed reactions in body and would also make the plasma membrane unstable. 
• It is readily soluble in water and needs large quantity of water to dilute and reduce the toxicity. 
• However, it is energy saving mechanism of excretion. 
• Hence all animals that have plenty of water available for dilution of ammonia, excrete nitrogenous wastes in the form of ammonia. Such animals are called ammonotelic. 
• 1 gm ammonia needs about 300– 500 ml of water for elimination.
• Ammonotelism is found in aquatic invertebrates, bony fishes, and aquatic / larval amphibians. 
• Animals without excretory system are also ammonotelic. e. g. Protozoa
• Ammonotelic animals excrete ammonia through general body surface (skin), gills and kidneys.

2. Ureotelism : 

• Elimination of nitrogenous wastes in the form of urea (H2N-CO-NH2) is called as ureotelism. 
• Urea is less toxic and less water-soluble than ammonia. Hence it can be concentrated to some extent in body. 
• Due to this, it requires less water for elimination. (Compared to ammonia, about 100 time less water in human, several hundred times in camel, kangaroo rat and shark). 
• As it is less toxic and less water soluble; hence, ureotelism is suitable for animals those need conservation of water to some extent. Hence it is common in terrestrial animals, as they have to conserve H2O. 
• It takes about 50 ml H2O for removal of 1 gm NH2 in form of urea. 
• Mammals, cartilaginous fishes (sharks and rays), many aquatic reptiles, most of the adult amphibians, etc. are ureotelic. 
• They convert ammonia to urea in liver by operating ornithine / urea cycle (Krebs and Hanseleit, 1932). 
• 3 ATP molecules are used to produce one molecule of urea.
• Sharks retain more urea in their body fluid (blood) to make their blood isotonic to surrounding marine water. This helps them to prevent possible loss of water by exosmosis.

3. Uricotelism (C5H4O3N4) :

• Elimination of nitrogenous wastes in the form of uric acid is called as uricotelism. 
• Uric acid is least toxic. Hence, it can be retained in the body for sometime in concentrated form. 
• It is least soluble in water. Hence minimum (about 5—10 ml for 1 gm) or no need of water for its elimination.
• Hence, animals those need to conserve more water follow uricotelism.
• Ammonia is converted into uric acid by ‘inosinic acid pathway’ in the liver of birds. 
• Birds, some insects, many reptiles, land snails, are uricotelic; because they need to conserve the water. However, theyhave to spend more energy.

Plasma creatinine :

• Plasma creatinine is produced from catabolism of creatinine phosphate during skeletal muscle contraction. 
• It provides ready source of high energy phosphate. 
• Normally blood creatinine levels remain steady because the rate of production matches it's excretion in urine. 
• Hence, level above normal is an indication of poor renal function.

Homeostasis :

• Excretory organs play an important role in maintenence of constant internal environment of the body called homeostasis.
• It requires osmoregulation, the process of controling solute concentrations and water balance. 
• It can be rightly said that composition of blood (and internal environment) is determined not by what mouth ingests but by what excretory organs retain.

Salt glands :

• Marine birdshave special glands called salt glands near nostrils.
• These are capable of secreting salts by active transport and help to manage osmotic balance. 
• Many marine organisms like sea turtles and marine iguanas also have such salt excreting glands.

Osmoconformers : 

• Animals which are  isoosmotic to the surrounding.

•  Marine organisms mostly are osmocomformers because their body fluids and external environment are isoosmotic in nature.

osmoregulators :

• Animals which are control internal environment independent of external environment.
• Fresh water forms and terrestrial organisms are osmoregulators.

Stenohaline organisms :

• Whether conformers or regulators, most organisms can tolerate only narrow range of salt concentrations. Such organisms are called stenohaline organisms. (steno : narrow)

Euryhaline organisms : 

• Those who are capable of handling wide changes in salinity are called euryhaline organisms 
• ex. barnacles, clams etc.

Unicellular forms :

• Unicellular forms have contractile vacuoles which collect and discharge waste products outside the cell. 

Excretion in sponges :

• Excretion in sponges takes place by diffusion of waste material in water which is discharged through osculum.

Nephridiopores. :

• True organs of excretion are found in those animals that show bilateral symmetry.
• Most common type is simple or branching tube that opens to exterior through pores called nephridiopores.

Two major types of nephridia :

1. Protonephridia 
2. Metanephridia 

1. Protonephridia : 

• These are network of dead end tubes called flame cells. 
• These are found mostly in animals that lack true body cavity e.g.Platyhelminthes. 
• Protonephridia are also found in rotifers, some annelids and Amphioxus.

2.Metanephridia : 

• These are unbranched coiled tubes that connect to body cavity through funnel like structures called nephrostomes. 
  
• Body fluid enters the nephridium through nephrostome and gets discharged through nephridiopore. eg. Earthworms. 

• In most of the insects, excretion takes place by set of blind ended tubules called malpighian tubules. 
• Crustaceans have green glands as excretory organs.
•  Members of phylum Echinodermata do not have any specialised excretory organs. 
• Waste materials directly diffuse into water or are excreted through tube feet. 
• Mammalian kidneys are a collection of functional units called nephrons, which are well designed to extract metabolic waste.