Nitrogenous Excretory Products
Nitrogenous waste
products are produced by the breakdown of proteins, nucleic acids etc. Ammonia
is produced by the removal of amino group (NH2) from amino acids, by
a process called deamination. Amino group is transferred to another molecule
for removal or reuse. Amino group which is not reused for recycling of amino
acids is dissolved in water and excreted to avoid toxic rise in the plasma.
Elevated levels of nitrogenous wastes can cause convulsions, coma and
eventually death. Ammonia may be excreted immediately or converted to urea or
uric acid. Lower quantities of nitrogen are excreted in the form of other
compounds such as creatine, a creatinine or trimethylamine oxide and in very
small quantities as amino acids, purine and pyrimidine. Metabolism of purine
and pyrimidine bases produces significant amount of nitrogenous wastes of
hypoxanthine, xanthine, uric acid, allantoin, urea and ammonia.
Nature of Excretory Products in Different Habitats
The exact nature of the
excretory product is determined mainly by the availability of water to the
organism i.e. habitat, and the extent to which the organism controls the water
loss. The correlation with habitat is (a) ammonia-aquatic (b) urea- aquatic and
terrestrial (c) uric acid - terrestrial.
Nitrogenous Excretory Products
Nitrogenous waste
products are produced by the breakdown of proteins, nucleic acids etc. Ammonia
is produced by the removal of amino group (NH2) from amino acids, by
a process called deamination. Amino group is transferred to another molecule
for removal or reuse. Amino group which is not reused for recycling of amino
acids is dissolved in water and excreted to avoid toxic rise in the plasma.
Elevated levels of nitrogenous wastes can cause convulsions, coma and
eventually death. Ammonia may be excreted immediately or converted to urea or
uric acid. Lower quantities of nitrogen are excreted in the form of other
compounds such as creatine, a creatinine or trimethylamine oxide and in very
small quantities as amino acids, purine and pyrimidine. Metabolism of purine
and pyrimidine bases produces significant amount of nitrogenous wastes of
hypoxanthine, xanthine, uric acid, allantoin, urea and ammonia.
Nature of Excretory Products in Different Habitats
The exact nature of the
excretory product is determined mainly by the availability of water to the
organism i.e. habitat, and the extent to which the organism controls the water
loss. The correlation with habitat is (a) ammonia-aquatic (b) urea- aquatic and
terrestrial (c) uric acid - terrestrial.
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Ammonia:
The major source of ammonia is the deamination of excess of amino acids.
Ammonia is extremely toxic and must be eliminated. Being very soluble, it can
be eliminated from the body rapidly and safely if diluted in a sufficient
volume of water. This presents no real problems to organisms which have ready
access to water but this applies only to those organisms living in freshwater.
It is therefore excreted rapidly as ammonium ions (NH4++) in most aquatic
organisms, from protozoa to amphibia, fishes, before reaching to concentrations
toxic to the organisms. About 500 ml water is needed to excrete 1 gram of
ammonia.
Urea:
It is less toxic than ammonia. About 50 ml of water is needed to excrete 1 gram
of urea.
Uric
acid: Uric acid and its salts are ideal excretory
products for terrestrial organisms and essential for organisms such as
land-living insects and birds which produce shelled eggs. They can be stored in
cells, tissues and organs without producing any toxic or harmful osmoregulatory
effects. Only 1 ml of water is required to eliminate I gram of uric acid.
Excretion in Various Groups of Animals
Animals excreting
ammonia, urea and uric acid are called ammonotelic, ureotelic and uricotelic
respectively. There is diversity in the excretory structures in the animals.
Excretion in Hydra, Planaria, earthworm, cockroach and man is discussed as the
main representative models.
Excretion in Hydra
Hydra is a freshwater
organism. For excretion there are no specialized structures in Hydra. Carbon
dioxide, salts and ammonia leave the cells by diffusion into the iso- osmotic
surroundings.
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Excretion in Planaria
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Excretion in Planaria
Planaria, which lives
in fresh water, has two strands of branching excretory tubes that open to the
outside of the body through excretory pores. The simple tubular excretory
system is called protonephridium, which is a network of closed tubules without
internal openings. Tubular system is spread throughout the body. Located along
the tubules are bulb-like flame cells.
Flame
Cells: The flame cells consist of cytoplasmic body and
some cytoplasmic prolongations. The cytoplasm is hollowed out to form a large
cavity. The cavity is continuous with the cavity of the finer tubules. As a
result nucleus is placed on one side. A tuft of cilia projects into the space
(cavity) of the cell. As the movement of the cilia resembles the flickering
flame under the microscope so it is called flame cell. The beating of
flame-cell’s cilia propels hypotonic fluid through the excretory canal and out
of the body through numerous nephridiopores. The system functions in
osmoregulation and also in excreting nitrogenous wastes. Fresh water flatworm
excretes very dilute urine.
Excretion in Earthworm
The body of earthworm
is divided into segments by a series of partitions called septa and each body
segment has a pair of tubular excretory structures called nephridia. The
nephridia are more complex than that of Planaria and are called metanephridia.
Each nephridium is a tubule open at both ends. Each nephridium begins with a
ciliated funnel the nephrostome, that opens from the body cavity of a segment
in a coiled tubule enlarged bladder that empties to the outside of the body via
an opening called the nephridiopore. Fluid from the body cavity flows through
the nephrostome into the nephridium. As fluid from the coelom is propelled
through the tubule by beating cilia, certain substances are re-absorbed and
carried away by a network of capillaries surrounding the tubule. This process
results in the formation of urine that contains only metabolic wastes, salts
and water. Urine is given out of the body through nephridiopore.
Excretion in Cockroach
Insects have an
excretory system quite different from other animals. The excretory ducts of
insects do not open to the outside of the body. Instead, the system consists of
a cluster of long, thin Malpighian tubules. These are the only excretory
structures in animal kingdom that are attached to the gut. Together the
Malpighian tubules and the gut function as the insect’s excretory system.
Malpighian tubules are present between the stomach and large intestine. They
originate as an outgrowth, from the beginning of the large intestine. They are
lined with glandular cells having a characteristic brush border. The number of
Malpighian tubules is variable, usually 15-20. They are blind at the distal end
and bathe into the haemolymph of the haemocoel.
Uric acid simply flows
from the surrounding haemolymph into the Malpighian tubules, and water follows
a salt gradient established by active transport of K. Water and salts are
reabsorbed in the rectum, but the uric acid crystals, leave the body at the
anus.
Excretion in Planaria
Planaria, which lives
in fresh water, has two strands of branching excretory tubes that open to the
outside of the body through excretory pores. The simple tubular excretory
system is called protonephridium, which is a network of closed tubules without
internal openings. Tubular system is spread throughout the body. Located along
the tubules are bulb-like flame cells.
Flame
Cells: The flame cells consist of cytoplasmic body and
some cytoplasmic prolongations. The cytoplasm is hollowed out to form a large
cavity. The cavity is continuous with the cavity of the finer tubules. As a
result nucleus is placed on one side. A tuft of cilia projects into the space
(cavity) of the cell. As the movement of the cilia resembles the flickering
flame under the microscope so it is called flame cell. The beating of
flame-cell’s cilia propels hypotonic fluid through the excretory canal and out
of the body through numerous nephridiopores. The system functions in
osmoregulation and also in excreting nitrogenous wastes. Fresh water flatworm
excretes very dilute urine.
Excretion in Earthworm
The body of earthworm
is divided into segments by a series of partitions called septa and each body
segment has a pair of tubular excretory structures called nephridia. The
nephridia are more complex than that of Planaria and are called metanephridia.
Each nephridium is a tubule open at both ends. Each nephridium begins with a
ciliated funnel the nephrostome, that opens from the body cavity of a segment
in a coiled tubule enlarged bladder that empties to the outside of the body via
an opening called the nephridiopore. Fluid from the body cavity flows through
the nephrostome into the nephridium. As fluid from the coelom is propelled
through the tubule by beating cilia, certain substances are re-absorbed and
carried away by a network of capillaries surrounding the tubule. This process
results in the formation of urine that contains only metabolic wastes, salts
and water. Urine is given out of the body through nephridiopore.
Excretion in Cockroach
Insects have an
excretory system quite different from other animals. The excretory ducts of
insects do not open to the outside of the body. Instead, the system consists of
a cluster of long, thin Malpighian tubules. These are the only excretory
structures in animal kingdom that are attached to the gut. Together the
Malpighian tubules and the gut function as the insect’s excretory system.
Malpighian tubules are present between the stomach and large intestine. They
originate as an outgrowth, from the beginning of the large intestine. They are
lined with glandular cells having a characteristic brush border. The number of
Malpighian tubules is variable, usually 15-20. They are blind at the distal end
and bathe into the haemolymph of the haemocoel.
Uric acid simply flows
from the surrounding haemolymph into the Malpighian tubules, and water follows
a salt gradient established by active transport of K. Water and salts are
reabsorbed in the rectum, but the uric acid crystals, leave the body at the
anus.
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| Excretion in Cockroach |
