EXCRETORY SYSTEM

Metabolism can be divided into catabolism and anabolism. During
catabolism, energy-containing compounds are degraded to produce energy
for the chemical, mechanical and electrical processes that occur in the
body. The compounds degraded by catabolism contain only carbon,
hydrogen and nitrogen. Before being catabolized, more complicated
molecules are reduced in size and other atoms, such as sulfur,
phosphorus and nitrogen, are removed. Only part of the energy contained
in these compounds, usually less than 50%, is converted into useful
energy. The rest is lost as heat. During anabolism new molecules are
synthesized, such as proteins, carbohydrates, nucleic acids and lipids.
In a growing animal anabolism can be intense, but in an adult, anabolism
just replaces compounds that are degraded or used. Catabolism must be
more intensive than anabolism because of the additional energy required
for mechanical and electrical processes, as well as the energy lost as
heat. Metabolism produces a number of byproducts, some of which must be
excreted. However, what is waste to one animal or organ, may be a
treasure to another. Carbon dioxide, for example, is a byproduct of
metabolism that is produced during oxidative catabolism, and is excreted
in large amounts via the lungs and skin. However, it is also very
important component of the carbonate buffering system of blood and is
used in the synthesis of many compounds. Water is produced as the last
step in oxidative catabolism and is excreted in large amounts by most
animals, but in desert animals it is highly conserved. The water
produced by oxidative catabolism is called metabolic water, and for some
desert animals the only water they have available is metabolic water.
Nitrogen is usually excreted, and in s0ome forms (i.e. ammonia) is
toxic. But elasmobranch fish (i.e. sharks, rays) use nitrogen in the
form of urea to maintain the osmotic pressure of the blood isosmotic
with seawater. In cows, and other ruminents, urea is secreted into the
rumen (via the salivary glands) where it is used as a source of nitrogen
by the symbiotic bacteria and protozoa. It is evident, therefore, that
what is excreted varies between animals, although the most common
excretory products are carbon dioxide and nitrogen. The topic of
excretion usually covers how animals deal with excess nitrogen. The body
is usually faced with an excess of amino acids derived from the diet and
turnover of cellular proteins. In carnivores, such as a trout or a cat,
the major source of energy is obtained by catabolism of amino acids,
which are in excess because of the animal\'s diet. Excess carbohydrate
and lipid can be stored as glycogen and triglycerides, but there is no
storage form for amino acids. The body deals with most excess amino
acids by deaminating them (removing the nitrogen) and using the carbon
skeleton to synthesize glucose. This is called gluconeogenesis which
occurs in the kidney and liver. An example is the deamination of serine
to produce pyruvate. Gluconeogenesis has been shown to be important in
several invertebrates, including snails, clams, insects, and vertebrates
including fish, birds, reptiles and mammals. It is an important way that
the body maintains the levels of glucose in the blood. Many animals
synthesize urea to detoxify ammonia. The biochemical pathway used is
called the urea cycle. The important point to notice is that part of
this pathway occurs in the mitochondria. We learned above that the
ammonia is produced in the mitochondria and disrupts ATP formation when
it diffuses out of the mitochondrion. In the mitochondrion, the
conversion of ornithine to citrulline involves the utilization of NH3 in
the formation of carbamyl-phosphate. The enzyme involved in this
reaction, carbamyl phosphate synthetase can amount to 15-20% of the
total protein in the mitochondrion, an indication of the importance of
detoxification of ammonia. The citrulline diffuses out of the
mitochondrion where it is converted to arginine. Arginase cleaves off
urea, producing ornithine which cycles back to the mitochondrion. Many
species of terrestrial invertebrates as well as birds and reptiles
detoxify ammonia by producing uric acid. The advantage to these animals
is the insoluble nature of uric acid. When it crystallizes out of
solution, the uric acid has no effect on the osmotic pressure of the
urine, and more water can be reabsorbed without an increase in osmotic
concentration which would oppose the uptake of water. The uric acid
pathway also begins in the mitochondrion with the conversion of
glutamate to lutamine by the addition of NH3. The glutamine diffuses
out of the mitochondrion and is converted to uric acid by the addition
of several nitrogen and carbon atoms from glycine and aspartate. The
nitrogen in uric acid therefore derives from several amino acids. This
pathway is part of the pathway