Climate Change 5: Evolution 1. Evolution Evidence 2. Natural Selection 3. Classification 4. Cladistics 6: Human Physiology 1.
Digestion 2. The Blood System 3. Disease Defences 4. Gas Exchange 5. Homeostasis Higher Level 7: Nucleic Acids 1. DNA Structure 2. Transcription 3. Translation 8: Metabolism 1. Metabolism 2. Cell Respiration 3. Photosynthesis 9: Plant Biology 1. Nitrogenous wastes tend to form toxic ammonia , which raises the pH of body fluids. The formation of ammonia itself requires energy in the form of ATP and large quantities of water to dilute it out of a biological system. Animals that live in aquatic environments tend to release ammonia into the water.
Animals that excrete ammonia are said to be ammonotelic. Terrestrial organisms have evolved other mechanisms to excrete nitrogenous wastes. The animals must detoxify ammonia by converting it into a relatively nontoxic form such as urea or uric acid.
Mammals, including humans, produce urea, whereas reptiles and many terrestrial invertebrates produce uric acid. Animals that secrete urea as the primary nitrogenous waste material are called ureotelic animals.
The urea cycle is the primary mechanism by which mammals convert ammonia to urea. Urea is made in the liver and excreted in urine. The urea cycle utilizes five intermediate steps, catalyzed by five different enzymes, to convert ammonia to urea, as shown in Figure 4.
The amino acid L-ornithine gets converted into different intermediates before being regenerated at the end of the urea cycle. Hence, the urea cycle is also referred to as the ornithine cycle.
The enzyme ornithine transcarbamylase catalyzes a key step in the urea cycle and its deficiency can lead to accumulation of toxic levels of ammonia in the body. The first two reactions occur in the mitochondria and the last three reactions occur in the cytosol. Urea concentration in the blood, called blood urea nitrogen or BUN, is used as an indicator of kidney function.
The theory of evolution proposes that life started in an aquatic environment. It is not surprising to see that biochemical pathways like the urea cycle evolved to adapt to a changing environment when terrestrial life forms evolved. Arid conditions probably led to the evolution of the uric acid pathway as a means of conserving water. Birds, reptiles, and most terrestrial arthropods convert toxic ammonia to uric acid or the closely related compound guanine guano instead of urea.
Mammals also form some uric acid during breakdown of nucleic acids. Uric acid is a compound similar to purines found in nucleic acids. It is water insoluble and tends to form a white paste or powder; it is excreted by birds, insects, and reptiles. Conversion of ammonia to uric acid requires more energy and is much more complex than conversion of ammonia to urea Figure 5.
Figure 5. Nitrogenous waste is excreted in different forms by different species. These include a ammonia, b urea, and c uric acid. Answer the question s below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times. Use this quiz to check your understanding and decide whether to 1 study the previous section further or 2 move on to the next section.
Skip to main content. Malpighian tubules work cooperatively with specialized glands in the wall of the rectum. Body fluids are not filtered, as in the case of nephridia. Instead, urine is produced by tubular secretion mechanisms by the cells lining the malpighian tubules that are bathed in hemolymph. Metabolic wastes, such as urea and amino acids, freely diffuse into the tubules, while ions are transported through active pump mechanisms.
The secretion of ions alters the osmotic pressure, which draws water, electrolytes, and nitrogenous waste uric acid into the tubules. Water and electrolytes are reabsorbed when these organisms are faced with low-water environments and uric acid is precipitated and excreted as a thick paste or powder.
By not dissolving wastes in water, these organisms are able to conserve water; this is especially important for life in dry environments.
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