Question

Which of the following statements is incorrect? (a) In ureotelic organisms, ammonia is not a product of metabolism. (b) In mammals, some amount of urea may be retained in the kidney matrix to maintain osmolarity. (c) In fish, kidneys do not play any significant role in the removal of ammonium ions. (d) Urea and uric acid are less toxic than ammonia.

Short answer

The incorrect statement is (a) In ureotelic organisms, ammonia is not a product of metabolism. Ammonia is produced but is converted into urea which is less toxic and then excreted.

Step-by-step solution

Understanding Ureotelic Organisms

Ureotelic organisms excrete nitrogenous wastes primarily in the form of urea. The statement says ammonia is not a product of metabolism in ureotelic organisms, which is true because ammonia, which is highly toxic, is converted into less toxic urea.

Examining Mammalian Kidney Function

Mammals are known to conserve water by using urea to maintain the osmolarity of the kidney matrix. So, some amount of urea is indeed retained in the kidneys for this purpose.

Identifying Fish Kidney Function

Fish, especially those living in aquatic environments, excrete ammonia directly into the water. Their kidneys do play a significant role in the removal of excess ions and water but they generally do not play a major role in the removal of ammonia, as it diffuses easily across their gills into the water.

Comparing Toxicity of Nitrogenous Wastes

Urea and uric acid are indeed less toxic than ammonia. Organisms that excrete urea or uric acid can conserve more water as these compounds are less soluble and less toxic, allowing them to be excreted in a more concentrated form.

Identifying the Incorrect Statement

Based on the provided information, statement (a) is the incorrect one. Ureotelic organisms do produce ammonia, but they convert it to urea before excretion.

Key concepts

Ureotelic Organisms

Ureotelic organisms, such as mammals, amphibians, and some marine species, are distinguished by their ability to convert highly toxic ammonia into a much less toxic compound called urea, which is then excreted from the body. This conversion is a key part of the nitrogenous waste management system within these organisms and is crucial for their survival.

In the context of the exercise, understanding that ureotelic organisms do in fact produce ammonia as a byproduct of protein metabolism is essential. However, instead of excreting it directly and risking toxicity, they have evolved a mechanism in their liver known as the urea cycle. During this cycle, ammonia is detoxified by being bound into urea, which is soluble and can be safely transported in the bloodstream to the kidneys for excretion.

This evolutionary strategy serves two primary purposes: first, it prevents the accumulation of toxic ammonia in bodily tissues, and second, as urea is less toxic, it can be concentrated to a higher degree, thus saving water - a process especially beneficial for terrestrial animals.

Mammalian Kidney Function

Mammalian kidneys are complex organs that play several vital roles in maintaining homeostasis, including the regulation of electrolyte balance, blood pressure, and water conservation. One of the key functions relevant to the exercise is the role of the kidneys in osmoregulation, which includes the management of nitrogenous waste products such as urea.The kidneys operate through a series of structures, including nephrons, which are the functional units responsible for filtering and purifying the blood. During this process, some urea is deliberately retained in the kidney's inner medulla to regulate osmolarity – the concentration of solutes in the body fluids. Urea's retention helps to establish a concentration gradient that enables the kidneys to reclaim water from urine and thus conserve it, which is especially important in mammals that live in arid environments or those who have limited access to water.By doing so, mammals can produce urine that is more concentrated than their blood plasma, indicating that water is being conserved effectively. The ability to concentrate urine is a critical adaptation feature that allows mammals to thrive in a variety of habitats.

Ammonia Toxicity

Ammonia is a nitrogenous waste product that arises from the breakdown of proteins and nucleic acids. While it is a common waste product in many organisms, it is also highly soluble in water and extremely toxic, particularly to neural tissues. Excess ammonia can disrupt cellular and organ function, leading to a condition known as ammonia toxicity. This condition can result in serious physiological disturbances including alkalosis, encephalopathy, and in severe cases, death.

In aquatic animals like fish, ammonia can diffuse directly into the surrounding water through their gills. This is an efficient means of ammonia excretion for animals living in an aqueous environment; however, for terrestrial animals, or those in water-scarce environments, this method is not feasible. As described in the textbook exercise, ureotelic organisms have evolved to convert ammonia to urea, diminishing its toxicity. The kidneys then play a crucial role in excreting urea, which is less toxic and conserves more water compared to the excretion of ammonia. This process showcases a vital evolutionary adaptation that facilitates survival in various environments in which the direct excretion of ammonia would be detrimental or inefficient.