Question

Select the true statement -

• A. In fishes kidney play a major role in ammonia excretion
• B. Sharks retain a large amount of urea in the blood as a major osmolyte to balance the osmolarity of the body fluids
• C. Most terrestrial reptile excrete ammonia
• D. Ammonia is $100,000$ times less toxic than urea

Answer 1

The correct option is B Sharks retain a large amount of urea in the blood as a major osmolyte to balance the osmolarity of the body fluids

Answer: (c)

Sharks retain more urea in their body fluid (blood) to make their blood isotonic to surrounding marine water (in order to maintain osmotic balance). This helps them to prevent possible loss of water by exosmosis.

The process of excreting ammonia is Ammonotelism. Many bony fishes, aquatic amphibians and aquatic insects are ammonotelic in nature. Ammonia, as it is readily soluble, is generally excreted by diffusion across body surfaces or through gill surfaces (in fish) as ammonium ions.

Ammonia is more toxic than urea, as it requires large amount of water for its excretion.

Terrestrial adaptation necessitated the production of lesser toxic nitrogenous wastes like urea and uric acid for conservation of water.