Question

The animal which retains urea for hypertonicity is (most appropriate) (a) Elasmobranch (b) Man (c) Bird (d) Amphibian

Short answer

(a) Elasmobranch

Step-by-step solution

Understanding the Terms

Firstly, understand the meaning of the key terms here. Urea is a water-soluble molecule made in the liver that the body produces as a waste product of protein metabolism. Hypertonicity refers to the state of a solution in relation to another one where it has a higher concentration of solute. In the context of biology, it generally means a state where the body fluid concentration is higher than usual.

Knowing about Different Animals' Excretory Habit

Some animals excrete their nitrogenous wastes as ammonia (ammonotelic) while others as urea (ureotelic) or uric acid (uricotelic). Humans, for example, are ureotelic while birds and reptiles excrete uric acid.

Analyzing the Options

Some animals, like marine fish (elasmobranchs e.g., shark) have adapted to retain urea in their tissues to counterbalance the osmotic pressure of the seawater. In these animals, the concentration of urea is quite high which results in hypertonicity. None of the other given options - humans, birds, or amphibians - have this adaptation.

Key concepts

Nitrogenous Wastes

Nitrogenous wastes are the byproducts of protein and nucleic acid metabolism in living organisms. The primary forms include:

• Ammonia: Highly toxic and requires a lot of water to be excreted, so it's usually found in aquatic animals like fish.
• Urea: Less toxic than ammonia and soluble in water, making it common in mammals such as humans who can afford to lose water during excretion.
• Uric Acid: Least toxic and water-efficient, often excreted by birds and reptiles.

Each type of waste is linked to the animal's habitat and lifestyle. For example, terrestrial mammals excrete urea as they can drink to compensate for water loss, whereas birds conserve water by excreting uric acid. Understanding the types of nitrogenous wastes helps to understand animals' adaptations to their environments.

Urea Retention

Urea retention is a fascinating adaptation found in some marine animals, particularly elasmobranchs like sharks. These animals have evolved a mechanism to retain high urea levels in their bodies. This process helps counteract the osmotic pressure of the surrounding seawater.
The retained urea increases the internal solute concentration and creates hypertonicity, preventing excessive water loss from their bodies. It's a smart adaptation that allows them to survive in salty environments. A reason why animals like humans, birds, or amphibians don't retain urea is because they live in varying environments where such an adaptation isn't necessary. Therefore, understanding urea retention offers insights into how organisms adjust to their environmental challenges.

Osmotic Pressure

Osmotic pressure is a critical concept in biology, particularly when discussing how organisms manage fluid balance. It is the pressure required to prevent the flow of water across a semipermeable membrane. The balance of osmotic pressure is vital for the survival of an organism.
For marine animals, this means adapting to the saltwater environment, which has a higher concentration of solutes compared to their internal body fluids. They either need to conserve water or counterbalance the osmotic pressure.

• In freshwater organisms, osmotic pressure issues arise from losing salts to their environment which is less concentrated than their body fluids.
• Conversely, marine animals face a risk of water loss and must retain solutes like urea to maintain homeostasis.

Learning about osmotic pressure gives insight into how different animals have evolved mechanisms to survive in various environmental conditions.

Animal Adaptations

Animal adaptations are the evolutionary processes that allow organisms to better survive in their habitats. When we refer to adaptations involving excretion, we talk about how animals have developed systems to efficiently manage waste while conserving water or coping with different environmental pressures.
Elasmobranchs, such as sharks, demonstrate a unique adaptation by retaining urea to deal with high saline environments.

• An understanding of these adaptations can help explain why different excretory products are used, depending largely on the animal's habitat.
• For instance, terrestrial animals tend to excrete urea or uric acid because these substances require less water for excretion.

Recognizing these adaptations reveals the interconnectedness of organisms and their environments and underscores the diversity of solutions life has evolved to deal with similar problems.