Problem 5 - Objective Biology for NEET

Question

Select the incorrect statement from the following: (a) Ammonia is readily soluble in water. (b) Ammonia is generally excreted by the process of diffusion. (c) Ammonia is excreted as ammonium ion through gill surface in fishes. (d) Ammonia is the major and urea and uric acid are the minor forms of nitrogenous waste excreted by animals.

Step-by-Step Solution

Answer

The incorrect statement is (d). Ammonia is a minor form of nitrogenous waste excreted by most animals, while urea and uric acid are the major forms of such waste.

Step 1: Analyzing the Statements

The options (a), (b) and (c) are correct as per the known characteristics of ammonia. Ammonia is indeed soluble in water, can be excreted by animal organism through the process of diffusion and is also excreted as ammonium ions through gill surfaces in fish.

Step 2: Identifying the Incorrect Statement

The incorrect statement is option (d), because urea and uric acid are the major, while ammonia is a minor form of nitrogenous waste excreted by most animals. Ammonia is highly toxic and therefore cannot be accumulated in large amounts in the bodies of most terrestrial animals. It is primarily excreted by aquatic animals.

Key Concepts

Nitrogenous Waste

Nitrogenous waste refers to the by-products generated during the metabolism of proteins and nucleic acids. Proteins are broken down into amino acids, which can lead to the formation of ammonia, a toxic compound. Since this compound is harmful, animals have developed different strategies to excrete it safely.

There are three primary forms of nitrogenous waste:

• Ammonia: Found mainly in aquatic animals that can afford to release it quickly into the surrounding water due to its toxicity.
• Urea: Less toxic and soluble, allowing it to be safely stored in the body for some time before excretion. Common in mammals and amphibians.
• Uric Acid: Conserves water, as it's excreted as a paste with minimal water loss. This is advantageous for birds and reptiles.

Each form of waste reflects the adaptation of the organism to its environment. Aquatic species prefer ammonia, while terrestrial and avian species typically handle waste in the form of urea or uric acid to manage water balance and toxicity levels.

Ammonia Solubility

Ammonia is known for its high solubility in water. This property makes it convenient for aquatic animals that live in a water-rich environment. They can efficiently excrete ammonia directly into the surrounding water, minimizing the exposure to its toxicity.

The solubility of ammonia results from its ability to form hydrogen bonds with water molecules. This feature allows ammonia to dissolve easily, facilitating the diffusion process across membranes. Hence, for aquatic organisms, ammonia serves as an optimal choice for nitrogenous waste elimination because they can swiftly diffuse it into their watery habitat.

However, for land-dwelling creatures, solubility becomes an issue, as retention of ammonia would lead to toxic build-up. These organisms have thus evolved to convert ammonia into less toxic compounds like urea or uric acid, which are more suitable for their environments.

Aquatic Animals

Aquatic animals have distinct advantages when it comes to excreting ammonia. Firstly, they reside in a medium (water) where ammonia can be readily dispersed. This venue allows for consistent and effective removal of ammonia from their systems.

Fish, for instance, utilize their gills for this purpose. The large surface area of gills ensures efficient diffusion of ammonia directly from their bloodstream into the water. This method is economical as it helps conserve energy while effectively removing the toxic gas.

Moreover, many freshwater fish excrete ammonia in the form of ammonium ions, which help them manage their body's ion balance. Unlike terrestrial creatures, these animals do not face the challenge of water loss during excretion, making ammonia a preferential waste in their unique environment.

Excretion Methods

The method of excretion varies greatly among different animal species and is typically aligned with their habitat and evolutionary history. Excretion involves the removal of waste products like nitrogenous waste from an organism’s body to maintain internal homeostasis.

• Diffusion: Common in simple aquatic animals, where waste diffuses across a thin membrane, often directly into the external environment.
• Kidneys: Employed by mammals for more complex waste processing. Kidneys filter out nitrogenous waste primarily in the form of urea.
• Specialized Structures: Insects and certain arthropods use Malpighian tubules to convert waste into uric acid.

Each excretion method is optimized to mitigate waste toxicity and conserve resources like water. In aquatic settings, diffusion suffices due to the abundant surrounding water. But for terrestrial organisms, special structures help manage waste amid minimal water availability, showcasing the diversity of excretion strategies across the animal kingdom.