Problem 17 - Objective Biology for NEET

Question

Nitrogenous waste is excreted mainly as (a) Urea in both frog and tadpole (b) Urea in frog and ammonia in tadpole (c) Uric acid in frog and urea in tadpole (d) Urea in tadpole and ammonia in frog

Step-by-Step Solution

Answer

The correct answer is (b) Urea in frog and Ammonia in tadpole.

Step 1: Identify Nitrogenous Waste in Frog and Tadpole

The first step to solve this problem is to understand the type of Nitrogenous waste that is excreted by a frog and a tadpole. A frog, being a terrestrial adult, mainly excretes Urea. On the other hand, a Tadpole, an aquatic larvae of a frog, mainly excretes Ammonia.

Step 2: Match the Options with the Known Information

Now, we need to look at the options and match with the information we have. Scroll through the options and find the one that says Urea for frogs and Ammonia for tadpoles.

Step 3: Choose the Correct Option

The option that correctly matches with the biological information that we know is option (b) 'Urea in frog and Ammonia in tadpole'. Hence, (b) is the correct answer.

Key Concepts

Ammonia Excretion in Tadpoles

Like many aquatic organisms, tadpoles excrete nitrogenous waste in the form of ammonia. Ammonia is a byproduct of protein metabolism and is toxic to living cells. However, due to the ample water surrounding them, tadpoles can afford to excrete ammonia directly into their environment. This method of excretion is known as ammonotelism.

The high water solubility of ammonia allows it to diffuse rapidly across skin and gill surfaces. In aquatic environments like ponds and streams, where tadpoles live, the diluted ammonia poses little risk to them. Furthermore, tadpoles have a high surface area to volume ratio, which enhances the excretion of ammonia. As excretion of ammonia requires lots of water, this process is perfectly suited to the aquatic lifestyle of tadpoles.

Urea Excretion in Frogs

When tadpoles undergo metamorphosis and become adult frogs, their method of nitrogenous waste excretion shifts from ammonotelism to ureotelism - the excretion of urea. This change is an important adaptation to terrestrial life. Urea is less toxic than ammonia and is excreted in a concentrated form, which helps in conserving water.

Frogs have well-developed kidneys that convert ammonia into urea before releasing it into the environment. Unlike ammonia, urea does not require as much water for its disposal, which is advantageous for an animal that might not have constant access to water. This adaptation supports frogs' varied habitats, which can range from moist to relatively dry environments. In periods of scarcity of water, urea can accumulate in the bladder to significant levels, offering a buffer against dehydration.

Metabolic Waste in Vertebrates

All vertebrates produce metabolic waste during the breakdown of proteins and nucleic acids. The primary forms of nitrogenous wastes in vertebrates are ammonia, urea, and uric acid. Each vertebrate adapts to excrete one of these compounds based on their habitat and evolutionary history.

• Ammonotelic organisms, primarily aquatic like fishes and tadpoles, excrete nitrogen as ammonia.
• Ureotelic organisms, such as mammals and adult amphibians, convert ammonia to the less toxic urea.
• Uricotelic organisms, which include birds and reptiles, excrete nitrogen in the form of uric acid crystals, requiring the least water for excretion.

These adaptations not only reflect the evolutionary and ecological niches occupied by the organisms but also highlight the balance between the need to eliminate waste and conserve water.