The excretory product in protozoans is

Analyze the question: It asks for the main nitrogenous excretory product of protozoans (e.g., Amoeba, Paramecium).

Concept: Animals excrete nitrogenous waste in different forms depending on their habitat and water availability:

• Ammonia (ammonotelic): very toxic and very soluble, so it needs plenty of water to flush it out. It is typical of aquatic organisms such as protozoans, freshwater fish, and tadpoles.
• Urea (ureotelic): less toxic, typical of mammals and amphibians.
• Uric acid (uricotelic): nearly insoluble, saves water, typical of birds, insects, and reptiles.

Solution:

1. Protozoans are single-celled organisms that live in water, in freshwater, in the sea, or in moist surroundings.
2. Protein breakdown inside the cell produces nitrogenous waste in the form of ammonia.
3. Because they live in water, ammonia diffuses directly across the cell membrane into the surrounding water, where it is diluted. They don't need to convert it into a less toxic form.
4. Excess water in freshwater protozoans is removed by the contractile vacuole, but the nitrogenous waste itself is mainly ammonia, which leaves by diffusion.

Amino acid (A) is a building block of proteins, not an excretory product. Uric acid (B) and urea (D) need more complex metabolic processes and are found in higher animals.

Answer: C. ammonia

Exam tip: Remember "Aquatic = Ammonia." Simple aquatic organisms excrete ammonia, while land animals that need to conserve water excrete urea or uric acid.