Nitrogenous Wastes – Ammonia, Urea & Uric Acid | Class 12 Biology Chapter 13
Every day, our body breaks down proteins (proteins are made of amino acids, the building blocks of proteins) to obtain energy, repair tissues, and build new cells.
During this process, nitrogen-containing waste products are formed. These wastes are toxic (poisonous to body cells) and must be removed quickly to prevent damage to organs.
The removal of these harmful nitrogen-containing substances is called nitrogenous excretion.
Figure 13.21 – Formation of Nitrogenous Wastes Caption: Breakdown of proteins produces nitrogenous wastes (ammonia, urea or uric acid) that must be removed from the body. Attribution: Kaiser Science / educational biology resources.
🟣 What are Nitrogenous Wastes?
Nitrogenous Wastes (waste products containing nitrogen) are formed when amino acids are broken down during metabolism.
The three major nitrogenous wastes are: 1️⃣ Ammonia 2️⃣ Urea 3️⃣ Uric Acid
🔄 Formation of Nitrogenous Wastes
Proteins
│
▼
Amino Acids
│
▼
Deamination
(Removal of amino group)
│
▼
Ammonia (NH₃)
│
───────────────
In Mammals
↓
Converted into Urea
───────────────
In Birds/Reptiles
↓
Converted into Uric Acid🧠 Important Term
Deamination Deamination (the removal of the amino group (–NH₂) from an amino acid) occurs mainly in the liver. This process produces ammonia, which is highly toxic.
Figure 13.22 – Deamination in the Liver Caption: Amino acids undergo deamination in the liver, producing ammonia which is then converted into urea. Attribution: Educational biology diagram (Amino acid catabolism).
🔴 1. AMMONIA (NH₃)
📖 Definition
Ammonia is the most toxic nitrogenous waste produced in living organisms. Even a small amount can damage body cells if it accumulates.
💡 Characteristics
✅ Highly toxic ✅ Very soluble in water (dissolves easily) ✅ Requires large amounts of water for excretion
Animals that Excrete Ammonia 🐟 Most bony fishes 🦐 Aquatic invertebrates 🐸 Tadpoles
These animals live in water, so ammonia is quickly diluted and carried away.
🟠 2. UREA (CO(NH₂)₂)
📖 Definition
Urea is a nitrogenous waste formed by converting ammonia into a less toxic substance in the liver. This process is called the Urea Cycle (a series of chemical reactions in the liver that convert toxic ammonia into urea).
💡 Characteristics
✅ Less toxic than ammonia ✅ Requires less water for excretion ✅ Easily transported in blood
Animals that Excrete Urea 🐄 Mammals 🐸 Adult amphibians 🦈 Sharks
These animals conserve more water than animals that excrete ammonia.
Figure 13.24 – Urea Formation in the Liver Caption: Toxic ammonia is converted into urea in the liver and then transported to the kidneys for excretion in urine. Attribution: Lumen Learning / educational nutrition resources.
🟢 3. URIC ACID
📖 Definition
Uric Acid is the least toxic nitrogenous waste. It is produced as a white crystalline paste, requiring very little water for removal.
💡 Characteristics
✅ Least toxic ✅ Conserves maximum water ✅ Removed as semi-solid crystals
Animals that Excrete Uric Acid 🐦 Birds 🦎 Reptiles 🐍 Snakes 🦋 Insects
These animals usually live where water conservation is very important.
📊 Comparison Table
| Feature | Ammonia | Urea | Uric Acid |
|---|---|---|---|
| Toxicity | Very High | Moderate | Very Low |
| Water Needed | Very High | Moderate | Very Low |
| Solubility | Very High | High | Low |
| Main Animals | Fish, Tadpoles | Mammals, Adult Amphibians, Sharks | Birds, Reptiles, Insects |
| Energy Needed to Produce | Low | Moderate | High |
Figure 13.25 – Types of Nitrogenous Wastes Caption: Comparison of ammonia, urea and uric acid with the animals that excrete them. Attribution: Vedantu / educational biology resources.
🧠 Memory Trick
🐟 Fish → A A = Ammonia Fish live in water, so they can remove toxic ammonia easily.
🐄 Mammals → U U = Urea Most mammals convert ammonia into urea.
🐦 Birds → UA UA = Uric Acid Birds conserve water by producing uric acid.
📦 Did You Know?
A bird’s white droppings are mainly uric acid crystals, not urine like in mammals. This adaptation helps birds conserve water, which is especially important during flight and in dry environments.
⭐ Board Exam Tips
Frequently Asked Short Questions ✅ Define nitrogenous waste. ✅ What is deamination? ✅ Differentiate between ammonia, urea, and uric acid. ✅ Why do birds excrete uric acid instead of ammonia?
Frequently Asked MCQs ✔ The most toxic nitrogenous waste is → Ammonia ✔ The least toxic nitrogenous waste is → Uric Acid ✔ Urea is produced mainly in the → Liver ✔ Which animals excrete uric acid? → Birds and reptiles
📝 Practice Questions
Short Questions
- What are nitrogenous wastes?
- Define deamination.
- Name the three nitrogenous wastes.
- Why is ammonia toxic?
- Why is uric acid suitable for desert animals?
Long Question Explain the formation of nitrogenous wastes and compare ammonia, urea, and uric acid with suitable examples.
📌 Quick Revision Box
✔ Proteins are broken down into amino acids. ✔ Deamination in the liver produces ammonia. ✔ Ammonia is highly toxic and needs lots of water. ✔ Mammals convert ammonia into urea. ✔ Birds and reptiles convert nitrogenous wastes into uric acid to conserve water. ✔ The type of nitrogenous waste excreted depends on the animal’s habitat and need for water conservatio
13.2.3 Nitrogenous Wastes & Their Excretion
(Prepared in Punjab Board past paper style for Class 12)
30 MCQs – Nitrogenous Wastes
1. The most toxic nitrogenous waste is: A) Urea B) Uric acid C) Ammonia D) Creatinine
Answer: C) Ammonia
2. The least toxic nitrogenous waste is: A) Ammonia B) Urea C) Uric acid D) Amino acid
Answer: C) Uric acid
3. Deamination mainly occurs in the: A) Kidney B) Liver C) Lungs D) Pancreas
Answer: B) Liver
4. Urea is produced in the: A) Kidney B) Liver C) Spleen D) Intestine
Answer: B) Liver
5. Animals that excrete ammonia are called: A) Ureotelic B) Uricotelic C) Ammonotelic D) Aminotelic
Answer: C) Ammonotelic
6. Mammals are: A) Ammonotelic B) Ureotelic C) Uricotelic D) None
Answer: B) Ureotelic
7. Birds and reptiles are: A) Ammonotelic B) Ureotelic C) Uricotelic D) Both A and B
Answer: C) Uricotelic
8. Which nitrogenous waste requires maximum water for its excretion? A) Urea B) Uric acid C) Ammonia D) Creatinine
Answer: C) Ammonia
9. Which nitrogenous waste requires minimum water for its excretion? A) Ammonia B) Urea C) Uric acid D) All equally
Answer: C) Uric acid
10. Conversion of ammonia into urea takes place through: A) Krebs cycle B) Urea cycle C) Calvin cycle D) Glycolysis
Answer: B) Urea cycle
11. Removal of amino group (–NH₂) from amino acid is called: A) Transamination B) Deamination C) Decarboxylation D) Amination
Answer: B) Deamination
12. Sharks excrete: A) Ammonia B) Urea C) Uric acid D) Both ammonia and urea
Answer: B) Urea
13. Tadpoles excrete: A) Urea B) Uric acid C) Ammonia D) Amino acids
Answer: C) Ammonia
14. Adult amphibians mainly excrete: A) Ammonia B) Urea C) Uric acid D) Both A and C
Answer: B) Urea
15. White droppings of birds mainly contain: A) Urea B) Ammonia C) Uric acid D) Creatinine
Answer: C) Uric acid
16. Which of the following is highly soluble in water? A) Uric acid B) Ammonia C) Both D) None
Answer: B) Ammonia
17. Energy requirement is highest for the production of: A) Ammonia B) Urea C) Uric acid D) All equal
Answer: C) Uric acid
18. Most bony fishes are: A) Ureotelic B) Uricotelic C) Ammonotelic D) None
Answer: C) Ammonotelic
19. Insects excrete: A) Ammonia B) Urea C) Uric acid D) Amino acids
Answer: C) Uric acid
20. Which statement is correct? A) Ammonia is least toxic B) Uric acid is most toxic C) Urea is moderately toxic D) All are equally toxic
Answer: C) Urea is moderately toxic
21. The process of converting ammonia into less toxic form is called: A) Deamination B) Detoxification C) Transamination D) Excretion
Answer: B) Detoxification
22. Which animals conserve maximum water by their excretory product? A) Fish B) Mammals C) Birds and reptiles D) Amphibians
Answer: C) Birds and reptiles
23. Amino acids are the building blocks of: A) Carbohydrates B) Lipids C) Proteins D) Nucleic acids
Answer: C) Proteins
24. Urea is less toxic than ammonia because: A) It is insoluble B) It is less soluble and less reactive C) It requires more water D) It is produced in kidney
Answer: B) It is less soluble and less reactive
25. Which of the following is NOT a nitrogenous waste? A) Ammonia B) Urea C) Uric acid D) Glucose
Answer: D) Glucose
26. Aquatic invertebrates mostly excrete: A) Urea B) Uric acid C) Ammonia D) Creatinine
Answer: C) Ammonia
27. The main advantage of uric acid is: A) High toxicity B) High solubility C) Water conservation D) Low energy cost
Answer: C) Water conservation
28. In humans, the major nitrogenous waste excreted is: A) Ammonia B) Urea C) Uric acid D) Amino acids
Answer: B) Urea
29. Which of the following animals is ureotelic? A) Bird B) Lizard C) Human D) Fish
Answer: C) Human
30. The type of nitrogenous waste excreted by an animal mainly depends upon: A) Size of the animal B) Availability of water in habitat C) Body temperature D) Type of food only
Answer: B) Availability of water in habitat
Quick Answer Key
1-C, 2-C, 3-B, 4-B, 5-C, 6-B, 7-C, 8-C, 9-C, 10-B, 11-B, 12-B, 13-C, 14-B, 15-C, 16-B, 17-C, 18-C, 19-C, 20-C, 21-B, 22-C, 23-C, 24-B, 25-D, 26-C, 27-C, 28-B, 29-C, 30-B
FAQ (Frequently Asked Questions)
Q1. What are nitrogenous wastes? Nitrogenous wastes are toxic nitrogen-containing compounds (ammonia, urea and uric acid) produced when proteins/amino acids are broken down in the body.
Q2. What is deamination? Deamination is the process of removal of the amino group (–NH₂) from an amino acid. It mainly occurs in the liver and produces ammonia.
Q3. Why is ammonia highly toxic? Ammonia is highly toxic because even a small amount can damage cells and tissues by raising the pH of body fluids and interfering with cellular processes.
Q4. Why do aquatic animals excrete ammonia? Aquatic animals live in water, so ammonia can be quickly diluted and washed away. They do not need to convert it into less toxic forms.
Q5. Why do mammals convert ammonia into urea? Mammals convert ammonia into urea because urea is less toxic and requires less water for excretion, helping in water conservation.
Q6. Why do birds and reptiles excrete uric acid? Birds and reptiles excrete uric acid because it is the least toxic and requires very little water. This helps them conserve water, which is important for flight and dry habitats.
Q7. What is the urea cycle? The urea cycle is a series of chemical reactions that take place in the liver to convert toxic ammonia into less toxic urea.
Q8. Differentiate between ammonotelic, ureotelic and uricotelic animals.
- Ammonotelic: Excrete ammonia (e.g., bony fishes, tadpoles)
- Ureotelic: Excrete urea (e.g., mammals, adult amphibians, sharks)
- Uricotelic: Excrete uric acid (e.g., birds, reptiles, insects)
Q9. Which nitrogenous waste is best for desert animals and why? Uric acid is best for desert animals because it requires the least amount of water for excretion, helping them survive in dry conditions.
Q10. Where is urea formed and where is it excreted? Urea is formed in the liver and excreted by the kidneys in urine.
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