CDS (II) 2022 General Knowledge · Question 22 of 120

Correct — A, Proteins. Urea is a nitrogen compound, and of the three classes of food only protein contains nitrogen. Carbohydrates and lipids are built from carbon, hydrogen and oxygen alone, so no amount of burning them can produce a nitrogenous waste. Protein is digested to amino acids; those the body does not need for building are deaminated in the liver, which strips off the amino group as ammonia. Ammonia is toxic even in small amounts, so the liver immediately converts it, through the urea cycle, into urea — far less toxic and readily soluble. The blood carries the urea to the kidneys, and NCERT describes the outcome in a line: 'nitrogenous waste such as urea or uric acid are removed from blood in the kidneys'.

• (b)Carbohydrates — Contain only carbon, hydrogen and oxygen. Their complete breakdown gives carbon dioxide and water, and there is no nitrogen in them from which urea could be made.
• (c)Lipids — Also carbon, hydrogen and oxygen only. Fat metabolism yields carbon dioxide, water and, when incomplete, ketone bodies — never urea.
• (d)Both Proteins and Carbohydrates — The designed trap, and it fails on the same point. Adding carbohydrate to the correct answer adds a nitrogen-free molecule to a nitrogen-based product.

Excretion removes the nitrogenous waste left over when the body breaks down protein. Which compound is excreted depends on how much water an animal can spare. Aquatic animals release ammonia directly, since water dilutes it instantly. Mammals convert it to urea, which is less toxic and needs a moderate amount of water. Birds and reptiles go further, to almost insoluble uric acid, which can be passed as a paste and saves water. Humans are ureotelic, and the conversion happens in the liver while the removal happens in the kidney.

The question can be settled by counting elements. Write out what each food class is made of — protein has nitrogen, carbohydrate and fat do not — and only one option survives. That single test also disposes of the compound option D, which exists to catch anyone answering by feel. Two related facts are worth carrying because they are asked in the same breath. The liver, not the kidney, is where urea is made; the kidney only filters it out. And a blood urea reading is used as an index of kidney function precisely because urea production is fairly steady while its clearance is not.

• Proteins contain carbon, hydrogen, oxygen and nitrogen; carbohydrates and fats contain no nitrogen.
• Excess amino acids are deaminated in the liver, releasing ammonia.
• The liver converts ammonia to urea through the urea cycle; the kidney then filters urea out of the blood.
• Human beings are ureotelic; fish excrete ammonia and birds and reptiles excrete uric acid.
• Nitrogenous wastes such as urea and uric acid are removed from blood in the kidneys.

• Selecting the compound option because more than one food class supplies energy; energy is not the question, nitrogen is.
• Placing urea formation in the kidney; it is made in the liver and only removed by the kidney.
• Confusing urea with uric acid, which is the bird and reptile product and in humans is linked to gout.

As a one-liner on the origin of urea, as an organ-of-formation question, or as a match of animal group to nitrogenous waste.

• practice — not a real PYQ

  In the human body, urea is formed in the

  • (a)kidney
  • (b)liver
  • (c)pancreas
  • (d)urinary bladder

  Answer(b) liver — the liver converts ammonia from deamination into urea, and the kidney only filters it out of the blood.

• practice — not a real PYQ

  Animals that excrete their nitrogenous waste mainly as uric acid are described as

  • (a)ammonotelic
  • (b)ureotelic
  • (c)uricotelic
  • (d)osmotic

  Answer(c) uricotelic — birds and reptiles excrete uric acid, which needs very little water; fish are ammonotelic and mammals ureotelic.