In humans, urea is mainly formed from the metabolism of which one of the following components of food?

Correct — C, Amino acids. Urea is a nitrogenous waste, so it can only come from a food component that contains nitrogen, and among the four offered only amino acids do. Fats and carbohydrates are built from carbon, hydrogen and oxygen alone, and their complete oxidation gives carbon dioxide and water with no nitrogen left over to dispose of. Proteins are chains of amino acids, and every amino acid carries an amino group. The body cannot store surplus amino acids, so when intake exceeds what is needed for building, the liver strips the amino group off in a reaction called deamination and sends the remaining carbon skeleton into energy metabolism. The nitrogen comes away as ammonia, which is highly toxic even in small concentrations, and the liver at once converts it through the urea cycle into urea — far less toxic, freely soluble, and easily filtered out by the kidneys into urine.

• (a)Fatty acids — Fatty acids contain carbon, hydrogen and oxygen and no nitrogen at all, so their breakdown yields carbon dioxide and water. Nothing in a fat can become urea.
• (b)Vitamins — Several vitamins do contain nitrogen, but they are micronutrients taken in milligram quantities and used as coenzymes and regulators rather than broken down in bulk. They contribute nothing measurable to the urea in urine.
• (d)Glucose — Glucose is a carbohydrate with the formula C6H12O6 — again no nitrogen. Its oxidation ends at carbon dioxide and water.

Excretion is the removal of the nitrogenous waste that protein metabolism generates, and different animals dispose of it in different forms depending on how much water they can spare. Bony fish release ammonia directly into the surrounding water. Birds, reptiles and insects convert it into uric acid, which is nearly insoluble and can be excreted as a paste with almost no water loss. Mammals, humans among them, take the middle route and make urea in the liver.

The question can be answered on composition alone, and that is the fastest route: circle the option containing nitrogen. It is worth knowing why the body bothers with the conversion at all. Ammonia is the immediate product of deamination and is so toxic that it cannot be allowed to accumulate in the blood, so the liver converts it into urea within the same organ that produced it. This is also why liver failure raises blood ammonia and why kidney failure raises blood urea — two different organs, two different points in the same pathway.

• Urea is the principal nitrogenous excretory product in humans; it is made in the liver and removed by the kidneys.
• Amino acids, the units from which proteins are built, each carry an amino group, and deamination in the liver releases that nitrogen as ammonia.
• Ammonia is converted into the far less toxic urea through the urea cycle, worked out by Hans Krebs and Kurt Henseleit.
• Carbohydrates and fats contain only carbon, hydrogen and oxygen, so their metabolism produces carbon dioxide and water rather than any nitrogenous waste.
• Uric acid, the other nitrogenous waste in humans, comes from the breakdown of nucleic acids and accumulates in the joints in gout.
• Animals excreting mainly urea are called ureotelic; ammonotelic animals release ammonia and uricotelic animals uric acid.

• Associating urea with carbohydrate because glucose is the body's usual fuel — fuel and waste are separate questions.
• Treating vitamins as a bulk source of anything, when they are needed in milligram amounts.
• Placing urea formation in the kidney; the kidney excretes it, the liver makes it.

Set so that the composition of the four food components answers it outright — only one of them contains nitrogen, which is what a nitrogenous waste must come from.

• practice — not a real PYQ

  In the human body, urea is synthesised chiefly in the

  • (a)kidney
  • (b)liver
  • (c)spleen
  • (d)pancreas

  Answer(b) liver — the urea cycle operates in liver cells, and the kidney's role is to filter the urea out of the blood.

• practice — not a real PYQ

  Birds and reptiles excrete nitrogenous waste mainly as

  • (a)ammonia
  • (b)urea
  • (c)uric acid
  • (d)creatinine

  Answer(c) uric acid — it is almost insoluble and can be voided as a paste, which conserves water; fish release ammonia and mammals make urea.