Where urea comes from — answer key

1. LQ1 · Practice

Model answer. Liver cells join amino acids together in chains to make proteins, such as the proteins of the blood plasma. This is the assimilation of amino acids.

A full answer includes

• I chose: it joins them together to make proteins.
• I did not choose urea: only excess amino acids are made into urea.

Also accept

• joins them into proteins

Watch for

• “It turns every one of them into urea.” — Only excess amino acids are deaminated to make urea. Needed ones are made into proteins.
• “It stores them until the body needs them.” — The body cannot store amino acids. Needed ones are built into proteins.
• “It sends them to the kidneys to excrete.” — The kidneys excrete urea, not amino acids the body needs.

2. LQ2 · Practice

Model answer. Urea is made in the liver, from excess amino acids. The blood carries it to the kidneys, which remove it and excrete it in urine.

A full answer includes

• I chose the liver.
• I did not choose the kidneys: they remove urea, they do not make it.

Also accept

• the liver

Watch for

• “The kidneys.” — The kidneys remove urea from the blood. It is made in the liver.
• “The bladder.” — The bladder only stores urine. Urea is made in the liver.
• “The small intestine.” — The small intestine absorbs amino acids. They are deaminated, and urea is made, in the liver.

3. LQ3 · Practice

Model answer. Deamination is the removal of the nitrogen-containing part of amino acids, in the liver, to form urea. The rest of each amino acid can be used in respiration.

A full answer includes

• I chose: removing the nitrogen-containing part of amino acids to form urea.
• I did not choose joining amino acids into proteins: that is assimilation.

Also accept

• removal of the nitrogen-containing part of amino acids to form urea

Watch for

• “Joining amino acids together in the liver to form new proteins.” — That is assimilation. Deamination removes the nitrogen-containing part.
• “Removing urea from the blood in the kidneys to form the urine.” — That is the kidneys' excretion of urea. Deamination makes the urea, in the liver.
• “Breaking down proteins in the gut to form many amino acids.” — That is digestion. Deamination happens later, in the liver, to excess amino acids.

4. LQ4 · Practice

Options as numbered on the pupil sheet: 1 — Its nitrogen-containing part is removed.; 2 — The blood carries the urea to the kidneys.; 3 — The urea leaves the body in urine.; 4 — Urea is formed.; 5 — An excess amino acid arrives in the liver..

Model answer. The excess amino acid arrives in the liver, where its nitrogen-containing part is removed (deamination) and turned into urea. The urea passes into the blood, which carries it to the kidneys, and the kidneys excrete it in urine.

A full answer includes

• I put the amino acid arriving in the liver first.
• I put the removal of its nitrogen-containing part second.
• I put the forming of urea third.
• I put the blood carrying the urea to the kidneys fourth.
• I put the urea leaving in urine last.
• I did not put the kidneys before the liver.

Also accept

• liver; deamination; urea; blood to kidneys; urine

Watch for

• “The kidneys make the urea.” — The liver makes urea; the kidneys remove it.

5. LQ5 · Practice

Model answer. Urea is toxic. The liver makes it every day, so if it were not removed its concentration in the blood would keep rising and harm the body's cells.

A full answer includes

• I chose: it is toxic, so a build-up in the blood harms the cells.
• I did not choose "it is a protein": urea is made from amino acids, but it is not a protein.

Also accept

• urea is toxic

Watch for

• “It is a fuel: a build-up makes cells respire too fast.” — Urea is not a fuel. The part of the amino acid used in respiration is the part left after deamination.
• “It is solid: a build-up blocks the blood vessels.” — Urea is dissolved in the blood plasma. The danger is that it is toxic.
• “It is a protein: a build-up thickens the blood plasma.” — Urea is a small waste molecule, not a protein. The danger is that it is toxic.

6. LQ6 · Practice

Options as numbered on the pupil sheet: 1 — breaking proteins into amino acids in the gut; 2 — making amino acids into the body's proteins; 3 — removing the nitrogen-containing part of an amino acid; 4 — removing waste of metabolism from the body.

Model answer. Digestion breaks food proteins into amino acids in the gut. In the liver, assimilation joins amino acids into the body's own proteins, and deamination removes the nitrogen-containing part of excess ones to make urea. Excretion removes waste of metabolism, such as urea, from the body.

A full answer includes

• I matched assimilation to making amino acids into the body's proteins.
• I matched deamination to removing the nitrogen-containing part of an amino acid.
• I matched excretion to removing waste of metabolism from the body.
• I matched digestion to breaking proteins into amino acids in the gut.
• I did not swap assimilation and deamination.

Also accept

• assimilation, deamination, excretion, digestion correctly matched

Watch for

• “Assimilation removes nitrogen.” — That is deamination. Assimilation builds proteins.
• “Excretion breaks proteins into amino acids.” — That is digestion, in the gut. Excretion removes waste of metabolism from the body.

7. LQ7 · Practice

Model answer. The amino acids from 15 g of protein are assimilated into new proteins, so 85 − 15 = 70 g of protein is in excess and is deaminated in the liver. Urea made = 70 × 0.34 = 23.8 g, about 24 g.

Checked automatically

• Mass of urea: 23.8 g (within ±0.25 g)
  • Common wrong answer 28.9 g: That uses all 85 g. The amino acids built into new proteins are not deaminated.
  • Common wrong answer 5.1 g: That uses the 15 g built into proteins. Use the excess instead.

A full answer includes

• Method: I found the protein deaminated: 85 − 15 = 70 g.
• Method: I multiplied by 0.34: 70 × 0.34.
• Answer: I gave about 24 g (23.8 g).
• I did not multiply all 85 g by 0.34.

Also accept

• 23.8 g (24 g)

Watch for

• “28.9 g.” — That is 85 × 0.34. The 15 g built into new proteins is not deaminated.
• “5.1 g.” — That is 15 × 0.34: the protein kept for growth. The excess, 70 g, is the part deaminated.

8. LQ8 · Practice

Model answer. Her blood urea stays within the normal range: it may be a little higher than before, but it does not keep rising. Her liver still deaminates excess amino acids and makes urea every day, but one healthy kidney can remove it as fast as it is made, so it does not build up. This matters because urea is toxic: if it built up in the blood it would harm her cells.

A full answer includes

• I predicted that her blood urea stays within the normal range (it may be a little higher than before).
• I explained that her liver still makes urea every day.
• I explained that her one kidney removes the urea as it is made.
• I explained that this matters because urea is toxic, so it must not build up.
• I did not predict a steady rise: that happens when both kidneys stop working.

Also accept

• stays in the normal range (may rise a little, does not keep rising); urea still made but removed by one kidney; toxic, so it must not build up

Watch for

• “It rises to a dangerous level, because she has half her kidneys.” — One healthy kidney can remove the urea her liver makes, so the level stays in the normal range.
• “It falls to zero.” — Her liver keeps making urea, so there is always some in her blood.

9. LQ9 · Practice

Model answer. Amino acids. All four substances contain carbon, hydrogen and oxygen, so those elements do not decide it. Urea also contains nitrogen, and of these four only amino acids have it, in their nitrogen-containing part. The liver removes that part from excess amino acids (deamination) and turns it into urea.

A full answer includes

• I chose amino acids.
• I gave the reason: urea's nitrogen can come only from them.
• I did not choose glucose: its carbon is not enough, because urea also needs nitrogen.

Also accept

• amino acids, because they contain nitrogen, as urea does

Watch for

• “Glucose, because it contains carbon, as urea does.” — Glucose does contain carbon, but so do all four substances. Urea also needs nitrogen, and glucose has none.
• “Fatty acids, because they contain hydrogen, as urea does.” — All four substances contain hydrogen. Urea also needs nitrogen, and fatty acids have none.
• “Glycerol, because it contains oxygen, as urea does.” — All four substances contain oxygen. Urea also needs nitrogen, and glycerol has none.

10. LT1 · Apply

Model answer. (a) Day 3 is anomalous: 90 g of protein gave only 18 g of urea, less than the 26 g from 75 g on day 2. Leaving it out, the more protein in the food, the more urea is excreted: from 17 g at 50 g of protein up to 44 g at 130 g. (b) An adult's body cannot store amino acids and needs only a fixed amount for new proteins, so the extra ones are deaminated in the liver. More nitrogen-containing parts are removed, so more urea is made, carried to the kidneys and excreted.

A full answer includes

• (a) I identified day 3 (90 g of protein, 18 g of urea) as the anomalous result.
• (a) I left day 3 out when I described the pattern.
• (a) I described that the more protein eaten, the more urea is excreted.
• (b) I explained that more protein gives more excess amino acids.
• (b) I explained that the liver deaminates the excess amino acids.
• (b) I explained that more urea is made, so more is excreted.
• I did not say that the kidneys make the urea.

Also accept

• (a) day 3 anomalous; more protein, more urea; (b) more excess amino acids deaminated, more urea made

Watch for

• “(a) Day 5 is anomalous, because it is the largest.” — Day 5 fits the trend: more protein than day 4, and more urea. Day 3 has more protein than day 2 but less urea.
• “(b) The kidneys make more urea.” — The liver makes the urea; the kidneys only excrete it.

11. LT2 · Apply

Model answer. (a) The blood arriving from the gut carries the amino acids absorbed from the meal; liver cells take many of them out, to make proteins or to deaminate. So the blood arriving has more amino acids. (b) The liver makes urea from the excess amino acids and it passes into the blood, so the blood leaving in the hepatic vein has more urea.

A full answer includes

• (a) I stated that the blood arriving from the gut has more amino acids.
• (a) I explained that the liver takes amino acids out of the blood.
• (b) I stated that the blood leaving in the hepatic vein has more urea.
• (b) I explained that the liver deaminates the excess amino acids to make urea.
• (b) I explained that the urea made passes into the blood leaving the liver.
• I did not say that the blood arriving has more urea.

Also accept

• (a) arriving has more amino acids; (b) leaving has more urea

Watch for

• “(b) The blood arriving has more urea.” — Urea is made in the liver, so the blood leaving it has more.

12. LT3 · Apply

Model answer. Less protein in the food means fewer excess amino acids reach the liver, so fewer are deaminated and less urea is made. His kidneys remove urea slowly, so making less of it lets them keep up better, and the toxic urea does not build up so much in his blood.

A full answer includes

• I explained that less protein means fewer excess amino acids.
• I explained that fewer amino acids are deaminated in the liver.
• I explained that less urea is made.
• I explained that his slow kidneys can then keep up better with the urea made.
• I explained that toxic urea does not build up so much in his blood.
• I did not say that less protein makes his kidneys work faster.

Also accept

• less protein; fewer excess amino acids; less urea made; slow kidneys keep up; toxic urea builds up less

Watch for

• “Less protein makes his kidneys work faster.” — Eating less protein does not repair the kidneys. It reduces the urea made, so less has to be removed.
• “Less protein means less urea is filtered, so more stays in the blood.” — The point is that less urea is made in the liver, so less is in the blood to begin with.