AQA · GCSE · Biology

    Course version: 8461 · Version from Sept 2018

    (HT only) The digestion of proteins from the diet results in excess amino…

    Full specification wording

    Maintaining water and nitrogen balance in the body (biology only): (HT only) The digestion of proteins from the diet results in excess amino acids which need to be excreted safely.

    Proteins in the diet are digested into amino acids, which are absorbed into the blood.

    Checked against the AQA 8461 specification (opens in a new tab) on .

    • Point 7 of 17
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Proteins in the diet are digested into amino acids, which are absorbed into the blood. The body uses amino acids to build new proteins, but any excess cannot be stored. The amino group is removed and converted into ammonia, which is toxic, and then into urea in the liver. Urea is less harmful and is carried in the blood to the kidneys, where it is removed in urine. The remaining parts of the amino acids can be used in respiration to release energy. This process excretes the nitrogen safely and helps maintain nitrogen balance in the body.

    What earns the marks

    • Dietary proteins are digested into amino acids, which are absorbed into the blood.
    • Excess amino acids cannot be stored by the body and must be broken down.
    • Deamination in the liver removes the amino group from excess amino acids.
    • The amino group is converted to ammonia and then to urea, which is less toxic.
    • Urea is transported in the blood to the kidneys and excreted in urine.

    Common mistakes and how to put them right

    • Thinking excess amino acids are stored like fat.

      Put it right: They are broken down in the liver by deamination.

    • Believing urea is formed directly from proteins in the stomach.

      Put it right: Proteins are digested to amino acids, and urea is made from excess amino acids in the liver.

    • Confusing ammonia and urea.

      Put it right: Ammonia is toxic and is converted to the less toxic urea before excretion.

    Exam tips

    • Name the liver as the organ where deamination of excess amino acids occurs.
    • State the sequence excess amino acids → ammonia → urea → kidneys → urine.
    • Explain that converting ammonia to urea makes the nitrogen waste safer to transport and excrete.

    Try the lesson

    The start of the Maintaining water and nitrogen balance in the body (biology only): (HT only) The digestion of proteins from the diet results in excess amino acids which need to be excreted safely. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Digestion of proteins from the diet produces amino acids. What happens to the excess amino acids that the body does not need?

    Listen

    Proteins in the diet are digested into amino acids, which are absorbed into the blood. The body uses amino acids to build new proteins, but any excess cannot be stored. The amino group is removed and converted into ammonia, which is toxic, and then into urea in the liver. Urea is less harmful and is carried in the blood to the kidneys, where it is removed in urine. The remaining parts of the amino acids can be used in respiration to release energy. This process excretes the nitrogen safely and helps maintain nitrogen balance in the body. A strong answer earns marks for these points. Dietary proteins are digested into amino acids, which are absorbed into the blood. Excess amino acids cannot be stored by the body and must be broken down. Deamination in the liver removes the amino group from excess amino acids. The amino group is converted to ammonia and then to urea, which is less toxic. A common slip: Thinking excess amino acids are stored like fat. Instead, they are broken down in the liver by deamination. Exam tip: Name the liver as the organ where deamination of excess amino acids occurs.

    See it

    A top-to-bottom flowchart with eight boxes linked by arrows. The first box says dietary proteins are digested into amino acids, leading to a second box where amino acids are absorbed into the blood. The third box states excess amino acids cannot be stored, leading to a fourth box where deamination in the liver removes the amino group. The fifth box says the amino group is converted to ammonia, which is toxic, leading to a sixth box where ammonia is converted to urea, which is less toxic. The seventh box says urea is carried in the blood to the kidneys, and the final box says urea is excreted in urine.
    It shows the sequence from digested dietary proteins to excess amino acids, deamination in the liver, conversion of ammonia to urea, and excretion of urea by the kidneys in urine.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Amino group converted to ammonia (toxic)
    2. 2. Urea excreted in urine
    3. 3. Ammonia converted to urea (less toxic)
    4. 4. Dietary proteins digested into amino acids
    5. 5. Amino acids absorbed into the blood
    6. 6. Urea carried in the blood to the kidneys
    7. 7. Excess amino acids cannot be stored
    8. 8. Deamination in the liver removes the amino group

    Key words

    Say what each word means before you turn the card.

    Deamination

    Try answering aloud or in your head before turning the card.

    Ammonia

    Try answering aloud or in your head before turning the card.

    Urea

    Try answering aloud or in your head before turning the card.

    Excretion

    Try answering aloud or in your head before turning the card.

    See every definition
    Deamination
    Excess amino acids are broken down in the liver by deamination, which removes the amino group.
    Ammonia
    The amino group removed from excess amino acids is converted into ammonia, which is toxic.
    Urea
    Ammonia is converted into urea in the liver, which is less harmful and is carried in the blood to the kidneys.
    Excretion
    Urea is removed in urine by the kidneys, which excretes the nitrogen safely and helps maintain nitrogen balance in the body.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-to-bottom flowchart with eight boxes linked by arrows. The first box says dietary proteins are digested into amino acids, leading to a second box where amino acids are absorbed into the blood. The third box states excess amino acids cannot be stored, leading to a fourth box where deamination in the liver removes the amino group. The fifth box says the amino group is converted to ammonia, which is toxic, leading to a sixth box where ammonia is converted to urea, which is less toxic. The seventh box says urea is carried in the blood to the kidneys, and the final box says urea is excreted in urine.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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