Complete revision notes for Topic 13 of the CIE IGCSE Biology course: Excretion in humans. Explanations with diagrams for every specification point. These notes are written for candidates taking the Extended paper.
Unlike plants, humans have organs which are specialised for the removal of certain
excretory products.
Excretion is the removal of the waste substances of metabolic reactions (the chemical
reactions that take place inside cells), toxic materials and substances in excess of
requirements.
NOTE the difference between excretion and egestion:
Excretion is the removal from the body of waste products of metabolic reactions,
toxic substances and substances in excess of requirements
Egestion is the expulsion of undigested food waste from the anus
State that carbon dioxide is excreted through the lungs
Carbon dioxide must be excreted as it dissolves in water easily to form an acidic solution
which can lower the pH of cells. This can reduce the activity of enzymes in the body which
are essential for controlling the rate of metabolic reactions. For this reason, too much
carbon dioxide in the body is toxic.
State that the kidneys excrete urea and excess water and salts
, Urea is toxic to the body in higher concentrations and so must be excreted.
State that urea is formed in the liver from excess amino acids
Describe the role of the liver in the assimilation of amino acids by converting them to
proteins, including plasma proteins, e.g. fibrinogen
Many digested food molecules absorbed into the blood in the small intestine are carried to
the liver for assimilation (when food molecules are converted to other molecules that the
body needs).
These include amino acids, which are used to build proteins such as fibrinogen, a protein
found in blood plasma that is important in blood clotting.
Define deamination as the removal of the nitrogen-containing part of amino acids to form
urea
Excess amino acids absorbed in the blood that are not needed to make proteins cannot be
stored, so they are broken down in a process called deamination.
Enzymes in the liver split up the amino acid molecules.
The part of the molecule which contains carbon is turned into glycogen and stored.
The other part, which contains nitrogen, is turned into ammonia, which is highly toxic, and
so is immediately converted into urea, which is less toxic.
The urea dissolves in the blood and is taken to the kidney to be excreted.
A small amount is also excreted in sweat.
In deamination, the nitrogen-containing amino group is removed and converted into
ammonia and then urea to be excreted.
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