Video Transcript
The diagram provided shows a
simplified outline of the ornithine, urea, cycle that occurs in the liver. What compound has been replaced by
X?
To work out which compound is
missing, let’s first learn a bit more about the ornithine cycle itself. Not all of the amino acid which are
formed during the digestion of proteins can be stored by the body. Excess amino acids are delivered to
the cells of the liver via a vein called the hepatic portal vein that runs from the
digestive system. The amino group, shown here in
pink, is removed from the amino acid in the liver, which produces an organic acid
that can be used by body cells and a toxic substance called ammonia.
As ammonia is toxic, it cannot be
stored by the cells of the human body, so it needs to be converted into another form
to be excreted safely. The cells of the liver carry out
this conversion through a process called the ornithine cycle, or sometimes called
the urea cycle.
The ornithine cycle involves carbon
dioxide, which is produced as a product of cellular respiration, and three amino
acids: ornithine, citrulline, and arginine. These substances and a number of
enzymes are used to convert toxic ammonia into a relatively harmless substance
called urea. It also produces water as a
by-product. Urea can then be transported to the
kidneys to be removed from the body as a part of urine. As it is a substance which needs to
be detoxified, we can deduce that compound X is ammonia.
