Download Lesson_3_liver_function

The Function
of the Liver
Objectives
• Describe the formation of urea in the liver,
including an outline of the ornithine cycle.
• Describe the roles of the liver in
detoxification.
Formation of Urea
• XS aa in the diet cant be stored as the amine
group makes them toxic.
• The aa is deaminated (removal of the amine
group) to form a keto acid and ammonia. The
ammonia then enters the ornithine cycle to
form the less toxic urea for excretion.
amino acid
ammonia + keto acid
deamination
urea
ornithine cycle
Deamination
• Produces ammonia which is highly toxic
and very soluble, so it must not be
allowed to accumulate.
• It also produces a keto acid which can
enter respiration directly to release
energy.
R
NH2
C
H
R
COOH +
O2
C
O
COOH +
NH3
The ornithine Cycle
The ornithine cycle
• The ammonia is combined with carbon dioxide
to produce urea.
• Urea is less soluble and less toxic than
ammonia.
• It can be passed back to the blood and
transported to the kidneys where it is filtered
into the urine.
• Urine is stored in the bladder until it is
released from the body.
2NH3 + CO2
CO(NH2)2 + H2O
Detoxification
• Toxins may be produced by our body (e.g.
hydrogen peroxide), may be taken in via our
diet (e.g. alcohol), or may be consumed
recreationally/medicinally (e.g. drugs).
• Toxins can be oxidised, reduced, methylated
or combined with another molecule to make
them harmless.
• Liver cells contain many enzymes that make
toxins less toxic e.g. catalase breaks down
hydrogen peroxide into ……..
Detoxification of Alcohol
• Ethanol (alcohol) depresses nerve
activity.
• It contains energy so can be used for
respiration.
• It is broken down by hepatocytes by
the enzyme ethanol dehydrogenase to
ethanal.
• Ethanal dehydrogenase then breaks the
ethanal down into ethanoate.
• Ethanoate combines with coenzyme A to
form acetyl coenzyme A, and this
molecule can enter the respiratory
pathway.
Detoxification of Alcohol
• The hydrogen ions released during this
process are used to reduce another coenzyme
NAD to form reduced NAD.
• NAD is also used in respiration to break down
fatty acids.
• If the liver has to detoxify too much alcohol
it has insufficient NAD to deal with the fatty
acids and so these are converted back to
lipids to be stored in the liver. This leads to
the “fatty liver” condition and can lead to
hepatitis or cirrhosis.
Ethanol
Ethanal
NAD Reduced
NAD
Ethanoic Acid
NAD Reduced
NAD
Acetyl Coenzyme A
To
respiration