Download F214: Communication, Homeostasis and Energy

F214: Communication, Homeostasis and Energy
4.2.1 Excretion
• define the term excretion;
• explain the importance of removing metabolic
wastes, including carbon dioxide and nitrogenous
waste, from the body;
• describe, with the aid of diagrams and photographs,
the histology and gross structure of the liver;
• describe the formation of urea in the liver, including
an outline of the ornithine cycle
• describe the roles of the liver in detoxification;
Excretion
“The removal of metabolic waste (unwanted
substances from cell processes) from the body”
• Urea from excess amino acids in the liver
• Carbon dioxide from respiration
Task
• Using textbooks, research the importance of
the excretion of both carbon dioxide and
nitrogenous compounds
Q. Why must these substances be removed and
where/how are these substances excreted?
Why Excrete Nitrogenous Compounds?
•
•
•
•
•
•
•
•
The body can’t store them
They have almost as much energy as carbohydrates
Transported to liver
Toxic amino acid group removed (deamination)
Amino group forms ammonia
Ammonia converted to urea
Urea transported to kidneys for excretion
Remaining Keto acid used in respiration or stored
Deamination: amino acid + oxygen  keto acid + ammonia
Urea formation: 2NH3 +
CO2
 CO(NH2)2
ammonia + carbon dioxide  urea
+ H2O
+ water
Why Excrete Carbon Dioxide?
• Excess carbon dioxide is toxic
• Carbon dioxide is carried in blood as hydrogencarbonate ions
producing hydrogen ions that compete with oxygen for
haemoglobin
• It combines with haemoglobin to form carbinohaemoglobin with
a lower affinity for oxygen than normal haemoglobin
• Excess carbon dioxide can cause respiratory acidosis- carbon
dioxide dissolves in blood plasma producing carbonic acid with
dissociates forming hydrogen ions which lower the pH of blood
Carbonic acid production:
CO2 + H2O  H2CO3
Dissociation of Carbonic acid:
H2CO3  H+ + HCO3-
Why Excrete Carbon Dioxide?
• Hydrogen ions lower the pH and make the blood more acidic
• Proteins in the blood act as buffers to resist the pH change, by
adding a hydrogen onto their COO (carboxyl group) to become
COOH (They can exist as COO in a neutral solution)
• The extra hydrogen ions are detected by the respiratory centre in
the medulla oblongata of the brain
• The brain causes an increase in breathing rate to remove the
excess carbon dioxide
• If the pH drops below 7.35, the result is slow, difficult breathing,
headache, drowsiness, restlessness, tremor, confusion, rapid
heart rate and blood pressure changes
• This is called respiratory acidosis
• Emphysema, chronic bronchitis, asthma and pneumonia, choking
or vomiting can also lead to it
Where is the liver located?
Where is the liver located?
Where is the liver?
The liver lies just beneath the diaphragm towards the right hand
side of the body- it is made up of several lobes
• one of the largest organs in the body
• 1450cm3 of blood passes through it in one minute
• Enormous variety of functions
• Uniform structure
The Liver
1. Stick in the liver diagram showing the blood
vessels
2. Give the function of:
• Hepatic artery
• Hepatic vein
• Hepatic portal vein
• Describe the location of the liver in the body
The Structure of the Liver
Task:
Your task is to describe, with the aid of diagrams and
photographs, the histology and gross structure of the liver.
You must then present your findings to the rest of the class.
You can choose how you would like to do this, and what
format you would like to use e.g. PowerPoint, information
board, poster etc.
Task Guide
• Decide on the format
• Research the liver on the internet using the
checklist provided (approx 1-3 hours)
• Gather appropriate images (approx 1 hour)
• Complete the project including the images and
information you have gathered (1-3 hours)
• Present your project
• You will be assessed by your ability to annotate
(label with descriptions) a diagram of the liver
Checklist
Blood flow to and from the liver including the
hepatic portal vein
Arrangement of cells inside the liver
Structure of liver cells including microvilli
The role of Kupffer cells
Blood Supply
• Blood arrives in two different blood vessels
• Hepatic artery leads from the aorta and delivers
oxygenated blood to the liver
• The hepatic portal vein leads from the small intestine and
delivers blood rich in absorbed nutrients (like glucose and
amino acids from the intestine) and insulin and glucagon
from the pancreas
• The hepatic portal vein carries about 3 times more blood
per minute than the hepatic artery
• Blood from the hepatic portal vein has already travelled
through a set of capillaries so is at a much lower pressure
than blood in the hepatic artery
• The hepatic vein carries blood away from the liver to the
vena cava which then transports it back to the heart- it
returns glucose to circulation and allows amino acids to
enter circulation
Histology of the Liver
• Made up of many lobules
• In the centre of each lobule is a branch of the
hepatic vein
• Between the lobules are branches of the
hepatic artery and the hepatic portal veinblood flows from here, through the lobules
and into the branch of the hepatic vein
• The lobules are made
up of many liver cells
called hepatocytes
arranged in rows that
radiate out from the
centre like spokes of a
wheel
• The channels which
carry blood between
these rows of cells are
called sinusoids
• Other channels carry
bile, which is
produced from some
of the hepatocytes:
these channels are
called bile canaliculi
Histology of the Liver
• The bile flows from the
centre of the lobule
towards the outside –
the opposite direction
to the blood flow,
where it enters a
branch of the bile duct
• The rows of
hepatocytes are never
more than two cells
thick, so that each cell is
close to the blood in
the sinusoids
Histology of the Liver
• The sinusoids are lined with
large, phagocytic
macrophages which capture
and destroy bacteria entering
the liver via the hepatic
portal vein from the intestine
• These cells are sometimes
called kupffer cells.
• They are very efficient- if a
bacterium comes into contact
with the membrane of a
kupffer cell, it is taken into
the cell by phagocytosis
within 0.01 second!!
F214: Communication, Homeostasis and Energy
4.2.1 Excretion
• define the term excretion;
• explain the importance of removing metabolic
wastes, including carbon dioxide and nitrogenous
waste, from the body;
• describe, with the aid of diagrams and photographs,
the histology and gross structure of the liver;
• describe the formation of urea in the liver, including
an outline of the ornithine cycle
• describe the roles of the liver in detoxification;
F214: Communication, Homeostasis and Energy
4.2.1 Excretion
• define the term excretion;
• explain the importance of removing metabolic
wastes, including carbon dioxide and nitrogenous
waste, from the body;
• describe, with the aid of diagrams and photographs,
the histology and gross structure of the liver;
• describe the formation of urea in the liver, including
an outline of the ornithine cycle
• describe the roles of the liver in detoxification;
F214: Communication, Homeostasis and Energy
4.2.1 Excretion
• define the term excretion;
• explain the importance of removing metabolic
wastes, including carbon dioxide and nitrogenous
waste, from the body;
• describe, with the aid of diagrams and photographs,
the histology and gross structure of the liver;
• describe the formation of urea in the liver, including
an outline of the ornithine cycle
• describe the roles of the liver in detoxification;
Control of
blood glucose,
amino acid and
lipid levels
Synthesis of
Fetal red blood
cells, bile,
plasma proteins
and cholesterol
Destruction of
red blood cells
Liver
Functions
Storage of
vitamins A, D,
B12, iron and
glycogen
Breakdown of
hormones
Detoxification
of alcohol and
drugs
Urea Formation
• Excess amino acids cannot be stored as the
amine groups are toxic
• Therefore it goes through two treatments in
the liver: Deamination and The Ornithine
Cycle
Deamination
• Ammonia is produced which is soluble and
toxic
• Keto acid is produced which enters respiration
directly to release energy
R
NH2
C
R
COOH
H
Amino Acid
+
O2
C
COOH
+
NH3
O
Keto Acid
Ammonia
The Ornithine Cycle
(ammonia’s conversion to urea)
• Ammonia is combined with carbon dioxide to produce urea
• Urea is less soluble and less toxic than ammonia so it can be
passed back into the blood to the kidneys
• The kidneys filter out the urea from the blood and store it as urine
in the bladder
2NH3
+
CO2
 CO(NH2)2
Ammonia + carbon dioxide 
urea
+
+
H2O
water
As you can see above, ammonia and carbon dioxide enter the cycle. ATP
is required and Urea is produced. Water is also produced as a by-product.
The body can manufacture ornithine, but it is abundant in meat, fish, dairy
and eggs
Detoxification
• Many dangerous substances are broken down
by the liver
• Some are made harmless, some are excreted
into bile
• Most of these processes take place in the
smooth endoplasmic reticulum in the
hepatocytes
• Toxins can be made harmless by oxidation,
reduction, methylation or combination with
another molecule
Detoxification
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•
•
•
Oxidation: loss of electrons
Reduction: gain of electrons
Methylation: the addition of a methyl group
Combination with another molecule
Methylation is
the addition of a
methyl group
Metabolism of Alcohol
• Alcoholic drinks contain ethanol C2H5OH
• Ethanol molecules are small and lipid soluble
so diffuse easily across the plasma
membranes and enter cells
• Ethanol is a toxic substance and can cause
damage to liver cells
• The liver avoids damage by breaking down
ethanol into harmless substances
Metabolism of Alcohol
• The enzyme alcohol dehydrogenase converts
ethanol into ethanal (acetaldehyde)
• The enzyme aldehyde dehydrogenase then
converts ethanal (acetaldehyde) into
ethanoate (acetate)
• This can then be combined with coenzyme A
to form Acetyl Coenzyme A, which then enters
the Kreb’s cycle and be metabolised to
produce ATP
• Ethanol is therefore a source of energy for
cells
Ethanol
Ethanal
NAD
Reduced
NAD
Ethanoate
NAD
Acetyl Coenzyme A
Reduced
NAD
• The Hydrogen atoms released are combined with another coenzyme called NAD
to form reduced NAD
• NAD is also needed to oxidise and break down fatty acids for respiration
• If the liver has to detoxify too much alcohol, it does not have enough NAD to deal
with the fatty acids, so they are changed back to lipids and are stored in
hepatocytes (liver cells), causing the liver to become enlarged- this is called ‘fatty
liver’ and can lead to liver cirrhosis.
Discuss in pairs...
1. Why must ammonia be converted to urea?
2. Explain why excess amino acids and alcohol
should not be secreted?
3. Suggest why the liver cells have large
numbers of mitochondria and ribosomes?
Write down your responses in pairs and pass it to another pair to look at- do
you have the same responses?
Discuss in pairs...
1. Why must ammonia be converted to urea?
Ammonia is highly soluble and very toxic- urea is less soluble
and less toxic
2. Explain why excess amino acids and alcohol should not be
secreted?
They contain valuable energy that can be used. Amino acids
can be converted into others
3. Suggest why the liver cells have large numbers of
mitochondria and ribosomes?
Mitochondria provide ATP for processes e.g. protein
synthesis. Ribosomes manufacture the enzymes needed by
liver cells
F214: Communication, Homeostasis and Energy
4.2.1 Excretion
• define the term excretion;
• explain the importance of removing metabolic
wastes, including carbon dioxide and nitrogenous
waste, from the body;
• describe, with the aid of diagrams and photographs,
the histology and gross structure of the liver;
• describe the formation of urea in the liver, including
an outline of the ornithine cycle
• describe the roles of the liver in detoxification;