Download Activity: Poisonous mushrooms worksheet

Volume 27, Number 4, April 2015
Activity
Poisonous mushrooms: -amanitin
Bill Indge
This worksheet accompanies the PowerPoint presentation and is designed to support the article
‘Edible and poisonous mushrooms’ by Chris McInerny in the April 2015 issue of BIOLOGICAL SCIENCES
REVIEW.
1 The deathcap is an extremely poisonous fungus. It produces a large number of toxic substances.
One of these is -amanitin. -amanitin is a cyclic polypeptide made up of eight amino acids.
(a) Copy and complete the diagram below to show the structure of an amino acid molecule.
(1 mark)
(b) Use your diagram to show how two amino acids can bind together with a peptide bond to form a
dipeptide.
(1 mark)
(c) In the simplest cyclic polypeptides, the amino acids bind together to form a ring. How many peptide
bonds would there be in a cyclic polypeptide containing eight amino acids? Explain how you arrived at
your answer. (2 marks)
2 The cyclic polypeptide -amanitin is not digested in the gut but it is absorbed as a polypeptide.
There are two main types of protein-digesting enzymes in the gut:

Endopeptidases break large polypeptides in the middle of the chain into smaller polypeptides.

Exopeptidases remove individual amino acids from the end of a polypeptide chain.
-amanitin is not broken down by endopeptidases. Use this information to suggest why exopeptidases
do not digest -amanitin.
(2 marks)
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3 The diagram below shows how bile salts are circulated through the liver and the intestine.
-amanitin circulates in a similar way.
(a) Describe the route by which bile salts produced in the liver enter the duodenum (the first section of
the small intestine).
(1 mark)
(b) Name the blood vessel through which bile salts are returned to the liver.
(1 mark)
4 There is a delay of 2–6 days between eating fungi containing -amanitin and the development of
symptoms associated with liver damage.
(a) Explain why this delay might suggest that it would be inappropriate to treat patients with
substances that remove poisons from the gut. (2 marks)
(b) In spite of this delay, patients are often given oral doses of activated charcoal. Activated charcoal
absorbs poisonous substances in the gut which are then safely removed from the body in faeces. Use
information provided in this question to explain why giving activated charcoal might still be a useful
treatment for a person who has eaten fungi containing -amanitin.
(2 marks)
5 It is suggested that -amanitin enters cells by facilitated diffusion, making use of the same transport
protein as bile salts. Scientists investigated factors affecting the rate of uptake of -amanitin through
liver cell membranes. In the first experiment they looked at the effect of the concentration of
-amanitin on its rate of uptake. Their results are shown in the table below.
Concentration of -amanitin/µmol dm–3
Philip Allan Updates © 2015
Rate of uptake of -amanitin/µmol min–1 mg–1 protein
50
1.0
100
2.2
150
3.0
200
3.8
400
4.4
600
5.0
800
5.0
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(a) Describe the effect of -amanitin concentration on its uptake by liver cell membranes.
(2 marks)
(b) Explain the change in the rate of uptake at concentrations of -amanitin between 400 and 800
µmol dm–3.
(2 marks)
6 In a second experiment, the scientists investigated the effect of bile salts on the rate of uptake of
amanitin by liver cell membranes. Their results are shown in the graph.
(a) Explain why the units for -amanitin are given per gram of protein. (2 marks)
(b) Explain how the results shown in the graph support the suggestion that -amanitin enters cells by
facilitated diffusion, making use of the same transport protein as bile salts.
(2 marks)
(c) The fetus is not harmed if a pregnant woman eats fungi containing -amanitin. Suggest why the
fetus is not poisoned by -amanitin.
(2 marks)
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Marking guidelines
1 (a)
(1 mark)
This is the basic structure but it would be more useful if students set this out as shown in the
PowerPoint presentation. This makes it easier to show how two amino acids are joined by
condensation.
(b) Peptide bond correctly shown between the COOH and NH2; (1 mark)
For further clarification see PowerPoint presentation.
(c) 8;
There is a peptide bond between the amino acids forming the polypeptide plus an additional bond
where the two end amino acids bind to form a ring;
(2 marks)
2 Cyclic polypeptide so no smaller polypeptides produced;
No free amino/carboxylic acid groups;
Will not bind with enzyme;
(max. 2 marks)
3 (a) (Stored in gall bladder) then (into duodenum) through bile duct;
(1 mark)
(b) Hepatic portal vein; (1 mark)
4 (a) Takes less than 2 days/short time for substances in gut to be absorbed;
Will pass out as faeces;
So would not expect any -amanitin to still be in gut;
(max. 2 marks)
(b) -amanitin is recycled/passes back into the gut;
Activated charcoal may remove the -amanitin that re-enters the gut;
So would not reach such high concentrations in the liver; (max. 2 marks)
5 (a) The rate of uptake increases and then levels out (as the concentration of -amanitin increases);
Between 400 and 600 µmol dm–3;
(2 marks)
(b) Because -amanitin is taken up by facilitated diffusion;
At concentrations above 400 µmol dm –3 the carriers are saturated with -amanitin;
(2 marks)
6 (a) Because this allows the results to be compared;
As uptake depends on the number of carrier proteins present/the number of carrier proteins may vary
from membrane to membrane; (2 marks)
(b) Because the graph shows that the rate of uptake is lower when bile salts are present;
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-amanitin and bile salts are competing for the same transport protein;
(2 marks)
(c) -amanitin cannot pass through the placental membranes;
No -amanitin/bile salt carrier proteins present; (2 marks)
This resource is part of BIOLOGICAL SCIENCES REVIEW , a magazine written for A-level students by subject experts.
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