Science in Practice
Making dough
To make bread the fungus yeast is used. Flour is mixed with a solution of yeast. Sugar is added to provide a food source for the microbe to respire. Warmth helps the microbe multiply faster, so the dough is left in a warm area for several hours to rise. As there is a lack of oxygen the yeast respires anaerobically to produce carbon dioxide and alcohol. The carbon dioxide makes the bread rise. When the dough is put in the oven the alcohol evaporates off. Some breads such as pitta and naan bread do not rise because no yeast is added.
Breadmaking is an ancient art. In the Bible there is mention of unleavened bread, this is like pitta bread and is made without using yeast.
You are going to find out how temperature affects the rising of dough.
Method
1 Place five tablespoons of flour into your container.
2 Add 30 cm3of yeast solution to the flour and stir until
it becomes a thick liquid that can be poured, add more flour or liquid as needed.
3 Carefully pour the liquid dough into three boiling
tubes so they have equal amounts in. Measure the height of the dough once it has settled.
4 Place one boiling tube into a beaker of water at 20 °C,
another in a beaker at 40 °C and the final one into one at 60 °C. Check the temperature in the beaker and change it to keep it near the right temperature.
5 After 30 minutes measure how much the height of
the dough has risen.
… yeast … yoghurt
51Bacteria are grown on the waste material from certain food processing industries. The bacteria are used to make animal food.
Questions
1 How have you made the test fair?
2 What pattern do your results show?
3 Explain your results.
4 Why is it difficult to draw a graph of the results?
5 Why do you not get drunk when you eat lots of bread?
6 Why are there holes in bread?
7 a) Explain how you could obtain more accurate results instead of measuring the
height of dough.
b) Explain why it is difficult to see a clear pattern with the results. What extra experiments would give a clearer pattern?
Marmite is made from yeast and soya sauce is made using bacteria and fungi.
Copy the sentences below into your exercise book. Complete the following passage using the words below:
infection symptoms droplets incubation period
The measles virus enters the body by the spread of in the air. The
virus enters the cells, this is called . The virus may stay in the cells for
a while until it starts to attack them, this is the .
The person now starts to show the , like a rash, sore throat and aches.
Smallpox is a disease that has been wiped out from Europe. It was once one of the biggest killers of people.
Year Deaths of vaccinated Deaths of non-vaccinated
people % people %
1901 10 36
1902 10 34
1903 3 5
1904 5 8
a What pattern is shown by the table?
b What evidence is there that vaccinations prevented smallpox?
c What evidence is there that as more people are vaccinated the number of
deaths from smallpox of all people is reduced?
d Why should this be?
e Explain how the vaccine causes the body to produce antibodies.
A student tried to find out which disinfectant was the most effective against a certain bacteria.
He made some agar plates and placed the bacteria on them. He then placed discs of four different disinfectants on the plate. He incubated the plate at 30 °C and looked at the results.
a Why is there a clear zone round the discs?
b How can you measure the zone round the discs?
c Which disinfectant is the most effective? Explain why.
d Why is there no zone round B?
e Give one way you will make the test fair for comparing the effect of the
disinfectants.
Practice questions
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A B
A dangerous disease breaks out on a ship. The ship is put into quarantine.
a Why is the ship put into quarantine?
b What precautions will scientists take when they board the ship?
c The scientists found a sample of a microbe and wanted to find out if it caused
the disease.
i How could they use mice to see if the microbe caused the disease?
ii Why might the use of mice be unacceptable?
Measles, mumps and rubella are given as a triple injection. Several years ago there was concern expressed about the possibility that the vaccine could cause autism. In one study, 75% of a very small sample suffered from autism after they had the MMR jab. In another study of 500 children with autism, half of them had developed autism before vaccination.
a What evidence is there that MMR could cause autism?
b What evidence is there that MMR is unlikely to cause autism?
c Why do scientists publish their findings through scientific papers before they
make the public aware of them?
The publicity in some of the papers resulted in parents getting very worried and not giving their child the MMR vaccine. This resulted in a measles epidemic in America with 120 children dying.
d Why did the measles epidemic occur?
Pasteur was famous for his work on microbes. He trialled a vaccine for anthrax on sheep in the following way:
He took 60 sheep for the trial. 10 sheep were left alone in the field.
He then split the remaining 50 and injected 25 with a weak form of anthrax.
He then injected these 25 with a strong form of anthrax and the ones he had not treated with a strong form of anthrax.
a What results would you expect if the vaccine worked?
b Why were 10 sheep not treated?
c Why did he use 25 sheep not five sheep?
d He vaccinated the 50 sheep with the strong vaccine alternately (i.e. one of the
untreated ones followed by one of the treated ones). Explain why.
Topic Summary
✩I can name three ways diseases are spread. page 34
✩I know that infectious diseases are caused by microbes. page 35
✩I know two ways the body fights off infection. page 36
✩I know one example of biological warfare. page 42
✩I know that the types of microbe that cause disease are bacteria, page 34
viruses and fungi.
✩I know two examples of viral and two of bacterial disease. page 38
✩I can explain how one disease is spread and the symptoms it has. page 40
✩I know that disinfectants kill microbes. page 48
✩I can explain how to use an agar plate to show how effective disinfectants are. page 49
✩I know the infection cycle explains how microbes cause disease. page 34
✩I can explain the difference between infection and symptoms. page 35
✩I know how white blood cells prevent disease. page 36
✩I know the role of antibodies. page 37
✩I know the difference between disinfectants and antiseptics. page 48
✩I can explain how vaccines work. page 47
✩I can explain the case for and against the use of vaccines. page 47
✩I can explain how to find out which strength of disinfectant is not page 49
effective against bacteria.
✩I can explain the evidence to support the germ theory for disease. page 33
✩I can use the immune response model to explain how allergies are caused, page 44
how vaccines work and why some people never catch a disease.
1 What did people think caused diseases in 1600?
2 Who discovered the process of
pasteurisation?
3 Name a food produced using microbes.
4 How is a cold spread?
5 How is malaria spread?
6 Name two ways the body stops
diseases entering.
7 Name two types of microbe.
8 What is the role of white blood cells in preventing disease?
9 What is meant by the incubation
period?
10 What is the difference between
symptoms and infection?
11 What does a microbe do to the body to
cause disease?
12 How does a vaccine prevent a person
getting a disease?
True or False?
If a statement is false then rewrite it so it is correct.
1 All microbes cause disease. 2 Dead bodies were catapulted into
castles in ancient times to spread disease.
3 White blood cells produce antibodies. 4 Infectious diseases can be spread. 5 HIV is the most common STD.
6 The MMR vaccine is for measles, mumps
and rubella.
7 HIV is the virus and AIDS is the
symptoms caused by the virus.
8 Bread rises because yeast expands. 9 There are over twenty different sexually
transmitted diseases.
10 Flu is a sexually transmitted disease.
Literacy Activity
Read the passage below then answer the questions.
Louis Pasteur was one of the great scientists. He was responsible for the ‘Germ Theory’; this stated that microbes caused diseases.
A colleague’s wine was going sour (turning to vinegar) at a local vineyard. Pasteur investigated and discovered that microorganisms had contaminated the wine. He developed a process to kill the bacteria and not ruin the taste, this was called pasteurisation.
He then investigated why silkworms were dying in silkworm nurseries in France. To do this he:
studied the silk moths’ cells and found microorganisms. injected fluid from the moths into healthy moths.
found the silk moths died and had the same microorganisms in their cells.
1 What does sour wine form?
2 What food material is pasteurised?
3 What is the benefit of pasteurising milk over boiling it?
4 How did Pasteur’s work on silk worms provide evidence to support the Germ Theory?
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