Communicable Diseases, Disease Prevention & the Immune System (OCR A Level Biology)

An individual’s immune responses can change throughout their lifetime.

Fig. 4.1 shows one person’s immune response to the influenza virus when they were first infected and when they were infected two years later by a new, mutated strain of the virus.

The influenza virus has many antigens to which the immune system can respond. Fig. 4.1 shows the response to four of these antigens (A–D).

Explain the differences in the person’s initial immune response to the influenza virus with their immune response two years later.

The specific immune response involves B and T lymphocytes.

There is variation in specific immune responses between individual animals.

Variation between immune responses can be influenced by genes and the environment.

Using examples, explain how both genes and environment can cause animals to vary in their specific immune responses.

It is possible to manufacture antibodies to treat certain diseases. These are known as synthetic antibodies.

DNL-Fab3, shown in Fig. 4.2, is an example of a synthetic antibody.

Fig. 4.2

State two conclusions that can be drawn from Fig. 4.2 about the differences between the way DNL-Fab3 functions and the functioning of normal antibodies.

Some microorganisms can be used by humans in industry. Some microorganisms are pathogenic.

Pathogenic microorganisms are transmitted in various ways.

Complete the following passage about the transmission of pathogenic microorganisms using the most appropriate terms.

Some pathogens are carried between host organisms by animals, which are often insects.

These animals suffer no symptoms of the disease and are known as ..................................... .

Other pathogens, such as P. infestans that causes potato blight, produce reproductive structures called ...................................., which can be carried on air currents to infect other hosts.

All organisms exchange gases with their environment.

Organisms can use simple diffusion to exchange gases when the diffusion pathway is less than 1mm.

A beet armyworm larva:
   •     has a cylindrical shape
   •     is 15mm long
   •     has a volume of 30mm³.

Calculate the diffusion pathway of the larva and state whether it could or could not rely on simple diffusion across its external surface to meet its gas exchange requirements.

Use the formula: Volume of a cylinder = πr²l

diffusion pathway = ........................................................ mm

larva ........................... rely on simple diffusion

Beet armyworm larvae eat a variety of plants, including tomato plants.

Scientists wanted to investigate how effective a chemical called methyl jasmonate was in stopping beet armyworm larvae from eating plants. They sprayed tomato plants with different concentrations of methyl jasmonate and recorded the final biomass of the plants.

The results are shown in Fig. 5.1.

Fig. 5.1

The scientists wrote the following hypothesis:

Plants use methyl jasmonate as a defence against herbivory.

What additional information do the scientists need to confirm their hypothesis?

[2]

(ii)

Suggest one valid conclusion it is possible for the scientists to draw from the results in Fig. 5.1.

 [1]

(iii)

The scientists also recorded the level of cannibalism amongst the beet armyworm larvae.

Cannibalism was measured as the number of beet armyworm larvae eaten by other beet armyworm larvae.

The results are shown in Fig. 5.2.

Fig. 5.2

Suggest one valid conclusion it is possible for the scientists to draw from their results shown in Fig. 5.2.

The potato plant, Solanum tuberosum, is a staple food plant in many parts of the world.

Potatoes are susceptible to infection by a pathogen called Phytophthora infestans, which causes a disease known as potato late blight. The most visible sign of the disease is a brown discolouration of the leaves.

Some varieties of potato are resistant to infection by P. infestans.

State two ways in which an individual S. tuberosum plant could respond to infection by P. infestans.

The resistance of different varieties of S. tuberosum to infection by P. infestans was investigated.

• Three different clones, A, B and C, of S. tuberosum were used.
• The clones were grown in adjacent fields over the same time period.
• The percentage of leaf area affected by the disease was estimated at regular intervals.

The results are shown in Fig. 18.

Fig. 18

Suggest why it is important to use clones in an investigation such as this.

 [2]

(ii)

State how a clone of potatoes could be produced for this investigation and explain why it is important to carry out this procedure under aseptic conditions.

 [2]

(iii)

The extent of infection is estimated by comparing the area under the curve from the graph. The area under the curve for clone B is 1250. (Units can be ignored in this instance.)

Using Fig. 18, calculate the approximate area under the curve, between day 35 and day 98, for clone C.

[3]

(iv)

Calculate the area under the curve for clone C as a proportion of the area under the curve for clone B.

 [1]

(v)

Using Fig. 18, suggest why the area under the curve is used as a measure of infection rather than the area of leaf that is visibly affected on a given day.

 [2]

(vi)

The clones were planted in adjacent fields in order to control variables such as temperature, wind speed and rainfall.

Suggest two other abiotic variables that this precaution was intended to control.

[2]

Plague is caused by the bacterium, Yersinia pestis.

(i)

The bacterium is a rod-shaped cell that is approximately 3 μm long.

Yersinia pestis is viewed using a light microscope with a magnification of 1250.

What would be the length of the cell in the image produced by this microscope?

Give your answer in mm.

[2]

(ii)

Photographs taken of the image obtained by the light microscope could be further enlarged using a projector.

Why might the enlarged image be unable to tell us more about the structure of Yersinia pestis?

[1]

(iii)

Outbreaks of plague still occur occasionally. Plague is transmitted by several methods including droplet infection, close contact between people and fleas moving between infected rats and people.

Suggest two ways to minimise the spread of an outbreak of plague.

Botulism is a condition resulting from the action of botulinum toxin. The main symptom of botulism is skeletal muscle weakness, which can be fatal.

Botulinum toxin is produced by the anaerobic bacterium Clostridium botulinum. What information does the word ‘anaerobic’ suggest about the bacterium?

[1]

The toxin is initially produced as a large single polypeptide that has low potency.

After the toxin has been acted upon by a protease, two chains are produced which remain connected by a disulfide bond. In this form it is far more toxic.

Describe the action of the protease when it acts on the toxin.

[1]

A mouse assay, using 99 mice, was used to determine the median lethal dose of the toxin.

Suggest what is meant by the term median lethal dose.

[1]

(ii)

The median lethal dose of the toxin is in the range of 5 – 50 ng kg^-1 body mass, depending on the toxin type and the method of introduction into the body.

Calculate the probable lethal dose of the least toxic botulinum toxin for an individual with a body mass of 85 kg.

Give your answer in μg.

[2]

(iii)

The toxin acts primarily at the cholinergic nerve terminals of stimulatory motor neurones. Part of the molecule binds irreversibly to specific receptors on the presynaptic membrane. The toxin–receptor complex is then taken into the cytoplasm of the neurone where the disulfide bond is broken, releasing the section of the molecule which acts to block the release of the neurotransmitter.

Explain why botulism can be fatal.

[2]

There are a number of different strains of the Clostridium botulinum bacterium. Different strains produce immunologically distinct forms of the toxin.

Explain why the toxins produced by the different strains are described as being ‘immunologically distinct’ and how they will be dealt with by the immune system.

There will be outbreaks of new infectious diseases in the future. They will arise from mutations in the genomes of existing organisms. The mutating organisms may not at present be pathogenic, or they may be animal pathogens that mutate to become able to infect humans.

What feature of a pathogen such as Mycobacterium tuberculosis could be altered by a mutation, making a vaccine ineffective?

Outline the processes that lead to the production of antibodies against an unfamiliar bacterium.

[3]

(ii)

Explain how helper T cells act to speed up these processes.

[2]

Fig. 16.1 shows the concentration of new antibodies in the blood of a person infected for the first time by a pathogen, on day 0. This is their ‘primary response’.
Fig. 16.1

On day 30, this individual was again infected with the same pathogen. Sketch a line on Fig. 16.1 to show the antibody concentration from day 30 onwards.

[2]

(ii)

Explain how memory cells caused the differences between the two lines on the graph.

[2]

It takes time for an effective vaccine to be prepared in quantity for a new strain of bacterium.

List two vulnerable groups of people for whom you would advise doctors to prescribe antibiotics although they are not yet showing symptoms of the new disease.

[2]

(ii)

Discuss the implications of the over-use of antibiotics when people do not show symptoms.

[4]