4.3 Infection and response — coverage pack

Answer

• A pathogen is a microorganism that causes infectious disease.
• Any one from viruses, bacteria, protists and fungi.

Method: Give the complete definition for one mark, then select any one of the four groups listed in the specification for the second mark.

Answer

• Both may reproduce rapidly inside the body.
• Bacteria may produce toxins.
• The toxins damage tissues and make the person feel ill.
• Viruses reproduce inside body cells and damage those cells.

Method: Start with the shared feature of rapid reproduction, then distinguish the mechanisms: bacterial toxins damage tissues, while viral reproduction takes place inside and damages host cells.

Answer

• Coughing releases pathogen-containing droplets into the air.
• Other people may inhale the droplets.
• Covering coughs, improving ventilation or reducing close contact can reduce airborne spread.
• Pathogens can pass by direct contact through contaminated hands or surfaces.
• Handwashing and cleaning shared surfaces can reduce contact transmission.
• The controls reduce the chance that pathogens move from an infected host to a new host.

Method: Treat the two observations separately. Link coughing to airborne droplets and inhalation, and link hands and surfaces to contact transmission. Give a control matched to each route and explain that it interrupts transfer between hosts.

Answer

• Red skin rash: measles.
• Mosaic leaf pattern: tobacco mosaic virus (TMV).

Method: Match the characteristic red rash to measles and the named mosaic leaf symptom to tobacco mosaic virus.

Answer

• TMV causes a mosaic pattern of discolouration on the leaves.
• The discoloured leaves carry out less photosynthesis.
• Less glucose is produced for processes needed for growth, so growth is reduced.

Method: Build a causal chain from the visible leaf symptom to reduced photosynthesis and then to reduced production of material needed for plant growth.

Answer

• Measles spreads when droplets from coughs or sneezes are inhaled.
• It causes fever and a red skin rash and may be fatal if complications occur.
• Vaccination can prevent measles.
• HIV spreads by sexual contact or exchange of body fluids such as blood.
• HIV attacks immune cells and may progress to AIDS, leaving the body unable to deal with other infections or cancers.
• Antiretroviral drugs can control HIV, and avoiding exchange of infected body fluids reduces transmission.

Method: Organise the comparison into three matched categories. For measles, link droplets, its characteristic symptoms and vaccination. For HIV, link body fluids, progressive immune-system damage and control by antiretroviral drugs or prevention of fluid exchange.

Answer

• Salmonella: fever, abdominal cramps, vomiting or diarrhoea.
• Gonorrhoea: a thick yellow or green discharge from the vagina or penis, or pain when urinating.

Method: Choose one accepted symptom from each disease, keeping food-poisoning symptoms separate from the characteristic gonorrhoea symptoms.

Answer

• Vaccinating poultry reduces Salmonella infection in the birds.
• This reduces the chance that poultry food products contain the bacteria.
• Hygienic preparation reduces contamination or transfer of bacteria onto food.
• Both measures reduce the chance that people ingest Salmonella bacteria.

Method: Cover control at two stages: vaccination reduces the bacterial source in poultry, while hygiene reduces transfer during food preparation. Link both controls to a lower chance of people ingesting the pathogen.

Answer

• A penicillin-resistant strain is not killed by penicillin.
• A different antibiotic that is effective against the strain may be needed.
• Successful antibiotic treatment removes or kills the infective bacteria in the patient.
• A condom reduces sexual contact and exchange of infected fluids.
• Using both measures treats the existing infection and reduces spread to new hosts.

Method: Separate failure of the unsuitable antibiotic from the two controls. Resistance explains why penicillin does not kill this strain; a suitable antibiotic targets the infection, while a condom interrupts sexual transmission.

Answer

• It is caused by a fungus.
• Purple or black leaf spots, yellowing leaves or leaves dropping early.

Method: Identify the disease as fungal, then give one of the leaf symptoms stated in the specification.

Answer

• The fungus damages leaves and can make them drop early.
• The plant has less healthy leaf area for photosynthesis.
• Less photosynthesis means less glucose is made for growth.

Method: Link the observed leaf damage to a smaller photosynthesising surface and then to reduced glucose production and growth.

Answer

• The fungal disease can spread in water and by wind.
• Wet, windy conditions can therefore transfer the pathogen between leaves or plants.
• Remove affected leaves to reduce the source of infection.
• Destroy the removed leaves so the fungus is not left nearby to spread.
• Use an appropriate fungicide to treat the plants and reduce further infection.

Method: Use both environmental clues to explain the spread route. A complete programme removes infected material, destroys it rather than leaving it near the plants, and uses fungicide as the second specified control.

Answer

• The pathogen is a protist.
• The vector is a mosquito.

Method: Distinguish the disease-causing organism from the carrier: the protist is the pathogen and the mosquito is the vector in its life cycle.

Answer

• Mosquitos act as vectors for the malarial protist.
• A net reduces the chance that a mosquito bites a person.
• This reduces transfer of the protist between the mosquito and human hosts.

Method: Name the vector, state how the net changes contact between vector and person, and link fewer bites to less pathogen transmission.

Answer

• Mosquitos are vectors in the malarial protist's life cycle.
• Nets reduce bites and therefore reduce transfer of the protist to or from people.
• Removing standing water prevents or reduces mosquito breeding.
• This reduces the number of vectors able to transmit the pathogen.
• The two actions act at different stages, reducing both vector numbers and contact with people.

Method: Explain each intervention separately, then combine the causal chains: breeding control reduces the vector population, while nets reduce bites from vectors that remain.

Answer

• Any two from phagocytosis, production of antibodies and production of antitoxins.

Method: Recall any two of the three immune-system actions named in the specification; each distinct action earns one mark.

Answer

• Nasal hairs and mucus trap particles and pathogens.
• Mucus in the trachea and bronchi also traps pathogens.
• Cilia move the mucus towards the throat.
• The mucus can be swallowed, so trapped pathogens are removed from the airways.

Method: Follow the pathogen through the respiratory route. State where it is trapped, then explain how cilia transport the mucus away from the lungs.

Answer

• Intact skin normally acts as a physical barrier to pathogen entry.
• The cut bypasses this non-specific barrier.
• White blood cells can engulf and digest bacteria by phagocytosis.
• White blood cells can produce antibodies specific to antigens on the bacterium.
• The antibodies help target the pathogen for destruction.
• White blood cells can produce antitoxins that neutralise or counteract the bacterial toxin.

Method: Begin with the barrier and explain why the cut matters. Then distinguish the three white-cell responses: engulfing the pathogen, making specific antibodies against it, and making antitoxins against its toxin.

Answer

• A small quantity of a dead or inactive form of the pathogen is introduced.
• White blood cells are stimulated to produce antibodies.

Method: Name both the vaccine material and the immune response; saying only that a vaccine contains medicine does not identify either required feature.

Answer

• The vaccine has already stimulated production of antibodies against that pathogen.
• The immune system recognises the same pathogen or its antigens on later exposure.
• White blood cells produce the correct antibodies rapidly.
• The pathogen is destroyed before it causes illness.

Method: Use a chronological explanation: first exposure through vaccination establishes the specific response, then later recognition leads to faster production of the correct antibodies and prevents disease.

Answer

• Vaccinated individuals are less likely to become ill after exposure.
• A rapid specific antibody response can destroy the pathogen before disease develops.
• High uptake leaves fewer susceptible hosts and reduces transmission through the population.
• This can also reduce exposure of people who have not been immunised.
• The programme requires enough vaccine, staff, access and public uptake to reach a large proportion of the population.
• Its overall benefit is greatest when the disease is serious and high coverage substantially reduces illness and spread.

Method: Balance benefits to the individual with the population effect, then consider practical limits on achieving high uptake. Finish with a reasoned judgement linked to disease severity and coverage rather than listing points without a conclusion.

Answer

• An effective antibiotic kills infective bacteria inside the body.
• A painkiller reduces symptoms such as pain but does not kill the pathogen.

Method: Contrast action on the cause of a bacterial infection with action on a symptom; do not claim that symptom relief removes the pathogen.

Answer

• The antibiotic would not kill the viral pathogen.
• Antibiotics act against bacteria rather than viruses.
• The painkiller could reduce pain or other symptoms.
• The painkiller would not kill the virus or cure the infection.

Method: Judge the two medicines separately. Reject the antibiotic because the pathogen is viral, and credit the painkiller only with symptom relief, not pathogen destruction.

Answer

• Antibiotics can kill infective bacteria but cannot kill viral pathogens.
• Using an antibiotic for a viral disease would not remove the cause of the infection.
• Different antibiotics are effective against different bacteria.
• Some bacterial strains are resistant to particular antibiotics.
• The chosen antibiotic must therefore be effective against the infecting bacterial strain.

Method: First classify the pathogen because antibiotic action is limited to bacteria. Then explain specificity and resistance: even for bacterial disease, the selected drug must work against that particular strain.

Answer

• Aspirin: willow.
• Penicillin: Penicillium mould.

Method: Match aspirin to its plant source and penicillin to the microorganism from which Fleming discovered it.

Answer

• Preclinical testing uses cells, tissues and live animals in a laboratory.
• Testing checks toxicity, efficacy and dose.
• Clinical trials then use healthy volunteers and patients.
• Trials begin with very low doses and, if safe, continue to find the optimum dose.

Method: Give the stages in order, including the preclinical test systems, the three properties examined, the clinical participants and the low-dose safety-first progression.

Answer

• Some participants receive the drug and others receive a placebo for comparison.
• The placebo controls for changes caused by expectations rather than the active drug.
• In a double-blind trial, neither the patients nor the people assessing them know who received the drug during the trial.
• This reduces bias in reporting and assessing outcomes.
• Comparing the groups helps judge the drug's efficacy and identify unwanted effects.
• Peer review allows other scientists to scrutinise the methods, evidence and conclusions before the findings are accepted.

Method: Explain the purpose of each design feature rather than merely naming it: placebo provides a control, blinding reduces bias, comparison supplies evidence of effect, and peer review checks whether the study supports its claims.

Answer

• It is produced by a single clone of cells.
• It binds to one specific binding site on one protein antigen.

Method: Give both defining features: common clonal origin and specificity to one antigen binding site.

Answer

• Stimulate mouse lymphocytes to make the required antibody.
• Combine the lymphocytes with tumour cells to form hybridoma cells.
• Select and clone a single hybridoma that makes the required antibody to produce many identical cells.
• Collect and purify the antibody produced.

Method: Present the production process in order from stimulated lymphocyte to fusion, single-cell selection, cloning and purification. Each stage ensures both scale and one antibody specificity.

Answer

• The lymphocyte makes the required specific antibody but does not divide indefinitely.
• The tumour cell can divide repeatedly but does not make the required antibody.
• Fusion produces a hybridoma with both useful properties.
• Selecting one hybridoma ensures the chosen cell makes the required antibody.
• Cloning produces many identical cells that all make the same antibody.
• The resulting antibody can be collected in large amounts and purified from other material.

Method: Contrast the two parent cells, then show how fusion combines their useful features. Finally link single-cell cloning to identical antibody specificity and repeated division to large-scale collection.

Answer

• Any two from pregnancy testing, measuring hormones or other chemicals in blood, detecting pathogens, or locating specific molecules in cells or tissues.

Method: Choose two distinct specified uses; examples from the same application should not be counted twice unless they perform different functions.

Answer

• Choose a monoclonal antibody specific to an antigen or binding site on the protein.
• Attach a fluorescent dye to the antibody.
• The antibody binds only where the target protein is present.
• The fluorescent signal reveals the protein's location in the tissue.

Method: Link specificity to selective binding, then link the attached fluorescent label to a visible signal at the target's position.

Answer

• The monoclonal antibody is specific to an antigen on the cancer cells.
• It binds to the cancer cells and carries the attached toxic drug to them.
• The toxic drug kills the cancer cells or stops them growing and dividing.
• Targeting can reduce damage to other cells compared with delivering the toxin throughout the body without a carrier.
• Monoclonal antibodies can still cause unexpected side effects.
• The treatment is useful only when its targeting benefit outweighs its side effects and other limitations.

Method: Describe specific binding and delivery before evaluating. The advantage follows from concentrating the attached treatment at target cells; the limitation follows from the specification's warning about side effects.

Answer

• Stunted growth: nitrate ion deficiency.
• Chlorosis: magnesium ion deficiency.

Method: Match nitrate deficiency to stunted growth and magnesium deficiency to chlorosis; the roles in protein and chlorophyll production explain these pairings.

Answer

• Compare the symptoms with a gardening manual or reliable website.
• Send or take infected plant material to a laboratory to identify the pathogen.
• Use an appropriate testing kit containing monoclonal antibodies.

Method: List the three specified identification routes. The question asks for identification, so repeating visual symptoms without using a reference, laboratory or test kit does not answer it.

Answer

• Similar symptoms such as discolouration or reduced growth can have more than one cause.
• Inspect the plants for pests such as aphids and for characteristic disease patterns such as TMV mosaic or rose black spot.
• Compare the full symptom pattern with a gardening manual or reliable website.
• Use a laboratory or a monoclonal-antibody test kit to identify a suspected pathogen.
• Test or change mineral supply: nitrate deficiency is linked to stunted growth because nitrate is needed for proteins.
• Magnesium deficiency is linked to chlorosis because magnesium is needed for chlorophyll, so evidence from several checks supports the diagnosis.

Method: Do not diagnose from one non-specific symptom. Combine pest inspection and characteristic examples with the Higher-tier identification methods, then use the distinct biological roles of nitrate and magnesium to assess deficiency explanations.

Answer

• Physical: cellulose cell wall, tough waxy cuticle or layers of dead cells around stems.
• Chemical: antibacterial chemicals or poisons that deter herbivores.

Method: Give one example from each named category, taking care not to count the same feature twice.

Answer

• The tough waxy cuticle forms a physical barrier on the leaf surface.
• It resists entry of microorganisms into living leaf tissues.
• Layers of dead cells form a barrier around the stem.
• When dead outer layers fall off, microorganisms on them can be removed from the plant.

Method: Explain each structure as a physical barrier, then add the distinctive role of shedding dead outer cells in removing attached microorganisms.

Answer

• The waxy surface is a physical defence that resists invasion by microorganisms.
• The antibacterial substance is a chemical defence that inhibits or kills bacteria.
• The hairs are a mechanical adaptation that deters animals.
• Leaf curling when touched is a mechanical adaptation that discourages or dislodges animals.
• The responses act against both microorganisms and herbivores.
• Using different kinds of defence reduces damage through more than one route.

Method: Classify all four features using the specification's categories, state the target or action of each, and finish by explaining why several defences protect against different threats.