AQA GCSE biology - Infection and response

What is a communicable disease?

A disease caused by a pathogen which can be passed between animals or plants

What is a pathogen?

A disease-causing microorganism e.g. bacteria

How do bacteria cause disease?

Once inside the body, they divide rapidly by binary fission. They kill cells and produce harmful toxins.

How do viruses cause disease?

They invade and reproduce inside living body cells, leading to cell damage

How do fungi cause disease?

They digest surrounding cells by secreting enzymes

Give three ways in which pathogens can be spread

• By air - flu, tuberculosis and the common cold are spread by droplet infection

• By water - fungal spores in water spread plant diseases

• By direct contact - common in plant diseases and sexually transmitted infections

Give four ways in which the spread of pathogens can be reduced

• Hygiene - hand washing, disinfectants, tissues

• Reducing contact with infected individuals - quarantine

• Removing vectors - use of pesticides and insecticides, removal of habitats

• Vaccination

How are viruses spread?

They are often airborne

How are bacteria spread?

Via direct contact

How are fungal infections spread?

Via direct contact

How are protists spread?

In a vector (any organism that can spread disease)

Why is it especially important to prevent the spread of viral diseases?

Scientists have not yet developed cures for many viral diseases

What is measles?

Measles is a serious viral disease that can cause blindness and brain damage. The main symptoms are a fever and a red skin rash.

Why is measles a serious illness?

It can be fatal if complications arise.

For this reason most young children are vaccinated against measles

How is measles spread?

By air - through the inhalation of droplets from coughs and sneezes

What is HIV/AIDDS?

HIV is a virus which attacks and damages the immune system until it can no longer function properly. AIDS is the condition resulting from a long-term HIV infection, when the body’s immune system becomes so badly damaged it can no longer deal with other infections or cancers. There is no cure/vaccine for HIV/AIDS

What can HIV manifest as?

A flu-like illness

How is HIV spread?

Direct sexual contact and the exchange of bodily fluids e.g. blood

How can the spread of HIV/AIDS be prevented?

• Use of condoms

• Screening of blood for transfusions

• Not sharing needles (e.g drug users)

• Bottle-feeding by HIV positive mothers

• Use of antiretroviral drugs to prevent the development of AIDS

What is tobacco mosaic virus (TMV)?

A plant pathogen which causes leaf discolouration when cells are damage (chloroplasts are killed so yellow leaves)

This affects the growth and yield of the plant due to a lack of photosynthesis

How is TMV spread?

• Direct contact between infected and healthy plants.

• Insects may act as vectors which transfer the virus between different plants

• Contact with contaminated tools

How can the spread of TMV be treated?

There is no cure, so farmers try to reduce infection too their crops, attempt to reduce the spread of the virus (by spacing plants and cleaning tools), or grow TMV resistant strains of crops

What is Salmonella?

A type of bacteria found in raw meat, poultry and eggs. If they enter the body via food poisoning, they can affect natural gut bacteria

What are the symptoms of salmonella food poisoning?

• Fever

• Abdominal cramping

• Vomiting

• Diarrhoea

May be fatal in very young or elderly populations die to the risk of dehydration

How can the spread of salmonella be limited/prevented?

• Vaccinating animals (e.g poultry in the UK) intended for consumption

• Keep raw meat away from cooked meat

• Disinfect hands and surfaces after contact with raw meat

• Thoroughly cook meat

What is the treatment for salmonella?

Drink plenty of fluids (no real treatment)

What is gonorrhoea?

A sexually transmitted bacterial infection caused by unprotected sex with an infected individual.

What are the symptoms of gonorrhoea?

• Thick yellow or green discharge from the penis/vagina

• Pain on urinating

• May be symptomless

What is gonorrhoea caused by?

Direct sexual contact with an infected individual.

Bacteria

How can the spread of gonorrhoea be prevented?

• Methods of barrier contraception such as using a condom

• Not sharing sex toys

How can gonorrhoea be treated?

As it is a bacterial disease, it can be treated using antibiotics (mainly penicillin).

However, many antibiotic-resistant strains of gonorrhoea have appeared

What is rose black spot?

A fungal disease

What are the symptoms of rose black spot?

Purple/black spots on leaves, yellow leaves, these leaves drop off early

How does rose black spot affect the growth of a plant?

Growth is stunted as photosynthesis is reduced

How is rose black spot spread?

Fungal spores are spread by the wind and in water

How can rose black spot be treated?

• Using fungicides

• Removing and destroying (e.g burning) the affected leaves

What is malaria?

Malaria is a disease caused by the protist pathogen Plasmodium protist. These are often found in areas with higher temperatures like Africa and Asia.

How is malaria spread?

Mosquitos act as vectors for transmitting this disease as they do not become ill.

Mosquitos suck blood containing the protists from an infected person.

What are the symptoms of malaria?

• Fever (recurrent episodes can be fatal)

• Sweats and chills

• Headaches

• Vomiting and diarrhoea

How can malaria be prevented?

• Use of mosquito nets to avoid being bitten

• Wearing protective clothing

• Preventing the vectors (mosquitos) from breeding

What are the body’s passive defence mechanisms?

• Skin

• Nose hairs

• Trachea/bronchus

• Stomach

• The blood (platelets)

How does the skin prevent pathogens from entering the body?

• Acts as a physical barrier preventing entry

• It is dry so pathogens can’t grow

• Secretes an antimicrobial oil which can kill pathogens

• Outer layer flakes off so pathogens fall off

How do nose hairs prevent pathogens from entering the body?

• Has mucus which traps pathogens

• Hairs filter out large particles and droplets

• Sneezing/blowing nose removes pathogens

How does the trachea and bronchi prevent pathogens from entering the body?

• Lining secretes mucus which traps pathogens

• Ciliates cells waft mucus into the mouth/back of the throat so it can be swallowed

How does the stomach prevent pathogens infecting the body?

• The lining secretes hydrochloric acid

• Acid kills most bacteria in food

What two types of white blood cells does the immune system contain?

• Phagocytes

• Lymphocytes

What do phagocytes do to protect us from disease?

Phagocytes surround any pathogens in the blood and engulf them. They are attracted to pathogens and bind to them.

How does phagocytosis work?

The phagocytes membrane surrounds the pathogen and enzymes found inside the cell break down the pathogen in order to destroy it

Why are phagocytes called ‘non-specific’?

They destroy all and any pathogens that they encounter

What are antigens?

Proteins on the surface of pathogens

How does antibody production protect us against disease?

White blood cells called lymphocytes produce antibodies which are complementary to a specific antigen on a pathogen. The binding of antibodies to antigens causes pathogens to clump together, making them easier to destroy.

What happens to successful lymphocytes?

They turn into memory cells, which remember their antibody. This means that in the case of a second infection, the correct antibodies can be produces rapidly, preventing the person getting the same disease again.

How can antibodies destroy microbes (3 ways)?

• Cell lysis (making the microbes burst)

• Aggregation (making the microbes stick together so they cannot reproduce)

• Marking (for destruction via phagocytosis)

How does antitoxin production protect us against disease?

Some bacteria produce toxins to make us feel unwell. Lymphocytes produce antitoxins, which bind to toxins released by pathogens and neutralise them.

Why are lymphocytes called ‘specific’?

Both the antibodies and antitoxins are highly specific to the antigen on the pathogen i.e the antibody has a shape which only fits one antigen

What is a vaccination?

It contains a dead or inactivated form of the pathogen which stimulates white blood cells to produce complementary antibodies to the pathogen. In the case of a second infection, memory cells can rapidly produce the correct antibodies and prevent illness.

What is herd immunity?

If a sufficiently high proportion of a population are immune to a disease (through vaccination), the spread of this disease will be limited

What happens if the number of people vaccinated against a specific disease drops in population?

This leaves the rest of the population at risk of mass infection, as they are more likely to come across people who are infected and contagious. This increases the number of infections, as well as the number of people who could die from a specific infectious disease.

What are the advantages of vaccinations?

• They have eradicated many deadly diseases e.g smallpox

• Many epidemics can be prevented by vaccinations

• Herd immunity protects those who cannot have vaccinations

What are the disadvantages of vaccinations?

• Not guaranteed to work - might not protect against multiple strains of a pathogen

• May be side effects or adverse reactions

• Memory cells eventually die, so you will need a booster to make more memory cells

What are antibiotics?

They are medicines that help to cure bacterial disease by killing infective bacteria inside the body.

How do antibiotics work?

Antibiotics e.g penicillin kill bacterial pathogens inside the body, but do not kill human cells. Whilst some antibiotics kill a wide range of bacteria, it is important that the right antibiotic is used for specific bacteria.

Why can antibiotics not be used to treat viral diseases?

Antibiotics have no effect on viral pathogens as they live inside the host’s (human) cells. Therefore it is difficult to design drugs that would kill the virus and not destroy human cells at the same time.

What is antibiotic resistance?

Antibiotic resistance occurs when mutations lead to individual bacteria being resistant to an antibiotic. These bacteria are able to survive, reproduce and pass on their alleles, leading to a greater proportion of antibiotic-resistant bacteria. This is concerning as some types of bacteria are becoming resistant to all known antibiotics, so the diseases that they cause cannot be cured.

How can we prevent antibiotic resistance?

• Avoid overuse and unnecessary use of antibiotics e.g antibiotics are often used for viral infections

• Finish antibiotic courses - to ensure all bacteria is killed

What effect do painkillers have on infectious diseases?

Painkillers can only treat the symptoms but do not kill pathogens

Traditionally, where were drugs extracted from?

Plants and microorganisms

What plant is the heart drug digitalis extracted from?

Foxgloves

What painkiller originates from a compound found in willow bark?

Aspirin

What antibiotic was discovered by Alexander Fleming from a type of mould?

Penicillin

How are most new drugs made?

Most new drugs are synthesised by chemists in the pharmaceutical industry. However, the starting point may still be a chemical extracted from a plant.

What are the four qualities of a good medicine?

• Effective

• Safe

• Stable

• Able to be taken in and removed easily

What three main factors are tested for when developing new drugs?

• Toxicity

• Efficacy

• Dose

What has to happen to new medical drugs before use?

They have to be tested and trialled before being used to check that they are safe and effective.

What are the three stages of drug development?

1. Pre-clinical testing - testing on human cells, then animals

2. Clinical testing - human trials (3 phases)

3. Peer review - wider testing before drug is licensed

How is pre-clinical testing carried out?

Drugs are first tested on live cells and tissues and then sometimes on animals to find the level of toxicity and correct dose these tests are used as a model to predict how the drug may affect humans.

How is clinical testing carried out (phase 1)?

Drugs are tested on a small number of healthy human volunteers, starting with low doses to find out if there are any side effects.

How is clinical testing carried out (phase 2)?

The drug is tested in low doses on about 100 human volunteers with the disease to find out if it is effective at treating the disease and to find the optimum dose.

How is clinical testing carried out?

The drug is tested on several hundred patients to find to optimum dose and its effectiveness compared with the existing drug for the disease.

What is a placebo?

It looks exactly like the drug but has no drug in it, and is sometimes given to the control group

What is the difference between a single-blind and a double-blind trial?

In a single-blind trial, only the doctor knows whether the patient is receiving the drug or placebo. In a double-blind trial, neither the patient nor the doctor knows. Double-blind trials help remove bias on the part of the doctor.

What is a peer review?

Where the results of drug trials are checked over by scientist knowledgeable in this field

What are monoclonal antibodies?

They are antibodies made by a single clone of specialised cells. They are very useful as they are specific and only bind to one specific antigen and the scientists who produce them can choose which antigen to target.

How are monoclonal antibodies?

1. Specific antigen is injected into an animal (e.g mouse)

2. Lymphocytes in the animal produce complementary antibodies. These are the extracted from the animal

3. These lymphocytes are fused with a tumour cell to make hybridoma cells which can divide outside the body to produce the antibody

4. The hybridoma cells are cultured - each single cell is cloned making many identical cells that all produce the same antibody

5. Large amounts of monoclonal antibodies can be collected and purified

What are the uses of monoclonal antibodies?

• For diagnosis such as in pregnancy tests.

• In laboratories to measure the levels of hormones and other chemicals in blood, or to detect pathogens.

• In research to locate or identify specific molecules in a cell or tissue by binding to them with a fluorescent dye.

• To treat some diseases: for cancer the monoclonal antibody can be bound to a radioactive substance, a toxic drug or a chemical which stops cells growing and dividing. It delivers the substance to the cancer cells without harming other cells in the body.

What are some issues with monoclonal antibodies?

They create more side effects than created.

They are not yet as widely used as everyone hoped when they were first developed

What do pregnancy kits test for?

hCG in urine

What does a pregnancy test consist of?

A stick containing monoclonal antibodies specific to hCG

• Monoclonal antibodies attached to a blue bead (free to move)

• Monoclonal antibodies fixed to test stick

Describe what happens to the test stick if a woman is pregnant

• hCG in urine binds to monoclonal antibodies attached to a blue bead

• Monoclonal antibodies with hCG diffuse up stick

• Monoclonal antibodies fixed to the stick bind to hCG

• Blue line forms

Describe what happens the test stick if a woman is not pregnant

No hCG in urine so a blue line is not formed

What is the advantage of using monoclonal antibodies to test for pathogens?

• Specific to one particular antigen

• Very accurate

• Quick results

Why can monoclonal antibodies be used to target cancer cells?

• Cancer cells have specific antigens called ‘tumour markers’ on their membranes

• Monoclonal antibodies are specific to one type of antigen so they can be targeted to tumour markers without damaging other cells

Describe how monoclonal antibodies can be used to diagnose cancer

• Monoclonal antibodies are bound to a radioactive substance

• They are injected into a patient’s bloodstream

• They bind to tumour markers on cancer cells

• Emitted radiation is detected using a specialised scanner enabling doctors to determine the location of cancer cells

How can monoclonal antibodies be used to target drugs to cancer cells?

• Monoclonal antibodies are attached to an anti-cancer drug

• Monoclonal antibodies and injected into the patient’s bloodstream

• They bind to tumour markers on cancer cells

• Anti-cancer drug destroys cancer cells without harming other cells in the body

Why are cancer treatments that use monoclonal antibodies favoured over traditional treatments?

• Radiotherapy and chemotherapy target rapidly dividing cells

• Healthy cells are damaged as a consequence, producing unpleasant side effects

• Monoclonal antibodies only target cancer cells, reducing damage to normal cells

How can monoclonal antibodies be used to locate blood clots?

• Monoclonal antibodies are bound to a radioactive substance

• They target and bind to specific proteins in blood clots

• Radiation emitted by monoclonal antibodies is detected, enabling the location of blood clots to be identified

How can plant diseases by detected?

• Stunted growth

• Spots on leaves

• Areas of decay (rot)

• Growths

• Malformed stems or leaved

• Discolouration

• The presence of pests

How can identification of plant disease be made?

• Reference to a gardening manual or website

• Taking infected plants to a laboratory to identify the pathogen

• Using testing kits that contain monoclonal antibodies

How do aphids cause damage to plants?

They use their sharp mouthparts to extract sap from the plant phloem, weakening the plant. They are also vectors who can transfer pathogens from diseased plants to healthy plants.

What are the symptoms of an aphid infestation?

• Curling yellow leaves

• Mould

• Stunted growth