GCSE Biology - Infection and Response

What are pathogens and how do they cause disease?

Pathogens are microorganisms that cause infectious disease (viruses, bacteria, protists, or fungi). Bacteria reproduce rapidly inside the body and produce toxins / poisons that damage tissues and make us feel ill. Viruses live and reproduce inside (living) cells, causing cell damage and destroying the host cells.

State three ways communicable diseases can be spread.

By direct contact (skin-to-skin, body fluids, or plant material left in fields). By water (drinking contaminated water or eating undercooked/contaminated food). By air / droplet infection (pathogens carried in tiny droplets expelled from coughs, sneezes, or talk).

Give four ways the spread of diseases can be reduced or prevented.

Simple hygiene measures (e.g., washing hands after using the toilet or before preparing food, using disinfectants). Isolating infected individuals (reduces contact with healthy individuals). Destroying or controlling vectors (e.g., using insecticides to kill mosquitoes or clearing standing water). Vaccination (immunising a large proportion of the population to protect individuals and reduce spread).

Describe the symptoms, spread, and prevention of measles.

Symptoms: Fever and a red skin rash (can be serious and fatal if complications arise). Spread: By inhalation of droplets from sneezes and coughs. Prevention: Most young children are vaccinated against measles (using the MMR vaccine).

Describe the symptoms, spread, and treatment of HIV/AIDS.

Symptoms: Initially causes a mild, flu-like illness. Late-stage HIV (AIDS) occurs when the body's immune system becomes so badly damaged it can no longer deal with other infections or cancers. Spread: By sexual contact / intercourse, or exchange of body fluids (e.g., drug users sharing needles or unscreened blood transfusions). Treatment: Successfully controlled with regular use of antiretroviral drugs to stop the virus replicating and attacking immune cells.

Describe the symptoms and effects of Tobacco Mosaic Virus (TMV) on plants.

Symptoms: Gives a distinctive 'mosaic' pattern of discolouration on the leaves. Effect: Affects and stunts the growth of the plant. Reason: Discoloured areas lack chlorophyll, so less light is absorbed, resulting in a lower rate of photosynthesis (which reduces the glucose available for growth).

Describe the spread, symptoms, and prevention of Salmonella food poisoning.

Spread: Bacteria ingested in food, or on food prepared in unhygienic conditions (such as raw poultry contaminating other food). Symptoms: Fever, abdominal cramps, vomiting, and diarrhoea caused by the bacteria and the toxins they secrete. Prevention: In the UK, poultry are vaccinated against Salmonella to control the spread; wash hands and surfaces well after handling raw chicken and cook thoroughly.

Describe the spread, symptoms, treatment, and prevention of Gonorrhoea.

Spread: A sexually transmitted disease (STD) spread by unprotected sexual contact. Symptoms: A thick yellow or green discharge from the vagina or penis, and pain on urinating (though many infected individuals show no symptoms). Treatment: Treated with antibiotics (originally easily treated with penicillin until many resistant strains appeared). Prevention: Use of a barrier method of contraception, such as a condom, and reducing the number of sexual partners.

Describe the symptoms, spread, and treatment of Rose Black Spot.

Symptoms: Purple or black spots develop on leaves, which often turn yellow and drop early. This reduces photosynthesis, meaning the plant does not grow or flower well. Spread: Spreads in the environment by water (rain splashes) or wind. Treatment: Using fungicides and/or removing and destroying the affected leaves.

Describe the cause, spread, symptoms, and prevention of Malaria.

Cause: The pathogens that cause malaria are parasitic protists. Spread: The malarial protist has a life cycle that includes the female Anopheles mosquito (which acts as a vector when it takes a blood meal). Symptoms: Causes recurrent episodes of fever and shaking, weakens the person, and can be fatal. Prevention: Preventing vectors from breeding by removing standing water, spraying pools with insecticides, and using mosquito nets to avoid being bitten.

Describe the non-specific defence systems of the human body against pathogens.

Skin: Acts as a physical barrier; produces antimicrobial secretions to destroy bacteria; healthy skin is covered with microorganisms acting as an extra barrier; platelets form clots to make a scab over cuts. Nose: Contains hairs and produces sticky mucus to trap particles and pathogens that might irritate the lungs. Trachea and Bronchi: Secrete mucus to trap pathogens from the air; lining is covered in cilia (tiny hair-like projections) which beat to waft mucus up to the back of the throat to be swallowed. Stomach: Produces hydrochloric acid which destroys the majority of pathogens in swallowed mucus or ingested food and drink. Eyes: Produce tears containing antiseptic chemical enzymes to destroy bacteria.

Describe the three ways white blood cells defend against pathogens.

Phagocytosis: Some white blood cells engulf / ingest pathogens and digest them using enzymes to destroy them. Antibody production: Some white blood cells (lymphocytes) produce specific chemicals called antibodies. These target unique proteins (antigens) on the pathogen, binding to and inactivating or destroying them. Antitoxin production: Some white blood cells produce antitoxins which bind to and neutralise the toxins / poisons released by pathogens.

Explain how vaccination prevents illness in an individual and reduces the spread of disease.

Individual Prevention: A small quantity of dead or inactive forms of a pathogen is introduced into the body (often by injection). This stimulates white blood cells (lymphocytes) to produce the specific antibodies needed to target the pathogen's antigens. Memory cells are formed and remain in the body. If the same live pathogen re-enters the body, white blood cells respond quickly to produce correct antibodies in large numbers, preventing infection. Reducing Spread (Herd Immunity): If a large proportion of the population is immune to a pathogen, the spread of the pathogen is very much reduced, protecting unvaccinated individuals.

What are antibiotics and how do they work?

Antibiotics (such as penicillin) are medicines that help to cure bacterial disease by killing infective bacteria inside the body. It is important that specific bacterial infections are treated by specific antibiotics. Their use has greatly reduced deaths from infectious bacterial diseases.

Explain why antibiotics cannot be used to treat viral infections.

Antibiotics cannot kill viral pathogens. Viruses live and reproduce inside body cells. It is very difficult to develop drugs that kill viruses without also damaging the body's host cells and tissues.

What is the purpose of painkillers and other symptoms-treating medicines?

Painkillers and other medicines are used to treat the symptoms of disease (e.g., relieving headaches or sore throats) but do not kill the pathogens. You have to wait for your immune system to overcome the pathogens to recover.

Explain how strains of bacteria become resistant to antibiotics.

Strains of bacteria can mutate, causing some individuals to become resistant to an antibiotic. When antibiotics are used, the non-resistant bacteria are killed, but the resistant bacteria survive. The resistant bacteria reproduce rapidly (by binary fission) with no competition, passing on the resistant gene. The antibiotic is no longer effective against this new strain of bacteria, which is of great concern.

State the traditional origins of digitalis, aspirin, and penicillin.

Digitalis: A heart drug traditionally extracted from foxgloves. Aspirin: A painkiller/anti-inflammatory drug originating from willow bark. Penicillin: Discovered by Alexander Fleming from the Penicillium mould.

State the three properties new drugs are extensively tested for.

Toxicity: To check whether the drug is safe / has harmful side effects. Efficacy: To check whether the drug actually works / cures the disease. Dosage: To find the optimum concentration of the drug that works best with the fewest side effects.

Describe the stages involved in preclinical and clinical testing of new drugs.

Preclinical Testing: Done in a laboratory using cells, tissues, and live animals to check for toxicity, efficacy, and safe dosage. Clinical Trials (on humans): Phase 1: Very low doses of the drug are given to healthy volunteers to check for safety and side effects. Phase 2: If safe, it is tested on a small number of patients with the disease to see if it works and to find the optimum dose. Phase 3 (Double-Blind / Placebo-Controlled): Tested on a larger group of patients. Some are given the real drug, others are given a placebo (inactive substance or existing drug). Neither the patients nor the doctors know who has the real drug until the trial is complete, to prevent bias.

What must happen before the results of testing and trials are published?

The results of drug trials must be published only after scrutiny by peer review. Other scientists working in the same area check the results to prevent false claims and ensure safety.

Describe how monoclonal antibodies are produced. (Biology HT Only)

Monoclonal antibodies are produced from a single clone of cells: A mouse is injected with a specific antigen to stimulate its lymphocytes to make the correct antibody. The mouse lymphocytes are collected and combined with a particular type of tumour cell (which divides rapidly but doesn't make antibodies). This forms a hybridoma cell, which can both divide rapidly and produce the specific antibody. Single hybridoma cells are cloned to produce many identical cells that all produce the same antibody. A large amount of the antibody can be collected, purified, and used.

Describe some of the ways in which monoclonal antibodies are used. (Biology HT Only)

Diagnosis: Used in pregnancy tests (to bind to hCG hormone) or to detect pathogens. Measuring levels: Used in laboratories to measure hormone levels and other chemicals in the blood. Research: Used to locate or identify specific molecules in a cell or tissue by binding to them with a fluorescent dye. Treating disease (e.g., cancer): The monoclonal antibody is bound to a radioactive substance, a toxic drug, or a chemical that stops cells growing and dividing. It delivers the toxic substance directly to the cancer cells by binding specifically to their antigens, without harming healthy body cells.

State seven ways plant diseases can be detected. (Biology HT Only)

Stunted growth, Spots on leaves, Areas of decay (rot), Growths (e.g., galls), Malformed stems or leaves, Discolouration, and The presence of pests (e.g., aphids).

State three ways plant diseases can be identified. (Biology HT Only)

Reference to a gardening manual or website. Taking infected plants to a laboratory to identify the pathogen using scientific techniques (such as DNA analysis). Using testing kits that contain monoclonal antibodies (to bind to specific plant pathogens).

Explain how plants are damaged by nitrate and magnesium ion deficiencies. (Biology HT Only)

Nitrate deficiency: Nitrates are needed from the soil for protein synthesis. A lack of nitrates results in limited protein synthesis, leading to stunted growth. Magnesium deficiency: Magnesium ions are needed from the soil to make chlorophyll. A lack of magnesium means the plant cannot make enough chlorophyll, so leaves turn yellow (called chlorosis) and photosynthesis is reduced, slowing growth.

Describe the physical and chemical defence responses of plants to resist invasion. (Biology HT Only)

Physical defences (barriers against pathogens): Cellulose cell walls strengthen plant cells to resist invasion. A tough waxy cuticle on leaves acts as a barrier to pathogens. Layers of dead cells around stems (such as bark on trees) form a barrier; when these dead cells fall off, pathogens fall off with them. Chemical defences: Production of antibacterial chemicals to destroy invading bacteria. Production of poisons to deter herbivores.

Describe the mechanical adaptations and mimicry used by plants for defence. (Biology HT Only)

Mechanical adaptations: Thorns and hairs to deter animals / make it painful to eat. Leaves which droop or curl when touched (frightens larger animals and knocks insects off). Mimicry: Tricking / mimicking unhealthy plants (e.g., having spots like butterfly eggs) or looking like stones so herbivores avoid eating them.