AQA Biology GCSE Topic 3: Infection and Response

Communicable Diseases (3.1)

Pathogens (3.1.1)

  • Definition: Microorganisms that cause infectious disease, including viruses, bacteria, protists, and fungi.

  • Modes of Transmission: Pathogens can spread via direct contact, water, or air.

Types of Pathogens

  1. Viruses

    • Very small, invade cells and replicate using the host's biochemistry.

    • Cell damage leads to illness.

  2. Bacteria

    • Small and multiply quickly via binary fission.

    • Produce toxins that harm host cells.

  3. Protists

    • Some are parasitic, damaging hosts.

  4. Fungi

    • Can be unicellular or multicellular with hyphae.

    • Produce spores that spread to other organisms.

Methods of Spread

  • Direct Contact: Touching contaminated surfaces, kissing, skin contact, infected fluids.

  • Water: Consumption or contact with contaminated water.

  • Air: Airborne pathogens via droplets from sneezing, coughing, or talking.

Reducing Disease Spread

  • Improving hygiene: Hand washing, disinfectants, proper food handling.

  • Reducing contact with infected individuals.

  • Removing vectors: Using pesticides and habitat removal.

  • Vaccination: Introduces harmless pathogens to build immunity.

Viral Diseases (3.1.2)

Measles

  • Symptoms: Fever, red skin rash, complications like pneumonia.

  • Transmission: Droplet infection.

  • Prevention: Vaccination for children.

HIV

  • Symptoms: Flu-like then attacks immune system, leading to AIDS.

  • Transmission: Sexual contact, blood exchange.

  • Prevention: Safe sex, clean needle use, antiretroviral drugs.

Tobacco Mosaic Virus

  • Symptoms: Leaf discolouration reduces yield.

  • Transmission: Contact between plants, insect vectors.

  • Prevention: Good hygiene, pest control, grow resistant strains.

Bacterial Diseases (3.1.3)

Salmonella Food Poisoning

  • Symptoms: Fever, cramps, vomiting, diarrhoea.

  • Transmission: Found in raw meat, eggs; spread through unhygienic conditions.

  • Prevention: Vaccination of poultry, hygiene practices, thorough cooking.

Gonorrhoea

  • Symptoms: Discharge, pain urinating.

  • Transmission: Sexually transmitted.

  • Prevention: Use of condoms, antibiotics.

Fungal Diseases (3.1.4)

Rose Black Spot

  • Symptoms: Spots on leaves, reduced photosynthesis.

  • Transmission: Spores in water or by wind.

  • Prevention: Use fungicides, strip affected leaves.

Protist Diseases (3.1.5)

Malaria

  • Symptoms: Fevers and shaking.

  • Transmission: Female Anopheles mosquito as vector.

  • Prevention: Insecticide nets, remove standing water, antimalarial drugs.

Human Defence System (3.1.6)

Non-Specific Defences

  1. Skin: Acts as physical barrier, secretes antimicrobial substances.

  2. Nose: Hairs and mucus trap particles.

  3. Trachea/Bronchi: Secrete mucus, cilia waft it upwards.

  4. Stomach: Hydrochloric acid kills pathogens.

Specific Immune Response

  • White Blood Cells

    • Phagocytosis: Engulf pathogens.

    • Antibody Production: Bind to antigens, clump pathogens. Leads to immunity on re-exposure.

    • Antitoxins: Neutralize pathogen toxins.

Vaccination (3.1.7)

  • Purpose: Provide immunity before infection.

  • Mechanism: Introduces harmless pathogen to stimulate antibody production.

  • Herd Immunity: Immunizing a large population reduces pathogen spread.

Antibiotics and Painkillers (3.1.8)

  • Antibiotics: Kill bacterial pathogens without harming body cells; ineffective against viruses.

  • Painkillers: Alleviate symptoms but do not treat causes.

  • Importance: Antibiotics reduce bacterial disease mortality.

Concerns of Antibiotic Resistance

  • Caused by overuse and incomplete treatment of antibiotics, leading to resistant strains.

Discovery and Development of Drugs (3.1.9)

  • Initial source: Many drugs derived from plants and microorganisms.

  • Testing phases: Preclinical (cell testing) and clinical trials (volunteers/patients).

  • Example Plants:

    • Aspirin (willow)

    • Digitalis (foxgloves)

  • Penicillin: Discovered by Alexander Fleming through mold observation.

Drug Testing Process

  • Include safety and efficacy evaluations, utilization of placebos in trials.

  • Double-blind studies enhance reliability of results.

Monoclonal Antibodies (3.2)

Production of Monoclonal Antibodies (3.2.1)

  • Produced from a single immune cell and target specific proteins.

  • Combination of mouse lymphocytes and tumor cells forms a hybridoma that rapidly divides

  • Results in identical antibodies, which are then purified.

Uses of Monoclonal Antibodies (3.2.2)

  1. Pregnancy Tests: Detect hCG hormone in urine using specific antibodies.

  2. Measurement in Laboratories: Monitor hormonal levels through binding with fluorescent markers.

  3. Research: Identify specific molecules on cells.

  4. Cancer Treatment: Target tumor markers to direct immune response or deliver therapeutics.

Plant Disease (3.3)

Detection and Identification (3.3.1)

  • Common signs include stunted growth, decay, discoloration, pests.

  • Methods of identification: gardening manuals, laboratory observation, monoclonal antibodies.

Nutrient Deficiencies

  1. Nitrate Deficiency: Prevents growth, impacts protein synthesis.

  2. Magnesium Deficiency: Causes chlorosis, affects chlorophyll production.

Plant Defense Mechanisms (3.3.2)

  • Physical Defenses: Tough cuticles, cellulose cell walls, layers of dead cells.

  • Chemical Defenses: Poisons deter herbivores, antibacterial compounds target bacteria.

  • Mechanical Defenses: Thorns, drooping leaves, mimicry to prevent predation.