AQA Biology GCSE Topic 3: Infection and Response Notes

Content in bold is for higher tier only. Content is for both separate science and double award students unless indicated in heading.

Communicable Diseases (3.1)

Communicable (Infectious) Diseases (3.1.1)

  • Pathogens, which include viruses, bacteria, protists, and fungi, are microorganisms that cause infectious disease.
  • They can infect plants or animals and spread through:
    • Direct contact: touching contaminated surfaces.
    • By water: drinking or coming into contact with dirty water.
    • By air: pathogens can be carried in droplets expelled through sneezing, coughing, or talking, which can be inhaled.

Types of Pathogens

  1. Viruses

    • Very small microorganisms.
    • They invade host cells and utilize the host's biochemistry to replicate, leading to the bursting of the cell.
    • This process releases viral copies into the bloodstream, damaging and destroying host cells, resulting in illness.

  2. Bacteria

    • Small single-celled organisms that proliferate rapidly through binary fission.
    • Produce toxins that can harm host cells and tissues.

  3. Protists

    • Some protists are parasitic, living on or inside host organisms and causing damage.

  4. Fungi

    • Can exist as single-celled organisms or multicellular organisms with hyphal structures.
    • Produce spores for reproduction, which can be dispersed to infect other organisms.

Reducing Disease Spread

  • Improving hygiene:
    • Regular hand washing, using disinfectants, isolating raw meat, and using tissues or handkerchiefs while sneezing.
  • Reducing contact with infected individuals: minimizes transmission risk.
  • Removing vectors: using pesticides or insecticides and clearing their habitats can help limit disease spread.
  • Vaccination:
    • Involves injecting a small amount of a harmless pathogen to stimulate immunity, preventing infection and transmission.

Viral Diseases (3.1.2)

Overview of Viral Diseases

  • Viruses pose a significant threat as they can penetrate various cell types, with few effective treatments available.
Key Viral Diseases

  1. Measles

    • Symptoms: Fever, red skin rash; potential complications include pneumonia, encephalitis, and blindness.
    • Spread: Droplet infection.
    • Prevention: Vaccination for young children.

  2. HIV

    • Symptoms: Initial flu-like symptoms; progresses to AIDS, compromising the immune system.
    • Spread: Through sexual contact or bodily fluid exchange.
    • Prevention:
      • Safe practices such as condom use, not sharing needles, blood screening, and mothers with HIV opting for bottle-feeding.
      • Use of antiretroviral drugs to limit viral replication.

  3. Tobacco Mosaic Virus (TMV)

    • Symptoms: Leaf discolouration, reduced photosynthesis; results in lower crop yields.
    • Spread: Contact between infected and healthy plants, aided by insect vectors.
    • Prevention: Implementing good field hygiene and pest control measures, utilizing TMV-resistant plant strains.

Bacterial Diseases (3.1.3)

Overview of Bacterial Diseases

  • Increasing antibiotic resistance poses a growing public health concern.
Key Bacterial Diseases

  1. Salmonella Food Poisoning

    • Symptoms: Fever, stomach cramps, vomiting, diarrhoea (attributed to toxins).
    • Spread: Found in raw meat and eggs; typically from unhygienic conditions.
    • Prevention: Vaccination of poultry, hygiene practices, avoiding cross-contamination, and thorough cooking.

  2. Gonorrhoea

    • Symptoms: Thick yellow or green discharge, pain with urination.
    • Spread: Sexually transmitted through unprotected sex.
    • Prevention: Use of contraception (e.g., condoms) and treatment with antibiotics; note the emergence of resistant strains.

Fungal Diseases (3.1.4)

Key Fungal Diseases

  1. Rose Black Spot
    • Symptoms: Purple or black leaf spots, reduced photosynthetic area; affected leaves yellow and drop prematurely.
    • Spread: Spore transmission via rain or wind.
    • Prevention: Fungicides and removal (burning) of infected leaves.

Protist Diseases (3.1.5)

Key Protist Diseases

  1. Malaria
    • Symptoms: Fever and shaking (when protists rupture red blood cells).
    • Spread: Vector is the female Anopheles mosquito; the protists reproduce sexually in the mosquito and infect humans through saliva during blood feeding.
    • Prevention: Use insecticide-treated nets, eliminate stagnant water to curb vector breeding, and travelers can take antimalarial medications.

Human Defence System (3.1.6)

Non-Specific Defence Mechanisms

  1. Skin

    • Acts as a physical barrier and secretes antimicrobial substances.
    • Hosts beneficial microorganisms that compete with harmful pathogens.

  2. Nose

    • Contains hairs and mucus filtering particles before they reach the lungs.

  3. Trachea and Bronchi

    • Produce mucus to trap pathogens; cilia help expel mucus upward for swallowing.

  4. Stomach

    • Secretes hydrochloric acid to kill pathogens in food and drink.

Specific Immune Response

  • Contains white blood cells that operate in three main ways:
    1. Phagocytosis:
    • Engulfing and digesting pathogens, preventing illness.
    1. Antibody Production:
    • Each pathogen has specific antigens; antibodies bind to these antigens, facilitating their destruction.
    • Once the body has fought a specific pathogen, it retains memory cells for faster response in future infections, providing immunity.
    1. Antitoxin Production:
    • Neutralizes toxins released by pathogens by binding to them.

Vaccination (3.1.7)

Overview of Vaccination

  • Vaccination allows for immunity to diseases before exposure.
  • Immunizing a high percentage of the population helps achieve herd immunity, limiting pathogen spread.
  • Vaccines consist of killed or inactivated pathogens, prompting the immune response to create antibodies.

Antibiotics and Painkillers (3.1.8)

Differences Between Antibiotics and Painkillers

  • Antibiotics

    • Target and kill bacterial pathogens, not effective against viruses since they rely on host cells.
    • Administered as pills, syrups, or intravenously.
    • Essential to prescribe the right antibiotic for the specific bacteria.
    • Have drastically reduced bacterial disease mortality rates; significant example: Penicillin.

  • Painkillers (e.g., Aspirin)

    • Only alleviate symptoms without addressing root causes of illness.
Advantages and Disadvantages of Vaccination
Advantages
  • Vaccinations have successfully eradicated diseases (e.g., smallpox) and diminished the occurrence of others (e.g., rubella).
  • Prevent epidemic occurrences through herd immunity.
Disadvantages
  • Not always effective in providing full immunity.
  • Can cause adverse reactions (e.g., fevers) though rare.
Antibiotic Resistance Concerns
  • Growing bacterial resistance to antibiotics due to:
    • Mutations: Resulting in some bacteria surviving antibiotic treatment.
    • Selective Pressure: Non-resistant bacteria die off, allowing resistant strains to propagate.
  • Prevention measures include:
    1. Curbing overuse of antibiotics to minimize unnecessary exposure.
    2. Completing prescribed antibiotic courses to eradicate all bacteria.

Discovery and Development of Drugs (3.1.9)

Discovery

  • Many drugs originate from plant and microbial sources.
  • Current drug synthesis is primarily chemical.

Testing of New Drugs

  • Toxicity: Assessing harmful effects.
  • Efficacy: Evaluating drug effectiveness in fulfilling its intended purpose.
  • Dosage: Determining appropriate quantity for effectiveness.
Testing Phases
  1. Preclinical Testing: Using cellular and animal models to gauge safety.
  2. Clinical Trials: Administering drugs to human volunteers.
    • Initial tests on healthy individuals at low doses to monitor side effects.
    • Following steps involve patients for effective dosage determination.
    • Randomized control trials (placebo vs. actual drug) to assess drug efficacy.
    • Types of trials:
      • Single-blind: Only the medic knows the treatment; patients are unaware.
      • Double-blind: Neither medic nor patient knows treatment, reducing bias.
  • Results undergo peer review to confirm reliability and reproducibility.

Monoclonal Antibodies (3.2 - Biology Only)

Producing Monoclonal Antibodies (3.2.1)

  • Monoclonal antibodies are specific antibodies produced from identical immune cells.
Production Steps
  1. Mice lymphocytes, stimulated to produce specific antibodies, are obtained.
  2. These lymphocytes are combined with rapidly dividing tumor cells to create hybridoma cells.
  3. Hybridoma cells replicate and produce clones of the identical antibody.
  4. Antibodies are harvested and purified for use.

Uses of Monoclonal Antibodies (3.2.2)

Applications

  1. Pregnancy Tests:

    • Detects hormone hCG in urine of pregnant women.
    • Testing device with mobile antibodies bound to blue beads interacts with hCG, forming complexes that indicate pregnancy via blue lines.

  2. Laboratory Diagnostics:

    • Measures hormone/chemical levels in blood; antibodies can be modified to bind specific molecules.
    • Utilized for HIV screening in blood donation.

  3. Research Applications:

    • Identifying specific molecules in cells or tissues through fluorescence detection.

  4. Cancer Treatment:

    • Targets tumor markers not present on normal cells; three methods of action include:
      a) Stimulating immune response to eliminate cancer cells.
      b) Blocking receptors to inhibit growth signals.
      c) Delivering toxic treatments selectively to cancer cells.
Advantages and Disadvantages of Monoclonal Antibodies
Advantages
  • Specific targeting of diseased cells minimizes harm to healthy cells.
  • Versatile in application across various disorders.
Disadvantages
  • Difficulty in linking monoclonal antibodies to therapeutic drugs.
  • High production costs and complexity.
  • Mouse-derived antibodies may trigger immune reactions in humans.

Plant Disease (3.3 - Biology Only)

Detection and Identification of Plant Diseases (3.3.1)

  • Plants can be afflicted by viral, bacterial, and fungal pathogens.
Common Signs of Plant Diseases
  • Stunted growth: Often indicates nitrate deficiencies.
  • Spots on leaves: Associated with infections like black spot fungus.
  • Areas of decay: Manifestations of various pathogens (e.g., blights on potatoes).
  • Abnormal growths: Caused by bacterial infections (e.g., crown galls).
  • Discolouration: May signify magnesium deficiency or viral infections.
  • Pests: Presence of insects like caterpillars.
Identification Methods
  • Utilize gardening manuals or websites.
  • Laboratory analysis of infected specimens.
  • Testing with monoclonal antibodies for pathogen identification.
Specific Plant Diseases to Learn
  • Tobacco mosaic virus (viral), black spot (fungal), and aphids (insect infestations).

Ion Deficiencies

  1. Nitrate Deficiency: Impedes growth and protein synthesis necessary for growth.
  2. Magnesium Deficiency: Results in chlorosis, affecting chlorophyll production and thus photosynthesis.

Plant Defence Responses (3.3.2)

Defence Mechanisms
  1. Physical Defences:
    • Tough, waxy cuticle inhibits entry into leaves.
    • Cellulose cell walls act as barriers against microbial invasion.
    • Layers of dead bark cells shed with pathogens to hinder entry.
  2. Chemical Defences:
    • Toxic compounds deter herbivory and microbial growth (e.g., alkaloids from foxgloves).
    • Antibacterial substances combat bacterial infections (e.g., from mint).
  3. Mechanical Defences:
    • Thorns and hairs deter larger herbivores.
    • Responsive movements in certain plants to displace pests (e.g., leaves curling to push insects away).
    • Mimicry strategies to reduce predation (e.g., resembling unhealthy plants to avoid feeding).
Overall Summary
  • The spectrum of communicable diseases covered through various pathways of transmission, defense mechanisms of the body, treatment methods, and plant diseases exhibit the breadth of biological knowledge pertinent to infection and response mechanisms.
    The interconnected concepts provide a comprehensive framework for understanding diseases and defense strategies across biological domains, both in animals and plants.