What causes communicable diseases, how do they spread, and how do plants and the body's non-specific defences resist them?
4.1.1 Communicable diseases: the range of pathogens (bacteria, viruses, fungi and protoctists) and the communicable diseases they cause in animals and plants; the means of transmission; the primary non-specific defences of plants and animals; and the role of phagocytes in the non-specific immune response.
A focused answer to the OCR H420 4.1.1 dot point on communicable diseases. Covers the four pathogen groups and example diseases, means of transmission, the primary non-specific defences of plants and animals, and the role of phagocytes in non-specific immunity.
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What this dot point is asking
OCR wants you to name the four groups of pathogens and the communicable diseases they cause in animals and plants, describe how communicable diseases are transmitted, describe the primary non-specific defences of plants and animals, and explain the role of phagocytes in the non-specific immune response.
The answer
Pathogens and the diseases they cause
A communicable disease is caused by a pathogen that can be passed between organisms. OCR requires four groups:
- Bacteria (prokaryotes): tuberculosis, bacterial meningitis; ring rot in potatoes.
- Viruses (non-living, replicate inside host cells): influenza, HIV/AIDS; tobacco mosaic virus in plants.
- Fungi: athlete's foot and ringworm in animals; black sigatoka in bananas.
- Protoctista (protozoa and similar): malaria (Plasmodium); potato/tomato late blight (Phytophthora).
Pathogens cause disease either by damaging host tissues directly (for example by using up nutrients or breaking down cells) or by producing toxins.
Transmission
Communicable diseases spread by direct transmission (contact, body fluids, droplet infection through coughs and sneezes) or indirect transmission (via contaminated water, food, vectors such as mosquitoes, or fomites). In plants, pathogens spread through the soil, by contact between plants, and by vectors such as insects. Factors that increase transmission include overcrowding, poor sanitation, poor nutrition, climate (warmth and water for vectors), and the movement of people, animals or plant material.
Primary non-specific defences
These act against all pathogens and do not improve with exposure.
In plants:
- physical barriers: the waxy cuticle and cellulose cell walls;
- on attack, callose is deposited in and around sieve plates and between cell walls to block the pathogen's spread, and lignin thickens walls;
- chemical defences: antimicrobial compounds such as terpenoids and phenols, and enzymes (for example chitinases and glucanases) that break down pathogen cell walls.
In animals:
- the skin is a tough physical barrier and produces sebum (inhibits pathogen growth);
- mucous membranes in the airways and gut trap pathogens, and cilia waft mucus away;
- stomach acid (low pH) kills most ingested pathogens;
- lysozyme in tears and saliva digests bacterial cell walls;
- blood clotting seals wounds, and inflammation (mast cells release histamine, causing vasodilation and increased permeability) brings white cells to the site.
Phagocytosis
If a pathogen breaches the barriers, phagocytes (neutrophils and macrophages) carry out the non-specific cellular response:
- The phagocyte is attracted to the pathogen by chemicals it releases (chemotaxis) and recognises its antigens as foreign (often after the pathogen is coated by opsonins).
- It engulfs the pathogen by endocytosis, enclosing it in a vesicle called a phagosome.
- A lysosome fuses with the phagosome; its hydrolytic enzymes (lysozymes) digest and destroy the pathogen.
- A macrophage then displays the pathogen's antigens on its surface (acting as an antigen-presenting cell), linking the non-specific response to the specific immune response.
Examples in context
Example 1. Malaria and its vector. Malaria is caused by the protoctist Plasmodium and transmitted indirectly by the female Anopheles mosquito (a vector); controlling the vector (nets, draining standing water) is therefore a key way to reduce transmission.
Example 2. Tobacco mosaic virus. This plant virus spreads by contact and through infected tools, mottling the leaves and reducing photosynthesis, illustrating that plants suffer communicable disease and rely on barriers and chemical defences because they have no specific immune system.
Try this
Q1. Name the pathogen group responsible for tuberculosis and the group responsible for malaria. [2 marks]
- Cue. Tuberculosis is bacterial; malaria is caused by a protoctist (Plasmodium).
Q2. Describe how callose helps defend a plant against a pathogen. [2 marks]
- Cue. Callose is deposited in and around the sieve plates and between cell walls, blocking the movement of the pathogen and sealing off infected areas.
Q3. State the name of the vesicle formed when a phagocyte engulfs a pathogen. [1 mark]
- Cue. A phagosome.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/02 20194 marksDescribe how a phagocyte destroys a pathogen during the non-specific immune response.Show worked answer →
Give the ordered steps and the named structures.
The phagocyte (for example a neutrophil or macrophage) is attracted to the pathogen by chemicals it releases (chemotaxis) and recognises the pathogen's antigens as foreign, often after it has been coated by opsonins.
It engulfs the pathogen by endocytosis, enclosing it in a vesicle called a phagosome. A lysosome fuses with the phagosome to form a phagolysosome, and its hydrolytic enzymes (lysozymes) digest and destroy the pathogen.
Markers reward recognition, engulfing to form a phagosome, lysosome fusion and enzymatic digestion. A macrophage may then present the antigens, linking to the specific response.
OCR H420/02 20214 marksDescribe two physical or chemical defences that plants use against pathogens, and two that animals use, to prevent infection.Show worked answer →
Give two clearly plant and two clearly animal defences, each with how it works.
Plant defences: the waxy cuticle and cell walls form a physical barrier; on infection, plants produce callose (deposited in sieve plates and between cell walls) to block the spread of the pathogen, and chemical defences such as antimicrobial compounds (for example terpenoids, phenols) and enzymes that break down pathogen cell walls.
Animal defences: the skin is a physical barrier (and produces sebum); mucous membranes trap pathogens, which cilia waft away; blood clotting seals wounds; stomach acid (low pH) kills ingested pathogens; lysozyme in tears and saliva digests bacterial walls.
Markers reward two valid plant defences and two valid animal defences, each with a brief mechanism.
Related dot points
- 4.1.1 The immune response: the structure and function of antibodies; the roles of B and T lymphocytes in the humoral and cell-mediated responses; the primary and secondary responses and the role of memory cells; the principles of vaccination and herd immunity; the differences between active, passive, natural and artificial immunity; and the development of antibiotic resistance.
A focused answer to the OCR H420 4.1.1 dot point on the specific immune response. Covers antibody structure, B and T lymphocytes, the primary and secondary responses, memory cells, vaccination and herd immunity, the four types of immunity, and how antibiotic resistance evolves.
- 4.2.1 Biodiversity: the levels of biodiversity (habitat, species and genetic); how to sample plants and animals (random sampling, quadrats, transects and mark-release-recapture); the calculation and interpretation of Simpson's index of diversity; and the ecological, economic and aesthetic reasons for maintaining biodiversity.
A focused answer to the OCR H420 4.2.1 dot point on biodiversity. Covers habitat, species and genetic diversity, sampling methods including quadrats, transects and mark-release-recapture, the calculation and interpretation of Simpson's index of diversity, and the reasons for maintaining biodiversity.
- 4.2.2 Classification and evolutionary relationships: the binomial system and the taxonomic hierarchy; the five kingdoms and the three-domain classification; the meaning of phylogeny; and how molecular evidence (DNA base sequences, amino acid sequences) and other evidence are used to clarify evolutionary relationships.
A focused answer to the OCR H420 4.2.2 dot point on classification. Covers the binomial system and taxonomic hierarchy, the five kingdoms and the three-domain system, the meaning of phylogeny, and how molecular and other evidence is used to establish evolutionary relationships.
- 4.2.2 Evolution: the process of evolution by natural selection acting on variation; the role of mutation in generating variation; the types of natural selection (directional, stabilising and disruptive); the evidence for evolution from fossils, comparative anatomy and molecular biology; and examples such as antibiotic resistance and industrial melanism.
A focused answer to the OCR H420 4.2.2 dot point on evolution. Covers natural selection acting on variation, mutation as the source of variation, directional, stabilising and disruptive selection, the evidence for evolution, and examples such as antibiotic resistance and peppered moths.
- 2.1.1 Cell structure: the ultrastructure of eukaryotic and prokaryotic cells, the function of organelles including the role of the rough endoplasmic reticulum and Golgi apparatus in producing and secreting proteins; the use, calibration and resolution of light and electron microscopes.
A focused answer to the OCR H420 2.1.1 dot point on cell structure and microscopy. Covers every required eukaryotic and prokaryotic organelle, the protein secretory pathway, the three microscopes, eyepiece-graticule calibration and the magnification equation.
Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)