What causes communicable diseases, how do they spread, and how do organisms defend against them?
Communicable diseases and the four types of pathogen (bacteria, viruses, fungi and protists), how pathogens are spread, examples of human and plant communicable diseases, and the non-specific physical and chemical defences of the human body and of plants.
A focused answer to the OCR Gateway GCSE Biology A topic B6 on communicable disease, covering the four types of pathogen, how diseases are spread, examples of human and plant diseases, and the non-specific physical and chemical defences of the human body and of plants.
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What this dot point is asking
OCR wants you to describe communicable diseases and the four types of pathogen, explain how pathogens spread, give examples of human and plant diseases, and describe the non-specific physical and chemical defences of the human body and of plants.
Communicable disease and types of pathogen
There are four main types of pathogen, each causing disease in a different way:
- Bacteria are small living cells that reproduce rapidly inside the body and can release toxins that make us feel ill (for example food poisoning, tuberculosis).
- Viruses are much smaller and reproduce inside the body's cells, damaging them when new viruses burst out (for example measles, flu, HIV).
- Fungi can grow on or in the body (for example athlete's foot), and cause important plant diseases (for example rose black spot).
- Protists are single-celled organisms, often spread by a vector, that cause diseases such as malaria (carried by mosquitoes).
How pathogens spread
Knowing how a disease spreads tells you how to reduce its transmission. The main routes are:
- Direct contact with an infected organism or contaminated surface.
- In the air, in tiny droplets released when an infected person coughs or sneezes (for example flu).
- In contaminated food or water (for example cholera in dirty water).
- By a vector, an organism that carries the pathogen, such as the mosquito that spreads malaria.
Spread is reduced by hygiene (washing, disinfecting), clean water and sanitation, isolating infected people, vaccination, and controlling the vector (for example draining standing water to reduce mosquitoes).
Plant communicable diseases
Plants get communicable diseases too. Two examples to know are rose black spot (a fungus that produces purple or black spots on leaves, which then turn yellow and drop, reducing photosynthesis and growth) and tobacco mosaic virus (a virus that gives a mosaic pattern of discolouration on leaves, reducing photosynthesis). Plant diseases reduce yields and so threaten food security.
The body's non-specific defences
Before the immune system acts, the body has non-specific defences that try to stop pathogens entering at all. These are physical and chemical barriers:
Plant defences
Plants also have defences against pathogens and pests:
- Physical barriers: a waxy cuticle on leaves, cellulose cell walls, and a layer of dead cells around stems (bark) that falls off, taking pathogens with it.
- Chemical defences: some plants make antibacterial chemicals or poisons to kill pathogens or deter the pests that spread them.
- Mechanical defences: thorns, hairs and leaves that droop or curl on touch deter animals.
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 20194 marksDescribe the four main types of pathogen, giving one example of a disease caused by each, and state two ways pathogens can spread between people.Show worked answer →
A B6 recall question on pathogens.
Types and examples (one disease each): bacteria, for example causing food poisoning (Salmonella) or tuberculosis; viruses, for example measles, flu or HIV; fungi, for example athlete's foot or rose black spot in plants; protists, for example malaria (spread by mosquitoes).
Ways of spreading (any two): by direct contact; in the air, in droplets when an infected person coughs or sneezes; in contaminated food or water; and by a vector such as an insect (for example the mosquito that spreads malaria). Reward the four pathogen types with a valid disease for each, and any two correct routes of transmission.
OCR 20214 marksDescribe two physical or chemical defences of the human body that help to stop pathogens entering, and describe one way a plant defends itself against pathogens.Show worked answer →
A B6 question on non-specific defences.
Human defences (any two): the skin acts as a physical barrier and produces antimicrobial secretions; the nose has hairs and mucus that trap pathogens; the trachea and bronchi produce mucus to trap pathogens and have cilia that waft it up and away from the lungs; the stomach produces hydrochloric acid that kills many pathogens in food; tears and saliva contain enzymes (lysozyme) that kill bacteria.
Plant defence (any one): a waxy cuticle and cell walls act as physical barriers; bark and a layer of dead cells protect the stem; some plants produce antibacterial chemicals or poisons to deter pests and pathogens. Reward two valid human defences (saying physical or chemical correctly) and one correct plant defence.
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