Summary notes for the communicable diseases module for A-level year 1 biology. Includes: Organisms that cause disease, transmission of pathogens, plant defences against pathogens, primary defences against disease, secondary non-specific defences, development and use of drugs.
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A-Level Biology Year 1- Communicable Diseases- Pathogens and Defence
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AQA
GCSE Biology
Communicable Diseases
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PLANT DEFENCE AGAINST PATHOGENS
Plants manufacture sugars during photosynthesis and then convert them into compounds such as proteins and oils, so
they’re a rich source of nutrients for many organisms like fungi, insects, bacteria and viruses. These may be pathogenic or
act as a vector.
They do not have an immune system but they have a range of structural, chemical and protein based defences which
detect and prevent invading organisms. This includes passive defence (preventing entry) and active defence (induced
when pathogen is detected)
PASSIVE DEFENCES ACTIVE DEFENCES
PHYSICAL DEFENCES PHYSICAL DEFENCES
• Cellulose cell wall- acting as a physical barrier and • Stomatal closure- controlled by guard cells, the stomata
contains chemical defences will close if pathogens are detected.
• Lignin thickening of cell walls- lignin is waterproof • Callose- a large polysaccharide deposited in sieve tubes. It
and extremely dif cult to digest. blocks the ow in the sieve tube and prevents a pathogen
• Waxy cuticles- prevents potentially infected water spreading.
collection on cells. • Tylose formation- balloon-like swelling lling the xylem
• Bark- physical barrier, contains chemical defences vessel . When fully formed, it plugs the vessel and stops it
from carrying water. This again prevents the spread of
. pathogens. It contains a high concentration of chemicals
OTHER ACTIVE DEFENCE PROCESSES that are toxic to pathogeny
CHEMICAL DEFENCES
• Cell walls can be thickened and strengthened with • Terpenoids- a range of essential oils with antibacterial and
cellulose. anti fungal properties. They also smell good! Ex. Menthols/
• An increase in chemical production. menthones produced by mint plants.
• Oxidative bursts produce highly reaction oxygen • Phenols- have antibiotic and anti fungal properties. Ex.
molecules which can damage invading cells. Tannins in bark inhibit attack by insects. The compounds
• Callose being deposited between cell walls and cell bind to salivary proteins and digestive enzymes,
membranes near the invading pathogen which deactivating them. Insects that ingest lots of tannins will die
impede cellular penetration at the infection site. It from a lack of growth. This helps prevent transmission.
strengthens the cell wall and blocks plasmodesmata. • Alkaloids- Nitrogen containing compounds. Ex. Caffeine,
• Necrosis is deliberate cell suicide where a few cells nicotine, cocaine, morphine, solanine. They have a very
are sacri ced to save the rest of the plant. It limits bitter taste to prevent herbivores from eating them , and act
the pathogens access to water and nutrients. on metabolic reactions by inhibiting enzyme action.
• Canker is a sunken necrotic lesion in the woody • Defensive proteins (defensins)- small cysteine-rich proteins
tissue which causes death of the cambium tissue, with broad anti-microbial activity. They act upon molecules
often in the main stem of branches. in the plasma membrane, inhibiting ion transport channels.
• Hydrolytic enzymes- found in the spaces between cells.
Including chitinases (breaks down fungal chitin),
glucanases (hydrolysing glycosidic bonds in glucans) and
lysozymes (degrade bacterial cell walls)
Tylose formation in a
xylem vessel.
Necrosis within a leaf.
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