chapter 12 p3 Flashcards
Key factors in reducing the spread of communicable diseases in humans include:
hand washing - regular hand washing is the single most effective way of preventing the spread of many communicable diseases
improvements in living and working conditions, for example, reducing overcrowding, ensuring good nutrition
disposal of both bodily and household waste effectively.
Vectors:
Wind
Water
Animals
Humans
wind
bacteria, viruses and fungal or oomycete spores may be carried on the wind, e.g. Black sigatoka blown between Caribbean islands, P. infestans sporangia form spores which are carried by the wind to other potato crops/tomato plants.
Water
spores swim in the surface film of water on leaves; raindrop splashes carry pathogens and spores, etc. Examples are spores of P. infestans (potato blight) which swim over films of water on the leaves.
Animals
insects and birds carry pathogens and spores from one plant to another as they feed. Insects such as aphids inoculate pathogens directly into plant tissues.
Humans
pathogens and spores are transmitted by hands, clothing. fomites, farming practices and by transporting plants and crops around the world. For example, TMV survives for years in tobacco products, ring rot survives on farm machinery, potato sacks, etc.
Factors affecting the transmission of communicable diseases in plants:
- planting varieties of crops that are susceptible to disease
- over-crowding increases the likelihood of contact
- poor mineral nutrition reduces resistance of plants
- damp, warm conditions increase the survival and spread of pathogens and spores
- climate change - increased rainfall and wind promote the spread of diseases; changing conditions allow animal vectors to spread to new areas; drier conditions may reduce the spread of disease.
Key factors in reducing the spread of communicable diseases in plants:
- Leave plenty of room between plants to minimise the spread of pathogens.
- Clear fields as thoroughly as possible - remove all traces of plants from the soil at harvesting.
- Rotate crops - the spores or bacteria will eventually die if they do not have access to the host plant.
- Follow strict hygiene practices - measures such as washing hands, washing boots, sterilising storage sacks, washing down machinery, etc.
- Control insect vectors.
Chapter 12.4 - Plant defences against pathogens
Plants have evolved a number of ways to defend themselves against the pathogens that cause communicable diseases.
The waxy cuticle of plant leaves, the bark on trees, and the cellulose cell walls of individual plant cells act as barriers, which prevent pathogens getting in.
Unlike animals, plants do not heal diseased tissue - they seal it off and sacrifice it.
Because they are continually growing at the meristems, they can then replace the damaged parts.
Recognising an attack:
Plants are not passive - they respond rapidly to pathogen attacks.
Receptors in the cells respond to molecules from the pathogens, or to chemicals produced when the plant cell wall is attacked.
This stimulates the release of signalling molecules that appear to switch on genes in the nucleus.
This in turn triggers cellular responses, which include producing defensive chemicals, sending alarm signals to unaffected cells to trigger their defences, and physically strengthening the cell walls.
Physical defences in plants
When plants are attacked by pathogens they rapidly set up extra mechanical defences.
They produce high levels of a polysaccharide called callose, which contains ß-1,3 linkages and B-1,6 linkages between the glucose monomers.
Scientists still do not fully understand the roles played by callose in the defence mechanisms of the plant but current research suggests that:
Scientists still do not fully understand the roles played by callose in the defence mechanisms of the plant but current research suggests that:
within minutes of an initial attack, callose is synthesised and deposited between the cell walls and the cell membrane in cells next to the infected cells.
these callose papillae act as barriers, preventing the pathogens entering the plant cells around the site of infection
large amounts of callose continue to be deposited in cell walls after the initial infection.
Lignin is added, making the mechanical barrier to invasion even thicker and stronger
callose blocks sieve plates in the phloem, sealing off the infected part and preventing the spread of pathogens
callose is deposited in the plasmodesmata between infected cells and their neighbours, sealing them off from the healthy cells and helping to prevent the pathogen spreading.
Chemical defences:
Many plants produce powerful chemicals that either repel the insect vectors of disease or kill invading pathogens.
Some of these chemicals are so powerful that we extract and use them or synthesise them to help us control insects, fungi and bacteria.
Some have strong flavours and are used as herbs and spices.
Examples of plant defensive chemicals include
- insect repellents
- insecticides
- antibacterial compounds including antibiotics
- antifungal compounds
- anti-oomycetes
- general toxins
- insect repellents
- for example, pine resin and citronella from lemon grass
- insecticides
- for example, pyrethrins - these are made by chrysanthemums and act as insect neurotoxins; and caffeine - toxic to insects and fungi
- antibacterial compounds including antibiotics
- for example, phenols - antiseptics made in many different plants;
- antibacterial gossypol produced by cotton;
- defensins - plant proteins that disrupt bacterial and fungal cell membranes;
- lysosomes - organelles containing enzymes that break down bacterial cell walls
- antifungal compounds -
- for example, phenols - antifungals made in many different plants;
- antifungal gossypol produced by cotton;
- caffeine - toxic to fungi and insects;
- saponins - chemicals in many plant cell membranes that interfere with fungal cell membranes;
- chitinases - enzymes that break down the chitin in fungal cell walls
- anti-oomycetes
- for example, glucanases - enzymes made by some plants that break down glucans; polymers found in the cell walls of oomycetes (e.g., Pinfestans)
- general toxins
- some plants make chemicals that can be broken down to form cyanide compounds when the plant cell is attacked.
Cyanide is toxic to most living things.
Mammals (for example humans) have two lines of defence against invasion by pathogens:
The primary non-specific defences against pathogens are always present or activated very rapidly.
This system defends against all pathogens in the same way.
Mammals have a specific immune response, which is specific to each pathogen but is slower to respond
Non-specific defences - keeping pathogens out:
The body has a number of barriers to the entry of pathogens:
skin
mucous membranes
Lysozymes
expulsive reflexes.
skin
The skin covers the body and prevents the entry of pathogens.
It has a skin flora of healthy microorganisms that outcompete pathogens for space on the body surface.
The skin also produces sebum, an oily substance that inhibits the growth of pathogens.
mucous membranes
Many of the body tracts, including the airways of the gas exchange system, are lined by mucous membranes that secrete sticky mucus.
This traps microorganisms and contains lysozymes, which destroy bacterial and fungal cell walls.
Mucus also contains phagocytes, which remove remaining pathogens.
Lysozymes
Lysozymes in tears and urine, and the acid in the stomach, also help to prevent pathogens getting into our bodies.
expulsive reflexes.
We also have expulsive reflexes.
Coughs and sneezes eject pathogen-laden mucus from the gas exchange system, while vomiting and diarrhoea expel the contents of the gut along with any infective pathogens.
Blood clotting and wound repair:
If you cut yourself, the skin is breached and pathogens can enter the body. The blood clots rapidly to seal the wound.
When platelets come into contact with collagen in skin or the wall of the damaged blood vessel, they adhere and begin secreting several substances.
The most important substances are:
thromboplastin, an enzyme that triggers a cascade of reactions resulting in the formation of a blood clot (or thrombus)
serotonin, which makes the smooth muscle in the walls of the blood vessels contract, so they narrow and reduce the supply of blood to the area.
wound repair
The clot dries out, forming a hard, tough scab that keeps pathogens out. This is the first stage of wound repair.
Epidermal cells below the scab start to grow, sealing the wound permanently, while damaged blood vessels regrow.
Collagen fibres are deposited to give the new tissue strength.
Once the new epidermis reaches normal thickness, the scab sloughs off and the wound is healed.
diagram of the blood clotting cascade
The inflammatory response
The inflammatory response is a localised response to pathogens (or damage or irritants) resulting in inflammation at the site of a wound.
Inflammation is characterised by pain, heat, redness, and swelling of tissue.
Mast cells are activated in damaged tissue and release chemicals called histamines and cytokines.
Mast cells are activated in damaged tissue and release chemicals called histamines and cytokines.
Histamines make the blood vessels dilate, causing localised heat and redness. The raised temperature helps prevent pathogens reproducing.
Histamines make blood vessel walls more leaky so blood plasma is forced out, once forced out of the blood it is known as tissue fluid.
Tissue fluid causes swelling (oedema) and pain.
Cytokines attract white blood cells (phagocytes) to the site. They dispose of pathogens by phagocytosis.
If an infection is widespread, the inflammatory response can cause a whole-body rash.
