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Symbiotic Relationships - Parasitism - Commensalism - Mutualism SYMBIOSIS refers to relationships between organisms of DIFFERENT species that show an intimate association with each other Symbiotic relationships provide at least ONE of the participating species with a nutritional advantage 3 types of symbiosis have been recognised depending on the nature of the relationship:
Humans are the INTERMEDIATE HOST and RESERVOIR of the parasite, and the mosquito is the DEFINITIVE HOST and VECTOR .
Female anopheline mosquitoes become infected only if they take a blood meal from a person whose blood contains mature male and female stages of the parasite.
A cycle of development and multiplication then begins with union of the male and female gametocytes in the stomach of the mosquito and ends with parasites, called sporozoites, in its salivary glands, which are infective to humans.
The time required for the complete maturation of the parasite in the mosquito varies and depends on the Plasmodium species and external temperature.
Lifecycle of Malaria Parasite 1 2 3
The gametocytes are ingested by the female mosquito in a bloodmeal from an infected human. The gametocytes fuse to produce a zygote.The zygote secrete a cyst containing sporozoites formed from meiotic divisions
Sporozites enters the liver cell and during the next two weeks the intracellular parasite reproduces by mitosis within a liver cell to form as many as 200,000 merozoites! On maturation, the merozoites rupture the liver cells and are are released into the blood where they invade human red blood cells
In the red blood cells, the parasite matures asexually to produce another 10-20 merozoites which in turn can rupture the red blood cell and invade more liver cells or red blood cells
Vertebrate hosts infected with microparasites mount an immunological response
Vertebrate hosts infected with ectoparasites have other behavioural strategies e.g.
Preening or grooming each other to remove ectoparasites e.g. chimpanzees.
Move away from the infected area e.g. caribou move to higher altitudes during the summer months when the mosquito population is particularly dense to avoid attacks
Plants respond to parasitic infection in several ways:
e.g. in tobacco plants, if just one leaf is infected with the tobacco mosaic virus, there is an increase in the defensive chemicals throughout the plant-protects the plant from a variety of parasites and from the effects of grazing by herbivores. In addition, the plant will often kill the cells in the area that has been infected by the parasite, causing localised cell death. This deprives the parasite of its source of food and prevents parasitic spread to other cells.
There may be several different organisms growing in an infected sample, although most will have appeared after the initial disease has weakened the host.
Koch's postulates need to be satisfied in order to identify the organism that is causing a disease
Koch was one of the original researchers into tuberculosis, in the 19th century. In an attempt to define what an infectious disease actually is, he formulated his famous postulates, which now bears his name. Basically if,
1. An organism can be isolated from a host suffering from the disease AND
2. The organism can be cultured in the laboratory AND
3. The organism causes the same disease when introduced into another host AND
4. The organism can be re-isolated from that host THEN
The organism is the cause of the disease and the disease is an infectious disease
Most plants have to search through the soil with their roots to find nitrogen which is a critical nutrient required for growth
Legumes, however, form symbiotic relationships with Rhizobium bacteria
The Rhizobium live in little nodules in the roots of the legumes and fix atmospheric nitrogen into ammonium or nitrate, forms of nitrogen that can be used by the plant i.e. Rhizobium turn air into fertiliser!
The plant benefits because it gains nitrogen.
The bacteria benefit because they get sugars and nutrients to survive