Symbiosis are the intimate relationships that exist between species.

Symbiotic Relationships

Parasitism

Commensalism

Mutualism

Symbiotic relationships give at least one of theparticipants a nutritional or survival advantage.

Parasitism

An interaction between two organisms where onespecies benefits (parasite) and the other species (host)is ‘harmed’ in some way.

ParasitesObligate

Parasites dependant on

the host for survival

(viruses)

Facultative

Parasites that do not

need a host to survive.

They can live with or

without them.

Outside or Inside ?

Ectoparasites

Live on surface of the host

Ticks

Fleas

leeches

Endoparasites

Live within their host

Tapeworm

Malaria (plasmodium)

Sleeping sickness

(trypanosome)

Parasite and Host relationship

Balance between receiving nutrients and damage done.

Most parasites do not kill their host.Host could have some means of defence.Death of the host means loss for parasite.Relationship usually stable between them.

Transmission of parasite to new hostDirect Contact

Parasite moves between hosts e.g rhinovirus, influenza, head lice.

Secondary host species or vectorsOrganisms that transmit parasites e.g. fleas and ticks. The infectious stage develops inside them and is passed on e.g. toxoplasmosis, malaria

Resistant stagesOffspring released by parasite are in dormant or resistant form. When consumed by a host they become active e.g. liver fluke (snail and sheep)

Host – parasite specificityParasites have often developed with hostParasites have often shared an evolutionary path.As hosts defence system affects parasites the

most well adapted survive.

Defence against parasitesAnimalsLymphocytes – antibodiesMacrophagesPreening/groomingMigration

PlantsChemicals Localised death of cells to isolateGalls

Plant parasites

Viruses

tobacco mosaic virus

potato leaf roll virusBacteria

fireblight in apples and pearsFungi

wheat rust

grey mould

Koch’s postulate for diseasesPathogen should be present in all diseased

organisms but not healthy onesPathogen should be able to be isolated and grown

in pure cultureA healthy animal inoculated with the pure culture

should get the same disease as original animalsThe pathogen should be able to be isolated from

the newly diseased organisms

Commensalism

An interaction between two organisms whereone species (commensal) benefits and the otherspecies (host) is unaffected.

CommensalismAre there really any pure commensal relationships?

MutualismAn interaction betweentwo organisms whereboth species benefitfrom the relationship.

MutualismOrganisms involved in mutualistic relationships often benefit from increased levels of metabolites, protection, removal of parasitic organisms.

Organisms also show complementary structures or behaviours.

MutualismLichens - fungi and algae

Sea anemone and hermit crab

MutualismPlants often have symbiosis with fungi

especially in root systems.

Ectomycorrhizae – fungus on outside of cells

forming covering sheath of mycelium.

Endomycorrhizae – fungus found inside cortex

cells.