Key Area 5: Symbiosis

Unit 3: Sustainability and Interdependence

Symbiosis• Symbiosis is the close relationship

between two different species that live in direct contact with each other.

• e.g. the relationship between nitrogen fixing bacteria (Rhizobia) and leguminous plant roost (peas, beans and clover)

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Symbiosis• There are two categories of

symbiosis:

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Symbiosis

Parasitism Mutualism

(Find out about commensalism)

Parasitism

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• Parasitism is a relationship between two organisms where only one organism benefits.

• A parasite lives on another organism called the host.

• The parasite obtains nutrition and food from the host which benefits the parasite.

• The host does not receive any benefit but is harmed or killed by the parasite.

Parasitism

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• Some parasites are ectoparasites and live on the outer skin of the host.

• e.g. ticks

• Leeches

fleas

Parasitism

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• Some parasites are endoparasites and live inside the host.

• e.g. tapeworm, malarial Plasmodium

Parasitism

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• Parasites can be obligate or facultative.

• An obligate parasite is one that can only survive with a host.

• A facultative parasite is one that can survive with or without a host.

• Only poor parasites will kill their host as this is not to their advantage, especially for those that are obligate.

Parasitism

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• A balance must exist between parasitic damage and the defence of the host.

• A relatively stable relationship must exist between the two.

• As the parasite and the host coexist and evolve, the disease the parasite causes may occur in a cycle.

Parasitism

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• Parasites can be transmitted to new hosts by a variety of mechanisms:– Direct contact– Releasing resistant stages– Using vectors

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Method

Example(s) of parasite

Notes on transmission (e.g. description /advantages/disadvantages

Direct ContactResistant Stages

Vectors

RingwormRabiesHIVSpores

and endospores in fungi and bacteria

Allows the parasite to survive in extreme conditions

MalariaSomething else carries the disease between infected individual and new host. Malaria uses the mosquito as a vector

Research a specific example

Primary and Secondary hosts

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• A primary host is used in a parasitic direct life cycle.

• Only one species of host is used in this case.

• A secondary host is a host that a parasite uses for a short time along with a primary host to ensure that the parasitic life cycle can be completed.

• This is an indirect life cycle.

Secondary hosts

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• A tapeworm is a parasite that uses a secondary life cycle.– Parasite –

tapeworm– Primary

host - human

– Secondary host - pig

Primary and Secondary hosts

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• Trypanosoma are parasites that use a secondary host– Parasite –

trypanosome– Primary host

– tsetse fly– Secondary

host - human

Mutualism

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• Mutualism is a specialist relationship between two organisms where both organisms benefit.

• e.g. lichens are composed of fungus and algae in a beneficial relationship

Mutualism

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• e.g. herbivores have special cellulose digesting microorganisms in their stomachs.

• As we depend on ruminants (sheep, cattle) for food and products this symbiosis is of great economic importance

Leguminous plants and Rhizobium bacteria

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• Rhizobium bacteria live freely in the soil.

• They are able to infect the roots of leguminous plants and form nodules.

• Inside the nodules the bacteria obtain energy from their host and supply nitrogen compounds in return.

Fungi and Algae (lichens)

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• Lichens are able to colonise areas where other plants are unable to grow.

• The fungal part of the lichen produces tough cellulose walls and secretes mineral digesting chemicals while receiving carbohydrate.

• The algal part of the lichen photosynthesises and received protection and essential minerals

Herbivores and gut bacteria

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• Herbivores cannot produce cellulose digesting enzymes.

• Bacteria which can do this are found in the alimentary tract of herbivores.

• These bacteria receive food and shelter and aid plant digestion.

Ants and Aphids

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• Some species of ant ‘farm’ aphids.• They move them from plant to plant

and protect them from predators.• In return the aphids secrete a sugary

liquid that the ants drink.

Terrestrial plants and pollinating animals

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• Many plants have evolved special flower shapes which allow only one kind of animal near them.

• The animal depends on the nectar it receives as a reward for its visit.

• The plant depends on the animal to transport its pollen between flowers.