1. Unit 3: Sustainability and Interdependence
Key Area 5: Symbiosis
Unit 3: Sustainability and Interdependence

2. 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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3. Symbiosis There are two categories of symbiosis:
Parasitism
Mutualism
(Find out about commensalism)
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4. Parasitism
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.
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5. Parasitism
Some parasites are ectoparasites and live on the outer skin of the host.
e.g. ticks
Leeches
fleas
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6. Parasitism Some parasites are endoparasites and live inside the host.
e.g. tapeworm, malarial Plasmodium
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7. Parasitism 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.
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8. Parasitism
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.
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9. Parasitism
Parasites can be transmitted to new hosts by a variety of mechanisms:
Direct contact
Releasing resistant stages
Using vectors
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10. Example(s) of parasite
Method
Example(s) of parasite
Notes on transmission (e.g. description /advantages/disadvantages
Ringworm
Rabies
HIV
Direct Contact
Research a specific example
Resistant Stages
Spores and endospores in fungi and bacteria
Allows the parasite to survive in extreme conditions
Something else carries the disease between infected individual and new host. Malaria uses the mosquito as a vector
Vectors
Malaria
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11. Primary and Secondary hosts
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.
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12. Secondary hosts
A tapeworm is a parasite that uses a secondary life cycle.
Parasite – tapeworm
Primary host - human
Secondary host - pig
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13. Primary and Secondary hosts
Trypanosoma are parasites that use a secondary host
Parasite – trypanosome
Primary host – tsetse fly
Secondary host - human
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14. Mutualism
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
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15. Mutualism
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
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16. Leguminous plants and Rhizobium bacteria
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.
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17. Fungi and Algae (lichens)
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
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18. Herbivores and gut bacteria
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.
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19. Ants and Aphids 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.
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20. Terrestrial plants and pollinating animals
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.
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