What is symbiosis?
Please give the answer in full explanation.

Symbiosis refers to a close physical and prolonged association between two or more organism belonging to different species. This interaction between two different organisms are usually of benefit to both the organism but sometimes may be of benefit to one and harmful or unaffected to the other; or harmful or unaffected to both of them.

For example, The symbiotic interaction between algae and fungi forms lichens in which both the organisms are benefited from each other. Algae synthesise food for a fungal partner while fungi help in the absorption of nutrients and protection of algae.

Another example is the formation of root nodules in leguminous plants. Here also both the partners are benefited by the interaction. Bacteria present in the root nodules fix atmospheric nitrogen and gives it to plants. Plants make food for the bacteria and give them a place to live.

Hope this helps.
Regards.