What is symbiosis?

Hint: Symbiosis is an interspecific relationship in which two different organisms interact with each other for food and shelter. Such a type of relationship is necessary for the different organisms to fulfill their needs.

Complete answer:
1. Symbiosis is interaction between two types of different organisms who live in close physical association, basically for their own advantage such as food and shelter.

2. There are major three types of Symbiotic relationships, such as:

A. Commensalism: It is a relationship between two species where one species is benefitted while the other neither gets any benefit nor is adversely affected under normal conditions.
Some of the smaller organisms live inside the bodies of larger animals in order to protect themselves from the enemies.
One example in animals is the interaction between sea anemone that has stinging tentacles and the clown fish that lives among them. The fish receives protection from predators which stay away from the stinging tentacles. But the anemone does not appear to derive any benefit by hosting the clown fish.

B. Mutualism: An association of two species in which both species are benefitted, is called mutualism.
Mutualism may not involve close physical association between the individuals of pairs of species or it may sometime do it. The condition in which there is close physical association between the individuals of a pair of species is called Symbiosis.
For example: Mycorrhiza, it is a relationship between roots of higher plants and fungal hyphae. The fungus helps in mineral nutrition for the plants with which they are associated and obtains, in turn carbohydrates from plants.

C. Parasitism: It is a relationship in which one organism, called parasite, obtains its nourishment either partially or completely from another living organism called host.
The example of endoparasites in humans include malarial parasites (Plasmodium) in blood, tapeworm (Taenia) and roundworm (Ascaris) in the human intestine etc.

Note: Humans also show symbiosis: When two species benefit from each other than the symbiosis is termed as mutualism. As in the human intestinal tract, the bacterium bacteroides thetaiotaomicron lives,it digests complex polysaccharide plant materials and converts it into monosaccharides.