There are certain organisms that lives together and provide food and shelter to each other. This kind of nutrition is known as symbiotic relationship.
Certain microorganisms like fungi that lives in the roots of trees and absorb the nutrients from the fungi in turn help the plants to absorb water and nutrients from the soil required for the proper growth of plants.The algae contains the green pigment called chlorophyll which helps the algae to produce food which is being provided to the fungi too. In return the fungi protects the algae from drying in sunlight by enclosing it inside. So algae provides food to fungi and in return the fungi provides protection to the algae.(eg: Lichens)
This relation also exists between a bee and a flower. The bee helps the plant in pollination (the process of transfer of seeds of one flower to another that helps in asexual reproduction in plants) whereas the flower provides pollen grains and nectar to the bee.
Atmospheric nitrogen cannot be used by plants for production of food. Therefore bacteria known as Rhizobium present in the roots of gram, peas, moongbeans and other legumes converts the atmospheric nitrogen into soluble form that dissolves in soil and can be easily absorbed by plants. But these bacteria cannot make their own food and hence the plants in turn provide them with the shelter and food synthesized by them. They exhibit a symbiotic relationship between them.