Biology-Symbiosis

Biology-Symbiosis

Close living arrangements between two different species is called symbiosis. The word comes form the Greek word meaning 「state of living together.」 Usually the two organisms are in close physical contact, with one living on or in the other. In some cases, however, the relationship is less intimate. Symbiosis is classified into: mutualism (once called symbiosis), commensalism, and parasitism. These relationships rage from mutually beneficial to harmful, or even fatal, for one of the species.

In mutualism both partners benefit form the relationship. One of the best-known mutual relationships is the one between nitrogen-fixing Rhizobium bacteria and several leguminous plants such as beans, peas, peanuts, and alfalfa.

Human beings, animals, and most plants need nitrogen to survive but cannot metabolize it from the air. Rhizobium bacteria, which live in the soil, enter the roots of legumes and produce nodules, or enlargements, in which they absorb nitrogen form the air and convert it into ammonia. This compound is then converted into some organic form, such as amino acids, which is shared by the bacteria and the host plant. By eating such leguminous plants, other organisms obtain a rich source of nitrogen-bearing compounds. The bacteria, in turn, benefit from the relationship by absorbing from the host plant nutrients that they cannot manufacture themselves.

Lichens, which consist of fungi and algae, are another well-known example of mutualism. Algae receive shelter and a moist environment by dwelling within the fungi. In turn, the algae provide the fungi with food through photosynthesis.