Class 7 Science

Plant Nutrition

Symbiosis or mutualism

Symbiosis is the combination of two Greek words ‘Sym’ means ‘with’ and ‘biosis’ means ‘living’, which means living together. In symbiosis or mutualism two different types of organisms live and work together for their mutual benefit from each other. They share shelter and nutrients, e.g. Lichens. Lichens are composite organisms composed of fungus and alga. Fungus is a saprophyte and alga is an autotroph. The Fungus supplies water and minerals to the cells of the alga while the alga supplies food; prepared by photosynthesis.

A bird sitting on the back of a rhino is an example of symbiosis. The bird gets worms to eat, while the rhino gets rid of those worms.


Replenishment of Nutrients in Soil

Some fungi live in the roots of the plans. Roots of the plants provide shelter and food to the fungus while fungus provides important nutrients to the plants.

Because of continuous farming nutrients keep declining in the soil. Hence, to replenish those nutrients fertilizers are added to the soil.

Plants require a lot of nitrogen to grow and to be healthy. Although, a lot of nitrogen is available in the atmosphere, but plants cannot absorb nitrogen in gaseous form. There is a bacterium called Rhizobium, which lives in the root nodules of legumes, such as gram, moong, etc. These bacteria can absorb nitrogen from the atmosphere and make it available for the plants. Rhizobium cannot make its food, hence, it provides nitrogen to the legumes and in return legumes provide them food and shelter. This is an example of symbiotic relationship. This has great importance for farmers. Fields; in which leguminous plants are grown from time to time, do not need addition of nitrogenous fertilizers.



Question 1: What is the meaning of word ‘Symbiosis’?

Answer: It is a relationship between two organisms in which both the organisms benefit each other.

Question 2: What is Lichen?

Answer: The lichen is a composite organism formed because of symbiosis of algae and fungi.

Question 3: Give an example of symbiotic relationship.

Answer: A small bird; called plover; cleans the crocodile’s teeth. The crocodile keeps its mouth open and the bird takes out meat fibres stuck between the teeth. The bird gets food in lieu of providing dentist’s services to the crocodile.

Question 4: How do nutrients get replenished in the soil?

Answer: There are two main means through which nutrients get replenished in the soil. One of them is the nitrogen fixation in soil. Nitrogen fixation replenishes nitrogenous nutrients in the soil. Another mean is decomposition of dead remains of plants and animals (or farm waste). Decomposition of dead remains replenishes various other nutrients in the soil.



Question 5: What is Rhizobium?

Answer: Rhizobium is a bacterium.

Question 6: What is the function of Rhizobium?

Answer: Rhizobium helps leguminous plants in nitrogen fixation in soil.

Question 7: How does Rhizobium help farmers?

Answer: By helping in nitrogen fixation, rhizobium increases soil fertility and thus helps farmers.

Question 8: Why do farmers prefer to sow leguminous plants?

Answer: Leguminous plants carry out nitrogen fixation in soil and thus improve soil fertility.

Question 9: In which form do plants absorb nitrogen?

Answer: Plants absorb nitrogen in the form of nitrates.