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State two economically important uses of:
(a) Heterotrophic bacteria
(b) Archaebacteria

The economic uses of the following are:
(a) Heterotrophic bacteria:
(i) Used as bio-fertilizers as they act as decomposers and help in the formation of humus and some of them fix nitrogen.
(ii) They help in the production of curd, various antibiotics etc. Many antibiotics are obtained from some species of bacteria

(b) Archaebacteria:
(i) Used for the production of biogas as they produce methane gas by acting on the excreta of animals.
(ii) They are used for the production of commercial products like heat stable enzymes.  


What is the nature of cell-walls in diatoms?

The cell walls of diatoms contain silica and are indestructible. The cell wall is made up of two thin overlapping walls which fit together such as a soap box. When the diatoms die, the silica in their cell walls gets deposited in the form of diatomaceous earth. 


How are viroids different from viruses?  



1. Larger in size.

1. Smaller in size.

2. Have protein coat.

2. Lack protein coat.

3. RNA of higher molecular weight.

3. RNA of low molecular weight.

4. May have DNA or RNA.

4. Only RNA is present.


Discuss how classification systems have undergone several changes over a period of time?

The classification systems have undergone many changes with time.
(i) The Aristotle made the first attempt of classification and classified plants as herbs, shrubs, and trees.
Animals, on the other hand, were classified on the basis of presence or absence of red blood cells. This system of classification failed to classify all the known organisms.

(ii) Linnaeus developed a two-kingdom system of classification. It consisted of kingdom Plantae and kingdom Animalia. However, this system did not differentiate between unicellular and multicellular organisms or between eukaryotes and prokaryotes. There were large numbers of organisms that could not be placed under the two kingdoms.

(iii) A five kingdom system of classification was proposed by R.H Whittaker in 1969. On the basis of characteristics, such as cell structure, mode of nutrition, presence of cell wall, etc., five kingdoms, Monera, Protista, Fungi, Plantae, and Animalia were formed.


Find out what do the terms ‘algal bloom’ and ‘red-tides’ signify.

Algal bloom refers to an increase in the population of algae or blue-green algae in water due to an enrichment of nutrients, resulting in discoloration of the water body. This causes an increase in the biological oxygen demand (BOD), resulting in the death of fishes and other aquatic animals.
Red tides are caused by red dinoflagellates (Gonyaulax) that multiply rapidly. Their large numbers in the sea makes the sea appear red in colour. These release a large amount of toxin which results in the death of other marine organisms. 
Both of these signify the pollution of the water bodies.