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State two economically important uses of:

(a) Heterotrophic bacteria

(b) Archaebacteria

(a) Heterotrophic bacteria
The majority are important decomposers.Many of them have a significant impact on human affairs. They are helpful in making curd from milk, production of antibiotics, fixing nitrogen in legume roots, etc.
(b) Archaebacteria
Archaebacteria differ from other bacteria in having a different cell wall structure and this feature is responsible for their survival in extreme conditions. Methanogens are present in the guts of several ruminant animals such as cows and buffaloes and they are responsible for the production of methane (biogas) from the dung of these animals.


What is the nature of cell walls in diatoms?

In diatoms the cell walls form two thin overlapping shells, which fit together as in a soap box. The walls are embedded with silica and thus the walls are indestructible. Thus, diatoms have left behind large amount of cell wall deposits in their habitat; this accumulation over billions of years is referred to as ‘diatomaceous earth’.


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

Algal bloom
A heavy growth of algae in and on a body of water; usually results from high nitrate and phosphate concentrations entering water bodies from farm fertilizers and detergents; phosphates also occur naturally under certain conditions.

Red tides
Phenomenon associated with population explosions (blooms) of certain types of dinoflagellates; red structures inside the dinoflagellates cause the water to have a reddish color.


How are viroids different from viruses?

There are 6 ways in which viroids are different to viruses:

(i) Contain single circular RNA of low molecular weight (300 to 400 nucleotides).

(ii) RNA is often internally paired (3D) and is therefore protected against cellular

(ii) Viroids contain no capsids or envelopes.

(iii) There is no requirement for helper viruses.

(iv) Viroids do not produce proteins.

(v) In viroids RNA is always copied in the host cell nucleus.

(vi) Viroids can only be identified by nucleotide sequencing techniques.


Describe briefly the four major groups of Protozoa.

All protozoans are heterotrophs and live as predators or parasites. They are believed to be primitive relatives of animals. There are four major groups of protozoans.

(i) Amoeboid protozoans:
These organisms live in fresh water, seawater or moist soil. They move and capture their prey by putting out pseudopodia (false feet) as in Amoeba. Marine forms have silica shells on their surface. Some of them such as Entamoeba are parasites.

(ii) Flagellated protozoans:
The members of this group are either free-living
or parasitic. They have flagella. The parasitic forms cause diseases such as sleeping sickness. Example: Trypanosoma.

(iii) Ciliated protozoans:
These are aquatic, actively moving organisms because of the presence of thousands of cilia. They have a cavity (gullet) that opens to the outside of the cell surface. The coordinated movement of rows of
cilia causes the water laden food to be steered into the gullet. Example: Paramoecium.

(iv) Sporozoans:
This includes diverse organisms that have an infectious
spore-like stage in their life cycle. The most notorious is Plasmodium (malarial parasite) which causes malaria which has a staggering effect on human population.


What do the terms phycobiont and mycobiont signify?

The algal component is known as phycobiont and fungal component as mycobiont, which are autotrophic and heterotrophic, respectively. Algae prepare food for fungi and fungi provide shelter and absorb mineral nutrients and water for its partner.
So close is their association that if one saw lichen in nature one would never imagine that they had two different organisms within them.


What are the characteristic features of Euglenoids?

Majority of the euglenoids are fresh water organisms found in stagnant water. Instead of a cell wall, they have a protein rich layer called pellicle which makes their body flexible. They have two flagella, a short and a long one. Though they are photosynthetic in the presence of sunlight, when deprived of sunlight they behave like heterotrophs by predating on other smaller organisms. Interestingly, the pigments of euglenoids are identical to those present in higher plants. Example: Euglena


Give a brief account of Viruses with respect to their structure and nature of genetic material. Also name four common viral diseases.

Viruses are obligate parasites. In addition to proteins viruses also contain genetic material that could be either RNA or DNA. No virus contains both RNA and DNA. A virus is a nucleoprotein and the genetic material is infectious. In general, viruses that infect plants have single stranded RNA and viruses that infect animals have either single or double stranded RNA or double stranded DNA. Bacterial viruses or bacteriophages (viruses that infect the bacteria) are usually double stranded DNA viruses. The protein coat called capsid made of small subunits called capsomeres. The capsomeres protect the nucleic acid. These capsomeres are arranged in helical or polyhedral geometric forms. Viruses cause diseases like mumps, small pox, herpes and influenza. AIDS in humans is also caused by a virus. In plants, the symptoms can be mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.

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