NCERT Solutions For Class 11 Biology Chapter 2 Biological Classification

The difference between viroids and viruses are as follows:

The phycobiont and mycobiont are the algal and fungal partners, respectively, which are in a symbiotic relationship with each other in nature. These are known as lichens and this is a mutually beneficial partnership. The algal partner, phycobiont (also called photobiont) is autotrophic and provides food to the fungi. The fungal or mycobiont partner provides shelter, nutrients and water to the algal partner.  

In diatoms, the cell walls form two thin overlapping shells, which fit together closely. The walls cannot be destroyed as they are embedded with silica. The cell walls show a wide diversity in form but are usually almost bilaterally symmetrical, hence the group name – diatom.

Plants are mostly autotrophic which means that they possess the pigment chlorophyll. With the help of chlorophyll, they manufacture their own food by performing photosynthesis. However, a few members of the plant kingdom are partially heterotrophic, for example, the insectivorous plants or parasites. These plants typically grow in nitrogen and mineral deficient soils and compensate for their nutritional requirement by their partial heterotrophic nature. These insectivorous plants have evolved mechanisms to trap insects from which they derive minerals and nutrition. Bladderwort and Venus flytrap are examples of insectivorous plants.

The other partially heterotrophic plant is Cuscuta that lives like a parasite on many trees and absorbs nutrition from the host plant.

Algal bloom: The algal bloom is a phenomenon occurs when contaminants like nitrates and phosphates from untreated sewage are released into water bodies. the presence of an excessive amount of nutrients such as nitrates and phosphates overstimulate the growth of algae.  This overstimulation of algal growth (algal bloom) reduces the dissolved oxygen content of the water, which in turn causes other life forms to perish. Algal bloom affects water quality and causes fish and other aquatic life mortality. Some algae can be toxic to humans too.

Algal blooms are often green but they could also be other colours like red or green depending upon the predominant species of algae causing the bloom.

Red Tides: Red or brown coloured algal blooms are called red tides. Red tides are caused by red dinoflagellates (example Gonyaulax) which accumulate due to rapid multiplication making the sea appear red. Toxins released by such large numbers of algae may kill other marine animals such as fish.

The two economic uses of heterotrophic bacteria are: 

•  Many heterotrophic bacteria help in decomposing dead and decaying organisms. This leads to the production of humus which increases soil fertility.
• Heterotrophic bacteria are also used in the production of antibiotics. Many known antibiotics are produced by Streptomyces, which are filamentous soil bacteria.

The two economic uses of archaebacteria are: 

• Methanogens are archaebacteria that produce methane gas as a metabolic bi-product. These bacteria are naturally found in the dung of cows and buffaloes. Some examples of methanogens are Methanococcus jannaschii and Methanobacterium thermoautotrophicum. Methanogens are used for commercial production of methane gas.
• Methanogens are also useful for the treatment of sewage in villages and municipal sewage treatment plants. Under anaerobic conditions soluble carbon compounds of wastes and wastewater are degraded stepwise to methane and other gases.

The debate whether viruses are living or non-living is an old one and till today, a consensus has not been reached.

Characteristics that classify viruses as living:

• They have complex molecule assemblies that include nucleic acids, proteins, carbohydrates, lipids and genetic material.
• They also make copies of themselves to reproduce.
• They are sensitive to external factors such as temperature, chemicals and radiations. 

Characteristics that classify viruses as non-living:

• They cannot replicate on their own - they need a host cell to provide them with the machinery to replicate.
• They can remain in the dormant state forever till they find the right host to multiply. 
• They have only one type of nucleic acid, i.e., either DNA or RNA. All the living things possess both the types of nucleic acid. 

Though it is challenging to categorise viruses, it can be said with enough confidence that viruses are some form of life. Nature is so diverse that the classification system that humans have made for their ease of use may not fit that diversity.

A complete virus particle is known as virion. It consists of nucleic acid, either DNA or RNA, surrounded by a coat of protein called capsid. These are formed from identical protein subunits called capsomeres. Proteins associated with nucleic acid are known as nucleoproteins, and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid. Viruses are classified into four types based on their structure:

Polyhedral / icosahedral Viruses: These are non-enveloped viruses in which the capsomeres are arranged in geometric shape around the genetic material which is either double or single-stranded DNA or RNA. The icosahedron shape has 20 equilateral triangle faces and 12 corners. Example: polio causing virus

Helical viruses: These viruses are non-enveloped with capsomeres arranged as a helix around the nucleic acid giving rise to elongated rods or flexible filaments. Example: tobacco mosaic virus

Complex viruses: The complex viruses have complex combinations of structures that may or may not be completely consistent between viruses of the same species. Example tailed bacteriophage, poxviruses.

Enveloped viruses: These viruses are helical and icosahedrons shapes but have an outer envelope that surrounds the virus capsomeres. The envelope is a lipid bilayer containing glycoproteins embedded in the lipid. Example: Herpes virus.

Genetic Material of Viruses:

• It can be RNA or DNA but not both at the same time in a virus.
• Viruses infecting plants have single-stranded RNA.
• Viruses infecting animals have either single or double-stranded RNA or double-stranded DNA.
• Bacteriophages usually have double-stranded DNA.

Four viral diseases: Rabies, Influenza, Herpes, HIV

Characteristic features of Euglenoids are:

• Euglenoids are a group of flagellates commonly found in stagnant pools of freshwater and ditches or damp soil.
• Instead of a cell wall, they have a protein-rich layer called pellicle which makes their body flexible. The pellicle is composed of proteinaceous strips underneath the cell membrane giving them a distinctive striation.
• They have one or two flagella that they use for swimming. Those have two flagella, one is short and another is long. Both the flagella are used for locomotion.
• They are photosynthetic in the presence of sunlight, however, when deprived of sunlight they behave like heterotrophs by predating on other smaller organisms.
• Euglena reproduces by fission. It splits lengthwise into two.
• The euglenoids have chlorophyll a and chlorophyll b and they store their photosynthetic product in an unusual form called paramylon starch.

(i) Mode of nutrition 

(ii) Mode of reproduction 

The four major groups of protozoans are as follows:

Amoeboid protozoans:

• Found in fresh water, sea water or moist soil
• Move and capture the prey by using pseudopodia (false feet) as in Amoeba
• Marine forms have silica shells on their surface
• Some are parasites e.g. Entamoeba

Flagellated protozoans:

• Free-living or parasitic
• Movement by flagella
• The parasitic forms cause diseases such as sleeping sickness. Example: Trypanosoma

Ciliated protozoans:

• Aquatic and very motile
• Presence of hair-like organelles called cilia on the pellicle (cell body). Cilia are shorter than flagella and occur in large numbers. Cilia are used for movement and food gathering.
• Presence of a cavity (gullet) that opens to the outside of the cell surface.
• Rows of cilia move in a synchronised manner for locomotion and also to steer food into the gullet. Example: Paramecium

Sporozoans:

• Parasitic protozoans
• They lack organelles for locomotion like cilia or flagella
• All sporozoans lack contractile vacuole
• They have complex life cycles a part of which is spent in host organisms
• Some sporozoans pass from host to host via an infectious spore stage
• Example: Plasmodium (malarial parasite) which causes malaria

The biological classification system has undergone tremendous change guided by scientific understanding and a need to define and categorize each organism precisely on the basis of its morphological, anatomical and genetic attributes. The steps given below show how the present system of classification has evolved into its form.

• The earliest methods of classification were based out of the need to use plants or animals and didn't have any scientific basis. 
• Aristotle defined a classification method whereby he used the morphological characters to classify plants and animals. 
• Linnaeus defined a two-kingdom system of plants and animals. this included all plants and animals irrespective of their varied forms and features. 
• R.H.whittaker (1969) proposed a five-kingdom classification system. This mode of classification system includes cell structure, thallus organisation, mode of nutrition, reproduction and phylogenetic relationship. In this classification system, a clear differentiation between photosynthetic and non-photosynthetic organisms was made.