Biology Class 9 Notes- Chapter 4

VIRUSES, BACTERIA AND CYANOBACTERIA

QNo.1: Describe the heterogenous group of micro-organisms.

Ans. Micro-organisms form a heterogenous group which includes viruses, bacteria, cyanobacteria, protozoa, unicellular algae and some fungi.

Ø  Bacteria and cyanobacteria are prokaryotes and studied under Kingdom Prokaryota (Monera).  Bacteria are heterotroph as well as autotroph but all members of cyanobacteria and algae are autotroph.

Ø  All micro-organisms other than bacteria and cyanobacteria are eukaryotes and studied under Kingdom Protoctista (Protista). Protozoa are heterotroph.

Ø  Viruses lie somewhere between living and non-living things.

QNo.2: What are viruses?

Ans.

Ø  Viruses (Latin word Viron = Poison) are the smallest, simplest and most primitive living things.

Ø  Viruses were first discovered by a Russian biologist, Iwanowky in 1892. He showed that disease in tobacco plant was due to something smaller than bacteria. He named them viruses. Later these viruses were named as Tobacco Mosaic Virus (TMV).

Ø  In the year 1935, electron microscope had been constructed and any object smaller than m (millimicron 1,000,000,000^th part of a meter) can be observed.

Ø  In the same year Wendell Stanley crystallized TMV and observed under electron microscope.

QNo.3: Define characteristics of viruses.

Ans.

Ø  Viruses are non-cellular obligate parasites having protein coat and nucleic acid core.

Ø  They cannot live or reproduce outside of living cell.

Ø  They range in size from 20 nm to 250 nm.

Ø  They are submicroscopic (too smaller to be seen with light microscope).

Ø  There is no sexual or asexual reproduction in viruses but they reproduce by replication.

Ø  Viruses use the enzymes of host cell for their protein synthesis and gene replication.

Ø  More complex viruses contain up to 200 genes.

Ø  Viruses either destroy the cell in which they occur or calm, integrated into host genomes and remaining stable for long periods of time.

QNo.4: Define structure of viruses.

Ans. The viruses appear like small rods, tadpoles, polyhedrals or little spheres.

Viruses may consist of:

a)      Nucleic acid: It may consist of single or several molecules of DNA or RNA. Smallest virus has four genes while the largest have up to 200 hundred genes. In bacteriophage long DNA molecule is coiled within the head.

b)     Capsid: The protein coat that encloses the nucleic acid is called capsid. Capsid is made up of protein subunits called capsomers. The number of capsomers is characteristic of a particular virus.

c)      Envelopes: Some viruses have membranes covering their capsids. These coverings are called viral-envelopes, which help them infect their hosts.

d)     Tail-fibers: Bacteriophage virus is made up of five separate proteins of which one type of protein makes tail-fibers.

QNo.5: What are the living and non-living characteristics of viruses?

Ans. Viruses are considered as living things because of following characteristics:

a)      Viruses with core of DNA or RNA surrounded by a protein coat somewhat resembles the chromosome of other living organisms.

b)      They have ability to reproduce (Property of replication, reproduction).

c)      Many of viruses can undergo mutations.

d)     They show genetic recombination.

Viruses are also considered as non-living because of following characteristics:

a)      Viruses have non-cellular structure.

b)      Undergo crystallization.

c)      Completely inactive outside host’s cell.

QNo.6: Define viral diseases.

Ans. 1. Animal Diseases

a.      Poliomyelitis: This disease is caused by polio virus. It is now largely under control by vaccination in developed countries. It remains a serious and common disease in tropics and less developed parts of the world.

b.      Colds: Colds are viral infections of upper respiratory tract. More than 200 of viruses that cause colds have been identified.

c.       Human Immune Virus (HIV): HIV was discovered in 1985. It is the cause of Acquired Immune Deficiency Syndrome (AIDS).

d.      Rabies: Rabies is also caused by virus.

e.       Flu: It is a most common disease in man. It is caused by Influenza virus.

2. Plant Diseases

a.       Tobacco Mosaic Virus (TMV): This virus effects the leaves of tobacco plant. Light green and yellow patches appear on the leaves.

QNo.7: What are bacteria?

Ans.

Ø  Bacteria, first discovered by Leeuwenhoek are the smallest oldest and simplest organisms.

Ø  Bacteria are the largest number of creatures present in every habitat of earth under all possible environmental conditions.

Ø  Bacterial cell ranges in size from 0.2 microns to 2 microns in width and 2 to 10 microns in length.

Ø  Bacteria are unicellular prokaryotes. Cell wall is made up of amino acids, sugar and sometimes chitin. Cell membrane is attached to cell wall at few places and has many pores. Cell membrane performs function of respiration as mitochondria are absent in them. It also acts as selective membrane. Cytoplasm is granular. It has no membranous structures like endoplasmic reticulum, golgi bodies, plastids and mitochondria.

Ø  Bacteria do not have well organized nucleus. However DNA is found as concentrated structure called chromatin bodies.

Ø  They reproduce by binary fission.

Qno.8: What are the forms (shapes) of bacteria?

Ans. Following are the shapes of bacteria:

a)      Cocci (Sing. Coccus = Greek Kokkos= Berry, rounded): They are spherical. According to cell arrangement they are:

1.      Monococcus: They are solitary (Single).

2.      Diplococci: They are in pair.

3.      Streptococci: They are I the form of chain.

4.      Staphylococci: They form cluster.

They are non-flagellated.

b)     Bcilli (Sing. Bacillus = Bakulus= A rod): They are rod shaped. They may be found in:

1.      Diplobacilli: In pairs.

2.      Streptobacilli: In chain.

c)      Spirilla (Sing. Spirillum = Gr. Sperira = A coil): They are spiral or coil shaped.

d)     Vibrio or Comma: They are slightly curved or comma shaped. E.g. Vibrio cholerea. They may be flagellated.

QNo.9: Define nutrition in bacteria.

Ans. According to mode of nutrition bacteria can be divided into following groups:

1.      Saprophytic bacteria: They get their food from dead organic matter. They live in the soil and have enzymes that break complex substances of humus into simpler compounds. Many other saprophytic bacteria cause decay of dead animal and plant material.

2.      Symbiotic bacteria: They are found associated with other living organisms. They obtain food from host without harming it. E.g. nitrogen fixing bacteria.

3.      Parasitic bacteria: They grow inside the tissue of other living organisms and obtain food at the expense of host.

4.      Autotrophic bacteria: These bacteria may be:

a.      Photosynthetic bacteria: Bacterial cell contains bacterio chlorophyll present in cytoplasm. They carryout photosynthesis.

b.      Chemosynthetic bacteria: They obtain their energy from oxidation of some inorganic substances like iron, hydrogen and sulphur compounds to synthesize their inorganic compounds.

QNo.10: Define respiration in bacteria:

Ans. On the basis of respiration bacteria can be classified into following groups:

1.      Aerobes: Require oxygen for respiration.

a.      Obligate aerobes: Bacteria which can survive only in the presence of oxygen.

2.      Anaerobes: Respire without oxygen.

a.      Obligate anaerobes: They are killed in the presence of oxygen

b.      Facultative anaerobes: They use oxygen but survive without it.

QNo.11: Write salient features of cyanobacteria (Blue green algae).

Ans.

Ø  Cyanobacteria (Blue green algae) are prokaryotes.

Ø  They may occur alone or in the form of colony.

Ø  Cell wall is double layered.

Ø  Protoplasm consists of two parts 1. Outer colored region chromoplasm 2. Inner colorless region centroplasm.

Ø  Found in fresh water, few are marine.

Ø  Total sexual reproduction is absent, only asexual reproduction is present.

Ø  Examples are Nostoc, Anabena.

QNo.12: Write a note on Nostoc.

Ans.

Ø  Taxonomic Position:

Kingdom	Prokaryotae (Monera)
Phylum	Cyanobacteria
Class	Cyanophyceae
Order	Nostocales
Family	Nostocaceae
Genus	Nostoc
Species	Nostoc commune

Ø  Structure:

·         The nostoc is filamentous. Filaments are intermixed forming a ball like structure.

·         It floats on water.

·         Each filament is unbranched chain of beads having row of rounded or oval cells. Each cell ha doubled layered wall. Outer layer is made up of cellulose and pectin while inner layer is made up of only cellulose.

·         Membranous organells of cell like endoplasmic reticulum, mitochondria, golgi bodies and vacuole are totally absent. However ribosomes, pseudo-vacuole and reserve food are present.

·         Centroplasm has central body called chromatin body consisting of DNA.

·         At interval larger, oblong, colorless and thick walled cells called Heterocyst are found. Each heterocyst is the center of nitrogen fixation.