Monday, July 11, 2011

Class IX, BIOLOGY, "Virus, Bacteria and Cyano Bacteria"


A large number of living things are present in this world. Some of them are large and some are small. Majority of the organisms are so small that they re not seen with naked eyes. For their observation, we need a light microscope or even an electron microscope. These microscopic organisms are called micro-organisms.

Micro-organisms As a Heterogeneous Group

Micro-organisms are a heterogeneous group. It includes different kinds of organism viruses, bacteria, cyanobacteria, protozoa, certain algae and some fungi. On the basis of structure they range from sub-cellular to cellular for example, viruses are sub-cellular and all other micro-organisms are cellular. Bacteria, and cyanobacteria are prokaryotes (without nucleus) where as algae, fungi and protozoa are eukaryotes (with nucleus). On the basis of mode of nutrition algae are autotrophic while fungi and protozoa are heterotrohic. Therefore, micro-organisms differ in their structure and mode of characteristics of viruses, they are studied in a separate group where as bacteria and cyanobacteria, being prokaryotes, are included in kingdom Monera.


1. Virus is a Latin word which means “Poison”. Viruses are so small that they can only be seen with electron microscope.
2. Viruses have charcteristics of both living and non-living things.
3. Structurally they are not like, cell and are only made up of proteins and nucleic acids.
4. When they enter the body of any living organisms, they reproduce there like living organism.
5. They look like non-living crystals when they are out of the body of a living organism.
6. That is why they are placed between living and non-living things.
7. All viruses are parasites and cause different diseases in their hosts.
8. Viruses were discovered by Iwanowsky in 1892 from infected tobacco leaves. In 1935 W.M. Stanley isolated viruses in crystalline form from infected leaves of tobacco and observed them under electron microscope.

Size of Virus

Viruses are of different sizes. Their size varies from 0.01um to 0.03um(um is micrometer = 1/10,00,000 meter)

Shape of Virus

Viruses are of different shapes some are rounded, few are rod shaped, few polyhedral while some viruses look like tadpoles.

Structure of Virus

Viruses have same biochemical nature. In spite of their different shapes, they are made up of only two parts, an outer “coat”, and an inner “core”. The core is made up of DNA or RNA (never both) and the coat is made of protein. The outer protein coat determines the shape of viruses. e.g. in bacteriophage (virus that lives in bacteria) protein coat consists of two parts, head and tail. DNA is present in the head region but the tail has only protein. Most of the animal viruses contain DNA whereas plant viruses have RNA core bacteriophage is also called phage virus.

Viral Diseases in Plants

Ring spot in different plants, yellow in sugar beet and mosaic disease in tobacco, potato, tomato, bean and cabbage are the various diseases of plants, caused by viruses.

Viral Diseases in Animals

Mouth and foot disease in cattle and cowpox in horses, buffalo and cows are caused by viruses.

Viral Diseases in Humans

In human beings, viruses produce common cold, influenza, small pox, yellow fever, polio, infectious hepatitis, cancer and AIDS.

Ways of Viral Transmission

1. Through droplets produced during coughing and sneezing.
2. Through contact.
3. By air, contaminated water and food.
4. Through insects.
5. By reuse of already used syringes.
6. By un-sterilized surgery equipments.


Bacteria are found every where in air, water, living and dead bodies of organisms and even in glaciers and hot springs. These are unicellular micro-organisms.

Discovery of Bacteria

Leeuwenhoek discovered bacteria in 1697 for the first time. Later, Louis Pasteur and Robert Koch worked on them. They discovered that bacteria produce many diseases in men and animals.

Size of Bacteria

Bacteria (singular : bacterium) range from 1um to 10um in length and from 0.2um to 1um in width and can be observed under light microscope.

Types of Bacteria

On the basis of shape and form, bacteria are of four types. These are as follows:
1. Rounded – Cocci (singular; coccus)
2. Rod-like – Bacilli (singular; bacillus)
3. Spiral shaped – Spirilla (singular; spirillum)
4. Comma like – Vibrios (singular; vibrio)
Bacteria occur both singly and in colonies. Cocci bacteria are found in groups of two or four, or in irregular groups and even in the form of long beads. Baccilli are found singly or may join end to end to form long threads. But Spirilla and Vibrios occur singly.

Structure of Bacteria

1. Bacteria are single celled prokaryotic organisms.
2. Bacterial cell is surrounded by a cell wall which is made of carbohydrates and amino acids.
3. Some bacteria have an additional slime capsule around their cell wall, which protects them and prevents them from drying.
4. Ribosomes help in synthesis of proteins. Nucleus is absent in bacterium. However, only a single large circular molecule of DNA is present which is surrounded by a clear zone of cytoplasm. It is known as nucleoid. This is not bounded by a nuclear membrane.
5. In addition to main bacterial DNA small, circular molecules of DNA called plasmids are also found. Plasmids play an important role in transmission of some heredity characteristics. Plasmids are also used a vectors in genetic engineering.
6. Motile (which can move) bacteria like bacilli are spirilla have one or more thread like flagella (singular; flagellum) which help them in their locomotion. Non motile bacteria like cocci lack flagella.

Economic Importance of Bacteria

It is generally thought that bacteria are fatal and harmful organisms and there is no beneficial aspect. But this is wrong impression. There are number of bacteria which are not only beneficial for mankind but are also essential for living system. Bacteria play very important role in the life of living organisms.

Beneficial Bacteria

Ecological Importance
These, along with fungi, help to decompose dead organisms and their refuse into simpler substances replenishing the raw materials in the soil and atmosphere and can thus purify the environment.

Bacteria and Nitrogenous Compounds in Soil
These bacteria are called nitrogen fixing bacteria. Another kind of bacteria live in the soil, called nitrifying bacteria which convert ammonia into nitrite and then to nitrate, enhancing the amount of nitrogen in the soil. In this way fertility of soil is increased.

Industrial and Commercial Purposes
1. These are used in manufacturing butter, cheese and yogurt.
2. These are used in processing of commercial fibers, leather, coffee, tobacco and vinegar.

Bacteria Synthesize Enzymes
Bacteria synthesize cellulose enzyme in the stomach of herbivore animals which helps in the digestion of food. Some bacteria also synthesize vitamin “B” and “K” in the large intestine of man and other mammals.

Bacteria as Bio-Insecticides
Recently the use of bacteria in bio-insecticides has become popular.

Harmful Bacteria

1. Bacterial decomposition on one hand is beneficial but on other hand causes damage to food, wood, clothes and other things.
2. Denitrifying bacteria in soil decrease the amount of nitrogen in soil and reduce the soil fertility. These are called identifying bacteria.
3. Many bacteria are harmful and cause many diseases in plants, such as canker disease in citrus fruits, rot and fire blight in peach, pear and apple, and potato scab in potato.
4. In animal like cattle bacteria cause T.B and anthrax. Bacteria also cause many diseases in man like T.B, Whooping Cough, Diphtheria, Typhoid, Pneumonia, Tetanus, Plague, Bacterial Dysentery, Cholera, Leprosy etc.

Ways of Bacteria Transmission

1. Whooping cough, Diphtheria, T.B and Pneumonia causing bacteria are transmitted from one person to other person through sneezes and cough droplets released in air.
2. Bacteria causing Typhoid and Cholera, are transmitted from one organism to another through contaminated water and food.
3. Plague and bacterial dysentery read through vectors like flies and animals.


1. Cyanobacteria are also called blue green algae. They are simplest living organisms which have the ability to manufacture their own food by photosynthesis.
2. Structurally they resemble bacteria. Bacteria and Cyanobacteria are prokaryotes and they are placed in kingdom Monera.
3. Generally Cyanobacteria are found in moist places like of trees, rocks and soil, fresh water and oceans.
4. Some of them are symbionts and some are epiphytes.
5. Cyanobacteria are usually unicellular and solitary.


A common example of cyanobacteria which has filamentous structure which is found in the form a ball is called Nostoc.

Characteristics of Nostoc

The important characteristics of Nostoc are:
1. It has a filamentous structure which form a ball like structure of Nostoc.
2. It floats on water.
3. Each filament of Nostoc is unbranched and has a single row of rounded or oval cells.
4. Each cell of Nostoc has double layered wall.
5. The protoplasm is differentiated into two parts.
6. Endoplasmic reticulum, mitochondria, golgi bodies and vacuoles are not present in the structure of Nostoc.
7. Heterocyst are found which help in nitrogen fixation.
8. Nostoc is an autotroph like other Blue-green-Algae.

Taxonomic Position of Nostoc

According to new classification, Nostoc belongs to kingdom prokaryota or Monera.

Structure of Nostoc

The structure of Nostoc is filamentous. The filaments are interring mixed in agelatinuous mass forming a ball like structure. It floats on water. A single filament looks like a chain of beads. Each filament is unbranched and has a row of rounded or oval cells

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