Microorganisms Class 8th Science Term 1 Tamilnadu Board Solution

Class 8th Science Term 1 Tamilnadu Board Solution

Question 1.

Choose the best answer:

a) Algal bloom leads to loss of species diversity which is known as _______. (Eutrophication, Pasteurisation)

b) Which one of the following cause amoebic dysentery? (Entamoeba histolytica, Plasmodium vivax, Plasmodium falciparum)

c) Milk is heated to 72o C for 30 minutes and then suddenly cooled to12o C. Microbes are killed without causing damage to the taste and quality of milk. Name this process (dehydration, pasteurisation)

d) Nodules are responsible for nitrogen fixation. Name the bacteria present inside the root nodules. (Acetobactor, Rhizobium, Clostridium)


a) Eutrophication

In some conditions, the algae produce thick material masses, called 'bloom' which covers the upper surface of a water body (usually standing water like pond, lake) like a carpet. This cuts off the oxygen supply to water under the bloom and thus decreasing oxygen content, leading to the death of aquatic species. This is called Eutrophication.

b) Entamoeba histolytica

Entamoeba histolytica is a protozoan, which contaminated food and water easily, if proper hygienic conditions are not maintained. Thus, the microorganisms can enter our alimentary canal and damage it, causing amoebic dysentery. In this, one experiences pain in abdomen and blood in stools.

c) Pasteurisation

Louis Pasteur developed the method of preservation of food (mainly milk) by process of heating at specified temperature for a given duration of time. The technique was thus named Pasteurisation.

d) Rhizobium

Nitrogen plays an important role in life processes, specially for plants. As plants cannot take in nitrogen directly, they are helped by some bacteria for this purpose. Rhizobium are present inside the root nodules and help in fixing the atmospheric nitrogen for the plant.

Question 2.

Living organisms show a great degree of diversity in their size. A considerable number of species are not visible to naked eye. They can only be seen with the help of a microscope. How will you measure the size of micro organisms?


Due to very small size, some organisms are not visible to naked eye and can only be seen under a microscope. Such organisms are thus called microorganisms. To measure them, units such as micron (10^-6 m, or tenth lac fraction of one metre), or millimicron (10^-9) are used.

Question 3.

Fungi do not possess chlorophyll. Hence, they are incapable of photosynthesis. So, they depend on either living organisms or non-living things. Name the two types of modes of nutrition in fungi.


Because of inability to synthesise their own food, fungi have adopted two modes of nutrition:-

i) When they depend on living organisms, it is called parasitic mode and fungi are called parasites. They use a living organism as host and eat the living tissues using their enzymes. This is harmful for the host and causes disease or illness.

ii) When they depend on non-living plants and animals, they act as saprophytes. The feed on organic substance which is soluble. By this mode they are helpful in decomposing the dead and decaying organisms and maintaining the flow of energy in ecosystem.

Question 4.

Communicable diseases are pathogenic diseases which are spread from person to person either directly or indirectly. Write the mode of transmission of the following diseases.

i) Cholera, typhoid

ii) Malaria


i) Cholera and typhoid are diseases caused by bacteria and spread through contaminated water, specially when there is absence of proper hygiene, or the water with standing water and it’s surroundings are not kept clean.

ii) Malaria is a disease caused by protozoan called Plasmodium, but spreads through vectors. Vectors are organisms , e.g. mosquitoes, that spread the disease from an infected person to a healthy individual (by , say mosquito bites) .

Question 5.

The conversion of sugar solution into alcohol and liberation of carbon dioxide is known as fermentation. Which microorganism is responsible for this process?


Fungi are responsible for fermentation. The sugar solution is converted to alcohol and carbon dioxide is released. The sugar solution is said to be fermented and gives out a typical odour. The process of fermentation is used for the production of wine and other alcoholic drinks.

Question 6.

i) Identify the following bacteria based on its shape.

ii) Draw and label the following parts of the bacteria.

a) Flagella b) pili c) cell wall cytoplasm cell membrane


i) Spherical or circle shaped – cocci

Rod shaped – bacilli

Spiral shaped – spirillium

Comma shaped - vibrio

Question 7.

In the world today many industries totally depend upon the microbial activities.

For example, oxalic acid is the fermented product of fungi Aspergillus niger.

Name any two bacteria used in industry with their product.


The role of microorganisms in industries is increasing day by day. Some industrially useful microorganisms are:-

• Bacillus megaterium- This bacterium is used for curing. Curing is the process where tea - tobacco leaves and coffee beans are fermented to give out the flavour.

• Acetobacter acetic - this fungi is used for the fermentation of sugar solution for the manufacturing of vinegar, which acts as a good preservative.

Question 8.

Farmers consider microorganisms such as bacteria, fungi and virus a menace on their fields and gardens because they cause disease to their crops. Name any one of the bacterial disease, viral disease and fungal disease.