NCERT Solutions for Class 8 Science Chapter 2 Microorganisms Friend And Foe

Q6.Name the microorganisms which can fix atmospheric nitrogen in the soil.

Answer :

Rhizobium, Azotobacter, blue green algae(cyanobacteria)

EXPLANATION:

Nitrogen fixation is the process by which atmospheric nitrogen is converted into ammonia and different nitrogen oxides. These nitrogenous compounds are used by plants for the synthesis of plant proteins and other compounds.

Ammonia is present in soil, is an important source of nitrogen for plants. Nitrogen promotes plant growth and improves fruit and seed production, resulting in a greater yield. Ammonia is a basic building block for ammonium nitrate fertilizer, which releases nitrogen for the growth of plants. Nitrogen supplied to the plants by ammonia , is a major component of chlorophyll by which plants use sunlight energy to produce glucose from water and carbondioxide.

Plants that are provided with a sufficient amount of soil nitrogen and nitrogen dioxide (NO2) has an increase in the uptake of nutrients, photosynthesis, and nutrient metabolism. Nitrogen oxide (NO) is involved in controlling cell differentiation and lignification, root and shoot development, flowering, growth and reorientation of pollen tubes, senescence and maturation, stomatal movement, plant-pathogen interactions and programmed cell death.

Q.1 Fill in the blanks.
(a) Microorganisms can be seen with the help of a ____________.
(b) Blue green algae fix __________ directly from air and enhance fertility of soil.
(c) Alcohol is produced with the help of __________.
(d) Cholera is caused by __________

Answer :

(a) Microorganisms can be seen with the help of a microscope.
(b) Blue green algae fix nitrogen directly from air and enhance fertility of soil.
(c) Alcohol is produced with the help of yeast.
(d) Cholera is caused by bacteria (Vibrio cholerae).

EXPLANATION:-

(a) A microscope is a laboratory instrument which is used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. The image of an object is magnified through lens in the microscope.

(b) Blue-green algae are also known as Cyanobacteria. They normally look green and sometimes turn bluish when they are dying. They are a group of prokaryotic, autotrophic microorganisms that contain the photosynthetic pigments (chlorophyll and phycocyanin). They can convert inert atmospheric nitrogen into an organic form, such as nitrate or ammonia. These fixed forms of nitrogen are used by plants for their growth.

(c) The primary role of yeast is to convert glucose into ethanol and carbondioxide. Ethanol is released by the yeast cells as a waste product.

(d) Cholera is an acute diarrheal illness caused by infection of the intestine with Vibrio cholerae bacteria. People get sick when they swallow contaminated food or water. Severe watery diarrhea accompanied by vomiting can quickly lead to dehydration and even death if untreated.

Q.2 Tick the correct answer. (a) Yeast is used in the production of
(i) sugar (ii) alcohol (iii) hydrochloric acid (iv) oxygen
(b) The following is an antibiotic
(i) Sodium bicarbonate (ii) Streptomycin (iii) Alcohol (iv) Yeast
(c) Carrier of malaria-causing protozoan is
(i) female Anopheles mosquito (ii) cockroach (iii) housefly (iv) butterfly
(d) The most common carrier of communicable diseases is
(i) ant (ii) housefly (iii) dragonfly (iv) spider
(e) The bread or idli dough rises because of
(i) heat (ii) grinding (iii) growth of yeast cells (iv) kneading
(f) The process of conversion of sugar into alcohol is called
(i) nitrogen fixation (ii) moulding (iii) fermentation (iv) infection

Answer :

a) (ii) alcohol
b) (ii) Streptomycin
c) (i) female Anopheles mosquito
d) (ii) housefly
e) (iii) growth of yeast cells
f) (iii) fermentation

EXPLANATION:-

a) Yeast is a microorganism, made up of just a single cell. Yeast cells grow through a process called budding in which a small yeast cell grows on the outside of a mature one, until it is fully grown and ready to separate. The primary role of yeast is to convert glucose into ethanol and carbondioxide. Ethanol is released by the yeast cells as a waste product.

b) Streptomycin is an antibiotic medication used to treat a number of bacterial infections. This includes tuberculosis, Mycobacterium avium complex, plague, rat bite fever etc. Dosage is based on the kind of infection, weight of the infected individual, medical condition of the individual, streptomycin blood levels, and the side effects.

c) Malaria is transmitted to humans by female mosquitoes of the genus Anopheles. Thus female Anopheles mosquito acts as the malaria vector. Female mosquitoes take blood for their egg production.

d) House flies can spread diseases such as food poisoning and dysentery. They contaminate food and surfaces by defecating on them.

e) Yeast cells grow through a process called budding in which a small yeast cell grows on the outside of a mature one, until it is fully grown and ready to separate.

f) The process of conversion of sugar into alcohol is called fermentation. This fermentation is carried out by the yeast.

Q.3 Match the organisms in Column A with their action in Column B.
A
I) Bacteria
II) Rhizobium
III) lactobacillus
IV) yeast
V) A protozoan
VI) A virus
B
a) fixing nitrogen
b) setting of curd
c) baking of bread
d) causing malaria
e) causing cholera
f) causing AIDS
g) producing antibodies

Answer :

A

i) Bacteria
ii) Rhizobium
iii) lactobacillus
iv) yeast
v) A protozoan
vi) A virus

B

e) causing cholera
a) fixing nitrogen
b) setting of curd
c) baking of bread
d) causing malaria
f) causing aids

EXPLANATION:-

1. Cholera is a bacterial disease which is spread through contaminated food and water. Cholera causes severe diarrhea and dehydration.

2 . Rhizobium is a bacteria which is associated with the formation of root nodules in plants. These bacteria live in a symbiotic relationship with legumes. They obtain nitrogen from the atmosphere and pass it on to the plant, thus allowing it to grow with proper metabolism.

3 . Lactobacillus is a rod-shaped, gram-positive, non-spore forming bacteria which helps in setting curd. Lactobacilli are very good for gut health. They produce lactic acid, which prevents many harmful bacteria from colonizing and infecting the intestines.

4 . Yeast is used for the leavening (rising the dough) of bread. Yeast uses glucose and oxygen in dough to produce more yeast cells and thus many yeast cells increases the production of carbon dioxide. Thus the carbon dioxide makes the dough rise which gives the bread a light and spongy texture.

5 . Malaria is caused by the protozoa Plasmodium which acts as a parasite. This protozoa is spread to the human through the bites of infected mosquitoes. Female anopheles mosquito carries the protozoa and while having its blood meals, it transfers this protozoa into human blood which lives inside human body as a parasite and later causes malaria.

6 . Acquired immunodeficiency syndrome (AIDS) is a chronic, potentially life-threatening condition caused by the human immunodeficiency virus (HIV). By damaging the immune system, HIV interferes with the body's ability to fight infection and disease.

Q.4 Can microorganisms be seen with the naked eye? If not, how can they be seen?

Answer :

Microorganisms cannot be seen with the naked eye because they are very small. They can be seen with the help of a microscope. It is an instrument that produces enlarged images of small objects, thus allowing the observer to see an exceedingly close view of minute structures at a scale convenient for examination and analysis.

A simple microscope consists of a single lens that magnifies the object. A compound microscope consists of two lens system and is used for better magnification. The magnifying power of a microscope is an expression of the number of times, the object being examined appears to be enlarged.

Q.5 What are the major groups of microorganisms?

Answer :

Major group of microorganisms are as follows

1) Bacteria:

Bacteria are small single-celled organisms. Their cell structure is simpler than that of other organisms as there is no nucleus or membrane bound organelles. Hence they are also called as prokaryotic organisms. Bacteria are classified into five groups according to their basic shapes: spherical shaped, rod shaped, spiral shaped, comma shaped and corkscrew shaped. They can exist as single cells, in pairs, chains or as clusters. They are found in every habitat like in soil, rock, oceans and even arctic snow. Some species can live under extreme conditions of temperature and pressure. Some live as host in other organisms including plants, animals and humans. They play an important role in the cycling of nutrients. Some species cause food spoilage and crop damage while others are incredibly useful in the production of fermented foods.

2) Fungi:

Fungi are eukaryotic organisms as their cells contain membrane-bound organelles and well defined nuclei. Fungi are the species of organisms, which includes the yeasts, rusts, smuts, mildews, molds, and mushrooms. They are of great environmental and medical importance. Many fungi are free-living in soil or water while others form parasitic or symbiotic relationships with plants or animals.

3) Protozoan:

A group of single-celled eukaryotes, either free-living or parasitic, which feed on organic matter such as other microorganisms or organic tissues and plant debris. It consists of the single-celled microscopic organisms which include amoebas, flagellates, ciliates, sporozoans, and many other forms. They live in a wide variety of moist habitats including fresh water, marine environments and the soil.

4) Algae:

Algae are a diverse group of aquatic organisms that have the ability to conduct photosynthesis. They play ecological roles as oxygen producers and as the food base for almost all aquatic life. They are economically important as a source of crude oil and as sources of food and a number of pharmaceutical and industrial products for humans. The study of algae is called phycology,

5) Virus:

A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all types of life forms, from animals and plants to microorganisms, including bacteria. They invade living normal cells and use those cells to multiply and produce other viruses like themselves. They are found in almost every ecosystem on earth and are the most numerous type of biological entity. The study of viruses is known as virology.

Q.7 Write 10 lines on the usefulness of microorganisms in our lives.

Answer :

Micro-organisms are too small to be seen through naked eyes. However, they are a crucial part of our ecosystem in the following ways:-


1) Alcoholic fermentation by yeast is widely used in the preparation of wine and bread.

2) Microbes are helpful in reducing pollution. They act as natural decomposers by breaking these complex substances like dead plants, animals and their excreta into simpler substances, which can be absorbed by plants. Hence they play a key role in recycling of nutrients.

3) They are also used to increase the soil fertility by fixing the atmospheric nitrogen with the help of Rhizobium, Azotobacter, blue green algae etc., thus enriching the soil with nitrogen content.

4) They also play an important role in the preparation of medicines. Antibiotics are chemicals produced by micro-organisms to kill bacteria. For e.g., Streptomycin is an antibiotic obtained from Streptomyces griseus.

5) Certain microbes are also used in the biological treatment of sewage and industrial effluents.

6) Bacteria present in our intestine helps in proper digestion. Gut bacteria controls diabetes, obesity, depression and colon cancer.

7) A bacterium called Lactobacillus promotes the formation of curd.

8) They are also used as preservatives for food items. For e.g., pickles

9)Processing and ageing of cheese is carried out by microorganisms.

10) Spirulina is a type of cyanobacteria which is extremely high in many nutrients. It is available in capsules, tablets, and powder and has been incorporated in certain foods and beverages such as energy bars etc

Q.8 Write a short paragraph on the harmful effects of microorganisms.

Answer :

Microorganisms cause diseases in both plants and animals. A microbe which is capable of causing infectious disease in an animal or plant is called a pathogen. Four groups of microbes contain pathogens: bacteria, fungi, protozoa and the viruses.

BACTERIA:

1. Bacteria causes toxicity as it produce toxins which can damage specific tissues in the body.
2. Bacteria shows invasiveness as it multiplies rapidly at the site of infection and overwhelm the body's defense mechanisms.
3. Bacteria can cause diseases in humans, such as cholera, diphtheria, dysentery, bubonic plague, pneumonia, tuberculosis (TB), typhoid, food poisoning etc.

FUNGI:

1. Fungi create harm by spoiling food, destroying timber, and by causing diseases of crops, livestock, and humans.
2. Penicillium and Aspergillus, spoil many stored foods.
3. Fungi cause the majority of plant diseases, which in turn cause serious economic losses.

PROTOZOA:

1. It causes many deadly human diseases such as sleeping sickness, amoebic dysentery, and malaria.
2. They also cause disease in plants. Phytomonas, a type of protozoa that is transmitted as durable zoospores that survive in a resting state in the soil for many years and further they transmit plant viruses.

VIRUS:

1. Some viruses can cause lifelong or chronic infections where the viruses continue to reproduce in the body despite the host's defense mechanisms. For e.g., AIDS
2. Most plant viruses are transmitted by insect vectors that cause damage to the plant.
3. It causes chickenpox, flu (influenza), herpes etc., in humans while it causes tobacco mosaic virus(TMV) and many other diseases in plants.

Though algae doesn't cause diseases but it also harms our environment and aquatic life in many ways:

ALGAE:

1. Algal blooms can reduce the ability of fish and other aquatic life to find food and can cause entire populations to leave an area or even die.
2. Nutrient pollution fuels the growth of harmful algal blooms which have negative impacts on aquatic ecosystems.

Q.9 What are antibiotics? What precautions must be taken while taking antibiotics?

Answer :

Chemical agents that kill or stop the growth of disease-causing microorganisms are called antibiotics.
It is a type of antimicrobial substance that is active against bacteria.
It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of such infections.
They may either kill or inhibit the growth of bacteria.
They are either produced in nature by soil bacteria and fungi or are synthesized artificially using the products obtained from bacteria and fungi by the process of fermentation.

Antibiotics should be taken under the supervision of a well-qualified doctor.
One must complete the course prescribed by the doctor.
They should be taken in the right amount and at the right time as prescribed.