NCERT Exemplar for Class 8 Science Solutions Chapter 2 Microorganisms : Friend and Foe

1. Which of the following reproduces only inside a host cell?  

(a) Bacteria 

(b) Virus 

(c) Amoeba 

(d) Fungus

Ans: (b) Virus 

Bacteria, protists, and fungi can reproduce by cell division without the requirement of a host body. In contrast, viruses are considered both living and non-living since they act like non-living organisms outside the host but reproduce inside the host, thus behaving like living organisms.

2. A disease in human beings caused by a virus is 

(a) typhoid 

(b) influenza

(c) dysentery 

(d) cholera   

Ans: (b) influenza

Influenza (also known as the flu) is a viral infection caused by the influenza virus. Diphtheria, cholera, and typhoid are all bacterial infections caused by Vibrio cholerae, Corynebacterium diphtheriae, and Salmonella typhi, respectively. 

3. Pathogenic microorganisms present in host cells are killed by medicines called

(a) pain killer 

(b) antibodies  

(c) antibiotics 

(d) vaccines 

Ans: (c) antibiotics 

Antibiotics are chemical compounds that can slow or stop the growth of pathogenic bacteria. They inhibit the bacteria from reproducing inside the host's body. Consumption of antibiotics kills the microorganisms that cause infection in host bodies.

4. The two microorganisms which live in symbiotic association in lichens are

(a) fungus and protozoa 

(b) alga and bacteria 

(c) bacteria and protozoa 

(d) alga and fungus 

Ans: (d) alga and fungus 

Lichens are symbiotic relationships between fungi (mycobionts) and photosynthetic partners (photobionts), which might be algae or bacteria (blue-green algae). Algae are autotrophic as they have chlorophyll and perform photosynthesis. Fungi are heterotrophic as they are dependent on others for their nutrient requirements. Algae provide food to the fungus. In return, the fungus provides water, minerals, and shelter to the algae. 

5. The gas released during the preparation of bread is 

(a) oxygen 

(b) carbon dioxide 

(c) nitrogen 

(d) sulphur dioxide

Ans: (b) carbon dioxide

Carbon dioxide gas is released due to the fermentation process conducted by yeast cells. Yeast converts sugars present in flour into carbon dioxide and ethanol (ethyl alcohol). Alcohol evaporates due to the heat of the oven, whereas carbon dioxide gas is trapped in the dough, which is released during the preparation of bread, which makes the bread dough fluffy. 

6. The disease caused by a protozoan and spread by an insect is __________.

(a) dengue 

(b) malaria 

(c) polio 

(d) measles 

Ans: (b) malaria 

Malaria is a mosquito-borne infectious disease caused by a plasmodium (protozoa) parasite. The primary host is mosquitoes (female Anopheles) and the secondary or intermediate host is humans. Dengue, Polio and Measles are all viral diseases that are caused by the dengue virus, poliovirus, and rubeola virus, respectively.  

7. Paheli dug two pits, A and B, in her garden. In pit A, she put a polythene bag packed with some agricultural waste. In pit B, she dumped the same kind of a waste but without packing it in a polythene bag. She then covered both the pits with soil. What did she observe after a month?  

(a) Waste in pit A degraded faster than that in pit B.

(b) Waste in pit B degraded faster than that in pit A. 

(c) Waste in both pits degraded almost equally.  

(d) Waste in both pits did not degrade at all.

Ans: (b) Waste in pit B degraded faster than that in pit A. 

Pit B's waste degraded faster than Pit A's because it was held open, and as a result, it absorbed oxygen far faster than Pit A. Oxygen helps in the decomposition of organic waste by microbes. The plastic in pit A prevented the proper flow of air. As a result, microbial degradation did not occur in pit A.

VERY SHORT ANSWER QUESTIONS 

8. Unscramble the jumbled words underlined in the following statements.  

(a) Cells of our body produce a santiidobe to fight pathogens.  

Ans: Antibodies 

Antibodies are proteins that are produced by our immune system in response to disease-causing microbes and remain in the body to fight against further infection caused by the same pathogens.

(b) Curbossulite is an air-borne disease caused by a bacterium.

Ans: Tuberculosis 

Tuberculosis (TB), a potentially serious infectious bacterial disease that mainly affects the lungs, is caused by a bacterium Mycobacterium tuberculosis. 

(c) Xanrhat is a dangerous bacterial disease? 

Ans: Anthrax

Anthrax is a serious bacterial disease caused by Bacillus anthracis, a spore-forming bacteria.

(d) Yeasts are used in the wine industry because of their property of meronettinaf.

Ans: Fermentation  

Fermentation is the process by which yeast converts sugars (such as glucose) into alcohol and releases carbon dioxide gas.

9. Suggest a suitable word for each of the following statements.

(a) Chemicals are added to food to prevent the growth of microorganisms.  

Ans: Preservatives 

Chemicals added to food are known as preservatives. These preservatives prevent microbial growth without altering the taste or appearance of food. These preservatives are usually added to jams, jellies, ketchup, etc.

(b) Nitrogen-fixing microorganisms present in the root nodules of legumes.

Ans: Rhizobium 

Rhizobium are the nitrogen-fixing bacteria, present in the root nodules of leguminous plants such as clover, alfalfa, beans, peas, and soybeans. Rhizobium fixes the atmospheric nitrogen into that form (i.e. nitrates, nitrates) which is usable for plants.

(c) The agent spreads pathogens from one place to another.  

Ans: Carrier/Vector 

Vector, also called a carrier, helps to transmit pathogens from one organism to another organism.

(d) Chemicals which kill or stop the growth of pathogens.  

Ans: Antibiotics 

Antibiotics are chemical compounds that can slow or stop the growth of pathogenic bacteria. They inhibit the bacteria from reproducing inside the host's body. Consumption of antibiotics kills the microorganisms that cause infection in host bodies.

10. Match the names of scientists given in Column A with the discovery made by them given in Column B. 

Ans: The correct matching for the given questions is as follows-

11. Name one commercial use of yeast.  

Ans: Yeast is the microorganism that is used for the large-scale production of alcohol.

12. Name the process in yeast that converts sugars into alcohol.

Ans: Fermentation is the process by which yeast converts sugars (such as glucose) into alcohol and releases carbon dioxide gas.

13. In the soil, which nutrient is enriched by blue-green algae (cyanobacteria)? 

Ans: In the soil, nitrogen gets enriched by nitrogen-fixing bacteria called cyanobacteria or blue-green algae. They convert atmospheric nitrogen to nitrites or nitrates that can be absorbed by the plants.

14. Why should we avoid standing close to a tuberculosis patient while he/she is coughing? 

Ans: Tuberculosis (TB), a potentially serious infectious bacterial disease that mainly affects the lungs, is caused by the bacterium Mycobacterium tuberculosis. A patient who is affected by tuberculosis may transfer the bacteria while coughing because it's an air-borne disease and spreads from person to person through tiny droplets released into the air via coughs and sneezes. Any healthy body may suffer from tuberculosis disease after coming into contact with this bacterium. That's why one should avoid standing near any tuberculosis patient.

15. Polio drops are not given to children suffering from diarrhoea. Why?  

Ans: Polio (poliomyelitis) is a life-threatening disease caused by the poliovirus. The Polio drop, which is an oral vaccine, is not given to children suffering from diarrhoea because it may be excreted from the body as a result of frequent motion. Diarrhoea is a disease that causes frequent motions in the human body. As a result, the polio drop won't be effective in fighting against the poliovirus due to diarrhoea. 

16. Paheli watched her grandmother making a mango pickle. After she bottled the pickle, her grandmother poured oil on top of the pickle before closing the lid. Paheli wanted to know why oil was poured. Can you help her understand why?

Ans: The oil poured on the tip of pickles while bottling forms a layer of protection between the air and the pickle from the microorganisms present in the air. The use of oil (or chemical preservatives) prevents the spoilage of pickles because bacteria cannot survive in such an environment. This increases the life of the pickle and doesn't let the bacteria destroy the pickle by attacking it.

SHORT ANSWER QUESTIONS

17. Match the microorganisms given in Column A to the group to which they belong in Column B. 

Ans: 

18. Classify the following into friendly and harmful microorganisms. Yeast, malarial parasite, Lactobacillus, bread mold, Rhizobium, Bacillus anthracis.

Friendly                               Harmful 

________                              ________

________                              ________

________                              ________

Ans:  Friendly microorganisms- Microorganisms which support human life are known as friendly or useful microorganisms.

Yeast- Yeasts are commercially used in the food industry. It helps in the preparation of bread loaf, and alcoholic beverages such as wine and beer.

Lactobacillus- Lactobacillus (Lactic Acid Bacteria) helps to make curd from milk.

Rhizobium- Rhizobium is a nitrogen-fixing bacteria present in the root nodules of leguminous plants and helps in fixing atmospheric nitrogen into nitrates and nitrites for plants use.  

Harmful microorganisms- These kinds of organisms don't help human life and cause several diseases in humans, plants, and animals, sometimes leading to even death.

Malarial parasite- Malaria parasite present in the saliva of female Anopheles mosquito causes malaria by entering in host bodies.

Bacillus anthracis- Bacillus anthracis, a rod-shaped bacterium, causes the deadly disease anthrax in livestock and occasionally in humans.

Bread mould- Bread mold (Rhizopus) spoils the bread and when this spoiled bread is consumed it can cause food poisoning.

19. While returning from the school, Boojho ate chaat from a street hawker. When he reached home, he felt ill and of stomach ache and fell ill. What could be the reason? 

Ans: This could be the result of chaat that might have been contaminated with bacteria, viruses, parasites, or toxins (pathogenic microbes). Often, street foods get contaminated by pathogenic or disease-causing microorganisms. The unhygienic conditions near the shop attract flies and other microbial organisms, which help pathogens grow. Sometimes the utensils that are used for serving food are also contaminated by pathogens. When someone eats these contaminated foods, he/she will suffer from some deadly and painful diseases like dysentery, cholera, diarrhoea, etc.

20. What will happen to 'pooris' and 'unused kneaded flour ' if they are left in the open for a day or two? 

Ans: The "unused kneaded flour," if kept open for a long time in warm conditions, may get contaminated by microbes which may cause fermentation of the dough. Thus, these microbes may spoil the taste and/or texture of food. But it's not necessarily true for pooris as they are deep-fried in oil, which may kill the microbes. As a result, they may last longer than the kneaded flour. 

21. (a) Name two diseases that are caused by viruses.  

Ans: Chicken Pox and Influenza are two viral diseases caused by the Varicella-zoster virus and Myxovirus influenzae virus, respectively.

(b) Write one important characteristic of a virus.   

Ans: A virus remains in a dead condition, but as soon as it comes into contact with a living organism, it becomes alive. That's why viruses are known as the connecting links between living and non-living things. They have no effect until they enter a host cell. 

LONG ANSWER QUESTIONS 

22. Observe Fig.2.1 and answer the questions that follow

(Image will be uploaded soon)

(a) Write the name of the disease.  

Ans: The name of the given plant is okra (lady’s finger) and the disease is yellow vein mosaic of lady’s finger, which is characterized by alternating yellow and green patches and chlorosis in the veins of the leaves.

(b) Name the causative agent of this disease?  

Ans: The causative agent of the disease is the yellow vein mosaic virus.

(c) How does the disease spread from one plant to another?  

Ans: The disease spreads from one plant to another by carriers which are flies and other insects. They sit on any diseased plant and then transfer it to other plants.

(d) Name any two plant diseases and the microbes that cause them.  

Ans: 

• Citrus canker is caused by bacterium-Xanthomonas axopodia.  

• Rust of Wheat is caused by Puccinia triticina which is a fungal disease. 

23. How do vaccines work? 

Ans: Vaccines contain disease-specific microbial strains that have been weakened or killed. When a vaccine is introduced into a healthy body, either orally or through injection, the body fights against the infection by producing suitable antibodies against the pathogen. These antibodies remain in the body, and the body also remembers how to fight when the same microbe enters the body again.

24. Observe the set-up given in Fig.  2.2 and answer the following questions.

(a) What happens to the sugar solution in A?  

Ans: Yeast causes fermentation and converts the sugar into alcohol (ethyl alcohol or ethanol) and carbon dioxide gas is released. 

(b) Which gas is released in A?  

Ans: During the process of fermentation, carbon-di-oxide gas is released from flask A.

(c) What changes will you observe in B when the released gas passes through it?

Ans: As soon as the carbon-di-oxide gas passes through the flask B, the lime water present in it turns milky.  

25. Observe Fig.2.3 and answer the following questions.

(a) Name the microorganism and the group to which it belongs.  

Ans: The given picture is of bread mold, which is also known as Rhizopus. Rhizopus/ bread mold belongs to the fungi group.

(b) Name the food item on which the organism grows.  

Ans: The organism Rhizopus grows on stale and moist bread.

(c) Does it grow well in dry or in moist conditions? 

Ans: The organism grows well in moist conditions.

(d) Is it safe to eat infected bread?

Ans: No, it's not safe to eat infected bread because the harmful toxins produced by bread mold can spoil the bread and its consumption can lead to food poisoning in the human body.

26. Give reasons for the following. 

(a) Fresh milk is boiled before consumption while processed milk stored in packets can be consumed without boiling.

Ans: Boiling fresh milk is necessary to kill the harmful or disease-causing microorganisms present in it. On the other hand, processed milk is stored in packets and is safe to consume directly because it has been pasteurized. Pasteurized milk is boiled at 70 degrees Celsius and then cooled rapidly. As a result of pasteurization, all the microorganisms die and the milk becomes consumable.

(b) Raw vegetables and fruits are kept in refrigerators whereas jams and pickles can be kept outside.  

Ans: The microorganisms present in the open atmosphere easily spoil the vegetable by attacking it. Refrigerators work at low temperatures, which inhibit the growth of microorganisms. On the other hand, jams and pickles contain preservatives such as sugar and salt, which save them from the attack of microorganisms and give them a long life. 

(c) Farmers prefer to grow beans and peas in nitrogen-deficient soils.  

Ans: Leguminous plants such as peas and beans have nitrogen-fixing bacteria called Rhizobium. Rhizobium present in the root nodules of leguminous plants helps in fixing atmospheric nitrogen and transforming it into nitrates and nitrites for plants' use. After harvesting, the roots are left in the soil, which results in the enrichment of the soil with nitrogen and increases its fertility. Therefore, farmers prefer to grow peas and beans in nitrogen-deficient soil.

(d) Mosquitoes can be controlled by preventing stagnation of water though they do not live in water. Why?  

Ans: Water stagnation is the process in which clean water gets collected in a small area. Although mosquitoes live on land, they lay eggs in clean water, and the stagnant water provides a breeding ground for them. Their larvae grow in clean water. Therefore, by preventing stagnation of water, we can prevent mosquitoes from breeding (mating). As a result, mosquitoes can be controlled. 

27. How can we prevent the following diseases?  

(a) Cholera 

Ans: Cholera- Cholera is a bacterial disease caused by a bacterium called Vibrio Cholerae. A person can get cholera by drinking water or eating food contaminated with the bacterium Vibrio Cholerae. We can prevent cholera by maintaining personal hygiene and good sanitation, drinking boiled water, and eating properly cooked food.

(b) Typhoid 

Ans: Typhoid- Typhoid is a bacterial disease caused by the bacterium Salmonella Typhi. A person can get infected when they ingest the typhoid bacterium by any means, such as food, water, etc. To prevent this disease, one should drink boiled or filtered water and eat properly cooked hygienic food.

(c) Hepatitis

Ans: Hepatitis- Hepatitis is a viral disease caused by the hepatitis virus. The symptoms of this disease appear after two weeks of infection. To prevent this disease, infants should be given vaccinations. Apart from this, proper sanitation practices and hygiene can prevent this disease. 

28. Complete the following cycle given as Fig.  2.4 by filling the blanks (Al, (b), (c), (d).

Ans: The above cycle is the Nitrogen cycle, a biogeochemical cycle that occurs in the environment. The Nitrogen which is present in the atmosphere cannot be consumed directly by trees and plants. There are some bacteria and blue-green algae (Cyanobacteria) present in the soil which fixes atmospheric nitrogen and transforms it into nitrates and nitrites for plant use. Below are the necessary missing components which help in fixing atmospheric nitrogen are: -

(a) Nitrogen is fixed by lightning.

(b) Blue-green algae (cyanobacteria) and nitrogen-fixing bacteria fix the atmospheric nitrogen.  

(c) Death and decaying organisms excrete nitrogenous waste.

(d) Bacteria transform compounds of nitrogen into a gaseous state by the process of denitrification.

Microorganisms are Classified as follows:

• Bacteria are a kind of prokaryotic organism that lacks a membrane-bound nucleus and organelles and makes up most microorganisms. They may live in dirt, water, and every other habit on the planet. They can be as small as a few micrometers long and come in various forms, including spheres, rods, and spirals. Escherichia coli, coliform bacteria, and other bacteria are examples.

• Fungi are eukaryotic organisms (those with a membrane-bound nucleus and organelles) that are often heterotrophic. Penicillium, bread mold, mushroom, and more examples

• Protozoa are a category of unicellular eukaryotic creatures that are heterotrophic (cannot make their food) in nature and can be parasitic or self-sustaining. Amoeba, paramecium, and other organisms are examples.

• Algae are a type of photosynthetic eukaryotic creature that is mainly found in water. Spirogyra, Chlamydomonas, and other bacteria are examples.

The virus is a microbe, but unlike the others in this category, it can only multiply in a host’s body, which can be animals, plants, or people. AIDS, herpes, rubella, zika, and other diseases are examples.

What is the Ecosystem of Microorganisms?

Microorganisms can be single-celled or unicellular, such as bacteria and protozoa, or multicellular, such as fungi and mammals. They can survive in water, air, and various other environments, including severe cold, hot springs, deserts, and marshy regions. Some even survive within the bodies of animals, including humans. Some thrive on the bodies of other creatures, while others develop on their own.

Four Types of Microorganisms are:

Bacteria 

Bacteria are unicellular prokaryotic microorganisms.

Some bacteria are beneficial to humans, while others are potentially hazardous.

Bacillus, Vibrio, Cocci, and Spirilla are the four primary kinds.

Fungi

Fungi are eukaryotic creatures that include yeasts, moulds, and mushrooms, as well as other microbes. These creatures belong to the fungus kingdom. The creatures in the Kingdom Fungi have a cell wall and are widely distributed. Among living creatures, they are categorized as heterotrophs.

Fermentation

Fermentation is a metabolic process in which sugar is transformed into acids, gases, or alcohol. Curd and alcoholic beverages are both made by fermentation. Fermentation takes place in yeast and bacteria, as well as in mammalian muscles. It's a glucose-breaking anaerobic route. Cellular respiration is the respiration that occurs at the cellular level in our bodies. It can happen when there isn't enough oxygen in the air or when there isn't enough. Glycolysis produces pyruvic acid, a 3-carbon molecule resulting from any sort of cellular respiration. Fermentation is one of the essential methods by which different cells deal with pyruvate.

Use of microorganisms in an environmentally friendly manner:

Microorganisms can be used in various ways, including the production of alcohol, bread, cake, and medications, as well as agriculture and environmental cleanup.

Making of Curd and Bread:

Lactobacillus, a bacteria, grows in milk and turns it into curd. Bacteria are also utilized to make cheese, pickles, and a wide range of other foods. Bacteria and yeast aid in the fermentation of sugar to alcohol, which is necessary for the rice batter to make idlis and dosa. When yeast is introduced to a dough, it quickly reproduces, creating carbon dioxide that causes the dough to rise. This feature of yeast is exploited in the production of bread, cakes, and other baked goods.

Vaccine

When a pathogen, a disease-causing microbe, enters our bodies, antibodies are produced against it. These antibodies remain in our systems and protect us from sickness indefinitely. A vaccine is a biological substance that causes the body to produce antibodies in response to a specific illness. As a result, immunization protects us from that particular illness. Many illnesses, including polio, TB, chickenpox, hepatitis, and others, are routinely vaccinated in children. This method has been used to eradicate smallpox. The Covid vaccination is presently doing the same.

Harmful Uses of Microorganisms:

They may affect humans, plants, and animals in a variety of ways and cause illnesses. Pathogens are disease-causing microbes, as previously stated.

1. Disease-Causing Microorganisms in Humans

• Human infections enter the body through the air, water, or food they consume. They can then infect others by contact with an infected person, an animal, or a bug.

• Infectious illnesses can transmit from an infected person to a healthy person by air, water, or touch. Plague, chickenpox, TB, the common cold, and the current Covid 19 infections are some examples.

• When disease-causing bacteria are conveyed by insects or animals, they are referred to as disease carriers. The female anopheles mosquito, for example, is the carrier of Plasmodium, the parasite that causes malaria.

• The prevention of these diseases follows a general pattern of isolating the infected person and his belongings, maintaining personal hygiene, following proper sanitary practices, drinking boiled and clean water, controlling mosquito breeding sites, eating properly cooked food, and, above all, vaccination against all of these diseases regularly.

2. Disease-Causing Microorganisms in Animals

Several of the bacteria are also responsible for animal illnesses. Listed below are a handful of the illnesses that may be seen in animals.

3. Disease-Causing Microorganisms in Plants

Several bacteria cause illnesses in potato, rice, wheat, sugarcane, and other plants, lowering the quantity and quality of agricultural yields. Chemicals that eliminate these microorganisms can be used to manage certain disorders.

4. Food Poisoning