Microorganisms: Friend and Foe
Welcome to HSLC Guru! In this article, we present a complete English-medium study guide for Class 8 General Science Chapter 2 — Microorganisms: Friend and Foe, designed strictly according to the ASSEB (Assam State School Education Board) syllabus. This chapter introduces the tiny living organisms that surround us, exploring how some help humans through fermentation, medicine and soil enrichment, while others cause disease in humans, animals and plants. Use this guide to revise the summary, master textbook questions, practise additional MCQs and refer to the glossary for quick recall before your examinations.
Chapter Summary
Microorganisms (also called microbes) are extremely tiny living organisms that cannot be seen with the naked eye and require a microscope for observation. They are classified into four major groups — bacteria, fungi, protozoa and some algae. Viruses are also microscopic but they reproduce only inside the host organism. Microorganisms live almost everywhere — in soil, water, air, hot springs, deep oceans, icy mountains and even inside the bodies of plants and animals. They can survive in extreme conditions where most other organisms cannot live.
Microorganisms are friends to humans in many ways. Yeast is used in the preparation of bread, cake, pastries and alcoholic drinks through the process of fermentation. Lactobacillus bacteria convert milk into curd. Bacteria and fungi are used in the production of cheese, vinegar, pickles and many other food items. Antibiotics such as penicillin (discovered by Alexander Fleming from the fungus Penicillium) are produced from microbes and used to cure many diseases. Vaccines are also prepared using weakened or dead microbes to protect us from infectious diseases. Rhizobium bacteria, found in the root nodules of leguminous plants, fix atmospheric nitrogen and increase soil fertility. Microbes also act as decomposers, breaking down dead plants, animals and waste into simple substances that enrich the soil.
Some microorganisms are harmful and act as foes by causing diseases known as communicable diseases. In humans they cause tuberculosis, cholera, typhoid, measles, chickenpox, polio, malaria, AIDS, common cold and many more. They spread through air, water, food, contact and insect carriers (vectors) such as the female Anopheles mosquito (malaria) and the female Aedes mosquito (dengue). In animals, the bacterium Bacillus anthracis causes the dangerous disease anthrax, and foot-and-mouth disease in cattle is caused by a virus. In plants, microbes cause diseases such as citrus canker, rust of wheat and yellow vein mosaic of bhindi, leading to heavy losses in agriculture. Harmful microbes also spoil food by producing toxic substances; food poisoning caused by such toxins can be very serious.
To protect food from spoilage, several food preservation methods are used. Common methods include salting (meat, fish, raw mangoes), sugar preservation (jams, jellies, squashes), oil and vinegar (pickles), heat and cold treatment (boiling, refrigeration, deep freezing), pasteurisation of milk (heating to about 70°C and sudden cooling, discovered by Louis Pasteur), dehydration (removing moisture) and the use of chemical preservatives like sodium benzoate and sodium metabisulphite. Microbes are also vital in maintaining the nitrogen cycle in nature: nitrogen-fixing bacteria convert atmospheric nitrogen into nitrates, plants absorb nitrates to make proteins, animals get nitrogen by eating plants, and decomposers return nitrogen back to the atmosphere, keeping the percentage of nitrogen in air constant at about 78%.
Textbook Questions and Answers
1-Mark Questions
Q1. What are microorganisms?
Answer: Microorganisms or microbes are extremely small living organisms that cannot be seen with the naked eye and can only be observed through a microscope.
Q2. Name the four major groups of microorganisms.
Answer: The four major groups of microorganisms are bacteria, fungi, protozoa and some algae.
Q3. Which microorganism is used for the preparation of curd?
Answer: The bacterium Lactobacillus is used for the preparation of curd from milk.
Q4. Name the scientist who discovered penicillin.
Answer: Penicillin was discovered by Alexander Fleming in 1929 from the fungus Penicillium notatum.
Q5. What is fermentation?
Answer: Fermentation is the process in which yeast or other microbes convert sugar into alcohol and carbon dioxide.
Q6. Name the bacterium that fixes atmospheric nitrogen in the soil.
Answer: Rhizobium, found in the root nodules of leguminous plants such as gram, pea and beans.
Q7. Which mosquito acts as the carrier of malaria?
Answer: The female Anopheles mosquito is the carrier of the malarial parasite Plasmodium.
Q8. What is pasteurisation?
Answer: Pasteurisation is the process of heating milk to about 70°C for 15-30 seconds and then suddenly cooling it to kill harmful microbes. It was discovered by Louis Pasteur.
Q9. What is a vaccine?
Answer: A vaccine is a preparation of dead or weakened microbes that, when introduced into the body, makes the body produce antibodies to fight against disease-causing microorganisms.
Q10. Name two diseases caused by viruses in humans.
Answer: Common cold, influenza, polio, measles, chickenpox and AIDS are diseases caused by viruses (any two).
2-3 Mark Questions
Q1. Where do microorganisms live?
Answer: Microorganisms live almost everywhere on the earth. They are found in air, water, soil, on and inside the bodies of plants and animals, in food, in hot springs, in deep oceans, in polar ice and even on rocks. They can survive in extreme conditions of temperature and dryness because some form protective coats called cysts.
Q2. What is the role of microorganisms in the preparation of bread?
Answer: Yeast is added to the flour used for making bread. Yeast multiplies rapidly and produces carbon dioxide gas during fermentation. The gas fills the dough and makes it rise, increasing its volume. This makes the bread soft, spongy and light when baked.
Q3. What are antibiotics? Give two examples.
Answer: Antibiotics are medicines produced from certain microorganisms that kill or stop the growth of disease-causing microbes. Examples include penicillin, streptomycin, tetracycline and erythromycin. Antibiotics should always be taken on the advice of a qualified doctor and the full course must be completed.
Q4. How are microbes useful in cleaning the environment?
Answer: Microbes act as natural decomposers. They break down the dead bodies of plants and animals, animal waste and garbage into simple substances such as carbon dioxide, water and nutrients. This decomposition recycles materials, removes waste from the environment and increases the fertility of the soil.
Q5. Differentiate between communicable diseases and non-communicable diseases with examples.
Answer: Communicable diseases are diseases caused by microorganisms that spread from an infected person to a healthy person through air, water, food or contact. Examples: cholera, tuberculosis, measles, chickenpox. Non-communicable diseases do not spread from one person to another; they are caused by lifestyle, deficiency or hereditary factors. Examples: diabetes, cancer, scurvy.
Q6. Why should we not use antibiotics unnecessarily?
Answer: Antibiotics taken unnecessarily or in wrong doses may kill the useful bacteria of our body and cause side effects. Microbes may also develop resistance, making the medicine less effective in future. Antibiotics do not work against viruses, so taking them for a viral cold is useless. They should always be taken on the advice of a doctor and the full prescribed course must be completed.
5-6 Mark Questions
Q1. Describe the useful role of microorganisms in our daily life.
Answer: Microorganisms are very useful to humans in the following ways:
- Preparation of curd and cheese: Lactobacillus converts milk into curd; bacteria are used in cheese-making.
- Preparation of bread, cake and alcohol: Yeast carries out fermentation, producing carbon dioxide and alcohol.
- Production of antibiotics: Penicillin, streptomycin and tetracycline are obtained from fungi and bacteria.
- Vaccine production: Vaccines for polio, smallpox, measles and rabies are prepared using microbes.
- Increasing soil fertility: Rhizobium and blue-green algae fix atmospheric nitrogen in the soil.
- Cleaning the environment: Microbes decompose dead organic matter and recycle nutrients.
- Biogas and sewage treatment: Bacteria are used to produce biogas and to clean sewage water.
Q2. Explain the harmful effects of microorganisms with examples in humans, animals and plants.
Answer: Microorganisms cause many diseases in living organisms.
- In humans: Bacteria cause tuberculosis, cholera and typhoid; viruses cause common cold, measles, chickenpox, polio and AIDS; the protozoan Plasmodium causes malaria; Entamoeba causes amoebic dysentery.
- In animals: The bacterium Bacillus anthracis causes anthrax in cattle and humans; foot-and-mouth disease in cattle is caused by a virus.
- In plants: Microbes cause citrus canker (bacteria), rust of wheat (fungus) and yellow vein mosaic of bhindi (virus), reducing crop yield.
- Food spoilage: Microbes spoil food and may produce toxins that cause food poisoning.
Q3. What is food preservation? Describe any four common methods of food preservation.
Answer: Food preservation is the process of treating and storing food so that it does not spoil and remains safe for a long time. Common methods are:
- Salting: Salt is added to meat, fish, raw mangoes and amla to prevent the growth of microbes.
- Sugar preservation: A high concentration of sugar in jams, jellies and squashes reduces moisture and stops microbial growth.
- Use of oil and vinegar: Microbes cannot live in oil and vinegar, so pickles last for a long time.
- Heat and cold treatment: Boiling kills microbes, while refrigeration and deep freezing slow down their growth. Pasteurisation is used to preserve milk.
- Dehydration: Removing moisture from food (sun-drying of fish, vegetables and grains) prevents microbes from growing.
- Chemical preservatives: Sodium benzoate and sodium metabisulphite are used in jams, sauces and squashes.
Q4. Explain the nitrogen cycle with the help of a description.
Answer: The atmosphere contains about 78% nitrogen, which plants and animals cannot use directly. The nitrogen cycle describes how nitrogen is circulated between the atmosphere, soil and living organisms.
- Nitrogen fixation: Bacteria such as Rhizobium in the root nodules of leguminous plants and blue-green algae convert atmospheric nitrogen into nitrates.
- Absorption by plants: Plants absorb these nitrates from the soil through their roots and use them to make proteins.
- Transfer to animals: Animals get nitrogen by eating plants or other animals.
- Decomposition: When plants and animals die, decomposer microbes break down their bodies and release nitrogen compounds back into the soil.
- Denitrification: Denitrifying bacteria convert these compounds back into atmospheric nitrogen, completing the cycle.
Thus, the percentage of nitrogen in the atmosphere remains almost constant.
Q5. How are diseases spread by microorganisms? Mention preventive measures.
Answer: Disease-causing microorganisms (pathogens) spread in several ways:
- Through air: Diseases like tuberculosis, common cold and measles spread through droplets when an infected person coughs or sneezes.
- Through water: Cholera, typhoid and hepatitis spread through contaminated drinking water.
- Through food: Eating spoiled or uncovered food may cause diarrhoea and food poisoning.
- Through contact: Diseases such as chickenpox and skin infections may spread by direct contact or by sharing belongings.
- Through carriers (vectors): Mosquitoes and houseflies act as carriers; the female Anopheles mosquito spreads malaria and the female Aedes mosquito spreads dengue.
Preventive measures: Maintain personal hygiene, drink boiled or filtered water, eat fresh and covered food, keep surroundings clean, use mosquito nets and repellents, get vaccinated against major diseases and consult a doctor when ill.
Additional Practice Questions
Multiple Choice Questions (MCQs)
Q1. Which of the following is not a microorganism?
(a) Bacteria (b) Fungi (c) Mango tree (d) Protozoa
Answer: (c) Mango tree.
Q2. The disease caused by a protozoan is —
(a) Tuberculosis (b) Malaria (c) Polio (d) Anthrax
Answer: (b) Malaria.
Q3. Yeast is used in the preparation of —
(a) Sugar (b) Bread (c) Curd (d) Cheese
Answer: (b) Bread.
Q4. The bacterium that fixes nitrogen is —
(a) Lactobacillus (b) Rhizobium (c) Bacillus anthracis (d) Streptococcus
Answer: (b) Rhizobium.
Q5. Which scientist discovered penicillin?
(a) Louis Pasteur (b) Edward Jenner (c) Alexander Fleming (d) Robert Koch
Answer: (c) Alexander Fleming.
Q6. The carrier of dengue fever is —
(a) Female Anopheles (b) Female Aedes (c) Housefly (d) Cockroach
Answer: (b) Female Aedes.
Q7. Pasteurisation of milk is done at about —
(a) 30°C (b) 50°C (c) 70°C (d) 100°C
Answer: (c) 70°C.
Q8. Anthrax disease in cattle is caused by —
(a) A virus (b) A fungus (c) A bacterium (d) A protozoan
Answer: (c) A bacterium.
Q9. Which of the following is a viral disease?
(a) Cholera (b) Typhoid (c) AIDS (d) Tuberculosis
Answer: (c) AIDS.
Q10. The percentage of nitrogen in the atmosphere is approximately —
(a) 21% (b) 78% (c) 0.03% (d) 50%
Answer: (b) 78%.
Fill in the Blanks
Q1. Microorganisms can be seen with the help of a __________.
Answer: microscope.
Q2. Microorganisms which cause diseases are known as __________.
Answer: pathogens.
Q3. __________ is used in the production of bread and cake.
Answer: Yeast.
Q4. The female __________ mosquito is the carrier of malaria.
Answer: Anopheles.
Q5. The process of converting sugar into alcohol by yeast is called __________.
Answer: fermentation.
True or False
Q1. Viruses can multiply on their own without a host.
Answer: False.
Q2. Antibiotics are effective against viral diseases like the common cold.
Answer: False.
Q3. Pasteurisation is used to preserve milk.
Answer: True.
Q4. Rhizobium is found in the root nodules of leguminous plants.
Answer: True.
Q5. Salting is a method of food preservation.
Answer: True.
Glossary
| Term | Meaning |
|---|---|
| Microorganism | A very small living organism that can be seen only with a microscope. |
| Bacteria | Single-celled microorganisms found everywhere; some useful, some harmful. |
| Fungi | A group of microbes such as yeast and moulds; some cause diseases, others used in food and medicine. |
| Protozoa | Single-celled microbes; some cause diseases like malaria and dysentery. |
| Virus | A microscopic agent that reproduces only inside the host cell and causes diseases like AIDS and polio. |
| Fermentation | Conversion of sugar into alcohol and carbon dioxide by yeast. |
| Antibiotic | Medicine produced from microbes that kills disease-causing bacteria. |
| Vaccine | Preparation of dead or weakened microbes used to develop immunity. |
| Pathogen | A microorganism that causes disease. |
| Communicable disease | A disease that can spread from one person to another through air, water, food or contact. |
| Vector | An animal carrier (like a mosquito) that spreads pathogens. |
| Pasteurisation | Heating milk to about 70°C and cooling rapidly to kill microbes; discovered by Louis Pasteur. |
| Preservation | The process of treating food so that it does not spoil for a long time. |
| Nitrogen fixation | Conversion of atmospheric nitrogen into nitrates by bacteria like Rhizobium. |
| Decomposer | A microbe that breaks down dead plants and animals into simple substances. |
| Rhizobium | A nitrogen-fixing bacterium living in root nodules of leguminous plants. |
| Anthrax | A serious disease in cattle and humans caused by Bacillus anthracis. |
| Rust of wheat | A fungal disease that affects wheat plants. |