Class 8 Science Chapter 2-Microorganisms (Oxford CBSE)

Class 8 Science Ch 2 Microorganisms Oxford

In Class 8 Science Ch 2 Microorganisms Oxford Book, we delve into the fascinating world of Microorganisms—tiny organisms that are not visible to the naked eye but play a significant role in our everyday lives. Microorganisms are present everywhere, from the air we breathe to the food we consume, and even within our own bodies. This chapter explores the different types of microorganisms, their characteristics, and how they impact both the environment and human life.

You’ll learn about the various kinds of microbes, including bacteria, fungi, algae, protozoa, and viruses. The chapter also covers the importance of microorganisms in various fields such as medicine, agriculture, and food production. Additionally, it explains how microorganisms can both be beneficial (such as in the production of antibiotics and in fermentation) and harmful (causing diseases in plants, animals, and humans).

Understanding these microscopic organisms opens up a world of applications in science and technology, and this chapter provides the foundational knowledge to better appreciate their diverse roles in nature.

1. Objective type questions

A. Fill in the blanks with the correct words.

1. A …….(microscopic slide/eyepiece lens) is used to hold a small sample of the specimen.

Ans: A microscopic slide is used to hold a small sample of the specimen.

2. Some microbes live alone, while others grow in groups called………(specimens/colonies)

Ans: Some microbes live alone, while others grow in groups called colonies.

3………….. (Chlamydomonas /Chlorella) is a unicellular algae.

Ans: Chlorella is a unicellular algae.

4. Fermentation by yeast produces……. (water and oxygen/alcohol and carbon dioxide).

Ans: Fermentation by yeast produces alcohol and carbon dioxide.

5. Shells of……… (diatoms/Chlorella) give a gritty texture to toothpaste and helps in cleaning teeth.

Ans: Shells of diatoms give a gritty texture to toothpaste and help in cleaning teeth.

6. When a disease-causing microbe enters our body, substances called……..(antibodies/vaccines) are produced.

Ans: When a disease-causing microbe enters our body, substances called antibodies are produced.

7…………… (Bacteria/Fungus) is used in the treatment of sewage.

Ans: Bacteria is used in the treatment of sewage.

8…….(Citrus canker/Rinderpest) is a plant disease caused by a pathogen.

Ans: Citrus canker is a plant disease caused by a pathogen.

9. Food …..(cultivation/preservation) involves killing microbes or preventing their reproduction.

Ans: Food preservation involves killing microbes or preventing their reproduction.

10. In preserve them. (freezing/canning), food items are stored in air tight cans to

Ans: In canning, food items are stored in airtight cans to preserve them.

    B. Write T for the True and F for the False statements. Correct the false statements.

    1. All microorganisms are harmful and cause diseases in plants and animals.

    Ans: F – Not all microorganisms are harmful; many are beneficial and play important roles in processes like digestion, nitrogen fixation, and decomposition.

    Correction: Many microorganisms are beneficial and essential for various ecological processes.

    2. Pathogens enter the body of a healthy person through air and water only.

    Ans: F – Pathogens can enter the body through air, water, food, and other means like direct contact, insect bites, etc.
    Correction: Pathogens can enter the body through air, water, food, or direct contact.

    3. When antibiotics are injected within a human body, the body produces substances called vaccines.

    Ans: F – Antibiotics do not produce vaccines. Vaccines stimulate the immune system to produce antibodies, while antibiotics are used to treat bacterial infections.
    Correction: When vaccines are injected into the human body, the body produces substances called antibodies.

    4. In nature, nitrogen is fixed during lightning or by nitrogen-fixing bacteria and blue green algae.

    Ans: T – Nitrogen is fixed during lightning or by nitrogen-fixing bacteria and blue-green algae in nature.

    5. When plants and animals die, sulphur in their bodies is decomposed and returned to the air.

    Ans: T – When plants and animals die, sulfur in their bodies is decomposed and returned to the air.

    6. Grazing by cattle causes a disease called fin rot.

    Ans: F – Grazing by cattle does not cause fin rot; fin rot is a disease that affects fish, not cattle.
    Correction: Grazing by cattle may cause diseases like foot and mouth disease, not fin rot.

    7. Foul odour is the only symptom that shows that a food has become unfit for consumption.

    Ans: F – Foul odor is not the only symptom; there can be changes in color, texture, and taste that indicate food is unfit for consumption.
    Correction: Foul odor is one of the symptoms, but changes in color, texture, and taste also indicate spoiled food.

    8. Boiling involves removal of water from food items.

    Ans: F – Boiling does not remove water from food; it involves cooking food in water or other liquids.
    Correction: Boiling involves cooking food in water or other liquids, not the removal of water.

    9. Pasteurization involves heating a foodstuff to a high temperature and then cooling it rapidly.

    Ans: T – Pasteurization involves heating a foodstuff to a high temperature and then cooling it rapidly to kill harmful microorganisms.

    10. All diseases are communicable diseases, caused by pathogens.

    Ans: F – Not all diseases are communicable; some are non-communicable, such as heart disease or diabetes.
    Correction: Not all diseases are communicable; some are non-communicable, caused by factors like genetics or lifestyle.

      C. Choose the correct option.

      1. Which of these is a thin sheet of glass that is used to cover the sample viewed under a microscope?

      a. Slide b. Coverslip

      c. Stage d. Mirror

      Ans: b. Coverslip
      (A coverslip is a thin sheet of glass used to cover the sample viewed under a microscope.)

      2. Which of these refers to a spherical bacteria?

      a. Bacilli b. Cocci

      c. Vibrio d. Spirilia

      b. Cocci
      (Cocci refers to spherical bacteria.)

      3. Which of these is the primary causative organism for swine flu?

      a. Protozoa b. Bacteria

      c. Virus d. Spirillum

      Ans: c. Virus
      (Swine flu is caused by a virus, specifically the H1N1 influenza virus.)

      4. Which of these microorganisms is used in the manufacture of alcoholic beverages?

      a. Bacteria b. Virus

      c. Fungi d. Yeast

      Ans: d. Yeast
      (Yeast is used in the manufacture of alcoholic beverages through fermentation.)

      5. Which of these diseases can be prevented by a vaccine?

      a. Cholera and ringworm

      b. Poliomyelitis and chicken pox

      c. Malaria and ringworm

      d. All of these

      Ans: b. Poliomyelitis and chicken pox
      (Both poliomyelitis and chicken pox can be prevented by vaccines.)

      6. Which of these is not true about bacteria?

      a. They are unicellular

      b. They are of different shapes

      c. They are all harmful to us

      d. They are among the oldest organisms found on the planet

      Ans: c. They are all harmful to us
      (Not all bacteria are harmful; many bacteria are beneficial.)

      7. Which of these are microscopic plants like organisms with chlorophyll?

      a. Bacteria b. Virus

      c. Protozoa d. Algae

      Ans: d. Algae
      (Algae are microscopic plant-like organisms with chlorophyll.)

      8. Which of these found in milk coagulates to form curd?

      a. Lactobacillus b. Casein

      c. Acid in milk d. None of these

      Ans: a. Lactobacillus
      (Lactobacillus is the bacteria that helps coagulate milk to form curd.)

      9. Which of these gets formed when fermentation takes place by yeast?

      a. Alcohol b. Carbon dioxide and water

      c. Alcohol and carbon dioxide d. Oxygen and water

      Ans: c. Alcohol and carbon dioxide
      (Fermentation by yeast produces alcohol and carbon dioxide.)

      10. Which of these microbes are commonly used in sewage treatment?

      a. Bacteria b. Virus

      c. Fungi d. Algae

      Ans: a. Bacteria
      (Bacteria are commonly used in sewage treatment to break down organic matter.)

        II. Very short answer type questions

        A. Give one word for the following.

        1. An instrument that makes smaller objects look larger

        Ans Microscope

        2. The object that is viewed under a microscope

        Ans: Specimen

        3. Organisms that are visible only through a microscope

        Ans: Microorganisms

        4. The protein present in milk

        Ans: Casein

        5. The process of conversion of a sugar into an acid or an alcohol

        Ans: Fermentation

        6. Medicines that destroy certain disease-causing microbes

        Ans: Antibiotics

        7. The process by which free atmospheric nitrogen is converted into nitrogen compounds

        Ans: Nitrogen fixation

        8. A natural cyclic process in which atmospheric nitrogen enters the soil and leaves into the atmosphere

        Ans: Nitrogen cycle

        9. Disease-causing microorganisms

        Ans: Pathogens

        10. Disease caused by eating foodstuffs that can cause illness

        Ans: Food poisoning

        III. Short answer type questions

        1. What is a microscope? What does it help us to view?
        Ans: A microscope is an instrument used to view objects that are too small to be seen with the naked eye. It helps us view microorganisms, cells, and other tiny structures.

        2. What are microorganisms? Name five major groups of microbes.
        Ans: Microorganisms are tiny living organisms that are usually too small to be seen without a microscope. Five major groups of microbes are:

        Bacteria ,Fungi, Algae, Protozoa, Viruses

        3. Where do fungi derive their nutrition from?
        Ans: Fungi derive their nutrition from decomposing organic matter, either as saprophytes (feeding on dead matter) or as parasites (feeding on living hosts).

        4. What is a virus? Give two examples of viruses.
        Ans: A virus is a microscopic infectious agent that can only replicate inside the living cells of a host organism. Two examples of viruses are:

        Influenza virus

        HIV (Human Immunodeficiency Virus)

        5. What is an antibiotic?
        Ans: An antibiotic is a substance used to kill or inhibit the growth of bacteria, used to treat bacterial infections.

        6. What is a vaccine? Name two diseases that can be prevented by a vaccine.
        Ans: A vaccine is a biological preparation that provides active immunity to a specific disease. Two diseases that can be prevented by a vaccine are:

        Polio

        Measles

        7. Define nitrogen fixation.
        Ans: Nitrogen fixation is the process by which atmospheric nitrogen (N₂) is converted into nitrogen compounds like ammonia, usually by nitrogen-fixing bacteria, so that it can be used by plants.

        8. What are pathogens? How do they get spread?
        Ans: Pathogens are microorganisms that cause disease. They can spread through air, water, food, direct contact, or by vectors like insects.

        9. What are communicable diseases? Give two examples of communicable diseases in animals.
        Ans: Communicable diseases are diseases that can be transmitted from one individual to another, either directly or indirectly. Two examples of communicable diseases in animals are:

        Rabies

        Foot-and-mouth disease

        10. Define food preservation. Name any three methods of food preservation.
        Food preservation is the process of treating and storing food in a way that prevents it from spoiling. Three methods of food preservation are:

        Drying

        Freezing

        Canning

        IV. Long answer type questions

        1. Explain how microscopic objects can be viewed under a microscope.

        Ans: To view microscopic objects under a microscope, follow these steps:

        • Prepare the slide: Place the specimen (object to be viewed) on a glass slide and cover it with a coverslip.
        • Place the slide on the stage: Position the slide on the microscope’s stage and secure it with the stage clips.
        • Adjust the light source: Use the mirror or built-in light to illuminate the specimen. Adjust the light intensity using the diaphragm.
        • Start with low magnification: Use the lowest power objective lens (usually 4x or 10x) and focus the specimen using the coarse focus knob.
        • Increase magnification: Once the specimen is focused at low power, switch to higher magnification (e.g., 40x, 100x) and fine-tune the focus using the fine focus knob.
        • View the specimen: At higher magnification, the specimen will appear larger, and its details can be clearly seen.

        2. Discuss with examples some commercial uses of microorganisms.

        Ans: Microorganisms have several commercial uses in different industries:

        • Food and Beverage Industry: Yeasts are used in the production of bread, alcoholic beverages, and fermented foods like yogurt and cheese.
        • Pharmaceutical Industry: Bacteria and fungi are used to produce antibiotics, such as penicillin, which help treat bacterial infections. Microorganisms also help in producing vaccines and other medicinal products.
        • Agriculture: Nitrogen-fixing bacteria (e.g., Rhizobium) are used in crop farming to improve soil fertility by fixing nitrogen from the air.
        • Waste Treatment: Certain bacteria are used in sewage treatment plants to break down organic waste, helping purify water.
        • Biotechnology: Microorganisms like E. coli are used in genetic engineering to produce substances such as insulin.

        3. How are microorganisms useful in the field of medicine?

        Ans: Microorganisms play an important role in medicine, including:

        • Antibiotic Production: Fungi like Penicillium and bacteria like Streptomyces produce antibiotics that treat bacterial infections.
        • Vaccine Development: Viruses are used to create vaccines that protect against diseases such as measles, polio, and influenza.
        • Probiotics: Beneficial bacteria, such as Lactobacillus, are used to restore balance in the gut, promoting digestive health.
        • Enzyme Production: Microorganisms are used to produce enzymes that aid in various medical treatments, such as digestive enzyme supplements.
        • Bioremediation: Microorganisms help in cleaning up pollutants from the environment, such as oil spills, contributing to environmental medicine.

        4. With the help of a labelled diagram, describe the nitrogen cycle.

        Ans: (Since I cannot provide a diagram here, I’ll describe the nitrogen cycle, and you can imagine or draw it based on the description.)

        The nitrogen cycle is a natural process that describes how nitrogen moves through the environment:

        • Nitrogen Fixation: Atmospheric nitrogen (N₂) is converted into ammonia (NH₃) by nitrogen-fixing bacteria present in the soil or in the root nodules of leguminous plants (e.g., peas, beans). Some nitrogen is also fixed during lightning.
        • Nitrification: Ammonia is oxidized by nitrifying bacteria into nitrites (NO₂) and then into nitrates (NO₃), which plants can absorb and use.
        • Assimilation: Plants absorb nitrates from the soil and use them to synthesize proteins and nucleic acids. Herbivores consume plants and assimilate nitrogen into their bodies.
        • Ammonification: When plants, animals, and microorganisms die, decomposers (bacteria and fungi) break down organic matter, releasing ammonia into the soil.
        • Denitrification: Denitrifying bacteria convert nitrates back into nitrogen gas (N₂), which is released into the atmosphere, completing the cycle.

        5. Describe briefly diseases caused by microbes in plants.

        Ans: Several diseases in plants are caused by different types of microbes:

        • Bacterial Blight (Rice): Caused by Xanthomonas oryzae, it leads to yellowing of the leaves, stunted growth, and reduced yield.
        • Rust (Wheat): Caused by Puccinia fungi, it forms rust-colored pustules on plant leaves, leading to leaf damage and reduced crop yield.
        • Citrus Canker: Caused by the bacterium Xanthomonas citri, it causes lesions on leaves, stems, and fruit, affecting fruit production.
        • Downy Mildew (Cucurbits): Caused by the oomycete Pseudoperonospora cubensis, it results in yellowing of leaves and stunted plant growth.

        6. What is food poisoning? How do we detect whether a foodstuff is fit for consumption or not?

        Ans: Food poisoning is an illness caused by consuming contaminated food that contains harmful microorganisms, toxins, or chemicals. Symptoms include nausea, vomiting, diarrhea, and stomach cramps.

        • Detection of Spoiled Food: Food spoilage can be detected by:
          • Smell: A foul or sour odor often indicates bacterial growth or contamination.
          • Appearance: Discoloration, mold growth, or sliminess can indicate spoilage.
          • Taste: If food tastes unusual, sour, or bitter, it may have gone bad.
          • Texture: Softening, wilting, or a slimy texture can indicate microbial growth.

        7. Write a short note on some common food preservation methods.

        Ans: Food preservation methods help extend the shelf life of food by inhibiting the growth of microorganisms:

        • Refrigeration and Freezing: Cooling food slows down the growth of bacteria and molds, while freezing prevents microbial growth altogether.
        • Canning: Involves sealing food in airtight cans and heating it to kill microorganisms. Common for fruits, vegetables, and meats.
        • Drying: Removes moisture from food, preventing the growth of bacteria, yeasts, and molds. This method is common for fruits, vegetables, and meats (e.g., jerky).
        • Pickling: Involves preserving food in an acidic solution, such as vinegar, which prevents the growth of harmful microorganisms.
        • Salting: Adding salt to food helps draw out moisture, which inhibits microbial growth. Common for fish and meats.
        • Fermentation: Uses microorganisms to produce beneficial bacteria that preserve food, such as in yogurt, cheese, and sauerkraut.

        These methods help keep food safe, nutritious, and extend its shelf life.

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