NCERT Class 8 Science Chapter 2 - Microorganisms: Friend and Foe

Chapter 2 - Microorganisms: Friend and Foe

Question and Answers
Question 1: Can microorganisms be seen with naked eye? If not, how can they be seen?
Answer: No, microorganisms cannot be seen with naked eye. This is because these microbes are too small. Some of these microbes can be seen with the help of magnifying glass. Others can be seen with the help of microscope.

Question 2: What are the major groups of microorganisms?
Answer: The major groups of microorganisms include
• Bacteria
• Fungi
• Protozoa
• Algae
• Virus

Question 3: Define
a) Antibiotics - The medicines which kill or stop the growth of the disease-causing microorganisms are called antibiotics.

b) Fermentation - The yeast reproduces in sugar solution and converts it into alcohol. This process is called fermentation.

Question 4: Write 10 lines on the usefulness of microorganisms in our lives.
Answer:
• Making curd from the milk - The bacterium called Lactobacillus reproduces in milk and helps to convert milk into curd.
• Making alcohol - The fungus called yeast reproduces in sugar solution and converts it into alcohol. This process is called fermentation.
• Making bread - The fungus called yeast reproduces in flour dough and produces carbon dioxide during respiration which makes dough soft and helps in making bread, cakes, biscuits, pastries etc.
• Making medicines - Some bacteria and fungi are used to make medicines which kill or stop the growth of disease causing microorganisms. Such medicines are called antibiotics. Example: Streptomycin, Tetracycline, Erythromycin etc.

Question 5: What are antibiotics? What precautions must be taken while taking antibiotics?
Answer: The medicines which kill or stop the growth of the disease-causing microorganisms are called antibiotics. It is important to remember that antibiotics should be taken only on the advice of a qualified doctor. Also we must complete the course prescribed by the doctor. If we take antibiotics when not needed or in wrong doses, it may make the drug less effective when we might need it in future. Also antibiotics taken unnecessarily may kill the beneficial bacteria in the body. Antibiotics, however, are not effective against cold and flu as these are caused by viruses.

Question 6: Define
a) Communicable Diseases - Microbial diseases that can spread from an infected person to a healthy person through air, water, food or physical contact are called communicable diseases. Example: cholera, common cold, chicken pox and tuberculosis.

b) Pathogens - The microorganisms which cause disease in human, plants and animals are called pathogens.

Question 7: Write a short paragraph on the harms caused by microorganisms.
Answer: Some microorganisms cause diseases in plants and animals. Such disease causing microorganisms are called pathogens. Some micro organisms grow in food substances and produce toxic substances and makes the food poisonous. Food poisoning causes illness and even death. Some microorganisms spoil materials like clothing, leather, wood etc.

Question 8: Define:
a) Preservatives - A preservative is a substance that is added to food substances or beverages to prevent decomposition of microbes and to avoid chemical changes taking place undesirably.

b) Pasteurisation - The process of heating milk to about 70°C for 15 to 30 seconds and then suddenly chilling it to prevent the growth of microbes is called Pasteurization.

Question 9: Name the microorganisms which can fix atmospheric nitrogen in the soil.
Answer: Rhizobium, blue green algae are the microbes which can fix atmospheric nitrogen in the soil.