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The Invisible Living World: Beyond Our Naked Eye – Class 8 Science notes, important questions and MCQs

NCERT Class 8 Science, chapter 2. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Microorganisms are living things too small to see without a microscope. Some are useful, some cause disease, and some spoil food.

Types and uses

Bacteria, fungi (yeast, moulds), protozoa, algae and viruses.
Useful: Lactobacillus turns milk into curd; yeast makes bread rise and makes alcohol (fermentation); antibiotics like penicillin come from moulds.
Rhizobium in legume roots fixes nitrogen; microbes decompose waste.

Harmful microbes and safety

Diseases: cholera, typhoid (bacteria); malaria (protozoa, spread by female Anopheles mosquito); common cold, COVID-19 (viruses).
Vaccines train the body to fight specific germs.
Food preservation: refrigeration, salting, sugar, pickling with oil and vinegar, boiling, pasteurisation.

Key terms

Microorganism: a living thing visible only under a microscope
Fermentation: conversion of sugar into alcohol and CO₂ by yeast
Antibiotic: medicine that kills or stops bacteria
Pasteurisation: heating milk to about 70 °C and cooling quickly to kill germs

Formulas

Sugar —yeast→ alcohol + CO₂

Important questions with answers

Name the five main groups of microorganisms.2 marks

Bacteria, fungi, protozoa, algae and viruses.

Give three ways in which microorganisms are useful.3 marks

Lactobacillus turns milk into curd; yeast is used in baking and making alcohol; microbes make antibiotics (penicillin from a fungus), fix nitrogen in soil and decompose waste.

Explain how food is spoiled by microbes and describe five methods of food preservation with examples.5 marks

Microbes grow on food and produce toxins, bad smell and taste. Preservation: (1) Salting – pickles, fish. (2) Sugar – jams, jellies (remove water). (3) Oil and vinegar – pickles (microbes cannot grow). (4) Heating and cooling – boiling milk, refrigeration. (5) Pasteurisation – heating milk to about 70 °C for 15–30 seconds and cooling quickly. Also airtight packing and chemical preservatives like sodium benzoate.

Quick revision

Lactobacillus forms curd.
Malaria is caused by a protozoan (Plasmodium).
Vaccines prevent diseases.

Exam tips

Antibiotics do not work against viruses (cold, flu).
Louis Pasteur discovered fermentation; Alexander Fleming discovered penicillin.

Practice MCQs

Curd is formed from milk by
1. yeast
2. Rhizobium
3. Lactobacillus
4. virus
Show answer
Answer: (c) Lactobacillus. Lactic acid bacteria.
Penicillin was discovered by
1. Louis Pasteur
2. Edward Jenner
3. Alexander Fleming
4. Robert Koch
Show answer
Answer: (c) Alexander Fleming. In 1928.
Malaria is caused by a
1. protozoan
2. virus
3. bacterium
4. fungus
Show answer
Answer: (a) protozoan. Plasmodium.
Bread dough rises because yeast produces
1. carbon dioxide
2. oxygen
3. nitrogen
4. water
Show answer
Answer: (a) carbon dioxide. CO₂ bubbles.
Yeast makes bread dough rise by releasing
1. oxygen
2. nitrogen
3. carbon dioxide
4. hydrogen
Show answer
Answer: (c) carbon dioxide. Fermentation gas.
Why are antibiotics not given for the common cold?
1. antibiotics are costly
2. cold is caused by bacteria
3. cold is caused by a virus
4. cold is not a disease
Show answer
Answer: (c) cold is caused by a virus. Antibiotics do not act on viruses.
Pasteurised milk is heated to about
1. 70 °C and cooled quickly
2. 30 °C
3. 150 °C
4. 0 °C
Show answer
Answer: (a) 70 °C and cooled quickly. Pasteurisation.
Rhizobium in the root nodules of legumes
1. causes disease
2. makes curd
3. fixes atmospheric nitrogen
4. produces antibiotics
Show answer
Answer: (c) fixes atmospheric nitrogen. Natural fertiliser.

All chapters of Class 8 Science

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.