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Microorganisms: Friend and Foe — Class 8 CBSE Science MCQs with Solutions
Free Class 8 CBSE Science Microorganisms: Friend and Foe MCQs with step-by-step solutions covering Types of microorganisms, Useful microorganisms, Harmful microorganisms, Food preservation & nitrogen cycle. Practise online on Prepizo — no login needed.
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Sample questions with solutions
Q1 — Types of microorganisms · easy · theory
Amoeba and Paramecium are examples of which group of microorganisms?
A. Protozoa ✓ Correct
B. Fungi
C. Algae
D. Bacteria
Solution: Amoeba and Paramecium are classic examples of protozoa, one of the four major groups of microorganisms.
Q2 — Types of microorganisms · easy · theory
Most microorganisms are too small to be seen with the unaided eye and can only be viewed with a:
A. Microscope ✓ Correct
B. Magnifying glass
C. Telescope
D. Pair of binoculars
Solution: Most microorganisms are too small to see with the naked eye and need a microscope; only a few, such as bread fungus, can be seen with a magnifying glass.
Q3 — Types of microorganisms · easy · theory
The four major groups of microorganisms are bacteria, fungi, protozoa and some:
A. Algae ✓ Correct
B. Viruses
C. Yeasts
D. Moulds
Solution: The four major groups are bacteria, fungi, protozoa and some algae. Viruses are microscopic too, but they are not counted among these four groups.
Q4 — Types of microorganisms · easy · theory
Chlamydomonas and Spirogyra are examples of which group of microorganisms?
A. Algae ✓ Correct
B. Protozoa
C. Fungi
D. Bacteria
Solution: Chlamydomonas and Spirogyra are named in the chapter as examples of algae.
Q5 — Useful microorganisms · easy · theory
Which bacterium multiplies in milk and converts it into curd?
A. Lactobacillus ✓ Correct
B. Rhizobium
C. Penicillium
D. Plasmodium
Solution: The bacterium Lactobacillus promotes curd formation by multiplying in milk and turning it into curd.
Q6 — Useful microorganisms · easy · theory
During baking, yeast produces which gas that makes the dough rise?
A. Oxygen
B. Carbon dioxide ✓ Correct
C. Nitrogen
D. Hydrogen
Solution: Yeast releases carbon dioxide during respiration, and these bubbles of gas make the dough rise.
Q7 — Useful microorganisms · easy · theory
The antibiotic penicillin was discovered by which scientist?
A. Louis Pasteur
B. Edward Jenner
C. Alexander Fleming ✓ Correct
D. Robert Koch
Solution: Alexander Fleming discovered penicillin in 1929 from a green mould that stopped the growth of bacteria.
Q8 — Useful microorganisms · easy · theory
Who discovered the vaccine for smallpox?
A. Edward Jenner ✓ Correct
B. Alexander Fleming
C. Louis Pasteur
D. Robert Koch
Solution: Edward Jenner discovered the smallpox vaccine in 1798.
Q9 — Useful microorganisms · easy · theory
Fermentation is the conversion of sugar into which of the following by yeast?
A. Curd
B. Alcohol ✓ Correct
C. Bread
D. Antibiotics
Solution: Fermentation is defined as the conversion of sugar into alcohol by yeast.
Q10 — Harmful microorganisms · easy · theory
Microorganisms that cause diseases are known by what special name?
A. Pathogens ✓ Correct
B. Antibodies
C. Preservatives
D. Decomposers
Solution: Disease-causing microorganisms are called pathogens. They enter the body through air, water, food or direct contact.
Q11 — Harmful microorganisms · easy · theory
The malaria-causing parasite is carried into a person by the bite of which insect?
A. Female Anopheles mosquito ✓ Correct
B. Female Aedes mosquito
C. Housefly
D. Male Anopheles mosquito
Solution: The female Anopheles mosquito carries the malaria parasite (Plasmodium) and spreads it while biting.
Q12 — Harmful microorganisms · easy · theory
Anthrax, a dangerous disease of both humans and cattle, is caused by a member of which group of microbes?
A. Bacteria ✓ Correct
B. Virus
C. Protozoa
D. Fungi
Solution: Anthrax is caused by the bacterium Bacillus anthracis, so it belongs to the bacteria group.
Q13 — Harmful microorganisms · easy · theory
The dengue virus is spread to humans by the bite of which insect?
A. Female Aedes mosquito ✓ Correct
B. Female Anopheles mosquito
C. Housefly
D. Male Aedes mosquito
Solution: The female Aedes mosquito carries and spreads the dengue virus.
Q14 — Harmful microorganisms · easy · theory
A housefly sits on garbage and excreta and then lands on our food. What is the best way to stop it from spreading disease this way?
A. Keep our food covered ✓ Correct
B. Sleep under a mosquito net
C. Boil our drinking water
D. Complete the full antibiotic course
Solution: The housefly picks up pathogens from filth and drops them on uncovered food, so keeping food covered prevents this contamination.
Q15 — Food preservation & nitrogen cycle · easy · theory
What are preservatives?
A. Bacteria that convert warm milk into curd
B. Chemicals that check the growth of microorganisms in food ✓ Correct
C. Vitamins added to increase the food value of a dish
D. Insects that carry disease-causing microbes to food
Solution: Preservatives are chemicals added to food to check the growth of microorganisms, so the food does not spoil.
Q16 — Food preservation & nitrogen cycle · easy · theory
Which of these chemicals are commonly used as preservatives in jams and squashes?
A. Penicillin and streptomycin
B. Chlamydomonas and Spirogyra
C. Sodium benzoate and sodium metabisulphite ✓ Correct
D. Lactobacillus and yeast
Solution: The chapter names sodium benzoate and sodium metabisulphite as the preservatives used in jams and squashes.
Q17 — Food preservation & nitrogen cycle · easy · theory
Pasteurisation, the process used to preserve milk, was discovered by:
A. Alexander Fleming
B. Edward Jenner
C. Robert Koch
D. Louis Pasteur ✓ Correct
Solution: Louis Pasteur discovered pasteurisation (he also discovered fermentation).
Q18 — Food preservation & nitrogen cycle · easy · theory
About what fraction of the atmosphere is nitrogen gas?
A. 78% ✓ Correct
B. 21%
C. 0.03%
D. 50%
Solution: The atmosphere is made up of about 78% nitrogen gas.
Q19 — Types of microorganisms · hard · theory
Which one of the following statements about microorganisms is correct?
A. Some microorganisms are single-celled, such as bacteria, while many algae and fungi are multicellular ✓ Correct
B. All microorganisms are single-celled and can be seen with the naked eye
C. Microorganisms live only inside the bodies of humans and animals
D. All microorganisms belong to a single group called algae
Solution: Microorganisms may be single-celled (like bacteria) or multicellular (like many algae and fungi), and they live almost everywhere, from ice-cold climates to hot springs, not only inside animals.
Q20 — Types of microorganisms · hard · theory
Which of the following correctly pairs a disease with the type of microbe that causes it?
A. Chicken pox is caused by a virus ✓ Correct
B. Malaria is caused by a bacterium
C. Typhoid is caused by a protozoan
D. Tuberculosis is caused by a fungus
Solution: Chicken pox is caused by a virus. Malaria is caused by a protozoan (not a bacterium), typhoid by a bacterium (not a protozoan), and tuberculosis by a bacterium (not a fungus).
Q21 — Types of microorganisms · hard · theory
Riya says malaria and dysentery are caused by the same group of microbes; Sameer says typhoid and tuberculosis are caused by the same group. Who is correct?
A. Both Riya and Sameer ✓ Correct
B. Only Riya
C. Only Sameer
D. Neither of them
Solution: Malaria and dysentery are both caused by protozoa, and typhoid and tuberculosis are both caused by bacteria, so both students are correct.
Q22 — Useful microorganisms · hard · theory
Ravi has a common cold and takes an antibiotic, but it does not cure him. What is the best reason for this?
A. The common cold is caused by a virus, and antibiotics do not work against viruses ✓ Correct
B. The dose of antibiotic he took was too small
C. Antibiotics only work against protozoa
D. The common cold is caused by a fungus that resists antibiotics
Solution: Antibiotics are not effective against cold and flu because these are caused by viruses.
Q23 — Useful microorganisms · hard · theory
A doctor tells a patient to finish the complete course of antibiotics and not stop as soon as he feels better. Why is completing the course important?
A. It makes the medicine taste better each day
B. Stopping early can let surviving microbes make the drug less effective later, so the full course must be taken ✓ Correct
C. It helps the body produce more carbon dioxide
D. It turns the antibiotic into a vaccine
Solution: Wrong or incomplete use of antibiotics lets microbes survive and makes the drug less effective in the future, so the full course must be completed on a doctor's advice.
Q24 — Useful microorganisms · hard · theory
Both antibiotics and vaccines protect us from microbial diseases, but they act differently. How does a vaccine work?
A. It directly kills the disease-causing microbes like an antibiotic does
B. It supplies extra nutrients to fight the disease
C. It introduces dead or weakened microbes so the body makes antibodies that stay and give future protection ✓ Correct
D. It adds preservatives that stop microbes from growing in the body
Solution: A vaccine works by introducing dead or weakened microbes, prompting the body to make antibodies that remain and protect against future infection.
Q25 — Useful microorganisms · hard · theory
Which of the following statements about useful microorganisms is correct?
A. Lactobacillus is used to make bread dough rise
B. Yeast converts milk into curd
C. Yeast produces carbon dioxide that makes dough rise ✓ Correct
D. Penicillin is used to preserve pickles
Solution: Yeast produces carbon dioxide during respiration and this gas makes the dough rise; the other statements wrongly swap the roles of these microbes.
Q26 — Useful microorganisms · hard · theory
A farmer's soil becomes more fertile because certain bacteria live in it. How do these bacteria increase the soil's fertility?
A. By fixing nitrogen from the atmosphere into compounds that plants can use ✓ Correct
B. By producing carbon dioxide in the soil
C. By making antibiotics in the soil
D. By decomposing plastic present in the soil
Solution: Some bacteria are biological nitrogen fixers that fix atmospheric nitrogen into compounds usable by plants, which raises soil fertility.
Q27 — Harmful microorganisms · hard · theory
A patient is found to be suffering from both typhoid and malaria. Which option correctly identifies the microbes responsible for each?
A. Typhoid by bacteria and malaria by protozoa ✓ Correct
B. Typhoid by virus and malaria by bacteria
C. Typhoid by protozoa and malaria by virus
D. Typhoid by bacteria and malaria by virus
Solution: Typhoid is caused by bacteria while malaria is caused by a protozoan (Plasmodium).
Q28 — Harmful microorganisms · hard · theory
A person ate food left uncovered, on which a housefly had earlier sat, and later fell ill. In spreading the disease this way, the housefly is best described as a:
A. Carrier of pathogens ✓ Correct
B. Pathogen itself
C. Preservative
D. Decomposer
Solution: The housefly does not itself cause the disease but transfers pathogens from filth onto food, so it acts as a carrier.
Q29 — Harmful microorganisms · hard · theory
Why can eating food that has been spoilt by certain microbes cause serious illness or even death?
A. Some microbes growing on the food produce toxic substances ✓ Correct
B. The food loses all of its nitrogen content
C. The microbes turn the food into antibiotics
D. The spoilt food becomes a vaccine
Solution: Certain microbes growing on food produce toxic substances (toxins) that cause food poisoning, which can be serious or even fatal.
Q30 — Food preservation & nitrogen cycle · hard · theory
What is the main reason food is preserved rather than left to spoil?
A. Microbes on spoilt food only change its colour but stay completely harmless
B. Preserved food supplies extra nitrogen that the body needs
C. Some microbes growing on food produce toxic substances that can cause serious illness ✓ Correct
D. Preservation is done mainly to make the food taste sweeter
Solution: Microbes spoiling food can produce toxic substances that cause food poisoning, so preservation stops their growth.