Microbes — Homi Bhabha Class 9 Biology MCQs with Solutions
Free Homi Bhabha Class 9 Biology Microbes MCQs with step-by-step solutions (16 questions). Part of Classification of Living Organisms. Practise online on Prepizo — no login needed.
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Questions with solutions
Q1 — Microbes · medium · theory
Organisms too small to be seen without a microscope are called:
A. vertebrates
B. microorganisms (microbes) ✓ Correct
C. mammals
D. angiosperms
Solution: Microorganisms are tiny living things visible only under a microscope.
Q2 — Microbes · medium · theory
Which of the following is used in the formation of curd from milk?
A. a fern
B. an alga
C. Lactobacillus bacteria ✓ Correct
D. a virus
Solution: Lactobacillus bacteria ferment milk sugar into lactic acid, forming curd.
Q3 — Microbes · hard · theory
A microbe that can reproduce only inside a living host cell is a:
A. fungus
B. protozoan
C. virus ✓ Correct
D. bacterium
Solution: Viruses are non-living outside a host and can multiply only inside a living cell.
Q4 — Microbes · medium · theory
Yeast is a microorganism used in the baking of bread because it produces:
A. nitrogen gas
B. oxygen gas
C. hydrogen gas
D. carbon dioxide gas ✓ Correct
Solution: Yeast ferments sugars, releasing CO₂ that makes the dough rise.
Q5 — Microbes · hard · theory
The antibiotic penicillin is obtained from a:
A. protozoan
B. green plant
C. virus
D. fungus (Penicillium) ✓ Correct
Solution: Penicillin, the first antibiotic, is produced by the mould Penicillium.
Q6 — Microbes · medium · theory
Which disease is caused by a protozoan and spread by mosquitoes?
A. Tuberculosis
B. Tetanus
C. Malaria ✓ Correct
D. Common cold
Solution: Malaria is caused by the protozoan Plasmodium, transmitted by the female Anopheles mosquito.
Q7 — Microbes · hard · theory
Nitrogen-fixing bacteria such as Rhizobium, found in the root nodules of legumes, help by:
A. producing oxygen
B. converting atmospheric nitrogen into a usable form ✓ Correct
C. destroying the plant
D. causing disease
Solution: Rhizobium fixes atmospheric nitrogen into compounds plants can absorb, improving soil fertility.
Q8 — Microbes · medium · theory
Microorganisms that break down dead plants and animals, returning nutrients to the soil, are called:
A. herbivores
B. decomposers ✓ Correct
C. producers
D. predators
Solution: Decomposers (many bacteria and fungi) recycle nutrients by breaking down dead matter.
Q9 — Microbes · hard · theory
Which of the following is a HARMFUL activity of microorganisms?
A. Fixing nitrogen in soil
B. Making curd
C. Producing antibiotics
D. Causing food to spoil and diseases to spread ✓ Correct
Solution: Some microbes spoil food and cause diseases, which are their harmful effects.
Q10 — Microbes · medium · theory
The main groups of microorganisms are bacteria, fungi, protozoa, some algae and:
A. ferns
B. mammals
C. viruses ✓ Correct
D. insects
Solution: Viruses are included among microorganisms along with bacteria, fungi, protozoa and some algae.
Q11 — Microbes · easy · theory
Which one of the following microbes is the smallest, can be seen only with an electron microscope, and is considered to lie on the borderline between living and non-living?
A. Virus ✓ Correct
B. Bacterium
C. Fungus
D. Protozoan
Solution: Viruses are the smallest microbes, visible only under an electron microscope. They are inert outside a host cell yet reproduce inside one, so they are placed on the borderline between living and non-living.
Q12 — Microbes · easy · theory
During bread-making the dough rises because yeast carries out fermentation. The gas mainly responsible for the rising is:
A. Oxygen
B. Hydrogen
C. Nitrogen
D. Carbon dioxide ✓ Correct
Solution: Yeast ferments the sugars in the dough and releases carbon dioxide gas, whose bubbles make the dough rise.
Q13 — Microbes · medium · theory
A farmer grows a crop of legumes (peas) and later finds the soil has become more fertile even though no fertiliser was added. The most likely reason is that:
A. The pea plants released extra oxygen into the soil
B. The pea plants absorbed nitrogen and removed it from the soil
C. Rhizobium bacteria in the root nodules fixed atmospheric nitrogen into a usable form in the soil ✓ Correct
D. Viruses in the roots destroyed all the weeds
Solution: Rhizobium bacteria living in the root nodules of legumes convert atmospheric nitrogen into nitrogen compounds that plants can use, naturally enriching the soil.
Q14 — Microbes · medium · theory
Assertion: Antibiotics such as penicillin can treat many bacterial diseases but are useless against the common cold. Reason: The common cold is caused by a virus, and antibiotics do not act on viruses. Which is correct?
A. Both are true, but the reason does not explain the assertion
B. The assertion is true but the reason is false
C. Both the assertion and the reason are true, and the reason correctly explains the assertion ✓ Correct
D. The assertion is false but the reason is true
Solution: Antibiotics act on bacteria but have no effect on viruses. Because the common cold is caused by a virus, antibiotics cannot cure it, so the reason correctly explains the assertion.
Q15 — Microbes · hard · theory
Malaria and the common cold are both diseases that spread among people, yet they are caused by very different microbes. Which pairing of the disease with its causative microbe is correct?
A. Malaria is caused by a protozoan (Plasmodium); the common cold is caused by a virus ✓ Correct
B. Malaria is caused by a virus; the common cold is caused by a bacterium
C. Malaria is caused by a bacterium; the common cold is caused by a protozoan
D. Malaria is caused by a fungus; the common cold is caused by a protozoan
Solution: Malaria is caused by the protozoan Plasmodium, spread by the female Anopheles mosquito, while the common cold is caused by a virus.
Q16 — Microbes · hard · theory
If all the decomposer microorganisms in an ecosystem were suddenly destroyed, the most direct consequence would be that:
A. Plants would immediately make much more food
B. Dead plants and animals would pile up and nutrients would not be returned to the soil ✓ Correct
C. Animals would begin to reproduce far faster
D. The air would quickly fill with oxygen
Solution: Decomposers (many bacteria and fungi) break down dead organisms and recycle their nutrients into the soil. Without them, dead matter would accumulate and soil nutrients would not be replenished.