Useful & Harmful Micro-organisms — Homi Bhabha Class 9 Biology MCQs with Solutions
Free Homi Bhabha Class 9 Biology Useful & Harmful Micro-organisms MCQs with step-by-step solutions (12 questions). Part of Biotechnology & Applied Biology. Practise online on Prepizo — no login needed.
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Questions with solutions
Q1 — Useful & Harmful Micro-organisms · medium · theory
The microorganism used to convert milk into curd is:
A. Plasmodium
B. a fern
C. a virus
D. Lactobacillus (bacteria) ✓ Correct
Solution: Lactobacillus bacteria ferment milk into curd.
Q2 — Useful & Harmful Micro-organisms · medium · theory
Yeast is useful in bakeries and breweries because it carries out:
A. photosynthesis
B. digestion
C. fermentation ✓ Correct
D. nitrogen fixation
Solution: Yeast ferments sugars to produce carbon dioxide and alcohol.
Q3 — Useful & Harmful Micro-organisms · hard · theory
Medicines that are produced from microorganisms and used to treat bacterial diseases are called:
A. vaccines
B. antibiotics ✓ Correct
C. hormones
D. vitamins
Solution: Antibiotics (e.g. penicillin) are made from microbes and kill or stop bacteria.
Q4 — Useful & Harmful Micro-organisms · medium · theory
Which of the following is a HARMFUL effect of microorganisms?
A. Spoiling food and causing diseases ✓ Correct
B. Fixing nitrogen in soil
C. Making curd
D. Producing antibiotics
Solution: Some microbes spoil food and cause diseases, which are harmful effects.
Q5 — Useful & Harmful Micro-organisms · hard · theory
Food is preserved by methods like refrigeration and salting mainly to:
A. slow down or stop the growth of microorganisms ✓ Correct
B. increase spoilage
C. add colour
D. add more microbes
Solution: Preservation prevents microbial growth that would otherwise spoil the food.
Q6 — Useful & Harmful Micro-organisms · medium · theory
Disease-causing microorganisms are known as:
A. decomposers
B. saprophytes
C. pathogens ✓ Correct
D. producers
Solution: Pathogens are microbes that cause disease in their host.
Q7 — Useful & Harmful Micro-organisms · hard · theory
The mould Penicillium is important because it produces the antibiotic:
A. vinegar
B. insulin
C. penicillin ✓ Correct
D. ethanol
Solution: Penicillium yields penicillin, the first widely used antibiotic.
Q8 — Useful & Harmful Micro-organisms · easy · theory
Milk is set into curd (dahi) by the action of which microorganism?
A. Lactobacillus bacteria ✓ Correct
B. Amoeba
C. Blue-green algae
D. Rhizobium bacteria
Solution: Lactobacillus bacteria convert the milk sugar (lactose) into lactic acid, which curdles the milk into dahi. Rhizobium fixes nitrogen in roots and is not involved in curd formation.
Q9 — Useful & Harmful Micro-organisms · easy · theory
Which microorganism is added to dough to make bread rise and become soft and spongy?
A. Protozoan
B. Alga
C. Yeast ✓ Correct
D. Virus
Solution: Yeast is a fungus that ferments the sugars in dough, releasing gas that makes the dough rise. Viruses and protozoans are not used in baking.
Q10 — Useful & Harmful Micro-organisms · medium · numerical
A bacterium divides into two once every 20 minutes. Starting from a single bacterium under ideal conditions, how many bacteria are present after 2 hours?
A. 64 ✓ Correct
B. 32
C. 12
D. 128
Solution: 2 hours = 120 minutes, giving 120 ÷ 20 = 6 divisions. The number doubles each time, so it is 2⁶ = 64. Picking 32 stops one division short (2⁵) and 12 wrongly multiplies 6 × 2.
Q11 — Useful & Harmful Micro-organisms · hard · theory
Penicillin, the first antibiotic discovered by Alexander Fleming, is obtained from which type of microorganism?
A. A protozoan called Plasmodium
B. A blue-green alga
C. A bacterium called Lactobacillus
D. A mould (fungus) called Penicillium ✓ Correct
Solution: Penicillin is produced by the mould Penicillium, a fungus. Fleming noticed that this mould killed bacteria around it, which led to the antibiotic. Lactobacillus is a helpful bacterium, not the source of penicillin.
Q12 — Useful & Harmful Micro-organisms · hard · theory
A student lists four processes: (i) fixing nitrogen in soil, (ii) causing tuberculosis, (iii) setting curd, (iv) spoiling stored food. Which pair shows micro-organisms acting in a HARMFUL way?
A. (iii) and (iv)
B. (i) and (ii)
C. (i) and (iii)
D. (ii) and (iv) ✓ Correct
Solution: Causing tuberculosis (a disease) and spoiling stored food are both harmful actions of micro-organisms. Fixing nitrogen and setting curd are useful roles, so the harmful pair is (ii) and (iv).