CBSE Important Questions Class 12 Biology Chapter 8 – Microbes in Human Welfare

Class 12 Biology Chapter 8 Microbes in Human Welfare is the eighth chapter of the NCERT Class 12 Biology textbook. It explains how microbes are useful to humans: in household products such as curd and bread, in industrial products like antibiotics, organic acids and enzymes, in sewage treatment, in biogas production, as biocontrol agents such as Bacillus thuringiensis and Trichoderma, and as biofertilisers.

These CBSE important questions for Class 12 Biology Chapter 8 cover Lactobacillus and curd, lactose intolerance, yeast and the five-kingdom classification, chemical pesticides versus biocontrol, the structure of a bacteriophage, how viruses depend on the host cell, why Bt spray is better than chemical pesticides, and the methanogens in a biogas plant. The set has multiple-choice, data-based and short-answer questions with answers for board exam revision.

Q1. Which of the following bacteria is used in the conversion of milk into curd?
(a) Rhizobium
(b) Lactobacillus
(c) Clostridium tetani
(d) Mycobacterium tuberculosis

Answer: (b) Lactobacillus
Lactobacillus (lactic acid bacteria) ferments the milk sugar into lactic acid. The lactic acid lowers the pH and makes the milk protein casein coagulate, forming curd. Rhizobium fixes nitrogen in root nodules, while Clostridium tetani and Mycobacterium tuberculosis cause tetanus and tuberculosis.

Q2. Observe the given graph and read the relevant information carefully. Two friends ‘A’ and ‘B’ consume 2 glasses of milk every day. Friend ‘A’ suffers from stomach pain, bloating, etc., whenever he consumes milk, whereas Friend ‘B’ remains completely alright. Among the two, whose body does not contain the milk-digesting enzyme?

Bar graph showing stomach pain and gas symptoms in friend A and friend B after drinking milk

(a) Friend ‘A’
(b) Friend ‘B’
(c) Both ‘A’ and ‘B’
(d) Neither ‘A’ nor ‘B’

Answer: (a) Friend ‘A’
Milk sugar is lactose, which is digested by the enzyme lactase. A person who lacks lactase cannot digest milk and suffers from stomach pain, bloating and gas (lactose intolerance). The graph shows that Friend A has a high frequency of these symptoms while Friend B has almost none, so Friend A's body does not contain the milk-digesting enzyme.

Q3. Yeast is an example of
(a) multicellular fungi.
(b) unicellular fungi.
(c) protozoa.
(d) blue green algae.

Answer: (b) unicellular fungi.
Fungi are eukaryotic organisms with a cell wall made of chitin. Multicellular fungi have thread-like hyphae that form a mycelium, whereas yeast consists of single oval cells, lacks true hyphae and reproduces by budding. So yeast is a unicellular fungus.

Q4. Which of the following organisms is paired with its correct group?

Organism Group
A Bacteria Protoctista
B Yeast Fungi
C Amoeba Cnidaria
D Roundworm Porifera

(a) A
(b) B
(c) C
(d) D

Answer: (b) B
Yeast belongs to Kingdom Fungi, so pair B is correct. Bacteria belong to Kingdom Monera (not Protoctista), Amoeba is a protozoan of Kingdom Protoctista (not Cnidaria), and roundworms belong to Phylum Nematoda (not Porifera).

Q5. A group of researchers tested two fields, one that used chemical pesticides and the other that used biocontrol agents. The table below depicts the change in pest populations after the treatment:

Field Initial Pest Count After 1 Month After 3 Months
Chemical Pesticides 500 50 700
Biocontrol 500 300 100

Analyse the data and identify the most accurate conclusion.
(a) Biocontrol is ineffective because pests were not eliminated immediately.
(b) Biocontrol leads to long-term pest suppression without resurgence.
(c) Chemical pesticides provide longer-lasting pest control.
(d) Both methods are equally effective in pest control.

Answer: (b) Biocontrol leads to long-term pest suppression without resurgence.
The data shows that chemical pesticides reduce the pest population quickly (from 500 to 50 in one month), but the pests come back strongly to 700 after three months. This suggests that the pesticides also killed the natural predators of the pests, so the pests could multiply unchecked. In contrast, biocontrol reduces pests more slowly (500 to 300 in one month) but keeps lowering the population to 100 after three months. This is a gradual yet sustainable suppression without resurgence, because the natural predators and biocontrol agents maintain the ecological balance. So biocontrol is more effective for long-term pest management.

Q6. Below is a diagram showing the structure of a bacteriophage. Six parts are labelled as A, B, C, D, E and F. Which one of the following options correctly identifies all these parts?

Structure of a bacteriophage with parts labelled A to F

(a) A – Head, B – Tail, C – Collar, D – Tail Pins, E – Base Plate, F – Tail Fibres
(b) A – Head, B – Collar, C – Tail, D – Base Plate, E – Tail Pins, F – Tail Fibres
(c) A – Head, B – Tail, C – Collar, D – Base Plate, E – Tail Fibres, F – Tail Pins
(d) A – Head, B – Collar, C – Tail, D – Tail Pins, E – Base Plate, F – Tail Fibres

Answer: (b) A – Head, B – Collar, C – Tail, D – Base Plate, E – Tail Pins, F – Tail Fibres
A is the head (capsid) that encloses the viral nucleic acid. B is the collar that connects the head to the tail. C is the long tubular tail through which the genome is injected into the host. D is the base plate at the end of the tail. E are the tail pins, the short pointed structures on the base plate that anchor the phage to the host. F are the long tail fibres that recognise and attach to the host cell.

Q7. Which among the following depends on the machinery of the host to multiply?
(a) Bacterium
(b) Fungus
(c) Virus
(d) Protist

Answer: (c) Virus
A bacterium is a prokaryote that carries out its own life processes. A fungus and a protist are eukaryotes with cell organelles, so they can replicate on their own. A virus has no cell organelles and shows no characteristics of life on its own; it enters a host cell and uses the host's cellular machinery to multiply.

Q8. The five kingdom classification was proposed by
(a) Mendel
(b) Robert Brown
(c) Robert Hooke
(d) Whittaker

Answer: (d) Whittaker
In 1969, R. H. Whittaker introduced a classification system based on cell structure, level of organisation, mode of nutrition and reproduction. He divided all living organisms into five kingdoms: Monera, Protista, Fungi, Plantae and Animalia.

Q9. Whittaker classified all the living organisms into ______ kingdoms.
(a) 4
(b) 5
(c) 2
(d) 6

Answer: (b) 5
Linnaeus had only two kingdoms, Plantae and Animalia. The discovery of micro-organisms led to the kingdoms Monera and Protista, and studies on nutrition and cell organisation separated Fungi. Whittaker's five kingdoms are Monera, Protista, Fungi, Plantae and Animalia.

Q10. Dr. Anand, a microbiologist, compares two methods for controlling caterpillar infestation in a Brassica farm – chemical pesticide spraying and application of Bacillus thuringiensis (Bt) spores. He also observes that Trichoderma fungi are naturally present in the root zone of healthy plants. Both Bt spray and chemical pesticides effectively kill caterpillars. However, Dr. Anand recommends Bt. Identify two specific groups of non-target organisms affected by chemical pesticides but not by Bt, and explain why conserving these organisms is important for long-term ecosystem stability.

Answer: Chemical pesticides harm (1) beneficial insects such as pollinators like bees, and (2) natural predators of pests such as ladybirds and parasitic wasps, whereas Bt is target-specific and kills only the caterpillars. Conserving these organisms is important because they maintain the ecological balance and provide natural pest control. This keeps the farm ecosystem stable and supports long-term sustainability without repeated chemical spraying.

Q11. In a biogas plant, cattle dung is used as a substrate for methane production by methanogens. After gas production, the remaining slurry is removed and applied as fertiliser. Despite passing through the digestive system of cattle, dung still supports active methanogen activity. What does this suggest about the survival of these micro-organisms during digestion, and how does the nature of cellulose as a substrate contribute to their continued activity in dung?

Answer: This suggests that methanogens survive passage through the digestive system of cattle (they live in the rumen) and remain active in the dung. Cellulose, their main substrate, is not completely digested in the rumen, so sufficient organic matter remains in the dung for continued microbial activity and methane production in the biogas plant.

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