NCERT Solutions: Microbes in Human Welfare

Q1: Bacteria cannot be seen with the naked eyes, but these can be seen with the help of a microscope. If you have to carry a sample from your home to your biology laboratory to demonstrate the presence of microbes under a microscope, which sample would you carry and why?
Ans: Curd is the best sample to carry for demonstrating microbes under a microscope. Curd contains large numbers of lactic acid bacteria, mainly Lactobacillus, which convert milk into curd. A single drop of curd contains millions of these bacteria, so they are easy to observe when a small smear or a drop is mounted on a slide and viewed under a compound microscope. Curd is also safe and easy to transport compared to other microbial samples.

These bacteria produce acids that coagulate and partially digest milk proteins. Because of their very large numbers, the bacterial cells become clearly visible at typical magnifications used in school laboratories.

Q2: Give examples to prove that microbes release gases during metabolism.
Ans: The following examples show that microbes release gases during metabolism:
1. During fermentation, bacteria and fungi (including yeast) break down sugars into simpler products and release carbon dioxide as a by-product. For example, fermentation of sugar by yeast produces alcohol and carbon dioxide.

Fig: Bacteria 

2. The dough used for making idli and dosa becomes soft and puffed because microbes present in the batter produce carbon dioxide. The carbon dioxide gets trapped as tiny bubbles in the batter, causing the dough to rise and become spongy when cooked.

Q3: In which food would you find lactic acid bacteria? Mention some of their useful applications.

Ans: Lactic acid bacteria are commonly found in curd. These bacteria (for example, species of Lactobacillus) ferment lactose in milk to lactic acid, converting milk into curd. Useful applications of lactic acid bacteria include:

- Formation of curd and other fermented dairy products.

- Acting as probiotics that help maintain a healthy gut flora and inhibit disease-causing microbes in the intestine.

- Enhancing nutritional value by increasing certain B-vitamins (for example, vitamin B₁₂) and improving digestibility of foods.

Fig: Lactic acid bacteria

Q4: Name some traditional Indian foods made of wheat, rice and Bengal gram (or their products) which involve use of microbes.
Ans: Wheat: Product: Bread, cake, etc.
Rice: Product: Idli, dosa
Bengal gram: Product: Dhokla, Khandvi
Q5: In which way have microbes played a major role in controlling diseases caused by harmful bacteria?
Ans: 

• Many microbes themselves are the source of medicines used to control diseases. Antibiotics are substances produced by certain micro-organisms (mainly bacteria and fungi) that either kill pathogenic bacteria or inhibit their growth.
• Examples of antibiotics include streptomycin, tetracycline and penicillin. Penicillium species produce penicillin, which interferes with the formation of bacterial cell walls. When a bacterium cannot maintain its cell wall, it becomes unable to survive and is destroyed.
• By using antibiotics appropriately, infections caused by many harmful bacteria can be treated, reducing illness and saving lives. It is important that antibiotics are used correctly to avoid the development of resistant strains.

Q6: Name any two species of fungus, which are used in the production of the antibiotics.
Ans: Antibiotics are produced by certain micro-organisms, including fungi. Two fungi used in antibiotic production are:

Q7: What is sewage? In which way can sewage be harmful to us? 

Ans: Sewage is the waste water and refuse carried away from homes, institutions and industries through drains and sewers. It contains both liquid and solid wastes that are rich in organic matter and microbes. Sewage can be harmful because:

- It often contains pathogenic micro-organisms that cause water-borne diseases (for example, cholera, dysentery and typhoid).

- If discharged untreated into natural water bodies it pollutes drinking water sources and harms aquatic life.

- Organic matter in sewage can lead to depletion of dissolved oxygen in water, affecting other organisms.

For these reasons, sewage must be properly collected, treated and safely disposed of.

Fig: Sewage

Q8: What is the key difference between primary and secondary sewage treatment?
Ans:

Q9: Do you think microbes can also be used as source of energy? If yes, how?
Ans: Yes. Microbes are used to produce biogas, which is a useful source of energy. In a biogas plant, animal dung is mixed with water to form a slurry and placed in an anaerobic digester where methane-producing microbes break down the organic matter in the absence of oxygen. The gases produced (mainly methane with some carbon dioxide) are collected through a pipe and used as fuel for cooking or lighting. The remaining digested slurry is rich in nutrients and is used as an organic fertiliser.

Fig: Microbes source of energy

Q10: Microbes can be used to decrease the use of chemical fertilisers and pesticides. Explain how this can be accomplished.
Ans: 

• Microbes are an important part of organic farming because they can supply nutrients and protect crops without the need for chemical fertilisers and pesticides.

Fig: Microbes

• Bio-fertilisers are preparations containing beneficial microbes that are introduced into seeds, roots or soil to increase the availability of nutrients to plants. They mobilise nutrients by biological activity rather than by chemical addition.
• Many bacteria and cyanobacteria fix atmospheric nitrogen into forms that plants can use. Examples include symbiotic Rhizobium (in root nodules of legumes), free-living nitrogen-fixers such as Azospirillum and Azotobacter, and cyanobacteria such as Anabaena, Nostoc and Oscillatoria. Use of these organisms reduces the need for synthetic nitrogen fertilisers.
• Microbes can also act as bio-pesticides. For example, Bacillus thuringiensis (Bt) produces toxins that kill insect larvae when ingested; dried Bt spores are mixed with water and sprayed on crops. Trichoderma species colonise plant roots and protect plants from pathogenic fungi. Baculoviruses are used to control specific insect pests. These biological agents reduce the need for chemical pesticides and are environmentally friendly.

Q11: Three water samples namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant were subjected to BOD test. The samples were labelled A, B and C; but the laboratory attendant did not note which was which. The BOD values of the three samples A, B and C were recorded as 20mg/L, 8mg/L and 400mg/L, respectively. Which sample of the water is most polluted? Can you assign the correct label to each assuming the river water is relatively clean?
Ans: Biological oxygen demand (BOD) measures the amount of dissolved oxygen required by microbes to decompose organic matter in water. Higher BOD indicates more organic pollution and greater microbial activity.
Hence, the correct labels are:

Q12: Find out the name of the microbes from which Cyclosporin A (an immuno-suppressive drug) and Statins (blood cholesterol lowering agents) are obtained.
Ans:

Ques 13: Find out the role of microbes in the following and discuss it with your teacher.
(a) Single cell protein (SCP)
(b) Soil
Ans: (a) Single cell protein (SCP)
A single cell protein is protein-rich biomass produced from microbes such as algae, yeast or bacteria and used as an alternative protein source in animal feeds or, in some cases, for human nutrition. Microbes like Spirulina can be grown on inexpensive substrates (molasses, waste water) to produce protein, vitamins and minerals. Other bacteria (for example, certain methylotrophs) can produce large amounts of biomass quickly and are grown on an industrial scale to obtain protein supplements.

Fig: Single Cell

(b) Soil

Microbes are essential for soil fertility. They decompose dead organic matter to form humus, improve soil structure and release nutrients in forms plants can absorb. Many microbes fix atmospheric nitrogen (for example, symbiotic Rhizobium in legume root nodules, free-living Azotobacter and cyanobacteria such as Anabaena and Nostoc) and thus supply nitrogen to plants. Other soil microbes help solubilise phosphorus and produce growth-promoting substances. Together, these activities maintain and improve soil productivity.

Q14: Arrange the following in the decreasing order (most important first) of their importance, for the welfare of human society. Give reasons for your answer. 

Biogas, Citric acid, Penicillin and Curd.

Ans: The decreasing order of importance is:

Penicillin - Biogas - Citric acid - Curd

Reasoning:

• Penicillin is very important as it is an antibiotic used to treat bacterial diseases and has saved many lives.

• Biogas is an eco-friendly source of energy and also provides nutrient-rich manure.

• Citric acid is widely used as a food preservative and in industries.

• Curd is a nutritious food product and beneficial for digestion.

Thus, all are important, and their importance cannot be strictly ranked as it depends on the context of use.

Q15: How do bio-fertilisers enrich the fertility of the soil?

Ans: Bio-fertilisers contain living beneficial microbes that increase soil fertility by biological processes. They are introduced into seeds, roots or soil where they:

- Fix atmospheric nitrogen and make it available to plants (for example, Rhizobium, Azotobacter, cyanobacteria).

- Solubilise and mobilise soil nutrients such as phosphorus and micronutrients so plants can absorb them more easily.

- Produce substances that promote plant growth and improve soil health.

Bio-fertilisers are cost-effective and eco-friendly alternatives to chemical fertilisers and help maintain long-term soil fertility.

Fig: Fertility of Soil