CBSE Class 12 Biology Chapter 10: Microbes in Human Welfare NCERT Solutions

For demonstrating the presence of microbes under a microscope, a sample of curd would be an excellent choice to carry from home to the biology laboratory. Curd is rich in various microorganisms, particularly Lactic Acid Bacteria (LAB), such as *Lactobacillus*. These bacteria are responsible for the fermentation process that converts milk into curd. A tiny drop of curd contains millions of these bacteria, making them easily observable and identifiable under a microscope, thus effectively demonstrating the presence of microbes.

Microbes release gases, primarily carbon dioxide, during their metabolic processes, especially fermentation. Here are some examples:

1. Fermentation of Sugars: Bacteria and fungi, like yeast, carry out fermentation. During the fermentation of sugars, they break down complex organic compounds into simpler ones, releasing significant amounts of carbon dioxide gas and a small amount of energy. For instance, the fermentation of glucose can be represented as:

   $$Glucose\; \backslash rightarrow\; Alcohol\; +\; CO\_2\; +\; Energy$$

2. Dough for Idli and Dosa: The batter used for making traditional South Indian foods like idli and dosa is fermented by a mixture of bacteria and yeast. These microorganisms produce carbon dioxide gas. This gas gets trapped within the gluten network of the dough, causing it to expand and become puffed up, giving the final product a light and spongy texture.

Lactic acid bacteria (LAB) are predominantly found in curd. They play a crucial role in the transformation of milk into curd. As these bacteria multiply, they produce lactic acid, which causes the milk proteins to coagulate and thicken, forming curd. Additionally, LAB are known to enhance the nutritional value of curd by increasing the content of certain vitamins, notably $B\_\{12\}$.

Useful applications of lactic acid bacteria include:

• Food Production: Essential for making curd, yogurt, and other fermented dairy products.
• Digestive Health: They are part of the normal gut flora in humans and help maintain a healthy digestive system. They can also inhibit the growth of pathogenic microorganisms in the gut.
• Nutrient Enhancement: Increase the levels of certain vitamins in fermented foods.

Several traditional Indian foods rely on microbial fermentation for their preparation. Here are some examples based on the primary ingredients:

• Wheat Products: Bread, cakes, biscuits, and various types of naan and roti often involve yeast fermentation, which produces carbon dioxide for leavening and contributes to flavor.
• Rice Products: Idli and dosa are classic examples. The batter made from rice and black gram is fermented by a combination of bacteria and yeast, resulting in a sour taste and spongy texture due to gas production.
• Bengal Gram (or its products) Products: Dhokla and Khandvi are popular Gujarati snacks prepared from fermented batter derived from Bengal gram flour or its derivatives. The fermentation process gives them their characteristic tangy flavor and airy or smooth texture.

Microbes play a pivotal role in controlling diseases caused by harmful bacteria primarily through the production of antibiotics. Antibiotics are medicinal substances derived from certain microorganisms (like bacteria and fungi) that possess the ability to kill or inhibit the growth of other pathogenic microorganisms. For example, the fungus Penicillium notatum produces penicillin, a potent antibiotic that effectively checks the growth of bacteria such as staphylococci. Antibiotics work by targeting specific structures or processes in bacterial cells, such as weakening their cell walls. This weakening makes the bacteria vulnerable to destruction by the host's immune cells, like white blood cells, or can lead to cell lysis (bursting of the bacterial cell).

Antibiotics are crucial medicines used to combat bacterial infections, and they are often produced from microorganisms, including fungi. Two notable species of fungi used in the production of antibiotics are:

1. Penicillin: Produced from the fungus Penicillium notatum.
2. Cephalosporin: Produced from the fungus Cephalosporium acremonium.