Biology Class 12 NCERT Solutions: Chapter 10 Microbes in Human Welfare Part 1

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Q.1 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?


  • Curd can be used as a sample for the study of microbes. Curd contains numerous lactic acid bacteria (LAB) or Lactobacillus.

  • These bacteria produce acids that coagulate and digest milk proteins. A small drop of curd contains millions of bacteria, which can be easily observed under a microscope.

Image of Lactobacillus under a Microscope

Image of Lactobacillus under a Microscope

Q.2 Give examples to prove that microbes release gases during metabolism.


The examples of bacteria that release gases during metabolism are:

  • Bacteria and fungi carry out the process of fermentation and during this process they release carbon dioxide.

  • Fermentation is the process of converting a complex organic substance into a simpler substance with the action of bacteria or yeast.

  • Fermentation of sugar produces alcohol with the release of carbon dioxide and very little energy.

Glucose fermentation

Glucose Fermentation

  • The dough used for making idli and dosa gives a puffed appearance. This is because of the action of bacteria which releases carbon dioxide.

  • This released from the dough gets trapped in the dough, thereby giving it a puffed appearance.

Q.3 In which food would you find lactic acid bacteria? Mention some of their useful applications.


  • Lactic acid bacteria can be found in curd. It is this bacterium that promotes the formation of milk into curd.

  • The bacterium multiplies and increases its number, which converts the milk into curd. They also increase the content of vitamin in curd.

  • Lactic acid bacteria are also found in our stomach where it keeps a check on the disease-causing micro-organisms.

Image of Lactic Acid Bacteria

Image of Lactic Acid Bacteria

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