Case Study Based Questions: The Invisible Living World: Beyond Our Naked Eye | Science Curiosity Class 8 - New NCERT PDF Download

Case Study 1

You have seen several kinds of plants and animals. However, there are other living organisms around us which we normally cannot see. These are called microorganisms or microbes. For example, you might have observed that during the rainy season moist bread gets spoilt and its surface gets covered with greyish white patches. Observe these patches through a magnifying glass. You will see tiny, black rounded structures. Do you know what these structures are and where do these come from? Water and soil are full of tiny organisms, though not all of them fall into the category of microbes. These microorganisms or microbes are so small in size that they cannot be seen with the unaided eye. Some of these, such as the fungus that grows on bread, can be seen with a magnifying glass. Others cannot be seen without the help of a microscope. That is why these are called microorganisms or microbes. Microorganisms are classified into four major groups. These groups are bacteria, fungi, protozoa and some algae. Viruses are also microscopic but are different from other microorganisms. They, however, reproduce only inside the cells of the host organism, which may be a bacterium, plant or animal. Common ailments like cold, influenza (flu) and most coughs are caused by viruses. Serious diseases like polio and chickenpox are also caused by viruses. Diseases like dysentery and malaria are caused by protozoa(protozoans)whereas typhoid and tuberculosis (TB)are bacterial diseases. You have learnt about some of these microorganisms in Classes VI and VII. Microorganisms may be single-celled like bacteria, some algae and protozoa, or multicellular, such as many algae and fungi. They live in all types of environments, ranging from ice cold climate to hot springs; and deserts to marshy lands. They are also found inside the bodies of animals including humans. Some microorganisms grow on other organisms while others exist freely.

Q1: …………………………………. only reproduce inside the cells of host organisms like bacteria, plants, or animals.
(a) Microbes
(b) Viruses
(c) Spores
(d) Bacillus
Ans: (b)

Q2: Which among the following is an example of a disease caused by bacteria?
(a) Flu
(b) Malaria
(c) Tuberculosis
(d) Covid 19
Ans: (c)

Q3: During the rainy season, moist bread gets spoiled and is covered with greyish white patches. What are these greyish white patches?
(a) Bacteria
(b) Viruses
(c) Dirt
(d) Fungus
Ans: (d)

Q4: Define microorganisms and state its major groups of classification.
Ans: Microorganisms are tiny living organisms that cannot be seen with our naked eyes. Microorganisms are classified into four major groups: bacteria, fungi, protozoa, and some algae.

Q5: Explain in detail where microorganisms are commonly found.
Ans: Microorganisms are present in all types of environments ranging from ice-cold climates to hot springs, deserts, and marshy lands. They can also be seen inside the bodies of animals. Some microorganisms exist freely, and some grow on other organisms.

Case study 2

Microorganisms are used for various purposes. They are used in the preparation of curd, bread and cake. Microorganisms have been used for the production of alcohol since ages. They are also used in cleaning up of the environment. For example, the organic wastes (vegetable peels, remains of animals, faeces, etc.) are broken down into harmless and usable substances by bacteria. Recall that bacteria are also used in the preparation of medicines. In agriculture they are used to increase soil fertility by fixing nitrogen. You have learnt in Class VII that milk is turned into curd by bacteria. Curd contains several micro-organisms. Of these, the bacterium, Lactobacillus promotes the formation of curd. It multiplies in milk and converts it into curd. Bacteria are also involved in the making of cheese, pickles and many other food items. An important ingredient of rava (sooji )idlis and bhatur as is curd. Can you guess why? Bacteria and yeast are also helpful for fermentation of rice idlis and dosa batter. Kneading of Yeast and sugar into the flour causes the flour to rise and double in size. Yeast reproduces rapidly and produces carbon dioxide during respiration. Bubbles of the gas fill the dough and increase its volume. This is the basis of the use of yeast in the baking industry for making breads, pastries and cakes. Commercial Use of Microorganisms: Microorganisms are used for the large-scale production of alcohol, wine and acetic acid(vinegar).

Yeast is used for commercial production of alcohol and wine. For this purpose, yeast is grown on natural sugars present in grains like barley, wheat, rice, crushed fruit juices, etc. Adding a spoon full of yeast powder into sugar solution gives out a characteristic smell after few hours. This is the smell of alcohol as sugar has been converted into alcohol by yeast. This process of conversion of sugar into alcohol is known as fermentation. Louis Pasteur discovered fermentation in 1857.

Q1: In the process of fermentation, the sugar is converted into…………………………..
(a) Vinegar
(b) Curd
(c) Salt
(d) Alcohol
Ans: (d)

Q2: Name the bacterium which, when added to milk, multiplies in it and converts milk into curd?
(a) Lactobacillus
(b) Streptococcus
(c) Salmonella
(d) Vibrio
Ans: (a)

Q3: Louis Pasteur, a French chemist, in 1857 discovered the process of ………………………….?
(a) Fermentation
(b) Pollination
(c) Saturation
(d) Composting
Ans: (a)

Q4: Explain the process of doubling of flour after the addition of yeast into it.
Ans: After adding yeast and sugar into the flour, the yeast reproduces rapidly and produces carbon dioxide during respiration. Bubbles of gas fill the dough, causing the volume of dough to double in size.

Q5: Name some of the common uses of microorganisms.
Ans: Microorganisms are useful for various purposes, such as in the preparation of curd, bread, and cake. They are used in the production of alcohol as well as medicines. Additionally, microorganisms are used to increase the fertility of the soil.

Case study 3

How do we preserve cooked food at home? You know that bread left unused under moist conditions is attacked by fungus. Microorganisms spoil our food. Spoiled food emits bad smell and has a bad taste and changed colour. Is spoiling of food a chemical reaction? Paheli bought some mangoes but she could not eat them for a few days. Later she found that they were spoilt and rotten. But she knows that the mango pickles that her grandmother makes does not spoil for a long time. She is confused. Let us study the common methods of preserving food in our homes. We have to save it from the attack of microorganisms. Chemical Method: Salts and edible oils are the common chemicals generally used to check the growth of microorganisms. Therefore, they are called preservatives. We add salt or acid preservatives to pickles to prevent the attack of microbes. Sodium benzoate and sodium metabisulphite are common preservatives. These are also used in jams and squashes to check their spoilage. Preservation by Common Salt: Common salt has been used to preserve meat and fish for ages. Meat and fish are covered with dry salt to check the growth of bacteria. Salting is also used to preserve amla, raw mangoes, tamarind, etc. Preservation by Sugar: Jams, jellies and squashes are preserved by sugar. Sugar reduces the moisture content which inhibits the growth of bacteria which spoil food. Preservation by Oil and Vinegar: Use of oil and vinegar prevents spoilage of pickles because bacteria cannot live in such an environment. Vegetables, fruits, fish and meat are often preserved by this method. Heat and Cold Treatments: You must have observed your mother boiling milk before it is stored or used. Boiling kills many microorganisms. Similarly, we keep our food in the refrigerator. Low temperature inhibits the growth of microbes. Pasteurised milk can be consumed without boiling as it is free from harmful microbes.

The milk is heated to about 700C for 15 to 30 seconds and then suddenly chilled and stored. By doing so, it prevents the growth of microbes. This process was discovered by Louis Pasteur. It is called pasteurisation. Storage and Packing: These days dry fruits and even vegetables are sold in sealed air tight packets to prevent the attack of microbes.

Q1: Which of the following is NOT an example of a chemical method of preservation?
(a) Salt
(b) Heating
(c) Sodium Benzoate
(d) Sodium Metabisulphite
Ans: (b)

Q2: ……………. of milk before storing is done to kill many microorganisms.
(a) Mixing
(b) Distilling
(c) Baking
(d) Boiling
Ans: (d)

Q3: Pasteurisation technique was discovered by which of the following scientists?
(a) Louis Pasteur
(b) Fanny Hesse
(c) Ronald Ross
(d) Harold Ginsberg
Ans: (a)

Q4: Explain the process of pasteurization in detail.
Ans: In the pasteurization process, milk is heated to about 70°C for 15 to 30 seconds and then suddenly chilled and stored. This process prevents the growth of microbes and extends the shelf life of the milk.

Q5: Name some of the common methods of food preservation at your home.
Ans: Some common methods of preservation include adding common salt to pickles, meat, and fish; using sugar; and preserving with oil and vinegar.