NCERT 8th Class (CBSE) Science: Microorganisms

Question: Which microorganism act as decomposers? How is this activity useful to us?

Answer: Microorganisms such as bacteria, fungi and protozoa help by decomposing the tissues of dead organisms to get their food. Without bacteria, piles of dead organisms would cover the earth. Usefulness of decomposing to us:

1. When microorganisms decompose organic matter, they break down plant and animal tissues into simple substances, which are restored to the soil, making it fertile.
2. Some bacteria decompose sewage and other waste in water. This is nature’s method of keeping the environment free from pollution.
3. Since algae can photosynthesize, they give off oxygen. It is thought that most of the oxygen that we have on earth was initially produced by algae in the oceans.

Question: Why viruses considered to be on the borderline of living and non-living?

Answer: Viruses cannot reproduce by themselves, respond to changes or use energy or grow. This seems to indicate that viruses are not living organisms. However, when a virus enters the living cell of an organism, say a human, it is able to reproduce. It uses the energy of the host cells for this purpose. After the formation of thousands of viruses, the host cell often dies. It bursts and new viruses spread and invade other cells. As a large number of host cells die, the person falls ill. Scientists regard viruses as a link between living and non-living. They have been placed on the dividing line between the living and the non-living.

Question: Discuss three important ways in which are useful to us and two ways in which they are harmful.

Answer: Microorganisms are useful in the following ways:

1. In food and beverage industry: Bacteria help us in making certain foods. Lactobacillus is a bacterium that sours milk and is used to make curd from milk. Cheese is also made by the action of bacteria on milk. Vinegar is produced by the action of bacteria. Bacteria are also used in the production of tea, coffee and cocoa. The process of fermentation of sugar by yeast is used in the manufacturing of alcohol and alcoholic drinks like beer and wines.
2. Making of medicines and vaccines: Bacteria and fungi are used to make medicines called antibiotics. These medicines kill or stop the growth of disease-causing microorganisms, and hence are used to cure diseases. Penicillin comes from a species of mould called Penicillium. Streptomycin, tetracycline and erythromycin are some other common antibiotics made from bacteria or fungi. Antibiotics are also used to control some diseases of animals and plants.
3. In agriculture: Some bacteria live in the root nodules of plants, such as gram, pea, beans etc. These bacteria absorb the nitrogen of the atmosphere and convert it into nitrates which serve as natural fertilizers for plants.
4. In cleaning the environment

Harmful Microorganisms

1. Microorganisms causes diseases: The microorganisms that cause diseases are known as pathogens or germs. To cause disease the germs have to first enter our body. They can enter through air, through food and water, through direct contact with a sick person, through insect such as mosquitoes and flies, through cuts and wounds. Microorganisms also cause diseases in animals and plants, for example- anthrax, foot and mouth disease, citrus canker that effects trees of citrus fruits, yellow vein mosaic, rust of wheat, etc.
2. Microorganism spoils food: Bacteria are responsible for spoilage of food. The souring of milk, putrefaction of meat, rotting of potatoes, vegetables and fruits are due to the action of bacteria. Growth of mould can make food unfit for consumption. Microorganisms growing on food sometimes produce toxic substances. These make the food poisonous. Consuming such food can cause a serious illness called food poisoning.

Question: Discuss the different ways in which communicable disease spread from person to person.

Answer: The microorganisms that cause diseases are known as pathogens or germs. To cause disease the germs have to first enter our body. They can enter in the following ways:

1. Through air: When a person suffering from common cold or flu sneezes, cough or spits germs are released in the air. A healthy person breathing this air can get infected. Some diseases that spread through air are common cold, flu, chickenpox, mumps, tuberculosis, measles and polio.
2. Through direct contact with a sick person, e.g. ringworm, common cold, flu, chickenpox.
3. Through insects such as mosquitoes, flies and fleas. For example, the female Anopheles mosquito carries the malaria germs. When it bites a healthy person, he / she gets infected with malaria.
4. Through foods and water, e.g. cholera, typhoid, hepatitis.

Question: Discuss five methods of food preservation.

Answer: The following are some common methods for food preservation:

1. Heating: Heating food to a high temperature kills microbes. For example, milk and water are boiled to kill microbes. Ultra-heat treatment (UHT) is the partial sterilization of milk by heating it for a short time, around 1-2 seconds, at a temperature exceeding 135°C. UHT milk available in tetrapacks has a shelf life of 6 months or more, until opened.
2. Pasteurization of milk: consists of heating it to a high temperature of 70°C for about half a minute and then cooling it quickly. This kills most of the bacteria without affecting the flavour. This method was invented by Louis Pasteur.
3. Cooling: Cooling food stops microbes from growing and reproducing, and hence preserves food. However, once the food is taken out of the freezer and warmed, microbes starts growing again.
4. Chemical preservatives: such as sodium benzoate and sodium metabisulphite help control microbial growth. They are used to preserve jams, squashes and ketchups. Other preservatives that are commonly used are: salt, sugar, oil and vinegar.
5. Drying or dehydration of food consists of removing water from it. This stops microorganisms from growing as they cannot grow without water. Cereals, pulses, spices and dry fruits are preserved by this method.
6. Canning: After sterilizing the food, it is canned in airtight containers. Many canned food items are available in the market.