CBSE Class 8 Science Chapter 2 Microorganisms: Friend And Foe Detailed Notes Groups Of Microorganisms
The microorganisms may be unicellular (single-celled) or multicellular (many-celled). These are classified into four major groups as described below
Bacteria are the simplest unicellular organisms with a rigid cell wall, like plant cells, an Example. Lactobacillus, E. coli, etc.
Fungi are a large group of organisms which do not have chlorophyll and hence, do not photosynthesise. Thus, they are heterotrophs, an Example. yeast, Penicillium, Aspergillus, Rhizopus (bread mould). Fungi may be unicellular or multicellular.
Protozoa is a group of single-celled microorganisms that have animal-like characteristics. They can move from place to place. Example, Amoeba and Paramecium. Diseases like dysentery and malaria are caused by protozoa.
Algae is a large group of single-celled, plant-like organisms. They have no true roots, stems or leaves. They can be unicellular or multicellular. Algae are photoautotrophic organisms that carry out photosynthesis. Example: Chlamydomonas, Spirogyra, etc.
Viruses
These are also microscopic organisms, but are different from other microorganisms. They reproduce only inside the cells of the host organisms. These host organisms can be a bacterium, plant or animal.
Class 8 Science Chapter 2 Microorganisms: Friend And Foe Notes: Where Do Microorganisms Live?
Microorganisms spread over the entire biosphere. Examples in soil, water, air, and inside the bodies of animals, including humans, etc. They are characterised by a very high degree of adaptability, as they can survive in all types of environments, ranging from ice-cold climates to hot springs and deserts to marshy lands.
Some microorganisms grow on other organisms, while others exist freely. Microorganisms like amoebas can live alone, while bacteria and fungi may live in colonies.
Microorganisms And Us
Microorganisms play a dual role in our lives. They not only offer various benefits, but also cause a significant number of diseases.
Friendly Microorganisms
Microbes are used for various purposes. Some common beneficial activities of microbes are discussed below
1. Microbes in Household Products
Some of the household products obtained from microbial activities are as follows
Curd contains several microorganisms. Of these, the bacterium Lactobacillus promotes the formation of curd. It multiplies in milk and converts milk into curd, The curd is added to run idlis and bhaturas to make them soft and spongy.
Baking Industry The dough for making bread, cakes and pastries is prepared by adding yeast to flour.
Yeast reproduces rapidly and produces carbon dioxide (CO2) during respiration; bubbles of this gas fill the dough and increase its volume. It adds sponginess to cake and bread.
Bacteria are also involved in the making of cheese, pickles and other food items. Bacteria, along with yeast, are helpful for the fermentation of rice idlis and dosa batter.
2. Commercial Use of Microorganisms
Microbes are used for the large-scale production of alcohols, wine and acetic acid (vinegar). Yeast is used for the commercial production of alcohol and wine. For this purpos,e yeast is grown on natural sugars present in grains like barley, wheat, rice, crushed fruit juices, etc.
The process of converting sugar into alcohol is called fermentation. Louis Pasteur discovered fermentation in 1857.
3. Medicinal Use of Microorganisms
When an individual falls ill, the doctors prescribe antibiotics for treatment. The source of these antibiotic medicines and vaccines is microorganisms. The details of antibiotic medicines and vaccines are given below
Antibiotics: The medicines, which are used to kill or stop the growth of the disease-causing microorganisms, are known as antibiotics.
These antibiotics in the form of tablets, capsules or injections are products of microorganisms (mainly bacteria and fungi), For example. penicillin, streptomycin, tetracycline, erythromycin.
The antibiotics are manufactured by growing specific microorganisms and are used to cure a variety of diseases. Antibiotics are even mixed with the feed of livestock and poultry to check microbial infection in animals.
They are also used to control many plant diseases. Antibiotics are not effective against cold and flu as these are caused by viruses.
Certain precautions to be taken for using antibiotics are as follows
It should be taken only on the advice of a qualified doctor in the proper dose. In wrong doses (or taken when not needed), it becomes less effective when taken in future
The person must complete the course as prescribed by their doctor.
It should not be taken unnecessarily, as it may kill useful bacteria in the body.
History Of Antibiotics: Penicillin
The first antibiotic, penicillin, was discovered by Alexander Fleming in 1929. He was working on a culture of disease-causing bacteria.
During the experiment, he observed that the presence of a mould in one of his culture plates prevented the growth of bacteria. Indeed, it also killed many of these bacteria. From this mould, penicillin was prepared.
Vaccine: It is a biological preparation that provides immunity (or protection) against a particular disease.
A vaccine consists of dead or weakened microbes which, when introduced in a healthy body, produces suitable antibodies to fight and kill the invading microbes. bacteria or microbes.
These antibodies remain in the body to protect from disease-causing microbes, thus developing immunity. The diseases which can be prevented by vaccination are polio, cholera, tetanus, rabies, TB, smallpox, measles and hepatitis.
Vaccination
- Edward Jenner discovered the smallpox vaccine in 1798.
- A worldwide campaign against smallpox has finally led to its eradication from most parts of the world.
- Pulse Polio Immunisation Programme was launched in India in 1995 for the eradication of polio. Children in the age group of 0 (zero) months to 5 years are administered polio drops (actually vaccine) every year.
4. Increasing Soil Fertility
Some bacteria and cyanobacteria (blue-green algae) can fix nitrogen from the atmosphere to enrich soil with nitrogen and increase its fertility.
These microbes are commonly called biological nitrogen fixers, e.g. Rhizobium.
5. Cleaning the Environment
Microorganisms decompose dead organic waste of plants and animals, converting them into simple substances. These substances are again used by other plants and animals. The microorganisms are used to degrade the waste and smelly substances from our biosphere and thereby clean the environment.
Harmful Microorganisms
The microorganisms are harmful in many ways. Some of these cause diseases in human beings, plants and animals. Such disease-causing microorganisms like bacteria, fungi, viruses and protozoans are called pathogens.
They enter the body by various means, where they multiply and cause harm to the body. Some of their activities are described below.
Disease-Causing Microorganisms in Humans
Certain microbial diseases are spread from an infected person to a healthy individual through air, water, food or physical contact. Such diseases are called communicable diseases, for Example. cholera, common cold, chickenpox and tuberculosis.
Transmission of Communicable Diseases
There are various ways through which communicable diseases can be transmitted. These are as follows
- When a person suffering from a common cold or flu sneezes, coughs or spits, fine droplets of moisture carrying viruses spread in the air. The virus may enter the body of a healthy person while breathing and cause infections, for Example tuberculosis.
- Through direct contact with an infected person, for Example, chickenpox, ringworm.
- Through the food we eat or the water we drink, for Example, cholera, typhoid.
- Through vectors or carriers, like some insects and animals. The organisms that transmit pathogens from an infected individual to a healthy one are known as carriers. For example,
The housefly is one such carrier. The flies sit on the garbage and animal excreta, accumulating pathogens on their bodies. When these flies land on exposed food, they can transfer these pathogens.
Another example of a carrier is the female Anopheles mosquito, which carries the parasite of malaria (Plasmodium). The female Aedes mosquito acts as a carrier of the dengue virus.
Prevention Of Communicable Diseases
Some methods of preventing the Occurrence and spread of communicable diseases are as follows
- Cover your mouth and nose with a handkerchief while sneezing.
- Keep your food covered and avoid consuming uncovered food items.
- Drink clean and safe water.
- Use mosquito nets and mosquito repellent creams to protect yourself from mosquito bites.
- Take the vaccination at the proper time.
- Avoid breeding of mosquitoes by not allowing water to collect anywhere, i.e. in coolers, tyres, etc. and by keeping surroundings clean and dry.
Some of the common diseases affecting humans, their mode of transmission and a few general methods of prevention are given in the table below.
Common human diseases caused by microorganisms
Disease-Causing Microorganisms In Animals
Several microorganisms not only cause diseases in humans, but also cause diseases in other animals. For example, Anthrax is a dangerous human and cattle disease caused by a bacterium. Some of the common animal diseases caused by microorganisms are given in the table below
Common Animal Diseases Caused by Microorganisms
Disease-Causing Microorganisms In Plants
Several microorganisms cause diseases in plants like wheat, rice, potato, sugarcane, orange, apple, etc.
These diseases reduce the yield of crops. They can be controlled by the use of certain chemicals which kill the microbes. Some of the common plant diseases caused by microorganisms are tabulated below.
Common plant diseases caused by microorganisms
Food Poisoning
Microorganisms that grow on food may produce toxic substances, thereby causing the spoilage of food.
The spoiled food starts giving a foul smell, bad taste, and its colour may also change “his spoiling of food is a chemical change. Consumption of spoiled food can lead to a condition called food poisoning.
The symptoms of food poisoning include vomiting, diarrhoea, headache, fever, etc. It can cause serious illness and even death. So we must preserve food to prevent it from getting spoiled.
Food Preservation
The process by which the spoilage of food is prevented using suitable chemical or physical methods is called food preservation. Some of these methods are given below
Chemical method. The chemicals used to check the growth of microorganisms are called preservatives. Salts, edible oils and acid preservatives are added to pickles to prevent the attack of microbes.
Sodium benzoate and sodium metabisulfite are common preservatives used to check the growth of microbes. They are added in jams and squashes to check their spoilage.
Preservation by common salt has been used to preserve meat and fish for ages. Salting is also used to preserve amla, raw mangoes, tamarind, etc.
Preservation by sugar: Jams, jellies and squashes are preserved by sugar. It reduces the moisture content, which inhibits the growth of bacteria which spoil food.
Preservation by oil and vinegar. Use of oil and vinegar prevents the spoilage of pickles because bacteria cannot live in such an environment. Vegetables, fruits, fish and meat are preserved by this method.
Heat and cold treatments. Boiling kills many microorganisms. Similarly, we keep our food in the refrigerator as low temperature inhibits the growth of microbes.
Pasteurisation is used for the preservation of milk. It involves the process of heating milk to about 70% for 15 to 30 seconds (killing most of the bacteria) and then cooling it quickly at a very low temperature.
This milk is then stored at a cold temperature. Pasteurised milk can be consumed without boiling as it is free from harmful microbes. The milk that comes in packets does not get spoiled as it is pasteurised. The process of pasteurisation was discovered by Louis Pasteur.
Storage and packing. These days, dry fruits and even vegetables are sold in sealed, air-tight packets. This helps to preserve food by keeping germs and microbes out. Airtight packets also prevent food from moisture and contamination.
Science Insight: Irradiation is a modern method of food preservation. The food is exposed to high-energy gamma rays or X-rays. These powerful radiations destroy the harmful microorganisms. The food can be preserved for years without refrigeration.
Nitrogen Fixation
Plants cannot directly utilise the nitrogen gas present in the atmosphere. It is taken up by plants from the soil only when it is converted to nitrogenous compounds. The process of conversion of atmospheric nitrogen into nitrogenous compounds is called nitrogen fixation.
Nitrogen can be fixed by
- Lightning is a natural process that converts nitrogen into a usable form.
- Rhizobium bacteria are involved in the fixation of nitrogen in leguminous plants. These bacteria live in the root nodules of leguminous plants such as beans and peas, with which it has a symbiotic relationship.
- Some of the blue-green algae can fix nitrogen, e.g. Anabaena, Nostoc, etc.
Although plants and animals continuously use up atmospheric nitrogen, it remains constant. This happens through a cycle of processes called the nitrogen cycle.
Class 8 Science Chapter 2 Microorganisms, Friend And Foe Notes With Practice Questions Nitrogen Cycle
It is a repeating cycle of processes during which nitrogen in the atmosphere circulates through living and non-living things. Nitrogen is one of the essential constituents of all living organisms.
It is a part of proteins, chlorophyll, nucleic acids and vitamins. Our atmosphere contains 78% nitrogen gas. The main steps in this cycle are as follows
- The atmospheric nitrogen is fixed through various means and is converted into usable compounds.
- These compounds are utilised by plants from the soil through their root system. This nitrogen is used for the synthesis of plant proteins and other compounds.
- Animals taking these plants as food get these proteins and other nitrogen compounds.
- When these plants and animals die, certain bacteria and fungi convert the complex nitrogenous compounds present in their bodies into simple nitrogenous compounds. These simple nitrogenous compounds are again absorbed by the plants for their growth.
- Certain other bacteria convert some of these compounds into nitrogen gas, which goes back into the atmosphere. As a result, the percentage of nitrogen in the atmosphere remains nearly constant.
Algae are A type of microorganism that can photosynthesise and form green patches on water.
Antibiotics are the medicines that kill or stop the growth of disease-causing microorganisms.
Antibodies are substances produced by our body to fight against the invading microorganisms.
Bacteria are A type of microorganism which can be found in all kinds of environments.
Carrier Animals that carry disease-causing microorganisms.
Communicable diseases are Microbial diseases that can spread from an infected person to a healthy person through air, water, food or physical contact.
Fungi are A type of microorganism which cannot photosynthesise.
Fermentation is A process of conversion of sugar into alcohol by bacteria and yeast.
Lactobacillus is A type of bacteria that promotes the formation of curd.
Microorganism: A living organism which cannot be seen with the unaided eye.
Nitrogen fixation is the Process of conversion of free nitrogen gas present in the atmosphere into nitrogenous compounds.
Nitrogen cycle: The nitrogen in the atmosphere is circulated again and again in a cyclic manner by the living and the non-living things. This circulation is called the nitrogen cycle.
Protozoa, A type of microorganism that has animal-like characteristics.
Pathogen, a disease-causing microorganism.
Preservatives are the chemical substances used to prevent the growth of microorganisms.
Pasteurisation Process of destroying microorganisms in food items, e.g. milk. It involves high heating followed by sudden cooling.
Rhizobium, A Bacteria that live in the roots of leguminous plants and is involved in the fixation of atmospheric nitrogen.
Virus: A type of microorganism which can reproduce only inside a host.
Vaccine: Weak or dead microbes that are injected into our body to fight against disease.
Yeast is A unicellular fungus that is used in the baking industry.