CBSE Class 8 Science, Microorganisms - Friend and Foe

   Microorganisms- A living organism that is very minute to be seen by the naked eye, especially a single celled organism, such as a bacterium is called microorganism. Microorganisms are classified into four major groups mainly bacteria, fungi, protozoa and some algae.
   Some of the microorganisms are unicellular like bacteria, while some have well developed, multi-cellular body like moulds.
   They can survive under different types of climatic environment, ranging from ice cold climate to hot summers and deserts to marshy, muddy lands. They are also found inside the bodies of animals including humans, air, water, soil, food and other things.
   Virus: You may be surprised to know that the viruses can be crystallized and stored like some non-living things, such as common salt and sugar. On the other hand, they multiply like living organisms when they are within a living body.
   Bacteria: Bacteria are single celled microorganisms. They are put under plant kingdom because they have rigid definite cell wall like plant cells. Bacteria can be seen under microscope which enlarges their image from a hundred to thousand times.
   Fungi: Fungi are non-green plants. They cannot synthesize their own food. They have to depend for their food on others. Most fungi take their food from dead organic matter. Some live on other organisms as parasites.
   Algae: Algae are green substances floating on the surface of a pond, lake, river, stagnant water, moist soil, stones. They tend to grow on wet surfaces. Therefore, they can synthesize their own food. They are found in water or in very moist places.
   Protozoa: Protozoa are unicellular animals. Some are free-living, others are parasites. Several parasitic protozoans cause diseases in human beings, domestic animals and plants. For example, Plasmodium, a protozoan, causes malaria.
   Microorganisms are commercially used for the large scale production of alcohol, wine and acetic acid (vinegar), while has medicinal uses too.
   Microorganisms can harmful too. Some of the microorganisms cause diseases in humans, plants as well as in animals. Such microorganisms which cause disease are known as pathogens. Some microorganisms spoil leather, food and clothing.
   Food Poisoning- Food poisoning can be caused as a result of consumption of food spoilt by some microorganisms. Microorganisms that grow on our food sometimes produce substances that are toxic in nature. These make the food poisonous and can cause serious illness to living organisms.
   Food Preservation- Microorganisms may grow on our food and spoil it. Spoiled food emits bad smell,bad taste and changed colour.
   Antibody- When a disease causing microbes enter our body, our internal defensive mechanisms produces substances called Antibodies to fight disease causing microbes.
   Vaccine- When a disease-carrying microbe enters our healthy body, the body produces antibodies, body fights and kills them by these antibodies. The body also remembers how to fight the microbe if it enters again. The antibodies remain in the body for a long time and protect us from the disease causing microbes. The substance which is injected into the body to trigger the body to initiate this entire process is called a vaccine.
   Pasteurization- this is a process in which milk is heated to about 70 degree Celsius for about 15 to 30 seconds and then suddenly chilled and stored. This process was discovered by Louis Pasteur.
   Nitrogen Cycle- Some bacteria and blue green algae present in the soil fix nitrogen from the atmosphere and convert into compounds of nitrogen. Once nitrogen is converted into these usable compounds, it can be utilised by plants from the soil through their root system. On the othe hand when plants and animals die, bacteria and fungi present in the soil convert the nitrogenous wastes into nitrogenous compounds to be used by plants again. Thus due to this Nitrogen Cycle, the percentage of nitrogen in the atmosphere remains more or less constant.