"Algae by weight represent almost of the biomass of all life on Earth. They produce more than half of all the oxygen we breathe."
A microorganism is a living organism that is very minute. It can only be seen with the aid of a microscope. Consider that the largest human cells are about the diameter of human hair. Bacteria cells are one-hundredth the size of a human cell and viruses are much, smaller again. A bacterium next to a human cell is like a tiny boat next to a big ship.
(i) Microorganisms are present everywhere in air, soil, water, foodstuffs, sewage waste as well as inside animal bodies. (ii) They can survive in all types of environments even in extremely harsh environmental conditions like hot springs, desert soil, saline water, ice-cold water and marshy lands. (iii) They are also present in dead and decaying organic matter. (iv) Some can live alone while others like bacteria and fungi may live in colonies. (v) Some microbes live as parasites either outside or inside the body of other organisms and cause diseases.
There are five major groups of microorganisms i.e. bacteria, virus, protozoan, algae and fungus.
Aspergillus
Penicillium
3. Algae: These are unicellular or multicellular eukaryotic autotrophic organisms, contain chlorophyll pigment and carry out photosynthesis. Cell wall of algae is mainly made up of cellulose. e.g. Chlamydomonas, Spirogyra, Chlorella.
4. Viruses: Viruses are acellular structures because they are not built up of cells. They are much smaller than cells. These are ultramicroscopic organisms, which require cells of host organisms to reproduce. e.g. Influenza virus, polio virus, AIDS etc. Structure of virus consist of nucleic acid and a protein coat. Nucleic acid can either be DNA or RNA. Microbes-friend and foe: It is almost impossible to think of any place without microorganisms. Most microorganisms are beneficial, for example, there are micro-organisms in our large intestine that synthesize vitamins. However, a tiny minority are pathogens (disease causing agents). These pathogens, often called germs, are a threat to all life forms.
Microorganisms are used for a number of purposes such as: (1) Preparation of curd, bread, cake, idli, dosa and cheese. (2) In industry - to produce alcohol, wine and acetic acid (vinegar). (3) In agriculture - to increase soil fertility. (4) In medicines - to produce antibiotics and vaccines. (5) Cleaning the environment.
(i) Formation of curd (dahi): Making of curd is a process in which the lactose (milk sugar) of the milk is converted into lactic acid by the action of certain bacteria (mainly Lactobacillus). This curd is further used in the preparation of idli-dosas and bhaturas.
Q. How does the addition of small amount of curd to milk converts the whole milk into curd?
Anaerobic respiration in yeast cell: Yeast can survive in the absence of air. They are called anaerobes. They get energy through anaerobic respiration. The glucose breaks down into alcohol and carbon dioxide in the absence of oxygen. The reaction is called anaerobic respiration or fermentation.
The wine and beer (brewery) industries are important industries which make use of the fungus yeast. Yeast is grown on natural sugars present in crashed fruit juices and grains like barley, wheat and rice. Yeast brings about conversion of sugars into alcohol and carbon dioxide.
All plants and animals need nitrogen, the gas that makes up 78% of the air. Some bacteria and Blue green algae are the only organisms that can take nitrogen directly from air. These microbes convert atmospheric nitrogen into compounds of nitrogen and so called as biological nitrogen fixers. Thus, they are very useful to agriculture, as they enrich the soil with nitrogen.
Anabaena
Nostoc
It is conducted by a variety of bacteria, both as free-living organisms and in symbiotic association with plants. e.g. The roots of leguminous plants like beans and pea, bear certain swollen structures called root nodules. The bacterium Rhizobium lives in the root nodules. Bacteria fix nitrogen for the plants, while the plants give food and shelter to the bacteria. This type of relationship is called as symbiotic association.
Nitrogen is an essential element for all living organisms because it is a principle component of proteins, chlorophyll, vitamin and nucleic acids. The circulation of nitrogen through the living and non-living components of the biosphere (air, soil, water, plants and animals) is called nitrogen cycle.
The bacterium called Rhizobium can fix atmospheric nitrogen and convert it into a soluble form. But Rhizobium cannot make its own food. So, it lives in the roots of gram, peas, moong, beans and other legumes and provides them with nitrogen.
Pulses (dals) are obtained from leguminous plants. In return, the plants provide food and shelter to the bacteria. They have a symbiotic relationship. This association is of great significance for the farmers. They do not need to add nitrogen fertilizers to the soil in which leguminous plants are grown.
Microbes are being employed for the production of both antibiotics and vaccines. Antibiotics: Antibiotics are produced by a number of microorganisms and inhibit the growth of other microorganisms even at very low concentrations. Fungi and bacteria are important microbes producing a variety of antibiotics.
Q. Why are children/infants given vaccination?
(i) Microbes, like bacteria and fungi, bring about the decay of dead plants and animals, both on land and in water. (ii) They act on dead plants and animals and convert various complex organic compounds into simple inorganic substances. Thus, microbes also help in recycling of nutrients. decompose polythene bags. (iii) They also help in sewage disposal by putrefaction (incomplete decomposition of dead organic matter).
Microorganisms can cause harm in many ways. Many microbes are the causative agents of various diseases that affect plants and animals. Such disease-causing microorganisms are called pathogens. They also cause damage of clothes, leather and spoilage of food.
(a) In human: Pathogens enter our body through various sources like air, water, food and infected person. The microbial diseases that can spread from an infected person to a healthy person through air, water, food, insects and physical contact are known as Communicable/Infectious diseases. e.g. Cholera, Common cold, Chicken pox, tuberculosis.
Q. Sneezing can cause infection to a healthy person. Explain?
Another example of a vector is mosquito, Female Anopheles mosquito, which is the vector for the parasite (Plasmodium), that is the causative agent of malaria. Similarly, Female Aedes mosquito is the vector of dengue virus which causes dengue fever.
(b) In animals: Microbes not only cause disease in humans but also in other animals. The following table shows the list of microorganisms affecting various animals.
Common animal diseases
(c) In plants: There are several microbes causing diseases in plants. The list of microorganisms affecting plants is shown below in the table.
Common plant diseases
The diseases in plants result in decrease in crop yield. Therefore, the plants have to be treated by chemicals to protect them from disease causing microorganisms.
The contamination of food by microorganisms results into food spoilage. The consumption of this spoiled food by humans or animals results into food poisoning. Microorganisms like bacteria and fungi are responsible for food poisoning. These microbes when grow on food substances produce toxin. These make the food poisonous causing vomiting, serious illness and even death.
Therefore, there is a need to preserve food from being spoiled.
The food that we eat is either in raw or cooked form. All types of food can be targets of microbes which result into food spoilage.
The process of treating and handling food with an aim to stop or slow down its spoilage while maintaining its nutritional value, texture, and flavour.
(i) Heat and cold treatment: Boiling of milk and liquid food products kills many microorganisms. Similarly, low temperature inhibits the growth of microbes.
Pasteurisation: This method was developed by scientist Louis Pasteur. It is used to store milk and liquid food products. In this the food is heated about for 15 to 30 seconds and then suddenly chilled and stored.
Low temperature Storage: Microorganisms present in food products cannot be destroyed by low temperature, but their metabolic activities can be arrested so that they can't act on food products at to temperature. Fruits, eggs and meat can be stored for long time.
Refrigeration: Fresh fruits, vegetables, fish, meat, eggs, etc. can be stored by keeping the temperature below .
Q. Why does the milk that comes in packet does not require boiling?
(ii) Chemical Method: Chemicals which are used to check the growth of microbes are called preservatives. Salts and edible oils are commonly used for this. The commonly used preservatives are sodium benzoate, sodium metabisulphite and potassium metabisulphite. This method is employed in the preservation of fruit juices, jams, squashes, fruits, etc.
(iii) Preservation by common salt: Common salt has been used to preserve meat and fish for ages. Concentrated solutions of common salt can kill microorganisms due to dehydration. Salting is also used to preserve amla (Indian gooseberry), tamarind and raw mangoes.
(iv) Preservation by sugar: Sugar reduces the moisture content which inhibits the growth of bacteria. Jams, jellies and squashes are preserved by sugar.
(v) Preservation by oil and vinegar: Use of oil and vinegar prevents spoilage of pickle because bacteria cannot live in this environment. Vegetables, fruits, fish and meat are often preserved by this method.
(vi) Preservation by radiation: In this method air tight packed food substances, are passed through a radiation beam, microorganisms present in food substances get destroyed due to radiation. This method is new and cheaper.
(vii) Storage and Packing: Dry fruits and vegetables are sold in sealed air tight packet to prevent the attack of microbes.
Q. Mango gets spoiled at room temperature within few days, but the mango pickle remains as such for a long time. Explain.
(i) This prevents the food from being spoiled by the action of microorganisms. (ii) It increases the storage period of food materials. (iii) It helps in obtaining the off-season food materials. (iv) It makes the transportation of food materials easier.
Anti - oxidants are also used to preserve food. They prevent the reaction of food with oxygen. e.g. chilli and pepper.
(Session 2025 - 26)