Microorganisms: Friend and Foe — NCERT Solutions, AI Tutor & Practice

Understand: Microorganisms (or microbes) are living organisms so tiny that they cannot be seen with the naked eye. They are observed only through a microscope.

Analyze: Based on their structure and characteristics, microorganisms are classified into four major groups:

1. Bacteria — e.g., Lactobacillus
2. Fungi — e.g., bread mould, yeast
3. Protozoa — e.g., Amoeba, Plasmodium
4. Algae — e.g., Chlamydomonas, Spirogyra

Conclude: Besides these four groups, viruses are also microorganisms, but they are different because they reproduce only inside the cells of a host (bacteria, plant, or animal).

Understand: Microorganisms are found almost everywhere on Earth.

Analyze: Their habitats include:

• Soil — bacteria and fungi live in and enrich the soil.
• Water — ponds, rivers, and oceans contain algae, bacteria, and protozoa.
• Air — many microbes float in the air.
• Bodies of other organisms — some live inside the bodies of humans, animals, and plants.
• Extreme conditions — some survive in ice-cold climates as well as in hot springs.

Conclude: Microorganisms may live singly (like Amoeba) or in colonies (like bacteria and fungi). Their ability to survive in extreme environments shows how adaptable they are.

Understand: Certain microorganisms are used to prepare and improve food.

Analyze:

1. Curd formation: The bacterium Lactobacillus converts milk into curd by multiplying in it.
2. Bread and cakes: Yeast reproduces rapidly and produces carbon dioxide during respiration. The gas bubbles cause the dough to rise, making bread soft and spongy.
3. Alcohol and wine: Yeast is used to convert sugars into alcohol through a process called fermentation.
4. Other foods: Bacteria are used in making cheese, pickles, and many fermented foods like idli and dosa batter.

Conclude: The process by which yeast converts sugars into alcohol and carbon dioxide is called fermentation, discovered by Louis Pasteur.

Understand: Antibiotics are medicines that kill or stop the growth of disease-causing microorganisms. They are produced from certain fungi and bacteria.

Analyze: Example — Penicillin, the first antibiotic, was discovered by Alexander Fleming in 1929 from a mould.

Precautions while using antibiotics:

1. Take antibiotics only on the advice of a qualified doctor and complete the full course prescribed.
2. Do not take antibiotics unnecessarily (e.g., for colds and flu caused by viruses), because they do not act against viruses, and overuse makes them less effective in future.

Conclude: Antibiotics are also mixed in the feed of livestock and used to control microbial diseases in plants and animals.

Understand: A vaccine is a preparation containing dead or weakened microbes that is introduced into the body to protect against a particular disease.

Analyze — how it works:

1. When a vaccine is introduced into the body, the body produces suitable antibodies to fight the (weakened) microbes.
2. These antibodies remain in the body even after the microbes are destroyed.
3. If the same disease-causing microbe later enters the body, the antibodies quickly fight and destroy it, preventing the disease.

Conclude: Vaccines protect us from diseases such as cholera, tuberculosis, smallpox, hepatitis, and polio. Edward Jenner discovered the vaccine for smallpox in 1798.

Understand: Plants cannot use the nitrogen gas present in the air directly. Certain microbes convert this nitrogen into usable forms.

Analyze:

1. Rhizobium bacteria live in the root nodules of leguminous plants (like beans, peas, gram). They fix atmospheric nitrogen into compounds that plants can absorb.
2. Some blue-green algae (cyanobacteria) also fix nitrogen directly from the air.
3. Microbes and fungi also decompose dead plants and animals, returning nutrients to the soil.

Conclude: In this way microorganisms increase soil fertility and maintain the nitrogen cycle in nature, ensuring nitrogen keeps circulating between the atmosphere, soil, plants, and animals.

Understand: Communicable diseases are diseases that spread from an infected person to a healthy person through air, water, food, or physical contact. Microorganisms causing such diseases are called pathogens.

Analyze: Examples:

| Disease | Causative Microbe | Mode of Transmission | |---------|------------------|---------------------| | Common cold | Virus | Air (droplets while sneezing/coughing) | | Tuberculosis (TB) | Bacterium | Air | | Malaria | Protozoan (Plasmodium) | Bite of female Anopheles mosquito |

Conclude: Insects and animals that carry disease-causing microbes are called carriers or vectors. For example, the housefly and the mosquito are common carriers.

Understand: Microorganisms that grow on food produce toxic substances that spoil it and can cause food poisoning when eaten.

Analyze — Preservation methods:

1. Chemical preservatives: Sodium benzoate and sodium metabisulphite prevent microbial growth in jams and squashes.
2. Preservation by salt (salting): Salt is added to pickles, meat, and fish to stop microbe growth.
3. Preservation by sugar: Sugar reduces moisture, checking bacterial growth (used in jams and jellies).
4. Preservation by oil and vinegar: Bacteria cannot survive in this environment (used in pickles).
5. Heat and cold treatments: Boiling, refrigeration, and freezing slow or stop microbial growth.
6. Pasteurisation: Milk is heated to about $70^\circ C$ for 15–30 seconds and then suddenly chilled, killing microbes.

Conclude: These methods work by removing moisture, heat, air, or by adding substances in which microbes cannot survive.

Understand: Pasteurisation is a method of preserving milk by killing harmful microbes present in it.

Analyze — the steps:

1. The milk is heated to about $70^\circ C$ for 15 to 30 seconds.
2. It is then suddenly chilled and stored.
3. This process prevents the growth of microbes and keeps the milk safe for a longer time.

Conclude: This method was discovered by Louis Pasteur, which is why it is called pasteurisation. Because of it, milk can be transported and stored without quickly turning sour.

Understand: Plants continuously absorb nitrogen compounds from the soil, so this nitrogen must be replenished.

Analyze:

• Excessive chemical fertilisers can pollute water sources, harm soil organisms, and reduce natural soil fertility over time.
• Natural alternative: Rhizobium bacteria in root nodules of leguminous plants and blue-green algae in the soil fix atmospheric nitrogen into usable compounds, naturally enriching the soil.
• Farmers can grow leguminous crops in rotation to naturally add nitrogen to the field.

Conclude: Using nitrogen-fixing microbes (biofertilisers) maintains the nitrogen cycle, keeps soil healthy, and reduces the need for harmful chemical fertilisers.

Understand: Nitrogen is essential for life (it is part of proteins and DNA). The atmosphere contains about 78% nitrogen gas, but it must be converted to usable forms and recycled.

Analyze — steps of the nitrogen cycle:

1. Nitrogen fixation: Rhizobium bacteria and blue-green algae convert atmospheric nitrogen into nitrogen compounds in the soil. Lightning also fixes some nitrogen.
2. Absorption by plants: Plants absorb these nitrogen compounds from the soil to make proteins.
3. Transfer to animals: Animals eat plants and obtain nitrogen compounds.
4. Decomposition: When plants and animals die, bacteria and fungi decompose them, releasing nitrogen compounds back into the soil.
5. Denitrification: Certain bacteria convert nitrogen compounds back into nitrogen gas, returning it to the atmosphere.

Conclude: This continuous circulation keeps the percentage of nitrogen in the atmosphere roughly constant.

Understand: Microbes can be either friends (useful) or foes (harmful) to us.

Analyze:

(a) Yeast — Useful. It is used in baking bread and in the production of alcohol through fermentation.

(b) Plasmodium — Harmful. This protozoan causes malaria in humans.

(c) Rhizobium — Useful. It fixes atmospheric nitrogen in the root nodules of leguminous plants, increasing soil fertility.

(d) Bacteria in curd (Lactobacillus) — Useful. It converts milk into curd.

Conclude: Whether a microbe is a 'friend' or 'foe' depends on its effect: some help in food, medicine, and soil fertility, while others cause diseases and food spoilage.