This MCQ module is based on: Useful and Harmful Microbes
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Useful and Harmful Microbes
🎓 Class 8
Science
CBSE
Theory
Ch 2 — Microorganisms: Friend and Foe
⏱ ~31 min
🌐 Language: [gtranslate]
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2.4.1 Useful Microorganisms — Our Tiny Helpers
While many people associate microorganisms with disease, the truth is that the vast majority of microbes are either harmless or actively beneficial. Some of the foods we eat every day exist only because of the work of helpful bacteria and fungi.
How Curd Is Formed from Milk L2 Understand
Have you ever wondered how liquid milk transforms into thick, tangy curd overnight? The answer lies in a tiny bacterium called Lactobacillus. Here is how it works:
- When a small amount of existing curd (which contains Lactobacillus bacteria) is added to warm milk, the bacteria get introduced into the milk.
- The bacteria feed on lactose (the natural sugar in milk) and multiply rapidly in the warm environment.
- As they feed, they produce lactic acid as a by-product.
- The lactic acid causes the milk proteins to coagulate (clump together and thicken), gradually converting the liquid milk into semi-solid curd.
- This entire process works best in warm conditions — which is why people typically set curd in a warm place and cover the container.
Yeast and Fermented Foods L2 Understand
Yeast is another incredibly useful microorganism — a single-celled fungus. When yeast is mixed into bread dough, it feeds on the sugar present and releases carbon dioxide gas. These tiny gas bubbles get trapped inside the dough, causing it to puff up and rise. After baking, the heat kills the yeast and evaporates the gas, leaving behind the soft, spongy texture we love in bread.
Many traditional Indian foods also rely on fermentation by microorganisms:
Idli & Dosa Batter
Rice and urad dal batter is left to ferment overnight. Natural bacteria and yeast produce gas, making the batter light and airy.
Dhokla
Fermented chickpea flour batter steamed to make a soft, spongy snack. Fermentation gives it a distinctive tangy flavour.
Bamboo Shoot Pickle
In Northeast India, bamboo shoots are fermented to create traditional preserved foods with a unique sour taste.
Activity — Make Curd at Home
L3 Apply
Think first: Would curd set faster in a warm kitchen or inside a cold refrigerator? Why?
- Boil about 250 ml of milk and let it cool until it is just warm (not hot — you should be able to dip your finger comfortably).
- Add one tablespoon of fresh curd to the warm milk and stir gently.
- Cover the vessel and place it in a warm spot (e.g., near the stove, wrapped in a towel).
- Check after 6 to 8 hours. Has the milk thickened into curd?
- Bonus experiment: Try the same process but place one vessel in a warm spot and another in the refrigerator. Compare results after 8 hours.
Expected result: The milk kept in a warm place will have set into thick curd, while the refrigerated milk will remain mostly liquid (or only slightly thickened). This confirms that Lactobacillus bacteria multiply and produce lactic acid faster in warm conditions. Cold temperatures slow down bacterial activity significantly, which is exactly why we store food in refrigerators — to prevent microbial growth and spoilage.
2.4.2 Nitrogen-Fixing Bacteria
Plants need nitrogen to grow — it is an essential component of proteins and chlorophyll. Although our atmosphere is about 78% nitrogen gas, plants cannot use nitrogen directly from the air. They need it in a "fixed" form dissolved in soil water. This is where certain remarkable bacteria come in.
Rhizobium bacteria live inside small, round swellings called root nodules found on the roots of leguminous plants (peas, beans, lentils, chickpeas, groundnuts). These bacteria trap nitrogen from the air and convert it into compounds that plants can absorb through their roots. This process is called nitrogen fixation.
Why Farmers Rotate Crops: Farmers have known for centuries that growing legumes (like moong, chana, or peas) in a field, then planting a different crop like wheat or rice afterwards, leads to a better harvest. Now we know the science behind it: the Rhizobium bacteria in the legume roots enrich the soil with nitrogen, acting as a natural fertiliser for the next crop. This practice is called crop rotation.
The Nitrogen Cycle (Simplified) L2 Understand
2.4.3 Microalgae — Tiny but Mighty
Microalgae are microscopic plant-like organisms that live in water, soil, and sometimes even on rocks and tree bark. Despite being invisible to the naked eye, they perform one of the most important functions on our planet: they carry out photosynthesis, using sunlight to produce food and releasing oxygen as a by-product.
Here is an astonishing fact: microalgae produce more than half of all the oxygen on Earth. That means every second breath you take contains oxygen generated by these tiny organisms floating in oceans, lakes, and rivers around the world.
Important Types of Microalgae L1 Remember
Spirulina
A spiral-shaped blue-green microalga rich in protein, vitamins, and minerals. Widely used as a health supplement and considered a "superfood."
Chlorella
A spherical, single-celled green microalga. Used as a dietary supplement and being researched as a potential future food source for growing populations.
Diatoms
Microalgae with stunning geometric glass-like shells made of silica. They play a crucial role in ocean food chains and even help clean water naturally.
Conservation Alert: Microalgae are threatened by water pollution, climate change, and habitat destruction. Since they produce more than half of Earth's oxygen and form the base of aquatic food chains, their decline would have devastating consequences for all life on our planet. Protecting water bodies from pollution is essential for the survival of these tiny but vital organisms.
Harmful Microorganisms
While many microbes are our allies, some can cause serious harm. Disease-causing microorganisms are called pathogens. Different types of microorganisms cause different diseases:
| Type of Microbe | Example Disease | How It Spreads |
|---|---|---|
| Virus | Common cold, influenza, chickenpox | Through air (coughing, sneezing), direct contact |
| Bacteria | Food poisoning, tuberculosis, cholera | Contaminated food/water, air, direct contact |
| Fungus | Athlete's foot, ringworm | Direct contact with infected skin, damp surfaces |
| Protozoa | Malaria, dysentery | Through mosquito bites, contaminated water |
Prevention: Simple practices can protect you from many microbial diseases — washing hands regularly with soap, drinking clean and boiled water, getting vaccinated, keeping food covered, and maintaining personal hygiene. These everyday habits form the first line of defence against harmful microorganisms.
Competency-Based Questions
Priya's grandmother sets curd every night by adding a spoonful of old curd to warm milk. One cold winter night, the milk did not turn into curd even after 10 hours. Priya's father, a farmer, always grows a crop of chickpeas before planting wheat in the same field.
Q1. L3 Apply Explain why the curd failed to set on the cold winter night. What could Priya's grandmother do differently to ensure success? (Short Answer — 2 marks)
Answer: Lactobacillus bacteria require warm temperatures to multiply and produce lactic acid efficiently. On a cold winter night, the low temperature slowed down bacterial activity dramatically, preventing the milk from fermenting into curd. To ensure success, Priya's grandmother could wrap the milk container in a thick towel or blanket to insulate it, or place it near a warm spot (such as near an oven or heater) to maintain the warmth needed for fermentation.
Q2. L4 Analyse Explain the scientific reason behind Priya's father growing chickpeas before wheat. How does this practice benefit the wheat crop? (Short Answer — 3 marks)
Answer: Chickpeas are leguminous plants that harbour Rhizobium bacteria in their root nodules. These bacteria carry out nitrogen fixation — they capture atmospheric nitrogen (N₂) and convert it into nitrogen compounds (like nitrates) that enrich the soil. When chickpeas are harvested and wheat is planted next, the wheat benefits from this naturally nitrogen-enriched soil, leading to better growth without needing as much artificial fertiliser. This practice of alternating legumes with other crops is called crop rotation.
Q3. L1 Remember Which of the following correctly matches the microorganism with its role?
Answer: C. Yeast is a single-celled fungus that feeds on sugar in bread dough and produces carbon dioxide gas, which causes the dough to rise and become fluffy. Lactobacillus makes curd (not malaria), Rhizobium fixes nitrogen (not bread), and Spirulina is a nutritious microalga used as a health supplement (not a disease-causing organism).
Q4. L5 Evaluate True or False: "All microorganisms are harmful and cause diseases." Support your answer with at least three examples of beneficial microbes. (3 marks)
Answer: False. The vast majority of microorganisms are either harmless or beneficial. Three examples of helpful microbes: (1) Lactobacillus — converts milk into curd through fermentation. (2) Rhizobium — fixes atmospheric nitrogen in the roots of leguminous plants, naturally enriching soil fertility. (3) Yeast — used in baking bread (produces CO₂ that makes dough rise) and in producing fermented foods. Additional examples include Spirulina (health supplement) and microalgae in general (produce more than half of Earth's oxygen).
Q5. L6 Create HOT: Microalgae produce more than 50% of Earth's oxygen, yet pollution threatens their survival. Suggest two actions that your school or community could take to protect local water bodies where microalgae live. (3 marks)
Hint: Think about what harms water quality — untreated sewage, chemical runoff from farms, plastic waste. Your suggestions could include organising community clean-up drives for nearby ponds/lakes, advocating for proper sewage treatment before discharge into water bodies, reducing use of chemical fertilisers near water sources, or conducting awareness campaigns about the importance of microalgae to our oxygen supply.
Assertion-Reason Questions
Assertion (A): Farmers often grow leguminous crops like peas or lentils in rotation with wheat or rice.
Reason (R): Rhizobium bacteria in the root nodules of legumes fix atmospheric nitrogen into the soil, naturally enriching it for the next crop.
Answer: A. Both are true and R correctly explains A. The practice of crop rotation with legumes exists precisely because Rhizobium bacteria naturally add nitrogen to the soil, reducing the need for artificial fertilisers and improving the yield of the subsequent non-leguminous crop.
Assertion (A): Curd sets faster in warm weather than in cold weather.
Reason (R): Lactobacillus bacteria multiply more rapidly at warm temperatures, producing more lactic acid that coagulates the milk proteins.
Answer: A. Both statements are true. The Reason correctly explains the Assertion — warmth accelerates the growth and metabolic activity of Lactobacillus, leading to faster lactic acid production and quicker curd formation. Cold temperatures slow bacterial metabolism significantly.
Assertion (A): Microalgae are important for the survival of life on Earth.
Reason (R): Microalgae produce more than half of all the oxygen in Earth's atmosphere through photosynthesis.
Answer: A. Both are true and R correctly explains A. Microalgae are vital because they are responsible for producing over 50% of Earth's oxygen through photosynthesis, making them indispensable for the survival of nearly all aerobic (oxygen-breathing) life forms on the planet.
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