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At first people thought all microbes were harmful. Later it was found that only a few cause disease. Most are harmless and many are very useful.
Cleaning the Environment
- Some microbes decompose dead matter.
- They break waste into simple substances.
- These mix into soil as nutrients.
- So microbes clean the environment.
Example 1 — Rotting Leaves
- Fallen leaves pile up in a garden.
- Microbes break them down over weeks.
- So the waste turns into nutrient-rich soil.
Example 2 — Animal Wastes
- Dung and other animal wastes are smelly.
- Microbes change them into harmless matter.
- So microbes recycle nutrients in nature.
Important Points
- Microbes acting this way are called decomposers.
- They cannot break down plastic, glass or polythene.
- This recycling keeps soil fertile for new plants.
Definition — Decomposition
The breaking down of dead plant and animal matter into simple substances by microbes.
Materials needed
Two flower pots marked A and B, garden soil, plant wastes (fruit and vegetable peels, fallen leaves), a polythene bag, an empty glass bottle and a broken plastic toy.
Procedure
Fill both pots half with soil.
In pot A, bury plant wastes such as peels and fallen leaves.
In pot B, bury the polythene bag, glass bottle and plastic toy.
Keep both pots aside for 3 to 4 weeks, then observe.
In pot A, bury plant wastes such as peels and fallen leaves.
In pot B, bury the polythene bag, glass bottle and plastic toy.
Keep both pots aside for 3 to 4 weeks, then observe.
Observation
In pot A the plant wastes have decomposed into manure. In pot B the polythene, glass and plastic remain unchanged.
Explanation
Microbes in soil can break down only natural matter like plant waste. They cannot break down polythene, glass or plastic. This proves microbes decompose organic waste but not man-made materials.
◆ Summary
- Pot A has plant waste
- Pot B has plastic
- Wait three weeks
- Pot A makes manure
- Pot B stays same
Making of Curd
- Bacteria turn milk into curd.
- A little curd is added to warm milk.
- Curd contains Lactobacillus bacteria.
- This bacterium sets the curd.
Example 1 — Setting Curd at Home
- Warm milk is mixed with a spoon of curd.
- Lactobacillus changes the milk sugar into lactic acid.
- So the milk sets into sour curd.
Example 2 — Idli and Bhatura
- Curd is added to make idli and bhatura batter.
- It makes them soft and spongy.
- So bacteria help make many foods.
Important Points
- This sugar-to-acid change is called fermentation.
- Idli and dosa batter also rise by fermentation.
- Warmth speeds up the setting of curd.
Definition — Fermentation
The breaking down of sugar by microbes, such as turning milk sugar into lactic acid to make curd.
Making of Bread
- Yeast is used to make bread.
- In dough, yeast multiplies quickly.
- It gives out carbon dioxide and alcohol.
- Gas bubbles make the dough rise.
Example 1 — Rising Dough
- Yeast is mixed in flour with sugar and warm water.
- Carbon dioxide bubbles fill the dough.
- So the dough swells and rises.
Example 2 — Holes in Bread
- The holes in bread are the trapped gas bubbles.
- They make the bread light and spongy.
- So yeast makes bread soft.
Important Points
- The holes in bread are trapped carbon-dioxide bubbles.
- Yeast also ferments sugar to make wine and beer.
- Lime water turns milky, proving the gas is carbon dioxide.
Definition — Yeast
A tiny fungus that gives out carbon dioxide gas, making bread, cakes and pastries rise.
Materials needed
Half a kilogram of white flour, some sugar, warm water and a small amount of yeast powder.
Procedure
Add sugar to the flour and mix with warm water.
Add a small amount of yeast powder.
Knead it all into a soft dough.
Keep the dough aside for about 2 hours and observe.
Add a small amount of yeast powder.
Knead it all into a soft dough.
Keep the dough aside for about 2 hours and observe.
Observation
After 2 hours the volume of the dough has increased. The dough has risen.
Explanation
Yeast respires using the sugar and gives out carbon dioxide gas. The gas bubbles fill the dough and make it expand. This proves that yeast is the microbe used to make bread, cakes and pastries.
◆ Summary
- Mix flour and sugar
- Add yeast and water
- Knead the dough
- Wait two hours
- Dough rises up
Increasing Soil Fertility
- Some soil microbes fix air nitrogen.
- They form nitrogen compounds in soil.
- This raises soil fertility.
- These are biological nitrogen fixers.
Example 1 — Rhizobium in Roots
- Rhizobium lives in root nodules of pulse plants.
- It fixes nitrogen from the air.
- So it makes the soil richer for the plant.
Example 2 — Crop Rotation
- Farmers grow pulses in turn with other crops.
- The nitrogen fixers refill the soil's nitrogen.
- So the next crop grows well without much fertiliser.
Important Points
- Plants cannot use nitrogen gas straight from the air.
- Crop rotation refills soil nitrogen without fertiliser.
- Microbes used this way are called biofertilisers.
Definition — Biological Nitrogen Fixers
Microbes that take nitrogen gas from the air and turn it into compounds that enrich the soil.
Microalgae
- Microalgae are tiny plant-like microbes.
- They grow in salt and fresh water.
- They make their own food.
- Examples: Chlorella, Spirulina, diatoms.
Example 1 — Spirulina
- Spirulina is a useful microalga.
- It is rich in protein, vitamin B12 and iron.
- So it has many health benefits.
Example 2 — Source of Oxygen
- More than half of Earth's oxygen comes from microalgae.
- They are food for humans and animals too.
- So we should conserve these microbes.
Important Points
- Over half of Earth's oxygen comes from microalgae.
- Spirulina is a "superfood", rich in protein and vitamin B12.
- So conserving microalgae protects oxygen and food supplies.
Definition — Microalgae
Tiny plant-like microscopic organisms that grow in water and make their own food.
⚠️ Did You Know — Harmful Microbes
A Few Are Harmful
Some bacteria and fungi spoil our food and make it go bad. A few microbes also cause diseases in humans, animals and plants. We notice these microbes through their actions.
Objective Type Questions Q1(j) - Page 34 Lakhmir Singh
Question
Name a biological nitrogen-fixer.
Show answer
Rhizobium.
Explanation
- Rhizobium lives in root nodules of legumes.
- It traps nitrogen from the air for plants.
Subjective Type Questions - Short Answer Q8 - Page 35 Lakhmir Singh
Question
A farmer was planting wheat crops and added nitrogen-rich fertiliser to the soil. In the neighbouring field, another farmer was growing bean crops but preferred not to add nitrogen fertiliser. Can you think of the reasons? (NCERT Book Question)
Show answer
Bean roots have Rhizobium bacteria that fix nitrogen from the air, so no extra nitrogen fertiliser is needed.
Explanation
- Beans are legumes with root nodules.
- The nodules hold Rhizobium, a nitrogen-fixing bacterium.
- This enriches the soil with nitrogen naturally.
- Wheat has no such bacteria, so it needs fertiliser.
Subjective Type Questions - Short Answer Q10 - Page 35 Lakhmir Singh
Question
Aanandi put sugar solution in test tubes A and B, and added yeast to test tube B. She fixed balloons on both and kept them warm. The balloon on B inflated. What is the explanation? (a) Water vapour filled it (b) Warm air expanded (c) Yeast produced a gas (d) Sugar reacted with air. (NCERT Book Question)
Show answer
(c) Yeast produced a gas inside test tube B which inflated the balloon.
Explanation
- Yeast uses the sugar and respires.
- It gives out carbon dioxide gas.
- This gas inflates the balloon on tube B.
- Tube A has no yeast, so its balloon stays limp.
Subjective Type Questions - Long Answer Q3(b) - Page 35 Lakhmir Singh
Question
How are microorganisms beneficial to us?
Show answer
Microbes help make curd and bread, decompose waste, fix nitrogen in soil and make medicines.
Explanation
- Lactobacillus sets milk into curd.
- Yeast makes bread soft and fluffy.
- Microbes decompose waste and clean the environment.
- Rhizobium fixes nitrogen and enriches the soil.
Subjective Type Questions - Long Answer Q4 - Page 36 Lakhmir Singh
Question
A student observes that when curd is left out for a day, it becomes more sour. What can be the possible explanations for this observation? (NCERT Book Question)
Show answer
The Lactobacillus bacteria in the curd keep multiplying and make more lactic acid, so the curd becomes more sour.
Explanation
- Curd contains Lactobacillus bacteria.
- In the warm air they keep growing in number.
- They change more milk sugar into lactic acid.
- More lactic acid makes the curd taste more sour.
Subjective Type Questions - Long Answer Q5 - Page 36 Lakhmir Singh
Question
Observe the set-up (warm sugar solution with yeast in flask A, connected to lime water in test tube B) and answer: (a) What happens to the sugar solution in flask A? (b) What do you observe in test tube B after four hours, and why? (c) What would happen if yeast was not added in flask A? (NCERT Book Question)
Show answer
(a) The yeast ferments the sugar and gives out carbon dioxide. (b) The lime water turns milky, because the carbon dioxide passes into it. (c) Without yeast, no gas forms and the lime water stays clear.
Explanation
- Yeast respires using the sugar in flask A.
- This releases carbon dioxide gas.
- The gas turns the lime water in tube B milky.
- With no yeast, there is no gas and no change.
Multiple Choice Questions Q5 - Page 36 Lakhmir Singh
Question
Which of the following increase the fertility of soil? A. Lactobacillus bacteria B. Rhizobium bacteria C. Spirogyra algae D. Blue-green algae (a) A and B (b) B and C (c) A and D (d) B and D
Show answer
(d) B and D.
Explanation
- Rhizobium bacteria fix nitrogen in root nodules.
- Blue-green algae also fix nitrogen.
- Both raise soil fertility.
