ScienceCBSE Class 8 · The Invisible Living World: Beyond Our Naked Eye

Microorganisms: Types, Habitats and Useful Roles in Class 8 Science

Updated 17 October 2026 · Fact-checked

Microorganisms are living beings too small to see with the naked eye. They include bacteria, protozoa, some algae and fungi, and they live in water, soil, air and inside bodies. Helpful ones decompose waste into manure, make bread soft (yeast), turn milk into curd (Lactobacillus), add nitrogen to soil (Rhizobium) and produce biogas.

Understand Microorganisms: Types, Habitats and Useful Roles

What are microorganisms? Organisms that cannot be seen without a microscope are called microorganisms or microbes. Some are unicellular (bacteria, protozoa such as Amoeba and Paramecium; yeast is a unicellular fungus), while others are multicellular (some algae; moulds are multicellular fungi). Bacteria have a nucleoid instead of a well-defined nucleus; fungal cells have a cell wall but no chloroplasts, so they cannot make food by photosynthesis. Viruses are microscopic and acellular: they multiply only inside a living cell of a plant, animal or bacterium, and may cause disease.

Where do they live? Activities 2.4 and 2.5 show that pond water and soil suspension contain Amoeba, Paramecium, algae, bread mould, other moulds and bacteria. They are found in water, soil, air, food, hot springs, snow-cold zones and inside our gut. Rotting fruit with a cotton-like growth is infected by microbes. Pickles and murabbas stay free of them because a high concentration of salt or sugar, with spices, does not let the microbes grow.

Cleaning the environment. Fungi and bacteria in soil break down fruit and vegetable peels, fallen leaves and dead animals into simpler, nutrient-rich substances. This is decomposition. The dark material that forms is manure, which makes the soil more fertile. Manure formation needs an optimal temperature and moisture. Some microbes live without oxygen and decompose plant and animal waste to give biogas, mainly carbon dioxide with a high proportion of methane, used for cooking, heating, electricity and vehicles.

Microbes in food. *Yeast* is a fungus. In dough with sugar, warm water and warmth, it respires and releases carbon dioxide bubbles that make the dough rise and soft, and a little alcohol that changes the smell. It is used for bread, cakes and pastries. *Lactobacillus* in curd feeds on the sugar of milk (lactose), multiplies and forms lactic acid, which sets and sours the milk; it grows well in warm conditions, so lukewarm milk sets into curd but cold milk does not. Lactobacillus also helps ferment the batter for idli and dosa and the dough for bhatura.

Nitrogen fixers. Microorganisms such as *Rhizobium* live in root nodules of legumes (peas, beans, lentils) and trap nitrogen from the air, which increases soil fertility. A bean grower can therefore use less nitrogen fertiliser than a wheat grower.

Microalgae. These microscopic plant-like organisms make food using sunlight, release oxygen (more than half of the Earth's oxygen supply), feed many aquatic animals, help clean water and are used for biofuel. Spirulina, Chlorella and diatoms are used as supplements. The chapter's account of Spirulina says it has more than 60 per cent protein by body weight and is a source of vitamin B12.

Scientists. Ananda Mohan Chakrabarty (1938 to 2020) developed in 1971 a bacterium that breaks down oil spills; the patent was granted in 1980.

Microorganisms: Types, Habitats and Useful Roles: formulas and key facts

Groups of microorganisms: Bacteria
Protozoa
Algae
Fungi (and viruses, which are acellular); Bacteria and protozoa are unicellular; algae and fungi may be unicellular or multicellular.
Bread dough with yeast: Sugar + warm water + yeast > carbon dioxide + a little alcohol; The gas bubbles make the dough rise; the alcohol changes the smell.
Curd formation: Lactose (milk sugar) + Lactobacillus (warm) > lactic acid > curd; Cold milk does not curdle in the same time.
Decomposition: Plant and animal waste + fungi and bacteria (optimal temperature, moisture) > simpler nutrient-rich manure; Nutrients return to the soil.
Biogas: Waste decomposed without oxygen > carbon dioxide + methane (mostly methane); Used as fuel.
Nitrogen fixation: Rhizobium in root nodules of legumes: nitrogen of air > soil nitrogen; Reduces the need of nitrogen fertiliser for beans, peas, lentils.

How to solve Microorganisms: Types, Habitats and Useful Roles questions

Use this method for activity-based and application questions on microbes.

1Identify the microorganism from the clue (single cell, thread-like, in root nodules, in curd).
2State the group (bacteria, protozoa, algae, fungi) and whether it is unicellular or multicellular.
3Link it to its process: decomposition, fermentation, nitrogen fixation, photosynthesis.
4For experiments, name the factor changed (warmth, yeast, sugar) and the observation (rise, curd, gas).
5Give the reason using the right term: carbon dioxide bubbles, lactic acid, nodules.
6State one benefit or harm in everyday life.

Microorganisms: Types, Habitats and Useful Roles in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Dough that rises

Suppose Neha kneads two lots of 200 g of flour, A with yeast and a pinch of sugar and B with only sugar, using warm water. After 5 hours, A's dough has doubled from 400 mL to its risen volume while B is unchanged at 400 mL. Imagine the risen dough measures 800 mL.

(a) By how much, and by what percentage, did A rise? (b) Why did B not rise? (c) What is the gas that caused the rise? (d) What would happen if A were kept in a refrigerator instead?

Try it first, then show the solutionSolution

Rise = 800 − 400 = 400 mL; percentage = 400 ÷ 400 × 100 = 100%.
B has no yeast, so no microorganism produces gas to puff up the dough.
Carbon dioxide, produced when yeast respires and breaks down sugar.
Yeast grows well in warm conditions; in the cold its activity would be much slower, so the dough would rise very little.

Answer: (a) 400 mL, 100%; (b) no yeast; (c) carbon dioxide; (d) very little rise because yeast needs warmth.

Memory hook: No yeast, no bubbles; no warmth, no rise.

Curd in different conditions

Imagine two bowls: bowl P has lukewarm milk and bowl Q has cold milk. A spoonful of curd is added to each. P is kept in a warm place and Q in the refrigerator for 8 hours.

(a) Which bowl forms curd? (b) Name the bacterium and the acid it produces. (c) What is the effect of cold on it? (d) Suppose 5 mL of curd is added to 500 mL of milk. What fraction of the mixture is curd, as a percentage?

Try it first, then show the solutionSolution

Bowl P forms curd because Lactobacillus grows well in warm conditions.
Lactobacillus produces lactic acid, which sours the milk.
Cold slows its growth, so Q does not set properly (it may only turn slightly sour).
Total volume = 505 mL (assuming the volumes add); 5 ÷ 505 × 100 ≈ 0.99%, about 1%.

Answer: (a) P; (b) Lactobacillus, lactic acid; (c) slows growth; (d) about 1%.

Memory hook: Warm milk, wake the bacteria.

Tricks to remember Microorganisms: Types, Habitats and Useful Roles

Short memory aids. Say each one aloud once, then use it on a question the same day.

BPAF
Bacteria, Protozoa, Algae, Fungi: the four kinds you saw under the microscope.
Use it when: Listing groups of microorganisms.
Yeast = bread, Lacto = curd, Rhizo = roots
Match each microbe to its job with the first letters: Y for bread's 'rise', L for 'lactic acid', R for 'roots'.
Use it when: Matching microbes to uses.
Peels to manure
Peels + soil + microbes + moisture + time = manure.
Use it when: Decomposition question.

Microorganisms: Types, Habitats and Useful Roles: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Identify the microbes · 3 marks · Short answerBoard level

Name the microorganism described: (i) lives in every kind of environment and inside the gut; (ii) makes bread and cakes soft; (iii) lives in the roots of pulse crops and gives them nutrients.

Try it first, then show the solutionSolution

(i) Bacteria. (1)
(ii) Yeast (a fungus). (1)
(iii) Rhizobium (a bacterium in root nodules that traps nitrogen from the air). (1)

Answer: (i) Bacteria; (ii) yeast; (iii) Rhizobium.

Example 2: Why pickles stay fresh · 1 mark · MCQBoard level

Pickles and murabbas do not get infected by microbes because
(a) they are kept in sunlight
(b) a high concentration of salt or sugar does not let microbes grow in them
(c) they contain no water at all
(d) microbes are not present in them

Try it first, then show the solutionSolution

Salt or sugar at a high concentration, with spices, acts as a preservative.

Answer: (b)

Example 3: Yeast and the balloon · 5 marks · Long answerHarder board level

Two test tubes A and B each hold two spoonfuls of sugar solution; B also has a spoonful of yeast. An incompletely inflated balloon is attached to each mouth. They are kept in a warm place. After 3 to 4 hours the balloon on B is inflated.
(a) What produced the gas? (1)
(b) Name the gas and the test to show it. (2)
(c) Why does A act as a comparison? (1)
(d) Why is the set-up kept warm? (1)

Try it first, then show the solutionSolution

(a) Yeast, which respired and broke down the sugar. (1)
(b) Carbon dioxide; shake the gas with lime water in a test tube: lime water turns milky. (2)
(c) A has the same sugar and warmth but no yeast, so any difference is due to yeast. (1)
(d) Yeast grows well in warm conditions. (1)

Answer: (a) Yeast; (b) carbon dioxide, lime water turns milky; (c) control without yeast; (d) yeast grows well in warmth.

Example 4: Planning an experiment on manure · 4 marks · Case-basedHarder board level

Read the passage and answer.

Snehal digs two pits. In pit A she puts fruit and vegetable peels mixed with dry leaves; in pit B she puts the same peels without the leaves. Both are covered with soil. After 3 weeks, A has dark, crumbly material while B is still wet and smelly.
(a) What factor is she testing? (1)
(b) Name the process and the organisms involved. (1)
(c) What might the dry leaves add? (1)
(d) Name two other conditions needed for the process. (1)

Try it first, then show the solutionSolution

(a) The effect of mixing dry leaves with the waste. (1)
(b) Decomposition by bacteria and fungi in the soil. (1)
(c) Dry leaves absorb extra moisture and let air reach the microbes, so decomposition is faster. (1)
(d) Optimal temperature and appropriate moisture. (1)

Answer: (a) Effect of dry leaves; (b) decomposition by bacteria and fungi; (c) leaves help microbes act on the waste; (d) temperature and moisture.

Example 5: Farmers and fertiliser · 4 marks · Case-basedHOTS / competency

Read the passage and answer.

One farmer grows wheat and adds nitrogen-rich fertiliser. A neighbour grows beans and adds none, yet her crop is healthy. Another farmer has a field where the soil is poor in nitrogen and wants to grow either wheat or lentils.
(a) Why does the bean farmer not need the fertiliser? (1)
(b) Name the microorganism and where it lives. (1)
(c) Which of wheat and lentils should be grown first, to raise soil fertility? Give a reason. (1)
(d) A student says 'Rhizobium are plants because they help plants.' Correct this. (1)

Try it first, then show the solutionSolution

(a) Bean plants have microbes in their root nodules that trap nitrogen from the air. (1)
(b) Rhizobium, a bacterium, in root nodules of legumes. (1)
(c) Lentils: legumes whose root nodules add nitrogen and so increase soil fertility. (1)
(d) Rhizobium is a bacterium, a unicellular microorganism, not a plant; helping plants does not make it one. (1)

Answer: (a) Nodule microbes supply nitrogen; (b) Rhizobium in root nodules; (c) lentils; (d) it is a bacterium.

Where students lose marks in Microorganisms: Types, Habitats and Useful Roles

The slips examiners see most often, and what to write instead.

Mistake: Writing that yeast is a bacterium.
Yeast and bacteria are both microscopic.
Wrong working
Yeast is a bacterium used in bread making.
Right working
Yeast is a fungus (unicellular).
Do this: Remember Y for yeast and F for fungus.
Usually costs: 1 mark.
Mistake: Confusing the gases or products of yeast and Lactobacillus.
Both are fermentation.
Wrong working
Lactobacillus produces carbon dioxide and alcohol.
Right working
Yeast gives carbon dioxide and a little alcohol; Lactobacillus gives lactic acid.
Do this: Yeast bubbles; Lactobacillus sours.
Usually costs: 1 mark.
Mistake: Forgetting the conditions.
Students state the process but not warmth, moisture or sugar.
Wrong working
Curd forms when curd is added to milk.
Right working
Curd forms when a little curd is added to lukewarm milk and kept warm, because Lactobacillus grows well in warmth.
Do this: Add the condition and the reason.
Usually costs: 1 mark.
Mistake: Calling all microorganisms harmful.
Disease is better known.
Wrong working
Microorganisms cause disease so we should destroy all of them.
Right working
Many microbes are useful: decomposers, yeast, Lactobacillus, Rhizobium; some cause disease.
Do this: Give both sides.
Usually costs: 1 mark.

Exam tips

For each microbe, give name, group, process, product and use in one line.
In experiments always say which tube or bowl is the control and why.
Use correct names: yeast is a fungus; Lactobacillus and Rhizobium are bacteria.
Lime water turns milky with carbon dioxide; write the test along with the gas.
Link a new case to the chapter's idea and give the reason, for example the effect of warmth on Lactobacillus.

Practice questions on Microorganisms: Types, Habitats and Useful Roles

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 2 marks · Very short answerBoard level
Name two unicellular and two multicellular microorganisms mentioned in the chapter.
Hint
Bacteria and Amoeba are unicellular; look at fungi and algae for multicellular.
Answer
Unicellular: bacteria, Amoeba, Paramecium, yeast. Multicellular: moulds (fungi), some algae (1 mark each pair).
Q2 · 1 mark · MCQBoard level
In which of the following does the fermenting microbe produce lactic acid?
(a) Bread dough with yeast
(b) Milk turning to curd
(c) Peels turning to manure
(d) Root nodules of a bean plant
Hint
Lactobacillus.
Answer
(b)
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Curd forms in warm milk but not in cold milk.
Reason (R): Lactobacillus grows well in warm conditions and forms lactic acid from the sugar in milk.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Does R explain the role of temperature?
Answer
(a) R explains A.
Q4 · 5 marks · Long answerHarder board level
Design an experiment to show that microorganisms need optimal temperature, air and moisture to grow. (a) State the question. (b) Describe a control and three set-ups, each changing one factor. (c) State what you observe in each. (d) State one safety rule.
Hint
Think of bread slices, as in the chapter: one near the sink, one in the refrigerator.
Answer
(a) Do temperature, air and moisture change the growth of microorganisms on bread? (1) (b) Four slices of the same bread kept for the same time: warm and moist (control); cold and moist (in the refrigerator); warm and dry; warm and moist in a sealed bag with little air. Each set-up differs from the control in one factor only (1 mark for the control and three correctly changed factors, 1 mark for same bread and same time). (c) Growth appears on the warm moist slice first; little on the cold one; little on the dry one; less in the sealed bag (1). (d) Do not open or smell sealed mouldy food; wash hands (1).
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
Curd left out for a day becomes more sour. A student offers two reasons. A cook asks whether adding more sugar to a dough would give a bigger rise without limit.
(a) Give two possible reasons for the sourness. (b) Why would a lot of extra sugar not always help the yeast? (c) Which gas comes out of the dough, and which substance gives the slightly different smell?
Hint
Bacteria multiply and keep forming acid. Recall pickles and sugar.
Answer
(a) The bacteria kept multiplying in the warm air and formed more lactic acid; more time allowed more acid to build up. (b) A very high concentration of sugar stops microbes from growing, as in murabbas, so too much sugar can slow the yeast. (c) Carbon dioxide; a small amount of alcohol gives the smell (1 mark each).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What are microorganisms?Show answer
Living beings too small to be seen with the naked eye.
2.Which gas makes dough rise?Show answer
Carbon dioxide, released by yeast.
3.What does Lactobacillus produce in milk?Show answer
Lactic acid, which forms curd and makes it sour.
4.What is decomposition?Show answer
The breakdown of complex substances from waste into simpler, nutrient-rich ones by microorganisms.
5.Where does Rhizobium live?Show answer
In the root nodules of legumes such as peas, beans and lentils.
6.What is biogas mainly made of?Show answer
Carbon dioxide and a high proportion of methane.

Microorganisms: Types, Habitats and Useful Roles: frequently asked questions

Are viruses microorganisms like bacteria?

The chapter says viruses are microscopic but acellular and reproduce only inside a host organism, so they differ from other microorganisms.

Why do pickles not rot?

A high concentration of salt or sugar, along with spices, does not let microorganisms grow on them.

What are microalgae used for?

They make food by photosynthesis and release oxygen, feed aquatic animals, help clean water, give biofuel and are used as supplements.

Why is idli batter fermented?

Some bacteria, such as Lactobacillus, help in the fermentation of batter for idli and dosa.