NCERT Solutions for Class 8 Science Chapter 2 The Invisible Living World Beyond Our Naked Eye (PDF) – 2027

You would find that almost nothing around you is really empty — every drop of water, every pinch of soil, the air, food and even your own body would be seen to be crowded with living beings.

You would see bacteria, Amoeba, Paramecium, green algae and moulds all around you, and you would see that every plant and animal body is built of tiny units called cells.

It changes all three ideas.

They interact in many ways.

• Living together and helping: Rhizobium lives in the root nodules of legumes and gives them nitrogen from the air; bacteria in our intestine help in digestion.
• Breaking down others’ remains: fungi and bacteria decompose dead leaves, peels, dung and dead animals and return nutrients to the soil.
• Being food for others: microalgae make their own food using sunlight and are eaten by many aquatic animals.
• Causing harm: some, like viruses, infect plant, animal or bacterial cells and may cause disease.

Sample answer (my questions):

• If a drop of pond water holds so many organisms, how many are there in the whole pond?
• Why does bread go mouldy in three days near the sink but not in the refrigerator?
• Are the microorganisms in soil from my garden the same as those in soil from a field?
• If bacteria live in my gut and help digestion, what happens to them when I fall ill?
• An onion peel cell has a cell wall and my cheek cell does not — what does the cheek cell use to keep its shape?
• Curd sets overnight in summer but takes much longer in winter. What exactly is temperature doing to the bacteria?

Yes — the letters of the book look larger when seen through the water-filled flask. The water in the round flask is thick in the middle and thin at the edges, like a lens, so the flask acts like a magnifying glass.

Yes. Details that the naked eye cannot separate become clear under a magnifying glass.

On an ant you can now make out:

• the body divided into distinct parts, with a narrow ‘waist’ between them;
• a pair of bent antennae that the ant keeps tapping the ground with;
• six jointed legs, each with tiny claws at the tip;
• fine hairs on the body, and the jaws in front.

The invention of the microscope. It was the microscope that first opened the hidden world of tiny living creatures.

Robert Hooke (1665) looked at a thin slice of cork through his microscope, saw small empty compartments and called each one a cell. Antonie van Leeuwenhoek (1660s) made better lenses and was the first to clearly see and describe tiny living things such as bacteria and blood cells — he is called the Father of Microbiology.

The smallest organisms most of us can see with the naked eye are tiny insects and mites — about the size of a full stop, or a little smaller. The eye can see only objects above a certain size (roughly a tenth of a millimetre). Anything smaller, such as bacteria and most other microorganisms, cannot be seen without a magnifying glass or microscope.

Both work in the same way — a curved piece of glass makes the object appear larger to the eye than it really is.

• Reading glasses use a lens that is thick in the middle and thin at the edge. Print that looked small and blurred is made to appear bigger and sharper, so the reader can make out the letters comfortably.
• A magnifying glass does the same for an object you hold under it. The ant, the grain of sand or the flower part fills a much larger part of your field of view, so details that were merged together become separate.

The stained onion peel under the microscope (Fig. 2.3c) looks very much like the wall made of bricks (Fig. 2.3d).

• Both are built from many small units of nearly the same shape — nearly rectangular cells, and rectangular bricks.
• The units are packed closely together with no space between them, in neat rows.
• Each unit has its own clear boundary — the cell wall in the peel, the edge of the brick in the wall.
• The whole structure is far bigger than any one unit, but it is made only of those repeated units.

An animal’s body is also made of cells. Onion peel is not a special case — every living being, plant or animal, is built of cells.

Like plant cells, animal cells have a cell membrane, cytoplasm and a nucleus, but they have no cell wall. Our own cheek cells (Activity 2.3) are an example.

You see polygon-shaped structures spread out on the slide, as shown in Fig. 2.4. These are cheek cells — they form the inner lining of your mouth.

In each cell you can make out three things:

• a thin outer boundary — the cell membrane;
• a round, deeply stained body near the middle — the nucleus;
• the space between the two, filled with cytoplasm.

Both are cells with the same three basic parts; the onion peel cell has one extra layer, and that changes how the whole tissue looks.

Each part does one essential job, and together they keep the cell — and therefore the organism — alive. Yes, these functions are essential; a cell could not survive if any of them stopped.

Yes. Even within one animal — even within one human body — cells differ widely in shape, size and structure.

• A muscle cell is shaped like a spindle — thick in the middle and tapering at both ends (Fig. 2.6a).
• A nerve cell is very long and has branches (Fig. 2.6b).
• Cheek cells, which you saw in Activity 2.3, are thin and flat and polygon-shaped.
• Some cells are round; others are long and thin.

Both are animal cells built on the same plan; they differ almost entirely in shape.

Yes — a cell’s shape and structure are directly related to the work it has to do. The unique shape, size and structure of cells help them carry out their specific functions.

Cells of the same kind group together, and different groups of cells do different jobs that together carry out a function of the body. For example, in digestion:

• muscle cells in the food pipe contract and relax in a wave-like manner and push the food down to the stomach;
• muscle cells in the stomach wall churn the food;
• other cells in the inner lining of the stomach produce digestive juices and acid that break down the food.

All these cells work together to make digestion possible.

They are partly like plant and animal cells and partly different — and the differences are exactly what separate one kind of microbe from another.

What is the same: like animal and plant cells, the cells of microorganisms are also surrounded by a cell membrane, and they contain cytoplasm.

What is different:

“In pond water I saw two kinds of organisms. One was a single cell with no fixed shape that slowly changed its outline as it moved — this matches Amoeba. The other was green and rounded and drifted about — a single-celled alga. In the soil suspension I found short rod-shaped bodies that were not green, and one branched thread with tiny round heads on it, which matches a mould. I did not see Paramecium.”

Yes — microorganisms can be found everywhere.

• In water, soil and air, and even in some food items.
• On the surfaces of leaves, stems, roots or any other part of a plant — you can explore these with a microscope.
• In extreme climatic conditions such as hot water springs and snow cold zones, as well as at moderate temperatures.
• Inside our own bodies, especially in our gut. As you studied in ‘Life Processes in Animals’ in Curiosity, Grade 7, our intestine has many bacteria that help in digestion.

The microbes came from the surroundings. Microorganisms are present everywhere — in air, water and soil — so they reach the fruit from the air, from the surface it is kept on and from hands or water that touch it. A ripe fruit is moist and sugary and the room is warm, so the microbes grow and multiply on it and form the powdery or cotton-like patch.

Because pickles and murabbas contain a lot of salt or sugar. The salt or sugar, added along with spices, acts as a preservative: a high concentration of salt or sugar does not allow microorganisms to grow on them.

Different microorganisms do different useful jobs for us. Yeast and Lactobacillus help make bread, idli, dosa and curd; bacteria in our gut help in digestion; Rhizobium in the root nodules of legumes traps nitrogen from the air; microalgae release oxygen and feed aquatic animals.

They clean the environment by decomposition: fungi and bacteria break down dead plants and animals, fruit and vegetable peels and dung into simple, nutrient-rich manure, and the nutrients return to the soil. Some bacteria also make biogas from waste, and some can break down oil spills.

Yes. The fruit and vegetable peels are no longer recognisable — they have turned into a dark-coloured material.

What you will notice after 2–3 weeks:

• The peels have lost their original colour and shape and become a dark, crumbly material.
• The total volume of the contents has gone down.
• The material smells earthy rather than of rotting fruit.

This dark material is manure. It is rich in nutrients and helps increase the fertility of the soil.

The microorganisms already living in the garden soil did it. In Activity 2.6 you saw that soil contains various kinds of microorganisms. Some of these — fungi and bacteria — act on the plant waste and slowly break it down into simpler, nutrient-rich manure.

Yes — they are making natural manure. The pit is Activity 2.7 done on a larger scale.

Fungi and bacteria in the pit slowly decompose the leaves and plant waste into dark, nutrient-rich manure. This manure is mixed into the soil and increases its fertility, so the waste is recycled instead of being burnt or thrown away.

Life as we know it could not continue. The most serious loss would be that nothing would decay — nutrients would be locked up in dead bodies for ever.

Different microorganisms help us make different foods. Yeast (a fungus) makes dough rise for bread, cakes and bhaturas; Lactobacillus (a bacterium) turns milk into curd and ferments the batter for idli and dosa.

Wherever something has to be broken down, cleaned up or built from waste, a microorganism is a possible answer. Chakrabarty’s bacterium of 1971 broke down oil spills; the same idea can be turned on many other problems.

• Household waste: kitchen waste and leaves can be turned into manure by microbes.
• Sewage and animal dung: bacteria can decompose them and give biogas as a fuel.
• Overuse of chemical fertilisers: Rhizobium in legumes adds nitrogen to the soil naturally.
• Dirty water: microalgae help clean water and can be used to make biofuel.
• Poor nutrition: Spirulina is a protein-rich food supplement.

Yes — but only in bowl A, the one with yeast.

Yeast is a microorganism (a fungus). In the warm, moist dough it respires — it breaks down the sugar to get energy and releases carbon dioxide. Bubbles of this gas are trapped in the dough, so the dough rises and becomes soft and fluffy. A small amount of alcohol formed at the same time gives the dough a different smell.

Sugar is added as food for the yeast. Warm water is used because yeast grows well in warm conditions.

Bowl A (lukewarm milk, kept warm) sets into curd; bowl B (cold milk, kept in the refrigerator) stays liquid. Here is Table 2.4 filled in.

The curd we added contains bacteria, mainly Lactobacillus. In the lukewarm milk of bowl A these bacteria feed on the milk sugar (lactose), multiply and ferment the milk. They produce lactic acid, which sets the milk into curd and makes it sour. These bacteria grow well only in warm conditions, so in the cold milk of bowl B they grow very slowly and the milk does not curdle (it may turn a little sour).

Fill the diagram like this:

(i) Prediction: the balloon on test tube B (with yeast) will inflate; the balloon on test tube A will not.

Correct explanation: (c) Yeast produced a gas inside the test tube B which inflated the balloon. Yeast respires and breaks down the sugar, releasing carbon dioxide. Tube A had the same sugar solution and warmth but no yeast, and its balloon did not inflate — so (a), (b) and (d) are wrong.

(ii) She wants to find out which gas the yeast produced — whether it is carbon dioxide. If the lime water turns milky, the gas is carbon dioxide.

Beans are legumes. Their roots have swollen root nodules containing Rhizobium bacteria, which trap nitrogen from the air and make it useful for the plant. So the bean crop gets nitrogen naturally and grows well without nitrogen fertiliser. Wheat has no such root nodules, so that farmer has to add nitrogen-rich fertiliser to get a good yield.

She is testing whether mixing dried leaves with the wet peels makes decomposition work better — that is, whether the dried leaves change how quickly and how well the waste turns into manure.

The two pits differ in only one thing — the dried leaves — so any difference after 3 weeks is due to them. Pit A is likely to give better manure faster, because dry leaves keep the heap loose and airy and soak up extra moisture, which helps the microorganisms decompose the waste.

Use bread as the food for the microbes, and set up four pieces that differ from one another in only one condition at a time.

What you need: four slices of the same bread, four clean transparent containers or polythene bags, a little water, a spoon or gloves.

How to read the result: A grows well. If B, C and D each grow much less than A, then each of the three conditions you removed — warmth, moisture, air — is needed. That is the conclusion the experiment is designed to reach.

Observation: after three days the slice near the sink has cottony or powdery patches of mould (white at first, later greenish-black) and smells musty. The slice in the refrigerator shows little or no mould.

Reasons: tiny fungal forms from the air settle on both slices. Near the sink it is warm and moist — the conditions in which microorganisms grow — so the mould grows and spreads. In the refrigerator the low temperature slows down their growth almost completely.

1. The Lactobacillus in the curd keeps feeding on the milk sugar (lactose) that is left and goes on producing lactic acid. More acid makes the curd more sour.
2. Outside the refrigerator it is warm, and these bacteria grow well in warm conditions, so they multiply faster and make more acid in a day. Microbes from the air may also settle on the curd and add to the souring.

(i) The yeast respires and breaks down the sugar in the solution, so the sugar is gradually used up. Carbon dioxide is released (bubbles and froth appear) and a little alcohol is formed.

(ii) The lime water in test tube B turns milky. This happens because the carbon dioxide from flask A passes through the tube into the lime water, and lime water turns milky with carbon dioxide.

(iii) Without yeast nothing would happen: the sugar would not be broken down, no carbon dioxide would form, and the lime water would stay clear.

Sample answer — my questions, with answers:

1. Why does a fungus not grow on pickles? — The high concentration of salt (or sugar in murabbas) does not allow microorganisms to grow.
2. When does curd set fastest? — In a warm place, because Lactobacillus grows well in warm conditions.
3. How long do peels take to turn into manure? — About 2–3 weeks in moist garden soil (Activity 2.7).
4. Where does Rhizobium live? — In the root nodules of legumes such as beans, peas and lentils.
5. Why not use cold milk to set curd? — In the cold, Lactobacillus grows very slowly, so the milk does not set.

The Ministry of New and Renewable Energy (MNRE) runs a biogas programme that helps families, farmers and dairies set up small biogas plants. It began in 1981–82 as the National Project on Biogas Development, later became the National Biogas and Manure Management Programme, and is now part of the National Bioenergy Programme. The plants are fed with cattle dung and kitchen or farm waste; bacteria decompose it without oxygen and give biogas for cooking and lighting, and the leftover slurry is used as manure. The government gives financial help for building the plants.

Importance: fermented foods keep longer than fresh food, are easier to digest, get their special sour taste and smell from fermentation, and are part of family and festival traditions of each region.

What to look at, in order:

Sample answer — steps for growing oyster mushroom:

1. Chop paddy or wheat straw and soak it in water.
2. Treat the wet straw with hot water or steam to kill unwanted microbes; cool and drain it.
3. Mix in the spawn (grain carrying the mushroom fungus) in layers.
4. Pack the mixture in polythene bags with small holes and keep them in a dark, humid room with moderate temperature.
5. After 2–3 weeks, when the bags turn white with fungal threads, open them to light and air; spray water to keep them moist.
6. Pick the mushrooms a few days later; two or three more crops follow. The used straw becomes manure.