NCERT Solutions Class 8 Science Chapter 2 the Invisible Living World Beyond Our Naked Eye

F R E E S T U D Y M AT E R I A L F O R E V E R Y S T U D E N T

CLASS 8 · SCIENCE

NCERT Solutions

Chapter 2: The Invisible Living
World: Beyond Our Naked Eye

NCERT Textbook — Curiosity

BOOK PAGES SECTIONS QUESTIONS MEDIUM

8 – 27 19 47 English

Solutions, notes, sample papers & more at 50 pages

Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

CLASS 8 · SCIENCE · CURIOSITY

NCERT Solutions — Chapter 2: The Invisible Living
World: Beyond Our Naked Eye
Complete NCERT Solutions for Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye
from the NCERT textbook Curiosity. The chapter takes you from a water-filled flask that behaves like a lens, to
the onion-peel and cheek cells you mount on a slide, to the bacteria, fungi, protozoa and microalgae that live
in a drop of pond water — and shows how these invisible organisms make our manure, our curd and our
bread.

TEXTBOOK BOOK PAGES

Curiosity (Class 8) 8 – 27

SECTIONS QUESTIONS

19 47

MEDIUM

English

Probe and ponder — Page 8
Chapter opening — the hidden world beyond the naked eye

PROBE AND PONDER

Q1 Have you ever wondered what you might see if the invisible world around you
became visible?

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.
This is exactly what you will see in this chapter when you actually look:

A single drop of pond or stagnant water on a slide shows Amoeba and Paramecium moving
about, and green single-celled algae (Activity 2.4, Table 2.1).
A drop taken from the top of a soil suspension shows bread mould, other moulds, algae and
bacteria of many shapes — spherical, comma, spiral and rod-shaped (Activity 2.5, Table 2.2).
A thin onion peel looks like a wall of neat rectangular bricks; a scrape from the inside of
your cheek shows polygon-shaped cells.
Fungal growth appears as a powdery or cotton-like patch on a rotting lemon or tomato, and
your own gut holds bacteria that help you digest your food.

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Why we cannot see them: the human eye can only see objects above a certain size.
These organisms are far below that size, so for a very long time they simply
remained unknown. A microscope magnifies them 100 to 400 times, which is what
makes them visible.

Q2 How do you think your observation of this hidden world might change the way you
think about size, complexity, or even what counts as ‘living’?

It changes all three ideas.

IDEA WHAT YOU WHAT THE HIDDEN WORLD SHOWS
THOUGHT
BEFORE

Size Living things are There is no lower size limit to life. Bacteria are far smaller than the
things you can see smallest speck your eye can catch, yet they eat, grow and multiply. At
the other end, the yolk of an ostrich egg is a single cell about 130 mm
to 170 mm across.

Complexity Something so small A cell is not just a simple bag of liquid. It has a cell membrane,
must be simple cytoplasm, a nucleus, and in plants plastids and a vacuole — each part
with its own special job. A one-celled Amoeba carries out every life
process inside that one cell.

What counts Living and non- Viruses are microscopic and acellular — they are not made of cells at
as ‘living’ living are clearly all, and they multiply only after entering a living cell. So the boundary
separate of ‘living’ turns out to be far less sharp than it looks.

Did you know? The change also works the other way. Once Hooke saw that cork
was made of small empty compartments, the idea that all bodies are built from cells
became possible — and that single idea explains a plant, an insect and you.

Q3 Have you thought how these tiny living beings interact with each other?

They interact constantly — with each other, with plants and animals, and with us. The
chapter gives real examples of each kind of interaction.

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

They live inside larger organisms and help them. Rhizobium lives in the swollen root
nodules of legumes such as beans, peas and lentils; it traps nitrogen from the air and makes
it useful to the plant, and the plant shelters and feeds the bacteria. Bacteria in our intestine
help in digestion.
They feed on the remains of other organisms. Fungi and bacteria act on fallen leaves, fruit
and vegetable peels and even dung, and break these complex substances into simpler,
nutrient-rich ones — decomposition. The nutrients go back to the soil, and plants use them
again.
They feed one another. Microalgae make their own food using sunlight and become food
for many aquatic animals — the beginning of the food chain in a pond.
They compete, and can be kept out. Pickles and murabbas do not spoil because a high
concentration of salt or sugar does not allow these organisms to grow on them.
Some of them attack. Viruses may infect plants, animals or even bacterial cells and cause
disease.

Why it matters: almost every one of these interactions ends with something being
broken down and passed on — nutrients back to the soil, sugar into curd, plant
waste into manure. That recycling is the microbes' main job in nature.

Q4 Share your questions

Write down the questions the pictures on this page actually raise for you — a good
question here is one you could later test with a slide, a microscope or a kitchen experiment.
How to frame one: take something you noticed, add why, how, where or what if, and check that
an observation could answer it.
Sample 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?

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

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Tip: keep this list in your notebook. At the end of the chapter the book asks you to

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Activity 2.1: Let us observe — Page 9 om
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A water-filled flask behaving like a lens
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reaches your eye as though it had come from a much bigger letter. So the flask of
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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Why the magnifying glass helps: two points on the ant's leg that are too close
together look like one point to the naked eye. The lens makes the ant occupy a much
larger part of your field of view, so those two points now fall far enough apart for the
eye to see them separately. That is all magnification does — and it is why each better
lens, from the magnifying glass to the microscope, showed people more than the
one before it.

Q3 For a long time, people were curious to explore the tiny organisms around them,
but they could not see them with their naked eyes. So, how did we finally discover
this invisible world? Do you know which scientific discovery helped us see the tiny
world for the first time?

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

SCIENTIST WHEN WHAT WAS DONE

Robert Hooke 1665 Published Micrographia with detailed drawings of tiny things nobody had
seen before. His microscope made things look 200 to 300 times bigger
than the unaided eye could see. Looking at a thin slice of cork he found
many small empty compartments that reminded him of a honeycomb, and
called each one a cell — the first use of the word in science.

Antonie van 1660s A Dutch scientist who made better lenses and built more useful
Leeuwenhoek microscopes. He was the first person to clearly see and describe tiny
living things such as bacteria and blood cells, and is known as the Father
of Microbiology.

Why it took so long: the difficulty was never a lack of curiosity — it was that no tool
existed to make these organisms big enough to see. Once lenses were improved
enough to be combined into a microscope, the whole world of microbes became
visible in a single generation.

In-text Questions — Page 9

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

How small a thing can the eye actually see?

Q1 Have you ever noticed the smallest organism around you that is visible to the
naked eye? Think about it — how small a thing can your eyes actually see?

The smallest living things most of us can spot unaided are creatures like a small ant, a
mite on a leaf, or a tiny insect on a flower — roughly the size of a full stop or a mustard
seed. Below about that size the eye can no longer separate an object from its background.

Why there is a limit: the human eye can only see objects that are above a certain
size. An object smaller than that forms too small an image for the eye to register as
a separate shape, so it simply disappears from view. This is not a failure of attention
— it is a fixed limit of the eye, and it is exactly why so many living things around us
stayed unknown for so long.

Try This: put a full stop on paper with a sharp pencil and step back slowly. The
distance at which the dot vanishes tells you the limit of your own eye. Now look at
the same dot through a magnifying glass — it becomes a rough, uneven blob. Every
organism in this chapter is smaller than that dot.

Q2 You might have seen some people using reading glasses. How does it help them see
better? Or what happens when we use a magnifying glass to observe something?

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.

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Why it happens: the piece of glass is curved — thick in the middle, thin at the edge,
like a lentil seed, which is why it is called a lens. Light from the object is bent as it
passes through the lens and enters the eye as if it had come from a much larger
object. Improving lenses in exactly this way, step by step, is what finally produced
the microscope.

Activity 2.2: Let us study a cell — Page 11
Section 2.1 What Is a Cell? — onion peel under the microscope

ACTIVITY

Q1 What similarities do you find in Fig. 2.3c and Fig. 2.3d?

Cell wall
Cell membrane
Nucleus

Cytoplasm

(c)

(d)

Fig. 2.3: (c) Structure of onion peel under the microscope; and (d) A wall made of brick.

The stained onion peel under the microscope (Fig. 2.3c) looks exactly 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.

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

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.

Why the book uses this picture: it fixes the central idea of the chapter in one image
— the cell is the basic unit of life, just as the brick is the basic unit of a wall. You
cannot understand a wall by studying mortar; you understand it by studying one
brick and then how the bricks are put together. The same is true of a leaf, an onion
or your own body.

Tip: the safranin does not create the cells — it only stains them a pinkish colour so
that their boundaries stand out against the background. The glycerin keeps the peel
from drying and makes the picture clearer.

Q2 What do you think the body of an animal is made of?

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.
You can prove it on yourself in the very next activity: scrape the inside of your cheek with the
blunt end of a clean toothpick, stain the material with methylene blue and look at it under the
microscope. You will see polygon-shaped cheek cells, which form the inner lining of your
mouth (Fig. 2.4).

What is the same and what differs: both onion peel cells and cheek cells have the
three basic parts — cell membrane, cytoplasm and nucleus. But the onion peel
cell has one extra outer layer, the cell wall, which the animal cell does not have. That
single difference is why the peel looks like a rigid brick wall and the cheek cells look
soft and irregular.

Activity 2.3: Let us investigate — Page 12

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Section 2.1 What Is a Cell? — human cheek cells
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Q2 What similarities and differences did you observe between the cells of onion peel in
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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

ONION PEEL CELL (PLANT) CHEEK CELL (ANIMAL)

Cell Present Present
membrane

Cytoplasm Present Present

Nucleus Present Present

Cell wall Present — an extra outer layer Absent

Shape Nearly rectangular, definite and regular Polygon-shaped, thin and flat

Arrangement Closely arranged in rows with no space between Loosely spread out or overlapping
them, like bricks on the slide

Stain used Safranin (red) Methylene blue

Why the difference in shape: the cell wall in the plant cell provides rigidity and
strength. A wall is stiff, so it holds the cell to a definite rectangular outline and lets
the cells pack compactly against one another. An animal cell has only the flexible cell
membrane, so it takes a softer, irregular outline and can change shape.

In-text Questions — Page 12
Section 2.1 — the parts of a cell and what each one does

Q1 What is the importance of these structures in a cell? What functions do they
perform? Are these functions important for the maintenance of life?

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.

Page 10 of 50

Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

PART OF WHAT IT DOES WHAT WOULD HAPPEN
THE CELL WITHOUT IT

Cell Encloses the cytoplasm and the nucleus, and The cell would have no
membrane separates one cell from another. It is porous, so it boundary; useful materials
allows the entry of materials essential for life would leak out and waste would
processes and the exit of waste material. collect inside

Cytoplasm Holds the other components of the cell and There would be no place for the
compounds such as carbohydrates, proteins, fats and cell's chemical work to happen
mineral salts. Most of the life processes take place
within the cytoplasm.

Nucleus Regulates all activities that occur within the cell, and The cell's activities would be
also regulates growth. uncontrolled and it could not
grow properly

Cell wall Provides rigidity and strength to the plant, which is A plant would have no firm
(plant cells) why plant cells are arranged compactly and look firm. structure to stand up with

Cell wall
Nucleus

Vacuole

Cell membrane

Chloroplast
Cytoplasm
(a) Animal cell (b) Plant cell

The same three basic parts — cell membrane, cytoplasm and nucleus — in an animal cell and a plant
cell. The plant cell has, in addition, a cell wall, chloroplasts and a large vacuole (Fig. 2.5).

The idea behind it: a cell is not just a simple bag of liquid. It is a complex structure
made of many different parts, each with its own special function, and it is this
division of work that allows the cell — and in turn the entire organism — to work.

In-text Questions — Page 13

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Section 2.1.1 Variation in shape and structure of cells

Q1 Do different animal cells also vary in their shape and structure?

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.

The number of cells also varies from organism to organism — from a single cell in a bacterium
or Amoeba to countless cells in a tree or a human being.

Why they vary: because they do different jobs. The unique shape, size and structure
of a cell help it carry out its own specific function — a point the next two questions
take further.

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Q2 The muscle cell and the nerve cell of a human are shown in Fig. 2.6a and (b). What
are the similarities and differences you see in them?

(a) Muscle cell

(b) Nerve cell

Fig. 2.6: Different types of cells in humans.

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

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

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MUSCLE CELL (FIG. 2.6A) NERVE CELL (FIG. 2.6B)

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Basic parts
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Cell membrane, cytoplasm, nucleus

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Function

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movement body to another

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many others, so a message can spread quickly through the body.

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Why do cells look so different
cell relate to its function?

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Yes —.ca cell's shape and structure are directly related to the work
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CELL ITS SHAPE THE FUNCTION THAT SHAPE MAKES
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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Why it happens: a shape is useful only for a particular kind of job. A message has to
travel a long distance, so the cell that carries it is drawn out into a long thread with
branches at the ends. A lining must cover an area without gaps, so its cells are flat
and broad. A cell that has to pull must be able to shorten, so it is slender and
flexible. Change the shape and the cell could no longer do its work — which is why
cells that do the same job look alike wherever you find them.

In-text Questions — Page 14
Section 2.2 What Are the Levels of Organisation in the Body of a Living Organism?

Q1 But how do these cells help in performing various functions in the body? Let us find
out.

Cells of one kind group together, and the group does a job that no single cell could do
alone. The digestive system, which you studied in Grade 7, shows this clearly.

The food pipe contains a group of muscle cells. These cells contract and relax in a wave-like
manner, pushing the food down to the stomach. The wave is possible only because the cells
are thin, flexible and spindle-shaped, and because they act one after another rather than all
at once.
The stomach wall also has muscle cells; these churn the food.
Other cells in the inner lining of the stomach produce digestive juices and acid that help
break down the food.

None of these cells digests food by itself. Digestion is possible because all of them work
together.

Cell Tissue Organ Organ system Organism

muscle cell muscle tissue stomach digestive system human being

The levels of organisation (Fig. 2.7), with the digestive system as the example.

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

The idea behind it: a group of similar cells forms a tissue; different tissues are
organised into an organ; several organs working together make an organ system;
and all the organ systems together make the complete organism. The cell is the
basic unit of life, just as a brick is the basic unit of a wall — but a single brick is not a
wall, and a single cell is not a stomach.

In-text Questions — Page 15
Section 2.3 What Are Microorganisms?

Q1 But what do their cells look like? Are they like the plant and animal cells we just
learnt about, or are they different?

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:

MICROORGANISM HOW ITS CELL DIFFERS

Fungi (yeast, moulds) Have a cell wall in addition to the cell membrane, but no chloroplasts — so
they cannot make their own food through photosynthesis

Bacteria Do not have a well-defined nucleus or a nuclear membrane. Instead they have a
nucleoid. This feature distinguishes them from the cells of yeast, protozoa, algae,
fungi, plants and animals

Protozoa (Amoeba, Single-celled, with a well-defined nucleus; carry out every life process in one
Paramecium) cell

Algae and microalgae Contain a green pigment, so they make their own food using sunlight

Some microbes are unicellular (bacteria, protozoa, and yeast among the fungi) and some are
multicellular (moulds, some algae).

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Why we need a microscope to tell them apart: all these differences lie inside a
body far below the size the eye can see. A microscope that magnifies 100 to 400
times shows the shape and the main parts. To see the smaller components inside a
cell you need an electron microscope, which magnifies the cell about 10,00,000
times.

Activity 2.6: Let us study — Page 16
Section 2.3 — recording what pond water and soil suspension contain

ACTIVITY

Q1 Did you also observe any of these microorganisms or something different? Record
in your notebook and discuss in your class.

Do the observation first, then record exactly what you saw — shape, whether it moves,
whether it is green — and only then try to name it. The two tables in the book (Tables 2.1
and 2.2) are the record made by a group of Grade 8 students; yours should be built the same
way.
What the book's record shows:

TABLE 2.1: ORGANISMS PRESENT IN POND WATER

S.NO. MICROORGANISM RECORDED DATA
OBSERVED

1. Amoeba (Protozoa) Single cell, moving, irregular shape

2. Paramecium (Protozoa) Single cell, moves from one place to another, movement takes
place with the help of specialised structures

3. Algae Single cell, looks green because of the presence of green
pigment, movement takes place with the help of specialised
structures

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

TABLE 2.2: ORGANISMS PRESENT IN SOIL SUSPENSION

S.NO. ORGANISM REMARKS

1. Bread mould Branched filament without chlorophyll having sac-like structure
(Fungi)

2. Mould (Fungi) Branched filament without chlorophyll having brush-like structure

3. Algae Spherical, presence of chlorophyll — a green pigment

4. Bacteria Spherical, comma, spiral or rod-shaped, one long hair-like structure and
many small hair-like projections around the cell

Sample answer: “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.”

Tip: use the four columns of the book's tables as your own headings — shape, single
cell or many cells, green or not green, moving or still. Those four observations are
enough to place almost anything you see into protozoa, algae, fungi or bacteria. And
follow the book's instruction in Activity 2.5: do not touch the soil with your bare
hands — use a spoon or gloves.

Why the soil is left to settle: when you stir soil in water, the heavy grains sink but
the very fine particles stay suspended — this is the soil suspension. A drop taken
from the top layer after settling has few heavy grains to block the view, so the tiny
organisms in it can actually be seen.

In-text Questions — Page 18
Section 2.4 How Are We Connected to Microbes?

Q1 Can we find microorganisms in other places, too?

Yes — microorganisms can be found everywhere.

In water, soil and air, and even in some food items.

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On the surfaces of leaves, stems, roots or any other part of a plant — you can explore these
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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Q2 Have you ever seen a lemon, tomato, orange, or any other food item rot after being
left outside for some time? If yes, you may have noticed a powdery or cotton-like
growth on them (Fig. 2.9). This happens because they have been infected by
microbes. But where did these microbes come from? How did they come in contact
with the food?

Fig. 2.9, page 18 — fruit with microorganism growing on it (redrawn sketch of the
textbook photograph).

They came from the surroundings — from the air, from the surface the fruit was kept on,
from the water it was washed in and from the hands that touched it. Microorganisms are
present everywhere: in water, soil and air, so a fruit lying in the open is never actually in a
microbe-free place.
What then happens on the fruit itself:

1. Very small fungal and bacterial forms settle on the fruit's skin from the air and from
whatever it rests on.
2. A ripe fruit gives them everything they need — it is moist, it is full of sugar, and the room is
warm. These are exactly the conditions in which microorganisms grow.

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

3. A bruise or a cut in the skin lets them reach the soft flesh inside.
4. They multiply. The fungus grows as branched filaments, which is why you see a cotton-like
or powdery patch — you are looking at a colony large enough for the naked eye, not at one
organism.

Why it looks like rotting: the microbes are feeding. They break down the complex
substances of the fruit into simpler ones — the same process by which they turn
fallen leaves into manure. The fruit softens, darkens and smells because its material
is being decomposed.

Check it yourself: keep two identical slices of the same fruit, one in the open and
one in the refrigerator. The cold one stays fresh far longer, because low temperature
slows the growth of these organisms — it does not remove them.

Q3 But why do microorganisms not infect the pickles and murabbas?

Because of what is added to them. The book's own answer: you add many spices with salt or
sugar, which act as preservatives. A high concentration of salt or sugar does not allow these
organisms to grow on them.

Why a strong salt or sugar solution stops them: a microorganism can only grow
where there is water it can use. In a pickle almost all the water is locked up in a very
strong salt solution, and in a murabba in a very strong sugar syrup, so almost no
free water is left for the microbe. Worse for the microbe, such a strong solution
outside its cell draws water out through its cell membrane, so the cell shrinks and
cannot multiply. The oil layer on top of a pickle helps further by keeping the surface
away from moist air.

Did you know? This is why our grandmothers always insisted on a dry spoon for the
pickle jar. A drop of water lowers the salt concentration just at that spot — and that
is exactly where the fungus appears first.

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Q4 How does the diversity of microorganisms play a role in our daily life? How do they
help clean the environment?

Different kinds of microbes do different jobs for us, and the most important of these jobs
is breaking down waste.

WHERE WHICH WHAT IT DOES FOR US
MICROBE

Soil and compost pit Fungi and Act on plant waste and dead animal bodies and break the complex
bacteria substances down into simpler, nutrient-rich manure —
decomposition. The nutrients go back to the soil.

Oxygen-free Certain bacteria Decompose plant and animal waste and release biogas — mainly
surroundings, methane with carbon dioxide — used for cooking, heating,
wastewater electricity and even vehicles.

Root nodules of Rhizobium Traps nitrogen from the air and makes it useful to the plant, so
legumes crops grow well without chemical fertilisers.

Kitchen Yeast, Makes bread, cakes and bhaturas rise; ferments idli and dosa
Lactobacillus batter; turns milk into curd.

Our gut Bacteria Help in digestion.

Water bodies Microalgae Make their own food using sunlight and release oxygen — more
than half of Earth's oxygen supply; feed aquatic animals; help
clean water and are used to make biofuel.

Polluted sites Specially Break down oil spills — the bacterium developed by Ananda
developed Mohan Chakrabarty in 1971.
bacteria

Why decomposition is the key service: the nutrients in a fallen leaf or a dead
animal are locked inside complex substances that a plant cannot take up. Microbes
break those substances into simple, nutrient-rich ones that dissolve and return to
the soil. Without this step the waste would pile up and the nutrients would never
come back into use — so microbes both clean the environment and recycle it.

Activity 2.7: Let us do — Page 18

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Section 2.4.1 Key players in cleaning the environment

ACTIVITY

Q1 After 2–3 weeks, observe the changes that have taken place. Do you observe any
difference in the contents of the container?

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.

Tip: keep the container in a shaded place and let it stay slightly moist but not
waterlogged. The book states that manure formation occurs at optimal
temperature and appropriate moisture level — too dry and the process stops, too
wet and air cannot get in.

Q2 But how did the peels of fruits and vegetables turn into manure?

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.

Fungi and bacteria
Peels, leaves, Manure — simple,
of the soil
dead matter nutrient-rich
warmth + moisture + air

nutrients return to the soil, plants use them again

Decomposition: microbes turn complex plant waste into simple nutrients, which go back to the soil
(Fig. 2.10).

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co m
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Why it happens: the peels are food for these microbes. To use that food they must

m l a se
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a e — decomposition and natural manure
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In-text Questions — Page 20
Sections 2.4.1 and 2.4.2 — a world without microbes, and microbes in the kitchen

Q1 Now, think what would have happened if microorganisms did not exist on Earth?

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.

WHAT MICROBES DO WHAT WOULD HAPPEN IF THEY DID NOT EXIST
NOW

Decompose fallen leaves, plant Dead plants, dead animals and waste would pile up and never break down.
waste, dung and dead animals Nutrients would never return to the soil, so the soil would lose its fertility and
plants would grow poorly.

Make manure and compost No natural manure and no compost pits; farmers would depend entirely on
other means.

Release biogas from waste in No biogas for cooking, heating, electricity or vehicles.
oxygen-free conditions

Rhizobium traps nitrogen from Legumes would get no nitrogen from the air, and crop rotation would not
the air in root nodules enrich the soil.

Yeast and Lactobacillus in food No curd, no idli, no dosa, no bhatura, no bread or cake — all of these depend
on fermentation.

Gut bacteria help in digestion Our own digestion would be affected.

Microalgae make food using More than half of Earth's oxygen supply would be gone, and the aquatic
sunlight animals that feed on microalgae would lose their food.

The one gain would be that the microbes which cause disease in plants, animals and humans
would also be absent. But weighed against everything above, that gain is very small.

The idea behind it: nature works in cycles. A plant takes nutrients from the soil, an
animal eats the plant, and both eventually die. Only the decomposers close that
cycle by returning the nutrients to the soil. Remove them and the cycle becomes a
one-way street that soon runs out.

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Q2 How does the diversity of microorganisms help in our kitchen?

Different microbes give us different foods — and the same diversity also lets us keep some
foods safe from microbes.

MICROORGANISM WHAT IT IS WHAT IT DOES IN THE KITCHEN

Yeast A unicellular Respires in the dough and releases carbon dioxide, which
fungus makes bread, cakes and pastries soft and fluffy; it also
produces a small amount of alcohol, which gives the dough
its smell

Lactobacillus A bacterium Ferments milk into curd and makes it sour by producing
lactic acid; also ferments the batter for idli and dosa and the
dough for bhatura

Salt and sugar (as Not microbes — A high concentration of salt or sugar does not allow
preservatives) they keep microorganisms to grow, which is why pickles and
microbes out murabbas keep

Why so many different results from the same idea: in every case the microbe is
simply feeding — breaking down sugar to release energy for its own growth. What
differs is the by-product it leaves behind. Yeast leaves carbon dioxide and alcohol,
so the dough puffs up. Lactobacillus leaves lactic acid, so the milk sets and turns sour.
The cook only has to supply the right microbe and the warmth it likes.

Be a scientist — Page 20
Ananda Mohan Chakrabarty and the oil-eating bacterium

BE A SCIENTIST

Q1 What are the other problems which you think can be solved with the help of
microorganisms?

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.

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Household and city waste. Kitchen waste, vegetable peels and fallen leaves can be
composted into manure instead of being dumped or burnt — as you did in Activity 2.7.
Sewage and wastewater. Bacteria that live in oxygen-free conditions can decompose the
waste in household wastewater and give biogas at the same time — the waste is cleaned
and a fuel is produced from the same process.
Dependence on chemical fertilisers. Growing legumes in rotation lets Rhizobium in the root
nodules trap nitrogen from the air, raising the nitrogen in the soil naturally.
Dirty water bodies. Microalgae help in cleaning water, and are also used to make biofuel.
Poor nutrition and livelihood. Spirulina is rich in protein — more than 60 per cent of its
body weight — and a good source of vitamin B12. Farming it in a glass tank is becoming a
feasible livelihood opportunity, so conserving microalgae supports both food security and
income.
Animal waste on farms. Helpful bacteria can decompose dung, turning a disposal problem
into manure and biogas.

Why microbes are so useful here: a microorganism does not need to be told what
to do — it is simply feeding. Give it the waste as food and the warmth, air and
moisture it needs, and the breaking-down happens by itself, without machinery. That
is why Chakrabarty's work is remembered as showing how microorganisms could be
used to solve environmental problems like pollution.

Did you know? Chakrabarty's discovery received a patent in 1980. A patent is a
copyright given to a person so that no one else can copy, use or sell his or her
invention without permission.

Activity 2.8: Let us perform — Page 20
Section 2.4.2 Microorganisms and food — yeast in dough

ACTIVITY

Q1 Did you find any change in the volume, smell, or texture of the dough?

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

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

BOWL A (FLOUR + SUGAR + YEAST + BOWL B (FLOUR + SUGAR + WARM
WARM WATER) WATER, NO YEAST)

Volume Has risen — the dough is clearly bigger than Almost unchanged
when it was kneaded

Texture Soft, spongy and fluffy; small holes are seen Firm and even, no holes
when it is torn open

Smell A different, slightly sharp smell The plain smell of dough

Tip: if you see no change after 4–5 hours, leave the dough for some more time. The
book says so — the yeast needs time to multiply before the change becomes visible.
Bowl B is not wasted effort: it is the comparison, and without it you could not be
sure the yeast caused the change.

Q2 Why does this happen? What is the role of yeast? Why did we add sugar and warm
water to the flour?

Yeast is a living microorganism — a fungus — and the dough rises because the yeast is
respiring inside it.
Follow what happens in bowl A:

1. Yeast belongs to the group of microorganisms called fungi. Like other organisms, it respires
and breaks down food to release energy for its growth and to carry out life processes (you
met this in ‘Life Processes in Animals’, Grade 7).
2. During this process carbon dioxide is released.
3. The kneaded dough is soft and stretchy, so the gas cannot escape freely. It collects as
thousands of tiny bubbles trapped inside.
4. Each bubble pushes the dough outwards, so the dough rises and becomes soft and fluffy.
The holes you see on tearing it open are those bubbles.
5. Yeast also produces a small amount of alcohol during this process, which is what gives the
dough its slightly different smell.

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Did you know? This special property of yeast is used in making breads, cakes and
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Activity 2.9: Let us prepare — Page 21
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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Section 2.4.2 Microorganisms and food — Lactobacillus and curd

ACTIVITY

Q1 Observe the changes in the glass bowls. Write your predictions and observations in
Table 2.4.

CHANGE IN THE CHANGE IN THE POSSIBLE
APPEARANCE OF MILK COLOUR OF MILK REASON

BOWL A BOWL B BOWL A BOWL B

Prediction

Observation

Table 2.4: Testing for curd formation using milk in different conditions (blank, as printed
on page 21).

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.

CHANGE IN THE CHANGE IN THE POSSIBLE REASON
APPEARANCE OF MILK COLOUR OF MILK

BOWL A BOWL B BOWL A BOWL
B

Prediction Will thicken and Will stay No real No Curd sets faster in a
set liquid change change warm place

Observation Milk has turned Milk has not Stays No Lactobacillus from the
into curd — thick curdled; it is white, change added curd multiplies
and set, tastes a still liquid, looks — still quickly in warm milk
little sour; some but it might slightly white and ferments it; in the
clear liquid may be a little sour more cold the same bacteria
separate at the opaque grow far too slowly
edge

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Tip: the spoonful of curd you add is the starter — it is not an ingredient of the milk
but a supply of living bacteria. That is why a fresh, live spoon of curd sets a bowl of
milk while a sour, old one often does not.

Q2 Do you know why this happens?

Because of the bacteria in the spoonful of curd you added, and because they can only
work when they are warm.

1. Curd contains several types of bacteria. One of them is Lactobacillus.
2. This bacterium feeds on the sugar in the milk (lactose) and multiplies.
3. As it feeds it ferments the milk to form curd. Unlike yeast, which produces alcohol, these
bacteria produce lactic acid.
4. The lactic acid makes the curd sour, and it is also what makes the milk thicken and set
instead of staying liquid.
5. These bacteria grow well in warm conditions. That is why curd is formed in bowl A but not
in bowl B.

Why bowl B is only slightly sour: the bacteria are present in bowl B too — you
added the same spoonful of curd. The refrigerator does not kill them; it only slows
them down so much that they produce very little acid in a few hours. Given a very
long time even refrigerated milk turns sour, which is why milk is not kept indefinitely
even in a fridge.

Compare the two activities: yeast (a fungus) and Lactobacillus (a bacterium) both
break down a sugar for energy. Yeast leaves behind carbon dioxide and a little
alcohol, so dough rises. Lactobacillus leaves behind lactic acid, so milk sets and turns
sour. Same idea, different by-product.

Keep the curiosity alive — Page 25

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

End-of-chapter question set

KEEP THE CURIOSITY ALIVE

Q1 Various parts of a cell are given below. Write them in the appropriate places in the
following diagram. Nucleus, Cytoplasm, Chloroplast, Cell wall, Cell membrane,
Nucleoid

Only in Animal Cell

Common to all three cells

Only in Bacterial Cell Only in Plant Cell

The diagram printed with question 1 on page 25 — eight blank lines grouped under four
headings.

Fill the diagram like this:

REGION OF THE DIAGRAM WHAT TO WRITE THERE

Common to all three cells Cell membrane, Cytoplasm

Only in Plant Cell Chloroplast, Cell wall

Only in Bacterial Cell Nucleoid

Only in Animal Cell Nucleus

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Animal cell

Nucleus

Cell membrane
Cytoplasm

Chloroplast
Nucleoid
Cell wall

Bacterial cell Plant cell

Where each part belongs. The overlap in the middle is what all three cells share.

Why each one goes where it does:

Cell membrane and cytoplasm — every cell has them. Like animal and plant
cells, the cells of microorganisms are also surrounded by a cell membrane.
Chloroplast — only plant cells have chlorophyll-containing plastids, so only they
can make food by photosynthesis. Fungal and bacterial cells have no chloroplasts.
Nucleoid — bacteria do not have a well-defined nucleus or a nuclear membrane;
instead they have a nucleoid. This is the feature that distinguishes bacteria from
the cells of yeast, protozoa, algae, fungi, plants and animals.
Nucleus — a well-defined nucleus is exactly what the bacterial cell lacks, so it
goes on the animal side of the diagram.
Cell wall — the animal cell has none, so it goes on the plant side.

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co m
m.
Be accurate about two of them. Strictly, the plant cell also has a nucleus, and the

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book itself says that plant, fungal and bacterial cells all have a cell wall. So of the
o g nucleoid in the
.c chloroplast is found in the plant cell alone and onlyathe
m
six, only the

l a se cell alone. Nucleus and cell wall are each shared by two of the three, and
g diagram has no space marked for a shared pair — so they are written in the
bacterial
athe
region that shows which cell does not have them.
co m
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Q2
a
Aanandi took two test tubes and marked them A and B. She put two spoonfuls of
sugar solution in each of the test tubes. In test tube B, she added a spoonful of
co m
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yeast. Then she attached two incompletely inflated balloons to the mouth of each

o m l a se
test tube. She kept the set-up in a warm place, away from sunlight. (i) What do you
.c will happen after 3 – 4 hours? She observed that the
a gballoon attached to test
m
se tube B was inflated. What can be a possible explanation for this? (a) Water
predict

a
agl evaporated in test tube B and filled the balloon with the water vapour. (b) The warm

s
atmosphere expanded the air inside the test tube B, which inflated the balloon. (c)
om a
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Yeast produced a gas inside the test tube B which inflated the balloon. (d) Sugar
reacted with warm air, which e
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glafilled with lime water. She removed the balloon from
produced gas, eventually inflating the balloon. (ii) She

a
took another test tube, 1/4
test tube B in such a manner that the gas inside the balloon did not escape. She
attached the balloon to the test tube with lime water and shook it well. What do
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g l a
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(a) is wrong. Test tube A holds the same sugar solution at the same warmth. If evaporation
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. c om
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(b) is wrong for the same reason — both test tubes are in the same mwarm place, so warm air
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m a s
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down the sugar to release energy, and releases carbon dioxide — the same gas that makes

m .
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The reasoning behind it: A and B are identical in every way except one — the yeast.
When two set-ups differ in only one thing and only one of them shows an effect, that
one difference must be the cause. Test tube A is the comparison, and it is what rules
out (a), (b) and (d) in a single stroke.

(ii) She wants to find out which gas the yeast produced — specifically, whether it is carbon
dioxide.
Lime water is the test for carbon dioxide: when carbon dioxide is shaken with clear lime water,
the lime water turns milky. So if the gas collected in the balloon turns the lime water milky, the
gas released by the yeast is carbon dioxide. She takes care not to let the gas escape while
transferring the balloon, because a test on the wrong gas — ordinary air — would prove
nothing.

Q3 A farmer was planting wheat crops in his field. He added nitrogen-rich fertiliser to
the soil to get a good yield of crops. In the neighbouring field, another farmer was
growing bean crops, but she preferred not to add nitrogen fertiliser to get healthy
crops. Can you think of the reasons?

Because the bean plant carries its own supply of nitrogen in its roots, and the wheat plant
does not.

Beans are a legume. The roots of certain legumes — beans, peas and lentils — have swollen
regions called root nodules.
These nodules contain Rhizobium bacteria, which live inside them.
These bacteria trap nitrogen from the air and make it useful for the plant.
So the bean crop gets its nitrogen naturally, and grows well without chemical fertilisers.
Adding nitrogen fertiliser would be an unnecessary expense.
Wheat has no root nodules and no Rhizobium, so it must take its nitrogen from the soil.
The farmer therefore adds nitrogen-rich fertiliser to get a good yield.

Why farmers rotate crops: the Rhizobium in the bean field goes on adding nitrogen
to that soil. So farmers grow legumes in rotation with other crops — this naturally
increases the nitrogen in the soil and keeps it healthy for the next crop, such as the
wheat. It is a way of letting a microorganism do the fertiliser's job.

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Q4 Snehal dug two pits, A and B, in her garden. In pit A, she put fruit and vegetable
peels and mixed it with dried leaves. In pit B, she dumped the same kind of waste
without mixing it with dried leaves. She covered both the pits with soil and
observed after 3 weeks. What is she trying to test?

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.
Notice how carefully the experiment is built: the two pits have the same kind of waste, the
same covering of soil, the same garden and the same three weeks. Only one thing is different
— the dried leaves. So whatever difference she finds after 3 weeks must be due to the dried
leaves alone.
What she is likely to find:

PIT A — PEELS + DRIED LEAVES PIT B — PEELS ONLY

After 3 Dark, crumbly, earthy-smelling manure; the A wet, packed, slimy mass; decomposition slower
weeks peels are no longer recognisable and less complete, with a bad smell

Why the dried leaves help: microorganisms need air and an appropriate moisture
level to do their work, and manure formation occurs at optimal temperature and
moisture. Wet peels alone settle into a solid, soggy lump — water fills every gap and
air cannot reach the microbes inside. Dry leaves mixed in keep the heap loose, so air
spaces remain, and they soak up the extra moisture. The fungi and bacteria then get
both the air and the moisture they need, and the waste is broken down faster and
more completely.

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Q5 Identify the following microorganisms: (i) I live in every kind of environment, and
inside your gut. (ii) I make bread and cakes soft and fluffy. (iii) I live in the roots of
pulse crops and provide nutrients for their growth.

CLUE MICROORGANISM HOW THE CLUE GIVES IT AWAY

(i) I live in every kind of Bacteria Bacteria are found everywhere — in water, soil
environment, and inside your and air, in hot water springs and snow cold zones
gut — and our intestine has many bacteria that help
in digestion

(ii) I make bread and cakes Yeast (a fungus) Yeast respires in the dough and releases carbon
soft and fluffy dioxide, whose bubbles make the dough rise; this
property is used in making breads and cakes

(iii) I live in the roots of Rhizobium (a It lives in the root nodules of legumes such as
pulse crops and provide bacterium) beans, peas and lentils, and traps nitrogen from
nutrients for their growth the air for the plant

Tip: answer (ii) as yeast, not just ‘fungus’ — yeast is the particular unicellular fungus
used for this, while mould is a multicellular fungus that spoils bread rather than
raising it.

Q6 Design an experiment to test that microorganisms need optimal temperature, air,
and moisture for their growth.

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.

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

SET-UP HOW IT IS PREPARED WHAT IS BEING EXPECTED
TESTED RESULT AFTER 4–
6 DAYS

A Sprinkle a few drops of water on the All three Cottony or powdery
(comparison) slice, leave it loosely covered in a warm, conditions present mould grows well
dark corner of the room

B Same moist slice, loosely covered, but Only the Very little or no
(temperature) kept in the refrigerator temperature is growth
changed

C (moisture) Dry the slice thoroughly in the sun first, Only the moisture Very little or no
then keep it loosely covered in the same is changed growth
warm corner

D (air) Moist slice sealed in a bag with the air Only the air is Much less growth
pressed out, kept in the same warm changed than A
corner

How to run it: keep all four at the same time; look at them once a day at the same hour; do not
open the containers; record what you see by sketching the patch of growth or by tracing its
outline on the container.
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.

Why only one thing is changed at a time: if you had put a dry slice in the
refrigerator, and it did not grow mould, you could not say whether the cold or the
dryness stopped it. Changing exactly one condition and keeping everything else the
same is what lets the result point to a single cause.

Handling: follow the book's own instruction from Activity 2.5 — do not touch the
mouldy material with bare hands; use a spoon or gloves. Seal the containers before
throwing them away, and wash your hands afterwards.

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

co m
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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Q8 A student observes that when curd is left out for a day, it becomes more sour. What
can be two possible explanations for this observation?

Explanation 1 — the Lactobacillus already in the curd goes on working. Curd contains
several types of bacteria, one of them Lactobacillus. It feeds on the sugar in the milk (lactose),
multiplies, and produces lactic acid as it ferments. Left out at room temperature there is still
some lactose remaining and the bacteria keep converting it, so more lactic acid collects in the
curd. More acid means more sourness.
Explanation 2 — room temperature is exactly what these bacteria like. These bacteria grow
well in warm conditions. Out of the refrigerator they multiply much faster than they did inside it,
so the number of bacteria — and therefore the rate at which acid is produced — rises sharply
over a day. Other acid-producing microorganisms from the air, the spoon or the vessel can also
settle in the open curd and add to the souring.

The common idea: in both explanations the sourness comes from lactic acid. The
first says the same bacteria produce more of it; the second says warmth lets more
bacteria produce it faster. This is also why curd is kept in the refrigerator once it has
set — the cold does not stop the bacteria, it only slows them enough that the curd
stays pleasant for a day or two.

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Q9 Observe the set-up given in Fig. 2.15 and answer the following questions. (i) What
happens to the sugar solution in flask A? (ii) What do you observe in test tube B
after four hours? Why do you think this happened? (iii) What would happen if yeast
was not added in flask A?

Warm sugar
solution + Yeast
Lime
water

A
B

Fig. 2.15: Experimental set-up.

The set-up: flask A holds a warm sugar solution with yeast; a delivery tube carries whatever
gas is formed from A into test tube B, which holds lime water.

carbon dioxide travels through the tube

Lime water

Warm sugar solution
+ yeast

A B

Fig. 2.15 set-up: the gas made by the yeast in flask A is bubbled through the lime water in test tube B.

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

(i) In flask A the yeast begins to respire. It breaks down the sugar of the solution to release the
energy it needs to grow, so the sugar is gradually used up. Carbon dioxide is released and a
small amount of alcohol is formed. You see bubbles rising through the liquid and a froth
collecting on top, the solution turns cloudy as the yeast multiplies, and it begins to smell slightly
of alcohol.
(ii) In test tube B the clear lime water turns milky after four hours.
This happened because the carbon dioxide released in flask A cannot escape anywhere else — it
travels along the delivery tube and bubbles through the lime water. Lime water turns milky
when carbon dioxide is passed through it, and that is the test that identifies the gas. So test
tube B proves that the gas coming out of A is carbon dioxide.
(iii) If yeast was not added to flask A, nothing would happen. There would be no organism to
break down the sugar, so no carbon dioxide would be produced, no bubbles would appear in
the sugar solution, no gas would pass down the tube, and the lime water in B would stay
clear. The sugar solution itself would be unchanged.

Why this set-up is convincing: it links a cause to an effect at two removes. The
yeast is in A, the milkiness is in B, and the only path between them is the tube. Part
(iii) is the control that closes the argument — remove the yeast and the whole chain
stops.

Discover, design, and debate — Page 27
Science · Society · Interdisciplinary Projects

DISCOVER, DESIGN, AND DEBATE PROJECTS

Q1 India has a long history of biogas production. One of our oldest biogas plant was set
up in late 1850s. Find out about the Biogas Program initiated by the Ministry of New
and Renewable Energy, Government of India.

Start from what the chapter already told you, then look up the programme. Biogas is the
mixture of gases released when certain bacteria decompose plant and animal waste, or
household wastewater, in an oxygen-free environment. It is mainly methane with carbon
dioxide, and has been used as a fuel for cooking, heating, generating electricity and even
running vehicles.
Where to look: the website and annual report of the Ministry of New and Renewable Energy
(MNRE), Government of India; your district's agriculture or rural development office; a nearby
village that has a working plant.
What to find out and record:

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

The name of the current national biogas programme and the year it began.
What size of plant a family gets, what it is fed with (cattle dung, kitchen waste, farm waste),
and how much gas it gives in a day.
What financial help the government gives towards building a plant, and who is eligible.
How many plants have been installed in your state, and the trend over the last few years.
What happens to the slurry left behind after the gas is taken out.

Why a biogas plant solves two problems at once: the waste that goes in would
otherwise rot in the open and be a nuisance. Inside the sealed plant the same
rotting is done by bacteria that live without oxygen, and the gas they release is
collected instead of being lost. The material left over is already decomposed — it is
manure. So the plant gives fuel and fertiliser from the same waste, which is why it
suits a village household so well.

Tip for the report: add one local fact — the number of plants in your own block, or a
photograph of a plant near you — and one clear diagram of how the dung goes in,
the gas comes off the top and the slurry comes out at the other end.

Q2 Fermented food items like fermented soya bean and fermented bamboo shoots are
consumed as traditional food in some parts of India. With the help of your parents
and teachers, list some traditional food items from your area that utilise the
process of fermentation. Investigate the ingredients used in the preparation of
these fermented food items; the method of preparing them; the microorganism
responsible for the fermentation of the food, and the cultural and nutritional
importance of the fermented food.

Make the list first, and for each food record the four things the question asks: ingredients,
method, microorganism, and cultural or nutritional importance.
Sample list (add your own local dishes):

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

FOOD MAIN METHOD IN SHORT MICROORGANISM
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INGREDIENTS MAINLY RESPONSIBLE

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Curd (dahi) Milk + a spoon of old Warm milk, add starter, Lactobacillus and other

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Idli and dosa Rice and urad dal, Soak, grind, keep the Lactobacillus, with yeasts

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salt batter

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dough covered in a
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Kanji Black carrot or Keep the jar in the sun Lactic acid bacteria

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Cultural and nutritional importance to write about: fermented foods keep far longer than

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

What is actually happening in every one of these: the microorganism is feeding
on the sugars or starch in the food. As it does so it leaves behind a by-product —
lactic acid in curd, kanji and bamboo shoots, carbon dioxide in idli batter and bread
dough. The acid gives the sour taste and, because most spoilage organisms cannot
grow in an acidic food, it also preserves it. That is why fermenting is both a way of
cooking and a way of storing.

Tip: interview one elder in your family for the method — ask especially about the
time and the place they keep it, and you will find they are describing exactly the
warmth and moisture the microbe needs.

Q3 Study the different parts of a macro fungus mushroom using a magnifying glass
and microscope. Take the help of students from senior classes and explore the
internal structure of different parts of mushrooms under the microscope in your
school laboratory.

What to look at, in order:

PART WITH THE NAKED EYE AND UNDER THE MICROSCOPE
MAGNIFYING GLASS

Cap The umbrella-shaped top; note its colour, A dense mass of fine threads packed
and whether the surface is smooth or scaly together

Gills (under Thin plates running from the stalk to the The surface carries tiny spores; a piece
the cap) rim like the spokes of a wheel mounted in water shows them clearly

Stalk The stem that holds the cap up; break it Long threads running lengthwise
and see that it is not hollow like a plant
stem

Base / Fine white cottony threads at the bottom, Branched filaments without chlorophyll —
mycelium spreading into the material it grew on exactly the description of fungi in Table 2.2

Spore print — worth doing: cut off the stalk, place the cap gills-down on a sheet of white paper,
cover it with a bowl and leave it overnight. In the morning lift the cap and you will find a pattern
of fine powder on the paper, in the shape of the gills. That powder is the spores. Look at a little
of it under the microscope.

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Why the mushroom has no green in it: a mushroom is a fungus. Fungal cells have
a cell wall but no chloroplasts, so a mushroom cannot make its own food through
photosynthesis. It has to grow on straw, wood or decaying matter and feed on it by
breaking it down — which is why you find mushrooms on rotting logs and in damp
compost, and why they are decomposers like the moulds in Table 2.2.

Tip: study a mushroom bought from the market or one your teacher gives you, and
handle it with a spoon or gloves as the book advises for soil, washing your hands
afterwards.

Q4 Interact with an entrepreneur and learn the steps for cultivation of mushroom.

Go with a written list of questions, and note the steps in the order the grower actually
does them.
The usual steps of oyster-mushroom cultivation, so that you can follow what you are
shown:

1. Prepare the substrate. Paddy straw or wheat straw is chopped and soaked in water.
2. Pasteurise it. The wet straw is treated with hot water or steam at a controlled temperature
for a few hours, then cooled and drained.
3. Mix in the spawn. Spawn is grain already carrying the growing mushroom fungus. It is
mixed through the cooled straw in layers.
4. Fill the bags. The mixture is packed into polythene bags with small holes punched for air,
and the bags are stacked in a dark room.
5. Maintain the room. Moderate temperature, high humidity kept up by spraying water, and
some fresh air. In two to three weeks the bags turn white as the fungal threads spread
through the straw.
6. Fruiting and harvest. The bags are opened to light and air; small pinheads appear and
grow into mushrooms in a few days. They are picked by twisting them off, and two or three
more flushes follow.
7. After the crop. The spent straw is not thrown away — it is already partly decomposed and
makes good manure.

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

Why the straw is pasteurised — the key step: straw is not sterile. It already carries
the moulds and bacteria you saw in Table 2.2, and those would grow much faster
than the mushroom and take the food first. Heating the wet straw to a controlled
temperature kills most of these unwanted microorganisms, without destroying the
straw itself. The mushroom spawn is then added into a substrate where it has
almost no competition, so it can spread through the straw undisturbed. Note that
pasteurisation does not kill everything — it only reduces the unwanted microbes
enough to give the mushroom a clear head start, which is why the grower still keeps
the room and the bags clean.

Questions worth asking the entrepreneur: where the spawn is bought and how it is stored;
what the room temperature and humidity are kept at; how many days from spawning to first
harvest; what a bag yields; what the biggest cause of crop failure is; where the mushrooms are
sold and at what price; what the cost of starting up was.

Link it back: mushroom growing, Spirulina farming and biogas are all the same idea
turned into a livelihood — give a chosen microorganism the conditions it needs,
keep the unwanted ones out, and harvest what it produces.

Chapter at a glance
The eye can only see things above a certain size. A curved piece of glass, thick in the
middle and thin at the edge — shaped like a lentil seed, which is why it was named a lens
— makes small things look bigger. Better lenses became microscopes, and microscopes
opened the hidden living world.
In 1665 Robert Hooke published Micrographia. His microscope magnified 200 to 300 times.
Looking at a thin slice of cork he saw many small empty compartments like a honeycomb,
and called each one a cell. Around the 1660s Antonie van Leeuwenhoek made better lenses
and was the first to clearly see and describe bacteria and blood cells — the Father of
Microbiology.
Every living body is built of cells. A typical cell has a cell membrane (porous — lets in what
life processes need and lets waste out), cytoplasm (where most life processes happen) and
a nucleus (regulates all the cell's activities and its growth). Plant cells have an extra cell wall
for rigidity, plus plastids (chloroplasts carry chlorophyll) and a large vacuole.
Shape follows function. Cheek cells are thin and flat and form a protective lining; nerve
cells are long and branched so messages travel quickly; muscle cells are thin, flexible and
spindle-shaped so they can contract and relax in a wave and push food down the food pipe.
Bodies are organised in levels: Cell → Tissue → Organ → Organ system → Organism. Life
in complex organisms begins from a single cell, the ‘egg’, which divides again and again —

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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

such organisms are multicellular.
Microorganisms are too small to be seen with the naked eye — bacteria, protozoa, fungi,
some algae. A microscope magnifies them 100 to 400 times; an electron microscope about
10,00,000 times. Bacteria have no well-defined nucleus or nuclear membrane — they have
a nucleoid instead. Viruses are microscopic and acellular, and multiply only inside a living
cell.
Microbes work for us: fungi and bacteria decompose plant and animal waste into nutrient-
rich manure; some bacteria make biogas (mainly methane with carbon dioxide) in an
oxygen-free environment; Rhizobium in the root nodules of legumes traps nitrogen from
the air; yeast (a fungus) makes dough rise; Lactobacillus turns milk into curd and ferments
idli–dosa batter; microalgae such as Spirulina release more than half of Earth's oxygen.

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co m
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m .co WHAT IT MEANS g
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TERM WHERE IT OR KEY

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COMES IN THE NUMBER
CHAPTER

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Lens A curved piece of glass, thick in the Page 9 Named after the lentil seed

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middle and thin at the edge, that makes it was shaped like

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small things look bigger

Cell
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living body is built from
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Cell membrane The porous outer layer that encloses Page 12 Present in animal, plant,

m . cytoplasm and nucleus, separates one
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and the nucleus; holds carbohydrates, here
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structure in bacteria
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Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye AglaSem · NCERT Solutions

TERM WHAT IT MEANS WHERE IT EXAMPLE OR KEY
COMES IN THE NUMBER
CHAPTER

Virus Microscopic and acellular; multiplies Page 17 May infect plants, animals
only after entering a living cell or bacterial cells

Decomposition Microbes breaking complex substances Page 19 Needs optimal temperature
of dead plants and animals into simpler, and appropriate moisture
nutrient-rich substances

Biogas A mixture of gases released by bacteria Page 20 Used for cooking, heating,
in an oxygen-free environment, mainly electricity and vehicles
methane with carbon dioxide

Rhizobium Bacteria living in the root nodules of Page 22 Beans, peas and lentils —
legumes that trap nitrogen from the air hence crop rotation
for the plant

Lactobacillus A bacterium that feeds on lactose, Pages 21–22 Also ferments idli and dosa
ferments milk into curd and produces batter and bhatura dough
lactic acid

Microalgae Microscopic plant-like organisms that Pages 22–23 Spirulina, Chlorella,
make their own food using sunlight Diatoms; over half of Earth's
oxygen

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