NCERT Solutions Class 9 Science Chapter 2 Cell the Building Block of Life

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 9 · SCIENCE

NCERT Solutions

Chapter 2: Cell: The Building
Block of Life

NCERT Textbook — Exploration

BOOK PAGES SECTIONS QUESTIONS MEDIUM

8 – 27 25 62 English

Solutions, notes, sample papers & more at 79 pages

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

CLASS 9 · SCIENCE · EXPLORATION

NCERT Solutions — Chapter 2: Cell: The Building Block
of Life
Chapter 2 of Exploration starts from a hard limit — the human eye cannot separate two points closer than 0.1
mm — and shows how the microscope pushed past it to reveal the cell. It then works inwards: the selectively
permeable cell membrane and osmosis, the rigid cell wall, the division of labour among organelles, how
cells divide by mitosis and meiosis, and the Cell Theory that ties all of biology together.

TEXTBOOK BOOK PAGES

Exploration (Class 9) 8 – 27

SECTIONS QUESTIONS

25 62

MEDIUM

English

Think It Over — Page 8
The four questions the chapter sets out to answer

THINK IT OVER

Q1 Where does a cell come from?

Every cell comes from an already existing cell that divides — never from non-living matter. This
is the third statement of the Cell Theory, added by Rudolf Virchow in 1855.

pre-existing cell → grows → divides → new cells

mitosis: 1 parent cell → 2 identical daughter cells

meiosis: 1 parent cell → 4 cells with half the chromosome number

Page 1 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Why it happens: a cell is not just a bag of chemicals. It needs a working membrane,
cytoplasm, ribosomes and above all a complete set of DNA instructions, and the only
place all of these are ready-made and correctly arranged is inside another living cell.
The chapter's own example proves the point: when Craig Venter's team made a
synthetic Mycoplasma mycoides genome in 2010, they still had to put it inside an
existing bacterial cell whose cytoplasm and membrane were intact. Only the DNA
was synthetic.

Did you know? Your own body began as a single fertilised egg. Every one of the
roughly 30 trillion cells you now carry traces back to that one cell through an
unbroken chain of divisions.

Q2 How have technological interventions facilitated the creation of new knowledge in
understanding the world beyond the naked eye?

Each new instrument pushed the limit of what could be seen, and every push produced new
biology.

TOOL SMALLEST THING NEW KNOWLEDGE IT CREATED
IT SHOWS

Unaided eye 0.1 mm (100 µm) Whole organisms and organs only

Robert Hooke's a few µm The box-like compartments in cork — he named
microscope, 1665 (200– them ‘cells’
300X)

School light microscope about 0.2 µm Nucleus, cell wall, chloroplasts, stages of cell division
(10X × 10X = 100X, up in an onion root tip
to 400X)

Electron microscope nanometre scale (1 nm Ribosomes, cristae of mitochondria, the lipid bilayer,
(beam of electrons) = one-billionth of a cytoskeleton, viruses; it confirmed the Golgi
metre) apparatus decades after Camillo Golgi first saw it

Page 2 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Why it happens: resolution is set by the wavelength of whatever is used to ‘look’.
Visible light has a wavelength of about 400–700 nm, so no light microscope can
separate two points much closer than about 200 nm, however strong the lens.
Electrons behave as waves of far shorter wavelength, so an electron microscope
resolves structures a thousand times finer. Scientists also improved two other things
— contrast (the brightness difference between parts of an object, which is why we
stain onion peel with safranin and cheek cells with methylene blue) and
magnification.

Did you know? The Golgi apparatus was doubted for decades because early
microscopes could not resolve it clearly. Only electron microscopy settled the
argument. Technology does not merely confirm knowledge — it decides which
questions can be answered at all.

Q3 How is the cell structural and functional unit of life?

It is the structural unit because every living body is built out of cells and nothing smaller, and
the functional unit because every life process is carried out inside a cell.

Structural: similar cells form a tissue, tissues form an organ, organs form an organ system —
nasal pores, nasal cavity, trachea and lungs form the respiratory system. Take the
organisation apart and you always arrive at cells.
Functional: respiration happens in mitochondria, protein synthesis on ribosomes,
photosynthesis in chloroplasts, digestion of waste in lysosomes, storage in vacuoles. All of
these sit inside one cell.
A single cell can be a complete organism — a bacterium or a yeast lives, feeds, grows and
reproduces on its own. Nothing smaller than a cell can do that.

Why it happens: the cell is the smallest unit that has all three things life needs at
once — a boundary that keeps the inside different from the outside (the membrane),
a set of instructions to run and copy itself (DNA), and machinery to turn those
instructions into work (ribosomes, enzymes, mitochondria). Break a cell open and
the parts stop working within minutes, because the boundary that held the right
concentrations together is gone. Viruses have DNA and protein but no membrane-
bound machinery of their own, which is exactly why they cannot live outside a host
cell.

Page 3 of 79

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Class 9 Science Chapter 2 Cell: The Building Block of Life
a g l AglaSem · NCERT Solutions

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Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Section 2.1 How to Study Cells?

Q1 What do we call the ability of the human eye to see two very close objects as
separate and distinct?

It is called the resolution of the eye, and the smallest separation it can still manage is the limit
of resolution.

near point of the human eye = 25 cm

limit of resolution of the human eye = 0.1 mm

0.1 mm = 0.1 × 1000 µm = 100 µm

Two dots on paper 0.1 mm apart, held 25 cm from the eye, are just seen as two. Bring them
closer than that and they merge into a single point.

Why it happens: resolution is not the same as magnification. Magnification only
makes the image bigger; resolution decides whether the two points stay
distinguishable when it is enlarged. Blowing up a blurred photograph gives a bigger
blur, not more detail. That is why scientists worked on all three features of a
microscope — resolution, contrast and magnification — and not on magnification
alone.

Check it yourself: a typical plant or animal cell is 10–100 µm across. The eye's limit
is 100 µm, so only the very largest cells could ever be at the edge of visibility — and
even then with no internal detail.

Q2 How have cell biologists studied the structure and function of cells that are much
smaller than the limit of resolution of the human eye?

By using lenses — a convex lens, or better, a combination of an objective lens and an eyepiece,
which is exactly what a microscope is.

The objective lens forms an enlarged image of the specimen; the eyepiece enlarges that
image again. The two magnifications multiply.
A school light microscope carries several objectives (10X, 40X) so the same slide can be
studied at different magnifications under visible light.

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Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Beyond the reach of light, scientists use an electron microscope, which uses a beam of
electrons instead of light and resolves structure at the nanometre scale (1 nm = one-
billionth of a metre). Fig. 2.4 in the book is a scanning electron micrograph of stomata on the
lower surface of a Colocasia leaf.

total magnification = magnification of eyepiece × magnification of objective

= 10X × 10X = 100X

so an onion cell of 200 µm appears 200 µm × 100 = 20 000 µm = 20 mm across

Why it happens: a lens bends light so that rays leaving a tiny object reach the eye as
if they came from a much larger one, at a much larger visual angle. Once that angle
is bigger than the eye's own limit, the detail becomes visible. Historically this is
exactly how the cell was discovered — Robert Hooke built his own microscope of
about 200–300X in 1665, looked at a thin slice of cork and saw small box-like
compartments which he named ‘cells’.

In-text Question — Page 10
Section 2.1 How to Study Cells?

Q1 Under the microscope, you will see a magnified image of an object. Can you
estimate its actual size?

Yes. Use the microscope's own field of view as a ruler.

1. Place a transparent millimetre ruler on the stage and measure the diameter of the circular
field of view — say 5 mm.
2. Convert it: 5 mm × 1000 = 5000 µm.
3. Replace the ruler with the specimen slide and count how many cells fit across that same
diameter in one straight line — say 25.
4. Divide.

estimated size of one cell = diameter of the visible field in µm ÷ number of cells along the

diameter

= 5000 µm ÷ 25

= 200 µm (that is 0.2 mm)

Page 6 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Why it happens: the eyepiece cannot tell you a size on its own, because it magnifies
the specimen and the ruler by exactly the same amount. But the field of view is a
fixed real length for a given objective, so once it has been measured in millimetres it
becomes a scale bar that stays valid for every slide viewed with that same objective.
This is why step 2 must be done before the ruler is removed, and why the objective
must not be changed afterwards.

Tip: the count must be along the diameter, in one straight line — not across the
whole circle. Counting along a chord instead of the diameter would use too short a
length and make the cells look bigger than they are.

Activity 2.1: Let us estimate the size of a cell — Page 10
Section 2.1 How to Study Cells?

ACTIVITY

Q1 Estimate the real size of the cell using the formula: Estimated size of the onion peel
cell = Diameter of the visible field in micrometre / Number of cells along the
diameter

Take the book's own readings: field diameter 5 mm, and 25 onion peel cells counted along that
diameter.

Step 1 — convert the field diameter to micrometres.

1 mm = 1000 µm

d = 5 mm = 5 × 1000 µm = 5000 µm

Step 2 — apply the formula.

estimated size of one cell = d ÷ n

= 5000 µm ÷ 25

= 200 µm = 0.2 mm

Page 7 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

field of view

5000 µm ÷ 25
= 200 µm per cell
(= 0.2 mm)
diameter = 5000 µm

25 cells fit along it
The field of view is used as a ruler: its known real diameter is shared out among the cells that fit
across it.

Why it happens: the method works because the cells and the field of view are
magnified by exactly the same factor. Dividing one by the other cancels the
magnification completely, so the answer is a real size even though it was obtained
from a magnified image. That is also why the answer is only an estimate — it
assumes the cells lie edge to edge with no gaps and that the count of 25 is exact.

Check it yourself: if 40 cells had fitted instead of 25, each cell would be 5000 µm ÷
40 = 125 µm. Smaller cells, more of them — the two numbers are inversely related.

Q2 If the estimated size of an onion peel cell is 200 µm, how much does a light
microscope magnify this cell?

The total magnification is the product of the two lenses.

Page 8 of 79

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Class 9 Science Chapter 2 Cell: The Building Block of Life
a g l AglaSem · NCERT Solutions

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Section 2.2.1 Cell membrane — The universal feature of a cell

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Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

They cross by diffusion, always from higher to lower concentration. In the alveolus, O₂ is
high in the inhaled air and low in the blood, while CO₂ is high in the blood and low in the air
— so the two gases move in opposite directions across the same membrane at the same
time.
The proteins embedded in the bilayer act like gatekeepers for everything that cannot
dissolve in lipid, which is what makes the membrane selectively permeable rather than simply
leaky.
The alveolar wall is only one cell thick, so the total distance a gas molecule has to cross is tiny
— this makes the exchange fast enough to keep up with breathing.

thickness of cell membrane = 7 to 10 nm

1 nm = 0.000001 mm = 10⁻⁶ mm

so the membrane is about 7 × 10⁻⁶ mm to 10 × 10⁻⁶ mm thick

Why it happens: diffusion slows down sharply as the distance grows, so a thin
barrier is not a design accident but a necessity. The lipid bilayer is the thinnest stable
sheet that will still hold the cell's contents in, and its oily core is what does the
sorting: anything that dissolves in oil crosses freely, anything that dissolves only in
water must be let through by a protein. No energy is spent — the concentration
gradient does all the work.

Activity 2.2: Let us experiment — Page 11
Section 2.2.1 Cell membrane — The universal feature of a cell

ACTIVITY

Q1 What do you observe?

The two potato pieces behave in opposite ways.

BEAKER LIQUID OBSERVATION AFTER ABOUT AN HOUR

A Plain water The potato piece swells; it feels firmer

B 20 per cent salt or sugar solution The potato piece shrinks; it feels soft and floppy

Page 10 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Beaker A — plain water Beaker B — 20% salt solution

water enters → piece swells water leaves → piece shrinks

Water always moves towards the more concentrated solution, so the same potato gains water in
beaker A and loses it in beaker B.

Tip: both pieces must be cut to roughly the same size and weighed before they go in,
otherwise there is nothing to compare the final weights with.

Q2 What do you infer? What do you expect in terms of changes in their weight?

The weight of the piece in Beaker A increases and the weight of the piece in Beaker B
decreases. The process responsible is osmosis.

gain in weight (A) = final weight − initial weight → a positive number

loss in weight (B) = final weight − initial weight → a negative number

Example: A 25.0 g → 27.2 g, change = +2.2 g

B 25.0 g → 22.6 g, change = −2.4 g

Since only water crossed the membrane, the change in weight is a direct measure of how much
water moved in or out.

Page 11 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Why it happens: the cell membrane of the potato cells is selectively permeable — it
lets water through but not the sugar or salt molecules. In beaker A the water outside
is more dilute (more water, less solute) than the cell sap inside, so water moves
inwards until the two concentrations approach each other. In beaker B the 20 per
cent solution outside is far more concentrated than the cell sap, so water moves
outwards. This net movement of water through a selectively permeable membrane
is osmosis, which is simply the diffusion of water. Ordinary diffusion needs no
membrane at all — you saw it in Grade 8 when dye spread through water (Activity
7.8) and fragrance spread through air (Activity 7.9). Osmosis is the special case
where the moving particles are water molecules and a selectively permeable
membrane stands in the way.

Did you know? This is exactly how a plant drinks. Soil water is dilute, root cell sap is
concentrated, so water enters root cells by osmosis — no pump and no energy
needed.

What if… — Page 12
Section 2.2.1 Cell membrane — The universal feature of a cell

WHAT IF…

Q1 …mung bean seeds are kept in a concentrated solution after soaking in water for 12
hours? What will happen to them?

They will shrink and go wrinkled, and they will not sprout.

1. During the 12 hours in plain water the seeds imbibe water and swell — the seed coat
stretches tight and the embryo becomes active.
2. Moved into a concentrated (hypertonic) salt or sugar solution, the solute concentration
outside is now far greater than inside the seed cells.
3. Water therefore moves out of the seed cells by osmosis. The swollen seeds lose water,
become smaller, softer and wrinkled.
4. Germination stops. Enzymes inside the seed need water to break down stored starch; with
the water drawn out, the reactions cannot run.

solute concentration outside > solute concentration inside → hypertonic

net movement of water = out of the cell → cell shrinks

Page 12 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Why it happens: soaking and shrivelling are the same physics running in opposite
directions. Water always moves from where it is more abundant (dilute solution) to
where it is scarcer (concentrated solution) across a selectively permeable membrane.
The seed does not ‘choose’; it simply sits at whatever the surrounding concentration
dictates.

Try This: soak two sets of mung beans for 12 hours, then keep one set in plain water
and the other in strong sugar solution. After a day, only the first set will show a white
radicle pushing out. It is the same reasoning that Deepa uses in Question 16 to
preserve amla and lemons.

Q2 …a cell is kept in salt or sugar solutions of different concentrations?

Three outcomes are possible, and which one occurs depends only on how the outside solute
concentration compares with the inside.

SOLUTION COMPARISON NET WATER MOVEMENT WHAT HAPPENS TO THE CELL

Isotonic outside = inside none (equal both ways) No change in size

Hypotonic outside < inside into the cell Cell swells; an animal cell may burst

Hypertonic outside > inside out of the cell Cell shrinks

Isotonic Hypotonic Hypertonic

no net movement water moves in water moves out
size unchanged cell swells cell shrinks

The same cell in three surroundings. Only the concentration difference decides which way water
moves.

Page 13 of 79

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Class 9 Science Chapter 2 Cell: The Building Block of Life
a g l AglaSem · NCERT Solutions

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Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Why it happens: an animal solves the same problems in a different way — it has a
skeleton and it can walk away from wind, drought or a predator. A plant is fixed in
one place, so its support has to be built into every single cell. The trade-off is
flexibility: because animal cells have no wall they can change shape, which is what
lets muscle contract, white blood cells squeeze between tissues and an embryo fold
into an organism.

Did you know? Cellulose in your diet cannot be digested by human enzymes, so it
passes through as roughage and helps digestion by adding bulk to the food.

Activity 2.3: Let us investigate — Page 13
Section 2.2.2 Cell wall — The outer covering of cells

ACTIVITY

Q1 Onion peel cells or Rhoeo leaf peel cells are box-shaped and regularly arranged,
whereas cheek cells are irregularly arranged. Why do you think this difference
exists?

Because plant cells have a rigid cell wall and animal cells do not.

ONION PEEL / RHOEO LEAF PEEL HUMAN CHEEK

Outermost covering Cell wall of cellulose, then cell membrane Cell membrane only

Shape Box-like, straight-edged, definite Rounded, irregular, no fixed outline

Arrangement Regular, packed edge to edge like bricks Scattered, often overlapping

Stain used Safranin Methylene blue

Why it happens: the wall is laid down while the cell is still attached to its neighbours
in the tissue, so adjoining walls are shared and the cells are cemented into a fixed
brick-like pattern which the peel keeps even after it is torn off. Cheek cells are
scraped loose from a moist surface layer, they have no wall to hold a shape, and
surface tension pulls each free cell towards a rounded outline. Their irregular
arrangement on the slide is partly the shape of the cells and partly the fact that they
are simply spread out in a drop of water rather than held in a tissue.

Page 15 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Tip: stain the two slides differently for a reason — safranin binds well to the cellulose
wall and the nucleus of plant cells, while methylene blue makes the nucleus of the
almost transparent cheek cell visible.

Q2 Prepare two slides of a Rhoeo leaf peel and human cheek cells again, and put 20 per
cent sugar solution on them. Observe them under a microscope after half an hour.
What do you observe?

Both cells lose water, but they look completely different afterwards.

Rhoeo (plant) cells: the outer boundary stays exactly where it was, but the inner contents
shrink away from it. A clear gap appears between the inner and the outer boundary. This
pulling away of the membrane and contents from the wall is called plasmolysis.
Cheek (animal) cells: the whole cell shrinks considerably and looks shrivelled, because there
is no outer wall to hold the shape.

plant cell in water plant cell in 20% sugar cheek cell in water cheek cell in sugar

wall and contents wall unchanged, round and full whole cell shrunk
both full contents pulled in

In a hypertonic solution both cells lose water; only the plant cell keeps its outline, because the wall
does not shrink.

Why it happens: the 20 per cent sugar solution is hypertonic to the cell sap, so
water leaves both cells by osmosis. In the plant cell the cellulose wall is rigid and
freely permeable — sugar solution seeps into the space between wall and
membrane, so while the protoplasm contracts the wall holds its original box shape.
In the cheek cell the membrane is the boundary, so when the contents lose water
the boundary itself caves in. This single observation is the cleanest proof in the
chapter that plant cells have something outside the membrane that animal cells
lack.

Page 16 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Check it yourself: flood the plasmolysed Rhoeo slide with plain water. The contents
swell back out to meet the wall — the process reverses, showing that plasmolysis
has not killed the cell.

Pause and Ponder — Page 14
Section 2.2.2 Cell wall — The outer covering of cells

PAUSE AND PONDER

Q1 What argument would you give for the necessity of a cell wall in plants usually fixed
in one place versus in animals usually moving from one place to the other?

The argument is about where the support and the escape route come from. A fixed organism
must build strength into every cell; a moving organism must instead keep every cell able to
change shape.

PROBLEM HOW A PLANT SOLVES IT HOW AN ANIMAL SOLVES IT

Standing upright, resisting Rigid cellulose wall in every cell; Internal skeleton and muscles
wind and rain turgid cells press against one
another

Too much water entering Wall pushes back and stops further Kidneys and blood keep body fluid
by osmosis entry — the cell cannot burst isotonic, so the problem does not arise

Escaping drought, heat or Cannot move; must endure it Moves away
a predator

Movement, growth of an Not needed — growth is by adding Wall-free cells change shape, slide past
embryo, healing cells at fixed growing points one another and migrate

Why it happens: a wall is not a free gift, it is a trade-off. It buys rigidity and
protection from bursting, but it costs mobility and flexibility. For a plant rooted in
soil, rigidity is worth far more than mobility, so the trade is a good one. For an
animal that must run, swallow, contract a muscle and heal a wound, a rigid box
around every cell would make all of that impossible — so animals keep a bare
membrane and pay for the strength elsewhere, with bone and cartilage. Notice that
fungi and bacteria, which are also non-motile or weakly motile, made the same
choice as plants and have walls too.

Page 17 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Q2 What consequences would you predict for a plant cell if its cell wall were to become
as flexible as a cell membrane?

The cell would behave exactly like an animal cell — and the whole plant would collapse.

1. In ordinary soil water (hypotonic), the cell would burst. Water keeps entering by osmosis
and only the rigid wall stops it. With a flexible wall there is nothing to build up the opposing
pressure, so the cell swells until the membrane tears.
2. The plant would lose its shape. A stem, leaf or petal is held up by millions of turgid walled
cells pressing on one another. Remove the rigidity and the plant would flop like a wilted one
even when fully watered.
3. Plasmolysis would become shrivelling. In concentrated solution the cell would shrink as a
whole instead of the contents pulling away from an unchanged wall — and it could not be
revived simply by adding water.
4. Root absorption would be less effective. The wall is freely permeable and acts as a wide,
open pathway for soil water and minerals to reach the membrane. A soft, membrane-like
covering would slow that route down.

Soil water is hypotonic to root cell sap

→ water enters continuously by osmosis

→ pressure builds up inside

rigid wall present: pressure balances, the cell becomes turgid and stops

rigid wall absent: pressure keeps rising → the cell bursts

Why it happens: the wall works as a pressure vessel. Once the cell is full, the wall's
inward push equals the osmotic pull inwards, and net water movement stops even
though the concentrations are still unequal. That balance is what we call turgor, and
turgor is the plant's substitute for a skeleton.

Q3 Why is it important to cut the two potato pieces in roughly equal size and measure
their initial weight before placing them in different liquids?

So that the only difference between the two set-ups is the liquid, and so that the change in
weight can actually be calculated.

Page 18 of 79

as e
Class 9 Science Chapter 2 Cell: The Building Block of Life
a g l AglaSem · NCERT Solutions

Equal size — a fair test. Osmosis depends on surface area and on the amount of tissue. A
co m
se
bigger piece has more membrane area and more cells, so it would exchange more water for
m.
o m
c the only variable. g l a
the same reason of size alone. Matching the sizes removes size as an explanation and leaves
the liquid.as a
m
se weight — a baseline. Weight gained or lost cannot be read from the final weight by
g l a
a itself. You need the difference.
Initial

co m
em
. ag
s
change in weight = final weight − initial weight
a
agl
m
Beaker A: 25.0 g → 27.2 g gives 27.2 − 25.0 = +2.2 g (water gained)
co
Beaker B: 25.0 g → 22.6 g gives 22.6 − 25.0 = −2.4 g (water lost)
em.
m l as
.co a g
a s emit happens: this is the logic of a controlled experiment. Every factor that could
a gl Why
affect the result — size, shape, the same potato, the same time, the same

m a s
agl
temperature — is deliberately kept the same, so any difference in the outcome must

m.co
be caused by the one factor that was changed. Without the initial weights the

l a se
g
experiment would give only a vague impression of ‘bigger’ and ‘smaller’; with them it
a
gives numbers that can be compared, repeated and checked by someone else.

co m
m .
Tip: pat both pieces dry with the same care before each weighing. Surface water
m as e
.co
clinging to a piece would be weighed as if it had entered the cells.
a g l
se m
g l a
a
se m
Activity 2.4: Let us study — Page 14
com g l a
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ase
agl

co m
m .
m as e
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se m
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a c
m .
m a s e
e m . co agl
g l as
a

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Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Section 2.3 The Cell Interior — A Coordinated Working System

ACTIVITY

Q1 Study the given diagrams of a bacterial cell, a plant cell, and an animal cell (Fig.
2.10a, b and c). Observe the different structures present in each of them. Record
your observations in Table 2.1.

Page 20 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Page 21 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Cell wall

Ribosome

Appendage for locomotion

Cytoplasm

Nucleoid (Genetic material)
(a)

Cell membrane
Chloroplast
Nucleus

Vacuole
Nucleolus

Ribosome
Endoplasmic reticulum

Mitochondria

Golgi body
Cytoplasm
Cell wall

(b)

Nucleus
Lysosome

Nucleolus
Vacuole

Endoplasmic reticulum

Golgi body

Mitochondria

Ribosome

Cell membrane Cytoplasm

(c)

Page 22 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Fig. 2.10 — (a) a typical bacterial cell, (b) a typical plant cell, and (c) a typical animal cell.

Table 2.1: Comparison of different kinds of cells based on their structure

S. CELL STRUCTURES BACTERIAL PLANT ANIMAL
NO. CELL CELL CELL

1. Cell membrane

2. Cell wall

3. Cytoplasm

4. Well-defined nucleus (genetic
material enclosed by a membrane)

5. Primitive nucleus or nucleoid
(genetic material without
membrane around it)

6. Membrane-bound organelles

Here is Table 2.1 completed from Fig. 2.10.

S. CELL STRUCTURES BACTERIAL PLANT ANIMAL
NO. CELL CELL CELL

1. Cell membrane Present Present Present

2. Cell wall Present Present Absent

3. Cytoplasm Present Present Present

4. Well-defined nucleus (genetic material Absent Present Present
enclosed by a membrane)

5. Primitive nucleus or nucleoid (genetic Present Absent Absent
material without membrane around it)

6. Membrane-bound organelles Absent Present Present

Two more structures are visible in Fig. 2.10 and worth noting: the bacterial cell has an
appendage for locomotion and free ribosomes; the plant cell alone has chloroplasts and a
large central vacuole.

Page 23 of 79

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Class 9 Science Chapter 2 Cell: The Building Block of Life
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co m
m.
Why it happens: the pattern in the table is not a list of random differences — it

m l a se
splits cells into two kinds. Rows 4, 5 and 6 all move together: a cell either encloses its
o
c
DNA in a .nuclear a g or it does
m
membrane and has membrane-bound organelles,

l a se That is because both features depend on the same ability, namely to build
g
neither.
ainternal membranes. Bacteria lack it, so their DNA lies free in the cytoplasm as a
nucleoid and all their chemistry happens in one open compartment. Rows 1 and 3
co m
.
are in every column because no cell can exist without a boundary and a fluid interior.
e m ag
g l as
a
Tip: row 2 is the row that separates plants from animals, and rows 4–6 are the rows
that separate bacteria from both. Read the table by rows, not by columns, and the
co m
logic appears.
em.
m l as
m .co a g
l a se
a g
In-text Question — Page 15

m a s
m .co agl
l a se
a g

co m
m .
m as e
.co a g l
se m
g l a
a
se m
com g l a
m . a
ase
agl

co m
m .
m as e
.co a g l
se m
g l a
a c
m .
m a s e
e m . co agl
g l as
a

co m
m .
m ase
.co


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Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Section 2.3 The Cell Interior — A Coordinated Working System

Q1 Which of the cells given in Fig. 2.10 fall under the categories of prokayotic and
eukaryotic cells?

Page 25 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Page 26 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Cell wall

Ribosome

Appendage for locomotion

Cytoplasm

Nucleoid (Genetic material)
(a)

Cell membrane
Chloroplast
Nucleus

Vacuole
Nucleolus

Ribosome
Endoplasmic reticulum

Mitochondria

Golgi body
Cytoplasm
Cell wall

(b)

Nucleus
Lysosome

Nucleolus
Vacuole

Endoplasmic reticulum

Golgi body

Mitochondria

Ribosome

Cell membrane Cytoplasm

(c)

Page 27 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Fig. 2.10 — (a) a typical bacterial cell, (b) a typical plant cell, and (c) a typical animal cell.

The bacterial cell (Fig. 2.10a) is prokaryotic. The plant cell (Fig. 2.10b) and the animal cell
(Fig. 2.10c) are eukaryotic.

pro = primitive, karyon = nucleus → prokaryotic = primitive nucleus

eu = true, karyon = nucleus → eukaryotic = true nucleus

What you can actually see in the figure that settles it:

Fig. 2.10a shows a nucleoid — genetic material lying naked in the cytoplasm — with
ribosomes scattered around and an appendage for locomotion. There is no nuclear
membrane and no organelle bounded by its own membrane.
Fig. 2.10b and 2.10c both show a nucleus with a nucleolus inside a nuclear membrane,
together with mitochondria, endoplasmic reticulum, Golgi body, lysosome and vacuole —
every one of them membrane-bound.

CHARACTERISTIC PROKARYOTIC CELL EUKARYOTIC CELL

Diameter of a typical cell 1 to 10 µm 10 to 100 µm

Membrane-bound nucleus Absent Present

Membrane-bound organelles Absent Present

Number of cells in an organism Usually unicellular Can be unicellular or multicellular

Why it happens: internal membranes are what make a eukaryotic cell possible. They
divide the cell into rooms, so incompatible reactions can run side by side at the same
time — digestive enzymes stay sealed in lysosomes instead of dissolving the cell,
energy release is confined to mitochondria, protein processing to the ER and Golgi.
A prokaryote has one open compartment, so all its chemistry must share a single
space; that is one reason it stays small. The size figures follow from the same logic:
1–10 µm against 10–100 µm is a tenfold difference in length, which is a
thousandfold difference in volume.

Page 28 of 79

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Class 9 Science Chapter 2 Cell: The Building Block of Life
a g l AglaSem · NCERT Solutions

co m
m.
Note on Table 2.2: the book's first row reads “Primitive nucleus — Present in

m l a se
prokaryotic, Absent in eukaryotic”. Read it as “primitive nucleus (nucleoid)”, which is
o g
.c only in prokaryotic cells; the last two rows then correctly
a
m
indeed present say that a

l a se
g
membrane-bound nucleus is present only in eukaryotic cells.
a

co m
e m . ag
as
In-text Question — Page 16
a
Threads of Curiosity — Nucleus: House of coded instructionsg l

m
THREADS OF CURIOSITY

co
e m.
comany other cells without nucleus?
Mature Red Blood Cells (RBCs) in humans do not have a nucleus (enucleate). Do you
g l as
. a
Q1

sem
know

la
agANSWER
m a s
.co agl
Yes. Several cells lose or never form a nucleus, and in each case the loss buys something useful.

a s emWHAT THE LOSS OF THE NUCLEUS ACHIEVES
gl
CELL FOUND IN
a
Mature red blood cell Human blood More room for haemoglobin, so more oxygen is carried

co m
m .
se
Platelets Human blood Small cell fragments that plug a wound quickly

omcells of l a
Sieve.ctube
m agthrough; a neighbouring
asephloem
Plant vascular An open channel for food to flow

agl
tissue companion cell keeps its nucleus and controls it

se m
Mature xylem vessel Plant vascular Dead, hollow tubes — a clear pipe for water
tissue
com l a
and tracheid

. Perfectly transparent, since a nucleus would scatter light a g
e m
as
agl
Mature lens fibre cells Eye lens
of the eye

. com
Why it happens: the nucleus is the control centre, so giving it up has a price — a cell
m
without a nucleus cannot repair itself or divide. A human RBCetherefore
s
m a
survives only
o 120 days and must be replaced from bone marrow.
.cabout agl The trade is worth it
m because the job is purely mechanical: carrying oxygen, plugging a leak, or acting as
l a se
ag
.c
a pipe needs no fresh instructions. Wherever a cell has one simple, fixed job and can
be replaced from elsewhere, evolution has often thrown the nucleus away.
s e m
m a
e m . co agl
g l as
a

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Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Did you know? A red blood cell's biconcave, nucleus-free shape also makes it
flexible enough to squeeze single file through capillaries narrower than the cell
itself.

In-text Question — Page 17
Lysosomes — The clean-up system

Q1 Cells produce waste materials and damaged, worn-out organelles during their
activities. How does the cell prevent these wastes from accumulating inside it?

It digests them, using lysosomes — single membrane-bound sacs filled with powerful enzymes.

unwanted proteins, carbohydrates, fats and damaged organelles

→ enclosed and broken down by lysosomal enzymes

→ simple products released into the cytoplasm

→ reused in other cellular processes

The enzymes can break down proteins, carbohydrates, fats and even worn-out parts of the
cell itself, keeping the cell clean and healthy.
The products of the breakdown are not thrown away — they are released into the cytoplasm
and used again as raw material.

Why it happens: the important detail is the single membrane around the lysosome.
Enzymes that can digest protein and fat would attack the cell's own structures if they
were loose in the cytoplasm, so they are kept sealed in a bag and only what is meant
to be destroyed is brought inside it. This is the same design principle you met with
the cell membrane — keep incompatible chemistry in separate compartments.
Recycling also makes sense economically: rebuilding a protein from its amino acids
costs the cell far less energy than making the amino acids from scratch.

Did you know? Human sperm cells carry lysosomal enzymes at their tip. When a
sperm meets an egg these enzymes break down the egg's outer layer so that
fertilisation can take place — the same digestive tool used constructively.

Page 30 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

In-text Question — Page 18
Plastids — Centre for food synthesis in the plant cells and beyond

Q1 Animals can obtain food from their surroundings, however, plants synthesise food
in the presence of sunlight. But where do plants synthesise their food and obtain
energy for cellular activities?

Plants make food in chloroplasts, a type of plastid, and then release energy from that food in
mitochondria, exactly as animals do.

sunlight + CO₂ + H₂O → (in the chloroplast, photosynthesis) → sugar

sugar + O₂ → (in the mitochondrion, cellular respiration) → ATP + CO₂ + H₂O

Inside the chloroplast is a semi-fluid substance, the stroma. Within it are disc-shaped
membrane structures containing chlorophyll, the green pigment that absorbs sunlight.
The sugars made during photosynthesis are stored in the stroma along with starch granules.
Chloroplasts are double-membrane-bound, just like mitochondria.
Other plastids do other jobs: chromoplasts carry yellow, orange or red pigments;
leucoplasts have no pigment and store starch, oils or proteins.

Why it happens: the question hides a common mistake — that plants use sunlight
directly to run their cells. They cannot. Sunlight is only captured, and its energy is
stored in the chemical bonds of sugar. Every activity of a plant cell, day or night, root
or leaf, is then paid for with ATP made by mitochondria breaking that sugar down.
That is why a plant cell contains both organelles, and why a root cell — which never
sees light and has no chloroplasts — still has plenty of mitochondria.

Did you know? Both mitochondria and plastids have their own DNA and ribosomes
and can make some of their own proteins — features shared with bacteria, which
suggests both organelles share an evolutionary history with single-celled organisms.

In-text Questions — Page 19

Page 31 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Plastids and Vacuoles

Q1 Are there any other plastids in plant cells that contain any pigments other than the
green pigments?

Yes — the chromoplasts.

PLASTID PIGMENT FUNCTION WHERE YOU FIND IT

Chloroplast Chlorophyll Photosynthesis; temporary Leaves, green stems
(green) storage of food

Chromoplast Yellow, orange or Gives bright colour Flower petals, fruits
red

Leucoplast None — Stores starch, oils or proteins Potato tuber, taro (Colocasia),
colourless seeds, roots

The Greek roots make the names easy: chroma means colour, leukos means white or colourless.

Why it happens: all three are plastids, that is, members of one family of double-
membrane organelles that make and store food. They differ only in which pigment
they hold and what they are asked to store. Because they belong to one family, a
plastid can change type — this is why a green tomato turns red as it ripens, as
chloroplasts are converted into chromoplasts, and why a potato exposed to light
turns green as leucoplasts develop chlorophyll.

Q2 How do flowers, fruits, and vegetables acquire varied colours?

From the pigments held in their chromoplasts — pigments other than chlorophyll, which may
be yellow, orange or red.

In the petals of a flower and in the flesh and skin of a fruit, the plastids carry these coloured
pigments instead of chlorophyll.
The bright colour is not decoration. It attracts pollinators to flowers so that pollination
happens, and it attracts fruit-eating animals, which then carry the seeds away and help in
seed dispersal.

Page 32 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

green fruit (chloroplasts, chlorophyll)

→ ripening: chlorophyll broken down, chromoplast pigments revealed and made

→ red, orange or yellow ripe fruit — a signal that the seeds are ready

Why it happens: a plant cannot move, so it must persuade animals to move on its
behalf. Colour is the cheapest long-distance signal it has. The timing is what makes it
work: the fruit stays green and unattractive while the seeds are still immature, and
turns colour only once they are ready to be dispersed. So the change of pigment is
really a piece of communication between a plant and an animal.

Did you know? Not every plant colour comes from chromoplasts. Many reds, blues
and purples — in jamun, beetroot or a rose — come from pigments dissolved in the
cell sap of the vacuole, not from plastids at all.

Q3 But where are water, minerals, and waste materials stored in the cell?

In the vacuole. In a mature plant cell there is usually one large central vacuole, surrounded by
a single selectively permeable membrane and filled with a watery fluid called cell sap.

It stores water, minerals, sugars and waste material.
By holding a large volume of water it maintains pressure inside the cell, which keeps the
plant cell firm.
Animal cells sometimes have vacuoles too. They are much smaller, and are used for the
temporary storage of materials.

Why it happens: a plant has no kidneys and no way of excreting most waste, so it
stores it instead — safely locked away in the vacuole where it cannot interfere with
the cytoplasm. The same sac solves a second problem at the same time. Filling one
big bag with water pushes the cytoplasm and organelles into a thin layer against the
wall, which puts them close to the surface for exchange, and it lets the cell grow
large cheaply — a cell can increase its volume by taking in water instead of by
making expensive new cytoplasm.

Page 33 of 79

ase
Class 9 Science Chapter 2 Cell: The Building Block of Life
a g l AglaSem · NCERT Solutions

co m
m.
Why do plants look wilted when they do not get enough water?
e
Q4

m l as
.co a g
a
s em
a gl the vacuole loses water, so the cells stop pressing outwards and the plant can no
Because
longer hold itself up.

. com ag
m membrane against the wall → cell turgid
enough water → vacuole full → cell sapepresses
s
a
→ plant stands firm agl
too little water → vacuole loses water → pressure inside falls → cell flaccid → leaves and
co m
e m.
m as
stem droop = wilting

.co a g l
a s em
a gl Why it happens: a young stem or a leaf has no wood to hold it up. Its stiffness
comes entirely from turgor pressure — the outward push of water-filled cells against
m a s
c o agl
their own walls, like air in a bicycle tyre. When the soil dries, the cell sap becomes
relatively more concentrated than m .
s e the soil solution, water is drawn out of the
aThe wall is still rigid, but a rigid wall around a half-
agl
vacuoles, and the ‘tyres’ go flat.
empty cell no longer presses against its neighbour, so the whole tissue goes limp.

. c om
m a s
Check it yourself: water a wilted money-plant in the evening andemlook at it next
. a gl by osmosis — proof
co It stands up again because the vacuoles have refilled
em
morning.

g l as that wilting is a water-pressure effect and not damage, provided it is caught early.
a
se m
com g l a
Pause and Ponder — Page 19m. a
ase
agl
Section 2.3.1 Why do eukaryotic cells need these organelles?

PAUSE AND PONDER

co m
m .
m as e
.co l
Do white flowers contain any pigment? Give reasons.
g
Q4

em a
a s
agl ANSWER
.c
A truly white flower contains no coloured pigment in its petals. Its plastids are leucoplasts —
s e m
com gla
plastids that lack pigment and are therefore colourless.
. a
a s em absorbs some wavelengths of light and reflects the rest.
Colour is produced when a pigment

a
Chlorophyll absorbs redgland blue and reflects green; a chromoplast pigment absorbs blue
and reflects yellow, orange or red.

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m .
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.co


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Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

A white petal absorbs almost nothing. Air spaces between its cells scatter and reflect all
wavelengths of visible light back together, and light containing all wavelengths is seen as
white.
So white is not a pigment at all — it is the appearance of a petal that has none.

all wavelengths absorbed → black

red and blue absorbed, green reflected → green

all wavelengths reflected → white

Why it happens: it is the same reason foam on milk, crushed ice and clouds look
white although water is colourless — countless tiny surfaces scatter every colour
equally. In a white petal the scattering surfaces are the air-filled gaps between cells.
A useful check: crush a white petal in water and no colour comes out, whereas a red
rose petal or a marigold petal releases its colour at once.

Did you know? Many ‘white’ flowers such as jasmine are far from plain to the insects
that visit them — their petals carry patterns visible in ultraviolet light, which bees
can see and we cannot. Some white flowers also carry colourless flavonoids that are
pigments in the chemical sense but absorb only in the UV.

Page 35 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Q5 Draw a well-labelled schematic diagram of a plant or an animal cell using these
clues — (i) Nucleus appears as a dark and round body inside the cell. (ii) ER spreads
like a network of extended nuclear envelope. (iii) Mitochondria and chloroplasts are
rod shaped. You may refer to Fig. 2.10.

Page 36 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Page 37 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Cell wall

Ribosome

Appendage for locomotion

Cytoplasm

Nucleoid (Genetic material)
(a)

Cell membrane
Chloroplast
Nucleus

Vacuole
Nucleolus

Ribosome
Endoplasmic reticulum

Mitochondria

Golgi body
Cytoplasm
Cell wall

(b)

Nucleus
Lysosome

Nucleolus
Vacuole

Endoplasmic reticulum

Golgi body

Mitochondria

Ribosome

Cell membrane Cytoplasm

(c)

Page 38 of 79

as e
Class 9 Science Chapter 2 Cell: The Building Block of Life
a g l AglaSem · NCERT Solutions

co m
Fig. 2.10 — (a) a typical bacterial cell, (b) a typical plant cell, and (c) a typical animal cell.

e m.
m l as
m .co a g
l a se
a g
Here is a schematic plant cell drawn to those three clues. Copy this layout, then label it.

co m
e m . ag
as
cell membrane
cell wall
a g l
nucleus (dark round body endoplasmic reticulum

co m
with nucleolus) (network from the nucleus)

em.
mitochondrion (rod)

m l as
m .co
chloroplast (rod)
a g
l a se
a g large central
vacuole

m a s
.co agl
cytoplasm

se m
g l a
a
Schematic plant cell: the nucleus is a dark round body, the ER spreads out from the nuclear envelope
as a network, and mitochondria and chloroplasts are drawn rod-shaped.
co m
m .
m as e
.co
What your diagram must show to earn full marks:
a g l
a s em
l
A double outer boundary — the thick cell wall outside and the thin cell membrane just
a g inside it.
A round, densely shaded nucleus with a smaller dark nucleolus inside it.
se m
com g l a
. a
The endoplasmic reticulum shown as a network of tubes and sheets continuous with the
m
ase
outer membrane of the nuclear envelope.

a gl
Rod-shaped mitochondria and rod-shaped chloroplasts (the chloroplasts shaded green).
The large central vacuole, with cytoplasm as a layer between it and the wall.

c o m
.
m the chloroplasts,
s e
Tip: if you draw an animal cell instead, remove the cell wall, remove

. om the outline rounded and irregular, keep the vacuole
cmake a glasmall, and add a lysosome
a s em and the Golgi apparatus.
agl c
m .
m a s e
In-text Question — Page 20 .co
m agl
l a se
a g

co m
m .
m ase
.co


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Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Section 2.4 How do Normal Cells Grow and Divide?

Q1 When you get a small cut on your skin, it heals after a few days. When hair fall out,
new hair grow back. How does this happen?

Because cells in the body grow and divide to replace old, dead or damaged cells.

damaged cell removed

→ nearby healthy cell copies its DNA

→ divides by mitosis into 2 identical daughter cells

→ new cells fill the gap and the wound closes

Skin cells and the cells at the base of a hair follicle divide continuously, because these tissues
are worn away all the time.
The daughter cells are genetically identical to the parent, so the new skin is real skin and the
new hair is the same kind of hair.
Division stops once the gap is filled and the new cells touch their neighbours — this is
contact inhibition.

Why it happens: notice what growth is not. Our body does not grow because cells
swell up; a cell can grow only up to a certain size, because beyond it the membrane's
surface area cannot supply the volume inside. Growth and repair therefore happen
by making more cells, not bigger ones. Roughly a few hundred billion cells — about 1
per cent of all the cells in the body — are replaced every single day.

Did you know? Cells that divide constantly are also the ones most affected by
treatments that block cell division. That is why chemotherapy, which targets rapidly
dividing cells, often causes hair loss.

Activity 2.5: Let us enhance our skills — Page 20

Page 40 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Section 2.4 How do Normal Cells Grow and Divide?

ACTIVITY

Q1 Leave the setup for 5–6 days and observe. Do you observe the roots growing?

Yes. Fresh white roots grow out from the base of the onion bulb into the water, usually a few
centimetres in 5–6 days.

The base of the bulb bearing the roots must stay immersed; the rest of the bulb stays above
the water.
Growth is fastest at the very tip. Cut 2–3 cm of the freshly grown roots for the slide, because
it is the tip that contains dividing cells.

2–3 cm of root cut → aceto-alcohol (glacial acetic acid : ethanol :: 1 : 3) for 24 hours

→ 70 per cent ethanol for preservation

→ dilute HCl for 10–15 minutes to soften the tissue

→ 2–3 drops aceto-carmine stain, 5–10 minutes, warmed gently

→ squash under a coverslip and observe

Why it happens: a root grows by adding new cells at its tip, in a region called the
meristem, so the tip is where cell division is actually going on. Each chemical in the
list has a job: aceto-alcohol fixes the cells, killing them instantly so that the stage of
division each was in is frozen exactly as it was; dilute HCl loosens the cement
between cells so the tissue can be spread; aceto-carmine stains chromosomes deep
red so they stand out; and the squash flattens the tip into a single layer, because
overlapping cells cannot be focused on together.

Tip: warm the slide gently over a spirit lamp — do not boil it. Overheating makes the
stain uneven and can destroy the chromosomes you are trying to see.

Page 41 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Q2 What do you observe? Do you observe the cells of the onion root tip? Are they
similar in structure? Do you find any structural differences in these cells? If yes,
why is it so?

The root tip cells are visible, and they are not all alike. Different cells show different
arrangements of stained material inside them.

Some cells show an ordinary round nucleus with a diffuse, thread-like chromatin network —
these are not dividing at that moment.
Others show deeply stained, rod-shaped chromosomes, sometimes lined up along the
middle of the cell, sometimes being pulled towards the two ends, and sometimes gathered
into two separate groups.

The reason is that the cells of a growing onion root tip divide continuously. At the moment the
tissue was fixed, each cell happened to be at a different point in the process, so the slide is a
snapshot of many stages of cell division at once — which is exactly what Fig. 2.17 shows.

Why it happens: chromatin and chromosomes are the same DNA in two different
states. In a non-dividing cell the DNA lies loose as chromatin, an entangled mass of
thread-like structures — good for reading instructions but hopeless for sharing out.
Whenever the cell is about to divide, the chromatin coils up tightly and organises
itself into short rod-shaped chromosomes, which can be moved about without
tangling or breaking. That is why chromosomes are visible only when a cell is about
to divide, and it is why a single slide of a root tip can show so many different-looking
cells even though they are all the same kind of cell.

Page 42 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Q3 Fig. 2.17 shows various stages of cell division. Can you identify which stage comes
first during cell division?

Fig. 2.17 — redrawn sketch of the textbook photomicrograph: onion root-tip cells in a
file, each drawn as the microscope shows it.

The first stage is the one in which the chromatin has just condensed into visible
chromosomes but they are still scattered, with the nuclear membrane breaking down. It
comes before any stage in which the chromosomes are neatly lined up or separated.
Use this order of events to place the cells in Fig. 2.17:

1. Cell with a plain nucleus and diffuse chromatin — the cell before division begins.
2. Chromosomes appear as distinct rod-shaped bodies, scattered in the cell; the nuclear
membrane disappears.
3. Chromosomes line up along the middle of the cell.
4. Chromosomes are pulled apart towards the two opposite ends.
5. Two groups of chromosomes gather at the ends, new nuclei form and a new wall grows
between them — two daughter cells.

Page 43 of 79

as e
Class 9 Science Chapter 2 Cell: The Building Block of Life
a g l AglaSem · NCERT Solutions

co m
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Why it happens: the sequence is forced by the logic of the job. Before DNA can be

m l a se
shared out it must first be packed into countable, movable units — so condensation
o g half goes to a
.c first. It must then be gathered at one place so that each
a
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l a se side — hence the lining up in the middle. Only then can the two sets be
g
different
aseparated, and only after they are separated can two new nuclei be built. You will
study the named stages of cell division in higher grades; at this level it is enough to
co m
read the sequence from what the chromosomes are doing.
e m . ag
g l as
a
Tip: when you see a squash slide, count how many cells show scattered
chromosomes and how many show separated groups. Stages that take longer
co m
appear in more cells — the picture is a record of time as well as of shape.
em.
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Section 2.4.1 Cell division

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PAUSE AND PONDER

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Q6
How does this relate to the concept of surface area?
co m
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a s em energy production happens on membrane surfaces, and many small mitochondria
gl give far more surface area than one big one of the same total volume.
Because

a
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a g l
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g l a
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total surface area = 8 × 4π(R/2)² = 8 × 4πR²/4 = 2 × 4πR²

s e m
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That is twice the surface area of the single large mitochondrion, for exactly the same

m .
volume.
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co m
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m ase
.co


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Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

one large mitochondrion eight small ones (four shown)

surface area 4πR² surface area 2 × 4πR², same volume

Dividing the same volume into smaller pieces multiplies the surface without changing the amount of
material.

Three further reasons follow from the same idea:

Delivery. ATP has to reach every corner of the cell. Many mitochondria can be positioned
exactly where energy is needed — in a muscle fibre they line up along the contracting fibres;
in a sperm they are packed into the tail.
Supply. Oxygen and glucose must diffuse into the mitochondrion. Diffusion is slow over long
distances, so a giant organelle would starve at its centre.
Safety and repair. If one small mitochondrion is damaged it can be digested by a lysosome
and replaced, with barely any loss. Losing one giant mitochondrion would leave the cell with
no energy supply at all.

Why it happens: the same principle explains the shape of a single mitochondrion.
Its inner membrane is folded into finger-like projections called cristae, which
increase the surface area available for the chemical reactions of cellular respiration
without increasing the organelle's size. Nature solves the surface-area problem twice
over — by folding inside each mitochondrion, and by having many of them.

Q7 If the skin cells start dividing by meiosis instead of mitosis, what do you think will
happen to a cut on the skin?

The cut would not heal properly. The skin would be replaced by cells carrying only half the
chromosome number, which cannot function as skin — and after one round of meiosis those
cells would stop dividing altogether.

Page 45 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Normal healing (mitosis):

skin cell (46 chromosomes) → 2 cells (46 each) → 4 → 8 … wound fills with true skin

If meiosis occurred instead:

skin cell (46) → 4 cells with 23 chromosomes each

half the genes missing → many proteins cannot be made → cells are not skin cells and soon

die

1. Wrong chromosome number. Each daughter cell would get only one of each pair of
chromosomes, so a large part of its genetic instructions would be missing. It could not build
the keratin, the pigment and the junction proteins that skin needs.
2. Wrong number of cells, and no continuation. Meiosis produces four cells and then stops
— gametes do not divide again. Healing needs division to be repeated until the gap is closed.
3. Result: the wound would stay open or be filled with weak, abnormal tissue; the skin would
lose its barrier function and infection would follow.

Why it happens: the two divisions are built for two different jobs, and the difference
is the point of the chapter. Mitosis maintains the genetic information across all body
cells, which is exactly what repair needs — the replacement must be an exact copy of
what was lost. Meiosis halves it deliberately, because gametes are going to be
combined with another gamete at fertilisation. Halving is right in the testes and
ovaries and catastrophic anywhere else, which is why the body confines meiosis
strictly to reproductive organs.

In-text Question — Page 23
Threads of Curiosity — Section 2.5.1 Do cells grow and reproduce forever?

THREADS OF CURIOSITY

Q1 How do cancer cells grow and spread?

They grow because they have lost the controls that stop normal cell division, and they spread
by invading nearby tissues and travelling to other parts of the body.

Page 46 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

normal cell: grow → work → age → die → replaced, all under control

cancer cell: control system breaks down

→ divides uncontrollably, ignores contact inhibition

→ mass of abnormal cells = tumour (benign or malignant)

→ malignant tumour invades nearby tissues

→ cells travel and form new tumours elsewhere

BENIGN TUMOUR MALIGNANT (CANCEROUS)
TUMOUR

Growth Stays as one local mass Invades the surrounding tissue

Spread to other No Yes — forms new tumours elsewhere
organs

Danger Usually low, unless it presses on an High
organ

Why it happens: in many animal cells, division normally stops when a cell comes
into contact with its neighbours — contact inhibition. A cancer cell no longer obeys
that signal, so it keeps dividing even when there is no room. It also escapes the
normal instruction to die at the end of its life span. The result is a growing mass
which takes food and oxygen away from healthy tissue, and, if malignant, breaks
through the boundaries between tissues and is carried to distant organs. Plant cells,
by contrast, do not show contact inhibition at all — their rigid cell walls hold them in
place and they follow a different pattern of growth, which is why the question of a
spreading tumour does not arise in plants in the same way.

Ready to Go Beyond — Page 23

Page 47 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Section 2.5.1 Do cells grow and reproduce forever?

READY TO GO BEYOND

Q1 How do cells monitor their growth to maintain a balance?

By balancing the making of new cells against the deliberate, controlled death of old ones. The
main mechanism is Programmed Cell Death (PCD).

PCD is a genetically regulated and organised process of selective cell destruction — the cell
dismantles itself neatly on instruction, instead of bursting and damaging its neighbours.
It is essential for three things: normal development, cellular quality control, and immune
function.
Contact inhibition works alongside it: in many animal cells, division stops when a cell comes
into contact with its neighbours, so a tissue stops growing once it is complete.
Every cell also has a definite life span. Mature red blood cells, for example, survive about 120
days and are then replaced.

rate of cell division = rate of cell death → tissue size stays constant

division > death → tumour

division < death → tissue wastes away

Why it happens: the striking example in the book is the human hand. As an embryo
develops, PCD removes the cells between the developing digits, and that is how
separate fingers are carved out of a paddle-shaped bud — without it we would have
webbed hands. So cell death here is not damage at all; it is a sculptor's tool. The
same process removes cells that have picked up faulty DNA before they can become
cancerous, which is why PCD counts as quality control. Problems arise at both
extremes: if cells do not die when they should, tumours form; if they die too early,
tissues break down.

Q2 Explore different ways by which cells maintain themselves.

A cell stays healthy by running several housekeeping systems at once. Here is what to explore,
with the part of the chapter each connects to.

Page 48 of 79

ase
Class 9 Science Chapter 2 Cell: The Building Block of Life
a g l AglaSem · NCERT Solutions

m
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WAY OF HOW IT WORKS ORGANELLE OR

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MAINTAINING PROCESS
ITSELF
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INVOLVED

se m
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gatekeepers; water balance by osmosis

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cdamaged
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ag
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carbohydrates, fats and
out parts

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sorted and packed for delivery
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se m
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se m
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broken down and rebuilt over time;owhat
g l a
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inseparable from the organelle
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.cTry This: take one organelle from Fig. 2.10 and trace what
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apparatus in turn will show you how tightly the systems depend on each other.
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s e m
m a
m . co agl
l a se
ag — Page 24
Revise, Reflect, Refine

co m
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m as e
.co


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Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

End-of-chapter questions

REVISE, REFLECT, REFINE

Q1 Differentiate between the following pairs of terms based on the clues given in
parentheses: (i) Cell membrane and cell wall (permeability) (ii) RER and SER
(structure) (iii) Chloroplasts and chromoplasts (pigments)

(i) Cell membrane and cell wall — permeability

CELL MEMBRANE CELL WALL

Permeability Selectively permeable — lets some substances Freely permeable — water and
through and blocks others dissolved minerals pass straight through

What that Water crosses; salt and sugar molecules are Does no sorting at all; the selection
means held back, which is what makes osmosis is left to the membrane just inside it
possible

Present in Every living cell Plants, fungi and bacteria only

(ii) RER and SER — structure

ROUGH ENDOPLASMIC RETICULUM SMOOTH ENDOPLASMIC
RETICULUM

Surface Ribosomes attached to its surface, so it looks rough under an No ribosomes on its surface, so it
electron microscope looks smooth

Main Protein synthesis and protein secretion (for example in Synthesis and storage of fats
function gland cells such as pancreatic cells) and hormones

(iii) Chloroplasts and chromoplasts — pigments

CHLOROPLAST CHROMOPLAST

Pigment Chlorophyll — a green pigment Pigments other than chlorophyll — yellow, orange or
red

Function of the Absorbs sunlight for Gives bright colour, which attracts pollinators and
pigment photosynthesis fruit-eating animals

Found in Leaves and other green parts Flower petals and fruits

Page 50 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Why it happens: in each pair the two structures are close relatives, and one small
difference decides the job. Membrane and wall are both boundaries, but only a
selective boundary can control the cell's internal composition — a freely permeable
wall could never do that, so the two are needed together. RER and SER are one
continuous network; attaching ribosomes to part of it turns that part into a protein
factory. Chloroplast and chromoplast are both plastids; swapping the pigment turns
a food factory into a signal.

Q2 Two similar animal cells are placed in two different solutions: Cell X is placed in
pure water. Cell Y is placed in a concentrated salt solution. Cells are observed after
some time. Cell X swells, and Cell Y shrinks. Which statement provides the correct
explanation for the above observations? (i) Salt molecules moved into Cell Y,
causing it to shrink. (ii) Water moved into Cell X and more water moved out of Cell Y
than the salt solution entered in it. (iii) Water moved into Cell X and moved out of
Cell Y through the cell membrane. (iv) Solute movement caused osmosis in both
cells.

The correct explanation is (iii) Water moved into Cell X and moved out of Cell Y through the
cell membrane.

Cell X in pure water: outside is hypotonic → water moves in → cell swells
Cell Y in concentrated salt: outside is hypertonic → water moves out → cell shrinks

In both cases the only substance crossing the membrane is water

Why the other options fail:

(i) is wrong because salt molecules do not cross the cell membrane — it is selectively
permeable. And even if salt entered, it would make the inside more concentrated and draw
water in, so the cell would swell, not shrink.
(ii) is wrong for the same reason: it assumes salt solution entered the cell. Nothing entered
Cell Y.
(iv) is wrong because osmosis is by definition the movement of water, not of solute. Solute
movement is diffusion, and here it does not happen at all.

Page 51 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Why it happens: the whole of osmosis rests on one asymmetry — the membrane is
open to the solvent and closed to the solute. Because the salt cannot move to even
out the concentrations, water has to move instead. Both cells in this question obey
exactly the same rule; only the direction of the concentration difference is reversed.
Note also what an animal cell risks: with no cell wall, Cell X in pure water can go on
swelling until it bursts.

Q3 Look at the diagram of a cell in Fig. 2.20. Identify the parts labelled from (a) to (g)
and correctly match them with their functions given below: (i) Controlling all the
activities of a cell. (ii) Site of cellular respiration. (iii) Storage organelle that also
provides rigidity to the cell. (iv) Separates the cell contents from surroundings. (v)
Provides structural rigidity to the cell. (vi) Packs and stores materials received from
ER. (vii) Helps in manufacturing food.

(d)
(a)
(e)
(b)

(f)

(c)

(g)

Fig. 2.20 — a cell with seven parts marked (a) to (g).

Fig. 2.20 is a plant cell — you can tell from the thick outer wall, the chloroplasts and the large
vacuole.

Page 52 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

LABEL PART FUNCTION

(a) Mitochondrion (ii) Site of cellular respiration

(b) Nucleus (i) Controlling all the activities of a cell

(c) Golgi apparatus (vi) Packs and stores materials received from ER

(d) Chloroplast (vii) Helps in manufacturing food

(e) Cell wall (v) Provides structural rigidity to the cell

(f) Cell membrane (iv) Separates the cell contents from surroundings

(g) Vacuole (iii) Storage organelle that also provides rigidity to the cell

(d) chloroplast

(a) mitochondrion
(e) cell wall
(b) nucleus

(f) cell membrane
(c) Golgi apparatus

(g) vacuole

The seven labelled parts of Fig. 2.20 and what each of them does.

Why it happens: two clues separate the trickiest pair, (e) and (f). Both arrows point
at the boundary, but (e) meets the thick outer orange layer and (f) meets the thin
line just inside it. Match that to the wording of the functions: “provides structural
rigidity” can only be the rigid cellulose wall, while “separates the cell contents from
surroundings” describes a boundary that controls exchange, which is the
membrane. Function (iii) is also worth noting — the vacuole is a storage organelle
and a source of rigidity, because the water it stores keeps up the turgor pressure
that makes the cell firm.

Page 53 of 79

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Class 9 Science Chapter 2 Cell: The Building Block of Life
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co m
m.
Which of the following option(s) of the pairs of cell organelles are correctly placed
se
Q4

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under the given categories? (i) Leucoplast / Cell wall (ii) Mitochondria / Ribosome
g
m .c wall / Golgi apparatus (iv) Lysosome / Endoplasmic reticulum
(iii) Cell a — the first

l a se being ‘Present in the plant cells’ and the second ‘Absent in the animal cells’.
column
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OPTION PRESENT IN THE PLANT ABSENT IN THE VERDICT

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CELLS? ANIMAL CELLS?

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a Ribosome Yes No — ribosomes are Wrong
present in animal cells

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animal cells
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agl exclusive to plants. Only three things in this chapter are: the cell wall, the plastids
m
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a se
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else — mitochondria, ribosomes, ER, Golgi apparatus, nucleus, cell membrane — is
. a g
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any eukaryotic cell. Once youashold that short list in mind, each option can be
checked in a second. a

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m ase
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Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Q5 Two students, Renu and Rohit, were having a discussion on the plastids. Renu
emphasised that all parts of the plants, even roots, contain plastids. However, Rohit
did not agree with the statement and told her that plastids are absent in plant
roots since the roots are underground and do not need to perform photosynthesis.
Who is correct? Justify your answer.

Renu is correct. Roots do contain plastids. Rohit has confused plastids with one particular kind
of plastid, the chloroplast.

Plastids are a family of organelles with three members: chloroplasts (green, for
photosynthesis), chromoplasts (yellow, orange or red) and leucoplasts (colourless, for
storage).
Roots lack chloroplasts because there is no light underground and no photosynthesis to do
— that part of Rohit's reasoning is right.
But roots are full of leucoplasts, which store starch, oils and proteins. Storing food is one of
the main jobs of a root.
Some roots even have chromoplasts. A carrot is orange because of the pigment in the
chromoplasts of its root; beetroot and sweet potato are coloured in the same underground
way.

plastids = chloroplasts + chromoplasts + leucoplasts

root: chloroplasts absent, leucoplasts present, chromoplasts sometimes present

→ therefore plastids are present in roots

Why it happens: plastids are interconvertible — one type can turn into another
when the cell's job changes. That is why a potato left in sunlight turns green
(leucoplasts developing chlorophyll and becoming chloroplasts) and why a green
tomato turns red as it ripens (chloroplasts becoming chromoplasts). Because the
whole family shares one origin, a plant cell almost always has plastids of some kind;
what varies is which type. Rohit's error is a useful one to remember: “no
photosynthesis” proves only that chloroplasts are absent, not that plastids are.

Check it yourself: put a drop of iodine solution on a thin slice of raw potato or arbi
(Colocasia). It turns blue-black, showing starch grains — starch stored inside
leucoplasts.

Page 55 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Q6 Mitochondria and chloroplasts are two important organelles in a plant cell. Discuss
how these two organelles are structurally and functionally similar to each other,
and different from each other.

Similarities

Both are double-membrane-bound organelles.
Both have their own DNA and their own ribosomes, so both can make some of their own
proteins.
Both share features with certain bacteria, which suggests both share an evolutionary
history with single-celled organisms.
Both are energy organelles: one captures energy, the other releases it. Both increase their
internal membrane surface — cristae in one, disc-shaped membranes in the other.
Both are found in plant cells.

Differences

MITOCHONDRION CHLOROPLAST

Pigment None Chlorophyll, a green pigment

Inner Inner membrane folded into finger-like Semi-fluid stroma containing disc-shaped
structure cristae; an intermembrane space membrane structures that hold chlorophyll

Process Cellular respiration — glucose and Photosynthesis — light energy is
carried out other molecules are broken down absorbed and sugar is made

Energy Releases stored chemical energy as ATP, Converts light energy into chemical energy
the energy currency in sugars

Occurrence In all eukaryotic cells — plant and animal Only in plants and some other
photosynthetic organisms; only in the
green parts

Storage Does not store food Sugars made are stored in the stroma
along with starch granules

Chloroplast: light energy + CO₂ + H₂O → sugar + O₂ (energy stored)

Mitochondrion: sugar + O₂ → ATP + CO₂ + H₂O (energy released)

Page 56 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Why it happens: the two organelles are the opposite halves of one cycle, and their
structures reflect that. Each folds its inner membrane because the reactions of both
photosynthesis and respiration happen on membranes, so extra membrane area
means a higher rate. The shared features — a double membrane, own DNA, own
ribosomes — are the reason biologists think both were once free-living bacteria that
came to live inside a larger cell. It also explains why a plant cell needs both: the
chloroplast can only bank the energy of sunlight in sugar, and only the
mitochondrion can withdraw it as ATP, which is the form every other part of the cell
can actually spend.

Q7 Which of the following pairs of cell organelles contains DNA? (i) Chloroplasts,
Ribosomes (ii) Mitochondria, Nucleus (iii) Golgi bodies, Ribosomes (iv) Nucleus,
Lysosomes

The correct pair is (ii) Mitochondria, Nucleus.

ORGANELLE CONTAINS WHAT IT CONTAINS INSTEAD / IN ADDITION
DNA?

Nucleus Yes Chromosomes made of DNA and specific proteins; the functional
segments of DNA are genes

Mitochondrion Yes Its own DNA and its own ribosomes — it makes some of its own
proteins

Chloroplast Yes Also has its own DNA and ribosomes

Ribosome No It is the site where protein is synthesised, not a store of DNA

Golgi apparatus No Modifies, sorts and packs proteins and lipids into vesicles

Lysosome No A single membrane-bound sac full of digestive enzymes

Options (i), (iii) and (iv) each pair one DNA-containing organelle with one that has none, so all
three are wrong. Note that chloroplasts do contain DNA — option (i) fails only because
ribosomes do not.

Page 57 of 79

Class 9 Science Chapter 2 Cell: The Building Block of Life AglaSem · NCERT Solutions

Why it happens: DNA is needed wherever proteins have to be made locally. The
nucleus holds the cell's master copy. Mitochondria and chloroplasts keep a small
private set because they also keep their own ribosomes and build a few of their own
proteins on the spot — the very feature that suggests they descend from ancient
free-living bacteria. Everything else in the cell receives its proteins ready-made from
the cytoplasm, so it has no use for DNA of its own.

Q8 A researcher carried out an experiment in which she took two carrots of similar
size. She placed one carrot in plain water and the other carrot in concentrated salt
solution (Fig. 2.21). After 24 hours she recorded her observations. (i) What
hypothesis does she want to test through this experiment? (ii) What would you
suggest for the improvement of this experiment? (iii) Why does the carrot in plain
water stay stiff and crunchy, but the carrot in concentrated salt solution become
rubbery and limp?

A B

Fig. 2.21 — experimental set-up having carrot (a) in plain water, and (b) in salt solution.

(i) The hypothesis. That water moves into or out of plant cells by osmosis according to the
concentration of the surrounding solution — so a carrot in a dilute (hypotonic) medium will gain
water and remain firm, while a carrot in a concentrated (hypertonic) medium will lose water and
become limp.

Page 58 of 79

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Class 9 Science Chapter 2 Cell: The Building Block of Life
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c om
(ii) Improvements.
.
m a s
Match the two carrots properly: use pieces of the same mass and same length, em cut from
carrots of c
. o same age and variety. a gl
em
the

l a
Weighs and measure each piece before and after the 24 hours, so the result is a number

agand not an impression. Record the change: final mass − initial mass.
State the salt concentration exactly (for example 20 per cent) and use the same volume of

. c om
liquid in both beakers, with both carrots fully immersed.
ag
s e m and cover them, so evaporation does not
Keep both beakers at the same temperature
a
agl
concentrate one solution.
Repeat with three or four carrots in each beaker and take an average — one carrot is not
enough evidence.
co m
se m.
Take readings at intervals (say every 4 hours) instead of only at the end, so the progress of

o m l a
g water is not counted.
.c pieces dry the same way before weighing, so clinging surface
the change is seen.

m a
se
Blot both
l a
g(iii) Why they feel different.
a
m a s
.co
Plain water: solute concentration outside < inside (hypotonic)
m agl
l a se
g
→ water enters the carrot cells by osmosis → vacuoles fill → cells become turgid
a
→ each cell presses against its rigid cell wall → tissue is stiff and crunchy

com
m .
m as e
.co
Concentrated salt: solute concentration outside > inside (hypertonic)
a g l
se m
a
→ water leaves the cells by osmosis → vacuoles empty → cells become flaccid

ag l → no pressure against the walls → tissue is soft, bendy and limp

se m
com g l a
m . a
ase
Why it happens: crunchiness is a mechanical property produced by water pressure.

a gl
A carrot snaps because millions of full, turgid cells push outwards against their walls,
so the tissue behaves like a bundle of inflated tubes and breaks cleanly instead of

c o m
bending. Draw the water out and the same walls are still there, but they are no
.
longer under pressure, so the carrot bends like an empty tube.mThis is exactly the
o m l a se when salt is rubbed
.c ag
wilting of Question 4 on page 19, and it is exactly what happens

se m on cut vegetables and water oozes out.
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a c
m .
m a s e
e m . co agl
g l as
a

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m .
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