NCERT Solutions Class 8 Science Chapter 13 Our Home Earth a Unique Life Sustaining Planet

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 13: Our Home: Earth, a
Unique Life Sustaining Planet

NCERT Textbook — Curiosity

BOOK PAGES SECTIONS QUESTIONS MEDIUM

210 – 228 15 38 English

Solutions, notes, sample papers & more at 38 pages

Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

CLASS 8 · SCIENCE · CURIOSITY

NCERT Solutions — Chapter 13: Our Home: Earth, a
Unique Life Sustaining Planet
The closing chapter of Curiosity pulls the whole year together and asks one question: why does life exist here
and nowhere else we know? The answer is not a single lucky fact but a chain of them — Earth's distance from
the Sun, its size, its atmosphere and magnetic field, the constant exchange between air, water, rock and living
things, and reproduction, which keeps that living part going.

TEXTBOOK BOOK PAGES

Curiosity (Class 8) 210 – 228

SECTIONS QUESTIONS

15 38

MEDIUM

English

Probe and ponder — Page 210
Chapter opener

PROBE AND PONDER

Q1 What do you think Earth would look like if there were no life on it at all?

It would still be a blue planet, but a bare one — rock, sand and water, with no green anywhere.
The changes go far deeper than colour, because much of what we think of as 'the Earth' is
actually the work of living things:

No green land. Forests, grasslands and crops are all life. The continents would look like the
bare rock and dust of the Mars pictures in this chapter.
Almost no oxygen in the air. The oxygen we breathe is released by plants during
photosynthesis. With no plants and no ocean plankton, the atmosphere would hold hardly
any of it — and with little oxygen there would be no ozone layer, so ultraviolet rays would
reach the surface unchecked.
No soil, only broken rock. Soil is rich in nitrogen and potassium because rock breaks down
slowly and the remains of plants and animals are added to it. Without life it would be lifeless
mineral dust.
Still oceans, still weather. Water would remain liquid, clouds would still form and rain
would still fall, because that depends on Earth's distance from the Sun and its atmosphere,
not on life.

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Why it happens: Earth's conditions made life possible, but life then changed the
Earth in return — the oxygen, the ozone shield and the soil are all products of living
things. That two-way link is exactly what the chapter calls the balance between living
and non-living parts of the planet.

Q2 Life on Earth has survived for billions of years. What allows it to keep going despite
major changes and disasters?

Two different kinds of reason work together — the planet keeps its conditions inside a narrow
range, and life itself keeps changing enough to fit whatever range it finds.
1. The physical conditions stay steady.

Earth stays in the habitable zone, and its orbit is nearly circular, so sunlight and heat hardly
vary through the year.
Its gravity holds the atmosphere, whose mild greenhouse effect keeps water liquid and
whose ozone layer blocks ultraviolet rays.
Its magnetic field turns away cosmic rays and the solar wind.

2. Reproduction lets life adapt. Parents pass on instructions (genes), so each kind continues.
But the instructions are not copied perfectly and, in sexual reproduction, instructions from two
parents are mixed. So every generation contains individuals that are slightly different from one
another. When the surroundings change, some of those individuals happen to cope better and it
is their line that continues.

Did you know? The chapter's own examples are camels, which over time developed
humps that store fat and let them survive in deserts, and bacteria that have become
resistant to antibiotics. Same mechanism, very different timescales.

Why it happens: A disaster does not have to be survived by every individual — only
by enough of them. Variation means a population is never a single design, so a
change that kills most members can still leave a few that carry on.

Q3 Why don’t dogs lay eggs? Or hens give birth to live chicks?

Because a dog and a hen use two different strategies for feeding the developing embryo, and
each animal inherits only its own kind's set of instructions.

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

HEN (A BIRD) DOG (A MAMMAL)

Where the Outside the body, after the egg is laid, Inside the female's body, until birth
zygote develops during hatching

How the embryo From the food the mother packs into The mother's body supplies food and
is fed the egg — it must last until hatching oxygen continuously

What the body Large yolk-filled eggs and a shell; the An internal space in which the embryo can
must have behaviour to sit on them grow, and a way to nourish it there

A hen's body is built to make big, food-filled, shelled eggs; a dog's body is not, and vice versa.
Reproduction can only pass on the instructions the parents already carry — it cannot swap one
whole body plan for another.

Why it happens: Both are answers to the same problem: how do you supply
nutrition to a growing embryo? A bird solves it in advance, all at once, by loading the
egg. A mammal solves it as it goes, through the mother's body. Neither is 'better' —
they simply cost the parent in different ways.

Q4 If a spaceship carried soil and water to Mars, could plants start growing there?

Not out in the open. Soil and water are necessary, but they are only two items on the list — and
Mars fails most of the others.

A PLANT NEEDS ON MARS TODAY

Water in the liquid state The atmosphere is 100 times thinner than Earth's and the planet is far colder
— poured water would mostly freeze or escape as vapour

Carbon dioxide and air pressure Carbon dioxide is there, but the air is far too thin

Sunlight Available, though weaker than on Earth

A temperature that does not freeze Well below freezing on average
the roots

Protection from ultraviolet rays and No ozone layer and no protective global magnetic field like Earth's
high-energy particles

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Inside a sealed, heated and pressurised chamber, yes. If you also carry the air, the pressure,
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the warmth and a light source — that is, if you carry a small piece of Earth's atmosphere with
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have written the answer to both questions.
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In-text Questions — Page 211
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13.1 Why Is Earth a Unique Planet?

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All of it lives in an extremely thin layer at the surface — the crust, together with the water on it
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Radius of the Earth ≈ 6400 km
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So the whole zone of life is about one three-hundredth of the Earth's radius
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thicker than the apple's skin (Fig. 13.1).
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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Crust — all life is here
only about 5–70 km thick

Upper mantle

Lower mantle

Outer core — molten iron
its movement gives Earth
its magnetic field

Inner core

Radius of the Earth ≈ 6400 km (layers not drawn to scale)

The layers of the Earth. Everything alive — every forest, ocean, city and person — is confined to the
green ring at the very edge.

Why it happens: Life needs liquid water, air and sunlight all in the same place. Only
the surface has all three: go a few kilometres down and there is no light and no free
oxygen; go far up and the air becomes too thin. So life is squeezed into the narrow
band where the geosphere, hydrosphere and atmosphere meet.

Activity 13.1: Let us find out — Page 212
13.1 Why Is Earth a Unique Planet?

ACTIVITY

Q1 List some features of the Earth that we often take for granted, but are interesting
and important to us. Write them in Table 13.1. We have filled in a few for you.

Here is Table 13.1 completed. The two filled rows are the book's; the rest are features you can
add, each with the reason it is remarkable.

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

S.NO. INTERESTING FEATURE OF THE WHY IT IS REMARKABLE
EARTH

1. The air we breathe doesn't fly off and Gas particles move freely and gases have no fixed
disappear into space. volume (Chapter 7), yet Earth's gravity is strong
enough to hold them.

2. We can stand on the ground held by Gravity is strong enough to keep us down, weak
gravity (Chapter 5), but our heart can enough that a fist-sized muscle can push blood
pump blood up to our head. upwards against it.

3. Water is found as ice, as liquid and as Earth's temperature sits right across water's
vapour — all on the same planet at the freezing and boiling points, which is why rivers,
same time. clouds and glaciers can all exist together.

4. The Sun does not burn us, even though The ozone layer absorbs the harmful ultraviolet
its light travels straight to us. part, and the magnetic field turns away high-energy
particles.

5. Day follows night with almost the same Earth's spin is steady and its orbit nearly circular, so
length of daylight in a given season, year heat arriving does not swing wildly.
after year.

6. Rain falls as fresh water even though Evaporation leaves the salt behind, so the water
most of Earth's water is salty. cycle keeps refilling rivers, lakes and groundwater
with usable water.

7. Dead leaves and animals do not pile up Decomposers break them down and return nitrogen
forever. and potassium to the soil for the next generation of
plants.

Check it yourself: A good entry for this table is something you would never notice
until it stopped. Ask of each item — 'what exactly is holding this steady?' If you can
name the reason, the row is worth writing down.

Activity 13.2: Let us find out — Page 213

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

13.2 What Do the Planets of Our Solar System Look Like?

ACTIVITY

Q1 Collect information about the temperature and size of the planets in the solar
system, and check if they have an atmosphere. You may collect this information
from books in your school library, trusted websites, or discuss with your teachers.
Fill out the missing information in Table 13.2.

Table 13.2 completed. The values already printed in your book are shown in normal type; the
ones you have to fill in are in orange.

S.NO. PLANET AVERAGE RADIUS, COMPARED HAS AN
TEMPERATURE (°C) TO THE EARTH ATMOSPHERE?

1. Mercury 170 0.38 No

2. Venus 450 0.95 Yes

3. Earth 15 1 Yes

4. Mars –65 0.53 Yes (about 100 times
thinner than Earth's)

5. Jupiter –110 11 Yes

6. Saturn –140 9 Yes

7. Uranus –195 4 Yes

8. Neptune –200 4 Yes

What the completed table shows. Read it as three patterns, not eight rows:

Temperature falls as you move outward — 170, 15, –65, –110, –140, –195, –200 — because
the energy a planet gets comes from the Sun and spreads out over a larger and larger
sphere with distance.
There is one exception, Venus at 450 °C. It is not the closest planet, yet it is the hottest. Its
thick carbon dioxide atmosphere traps the heat.
Size sorts the planets into two families. Radii of 0.38 to 1 are the small rocky planets
(Mercury, Venus, Earth, Mars); radii of 4 to 11 are the large gas planets (Jupiter, Saturn,
Uranus, Neptune).

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Tip: Different sources give slightly different average temperatures, especially for
Mercury, whose day and night sides differ enormously. Write down the source you
used next to your value — that is how scientists report data.

In-text Questions — Page 213
13.2 What Do the Planets of Our Solar System Look Like?

Q1 Venus, the second planet from the Sun has the highest average temperature and is
the hottest planet. Why is this so?

Because of its atmosphere, not its distance. Venus's air is almost entirely carbon dioxide, and
carbon dioxide does not let heat escape.
Follow the energy in three steps:

1. Sunlight passes through the atmosphere and warms the surface of the planet.
2. The warmed surface gives off its own radiation, back towards space.
3. Carbon dioxide in the thick atmosphere absorbs that outgoing radiation instead of letting it
through. The heat stays in.

This is the greenhouse effect. On Venus it is so strong that the planet reaches about 450 °C —
hotter than Mercury, even though Mercury is closer to the Sun and receives more sunlight.

Sun
carbon dioxide layer

heat given off by the
surface is absorbed
and sent back down

warmed surface

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

The greenhouse effect. Sunlight gets in easily; the heat the surface radiates back does not get out
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easily.

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Why it happens: A planet's temperature is set by the balance between the energy it
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atmosphere decides how much can get out. Venus receives less than Mercury but
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loses far less, so it settles at a much higher temperature.
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Did you know? The same effect on Earth is mild, and we depend on it — without it
the Earth would lose its heat to space and become too cold for liquid water. The
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A step further — Page 215
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A STEP FURTHER

Did Mars ever support life?
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Mars lies at the edge of the Sun's habitable zone, so it is right at the boundary of where
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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Tip: Notice how the chapter ends this box — 'science doesn't always have final
answers'. Saying 'we do not know yet, and here is what would settle it' is a scientific
answer, not a failure to answer.

In-text Questions — Page 215
13.3.2 Size of the Earth

Q1 Is the temperature or distance from the Sun, the only factor that makes the Earth
habitable?

No. Distance from the Sun is the most important reason, because it is what allows water to
stay liquid — but at least four other conditions must hold at the same time.

FACTOR WHAT IT DOES

Distance from the Places Earth in the habitable zone, so water is neither all evaporated nor all frozen
Sun

Nearly circular Keeps sunlight and heat almost steady through the year, preventing extreme
orbit summers and winters at most places

Size of the Earth Gravity strong enough to hold an atmosphere, not so strong that our bones would
be crushed

The atmosphere Oxygen for respiration, an ozone layer that blocks ultraviolet rays, and a mild
greenhouse effect that keeps the planet warm

Magnetic field Deflects cosmic rays and the solar wind, protecting the atmosphere and the ozone
layer

Why it happens: These conditions are not a list of separate advantages — they
depend on one another. The right size gives the atmosphere; the atmosphere gives
the greenhouse effect and the ozone layer; the magnetic field is what keeps the
atmosphere from being stripped away. Remove any one and the others start to fail
too.

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Q2 What would happen if the size of the Earth were too small or too big?

Either way we would lose the balance we depend on — a small Earth would lose its air, a large
Earth would crush what lives on it.
If the Earth were much smaller (with the same average density):

Its gravity would be too weak to hold on to the gases of the atmosphere, and they would
escape into space.
With no atmosphere there would be no greenhouse effect to keep the surface warm, no
oxygen to breathe and no ozone layer to block ultraviolet rays.
With no air pressure, liquid water at the surface could not last.

This is not a thought experiment — it is what we actually see. Mars, smaller than Earth, has an
atmosphere 100 times thinner; Mercury, smaller still, has none at all.
If the Earth were much larger, gravity would be much stronger. It would pull us downward
with so large a force that our bones could get crushed under our own weight, and moving,
standing and pumping blood upward would all become far harder.

Why it happens: Gas particles move freely and are always trying to escape. Whether
a planet keeps its atmosphere is a tug-of-war between the speed of those particles
and the pull of the planet's gravity — and gravity depends on how much mass the
planet has. Earth's size happens to win that tug-of-war without winning it so heavily
that life is flattened.

In-text Questions — Page 217
13.3.3 Magnetic field of the Earth

Q1 Does the magnetic field of the Earth have any role in sustaining life on Earth?

Yes — it acts as a protective shield around the whole planet.
The chain of reasoning runs like this:

1. Molten iron moving in the Earth's core is believed to be the origin of Earth's magnetic field,
which is why the Earth behaves like a giant magnet and a freely suspended magnet settles in
a fixed direction.
2. Earth is constantly hit by tiny, high-energy particles from space — cosmic rays from far
across the universe and the solar wind from the Sun.

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

3. These particles can damage the atmosphere and reduce the ozone layer, which would let in
more harmful ultraviolet rays.
4. The magnetic field pushes many of these particles away from the Earth, so the atmosphere
— and therefore life — stays safe.

magnetic field

solar wind and

Earth atmosphere and
ozone layer stay
protected

cosmic rays

High-energy particles arriving from the Sun and from deep space are turned aside by Earth's
magnetic field before they can reach the atmosphere.

Why it happens: The particles in the solar wind and in cosmic rays carry electric
charge, and a magnetic field changes the path of a moving charged particle. So the
field does not have to stop them — it only has to bend them around the planet, and
that is enough.

Q2 But, how is life supported and sustained on Earth?

Not by any single thing. Life is sustained by the constant exchange between four systems — the
atmosphere, the hydrosphere, the geosphere and the biosphere.

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

ATMOSPHERE
air

BIOSPHERE HYDROSPHERE
all living things water

GEOSPHERE
rock and soil

The four Earth systems, each feeding the others. A change forced on any one of them travels round
the circle.

Air, water and sunlight. The atmosphere holds the oxygen that humans, animals and plants
use for respiration. In sunlight, plants take carbon dioxide from the air and water from the soil
and prepare food by photosynthesis, releasing the oxygen that respiration needs. The
greenhouse effect keeps the temperature just high enough for water to stay liquid; water
vapour forms clouds, brings rain and snow, and refills rivers, lakes and groundwater.
Soil, rocks and minerals. The geosphere provides soil rich in nitrogen and potassium —
nutrients that come from the slow breakdown of rocks and the remains of plants and animals —
along with salt, coal, oil and metals such as iron and copper.
Living things. In the biosphere, plants make food, animals eat plants or other animals, and
decomposers break down dead matter and return the nutrients to the soil.

Why it happens: Every one of these is a loop, not a one-way supply. Oxygen
released by plants is used by animals, whose carbon dioxide goes back to plants.
Water that leaves the ocean returns as rain. Nutrients taken from the soil are
returned by decomposers. Because the materials cycle rather than run out, life can
continue indefinitely on a planet of fixed size.

In-text Questions — Page 219

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

13.4.4 The importance of balance
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Have you ever wondered how so many things on Earth stay in balance?
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They stay in balance because they are not independent. Land, air, water and living things

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Take the chapter's own example and follow it through:
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→ fewer trees releasing water vapour and absorbing carbon dioxide

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→ the whole local system shifts, not just the trees

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makes the forest itself more secure.

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Tip: This is also why 'protecting clean air, water, soil, and all forms of life' is described
in the chapter as essential rather than merely important. You cannot look after one
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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

13.5 What Keeps Life from Disappearing?

Q1 But how can the same process lead both to similarity (an animal gives birth to
similar individual, such as a cow gives birth to a cow) and variation (shows different
characteristics like difference in colour and height of individuals of some kind)?

Because reproduction passes on a complete set of instructions — but not always the same
complete set.
Where the similarity comes from. Genes are the instruction manual for building a body, and
every offspring receives a full set for its own kind. That is why a calf grows into a cow and a
kitten into a cat: the instructions for making blood, bone, muscle and skin in a cow are all there,
and they are cow instructions.
Where the variation comes from. Two things:

In sexual reproduction each gamete carries only half the parent's instructions, and it is a
different half each time. When a sperm and an egg join, that particular mixture has never
occurred before — which is why one child may inherit a nose like the mother's and another
eyes like the father's, and why brothers and sisters look different.
Small changes can also appear in the instructions as they are passed down.

Why it happens: Similarity and variation are not two processes fighting each other;
they are the same copying process seen at two levels. At the level of the kind, the
manual is passed on whole — so a cow gives birth to a cow. At the level of the
individual, the manual is reshuffled — so no two cows are quite alike. Life needs
both: without similarity a kind would not persist, without variation it could never
adapt.

Did you know? This also explains the two types of reproduction the chapter goes on
to describe. In asexual reproduction there is only one parent, so there is no
reshuffling and the young are near-exact copies. In sexual reproduction there are
two, so variation is built in.

In-text Questions — Page 221

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

13.5.1 Asexual reproduction

Q1 How do bamboo and sugarcane grow into new plants? I have never seen their
seeds.

They multiply by vegetative propagation — a piece of the parent plant itself grows into a new
plant, so no seed is needed.

Sugarcane is grown from short pieces of stem called setts. Each piece carries a node with a
bud (an 'eye'). Laid in moist soil, the bud sends up a shoot and the node sends down roots.
Bamboo spreads mainly through underground stems (rhizomes) that creep sideways and
send up new culms; a cut piece of culm with a node can also root.

Why it happens: A node of a stem is not just packaging — it carries a bud of living,
dividing tissue plus stored food. Given moisture, air and warmth, that tissue can
build roots downward and a shoot upward, which is everything a plant needs to
become independent. This is asexual reproduction: a single parent, and offspring
that are exact copies in terms of the instructions inside the cells.

Did you know? Because every cane in a field comes from the same parent, they are
all identical — which is why a whole sugarcane crop can ripen together, and also why
a single disease can sweep through the entire field.

Q2 Can you observe and list some plants around you that grow this way?

Many of the plants in a kitchen, a garden or a field are grown from parts, not seeds. Look for the
part that is planted — that tells you which type of vegetative propagation it is.

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

PLANT PART THAT IS PLANTED WHERE YOU CAN SEE
IT

Potato Tuber with 'eyes' (buds) A sprouting potato in the
kitchen

Ginger, turmeric Rhizome (underground stem) Kitchen, kitchen garden

Onion, garlic Bulb Kitchen, market

Sugarcane Stem cutting with a node Sugarcane fields

Money plant, rose, hibiscus, Stem cutting School garden, balcony
bougainvillea pots

Banana Sucker growing from the base Banana plantation,
backyard

Bryophyllum Leaf — buds grow at the notches of the margin Garden hedges, pots

Sweet potato, dahlia Root Fields, flower beds

Grass, mint Runner — a stem creeping along the ground Lawn, kitchen garden
that roots at nodes

Check it yourself: Walk round your school garden with this table. For every plant,
ask 'if I cut a piece and planted it, would it grow?' Then try it with one — a money
plant cutting in a glass of water shows roots within about a week.

Q3 You might wonder if both parents pass on their genetic material for making a new
organism, won’t the child end up with double the amount of instructions? And
would not this keep doubling every generation?

No — and the reason is that each parent does not pass on its whole set. Each makes specialised
reproductive cells called gametes, and a gamete carries only half the parent's genetic material.

male gamete (half a set) + female gamete (half a set)

= new cell with one complete set

half from each parent — not double

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Father
full set sperm cell
half a set
Zygote
one full set
egg cell
Mother half a set
full set

Halving at each parent, then joining: the amount of genetic material stays the same generation after
generation.

Why it happens: If gametes carried a full set, the amount would indeed double
every generation — 1, 2, 4, 8 — which is impossible. Halving before joining is what
keeps the total constant. And because it is a different half each time, the same step
that keeps the quantity steady is also the step that produces variation.

Activity 13.3: Let us find out — Page 221

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

13.5.1 Asexual reproduction
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ACTIVITY

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a s emsome parts of plants like stem cutting of a money plant, the ‘eyes’ of a
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sprouted potato, or a piece of ginger (Fig. 13.11b). Plant each of them separately in
moist soil (not too deep). For money plant, you can just put a cutting in a glass

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

PLANT PART ROOTS APPEAR SHOOT APPEARS FIRST NEW LEAF

Money plant, stem cutting in water about 5–10 days already present about 2–3 weeks

Potato, sprouted 'eyes' about 7–14 days about 7–10 days about 2–3 weeks

Ginger, piece of rhizome about 10–20 days about 2–3 weeks about 3–4 weeks

Why it happens: None of these pieces is a seed. Each already carries a bud — a
small mass of dividing cells — and a store of food. Given water, air and warmth, the
dividing cells build roots first, because the piece must secure its own water supply
before it can afford leaves. Only then does the shoot expand and the first leaf open,
after which the plant can start making its own food by photosynthesis.

Check it yourself: Keep one money plant cutting with a node and one without. Only
the one with a node will root. That single comparison shows that it is the node, not
just any piece of stem, that carries the growing tissue.

In-text Questions — Page 223
Sexual reproduction in animals

Q1 What are the advantages and disadvantages of giving birth to young ones vs. laying
eggs?

Both are solutions to one problem — how to feed and protect a developing embryo. Each buys
safety at a different price.

GIVING BIRTH TO YOUNG ONES LAYING EGGS

Advantages The embryo develops inside the mother, so The mother is not carrying the young,
it is protected from weather, predators and so she can move, feed and escape
drying out. Food and oxygen are supplied normally. Many eggs can be produced
continuously, so the young one can be born at one time, and in water thousands
at a well developed stage. can be released at once.

Disadvantages Only a few young at a time. The mother All the food for the whole of
carries the weight and the cost, and the development has to be packed into the
long development ties her to it. If the egg in advance. Once laid, the egg is
mother dies, the young die with her. exposed — many eggs are eaten or
destroyed, so most never hatch.

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Why it happens: Notice the trade-off: internal development spends a great deal of
care on very few offspring, while egg laying spends very little on very many. Both
work, and which one succeeds depends on the surroundings. In water, where eggs
and gametes can simply be released together, laying thousands is efficient; on land,
where an embryo would dry out, keeping it inside or sealing it in a shell is the safer
route.

Q2 Do you think animals like dogs, cows, or humans could lay eggs like birds? Why or
why not?

No, they could not. Laying eggs is not a choice an animal makes; it is the way its whole body is
built, and that is fixed by the instructions it inherits.
Three reasons, in order of how deep they go:

The body has no machinery for it. A hen's body forms a large egg loaded with enough food
for the entire development of the embryo, and then wraps it in a shell. A dog, a cow or a
human makes a tiny egg cell with almost no stored food, because the food is meant to come
later, from the mother's body.
The nutrition strategy is the opposite one. In mammals the zygote develops inside the
female and the mother's body provides all the food and oxygen the baby needs until it is
born. An egg laid outside would have no such supply.
The instructions cannot be swapped. A calf grows into a cow because it inherits the
complete set of cow instructions. Reproduction can pass on small changes, and over many
generations these can add up — but no single generation can be handed a different body
plan.

Why it happens: Reproduction copies and reshuffles the instructions a parent
already has. It does not invent new ones to order. A whole new way of reproducing
would require a very large number of coordinated changes, and those appear, if at
all, only over enormous stretches of time.

Keep the curiosity alive — Pages 226–227

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

End-of-chapter questions

KEEP THE CURIOSITY ALIVE

Q1 What is one major reason Mars cannot currently support life like Earth? (i) It has too
many volcanoes. (ii) It is too close to the Sun. (iii) It lacks a thick atmosphere and
liquid water. (iv) Its magnetic field is too strong.

(iii) It lacks a thick atmosphere and liquid water.

Why it happens: Mars's atmosphere is about 100 times thinner than Earth's. With so
little air there is neither enough pressure nor enough greenhouse warming to keep
water liquid at the surface, and no ozone layer to block ultraviolet rays. Liquid water
is, as far as we know, essential for life to evolve — so its absence is the decisive
difference.

Why the others are wrong:

(i) Volcanoes are not what makes a planet uninhabitable — Earth has plenty of them.
(ii) Mars is farther from the Sun than Earth, not closer; it lies at the outer edge of the
habitable zone.
(iv) A strong magnetic field is a protection, not a problem — and Mars's problem is that it
lacks the protection Earth's field provides.

Q2 Which of these is an example of geodiversity? (i) Variety of bird chirping in a forest.
(ii) Different landforms like mountains, valleys, and deserts. (iii) Changing weather
during monsoons. (iv) Number of different types of fish in a pond.

(ii) Different landforms like mountains, valleys, and deserts.

Why it happens: Geodiversity is the variety of the non-living parts of the Earth —
landforms, rocks and soils — together with the processes that shape and alter them.
Mountains, valleys and deserts are exactly that.

Why the others are wrong: (i) and (iv) describe the variety of living things, which is
biodiversity. (iii) describes changing weather, which belongs to the atmosphere, not to the solid
Earth.

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Tip: Geodiversity matters because it creates unique habitats. A desert, a river valley
and a mountain slope support completely different communities — so variety in the
rock beneath produces variety in the life above.

Q3 If the Earth were smaller with the same density, what might happen to its
atmosphere? (i) It would become thicker and hotter. (ii) It would escape into space
due to weaker gravity. (iii) It would become frozen. (iv) It would cause stronger
winds.

(ii) It would escape into space due to weaker gravity.

Why it happens: A smaller planet of the same average density has less mass, so it
pulls with a weaker gravitational force. Gas particles move freely and are always
tending to escape; holding an atmosphere is a tug-of-war between that motion and
the planet's gravity. Weaken the gravity and the gases win — they drift away into
space.

Check it yourself: The evidence is in the solar system. Mars is smaller than Earth
and its atmosphere is 100 times thinner; Mercury is smaller still and has no
atmosphere at all.

Q4 In sexual reproduction, why are offspring different from their parents? (i) They grow
in different climates. (ii) They eat different food. (iii) They acquire new instructions
after birth. (iv) They get mixed instructions (genes) from both parents.

(iv) They get mixed instructions (genes) from both parents.

Why it happens: Each gamete carries only half of a parent's genetic material, and it
is a different half every time. When a sperm and an egg join, the offspring receives a
complete set — but a mixture that has never existed before. That is why a child may
inherit a nose like the mother's and eyes like the father's, and why brothers and
sisters differ from each other.

Page 23 of 38

as e
a g l
Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Why the others are wrong: Climate and food can affect how an individual grows, but they are
co m
not what is inherited. And (iii) is simply not how genes work — the instructions are received at
em.
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the moment the gametes join, not acquired afterwards.

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Q5 You notice tiny green plants growing in cracks on your school wall after the

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plants grow there?
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The seeds were almost certainly carried there — most likely by birds, and some by the
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How they arrived. When birds or animals eat fruits, the seeds are dropped far from the parent

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plant. The chapter's own example is a banyan seed, dropped by a bird that ate the fruit and
l
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a can also be blown into a crack by the wind, and spores may be washed down by rain.
the seed, sprouting in a crack in a wall after the rains. Light seeds of grasses and ferns

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Why they could grow there. A crack after the monsoon happens to supply everything a
germinating seed needs:
a s em
a l crack and the shade keeps it from drying out quickly.
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Air — the crack is open, so oxygen reaches the seed.

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A little soil — dust, bits of leaf and droppings collect in the crack and supply nutrients.

s e m away.
A stable anchorage — the narrow gap grips the seed so it is not washed

. com — available as soon as the first leaves open.
Sunlight
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that store to push out roots and a shoot — it does not need soil to start, only water,
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air and warmth. That is why germination in a bare crack is possible at all; the plant
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then survives only if the crack can keep supplying the little water and nutrients it
needs.
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A city has recently cut down a large patch of forest to build
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Q6

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Removing a forest does not remove one thing — it breaks several links at once, and the effects
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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Trees release large amounts of water vapour into the air. With the trees gone there is less
vapour, so fewer clouds form and local rainfall tends to fall.
Shade is replaced by roads and buildings, which absorb and hold heat, so the area becomes
hotter.
The forest was absorbing carbon dioxide as it grew. That absorption stops, adding to the
greenhouse gases in the air.

Effect on biodiversity. When natural habitats are destroyed, plants and animals may disappear,
upsetting the ecosystem. Following Chapter 12: if the plants that herbivores eat vanish, deer
and insects struggle to survive; without herbivores, predators lose their food too. Every type of
living thing has a role, and losing even a few weakens nature's ability to support life.
Effect on water availability and quality.

Availability: roots and leaf litter used to slow the rain so it could soak in and refill
groundwater. On bare ground and concrete the same rain runs straight off, so wells and
springs get less. Less rainfall makes this worse.
Quality: with no roots holding it, soil is washed into streams — the water becomes muddy.
Run-off from roads and construction carries oil, dust and waste with it, and there is no
longer any vegetation to filter it.

Why it happens: A forest is not only a set of trees; it is the part of the local system
that stores water, holds soil, cools the air and feeds the food chain. Take it away and
each of those services stops together — which is exactly what the chapter means by
saying that a small change in one part of the Earth system affects rainfall, soil, air
quality and animals at the same time.

Q7 A friend says, “The Earth has always had climate changes in the past, so today’s
global warming is nothing new.” How would you respond using what you’ve learnt
in this and other chapters of your science book?

Your friend is right about the first half and wrong about the conclusion. The Earth's climate
has changed before — but what is happening now differs in its cause and in its speed.
1. The cause is different. The chapter is specific about where today's extra heat comes from.
Burning fossil fuels like coal and oil releases greenhouse gases such as carbon dioxide and
methane, which trap more of the heat the Earth gives off. Normally trees, plants and even tiny
ocean planktons absorb carbon dioxide as they grow and Earth keeps a balance. But burning
fossil fuels releases carbon that has been locked underground for millions of years, and the
Earth cannot absorb it fast enough — so the heat builds up.

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

2. The speed is different. Past changes generally unfolded over very long periods. That matters
because adaptation, as this chapter explains, works through reproduction over many
generations. A change that arrives faster than species can adapt to it leads to their
disappearance rather than their adjustment.
3. What is at stake now is a system we depend on. Even a small increase in temperature can
melt ice caps, raise sea levels which could flood many coastal cities, cause extreme weather, and
lead to many plants and animals disappearing. Together with biodiversity loss and pollution,
this forms what the chapter calls the triple planetary crisis.

Did you know? This is why countries have made global agreements. The Paris
Agreement (2015) set a goal of limiting global warming to below 1.5 °C; the book
records that as of 2025 the world is not on track to meet that goal, and that much
more action is needed.

Why it happens: 'It has happened before' tells you that the climate can change; it
does not tell you what is changing it this time. In science the useful question is not
whether an effect is new but what is causing it — and here the cause is identifiable,
measurable and, unlike a volcano or an orbit, something people can decide to
change.

Q8 Imagine Earth’s magnetic field suddenly disappeared. What kinds of problems could
arise for life on Earth? Explain.

The shield that keeps high-energy particles away from the planet would be gone, and the
damage would work its way inward, step by step.

1. Cosmic rays and the solar wind would reach the atmosphere directly. These tiny, high-
energy particles come from far across the universe and from the Sun. At present the
magnetic field pushes many of them away.
2. The atmosphere would be damaged and the ozone layer reduced. That is the specific
harm the chapter names.
3. More harmful ultraviolet rays would reach the surface. With a thinner ozone shield, UV
rays that damage living cells would get through — harming skin and eyes, damaging crops,
and affecting the plankton at the base of ocean food chains.
4. Over long periods the atmosphere itself could be stripped away, and with it the
greenhouse warming that keeps water liquid.

Everyday technology would suffer too — compasses would no longer point north, and the
particle storms from the Sun would disturb satellites and communications.

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Why it happens: The particles in cosmic rays and the solar wind carry electric
charge, and a magnetic field bends the path of a moving charged particle. Remove
the field and nothing bends them, so their full energy arrives at the top of the
atmosphere. The atmosphere then has to absorb that energy — and absorbing it is
precisely what breaks up molecules such as ozone.

Tip: Mars is the natural comparison. It has no protective global magnetic field like
Earth's, a very thin atmosphere and no ozone layer — a picture of what the end of
this chain looks like.

Q9 You are tasked with designing a new settlement for humans on Mars. Name three
things you would need to recreate from Earth to support human life there. Which of
these do you think is the hardest to replicate, and why?

The three I would recreate:

WHAT TO WHY IT IS NEEDED HOW IT MIGHT BE DONE
RECREATE

A breathable Oxygen for respiration; pressure to Sealed, pressurised domes; oxygen
atmosphere at Earth- keep water liquid and to keep our produced from carbon dioxide, and from
like pressure bodies working plants grown inside

Liquid water Drinking, growing food, and every Extract ice from beneath the Martian
process inside the body surface, melt it, and recycle every drop
used

Protection from Mars has no ozone layer and no Build under a thick layer of Martian soil
radiation and cold protective global magnetic field, or inside caves; heat the habitat from a
and is far below freezing nuclear or solar power source

The hardest to replicate: protection from radiation.

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Why it happens: Air and water can be manufactured, stored and recycled inside a
sealed dome — they are engineering problems of a size we already understand. But
Earth's protection comes from something planet-sized: a magnetic field generated
by molten iron in a core thousands of kilometres across, backed by an ozone layer
held in place by a whole atmosphere. You cannot carry that, and you cannot build it.
All you can do is hide behind mass — metres of soil or rock over every room — which
makes the settlement heavy, dark and enormously expensive to build.

Tip: A good way to rank the three is to ask, for each, 'is this a supply problem or a
planet problem?' Supply problems (air, water, food, heat) can be solved by carrying,
making and recycling. Planet problems (gravity, magnetic field, atmosphere as a
whole) cannot.

Q10 In a village, the temperature has been increasing and rainfall has become
unpredictable over the past few years. What could be causing this change?
Suggest two ways the village could adapt to these new conditions.

What could be causing it. These are exactly the signs the chapter describes as climate change
— long-term changes in temperature, rainfall and weather patterns. Two kinds of cause are
likely to be acting together:

Global: greenhouse gases released by burning fossil fuels trap more of the heat the Earth
gives off, and the resulting global warming changes weather patterns everywhere —
including the timing and amount of rainfall.
Local: if trees around the village have been cut for fields, fuel or building, less water vapour
enters the air and the bare ground heats up more. Both make the village hotter and its rain
less dependable.

Two ways the village could adapt.

1. Harvest and store the rain when it comes. Repair the village pond, build check dams
across the seasonal stream, and add rooftop rainwater harvesting with recharge pits. If the
rain is unpredictable, the answer is to hold on to it when it does fall instead of letting it run
off.
2. Change what is grown and how. Shift part of the land to crops that need less water and
tolerate heat — millets such as bajra and jowar, and pulses — and water them with drip
irrigation rather than flooding the field. Mulching the soil and planting shade trees along the
field boundaries reduce evaporation and cool the ground.

Page 28 of 38

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

co m
m.
Did you know? Adapting is not the same as solving. These steps help the village live

m l a se
with the change; reducing the cause needs the wider actions the chapter lists —
o
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cleaner and .c renewable energy such as solar and wind, better useagof energy, and
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environmentally friendly ways to travel.
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s
If there were no atmosphere on the
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water on the planet? Explain.

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Yes — and in each of the three ways the question names. The atmosphere is not just
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something we
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Life. e
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would strike the surface directly and damage living cells. Life as we know it could not exist.

m a s
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Temperature. Without an atmosphere there would be no greenhouse effect. The Earth would
radiate its heat straight out to space andmbecome too cold, and with nothing to spread the heat
l a se while the dark side froze.
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around, the sunlit side would be scorching
Water. This is the part that surprises people. With no atmosphere there is no air pressure
holding water down, so liquid water at the surface would boil away into space; whatever
com
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remained would freeze in the cold. And there would be no water cycle at all — no evaporation

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into air, no clouds, no rain or snow, so rivers, lakes and groundwater would never be refilled.

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The water cycle runs on

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Why it happens: The atmosphere does three separate jobs — it supplies gases, it
traps just enough heat, and it provides the pressure and the space in which water
can change state and travel. Because all three depend on the same layer of gas,
losing it does not cause three separate problems; it causes one collapse with three
faces.

Q12 Discuss five examples of vegetative propagation.

Vegetative propagation is asexual reproduction in plants: a leaf, stem or root of the parent
grows into a whole new plant. Five clear examples, each using a different part:

PLANT PART USED WHAT HAPPENS

1. Potato Tuber (underground The 'eyes' on the tuber are buds. A cut piece with an eye,
stem) planted in moist soil, sends out a shoot and roots.

2. Ginger Rhizome Each piece carries a bud; planted, it produces a new ginger
(underground stem) plant.

3. Money Stem cutting A cutting with a node, placed in water or moist soil, grows
plant roots from the node and then new leaves.

4. Sugarcane Stem cutting (sett) The bud at the node grows into a new cane — which is how the
with a node whole crop is planted.

5. Leaf Tiny buds grow at the notches along the leaf margin; a fallen
Bryophyllum leaf produces several small plants.

Other examples worth adding: onion and garlic from bulbs, sweet potato and dahlia from
roots, banana from suckers, and grass and mint from runners.

Why it happens: Each of these parts carries a bud — a group of cells that can still
divide — along with stored food. That is enough to build roots and a shoot without
any seed, fertilisation or pollination. Since there is a single parent and no mixing of
instructions, every new plant is an exact copy of the parent.

Tip: Vegetative propagation is used by farmers precisely because the plants are
copies. A mango variety grown this way keeps exactly the fruit quality of the parent
tree, which growing from seed would not guarantee.

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Discover, design, and debate — Page 227
Science • Society • Interdisciplinary Projects

DISCOVER, DESIGN, AND DEBATE PROJECTS

Q1 Design an ‘Earth Survival Kit’. Imagine you’re building a tiny model of Earth for
another planet. What must it have to support life, and why?

How to approach it. Do not start by listing objects. Start with the four Earth systems in Section
13.4 — atmosphere, hydrosphere, geosphere, biosphere — and ask what the kit must supply in
place of each. Then add the two protections Earth gives for free: warmth and shielding. A good
kit also explains how each item is replenished, because a supply that runs out is not survival.
Sample answer — the Earth Survival Kit:

ITEM IN THE KIT WHICH EARTH WHY IT IS NEEDED
SYSTEM IT
REPLACES

A sealed, pressurised dome Atmosphere Oxygen for respiration, carbon dioxide for
filled with a breathable gas photosynthesis, and pressure so that water
mixture can stay liquid

A water tank with a recycling Hydrosphere Drinking, growing food, transporting
and purification unit nutrients; every drop must be used again

Soil, or a nutrient bed with Geosphere Plants need nutrients and an anchorage for
nitrogen and potassium roots

Seeds and plants, plus Biosphere Plants make food and release oxygen;
decomposing microbes decomposers return nutrients to the soil so
the loop closes

A light and heat source (solar The Sun plus the Photosynthesis needs light; without warmth
panels, a heater) greenhouse effect the water freezes

Thick shielding — metres of Ozone layer and To block ultraviolet rays and high-energy
soil or rock over the dome magnetic field particles

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Why it happens: The hardest part of the design is not any single item but making
the loops close. On Earth nothing is consumed for good — oxygen, water and
nutrients all cycle. A kit that only carries supplies has a fixed lifetime; a kit in which
plants, animals and microbes feed one another can keep going. That is why the
chapter insists that life is sustained by connections, not by resources alone.

Q2 India is planning for a challenging lunar mission, Chandrayaan-4, which will bring
back samples of soil from the Moon. If the Moon had water, could plants grow in
that soil? Think of some experiment that could help you explore whether plant
growth is possible on the Moon.

Water alone would not be enough — but lunar soil plus water plus Earth-like air, warmth and
light might well work, and that is exactly what an experiment should test.
What lunar soil has and lacks. It is crushed rock and dust, so it contains minerals. What it does
not have is the part of Earth's soil that comes from life — the nitrogen and other nutrients
released by the breakdown of plant and animal remains. It also has no decomposers, and it has
been exposed to raw solar particles for billions of years.
A fair experiment you could design (using a small quantity of returned sample, or a simulated
lunar soil made from crushed volcanic rock):

1. Question: Can seeds germinate and grow in lunar soil given water, air and light?
2. Set-up: Four identical sealed transparent containers, each with the same mass of soil, the
same number of seeds of a fast-growing plant such as moong or mustard, and the same
water, temperature and light.

A — lunar (or simulated lunar) soil
B — ordinary garden soil, the control
C — lunar soil with a little compost added
D — lunar soil with no water added, to confirm water is the limiting factor

3. Keep everything else the same — only the soil differs. Change one thing at a time.
4. Measure: how many seeds germinate, on which day, the height after two weeks, the
number of leaves, and the colour of the leaves.
5. Repeat each container at least three times, because a single pot can fail for reasons that
have nothing to do with the soil.

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Why it happens: A seed germinates on its own stored food, so it may well sprout in
almost any damp medium — including lunar dust. The real test comes after that,
when the seedling must draw nutrients from the soil. Comparing A with C tells you
whether the missing ingredient is nutrients; comparing A with B tells you how far
behind Earth soil it is; D confirms that you have not simply been watching the seeds'
own food store.

Tip: Whatever the result, the experiment says nothing about growing plants on the
Moon in the open. There is no atmosphere there, so this test is really about a sealed
lunar greenhouse.

Q3 Flowers are often brightly coloured and have a pleasant smell. How do you think
these features help the plant reproduce?

They are advertisements — aimed at insects, birds and other animals that will carry pollen
from one flower to another.
A plant cannot move, but its male gametes must reach another flower for fertilisation to
happen. Pollen is carried by wind, insects or animals; this process is pollination. Bright colour
and scent solve the plant's problem of attracting a carrier:

Colour is visible from a distance and tells an insect where to come. It works in daylight.
Scent works where colour cannot — at night, and among leaves. Flowers that open at night,
like raat ki rani, are usually pale but strongly scented.
Nectar is the reward that makes the visit worthwhile, so the animal returns and visits other
flowers of the same kind.

As the insect pushes in for nectar, pollen from the anther sticks to its body, and it rubs off on the
next flower it enters. Male and female gametes then combine — that is fertilisation — forming a
zygote that becomes the seed, while the fleshy part of the flower around the ovule develops into
a fruit.

Why it happens: Colour and scent cost the plant energy to make, so they must be
earning something. They are only found in plants pollinated by animals. Wind-
pollinated plants — grasses, maize, many trees — have small, dull, scentless flowers,
because there is no point advertising to the wind. That contrast is the strongest
evidence that these features exist for pollination.

Page 33 of 38

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

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Check it yourself: Sit near a flowering plant for ten minutes and count the visitors.

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Why do animals like fish and frogs lay hundreds or even thousands of eggs at a
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Q4

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Because in fish and frogs almost nothing protects the eggs — so survival is a matter of
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uncared for, and eaten by other fish, insects and birds. Out of a thousand eggs, only a handful

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ADVANTAGES OF LAYING VERY MANY EGGS a DISADVANTAGES

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A population can recover quickly after a bad year Numbers swing wildly with conditions rather
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a g l Page 34 of 38

Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Why it happens: A parent has a limited amount of energy for reproduction and can
spend it in one of two ways: many cheap offspring with no protection, or few
expensive offspring with a great deal of protection. Water makes the first strategy
workable, because eggs do not dry out and gametes can meet by simply being
released. On land the first strategy fails, so land animals almost always take the
second.

Q5 Birds like sparrows build nests and care for their eggs and chicks, while reptiles like
snakes usually lay their eggs and leave them without protection. How might this
difference in parental care affect the chances of survival for the young ones in each
case?

Each individual sparrow chick has a far better chance of surviving than each individual
snake hatchling — but the snake makes up for it with numbers.

SPARROW — WITH PARENTAL SNAKE — WITHOUT PARENTAL
CARE CARE

Number of eggs Few Usually many more

While the egg The nest hides it, the parents warm it Buried or hidden and then abandoned;
develops and drive off predators many are eaten or destroyed

After hatching Parents bring food and shelter until The hatchling must find its own food
the chick can fly and feed itself and escape predators from day one

Chance that any one High Low
young survives

Cost to the parents Very high — weeks of feeding and Low — the work ends when the eggs are
guarding laid

Why it happens: Both strategies survive the same test: enough young reach
adulthood to keep the population going. The sparrow reaches that number by
protecting a few very well; the snake reaches it by producing many and protecting
none. Care is not simply 'better' — it is bought with the parents' own time and
safety, and a parent busy guarding a nest is a parent that is not feeding or escaping.
Which strategy wins depends on how dangerous the surroundings are and on how
much a parent can afford to give.

Page 35 of 38

Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Did you know? The same trade-off appeared in the previous question about fish
and frogs, and again in the comparison between laying eggs and giving birth to
young ones. It is one of the recurring patterns of biology — invest a little in many, or
a lot in few.

Chapter at a glance
All life on Earth exists in an astonishingly thin shell. From the highest peak to the deepest
ocean trench is only about 20 km against an Earth radius of about 6400 km — if Earth were
an apple, the crust would be the apple's skin.
Earth lies in the habitable zone (or 'Goldilocks zone'), the range of distances from a star
over which water stays liquid. Its nearly circular orbit keeps sunlight and heat steady
through the year, so most places escape extreme summers and winters.
Earth is also the right size. Its gravity is strong enough to hold on to the atmosphere —
Mars's atmosphere is 100 times thinner and Mercury has none — but not so strong that our
own weight would crush us.
The atmosphere works in three ways: oxygen for respiration, the ozone layer (a three-atom
form of oxygen) that blocks harmful ultraviolet rays, and a mild greenhouse effect in which
gases like carbon dioxide absorb the radiation Earth gives off, keeping it warm enough for
liquid water. On Venus a thick carbon dioxide atmosphere pushes the same effect to 450 °C.
Molten iron moving in Earth's core is believed to give Earth its magnetic field, which
deflects cosmic rays and the solar wind that would otherwise strip the atmosphere and
thin the ozone layer.
Life is sustained by the interaction of four systems — atmosphere (air), hydrosphere
(water), geosphere (rock, soil and minerals) and biosphere (all living things and where they
live). Damage to one reaches the others.
Reproduction keeps life from disappearing. Asexual reproduction (vegetative propagation,
budding in Hydra, regeneration in Planaria) gives exact copies; sexual reproduction
combines half the instructions from each parent through gametes, so offspring are similar
but never identical — and that variation lets life adapt.
The triple planetary crisis — climate change, biodiversity loss and pollution — is now the
biggest threat to that balance. The Paris Agreement (2015) set a goal of limiting global
warming to below 1.5 °C; as of 2025 the world is not on track to meet it.

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Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

Quick revision

TERM WHAT IT MEANS WHY IT MATTERS FOR LIFE WHERE IT
APPEARS IN
THE CHAPTER

Crust The thin, solid outer layer of the Everything alive lives on or just Fig. 13.1, Section
Earth above it — it is as thin, relatively, as 13.1
an apple's skin

Habitable zone The range of distances from a star Liquid water is, as far as we know, Fig. 13.4, Section
(Goldilocks over which water stays liquid essential for life to evolve 13.3.1
zone)

Greenhouse Gases such as carbon dioxide Mild on Earth, it keeps the planet Fig. 13.2, Section
effect absorb the radiation the Earth warm enough for liquid water; 13.2
gives off after the Sun warms it, runaway on Venus, it makes 450 °C
so heat does not escape

Ozone layer A part of the atmosphere Acts as a shield that blocks harmful Section 13.3.2
containing ozone, a three-atom ultraviolet rays which damage living
oxygen molecule cells

Cosmic rays and High-energy particles from deep They can damage the atmosphere Section 13.3.3
solar wind space and from the Sun and reduce the ozone layer; Earth's
magnetic field pushes most of
them away

Magnetic field The region around the Earth A protective shield for the Section 13.3.3
of the Earth where its magnetism is felt, atmosphere, and so for life
believed to arise from molten iron
moving in the core

Hydrosphere All the water of the Earth — Water is a good solvent, carries Section 13.4.1
ponds, lakes, rivers, springs, seas, nutrients in plants, regulates body
oceans and groundwater; about temperature in animals, and is
70 per cent of the surface home to millions of species

Geosphere The solid part of the Earth — Supplies nutrients such as nitrogen Section 13.4.2
rocks, soils and minerals and potassium, and materials like
salt, coal, oil, iron and copper

Geodiversity The variety of landforms, rocks Creates the different habitats in Fig. 13.9, Section
and soils, together with the which different kinds of life can 13.4.2
processes that shape them thrive

Biosphere All living beings together with This is where living things interact Section 13.4.3
the places where they live — with their surroundings to survive
land, water and air and grow

Page 37 of 38

Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet AglaSem · NCERT Solutions

TERM WHAT IT MEANS WHY IT MATTERS FOR LIFE WHERE IT
APPEARS IN
THE CHAPTER

Genetic material Instructions stored inside every They make a calf grow into a cow; Section 13.5
(genes) cell that tell it how to develop small changes in them, passed on,
allow adaptation

Gametes Specialised reproductive cells Half from each parent gives a Section 13.5.2
carrying only half the parent's complete set, so instructions do not
genetic material double every generation

Vegetative Asexual reproduction in which a How bamboo, sugarcane, potato, Fig. 13.11,
propagation leaf, stem or root planted in soil ginger and money plant multiply Section 13.5.1
grows into a new plant without seeds

Triple planetary Climate change, biodiversity loss The three biggest environmental Section 13.6
crisis and pollution taken together challenges now threatening life on
Earth

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