Our Home: Earth, a Unique Life Sustaining Planet | CBSE Class 8 Science Notes

This note covers Earth’s position and size, liquid water, the atmosphere, the greenhouse effect, protection from harmful radiation and particles, connected Earth systems, reproduction in plants and animals, environmental threats, and ways to protect life.

Why is Earth a unique home for life?

Life on a thin outer layer

Earth supports life across mountains, oceans, deserts, and forests. There are perhaps billions of planets in the universe, but Earth is the only one where life, as we know today, exists and thrives in all its forms.

The crust is Earth’s thin outer layer. The mountains, rivers, forests, animals, and people familiar to us occupy a very small part of the planet compared with its overall size. An apple provides a useful comparison: its thin skin represents the thinness of Earth’s crust.

A cutaway drawing of Earth appears beside a cut apple. A magnified portion shows the surface landscape. The thin outer edge of Earth is compared with the apple’s skin.

See Fig. 13.1 in your NCERT textbook

Resources and observations

Earth provides air to breathe, water to drink, and soil in which crops grow. Rock and timber supply materials for homes, buildings, and roads. These resources connect our everyday needs with the conditions that sustain other living beings.

Satellite images, pictures obtained using instruments carried by satellites, help investigate Earth. One false colour image of Earth was produced from nearly 3000 smaller images obtained by an Indian Space Research Organisation Earth Observation Satellite.

It is a false colour image: scientists use colours to represent different kinds of information. Such images help study land plants and tiny ocean organisms. They can also reveal ocean temperature, oil spills, and wind direction.

Understanding Earth therefore involves both its broad features and the connections between them. Air, water, soil, and living beings must be considered together when explaining why the planet supports life.

How do the planets and their temperatures compare?

Order and broad groups

The solar system includes the Sun and the planets that move around it. An orbit is the path a planet follows around the Sun. The eight planets move in nearly circular orbits.

In increasing distance from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Mercury, Venus, Earth, and Mars are relatively small and rocky. Jupiter, Saturn, Uranus, and Neptune are large and mostly made of gases.

All these planets receive energy from the Sun. A planet nearer the Sun would generally be hotter, while one farther away would generally be colder. However, distance does not explain every difference in planetary temperature.

Why Venus is the exception

Venus is the hottest planet, although Mercury is closer to the Sun. Its thick atmosphere traps heat. The atmosphere is the layer of gases surrounding a planet. Venus’s atmosphere is almost entirely carbon dioxide, a gas that helps retain heat.

The average temperatures are 170 °C for Mercury, 450 °C for Venus, and 15 °C for Earth. Here °C means degrees Celsius, the temperature unit. These are average temperatures, not a statement that every place on each planet has that temperature.

A planet’s radius is the distance from its centre to its surface. When Earth’s radius is taken as 1, Venus’s radius is 0.95. These relative values compare size; they are not distances from the Sun.

Note: The temperature pattern with distance is generally correct, but Venus is an exception. Its heat-trapping atmosphere makes it hotter than Mercury. Distance and atmosphere both matter when explaining planetary conditions.

How do distance and the greenhouse effect allow liquid water?

The habitable zone

Earth lies at a distance from the Sun where temperatures allow liquid water. The habitable zone, also called the Goldilocks zone, is the range of distances from a star over which water remains liquid.

If Earth were closer to the Sun, it would be too hot and water would evaporate, changing into vapour. If it were farther away, it would be too cold and water would freeze. Such extremes would prevent most life forms, especially plants, animals, and humans, from thriving.

Some microbes, living organisms too small to see with the naked eye, such as certain bacteria, can survive in frozen environments. From what we know so far, however, liquid water is essential for life to evolve. Earth’s distance allows water to remain mostly liquid.

A star appears beside coloured regions labelled “Too hot”, “Just right”, and “Too cold”. The habitable zone is marked around the middle region.

See Fig. 13.4 in your NCERT textbook

Two different ways to keep warm

Radiation is a way energy travels, including energy arriving from the Sun and energy given off by warmed Earth. The greenhouse effect occurs when atmospheric gases absorb radiation given off by Earth after sunlight warms it, trapping some heat.

This effect, though mild on Earth, keeps temperatures high enough for liquid water. Without an atmosphere, Earth would lose heat to space and become too cold. A plant greenhouse also stays warm, but its mechanism differs.

How do Earth’s size, atmosphere, and magnetic field protect life?

Gravity and the atmosphere

Gravity is the pull that draws objects towards Earth. Earth’s size allows it to hold an atmosphere. If Earth were much smaller, with the same average density, or mass per unit volume, its gravity would be too weak to retain atmospheric gases.

The atmosphere on Mars is 100 times thinner than Earth’s. A suitable distance from the Sun alone therefore does not provide all the conditions needed for life like Earth’s.

If a planet were too large and its gravity much stronger, it would perhaps pull living beings down with enough force that their bones could get crushed. Both sides of this comparison matter when considering Earth’s suitability.

Different protective roles

Atmospheric oxygen allows us to breathe and is needed by almost all forms of life on Earth. Some oxygen becomes ozone, a form of oxygen whose molecules contain three oxygen atoms. The ozone layer blocks harmful ultraviolet (UV) rays, radiation from the Sun that can damage living cells.

A magnetic field is the region around a magnet where its effect is felt. Earth behaves like a giant magnet. It is believed that movement of molten iron in Earth’s core may be the origin of its magnetic field.

Cosmic rays are high-energy particles arriving from far across the universe. The solar wind consists of particles arriving from the Sun. These particles can damage the atmosphere, reduce the ozone layer, and allow more harmful UV rays through.

Earth’s magnetic field pushes many harmful particles away. It therefore helps protect the atmosphere and life. Its role should be distinguished from the ozone layer’s protection against ultraviolet radiation.

What does exploring Mars tell us about habitability?

Habitability means suitability for supporting life. Mars lies at the edge of the Sun’s habitable zone. Spacecraft have travelled to Mars, and rovers, vehicles used to explore its surface, have investigated conditions there.

No proof of life has been found yet. Scientists think Mars may have had liquid water in the past, maybe even lakes, and conditions that could support simple life forms. These possibilities explain why Mars remains an important object of investigation.

Possibility and evidence

A possible past environment suitable for life is different from proof that life existed. Exploring Mars helps scientists search for new clues. Scientific explanations remain open to change when new evidence becomes available.

India’s Mangalyaan, the Mars Orbiter Mission, was launched in 2013 by the Indian Space Research Organisation. It carried instruments to study the atmosphere, surface, and signs of past water on Mars.

These investigations help address whether Mars was ever suitable for life. They do not turn a possible history of liquid water into a confirmed history of living organisms.

Several conditions work together

Earth provides a useful comparison because its position, liquid water, atmosphere, size, and magnetic field work together. Studying a planet’s temperature is therefore one part of assessing its suitability for life.

Earth’s orbit is almost circular. This keeps sunlight and heat nearly steady throughout the year, preventing extreme summers and winters at most places. The words “nearly” and “at most places” matter: this does not mean every place has identical conditions throughout the year.

How do air, water, and sunlight sustain living beings?

Food, oxygen, and water

Photosynthesis is the process by which plants use sunlight, carbon dioxide, and water to prepare food, releasing oxygen. Respiration is the life process through which organisms release energy from food. Humans, animals, and plants use atmospheric oxygen for respiration.

1. Plants take carbon dioxide from the surrounding air.
2. They obtain water from the soil.
3. In the presence of sunlight, they use these materials to prepare food.
4. Oxygen is released during the process and is available for respiration.

The hydrosphere comprises Earth’s water, including ponds, lakes, rivers, springs, seas, oceans, and groundwater. Water covers about 70 per cent of Earth’s surface. Its abundance makes Earth appear blue from space, giving it the name Blue Planet.

Water is a good solvent, a substance in which other substances dissolve. It transports nutrients, substances needed for growth and life, from soil to leaves. In animals, water helps regulate body temperature, aids digestion, and maintains hydration, the supply of water within the body.

Habitats and replenishment

Water supports millions of life forms, from tiny plankton, small organisms living in water, to giant whales. Many deep-ocean organisms are still being discovered. Freshwater also supports crop growth and people’s needs.

Water vapour in the air forms clouds and brings rain or snow. This replenishes rivers, lakes, and underground water. Rainfall influences the kinds of plants and animals that can live in a place.

Moving air helps shape weather and rainfall. These affect farming, water supply, and life on land. Air, water, and sunlight therefore support life through linked processes rather than through isolated contributions.

How do soil, rocks, and living organisms keep Earth in balance?

The geosphere and geodiversity

The geosphere consists of Earth’s solid parts, including rocks, soils, and minerals. Minerals are naturally occurring materials in Earth’s solid parts. Soil supplies nutrients such as nitrogen and potassium that plants need for growth.

These nutrients come from the slow breakdown of rocks and from plant and animal remains. Soil is therefore connected both to the non-living ground beneath us and to organisms that lived before.

Geodiversity is the variety of landforms, rocks, and soils, together with the processes that shape and change them. This variety creates different habitats, places where organisms live. The non-living environment helps shape the kinds of life that thrive.

A landscape drawing shows mountains, a winding river, vegetation, and a coast. A cutaway below the surface shows layers of solid material beneath the varied landscape.

See Fig. 13.9 in your NCERT textbook

The biosphere and interdependence

The biosphere includes living beings and the places where they live. It extends across land, water, and air wherever life interacts with its surroundings. An ecosystem comprises living beings interacting with one another and their non-living environment.

Plants prepare food through photosynthesis. Animals eat plants or other animals. Decomposers are organisms that break down dead matter and return nutrients to soil. These relationships help sustain the supply and reuse of materials needed by life.

Earth’s systems affect one another. Cutting down a forest can affect rainfall, soil, air quality, and animals living there. Protecting clean air alone is therefore insufficient if water, soil, or living communities are being damaged.

Balance depends on connected systems continuing to support one another. Protecting air, water, soil, and the variety of living beings helps keep the planet habitable for the future.

Why are reproduction and inherited instructions essential?

Continuity and resemblance

Reproduction is the process through which living beings produce new individuals of their kind. Without it, plants and animals would eventually disappear. Reproduction maintains the continuity of life by producing another generation.

Offspring are the young individuals produced by parents. A cow produces a calf that develops into a cow; a cat produces kittens that develop into cats. Parents pass on genetic instructions, inherited information that guides development.

Genes carry inherited instructions involved in development and characteristics. These instructions guide the formation of structures such as bones, muscles, and skin. Inheritance means the passing of such instructions from parents to offspring.

Similarity and variation

Variation means differences in characteristics among individuals of the same kind, such as differences in colour or height. Reproduction can preserve similarities while also allowing differences to appear.

Sometimes changes in inherited instructions help a plant or animal survive better in a new environment. Over many generations, small differences can accumulate into larger differences and even new kinds of living beings. Such outcomes are possibilities over generations, not changes guaranteed for every offspring.

In asexual reproduction, one parent produces new individuals with the same genetic instructions. In sexual reproduction, instructions from two parents combine, producing offspring that share features with both but are not exactly like either.

A trait is a characteristic, such as eye colour or hair type. Combining instructions can retain useful traits while allowing new ones to appear. This explains how reproduction contributes both to continuity and to changes in living populations over time.

How does asexual reproduction produce new individuals?

Vegetative propagation in plants

Vegetative propagation is reproduction in which a new plant grows from a part such as a leaf, stem, or root. Many plants can reproduce this way. Examples for observation include a money-plant stem cutting, the eyes of a sprouted potato, and a piece of ginger.

The potato’s eyes are the places from which sprouts grow. These examples allow new growth to be observed without beginning with seeds. The developing plant still needs suitable conditions, including water, air, and sunlight.

1. Take a money-plant stem cutting, a sprouted potato with eyes, or a piece of ginger.
2. Plant the pieces separately in moist soil, without placing them too deep.
3. Provide the water, air, and sunlight needed for growth.
4. Observe the pieces every day and note the appearance of roots, stems, and leaves.
5. Record how many days each change takes, including the appearance of the first new leaf.

Daily records distinguish an observed result from an assumed timetable. Different changes should be recorded when they actually appear, rather than assigning an invented number of days to root or leaf formation.

Microbes and simple animals

A single-celled organism consists of one cell, while a multicellular organism consists of many cells. Bacteria and amoebae can divide into two identical individuals. Some multicellular organisms, such as algae, can regrow from small cut parts.

Hydra produces small buds, outgrowths on its body that detach and develop into new individuals. Planaria, a kind of flatworm, can regrow from a body fragment. This ability is called regeneration, regrowth from a remaining part.

These examples show that asexual reproduction occurs beyond flowering plants. A single parent can give rise to new individuals through different processes, without combining genetic instructions from two parents.

How do gametes support sexual reproduction in plants?

Half the instructions from each parent

Gametes are specialised reproductive cells carrying half the parent’s genetic material. When male and female gametes unite, the resulting cell has a complete set of instructions, with half supplied by each parent.

Fertilisation is the joining of male and female gametes. The cell formed is a zygote. Gametes prevent the amount of genetic material from doubling in every generation while allowing instructions from two parents to combine.

1. The parents produce specialised reproductive cells called gametes.
2. Each gamete carries half the parent’s genetic material.
3. A male gamete and a female gamete unite during fertilisation.
4. The resulting zygote receives a complete set of genetic instructions, half from each parent.

Offspring receive different combinations of instructions. This helps explain why babies do not look exactly like either parent and why brothers and sisters can differ in eye colour, hair type, and other traits.

Flowers, seeds, and new plants

Flowering plants have parts involved in producing male and female gametes. Pollen consists of grains produced in flowers. Wind, insects, or other animals can carry pollen to another flower; this transfer is called pollination. Pollination and fertilisation are distinct processes.

Sexual reproduction in flowering plants leads to seeds and fruits. When birds or animals eat fruit, seeds often get dropped far from the original plant. This helps plants spread to other places.

A banyan seed eaten with fruit and later excreted by a bird might sprout in a wall crack after rain. Germination is the beginning of growth from a seed. With water, the seed uses stored nutrients to develop roots and shoots, followed by the first leaves.

This sequence connects reproduction with the wider environment: animals can help move seeds, while water and stored nutrients support the start of growth at the new location.

How do animals differ in fertilisation and development?

Fertilisation in water or within the female

In animals, the male gamete is called a sperm and the female gamete an egg. Their union forms a zygote. An embryo is an early developing organism formed as the zygote develops.

In fish or frogs, males and females release sperm and eggs into water, where fertilisation may occur. The zygote’s development into an embryo also takes place in water in these examples.

In birds and mammals, including humans, sperm are deposited inside the female’s body. They move towards an egg, and fertilisation takes place inside the body. What follows differs between birds and most mammals.

The drawing labels an egg cell, a sperm cell, fertilisation, and a zygote. Arrows connect the two reproductive cells with the cell formed by their union.

See Fig. 13.14 in your NCERT textbook

Supplying the developing young

Birds lay fertilised eggs. The developing embryo uses food supplied within the egg until hatching, when the young bird emerges. The female must therefore provide nourishment that will support development within the egg.

In most mammals, the zygote develops into an embryo inside the female’s body. The mother supplies food and oxygen until the young one is born. This differs from supplying nourishment within an egg that has been laid.

The place of fertilisation and the place of later development must be considered separately. Birds have fertilisation inside the female but lay eggs for subsequent development.

Both methods provide for the developing young, but their arrangements for food supply differ. Reproduction therefore involves more than the joining of gametes: continued development also requires a supply of nourishment.

What threatens Earth’s balance, and how can we protect it?

The triple planetary crisis

Biodiversity is the variety of living beings. Pollution is contamination of air, water, or soil by harmful substances or waste. Climate change, biodiversity loss, and pollution together form the triple planetary crisis.

Global warming is an increase in Earth’s temperature. Climate change means long-term changes in temperature, rainfall, and weather patterns. Burning fossil fuels, such as coal and oil formed from material stored underground over millions of years, releases extra greenhouse gases.

1. Burning coal and oil releases greenhouse gases such as carbon dioxide and methane.
2. These gases trap additional heat in the atmosphere.
3. Plants and tiny ocean plankton absorb carbon dioxide as they grow, but cannot absorb the extra release fast enough.
4. Heat builds up, contributing to global warming and changes in climate.

Even a small temperature rise can melt ice caps, raise sea levels, cause extreme weather, and lead to plants and animals disappearing. Higher seas could flood many coastal cities. These changes also affect crops, water supplies, habitats, and human health.

Habitat destruction can disrupt feeding relationships. If grasses disappear, deer or grasshoppers struggle. Herbivores are plant-eating animals. Their loss also removes food for predators, animals that feed on other animals, such as tigers or foxes.

Reducing damage

Factory, vehicle, and fuel-burning emissions can harm breathing and crops. Factory, farm, and plastic waste can damage aquatic life and make water unsafe. Excess fertilisers and poor waste disposal pollute soil and can spread harmful substances through feeding relationships.

Protection includes better waste management, sustainable farming, and preserving biodiversity. Sustainable use means using resources responsibly so that they continue to support life in the future. Diverse ecosystems are stronger and more balanced.

Renewable energy comes from sources such as sunlight and wind that are naturally replenished. Using these sources, improving energy use, and choosing environmentally friendly travel help reduce greenhouse gas emissions.

Reusing, repairing, and recycling clothes and plastic reduce waste and pollution. Saving water and energy, sharing ideas, and encouraging others also help. Local communities and global leaders both have roles in protecting the connected systems on which life depends.

Glossary

• Habitable zone — Range of distances from a star over which water can remain in liquid form.
• Atmosphere — Layer of gases surrounding Earth and helping sustain conditions needed for life.
• Greenhouse effect — Warming caused when atmospheric gases absorb radiation given off by the warmed Earth.
• Ozone layer — Part of the atmosphere containing ozone that shields living cells from harmful ultraviolet rays.
• Magnetic field — Region where magnetic effects act; Earth’s field pushes many harmful particles away.
• Hydrosphere — All Earth’s water, including water in rivers, lakes, oceans, springs, and underground sources.
• Geosphere — Solid parts of Earth, including the rocks, soils, and minerals beneath our feet.
• Geodiversity — Variety of landforms, rocks, and soils, including processes that shape and alter them.
• Biosphere — All living beings together with the places where they live and interact with their surroundings.
• Vegetative propagation — Asexual reproduction in which a new plant grows from a leaf, stem, or root.
• Gamete — Specialised reproductive cell carrying half the parent’s genetic material for sexual reproduction.
• Fertilisation — Joining of male and female gametes to form a cell called the zygote.
• Variation — Differences in characteristics among individuals of the same kind, such as colour or height.
• Climate change — Long-term changes in temperature, rainfall, and weather patterns that affect Earth’s living systems.
• Triple planetary crisis — The combined environmental challenges of climate change, biodiversity loss, and pollution threatening life on Earth.

Common errors and misconceptions

• Misconception: The nearest planet to the Sun must be the hottest. Correct: Venus is hotter than Mercury because its thick atmosphere traps heat; distance alone does not explain temperature.
• Misconception: The greenhouse effect is entirely harmful. Correct: Earth’s mild greenhouse effect helps maintain liquid water. Extra greenhouse gases trap additional heat and contribute to global warming.
• Misconception: Scientists have proved that Mars supported life. Correct: No proof of life has been found yet. Past liquid water and conditions suitable for simple life remain possibilities.
• Misconception: The ozone layer and magnetic field protect Earth in exactly the same way. Correct: Ozone blocks harmful ultraviolet rays; the magnetic field pushes many harmful high-energy particles away.
• Misconception: Every parent contributes its full genetic material through a gamete. Correct: Each gamete carries half. Their union restores a complete set of instructions in the zygote.
• Misconception: Plants reproduce through seeds alone. Correct: Many plants also reproduce through vegetative propagation, as observed with money-plant cuttings, sprouted potato eyes, and ginger pieces.
• Misconception: Fertilisation and later development happen in the same place in every animal. Correct: Birds have fertilisation inside the female but lay eggs, while most mammals support embryo development inside the female.

Exam-style questions with model answers

Q1. Mercury is nearer the Sun than Venus. Their average temperatures are 170 °C and 450 °C respectively, where °C means degrees Celsius. Venus has a thick atmosphere almost entirely made of carbon dioxide. Use these facts to explain why distance alone does not determine temperature. [2 marks]

1. Venus is hotter despite being farther from the Sun: its average temperature is 450 °C, compared with Mercury’s 170 °C.
2. Venus’s thick carbon dioxide atmosphere traps heat, showing that atmospheric conditions also influence a planet’s temperature.

Q2. In a proposed explanation, Earth becomes much smaller while keeping the same average density, meaning mass per unit volume. Smaller size would weaken gravity, the pull that holds objects and gases towards Earth. The atmosphere supplies oxygen for respiration and retains heat through the mild greenhouse effect, helping maintain liquid water. Explain the consequences for the atmosphere and life in three linked points. [3 marks]

1. With much smaller size at the same average density, Earth’s gravity would become weaker, reducing its ability to hold atmospheric gases close to the planet.
2. If the gravitational pull became too weak to retain these gases, they would escape into space, and Earth would lose its atmosphere.
3. Losing the atmosphere would remove conditions that sustain life, including oxygen for respiration and the mild greenhouse effect that helps maintain temperatures suitable for liquid water.

Q3. A planet has liquid water, an oxygen-containing atmosphere, soil, plants, animals, and decomposers. Plants use sunlight, carbon dioxide from air, and water to make food, releasing oxygen used in respiration. Water carries soil nutrients to leaves, and decomposers return nutrients to soil. Explain four connections that support life. Define photosynthesis and decomposers in your answer. [4 marks]

1. Photosynthesis is the process in which plants use sunlight, carbon dioxide from air, and water to prepare food. It connects the atmosphere, water, and living plants.
2. Plants release oxygen during photosynthesis. Atmospheric oxygen supports respiration in humans, animals, and plants, linking food production with another essential life process.
3. Water transports nutrients from soil to plant leaves. Soil and water therefore work together in supplying materials that support plant growth.
4. Decomposers are organisms that break down dead matter. By returning nutrients to soil, they connect the remains of living beings with resources available for further growth.

Q4. A money-plant stem cutting, sprouted potato eyes, and a ginger piece are available. Plan five steps to investigate vegetative propagation, meaning growth of new plants from plant parts. Include planting conditions and observations without inventing growth times. [5 marks]

1. Select the money-plant cutting, sprouted potato eyes, and ginger piece as the plant parts to investigate. Keep each example separate so its new growth can be followed.
2. Plant the parts separately in moist soil. Avoid planting them too deeply, so that the investigation follows suitable conditions for observing their growth.
3. Provide the water, air, and sunlight needed by the growing plants. Continue maintaining these conditions while the investigation is in progress.
4. Observe each plant part every day. Look for the development of roots, stems, and leaves rather than assuming that growth has already occurred.
5. Record the days on which changes actually appear, including the first new leaf. Do not assign a fixed growth period before making observations.

Q5. Each parent produces gametes, specialised reproductive cells containing half its genetic material. Male and female gametes unite during fertilisation. Explain in three points how the offspring receives a complete set of instructions and can differ from its parents. [3 marks]

1. Each gamete contains half the parent’s genetic material. This means that sexual reproduction does not involve combining two complete parental sets in every generation.
2. During fertilisation, male and female gametes unite to form a zygote, the resulting cell with a complete set of instructions, half supplied by each parent.
3. The offspring receives a combination of instructions from both parents. Different combinations help explain why it shares parental traits but does not look exactly like either parent.

Q6. Birds and most mammals have fertilisation inside the female. Birds lay fertilised eggs containing food for the embryo, while most mammals supply food and oxygen to the embryo inside the mother. An embryo is the early developing organism. Compare their provision for development in two points. [2 marks]

1. Birds provide food within the laid egg, supporting the developing embryo until the young bird hatches.
2. In most mammals, development occurs inside the mother, whose body supplies food and oxygen until birth.

Q7. Coal and oil burning releases extra greenhouse gases, which trap heat. Plants and ocean plankton, small water-living organisms, cannot absorb the extra carbon dioxide fast enough. Habitat destruction and factory, farm, and plastic waste also harm living beings. Explain the three parts of the triple planetary crisis and give one matching protective action for each. [6 marks]

1. Climate change involves long-term changes in temperature, rainfall, and weather patterns. Extra greenhouse gases trap additional heat, contributing to warming that affects water, crops, habitats, and health.
2. Reduce greenhouse gas emissions by using renewable energy such as solar or wind power. Improving energy use and choosing environmentally friendly travel also contribute.
3. Biodiversity loss means a reduction in the variety of living beings. Habitat destruction can remove organisms and disturb feeding relationships on which other organisms depend.
4. Preserve habitats and biodiversity, with local communities helping to use resources sustainably. Diverse ecosystems are stronger and more balanced, supporting continued connections among living beings.
5. Pollution contaminates air, water, and soil. Factory, farm, and plastic waste can harm aquatic life, make water unsafe, and damage the conditions needed for growth.
6. Improve waste management and reduce waste by reusing, repairing, and recycling items such as clothes and plastic. Sustainable farming also helps protect soil and water.

Key takeaways

• Earth’s distance from the Sun permits liquid water, while its size, atmosphere, and magnetic field also contribute to conditions suitable for life.
• The mild greenhouse effect helps keep Earth warm; additional greenhouse gases trap extra heat and contribute to global warming.
• Ozone blocks harmful ultraviolet rays, while Earth’s magnetic field pushes many harmful high-energy particles away from the planet.
• The atmosphere, hydrosphere, geosphere, and biosphere interact, so damage to one system can affect the others.
• Asexual reproduction involves one parent; sexual reproduction combines genetic instructions from two parents and allows different combinations of traits.
• Gametes each carry half the parent’s genetic material, and fertilisation produces a zygote with a complete set.
• Birds supply nourishment within laid eggs, while most mammals supply food and oxygen during development inside the mother.
• Climate change, biodiversity loss, and pollution form the triple planetary crisis; protecting life requires action on all three.

Test yourself

What is the Goldilocks zone?

It is another name for the habitable zone, the range of distances from a star over which water remains liquid.

Why is Earth called the Blue Planet?

Water covers about 70 per cent of Earth’s surface, making the planet appear blue when seen from space.

How does a plant greenhouse retain warmth?

It is a closed space, usually with glass walls, that retains warmed air so heat does not escape easily.

What is known about life on Mars?

No proof of life has been found yet. Scientists think Mars may have had liquid water and conditions suitable for simple life in the past.

How do geodiversity and biodiversity differ?

Geodiversity concerns varied landforms, rocks, soils, and the processes shaping them. Biodiversity concerns the variety of living beings.

Why does the amount of genetic material not double at every generation?

Each gamete carries half the parent’s genetic material. The joining of two gametes restores a complete set in the zygote.

How can a banyan seed reach a crack in a wall?

A bird may eat the fruit and excrete the seed elsewhere. The seed might then sprout in a wall crack after rain.

Name the three challenges in the triple planetary crisis.

The three challenges are climate change, biodiversity loss, and pollution, which threaten the balance sustaining life on Earth.