Our Home: Earth, a Unique Life Sustaining Planet — Class 8 Science
Curiosity · Chapter 13
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6- Mathematics
- Science
- + 3 more
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
2- Mathematics
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
Chapter summary
Key ideas & terms
- Habitable zone
- The range of distances from a star, also called the Goldilocks zone, where water can remain liquid and life can exist.
- Greenhouse effect
- The trapping of heat by atmospheric gases such as carbon dioxide, which keeps a planet warm.
- Ozone layer
- A layer of the atmosphere that shields the Earth by blocking harmful ultraviolet rays from the Sun.
- Biosphere
- All living beings on Earth together with the land, water, and air where they live.
- Asexual reproduction
- Reproduction by a single parent that produces offspring that are exact copies of itself.
- Sexual reproduction
- Reproduction in which genetic material from two parents combines through gametes, producing varied offspring.
Extra questions & answers
Why is Earths position in the solar system important for life?
Earth lies in the habitable zone, at just the right distance from the Sun for water to remain in liquid form, which is essential for life.
What is the greenhouse effect and why is it important on Earth?
It is the trapping of heat by atmospheric gases like carbon dioxide, which keeps Earth warm enough for water to stay liquid.
Why is Venus the hottest planet despite not being closest to the Sun?
Its thick atmosphere of carbon dioxide traps heat through a strong greenhouse effect, making it hotter than Mercury.
How does the size of the Earth help make it habitable?
Earths size gives gravity strong enough to hold an atmosphere but not so strong that it would crush living things.
What is the role of the ozone layer?
It acts like a shield, blocking harmful ultraviolet rays from the Sun that can damage living cells.
How does Earths magnetic field protect life?
It pushes away harmful high-energy particles such as cosmic rays and the solar wind, protecting the atmosphere and life.
Name the four interacting Earth systems that sustain life.
The atmosphere, the hydrosphere, the geosphere, and the biosphere.
How does asexual reproduction differ from sexual reproduction?
Asexual reproduction involves one parent and produces identical offspring, while sexual reproduction combines genetic material from two parents to produce varied offspring.
What is the role of gametes in sexual reproduction?
Gametes carry half of each parents genetic material, so when male and female gametes join, the new cell gets a complete set of instructions.
What is the triple planetary crisis?
The combined challenges of climate change, biodiversity loss, and pollution that together threaten life on Earth.
Probe and ponder
What do you think Earth would look like if there were no life on it at all?
If there were no life, Earth would look bare and rocky like the Moon or Mars. There would be no green forests, no blue-green oceans full of life, and no oxygen-rich air. Land would be mostly bare rock and dust, water might still be there but without plants and plankton it would not support food chains, and the sky and soil would look very different without life changing them.
Life on Earth has survived for billions of years. What allows it to keep going despite major changes and disasters?
Life keeps going because Earth provides stable conditions in the habitable zone with liquid water, a protective atmosphere and magnetic field, and because living things reproduce. Reproduction passes genes to the next generation and, through sexual reproduction, creates variations that help some organisms survive new conditions and adapt over time.
Why don’t dogs lay eggs? Or hens give birth to live chicks?
This is decided by the way each animal reproduces. Dogs are mammals where fertilisation is internal and the embryo develops inside the mother’s body with nourishment from the mother, so they give birth to live young. Hens are birds where the fertilised egg is laid outside and the embryo develops inside the egg using stored food until hatching, so they lay eggs.
If a spaceship carried soil and water to Mars, could plants start growing there?
Probably not on its own. Mars is at the edge of the habitable zone, has an atmosphere about 100 times thinner than Earth’s, no breathable oxygen, very low temperature, high ultraviolet radiation and no strong magnetic field to shield from solar wind and cosmic rays. Soil and water alone would quickly freeze or dry, and without a thick atmosphere, suitable temperature and protection, plants could not grow without an artificial habitat that recreates Earth’s conditions.
Keep the curiosity alive
1. 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.
Correct option is (iii) It lacks a thick atmosphere and liquid water. Mars has a very thin atmosphere about 100 times thinner than Earth’s, no stable liquid water on the surface, very cold average temperature of about
2. 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.
Correct option is (ii) Different landforms like mountains, valleys, and deserts. Geodiversity means the variety of non-living parts of Earth such as rocks, soils, minerals and landforms, while options (i) and (iv) are examples of biodiversity and (iii) is weather.
3. 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.
Correct option is (ii) It would escape into space due to weaker gravity. With the same density, a smaller Earth would have weaker gravity and could not hold gases, just as Mars has a thin atmosphere and Mercury has almost none.
4. 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.
Correct option is (iv) They get mixed instructions (genes) from both parents. Each parent makes gametes with half the genetic material, and when sperm and egg combine the zygote gets a new combination of genes, so children share traits with both parents but are not identical to either.
5. You notice tiny green plants growing in cracks on your school wall after the monsoon. Where do you think the seeds came from? What conditions helped these plants grow there?
The seeds were probably carried by wind, water or by birds and animals that ate fruits and dropped seeds in excreta. They lodged in cracks where dust and soil collect. Monsoon provided water, warm temperature, air and light, and even a little soil in the crack gave nutrients, so the seeds could germinate and grow.
6. A city has recently cut down a large patch of forest to build new roads and buildings. Discuss the possible effects this could have on the local climate and biodiversity? How might this affect water availability or quality in the area?
Cutting forest removes trees that absorb
7. 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?
It is true that climate has changed slowly over millions of years, but today’s warming is much faster and is mainly caused by humans burning fossil fuels like coal and oil. This releases extra
8. Imagine Earth’s magnetic field suddenly disappeared. What kinds of problems could arise for life on Earth? Explain.
Earth’s magnetic field acts like a shield against solar wind and cosmic rays. Without it, high-energy particles would hit the atmosphere, damage the ozone layer made of
9. 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?
Three essentials are: (1) a breathable atmosphere with oxygen and safe pressure, (2) liquid water and a water cycle, and (3) protection from radiation and a suitable temperature with food production. The hardest is likely recreating a thick, oxygen-rich atmosphere with an ozone layer and magnetic shielding, because we cannot easily create a planet-wide air envelope and magnetic field; we would need sealed habitats with artificial air, water recycling and radiation shielding for a long time.
10. 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.
This may be due to local deforestation and global climate change from increased greenhouse gases. The village could adapt by (1) conserving water through rainwater harvesting, small check dams and growing drought-tolerant crops, and (2) planting more trees, using renewable energy and improving soil health to keep the local climate more stable.
11. If there were no atmosphere on the Earth, would it affect life, temperature, and water on the planet? Explain.
Yes, greatly. Without atmosphere there would be no oxygen for respiration and no
12. Discuss five examples of vegetative propagation.
Vegetative propagation is asexual reproduction where a part of the plant grows into a new plant: (1) Money plant stem cutting grows roots in water or moist soil, (2) Potato tuber eyes sprout into new potato plants, (3) Ginger rhizome pieces with buds grow into new ginger plants, (4) Sugarcane stem cuttings with nodes grow into new canes, (5) Bamboo grows from rhizomes or stem cuttings. In each case the new plant is an exact copy of the parent.
Discover, design, and debate
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?
A model Earth kit must have: a heat source at the right distance for liquid water, water covering most of the surface as hydrosphere, an atmosphere with oxygen and
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.
Even with water, Moon soil lacks organic matter, nutrients and air, has extreme temperature swings and no atmosphere or magnetic protection. To test, we could take returned lunar simulant soil, add water and try growing seeds like moong in it versus Earth soil as control, keeping light, temperature and water same. Observe germination and growth. We would likely need to add nutrients and provide air and shielding, showing water alone is not enough.
Flowers are often brightly coloured and have a pleasant smell. How do you think these features help the plant reproduce?
Bright colours and scent attract pollinators like bees, butterflies and birds. When they visit for nectar, pollen from the anther sticks to them and is carried to another flower’s ovule. This pollination allows male and female gametes to combine and form seeds, so these features increase chances of sexual reproduction.
Why do animals like fish and frogs lay hundreds or even thousands of eggs at a time, while other animals lay only a few? What might be the advantages and disadvantages of laying so many eggs?
Fish and frogs lay eggs in water where fertilisation and development happen outside the body with no parental care, so many eggs are eaten or die. Laying hundreds increases the chance that some survive. Advantage is higher chance of survival for the species; disadvantage is high energy cost and most young die. Animals like birds and mammals lay few eggs or give birth to few young but provide food, warmth and protection, so survival rate per young is higher.
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?
Sparrows keep eggs warm, protect them from predators and feed chicks until they can fly, so fewer young die and survival is higher. Snakes leave eggs, so many eggs and hatchlings are lost to temperature changes and predators. To compensate, reptiles often lay more eggs, while birds can succeed with fewer eggs because of active care.