NCERT Solutions: Earth, a Unique Life Sustaining Planet

Probe and Ponder (Page 210)

Q1: What do you think Earth would look like if there were no life on it at all?
Ans: Earth would appear as a barren, rocky world with large stretches of empty land, dried-up riverbeds and oceans, no green forests or animals, and a much quieter surface. Without plants to hold soil together, erosion and dust storms would be common; without life producing oxygen, the atmosphere would be thinner and different in composition. Overall, it would look more like Mars or the Moon - lifeless and still.

Q2: Life on Earth has survived for billions of years. What allows it to keep going despite major changes and disasters?
Ans:  Earth's balance of air, water, sunlight, and soil, plus its position in the habitable zone, helps life adapt. Reproduction passes on traits that help survive changes, like disasters or climate shifts. Ecosystems recover through interactions between living things (like plants regrowing after a fire) and non-living parts (like nutrient recycling in soil).

Q3: Why don't dogs lay eggs? Or hens give birth to live chicks?
Ans: Because of differences in how these groups of animals evolved and reproduce. Dogs are mammals: their embryos develop inside the mother and get nourishment directly, so they give birth to live young. Hens are birds: their embryos develop inside eggs outside the mother, with stored food in the egg. Each method works for the life-history and survival strategies of that group.

Q4: If a spaceship carried soil and water to Mars, could plants start growing there?
Ans: Maybe simple plants or microbes could grow if we add the right conditions, like protection from Mars' thin atmosphere, extreme cold, and radiation. But most Earth plants would struggle without enough air pressure, liquid water stability, or Earth's magnetic shield. Experiments show some bacteria survive, but full growth needs more, like a greenhouse dome.

Q5: Share your questions ___________
Ans: Here are some fun questions from the chapter: 

• Why is Earth called the Blue Planet? 
• How does the ozone layer protect us? 
• What would happen if Earth's magnetic field disappeared? 
• Could life exist on other planets in the habitable zone?

InText (Page 212 - 221)

Q1: What makes the Earth unique for living beings to grow and survive? (Page 212)
Ans: Earth supports life because many favourable factors occur together:

• Correct distance from the Sun: Earth is in the habitable zone (about 93 million miles), so temperatures allow liquid water to exist.
• Suitable temperature range: Surface temperatures on Earth (roughly -88°C to 58°C in extremes) allow water in liquid form in many places.
• Protective atmosphere: A mix of nitrogen and oxygen supports breathing, regulates climate, and, with the ozone layer, shields harmful solar ultraviolet radiation.
• Magnetic field: It deflects charged particles from the Sun and helps protect the atmosphere.
• Active geology: Plate tectonics and volcanic activity recycle nutrients and help long-term climate stability.
• Stable tilt and Moon: The Moon helps stabilise Earth's tilt, producing predictable seasons.
• Chemical ingredients: Elements such as carbon, hydrogen, oxygen and nitrogen are available for building living molecules.

Q2: Is the temperature or distance from the sun the only factor that makes the Earth habitable? (Page 215)
Ans: No. Distance and temperature are crucial because they allow liquid water, but other factors are also essential. These include a protective atmosphere and ozone layer, a magnetic field to block charged particles, active geology to recycle materials, suitable gravity to hold an atmosphere, and a stable rotation and tilt. All these together create the right conditions for diverse life.

Q3: What would happen if the size of the Earth were too small or too big? (Page 215)
Ans: If Earth were significantly smaller or larger, its habitability would be compromised due to changes in gravity, atmosphere, and geological processes. A smaller Earth, with weaker gravity, would struggle to retain a dense atmosphere, leading to insufficient pressure for liquid water and poor protection from solar radiation, much like Mars. It might also lack the mass to sustain plate tectonics, limiting nutrient cycling and climate regulation. Conversely, a larger Earth with stronger gravity could trap a thick, oppressive atmosphere, like Venus, potentially causing extreme temperatures and pressure that crush life. Increased geological activity might lead to excessive volcanism, disrupting stable climates. Both scenarios would likely prevent the delicate balance of conditions, liquid water, breathable air, and stable ecosystems required for life as we know it.

Q4: Does the magnetic field of the Earth have any role in sustaining life on Earth? (Page 217)
Ans: Yes. The Earth's magnetic field shields the planet from harmful solar wind and many cosmic charged particles. By deflecting these particles, the magnetic field helps prevent the atmosphere from being stripped away and reduces radiation at the surface, both of which are important for protecting living organisms and maintaining a stable climate.

Q5: How is life supported and sustained on Earth? (Page 217)
Ans: Life is supported by several key factors:

• The presence of water, which is essential for the chemical reactions of living cells.
• The atmosphere, which supplies gases (oxygen and carbon dioxide), regulates temperature, and blocks some harmful radiation.
• The Sun's energy, which powers photosynthesis and drives weather and climate systems.
• The Earth's magnetic field, which helps protect life from charged particles from space.

Q6: How do bamboo and sugarcane grow into new plants? (Page 221)
Ans: Bamboo and sugarcane reproduce by vegetative propagation. This is a type of asexual reproduction where new plants grow from parts of the parent plant rather than from seeds. For example, sugarcane produces new shoots from nodes on its stem, and bamboo spreads through underground rhizomes that produce new shoots at some distance from the parent plant.

Bamboo and Sugarcane

Keep the Curiosity Alive (Page 226 - 227)

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.
Ans: (iii)
Explanation: Mars currently has a very thin atmosphere that cannot hold heat or stable liquid water on the surface. Without a thick atmosphere and liquid water, conditions are too cold and dry for most Earth-like life.

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.
Ans: (ii)
Explanation: Geodiversity refers to the variety of rocks, soils and landforms. Different landforms such as mountains, valleys and deserts are clear examples of geodiversity because they create distinct habitats and landscapes.

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.
Ans: (ii)
Explanation: A smaller Earth would have weaker gravity and would find it harder to hold on to gases. Over time, the atmosphere could escape into space, leaving the planet thin-airred and less able to support liquid water and life.

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.
Ans: (iv)
Explanation: Offspring receive a combination of genetic material (genes) from both parents by the fusion of gametes. This mixing produces individuals that share traits with both parents but are not identical to either, which creates variation in the population.

Q5: 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?
Ans: The seeds most likely arrived from nearby plants, carried by wind, birds or insects. The monsoon provided moisture needed for germination. The crack trapped some dust or organic matter, supplying small amounts of nutrients and shelter. Sunlight and air at the wall surface allowed the seedlings to photosynthesise and grow. Together, moisture, trapped soil, seeds and sunlight formed a small, favourable microhabitat.

Q6: 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?
Ans: 

• Effects on local climate: Removing trees reduces shade and evapotranspiration, often raising local temperatures and altering local rainfall patterns. This can make conditions drier and hotter in the area.
• Effects on biodiversity: Clearing forest destroys habitats and food sources, causing loss of plant and animal species and reducing overall biodiversity. Many animals may be forced to move or may die out locally.
• Effects on water availability and quality: Trees help retain soil and water; without them, soil erosion increases, which reduces groundwater recharge and leads to less reliable water supplies. Eroded soil can enter rivers and lakes, making water muddy and reducing quality for humans and wildlife.

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?
Ans: It is true that Earth has had natural climate changes in the past (for example, ice ages). However, current global warming is different because it is happening much faster and is largely caused by human activities, such as burning fossil fuels and deforestation. These activities increase greenhouse gas levels in the atmosphere, which trap more heat and change weather patterns. The rapid rate and human cause make today's warming a serious problem that needs action to reduce emissions and protect ecosystems.

Q8: Imagine Earth's magnetic field suddenly disappeared. What kinds of problems could arise for life on Earth? Explain.
Ans: Without the magnetic field, charged particles from the Sun and cosmic rays would strike the atmosphere more directly. This could lead to gradual loss of atmosphere, damage to the ozone layer, and higher levels of harmful radiation at the surface. The result would be increased risks to living cells (including higher cancer risks), harm to plants and animals, and disruption of satellites and communication systems. Over long periods, such changes would make Earth less hospitable for life.

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?
Ans: Three things needed:

• Liquid water: For drinking, hygiene and growing food.
• Breathable atmosphere: An oxygen-rich environment at a suitable pressure for human respiration.
• Temperature control and shelter: Habitats that keep temperature and pressure stable and protect from radiation.

Hardest to replicate: A stable, breathable atmosphere is likely the most difficult. Mars' atmosphere is very thin and mostly carbon dioxide, so creating and maintaining an Earth-like mix at the right pressure for a whole settlement, while protecting it from leaks, solar radiation and long-term loss, is a major technological and logistic challenge.

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.
Ans: Causes: The changes could be due to broader climate change caused by increased greenhouse gas emissions, and local factors such as deforestation or land-use changes that alter local climate and water cycles.
Adaptation strategies:

• Water conservation: Build rainwater harvesting systems and small check dams to store water when it falls and use it during dry periods.
• Sustainable agriculture: Plant drought-resistant crops, use mulching and drip irrigation to save water, and adopt soil conservation practices to retain moisture.

Q11: If there were no atmosphere on the Earth, would it affect life, temperature, and water on the planet? Explain.
Ans: Without an atmosphere:

• Life: Most life forms would not survive because there would be no breathable air and no protection from harmful solar radiation.
• Temperature: There would be extreme temperature swings - very hot in daylight and freezing by night - because the greenhouse effect and atmospheric circulation that moderate temperatures would be absent.
• Water: Liquid water could not persist; it would either freeze or evaporate into space because of the low pressure and lack of atmospheric water vapour and a normal water cycle.

Q12: Discuss five examples of vegetative propagation.
Ans: Vegetative propagation is a form of asexual reproduction in plants where new plants grow from parts of the parent plant. Five examples are:

• Money plant: A stem cutting with a node forms roots when placed in water or soil and grows into a new plant.
• Potato: Sprouted 'eyes' on a potato tuber can be planted; each eye grows into a new plant.
• Ginger: Pieces of ginger rhizome with buds, when planted, produce new shoots and roots.
• Sugarcane: Stem cuttings containing nodes are planted; each node can form roots and shoots to give a new plant.
• Bamboo: Bamboo spreads through underground rhizomes and stem sections that develop into new plants.

Discover, Design, and Debate (Page 227)

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?
Ans: My kit would include: 

1. Liquid water - essential for all life processes and for growing plants.
2. Breathable air with oxygen and some carbon dioxide - for respiration and photosynthesis.
3. Soil with nutrients - to grow plants and recycle organic matter.
4. A sunlight simulator - to provide energy for photosynthesis and warmth.
5. A magnetic shield model or radiation protection - to reduce harmful charged particles and protect the atmosphere and living beings.

These parts recreate the basic balance needed for life: water and air for metabolism, soil for food production, sunlight for energy, and protection from space radiation.

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. 
Ans: Plants might grow if water and nutrients are present, but lunar soil (regolith) lacks organic matter and has sharp, fine particles and different chemistry. A simple experiment: take pots with three types of medium - pure lunar-like soil (use fine sand or volcanic ash to simulate it), a 50:50 mix of lunar-like soil and fertile Earth soil, and pure Earth soil as control. Plant the same type of seed (for example, bean seeds) in each, keep equal water and light, and observe germination, growth and health over several weeks. Comparing results will show whether lunar-like soil needs added nutrients or treatment to support plants.

Q3: Flowers are often brightly coloured and have a pleasant smell. How do you think these features help the plant reproduce?
Ans: Bright colours and pleasant smells attract pollinators such as bees, butterflies and birds. When pollinators visit for nectar, they pick up pollen from one flower and carry it to another, helping fertilisation and the production of seeds. These features therefore increase the chances of successful reproduction.

Q4: 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?
Ans: Fish and frogs often release many eggs because most eggs and young are vulnerable to predators, disease and environmental hazards. Laying many eggs increases the chance that at least some will survive to adulthood.
Advantages: Higher probability that some offspring survive without parental care.
Disadvantages: Most offspring may die, so a large amount of energy is invested with low individual survival; there is little parental protection.

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?
Ans: Parental care, as in sparrows, increases the survival chances of young because parents feed, warm and protect them from predators, giving higher growth and survival rates. In contrast, species that leave eggs (like many snakes) rely on producing many eggs or hiding them well; fewer young survive on average, but this strategy requires less parental investment and can succeed if enough young hatch and reach maturity.