Our Home: Earth, a Unique Life Sustaining PlanetClass 8 Science NCERT Solutions
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Q1Discover, 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?
Solution
An 'Earth Survival Kit' for a tiny model of Earth would need the following:
- Water: Essential for all known life forms as a solvent, transport medium, and participant in biochemical reactions.
- Soil: Provides nutrients and support for plants, which form the base of the food chain.
- Seeds: Allows for the growth of plants, which produce oxygen and provide food for other organisms.
- Microorganisms: Decomposers that break down organic matter and recycle nutrients in the ecosystem.
- A closed container: To simulate the atmosphere and maintain a stable environment with the right mix of gases.
- A light source: To provide energy for photosynthesis.
- A temperature control mechanism: To regulate the temperature to allow water to remain in liquid form.
Q2Discover, design, and debate
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.
Solution
If the Moon had water, whether plants could grow in the lunar soil (regolith) is uncertain and would depend on several factors. Lunar regolith lacks the organic matter and nutrients found in Earth soil, and it may contain toxic compounds. An experiment to explore plant growth on the Moon could involve:
- Collecting lunar soil samples: Obtain samples of lunar regolith from the Chandrayaan-4 mission.
- Adding water and nutrients: Mix the lunar soil with water and essential plant nutrients to create a growth medium.
- Planting seeds: Plant seeds of various plant species (e.g., fast-growing plants like radishes or lettuce) in the prepared lunar soil.
- Controlling environmental conditions: Maintain controlled environmental conditions, including temperature, light, and humidity, similar to what plants need on Earth.
- Monitoring plant growth: Observe and record plant growth parameters, such as germination rate, plant height, leaf size, and overall health, over a period of time.
- Comparing with control group: Compare the growth of plants in lunar soil with a control group of plants grown in Earth soil under the same conditions.
- Analyzing soil composition: Analyze the chemical composition of the lunar soil before and after the experiment to assess nutrient uptake and any potential toxic effects.
Q3Discover, design, and debate
Flowers are often brightly coloured and have a pleasant smell. How do you think these features help the plant reproduce?
Solution
The bright colors and pleasant smells of flowers play a crucial role in attracting pollinators, such as insects, birds, and other animals. Pollinators visit flowers to feed on nectar or collect pollen, and in the process, they transfer pollen from one flower to another. This transfer of pollen is essential for pollination, the first step in sexual reproduction in flowering plants. The bright colors act as visual cues, making the flowers more noticeable to pollinators, while the pleasant smells serve as olfactory attractants, drawing pollinators from a distance.
Q4Discover, design, and debate
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?
Solution
Animals like fish and frogs lay hundreds or thousands of eggs at a time due to the high mortality rate of their offspring. Many of the eggs and young are vulnerable to predators, environmental hazards, and disease. Laying a large number of eggs increases the chances that at least some offspring will survive to adulthood.
Advantages of laying many eggs:
- Increased probability of survival: More eggs increase the likelihood that some offspring will survive to reproduce.
- Wider dispersal: A large number of eggs can be dispersed over a wider area, reducing competition and increasing the chances of finding suitable habitats.
Disadvantages of laying many eggs:
- Less parental care: Animals that lay many eggs typically provide little or no parental care, leaving the offspring vulnerable.
- Smaller egg size: With a large number of eggs, each egg may contain fewer nutrients, potentially reducing the survival chances of individual offspring.
- Energy expenditure: Producing a large number of eggs requires a significant amount of energy from the parent.
Q5Discover, design, and debate
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?
Solution
The difference in parental care between birds and reptiles significantly affects the survival chances of their young. Birds that build nests and care for their eggs and chicks provide protection from predators, regulate temperature, and ensure a consistent food supply. This parental care increases the likelihood that the young will survive to adulthood.
Reptiles, such as snakes, typically lay their eggs and leave them without protection. The eggs are vulnerable to predators, temperature fluctuations, and desiccation. Consequently, a smaller proportion of reptile eggs hatch, and fewer young survive to adulthood compared to birds with parental care. The lack of parental care is compensated by laying a large number of eggs.
Q1Keep the curiosity alive
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.
Solution
The correct answer is (iii) It lacks a thick atmosphere and liquid water. Mars has a very thin atmosphere compared to Earth, and although there is evidence of past water, it does not currently have liquid water on its surface in large quantities. These conditions make it difficult for life as we know it to exist.
Q2Keep the curiosity alive
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.
Solution
The correct answer is (ii) Different landforms like mountains, valleys, and deserts. Geodiversity refers to the variety of landforms, rocks, and soils on Earth.
Q3Keep the curiosity alive
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.
Solution
The correct answer is (ii) It would escape into space due to weaker gravity. If Earth were smaller, its gravity would be weaker, and it would not be able to hold onto its atmosphere as effectively. The gases would escape into space.
Q4Keep the curiosity alive
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.
Solution
The correct answer is (iv) They get mixed instructions (genes) from both parents. Sexual reproduction involves the combination of genetic material from two parents, resulting in offspring that have a mix of traits from both, leading to differences.
Q5Keep the curiosity alive
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?
Solution
The seeds likely came from various sources, such as wind dispersal from nearby plants, birds or animals that may have carried them, or even from soil or debris that accumulated in the cracks. The monsoon season provides ample moisture, which is essential for seed germination. The cracks in the wall may also offer some protection from harsh sunlight and wind, creating a more favorable microclimate for the young plants to establish themselves. Additionally, any accumulated organic matter in the cracks could provide nutrients for the plants to grow.
Q6Keep the curiosity alive
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?
Solution
Deforestation can significantly impact the local climate and biodiversity. The loss of trees reduces the amount of carbon dioxide absorbed from the atmosphere, potentially leading to increased local temperatures and changes in rainfall patterns. It can also lead to a decrease in local biodiversity as habitats are destroyed, and species are displaced or become extinct. Water availability may be affected as forests play a crucial role in regulating water cycles; deforestation can lead to increased runoff, soil erosion, and decreased groundwater recharge. Water quality may also decline due to increased sedimentation and pollution from construction activities.
Q7Keep the curiosity alive
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?
Solution
While it is true that the Earth has experienced climate changes in the past, the current global warming trend is different in several key aspects. The rate of change is much faster than natural climate variations, and the primary cause is human activities, particularly the burning of fossil fuels, which releases large amounts of greenhouse gases into the atmosphere. Although natural climate changes have occurred, the current rate and magnitude of warming, driven by human activities, are unprecedented and pose significant threats to ecosystems and human societies. The consequences of this rapid warming, such as rising sea levels, extreme weather events, and biodiversity loss, are also of great concern.
Q8Keep the curiosity alive
Imagine Earth's magnetic field suddenly disappeared. What kinds of problems could arise for life on Earth? Explain.
Solution
If Earth's magnetic field suddenly disappeared, life on Earth would face several significant challenges. The magnetic field acts as a protective shield, deflecting harmful high-energy particles from the Sun (solar wind) and cosmic rays from deep space. Without this shield, these particles would bombard the Earth's atmosphere, potentially damaging the ozone layer, which protects life from harmful ultraviolet (UV) radiation. Increased UV radiation could harm living cells, disrupt ecosystems, and increase the risk of skin cancer. The atmosphere itself could also be eroded by the solar wind over time, leading to significant changes in the planet's climate and habitability.
Q9Keep the curiosity alive
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?
Solution
To support human life on Mars, I would need to recreate:
- A breathable atmosphere: Mars has a very thin atmosphere composed mainly of carbon dioxide, which is not breathable for humans. A system to produce and maintain an atmosphere with sufficient oxygen and pressure would be essential.
- Liquid water: Water is crucial for drinking, growing food, and various industrial processes. A reliable source of liquid water, either extracted from Martian ice or imported from Earth, would be necessary.
- Protection from radiation: Mars lacks a global magnetic field and has a thin atmosphere, offering little protection from harmful solar and cosmic radiation. Shielding structures or habitats to minimize radiation exposure would be vital.
Replicating a breathable atmosphere would likely be the most challenging. It requires creating a closed-loop system that can efficiently produce oxygen, remove carbon dioxide, and maintain a stable atmospheric pressure. This would involve complex technologies and significant energy resources, and any failure could have catastrophic consequences for the inhabitants.
Q10Keep the curiosity alive
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.
Solution
The increasing temperature and unpredictable rainfall could be caused by climate change, driven by increased greenhouse gas emissions from human activities such as burning fossil fuels and deforestation. Deforestation in nearby areas could also contribute to these changes.
Two ways the village could adapt to these new conditions are:
- Implementing water conservation techniques: This could include rainwater harvesting, efficient irrigation methods (such as drip irrigation), and promoting water-wise farming practices to conserve water resources during periods of drought.
- Promoting climate-resilient agriculture: This could involve planting drought-resistant crops, diversifying crop varieties, and adopting soil conservation techniques to improve water retention and reduce soil erosion. Agroforestry, which integrates trees into farming systems, can also help to moderate temperatures and improve soil health.
Q11Keep the curiosity alive
If there were no atmosphere on the Earth, would it affect life, temperature, and water on the planet? Explain.
Solution
If there were no atmosphere on Earth, it would drastically affect life, temperature, and water:
- Life: The absence of an atmosphere would make it impossible for most life forms to survive. There would be no breathable air for animals and humans, and plants would not have the carbon dioxide they need for photosynthesis. The ozone layer, which protects life from harmful ultraviolet (UV) radiation, would also be absent.
- Temperature: Without an atmosphere, there would be no greenhouse effect to trap heat. The Earth's surface temperature would fluctuate wildly between extreme hot during the day and extreme cold during the night, making it uninhabitable for most organisms.
- Water: Liquid water would likely not exist on the surface. The lack of atmospheric pressure would cause water to evaporate quickly into space. The absence of an atmosphere would also prevent the formation of clouds and rainfall, further disrupting the water cycle.
Q12Keep the curiosity alive
Discuss five examples of vegetative propagation.
Solution
Five examples of vegetative propagation are:
- Stem cuttings: Many plants, like roses and sugarcane, can be propagated by cutting a section of the stem and planting it in soil. The cutting develops roots and grows into a new plant.
- Rhizomes: Plants like ginger and turmeric have underground stems called rhizomes. These rhizomes can be divided and planted to produce new plants.
- Tubers: Potatoes have 'eyes' or buds on their surface. These tubers can be cut into pieces, each containing an eye, and planted to grow into new potato plants.
- Bulbs: Onions and garlic reproduce through bulbs, which are underground storage structures. The bulbs can be separated and planted to produce new plants.
- Runners: Strawberry plants produce runners, which are horizontal stems that grow along the ground. At certain points, the runners develop roots and shoots, forming new plantlets that can be separated from the parent plant.