Class 8 Science Chapter 13 Our Home : Earth, a Unique Life Sustaining Planet (New Course)

Key Concept: Atmosphere, Oxygen, Ozone Layer

Correct Answer option (a) Increase in skin-related health issues among living organisms.
[Solution Description] The ozone layer plays a crucial role by absorbing harmful ultraviolet (UV) radiation from the Sun, thus protecting living organisms from potential cellular damage caused by UV exposure. Without ozone, there would be an increase in UV radiation reaching the Earth’s surface, leading to heightened risks of skin cancer and other health issues for humans and animals, along with detrimental effects on plant life, which rely on specific light wavelengths for photosynthesis. Ecosystems could face severe disruptions due to these factors.

Key Concept: Interdependence of Plants and Animals

c) Plants supply oxygen and food; animals assist in pollination and seed dispersal
[Solution Description] In an ecosystem, plants and animals exhibit a mutual dependence. Plants provide oxygen and food for animals through the process of photosynthesis. In return, animals contribute to the life cycle of plants by assisting in pollination, seed dispersal, and providing carbon dioxide required for photosynthesis through respiration. This interdependence maintains ecological balance.

Key Concept: Zygote Formation

d) In water, where fertilisation occurred
[Solution Description]
In aquatic animals like fish and frogs, both the sperm and eggs are released into the water where fertilisation occurs. Once fertilisation takes place, the zygote develops into an embryo and continues its development in the same water environment until it matures further.

Key Concept: Concept: Regeneration

c) Planaria
[Solution Description]
Planaria, a type of flatworm, has remarkable regenerative abilities. If a planarian is cut into pieces, each piece can potentially grow back into a whole organism, illustrating its powerful regeneration capability.

Key Concept: Examples of Vegetative Propagation

d) Seed germination
[Solution Description]
Examples of vegetative propagation include stem cutting of a money plant, using the eyes of a sprouted potato, and a piece of ginger. Seed germination does not involve vegetative propagation as it is a sexual reproduction process involving seeds.

Key Concept: Magnetic Field Origin, Cosmic Rays Interaction

b) Electrical currents generated by movements in the outer core deflecting charged particles
[Solution Description] The Earth’s magnetic field is believed to be generated by the dynamo effect, which involves the movement of molten iron within the Earth’s outer core. This motion produces electric currents, which in turn generate magnetic fields. These fields combine to create a dipole magnetic field that extends into space. As cosmic rays, which are high-energy particles from outer space, approach Earth, they encounter this magnetic field. Due to their charge, these particles experience a force perpendicular to both their velocity and the magnetic field, causing them to spiral along the magnetic field lines. Many of these particles are deflected away from Earth, thus protecting the planet from harmful radiation.

Key Concept: Atmospheric pressure on Mars compared to Earth

d) Mars has an atmosphere that is 100 times thinner than Earth’s
[Solution Description]
Mars’ atmosphere is significantly thinner than Earth’s. It is approximately 100 times less dense than Earth’s atmosphere, which means it exerts much less pressure on the surface of Mars than what we experience here on Earth.

Key Concept: Climate Change Impact

c) Increased social media usage
[Solution Description] Climate change directly results in altered crop growth patterns, impacts on water supply, changes in wildlife habitats, and affects human health due to temperature extremes. Increased social media usage is not a direct result of climate change.

Key Concept: Gravity and Atmospheric Retention, Earth’s Size

c) Its gravity would double, thus holding gases more effectively.
[Solution Description]
The gravitational force $F_g$ on the surface of a planet can be given by Newton’s law of gravitation:
$F_g = \frac{G M m}{r^2}$
where $G$ is the gravitational constant, $M$ is the mass of the planet, $m$ is the mass of the object being attracted, and $r$ is the radius of the planet.
For a sphere of uniform density $\rho$, the mass $M$ is:
$M = \frac{4}{3} \pi r^3 \rho$
Substituting for $M$ in the formula for gravity:
$F_g = \frac{G \left(\frac{4}{3} \pi r^3 \rho\right) m}{r^2} = \frac{4 \pi G \rho m r}{3}$
If the radius doubles, i.e., $r’ = 2r$, and density remains the same:
$F_{g’} = \frac{4 \pi G \rho m (2r)}{3} = \frac{8 \pi G \rho m r}{3} = 2 F_g$
Thus, the gravity on the surface of the new planet would be twice that on Earth. Stronger gravity means it can hold onto an atmosphere more effectively than Earth can.

Key Concept: Earth’s size, Stable atmosphere, Gravity

b) The current size ensures gravity strong enough to retain essential gases
[Solution Description] The Earth’s gravity is directly related to its mass and size. If Earth were smaller, it would have less gravitational force, causing atmospheric gases to escape into space, as observed in Mars. Conversely, if Earth were larger, the increased gravity could compress or destabilize the atmosphere. Therefore, Earth’s size balances these forces perfectly, allowing it to retain a life-sustaining atmosphere.

Key Concept: Concept of Reproductive Cells in Plants

b) Anther
[Solution Description] In flowering plants, the pollen grains found inside the anther of a flower contain the male gametes.

Key Concept: Geodiversity and Its Importance

d) By creating varied habitats with numerous ecological niches
[Solution Description] Geodiversity refers to the variety of landforms, rocks, soils, etc., on Earth, along with the processes that shape them. This diversity helps create unique habitats where different types of life can thrive by providing various environmental conditions necessary for diverse ecosystems.

Key Concept: Pollution, Ecosystem Impact, Biodiversity Loss

Correct answer option d) Pollution $\rightarrow$ Ecosystem disturbance $\rightarrow$ Resource scarcity $\rightarrow$ Biodiversity loss
[Solution Description] Pollution from industrial waste, agricultural runoff, and plastic debris influences terrestrial and aquatic ecosystems. Pollutants can lead to soil degradation and water contamination. For instance, excess fertilizers in water bodies can cause algal blooms, depleting oxygen levels, and harming aquatic life. On land, soil pollution reduces crop yields, impacting food supply for herbivores. Consequently, predators depending on these herbivores face starvation. This disruption extends through the food chain, reducing biodiversity. Therefore, pollution triggers a cascade starting with ecosystem disturbance, leading to resource scarcity, and culminating in biodiversity loss.

Key Concept: Concept of Greenhouse Gases and Global Warming

c) Carbon Dioxide
[Solution Description] Burning fossil fuels releases several greenhouse gases, with carbon dioxide ($CO_2$) being a major contributor. Carbon dioxide is known for its ability to trap heat in the Earth’s atmosphere, leading to an increase in global temperatures, which is a significant factor in global warming.

Key Concept: Genetic Material Transmission

c) Carry half of the parent’s genetic material
[Solution Description] Gametes are special cells used during sexual reproduction that carry half of the parent’s genetic material. When male and female gametes join, they form a new cell with a complete set of instructions — half from each parent — ensuring genetic diversity and continuity of life.

Key Concept: Role of Decomposers in Ecosystem Balance

b) They break down dead material and recycle nutrients.
[Solution Description]
Decomposers, such as bacteria and fungi, break down dead organisms and organic waste materials into simpler substances. This process returns essential nutrients to the soil, making them available for plant uptake. By recycling nutrients, decomposers ensure that nutrient cycles continue, supporting plant growth and sustaining life across ecosystems.
Furthermore, decomposers help reduce waste accumulation in nature by breaking it down into manageable components. Their role is crucial for nutrient cycling and ecosystem productivity.

Key Concept: Magnetic field of the Earth, Position of the Earth

Correct Answer option b) Deflects harmful particles and radiation
[Solution Description]
Earth’s magnetic field deflects high-energy particles from space, such as cosmic rays and solar winds, which could strip away the atmosphere or increase harmful UV radiation on the surface. Earth’s favorable position also keeps it within a safe range where these fields can effectively protect the planet from extensive atmospheric erosion and biological damage.

Key Concept: Differences in Temperature and Atmospheric Conditions

c) Venus
[Solution Description] Venus has the highest average temperature among all planets because its thick atmosphere, primarily composed of carbon dioxide, traps heat effectively through a process known as the greenhouse effect. This trapping of heat makes it even hotter than Mercury.

Key Concept: Air, water, and sunlight

c) It supplies the energy required for chemical reactions
[Solution Description]
Sunlight is a crucial component of photosynthesis. In this process, plants use light energy from the sun to convert carbon dioxide and water into glucose and oxygen. Specifically, sunlight provides the energy needed to drive the chemical reactions that transform these substances. Without sunlight, the energy necessary for these reactions would not be available, thereby halting photosynthesis.

Key Concept: Pollution

b) Carpooling and using public transport
[Solution Description] Air pollution from vehicles is a significant environmental problem. To reduce this pollution, using cleaner energy sources for transportation is essential. One effective way is by reducing the use of fossil-fuel-based vehicles and opting for public transportation, carpooling, or electric vehicles. Such actions help lower emissions and improve air quality.

Key Concept: Role of Magnetic Field in Life Sustainability

c) It deflects harmful particles from space.
[Solution Description] Earth’s magnetic field contributes to life’s sustainability by acting as a protective barrier against the solar wind and cosmic rays from space, which are high-energy particles. By deflecting these particles, the magnetic field prevents them from stripping away the atmosphere or damaging the ozone layer. A stable atmosphere with an intact ozone layer limits harmful ultraviolet radiation reaching the surface, thus facilitating conditions suitable for life to flourish.

Key Concept: Planetary Temperature Comparison

d) Neptune
[Solution Description] According to Table 13.2, Neptune has the lowest average temperature at -200°C. The given temperatures for each planet indicate that as the distance from the Sun increases, generally, the temperature decreases. Among all the planets listed, Neptune is the farthest and hence has the coldest temperature.

Key Concept: Size of the Earth, Atmospheric Retention

c) Atmosphere might escape, hindering life sustainability
[Solution Description]
If Earth were significantly smaller while maintaining the same density, its gravitational pull would be weaker. This weaker gravity would have difficulty retaining the atmospheric gases needed for life. A thin atmosphere would mean insufficient protection from harmful solar radiation and ineffective climate regulation. Furthermore, without enough atmospheric pressure, liquid water cannot remain stable, hampering the development and sustenance of life.

Key Concept: Zygote formation

c) A zygote is formed
[Solution Description] The immediate result of the fusion of male and female gametes (sperm and egg) is the formation of a zygote. This zygote contains a full complement of chromosomes, half contributed by each parent.

Key Concept: Pollution Impact

c) Causes respiratory problems
[Solution Description] Air pollution, caused by emissions from factories, vehicles, and other sources, introduces harmful substances into the atmosphere. These pollutants can cause respiratory issues in humans, such as asthma or other breathing difficulties, due to their inhalation and interaction with lung tissues.

Key Concept: Earth’s Surface and Climate

a) It allows Earth to remain within the habitable zone
[Solution Description] The Earth’s nearly circular orbit keeps it at a constant distance from the Sun throughout the year. This constancy in distance ensures that Earth experiences relatively stable temperatures, avoiding extreme cold or heat that would be inhospitable for life. As a result, this stable climate facilitates the consistent presence of liquid water, which is essential for sustaining life.

Key Concept: Asexual reproduction via regeneration

c) Regeneration
[Solution Description] Planaria have the remarkable ability to regenerate lost body parts. This regenerative capability allows them to reform entire bodies from fragments, thus serving as a mechanism for asexual reproduction.

Key Concept: Earth’s Magnetic Field, Protection from Cosmic Rays

c) Assertion is true, but Reason is false.
[Solution Description]
The assertion is true as Earth’s magnetic field does indeed protect us from harmful cosmic rays and solar wind by deflecting these charged particles away from the planet. This process helps maintain the stability of Earth’s atmosphere and protects life on Earth from potentially dangerous radiation.
However, the reason given is false. The deflection of charged particles occurs not because of Earth’s gravitational pull but due to electromagnetic forces. Charged particles interact with the magnetic field, and their paths are altered by the Lorentz force, which acts perpendicularly to both the magnetic field and the particle’s velocity. Thus, while the assertion is correct, the reason provided is incorrect.
Therefore, the correct answer is: Assertion is true, but Reason is false.

Key Concept: Climate Change

Correct Answer option a) Carbon dioxide
[Solution Description] When fossil fuels like coal and oil are burned, they release greenhouse gases, primarily carbon dioxide, into the atmosphere. This contributes significantly to global warming.

Key Concept: Special cells for reproduction

d) Carrying half of the parent’s genetic material
[Solution Description] In sexual reproduction, gametes are specialized reproductive cells that carry half of the parent’s genetic material. This ensures that when male and female gametes unite during fertilization, the resulting cell has a complete set of genetic instructions—half from each parent. This mechanism prevents the doubling of genetic material across generations.

Key Concept: Zygote Development

b) Either inside or outside the parent’s body
[Solution Description] In animals, the development of a zygote into an embryo can occur either inside the body (internal development) or outside the body (external development), depending on the species.

Key Concept: Overview of planets: size and composition

c) Jupiter
[Solution Description]
Jupiter, Saturn, Uranus, and Neptune are known as gas giants and are large planets mostly composed of gases. Among these, Jupiter is the largest planet in our solar system. It is primarily composed of hydrogen and helium, making it a gas giant.

Key Concept: Impact of Ozone Layer Depletion

c) Increased exposure to ultraviolet rays causing health issues
[Solution Description] Significant depletion of the ozone layer would result in more ultraviolet rays reaching Earth’s surface. This increase in UV radiation exposure could lead to higher rates of skin cancer, eye cataracts, and other health issues among humans and wildlife, as well as affecting ecosystems and food chains.

Key Concept: Pollination and Fertilisation

a) They carry pollen from the anther of one flower to the stigma of another.
[Solution Description] Insects assist in the transfer of pollen from the male part of one flower to the female part of another flower. This is crucial for fertilisation because it allows the sperm cells within the pollen to reach the ovules in the other flower. Without this transfer, fertilisation may not occur, and seeds would not form.

Key Concept: Photosynthesis and Air

d) They release oxygen during photosynthesis
[Solution Description] Plants take carbon dioxide from the air and water from the soil, using sunlight to perform photosynthesis. During this process, they produce glucose and release oxygen as a byproduct. This oxygen is then added to the atmosphere, crucial for the respiration of most living organisms.

Key Concept: Photosynthesis Process

b) Carbon dioxide
[Solution Description] Plants use carbon dioxide from the atmosphere during photosynthesis. In the presence of sunlight, carbon dioxide and water are converted into glucose and oxygen by plants.

Key Concept: Atmospheric Composition and Oxygen’s Role, Ozone Layer Formation

d) Preventing excess oxygen loss to maintain balance for ozone creation
[Solution Description] The Earth’s specific size ensures an appropriate level of gravity, which retains the necessary gases, including oxygen. This retention allows some of the oxygen molecules ($O_2$) to convert into ozone ($O_3$) through photochemical reactions driven by solar UV radiation. The ozone layer forms as these $O_3$ molecules accumulate at high altitudes, creating a protective shield that absorbs harmful ultraviolet rays from the sun, thus making Earth habitable.

Key Concept: Habitable Zone

b) It is positioned such that it never experiences extreme temperatures
[Solution Description] The Earth is located in what is known as the habitable zone or Goldilocks zone, which is the perfect distance from the Sun. This unique positioning means that the Earth receives just the right amount of heat and light, allowing water to exist in its liquid state. Liquid water is crucial for all known forms of life, making this feature essential for supporting life on Earth.

Key Concept: Temperature Variations

a) Greater distance from the Sun
[Solution Description] Neptune is farther away from the Sun compared to Jupiter, which results in it receiving less solar energy. This greater distance leads Neptune to be one of the coldest planets in the solar system. While other factors such as internal heat and atmospheric composition play roles, the primary reason for its coldness is its distance from the Sun.

Key Concept: Habitable Zone, Liquid Water

Correct Answer option b) No, the temperatures are too low for liquid water.
[Solution Description]
A planet located at twice the Earth-Sun distance ($2$ AU) would receive significantly less solar energy compared to Earth. According to the inverse square law, the energy received is inversely proportional to the square of the distance from the Sun.
Calculation:
Energy received at $2$ AU = $\frac{1}{{2}^2}$ = $\frac{1}{4}$ or $25\%$ of the energy Earth receives.
With such reduced solar irradiation, it’s highly probable that any water present on the planet’s surface would freeze, given the lower temperatures expected at that distance.
Therefore, under similar atmospheric conditions to Earth, liquid water is unlikely to exist on the surface due to insufficient warmth to prevent freezing.

Key Concept: Atmospheric Characteristics of Planets

c) Mercury
[Solution Description] Among the planets listed, Mercury does not have a substantial atmosphere. Its proximity to the Sun and low gravity contributes to this lack of atmosphere, as any gases that might form an atmosphere are quickly lost to space.

Key Concept: Comparison of Earth’s crust to an apple’s skin

d) The skin of the apple
[Solution Description] The statement compares Earth’s crust to an apple’s skin. The crust is the thin outermost layer of the Earth where all terrestrial life resides. This comparison highlights how relatively thin the crust is in relation to the entire size of the planet.