Earth as a System: Energy, Matter, and LifeClass 9 Science NCERT Solutions
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Solution 1 of 9
Q1Revise, Reflect, Refine
Choose the most appropriate option to describe the role of biogeochemical cycles in an ecosystem.
(i)
To provide food directly to all organisms.
(ii)
To recycle essential nutrients between biotic and abiotic components.
(iii)
To create new elements for use by living things.
(iv)
To remove pollutants and toxins from the organism.
Solution
The most appropriate option is (ii) To recycle essential nutrients between biotic and abiotic components.
Explanation:
As stated in the chapter, biogeochemical cycles describe the continuous movement of matter and energy between the non-living (abiotic) and living (biotic) components of the Earth. This process ensures that essential nutrients like carbon, nitrogen, and oxygen are recycled and remain available to support life. These cycles do not create new elements but rather circulate existing ones.
Q2Revise, Reflect, Refine
Which of the following is primarily responsible for warming of the Earth?
(i)
Solar radiation is immediately absorbed by carbon dioxide, which then releases it as heat.
(ii)
The atmosphere's tiny particles absorb incoming solar radiation, which directly heats the Earth.
(iii)
The Earth's surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases.
(iv)
The Earth's environment is heated only by the solar radiation reflected by the clouds.
Solution
The correct option is (iii) The Earth's surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases.
Explanation:
The chapter explains that visible light from the Sun reaches and warms the Earth's surface. The surface then re-radiates this energy as infrared radiation (heat). Greenhouse gases in the atmosphere, such as carbon dioxide (), methane (), and water vapour, absorb a portion of this outgoing heat, preventing it from escaping into space. This process, known as the greenhouse effect, keeps the Earth warm enough to support life.
Q3Revise, Reflect, Refine
Explain how climate change affects the water cycle. Illustrate with examples.
Solution
Climate change, primarily driven by global warming, significantly affects the water cycle by altering the rates of evaporation, condensation, and precipitation. The chapter illustrates this with the following examples:
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Intensified Rainfall and Droughts: A warmer atmosphere can hold more moisture. This leads to increased evaporation from water bodies, which can result in heavier rainfall and more intense storms in some regions (like intensified monsoons in India), causing floods. Conversely, other areas may experience prolonged droughts as weather patterns shift.
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Melting of the Cryosphere: Rising temperatures accelerate the melting of glaciers and polar ice caps (the cryosphere). This initially increases the flow of water into rivers but, in the long run, contributes to rising sea levels, threatening coastal cities like Mumbai and Chennai.
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Reduced Groundwater Recharge: Sudden, intense rainfall leads to a higher proportion of water running off the surface into rivers, rather than infiltrating the ground. This increased runoff can cause soil erosion and reduces the recharging of groundwater aquifers, making it difficult to sustain agriculture, especially during dry seasons.
Q4Revise, Reflect, Refine
Describe how albedo affects the Earth's surface temperature and its climate.
Solution
Albedo is the fraction of solar radiation that is reflected by a surface. It plays a crucial role in determining the Earth's surface temperature and climate by influencing how much solar energy is absorbed versus reflected.
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High Albedo Surfaces: Surfaces with high albedo, such as snow and ice, are light-coloured and reflect a large proportion of incoming sunlight. As they absorb less energy, these surfaces remain cooler. The high albedo of polar regions contributes to their very cold climates. For example, the chapter states that snow has an albedo of 0.80-0.90, meaning it reflects 80-90% of sunlight.
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Low Albedo Surfaces: Surfaces with low albedo, such as ocean water and black soil, are dark-coloured and absorb a large proportion of incoming sunlight. This absorption of energy causes them to heat up more. For instance, oceans and forests have low albedo, absorb more heat, and thus have a warming effect on the local and global climate.
Therefore, changes in the Earth's surface cover, such as the melting of ice (replacing a high-albedo surface with a lower-albedo one like water), can lead to increased absorption of heat, creating a feedback loop that accelerates warming.
Q5Revise, Reflect, Refine
How are mountain and valley breezes formed? Suppose there are two mountains, one covered with grass and another covered with barren rocks; would the temperature of the two mountain breezes be different? If so, how?
Solution
Mountain and valley breezes are local winds caused by the uneven heating and cooling of mountain slopes and valley floors.
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Valley Breeze (Daytime): During the day, mountain slopes that face the sun heat up more rapidly than the valley floor. The air above the slopes becomes warm, less dense, and rises, creating a low-pressure area. To replace this rising air, cooler, denser air from the valley moves up the slopes. This upward flow of air is called a valley breeze.
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Mountain Breeze (Night-time): After sunset, the mountain slopes lose heat faster through radiation and become cooler than the valley floor. The air in contact with the slopes cools, becomes denser, and flows down into the valley under the influence of gravity. This downward flow of cool air is known as a mountain breeze.
Comparison of Two Mountains:
Yes, the temperature of the mountain breezes from the two mountains would be different.
- The mountain covered with barren rocks has a lower specific heat capacity and a lower albedo (if dark) compared to the grass-covered mountain. This means it will heat up more during the day and cool down much faster and to a lower temperature at night.
- The mountain covered with grass will retain some heat due to the moisture in the soil and vegetation, and thus will not cool as rapidly or as much as the barren rock.
Therefore, the mountain breeze flowing down from the barren rock mountain would be significantly cooler than the breeze from the grass-covered mountain because the rock surface would have reached a lower temperature at night.
Q6Revise, Reflect, Refine
You have witnessed weather phenomena, such as winds, storms, rainfall, etc. Which atmospheric layer is mainly responsible for such phenomena and what is the primary reason for its occurrence?
Solution
The atmospheric layer mainly responsible for weather phenomena such as winds, storms, and rainfall is the troposphere.
Primary Reason:
The primary reason for the occurrence of these phenomena is the uneven heating of the Earth's surface by the Sun. The troposphere is heated from below by the Earth's surface, which absorbs solar radiation. This heating is not uniform across the globe.
- Warm air near the surface becomes less dense and rises.
- As it rises, it cools, and the water vapour in it may condense to form clouds and precipitation (rain, snow).
- Cooler, denser air from higher altitudes or other regions sinks to take its place.
This continuous vertical movement of air (convection), combined with pressure differences caused by uneven heating, drives winds and creates the dynamic weather patterns we experience in the troposphere.
Q7Revise, Reflect, Refine
Explain the processes involved in the nitrogen cycle. How would life on Earth be affected if nitrogen were not cycled?
Solution
The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms, circulating between the atmosphere, terrestrial, and marine ecosystems. The main processes are:
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Nitrogen Fixation: The conversion of atmospheric nitrogen gas (), which is unusable by most organisms, into ammonia (). This is primarily done by nitrogen-fixing bacteria (e.g., Rhizobium in legume root nodules) and can also occur through lightning.
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Nitrification: Bacteria in the soil convert ammonia into nitrites () and then into nitrates (). Nitrates are the form of nitrogen most easily absorbed by plants.
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Assimilation: Plants absorb nitrates or ammonia from the soil and use them to build essential organic molecules like proteins and nucleic acids (DNA).
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Ammonification: When plants and animals die, or when animals excrete waste, decomposers (bacteria and fungi) break down the organic matter and release the nitrogen back into the soil as ammonia ().
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Denitrification: Denitrifying bacteria convert nitrates () back into atmospheric nitrogen gas (), completing the cycle.
Effect on Life if Nitrogen Were Not Cycled:
If nitrogen were not cycled, life on Earth would be severely affected, likely leading to a collapse of most ecosystems. Nitrogen is a fundamental component of proteins, which are essential for all biological structures and functions (e.g., enzymes, muscles), and nucleic acids (DNA and RNA), which carry genetic information. Without a continuous supply of usable nitrogen, plants would be unable to grow. This would lead to the starvation of herbivores that depend on plants, and subsequently, the carnivores that feed on them. The entire food web would break down, and life as we know it would cease to exist.
Q8Revise, Reflect, Refine
What are the impacts of deforestation on the Earth's oxygen and carbon cycles? What are the other consequences of deforestation?
Solution
Deforestation has significant impacts on the Earth's oxygen and carbon cycles.
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Impact on Carbon Cycle: Forests are major carbon sinks, meaning they absorb large amounts of carbon dioxide () from the atmosphere through photosynthesis. Deforestation, or the clearing of forests, reduces the Earth's capacity to absorb . Furthermore, when trees are burned or left to decay, the carbon stored in them is released back into the atmosphere as . This dual effect leads to an increase in atmospheric concentration, enhancing the greenhouse effect and contributing to global warming.
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Impact on Oxygen Cycle: Photosynthesis is the primary process that replenishes oxygen () in the atmosphere. By removing trees, deforestation reduces the rate of photosynthesis on a large scale, thereby decreasing the amount of oxygen being released into the atmosphere.
Other Consequences of Deforestation:
Besides disrupting the carbon and oxygen cycles, deforestation has several other negative consequences as mentioned in the chapter:
- Altered Water Cycle: Reduced transpiration from trees can lead to a decline in local rainfall and altered weather patterns.
- Increased Soil Erosion: Tree roots hold soil in place. Without them, soil is more easily eroded by wind and rain, leading to land degradation and sedimentation in rivers.
- Habitat Loss and Decline in Biodiversity: Forests are home to a vast number of species. Clearing them destroys habitats, leading to a loss of biodiversity as many species lose their homes and food sources.
- Altered Surface Albedo: The removal of a dark forest canopy and its replacement with lighter-coloured soil or grassland can change the surface albedo, affecting local and regional temperatures.
Q9Revise, Reflect, Refine
Explain with suitable diagram the path that carbon takes to go back to the atmosphere. You may start from plants using from the atmosphere.
Solution
The path of carbon from plants back to the atmosphere involves several key processes, as shown in the carbon cycle (refer to Fig. 13.13 in the textbook).
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Photosynthesis: Plants absorb carbon dioxide () from the atmosphere and use it to create organic compounds (like glucose) for energy and growth. The carbon is now part of the plant's biomass.
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Respiration: Plants themselves release some of this carbon back into the atmosphere as through the process of respiration.
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Consumption: Animals (herbivores) eat plants, transferring the carbon from the plants to their own bodies. Carnivores then eat these herbivores, moving the carbon further up the food chain. All these animals also release through respiration.
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Decomposition: When plants and animals die, decomposers (like bacteria and fungi) break down their organic matter. During this process, the decomposers respire, releasing the stored carbon back into the atmosphere as .
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Combustion: Over millions of years, some dead organic matter can be converted into fossil fuels (coal, oil, gas). When humans burn these fossil fuels for energy, the stored carbon is rapidly released into the atmosphere as .
Diagrammatic Path:
Atmosphere ()
\downarrow (Respiration) \rightarrowCO_2 (Consumption)
Animals (Organic Carbon)
\rightarrow Atmosphere ()
\downarrow (Decomposition/Respiration) \rightarrowCO_2 (Fossilization over millions of years)
Fossil Fuels
\rightarrow Atmosphere ()