Class 9 - Chapter 13: Earth as a System: Energy, Matter, and Life

Exercise Solution

1. Choose the most appropriate option to describe the role of biogeochemical cycles in an ecosystem.

Answer: (ii) To recycle essential nutrients between biotic and abiotic components.

Biogeochemical cycles continuously circulate nutrients such as carbon, nitrogen, oxygen and water between living organisms and the physical environment. This recycling maintains ecological balance and ensures the availability of essential nutrients.

2. Which of the following is primarily responsible for warming of the Earth?

Answer: (iii) The Earth's surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases.

The greenhouse effect occurs when gases such as carbon dioxide, methane and water vapour trap outgoing infrared radiation, keeping the Earth warm enough to support life.

3. Explain how climate change affects the water cycle. Illustrate with examples.

Climate change alters temperature and rainfall patterns, affecting the water cycle. Higher temperatures increase evaporation rates, leading to more intense rainfall in some regions and droughts in others. Melting glaciers increase river flow initially but may reduce water availability in the future.

Examples:

• Increased flooding due to heavy rainfall.
• Longer drought periods in arid regions.
• Melting Himalayan glaciers affecting river systems.

4. Describe how albedo affects the Earth's surface temperature and its climate.

Albedo is the fraction of solar radiation reflected by a surface.

• High-albedo surfaces (snow, ice) reflect more sunlight and remain cooler.
• Low-albedo surfaces (oceans, forests) absorb more heat and become warmer.

A decrease in Earth's albedo due to melting ice increases heat absorption, leading to further warming and climate change.

5. How are mountain and valley breezes formed? Suppose there are two mountains, one covered with grass and another covered with bare rocks; would the temperature of the mountain breezes be different? If so, how?

Valley Breeze:

During the day, mountain slopes heat up faster than valleys. Warm air rises along the slopes, producing a valley breeze.

Mountain Breeze:

At night, mountain slopes cool rapidly. Cool dense air flows downward into valleys, producing a mountain breeze.

Yes, the temperatures would differ. Bare rocks heat and cool faster than grassy surfaces. Therefore, breezes from the rocky mountain would generally be warmer during the day and cooler at night compared to breezes from the grass-covered mountain.

6. 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?

The Troposphere is mainly responsible for weather phenomena.

This layer contains most of the Earth's air, water vapour and dust particles. Unequal heating of the Earth's surface creates differences in temperature and pressure, leading to winds, clouds, storms and rainfall.

7. Explain the processes involved in the nitrogen cycle. How would life on Earth be affected if nitrogen were not cycled?

Processes in the Nitrogen Cycle:

1. Nitrogen Fixation: Atmospheric nitrogen is converted into usable compounds by bacteria and lightning.
2. Nitrification: Ammonia is converted into nitrites and nitrates.
3. Assimilation: Plants absorb nitrates and use them to make proteins.
4. Ammonification: Decomposers convert dead organic matter into ammonia.
5. Denitrification: Bacteria convert nitrates back into atmospheric nitrogen.

Without nitrogen cycling, plants would not receive usable nitrogen compounds. Protein synthesis would stop, food chains would collapse, and life on Earth would become unsustainable.

8. What are the impacts of deforestation on the Earth's oxygen and carbon cycles? What are the other consequences of deforestation?

Effects on Oxygen Cycle:

• Reduced photosynthesis decreases oxygen production.

Effects on Carbon Cycle:

• Less carbon dioxide is absorbed by plants.
• Atmospheric CO₂ levels increase.
• Global warming intensifies.

Other Consequences:

• Loss of biodiversity.
• Soil erosion.
• Floods and droughts.
• Climate change.
• Destruction of wildlife habitats.

9. Explain with suitable diagram the path that carbon takes to go back to the atmosphere. You may start from plants using CO₂ from the atmosphere.

Carbon Cycle:

Atmospheric CO₂
       ↓
 Photosynthesis
       ↓
     Plants
       ↓
 Animals eat plants
       ↓
 Respiration
       ↓
Atmospheric CO₂

Dead Plants & Animals
       ↓
 Decomposition
       ↓
Atmospheric CO₂

Fossil Fuels
       ↓
 Combustion
       ↓
Atmospheric CO₂

Carbon returns to the atmosphere through respiration, decomposition and combustion of fossil fuels.

10. Why is an excess of CO₂ in the atmosphere considered undesirable even though it is required by plants?

Although plants require CO₂ for photosynthesis, excessive CO₂ strengthens the greenhouse effect, causing global warming, climate change, glacier melting, rising sea levels and disturbances in ecosystems.

11. How is heat lost from the surface of the Earth? What is its significance?

The Earth loses heat mainly by emitting infrared radiation into space.

Significance:

• Prevents excessive heating of Earth.
• Maintains energy balance.
• Helps regulate global climate and temperature.

12. If the Earth were a flat disc instead of a sphere, how would the patterns of solar radiation and temperature be different?

If Earth were flat:

• Sunlight would be distributed more uniformly.
• Temperature differences between regions would be smaller.
• Seasons and climatic zones would be greatly altered.
• Atmospheric circulation patterns would be very different.

The spherical shape of Earth is responsible for unequal heating, which drives weather and climate systems.

13. Suppose there is a rise in atmospheric temperature on Earth. How would this affect the cryosphere, hydrosphere and biosphere?

Cryosphere:

• Melting glaciers and polar ice caps.
• Reduction in snow cover.

Hydrosphere:

• Sea level rise.
• Changes in rainfall patterns.
• More floods and droughts.

Biosphere:

• Loss of biodiversity.
• Habitat destruction.
• Migration or extinction of species.
• Reduced agricultural productivity.

14. Explain how the Earth's atmosphere helps in maintaining suitable temperature for life to survive on the Earth.

The atmosphere acts like a protective blanket around Earth.

• It absorbs and retains some outgoing heat.
• Greenhouse gases prevent extreme cooling.
• It distributes heat through winds and weather systems.
• It protects Earth from harmful solar radiation.

As a result, Earth's temperature remains suitable for living organisms.

15. Describe the interrelationship between the different spheres of the Earth. Illustrate with example how these spheres function in a delicate balance.

The Earth consists of four major spheres:

• Atmosphere – air
• Hydrosphere – water
• Lithosphere – land
• Biosphere – living organisms

These spheres continuously interact with one another.

Example:

• Plants (biosphere) absorb water from the hydrosphere and minerals from the lithosphere.
• They use carbon dioxide from the atmosphere for photosynthesis.
• Plants release oxygen back into the atmosphere.
• Animals depend on plants and water for survival.

Any disturbance in one sphere affects the others. For example, deforestation (biosphere) increases atmospheric CO₂, alters rainfall patterns (hydrosphere), and causes soil erosion (lithosphere). Thus, all spheres work together in a delicate balance to sustain life on Earth.