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Earth as a System: Energy, Matter and Life - Biogeochemical Cycles

Grade 9CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Biogeochemical cycles represent the movement of nutrient elements like Carbon, Nitrogen, Oxygen, and Water through the biotic (biosphere) and abiotic (lithosphere, hydrosphere, atmosphere) components of Earth.

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The Water Cycle involves the continuous movement of water through evaporation, transpiration, condensation, and precipitation. Water acts as a universal solvent, transporting nutrients across different spheres.

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The Nitrogen Cycle is crucial because N2N_2 makes up 78%78\% of our atmosphere, but plants cannot use it directly. It must be converted into nitrates (NO3−NO_3^-) or nitrites (NO2−NO_2^-) through Nitrogen Fixation by bacteria like RhizobiumRhizobium.

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The Carbon Cycle maintains the balance of CO2CO_2 in the atmosphere. Carbon is fixed via photosynthesis into glucose (C6H12O6C_6H_{12}O_6) and released back through respiration, decomposition, and combustion of fossil fuels.

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The Oxygen Cycle is driven by living organisms. Oxygen (O2O_2) is used in respiration and combustion, and it is replenished in the atmosphere primarily through the process of photosynthesis.

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The Greenhouse Effect is the trapping of heat by gases like CO2CO_2 and CH4CH_4. While necessary for life, an increase in these gases leads to Global Warming.

📐Formulae

6CO2+6H2O+Sunlight→C6H12O6+6O26CO_2 + 6H_2O + \text{Sunlight} \rightarrow C_6H_{12}O_6 + 6O_2

C6H12O6+6O2→6CO2+6H2O+EnergyC_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}

N2→Nitrogen FixationNH3/NO3−N_2 \xrightarrow{\text{Nitrogen Fixation}} NH_3 / NO_3^-

C+O2→CO2+HeatC + O_2 \rightarrow CO_2 + \text{Heat}

💡Examples

Problem 1:

Calculate the approximate percentage of Nitrogen and Oxygen combined in the Earth's atmosphere.

Solution:

Combined percentage ≈99%\approx 99\%

Explanation:

Nitrogen (N2N_2) constitutes approximately 78%78\% and Oxygen (O2O_2) constitutes approximately 21%21\% of the atmosphere. Using simple addition: 78%+21%=99%78\% + 21\% = 99\%

Problem 2:

Explain the role of Rhizobium in the Nitrogen cycle.

Solution:

Converts atmospheric N2N_2 into usable nitrates (NO3−NO_3^-).

Explanation:

RhizobiumRhizobium bacteria live in the root nodules of leguminous plants. They perform biological nitrogen fixation, breaking the strong triple bond of N≡NN \equiv N to form nitrogenous compounds that plants can absorb for protein synthesis.

Problem 3:

What happens to the Carbon cycle if there is massive deforestation?

Solution:

Increase in atmospheric CO2CO_2 levels.

Explanation:

Plants act as 'Carbon Sinks' by absorbing CO2CO_2 for photosynthesis. If trees are removed, the rate of CO2CO_2 absorption decreases while respiration and combustion continue, leading to an imbalance in the formula 6CO2+6H2O→C6H12O6+6O26CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2 resulting in enhanced greenhouse effect.