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Organisms and their Environment - Nutrient cycles (Carbon and Nitrogen)

Grade 11IGCSEBiology

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

🔑Concepts

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The Carbon Cycle: The movement of carbon between the atmosphere, biosphere, oceans, and geosphere. Key processes include photosynthesis, respiration, feeding, decomposition, fossilization, and combustion.

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Photosynthesis: Green plants and algae remove CO2CO_2 from the atmosphere to produce glucose (C6H12O6C_6H_{12}O_6) and O2O_2.

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Respiration: All living organisms release CO2CO_2 back into the atmosphere as a byproduct of breaking down glucose for energy (ATPATP).

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The Nitrogen Cycle: The process by which nitrogen is converted between its various chemical forms. N2N_2 gas makes up 78%78\% of the atmosphere but is unreactive and cannot be used directly by most organisms.

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Nitrogen Fixation: Conversion of atmospheric N2N_2 into ammonia (NH3NH_3) or ammonium ions (NH4+NH_4^+) by nitrogen-fixing bacteria (e.g., Rhizobium in root nodules) or lightning.

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Nitrification: The conversion of ammonium ions (NH4+NH_4^+) into nitrites (NO2−NO_2^-) and then into nitrates (NO3−NO_3^-) by nitrifying bacteria.

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Assimilation: Plants absorb nitrates (NO3−NO_3^-) from the soil to build proteins and DNA; animals obtain nitrogen by feeding on plants.

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Denitrification: The conversion of nitrates (NO3−NO_3^-) back into nitrogen gas (N2N_2) by denitrifying bacteria, usually in anaerobic (waterlogged) soil conditions.

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Decomposition: Decomposers (fungi and bacteria) break down dead organic matter and urea, releasing nitrogen as ammonium ions (NH4+NH_4^+) in a process called ammonification.

📐Formulae

6CO2+6H2O→light, chlorophyllC6H12O6+6O26CO_2 + 6H_2O \xrightarrow{\text{light, chlorophyll}} 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}

CH4+2O2→CO2+2H2O (Combustion example)\text{CH}_4 + 2O_2 \rightarrow CO_2 + 2H_2O \text{ (Combustion example)}

N2→NH3/NH4+ (Nitrogen Fixation)N_2 \rightarrow NH_3 / NH_4^+ \text{ (Nitrogen Fixation)}

NH4+→NO2−→NO3− (Nitrification)NH_4^+ \rightarrow NO_2^- \rightarrow NO_3^- \text{ (Nitrification)}

💡Examples

Problem 1:

Explain how the combustion of fossil fuels affects the concentration of CO2CO_2 in the atmosphere and its consequence on the environment.

Solution:

Combustion releases CO2CO_2 that was 'locked' underground for millions of years. The reaction is: Fossil Fuel+O2→CO2+H2O\text{Fossil Fuel} + O_2 \rightarrow CO_2 + H_2O. This increases atmospheric CO2CO_2 concentration.

Explanation:

Increased CO2CO_2 (a greenhouse gas) enhances the greenhouse effect, leading to global warming, rising sea levels, and climate change.

Problem 2:

A farmer notices that crops grow poorly in waterlogged soil. Identify the nitrogen cycle process responsible for this and name the ions lost.

Solution:

The process is denitrification. The ions lost are nitrates (NO3−NO_3^-).

Explanation:

In anaerobic conditions (like waterlogged soil), denitrifying bacteria convert nitrates (NO3−NO_3^-) into nitrogen gas (N2N_2), which escapes into the atmosphere, reducing soil fertility.

Problem 3:

Identify the role of Rhizobium bacteria in the nitrogen cycle and where they are typically found.

Solution:

They perform nitrogen fixation, converting N2N_2 to NH3NH_3. They are found in the root nodules of leguminous plants.

Explanation:

These bacteria have a mutualistic relationship with legumes (like peas and beans), providing the plant with usable nitrogen in exchange for carbohydrates.

Nutrient cycles (Carbon and Nitrogen) Revision - Grade 11 Biology IGCSE