Nutrient Cycles | Biology for GCSE/IGCSE - Year 11 PDF Download

The Carbon Cycle

• Nutrients like carbon and nitrogen are finite resources, not endlessly available.
• Due to their limited availability, these elements must be recycled to support the growth of new organisms.
• Plants absorb carbon dioxide from the atmosphere during photosynthesis.
• Animals acquire carbon by consuming plants (and microorganisms).
• Carbon is released back into the atmosphere as carbon dioxide through respiration by plants, animals, and microorganisms.
• In environments lacking decomposing microorganisms, deceased organisms can undergo transformation into fossil fuels over millions of years under significant pressure.
• Combustion of fossil fuels leads to the release of carbon dioxide into the atmosphere.
• Increased usage of fossil fuels contributes to rising levels of atmospheric carbon dioxide.
• Widespread deforestation diminishes the number of plants available to absorb carbon dioxide through photosynthesis.
• Deforestation, particularly for land conversion purposes rather than timber extraction, further exacerbates the problem by releasing additional carbon dioxide into the atmosphere through burning.

The Nitrogen Cycle

The nitrogen cycle encompasses the following stages:

• Nitrogen Fixation: Nitrogen fixation involves converting atmospheric nitrogen into forms usable by plants, carried out by nitrogen-fixing bacteria like Rhizobium.
• Nitrification: Nitrification is the process where ammonia is converted into nitrites and then nitrates by nitrifying bacteria such as Nitrosomonas and Nitrobacter.
• Assimilation: During assimilation, plants and animals absorb nitrates and ammonia to synthesize proteins and nucleic acids.
• Ammonification: Ammonification involves the decomposition of organic nitrogen compounds into ammonia by decomposers like bacteria and fungi.

These stages collectively highlight the intricate processes involved in the nitrogen cycle:

• Nitrogen and Protein Synthesis
  • Nitrogen is a crucial element necessary for the formation of proteins.
• Challenges in Nitrogen Absorption
  • Plants and animals face difficulty in absorbing nitrogen directly from the air due to the stability of N₂ gas.
  • The strong triple covalent bond between nitrogen atoms requires significant energy to break.
• Processes of Nitrogen Conversion
  • Nitrogen-fixing bacteria, present in soil and root nodules of specific plants like peas and beans, convert N₂ gas into nitrates, aiding absorption.
  • Lightning plays a role in fixing N₂ gas by breaking its bond, leading to the formation of nitrous oxides that dissolve in rainwater and enter the soil.
• Nitrogen Utilization by Plants and Animals
  • Plants absorb nitrates from the soil, utilizing the nitrogen to synthesize proteins.
  • Animals acquire the necessary nitrogen by consuming plants or other animals rich in proteins.
  • Animal waste, such as urine and feces, returns nitrogen to the soil in the form of ammonium compounds.
• Nitrogen Recycling Process
  • Upon death, plants and animals decay, releasing proteins as ammonium compounds back into the soil with the help of decomposers.
  • Nitrifying bacteria convert ammonium compounds into nitrites and then nitrates, facilitating plant absorption in a continuous cycle.
• Impact of Denitrifying Bacteria
  • Denitrifying bacteria, found in poorly aerated soil, reverse the process by converting nitrates back into N2 gas, reducing nitrogen availability.
  • Farmers can mitigate the presence of denitrifying bacteria through soil management practices like ploughing.

The Nitrogen Cycle

• Plants take in nitrates from the soil and utilize nitrogen to produce proteins.
• Animals acquire the necessary nitrogen by consuming plants or other animals which contain proteins.
• Animal waste, such as urine and feces, returns nitrogen to the soil in the form of ammonium compounds.
• Upon the death and decomposition of plants and animals, proteins are broken down into ammonium compounds by decomposers.
• Plants are unable to directly absorb ammonium compounds. Nitrifying bacteria convert these compounds into nitrites and then nitrates, which can be taken up by plants.
• Denitrifying bacteria found in poorly aerated soil extract nitrates from the soil and convert them back into N2 gas.
• Farmers can mitigate the presence of unhelpful denitrifying bacteria by ploughing and aerating the soil.

Additional Information

• The nitrogen cycle is crucial for the continuous recycling of nitrogen in ecosystems.
• Nitrogen gas makes up about 78% of Earth's atmosphere, but most organisms cannot use it directly.
• Nitrogen-fixing bacteria in the soil and root nodules of certain plants play a vital role in converting atmospheric nitrogen into forms usable by plants.
• Nitrogen is an essential component of amino acids, the building blocks of proteins, which are essential for the growth and development of all living organisms.

Significance of the Nitrogen Cycle

• The nitrogen cycle ensures that nitrogen is continuously available for the growth of plants, which form the base of the food chain.
• It helps maintain the balance of nitrogen in various ecosystems, preventing nutrient imbalances that could harm plant and animal life.