Nutrient cycles

Paper 1Paper 2Paper 3Paper 4

The Carbon Cycle

The carbon cycle describes how carbon atoms move between the atmosphere, living organisms, and the Earth's crust. Carbon is the fundamental building block of all organic molecules (carbohydrates, lipids, proteins, nucleic acids). The cycle ensures that carbon is recycled rather than lost.

1. Photosynthesis (Removal of CO_2):
Producers (plants, algae) take in carbon dioxide (CO_2) from the atmosphere. Using light energy, they convert it into glucose and other organic compounds. This process removes carbon from the atmosphere.

2. Respiration (Release of CO_2):
All living organisms (producers, consumers, decomposers) perform respiration to release energy. During aerobic respiration, organic compounds are broken down with oxygen to produce carbon dioxide (CO_2), water, and energy. This process adds carbon back to the atmosphere.

3. Feeding / Nutrition (Transfer of Carbon):
Carbon moves between trophic levels through feeding. Herbivores eat plants, incorporating plant carbon into their own bodies. Carnivores eat herbivores. The carbon is transferred as organic molecules (carbohydrates, fats, proteins).

4. Decomposition / Decay:
When organisms die, decomposers (bacteria and fungi) break down their organic matter. They respire the organic carbon, releasing CO_2 back into the atmosphere. Some carbon remains in the soil as humus.

5. Formation of Fossil Fuels:
In anaerobic conditions (lack of oxygen), dead organisms do not fully decompose. Instead, they are compressed over millions of years to form fossil fuels (coal, oil, natural gas). The carbon is stored long-term in the Earth's crust.

6. Combustion / Burning:
When fossil fuels or biomass (wood) are burned, the organic carbon reacts with oxygen rapidly. This releases large amounts of CO_2 back into the atmosphere almost instantly.

Carbon Cycle Processes

Photosynthesis: The process by which green plants use sunlight to synthesize foods from carbon dioxide and water. It is the primary entry point of carbon into the biological cycle.

Respiration: The metabolic process where cells break down glucose to release energy, producing carbon dioxide as a waste product. Occurs in all living cells.

Combustion: The rapid chemical combination of a substance with oxygen, releasing heat and light. In the context of nutrient cycles, this refers to burning fossil fuels or biomass.

Describing Carbon Cycle Processes

Question: Describe two processes that add carbon dioxide to the atmosphere.

Model Answer:

Respiration: Living organisms break down organic molecules (like glucose) to release energy, producing carbon dioxide as a waste product which is released into the air.
Combustion: Burning fossil fuels or biomass causes the organic carbon stored in them to react with oxygen, releasing carbon dioxide gas into the atmosphere.

⚠︎ Carbon Cycle Confusion

Mistake: Confusing photosynthesis with respiration regarding carbon flow.
Correction: Photosynthesis removes CO_2 from the atmosphere (it is an uptake process). Respiration, decomposition, and combustion add CO_2 to the atmosphere (they are release processes).

Mistake: Thinking plants only photosynthesize and do not respire.
Correction: Plants perform both photosynthesis and respiration. They photosynthesize during the day but respire continuously (day and night).

Describing Carbon Release

When to use: When asked to explain how carbon returns to the atmosphere.
Why examiners accept this: Examiners look for specific keywords linking the process to the release of gas. Simply saying 'burning' is often insufficient; you must specify combustion of fossil fuels or biomass to distinguish it from natural fires, and mention the production of carbon dioxide.
Example: Instead of 'Animals breathe out carbon,' write 'Respiration by animals releases carbon dioxide into the atmosphere.'

Carbon Cycle Practice

Name two processes that remove carbon dioxide from the atmosphere.

Photosynthesis; Absorption by oceans (or formation of fossil fuels if referring to long-term removal, but photosynthesis is the primary biological answer).

Explain how combustion affects the carbon cycle.

Combustion releases carbon that was stored in fossil fuels/biomass back into the atmosphere as carbon dioxide rapidly.

The Nitrogen Cycle

Nitrogen is essential for making amino acids (the building blocks of proteins) and nucleic acids. Although the atmosphere is 78% nitrogen gas (N_2), plants cannot use it directly because the triple bond between nitrogen atoms is very strong and stable. Nitrogen must be 'fixed' into reactive compounds like nitrates (NO_3^-).

1. Nitrogen Fixation (Atmosphere to Soil):
Nitrogen gas (N_2) is converted into nitrogen oxides or nitrates.

Lightning: The high energy of lightning causes nitrogen and oxygen in the air to react, forming nitrogen oxides. These dissolve in rainwater to form nitrate ions, which fall into the soil.
Bacteria (Nitrogen-fixing bacteria): Certain bacteria convert atmospheric N_2 directly into ammonia/ammonium ions (NH_4^+). These can be free-living in the soil or live symbiotically in root nodules of legumes.

2. Absorption by Plants:
Plants absorb nitrate ions (NO_3^-) from the soil through their roots. They use these nitrates to synthesize amino acids. These amino acids are then linked together to form proteins, which the plant uses for growth and repair.

3. Feeding and Digestion:
Animals eat plants (or other animals). The plant proteins are digested in the animal's gut and broken down into amino acids. The animal absorbs these amino acids and reassembles them to form animal proteins specific to its own body.

4. Deamination and Excretion:
Animals do not need all the amino acids they consume for protein synthesis. Excess amino acids undergo deamination in the liver. The nitrogen-containing part is removed and converted into urea (in mammals) or ammonia, which is excreted in urine/feces. This waste contains nitrogen compounds that return to the soil.

5. Decomposition:
Dead organisms and waste products (urine/feces) are decomposed by bacteria and fungi. These decomposers break down the organic nitrogen (proteins/urea) into ammonium ions (NH_4^+). This process is called ammonification.

6. Nitrification:
Nitrifying bacteria in the soil convert the ammonium ions (NH_4^+) from decomposition/excretion back into nitrate ions (NO_3^-). This makes the nitrogen available again for plants to absorb. There are two stages of nitrification involving different bacteria.

7. Denitrification:
In waterlogged (anaerobic) soils, denitrifying bacteria convert nitrate ions (NO_3^-) back into nitrogen gas (N_2), which is released into the atmosphere. This removes nitrogen from the soil ecosystem.

Key Nitrogen Terms

Nitrogen Fixation: The conversion of atmospheric nitrogen gas (N_2) into ammonia or nitrates by bacteria or lightning.

Deamination: The process in the liver where excess amino acids are broken down, removing the nitrogen-containing group (which becomes urea/ammonia).

Nitrification: The bacterial conversion of ammonium ions to nitrate ions in the soil.

Denitrification: The bacterial conversion of nitrate ions back to nitrogen gas in anaerobic conditions.

Describing Nitrogen Transfer

Question: Describe how nitrogen from the atmosphere ends up in animal muscle protein.

Model Answer:

Fixation: Nitrogen-fixing bacteria convert atmospheric N_2 into ammonium/nitrates in the soil.
Absorption: Plants absorb nitrate ions from the soil and use them to make amino acids, which are linked to form plant proteins.
Feeding: Animals eat the plants. The plant proteins are digested into amino acids.
Synthesis: The animal absorbs these amino acids and uses them to synthesize new animal proteins (e.g., muscle protein).

⚠︎ Nitrogen Cycle Confusion

Mistake: Thinking nitrogen-fixing bacteria remove nitrates from the soil.
Correction: Nitrogen-fixing bacteria take nitrogen gas (N_2) from the air and convert it into ammonium/nitrates. They do not use soil nitrates; they create them.

Mistake: Confusing Nitrification with Denitrification.
Correction:

Nitrification: Ammonium \rightarrow Nitrate (Makes nitrogen available to plants).
Denitrification: Nitrate \rightarrow Nitrogen Gas (Removes nitrogen from soil, returns it to air).

Mistake: Forgetting that plants make proteins.
Correction: Plants do not just 'absorb' nitrogen; they actively use nitrate ions to build amino acids and then proteins. This is a key distinction from animals who eat proteins.

Describing Nitrogen Fixation

When to use: When asked to explain how nitrogen enters the biological cycle.
Why examiners accept this: Examiners require the distinction between abiotic (lightning) and biotic (bacteria) fixation. For lightning, you must mention the reaction of nitrogen and oxygen due to high energy. For bacteria, you must specify they convert N_2 to ammonium/nitrates, not that they 'eat' nitrogen.
Example: 'Lightning provides energy for nitrogen and oxygen to react, forming nitrogen oxides which dissolve in rain as nitrates.'

Nitrogen Cycle Practice

State the role of decomposers in the nitrogen cycle.

Decomposers break down dead organic matter and waste (urea/feces) into ammonium ions.

Explain why denitrification is a problem for agriculture.

Denitrification converts useful nitrate ions in the soil back into nitrogen gas, which escapes to the atmosphere. This reduces the amount of nitrogen available for plants to grow, potentially reducing crop yields.