19.3 Nutrient Cycles Revision Notes
1. Overview
Nutrient cycles describe how essential elements like carbon and nitrogen move between the biotic (living) and abiotic (non-living) parts of an ecosystem. Because the Earth is a closed system with a finite supply of matter, these cycles are vital for recycling atoms so they can be reused by new generations of organisms to build cells and fuel life processes.
Key Definitions
- Decomposition: The process by which microorganisms (decomposers) break down dead organic matter and waste, releasing nutrients back into the soil or atmosphere.
- Fossilization: The process where dead organisms are preserved over millions of years under specific conditions (high pressure and heat) to form fossil fuels like coal, oil, and gas.
- Combustion: The chemical process of burning a fuel in the presence of oxygen, releasing energy and carbon dioxide.
- Nitrogen Fixation: The conversion of inert atmospheric nitrogen gas ($N_2$) into nitrogen-containing compounds (like ammonia or nitrates) that plants can use.
- Nitrification: The biological conversion of ammonium ions into nitrites and then into nitrates by specialized bacteria.
- Denitrification: The process where specialized bacteria convert nitrates in the soil back into nitrogen gas, which is released into the atmosphere.
- Deamination: The removal of the nitrogen-containing part of amino acids to form urea.
Core Content: The Carbon Cycle
The carbon cycle involves the movement of carbon through the atmosphere, living organisms, and the earth.
Main Processes in the Carbon Cycle:
- Photosynthesis: Green plants and algae "fix" carbon by taking $CO_2$ from the atmosphere and converting it into glucose ($C_6H_{12}O_6$). This is the only process that removes $CO_2$ from the air.
- Respiration: All living organisms (plants, animals, and decomposers) release energy from food, which produces $CO_2$ as a waste product, returning it to the atmosphere.
- Feeding: Carbon is transferred through food chains. When an animal eats a plant, the carbon compounds are digested and used to build the animal's tissues.
- Decomposition: When organisms die or produce waste (feces/urine), decomposers (fungi and bacteria) break down the organic molecules. As they do this, they respire, releasing $CO_2$ back into the air.
- Formation of Fossil Fuels: If dead organisms do not decay fully (e.g., in acidic bogs or deep ocean sediments), they may be buried. Over millions of years, they form coal, oil, or natural gas.
- Combustion: When we burn wood or fossil fuels, the stored carbon reacts with oxygen to release $CO_2$ back into the atmosphere.
Extended Content (Extended Curriculum Only)
The nitrogen cycle is more complex because nitrogen gas ($N_2$) in the air is very unreactive and cannot be used directly by most living things.
The Nitrogen Cycle Step-by-Step:
- Nitrogen Fixation: $N_2$ gas is converted into usable forms.
- Lightning: Provides enough energy to react $N_2$ with $O_2$ to form nitrogen oxides that dissolve in rain.
- Nitrogen-fixing bacteria: Found in soil or in root nodules of legumes (e.g., peas/beans), they convert $N_2$ gas into ammonium ions.
- Absorption by Plants: Plants take up nitrate ions ($NO_3^-$) from the soil through root hair cells via active transport.
- Production of Proteins: Plants use the nitrogen from nitrates to make amino acids, which are then folded into proteins.
- Feeding and Digestion: Animals eat plants, digest the plant proteins into amino acids, and then use those amino acids to build their own animal proteins.
- Deamination: If animals have excess amino acids, the liver removes the nitrogen group (deamination) to form urea, which is excreted.
- Decomposition: When organisms die or excrete waste, decomposers break down proteins and urea into ammonium ions.
- Nitrification: Nitrifying bacteria convert ammonium ions into nitrites, and then into nitrates. This requires aerobic conditions (oxygen).
- Denitrification: Denitrifying bacteria (found in waterlogged, anaerobic soils) convert nitrates back into nitrogen gas, reducing soil fertility.
Roles of Microorganisms:
- Decomposers: Break down protein/urea $\rightarrow$ ammonium ions.
- Nitrifying bacteria: Convert ammonium ions $\rightarrow$ nitrates.
- Nitrogen-fixing bacteria: Convert $N_2$ gas $\rightarrow$ ammonium/nitrogen compounds.
- Denitrifying bacteria: Convert nitrates $\rightarrow$ $N_2$ gas.
Key Equations
| Process | Equation |
|---|---|
| Photosynthesis | $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$ |
| Aerobic Respiration | $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$ |
| Combustion (Generic) | $Fuel + O_2 \rightarrow CO_2 + H_2O$ |
- Symbols: $CO_2$ (Carbon Dioxide), $H_2O$ (Water), $C_6H_{12}O_6$ (Glucose), $O_2$ (Oxygen).
Common Mistakes to Avoid
- ❌ Wrong: Thinking plants only perform photosynthesis.
- ✅ Right: Plants perform both photosynthesis and respiration. They release $CO_2$ at night when they cannot photosynthesize.
- ❌ Wrong: Confusing nitrogen-fixing bacteria with nitrifying bacteria.
- ✅ Right: Nitrogen-fixing bacteria turn gas into solids; Nitrifying bacteria turn one soil solid (ammonium) into another (nitrate).
- ❌ Wrong: Saying "animals breathe out carbon."
- ✅ Right: Animals respire and exhale carbon dioxide.
Exam Tips
- Command Words: If asked to "Describe" the carbon cycle, list the processes. If asked to "Explain," you must say how the carbon moves (e.g., "Carbon is released as $CO_2$ via respiration").
- The "Decomposer" Trap: Exams often ask how carbon is returned to the atmosphere from dead leaves. Students forget that decomposers are living things that respire.
- Real-World Context: You may be asked why farmers "plough" their fields. The answer is to add oxygen to the soil to encourage nitrifying bacteria and discourage denitrifying bacteria (which hate oxygen).
- Typical Questions: Expect to label a diagram of the cycles or identify which group of bacteria is responsible for a specific arrow in the nitrogen cycle.