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.

A flowchart showing the Atmosphere ($CO_2$) at the top. Arrows point AWAY from the atmosphere labele — A flowchart showing the Atmosphere ($CO_2$) at the top. Arrows point AWAY from t...

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.