CIE iGCSE Biology-19.3 Nutrient cycles- Study Notes- New Syllabus
CIE iGCSE Biology-19.3 Nutrient cycles- Study Notes – New syllabus
CIE iGCSE Biology-19.3 Nutrient cycles- Study Notes -CIE iGCSE Biology – per latest Syllabus.
Key Concepts:
Core
- Describe the carbon cycle, limited to: photosynthesis, respiration, feeding, decomposition, formation of fossil fuels and combustion
Supplement
- Describe the nitrogen cycle with reference to:
• decomposition of plant and animal protein to ammonium ions
• nitrification
• nitrogen fixation by lightning and bacteria
• absorption of nitrate ions by plants
• production of amino acids and proteins
• feeding and digestion of proteins
• deamination
• denitrification - State the roles of microorganisms in the nitrogen cycle, limited to: decomposition, nitrification, nitrogen fixation and denitrification (generic names of individual bacteria, e.g. Rhizobium, are not required)
The Carbon Cycle
What Is the Carbon Cycle?
The carbon cycle is the natural circulation of carbon through the Earth’s ecosystems – between the air, plants, animals, and the ground.
It ensures carbon is recycled and reused by living organisms and the environment.
🔄 Key Processes in the Carbon Cycle:
1. Photosynthesis
- Green plants take in carbon dioxide (CO₂) from the air.
- They use sunlight to make glucose (food), storing carbon in their biomass.
- CO₂ → Glucose in plants
2. Feeding (Consumption)
- Animals eat plants or other animals.
- Carbon from plants is transferred into animals through food chains.
3. Respiration
- All living organisms (plants, animals, decomposers) release CO₂ during respiration.
- Glucose → CO₂ + energy (in cells)
4. Decomposition
- Decomposers like fungi and bacteria break down dead organisms.
- This releases CO₂ back into the atmosphere.
- Dead matter → CO₂
5. Formation of Fossil Fuels
- Some dead organisms are buried without decomposing and become fossil fuels over millions of years.
- Dead organisms → fossil fuels
6. Combustion (Burning)
- Humans burn fossil fuels, releasing stored carbon rapidly as CO₂.
- Fossil fuels + O₂ → CO₂ + energy
| Process | What Happens |
|---|---|
| Photosynthesis | Removes CO₂ from air, stores it in plants |
| Feeding | Passes carbon along food chains |
| Respiration | Returns CO₂ to air from living organisms |
| Decomposition | Returns CO₂ to air from dead organisms |
| Fossil fuel formation | Stores carbon underground over millions of years |
| Combustion | Releases stored CO₂ into air quickly |
📌 Final Thought:
The carbon cycle helps balance CO₂ levels in the atmosphere.
Human activities like burning fossil fuels can disrupt this balance, leading to climate change.
The Nitrogen Cycle
What Is the Nitrogen Cycle?
The nitrogen cycle is the natural process by which nitrogen moves between the air, soil, plants, animals, and decomposers to help make essential substances like proteins and DNA.
Nitrogen gas makes up about 78% of the air, but plants and animals can’t use it directly – it must be converted into other forms first.
🔄 Key Processes in the Nitrogen Cycle
1. Decomposition (Ammonification)
- Dead plants, animals, and waste products are broken down by bacteria and fungi.
- Proteins are converted into ammonium ions (NH₄⁺).
- Protein → Ammonium
2. Nitrification
- Nitrifying bacteria convert ammonium into nitrate in two steps:
- Ammonium (NH₄⁺) → Nitrites (NO₂⁻) → Nitrates (NO₃⁻)
- Plants can now absorb the nitrates.
3. Nitrogen Fixation
- Converts nitrogen gas (N₂) into useful compounds by:
- Lightning: Forms nitrates that dissolve in rain and enter the soil.
- Nitrogen-fixing bacteria: Found in soil or legume root nodules (e.g., peas, beans).
- N₂ → Ammonium or Nitrate
4. Absorption by Plants
- Plant roots absorb nitrate ions (NO₃⁻) from the soil to build amino acids and proteins.
5. Production of Proteins
- Plants use absorbed nitrates to make amino acids → proteins.
- These are used for growth and metabolism.
6. Feeding and Digestion
- Animals eat plants or other animals and digest proteins into amino acids, which are used to build animal proteins.
7. Deamination (in animals)
- Excess amino acids are broken down in the liver.
- Nitrogen is removed and converted into urea, which is excreted in urine.
8. Denitrification
- Denitrifying bacteria in waterlogged or low-oxygen soils convert nitrates (NO₃⁻) back into nitrogen gas (N₂), releasing it into the air.
| Process | What Happens |
|---|---|
| Decomposition | Proteins → Ammonium (NH₄⁺) |
| Nitrification | Ammonium → Nitrites → Nitrates |
| Nitrogen fixation | N₂ → Ammonium or Nitrate (via lightning or bacteria) |
| Absorption by plants | Nitrates → taken in by roots |
| Protein production | Plants convert nitrates → amino acids → proteins |
| Feeding & digestion | Animals eat plants/animals → digest proteins |
| Deamination | Excess amino acids → urea (excreted) |
| Denitrification | Nitrates → Nitrogen gas (returned to air) |
🧠 Final Thought:
The nitrogen cycle ensures nitrogen is constantly recycled for plant growth, protein formation, and ecosystem health.
Without it, plants couldn’t grow and animals couldn’t make the proteins they need to survive.
Roles of Microorganisms in the Nitrogen Cycle
Microorganisms (especially bacteria) play essential roles in converting nitrogen into forms that plants and animals can use.
Here are their roles in four key processes:
1️⃣ Decomposition (Ammonification)
- Microorganisms break down dead organisms and waste like urine and faeces.
- Proteins and urea are broken into ammonium ions (NH₄⁺).
- Microorganism role: Decomposers convert organic nitrogen into ammonium in the soil.
2️⃣ Nitrification
- A two-step process where microorganisms convert ammonium ions into nitrates:
- NH₄⁺ → NO₂⁻ → NO₃⁻
- Microorganism role: Nitrifying bacteria perform this oxidation to make nitrogen plant-available.
3️⃣ Nitrogen Fixation
- Certain microorganisms convert nitrogen gas (N₂) into ammonium or nitrate ions.
- This can happen in the soil or in root nodules of legumes.
- Microorganism role: Nitrogen-fixing bacteria trap atmospheric nitrogen and turn it into plant-usable forms.
4️⃣ Denitrification
- Microorganisms convert nitrates (NO₃⁻) back into nitrogen gas (N₂), especially in waterlogged soils.
- Microorganism role: Denitrifying bacteria remove nitrogen from the soil and return it to the atmosphere.
| Process | Microorganism Function |
|---|---|
| Decomposition | Break down proteins and urea into ammonium (NH₄⁺) |
| Nitrification | Convert ammonium → nitrite → nitrate (via nitrifying bacteria) |
| Nitrogen fixation | Convert nitrogen gas into ammonium/nitrate (via nitrogen-fixing bacteria) |
| Denitrification | Convert nitrate into nitrogen gas (via denitrifying bacteria) |
🧠 Summary:
Microorganisms are essential for driving the nitrogen cycle – they convert nitrogen into usable forms for plants and animals and return it to the atmosphere to keep the cycle going.