3.8.9Additional lessons
Nutrient Cycling — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 99 of 142
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Autotrophs and Heterotrophs
Photosynthesis
- 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light).
- Aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (energy released).
Photosynthesis
- Photosynthesis Producers take up carbon dioxide and release oxygen, and this is the source of the oxygen in Earth's atmosphere.
Aerobic respiration
- Aerobic respiration Organisms use oxygen to oxidise carbon compounds, releasing carbon dioxide that producers can reuse.
Linked cycles
- Linked cycles The waste product of each process is the raw material of the other, so gases are exchanged between producers and consumers.
Respiration in plants
- Respiration in plants Plants respire day and night;
- in light they photosynthesise faster than they respire, giving a net release of oxygen.
Balance
- Balance Over long periods the carbon dioxide used by producers is roughly balanced by that released by respiration, decay and combustion.
Part 2 of 4Recycling Minerals
Elements such as nitrogen and phosphorus pass from soil to producers, then to consumers, then back to the soil through decomposition.
1
Producers absorb mineral ions such as nitrate and phosphate from the soil and build them into proteins and nucleic acids.
2
Consumers obtain these compounds by feeding and then use them to make their own tissues.
3
Dead organisms and faeces are digested by saprotrophs, which release minerals back into the soil.
4
Producers absorb the released ions again, completing the cycle.
5
Removing crops or timber takes minerals out of the cycle, which is why fertilisers are needed in farming.
Part 3 of 4Open Energy, Closed Cycles
Energy and matter behave differently in ecosystems, and comparing them shows why cycling matters.
Open Energy, Closed Cycles
Energy flow
- Energy flow Energy enters as light, passes along food chains and leaves as heat, so it cannot be recycled.
Nutrient cycles
- Nutrient cycles Atoms of carbon, nitrogen and phosphorus are finite on Earth and are used repeatedly by successive organisms.
Decomposer limits
- Decomposer limits Cycling is faster in warm, moist and well-oxygenated conditions where decomposers are most active.
Self-sufficiency
- Self-sufficiency An ecosystem with efficient cycling needs little external nutrient input and is more stable.
Disruption
- Disruption Harvesting, deforestation or leaching removes nutrients faster than decomposers can replace them and lowers productivity.
| Feature | Energy | Nutrients |
|---|---|---|
| Source | Sunlight (or chemical oxidation) | Soil, water and air |
| Movement | One way, ending as heat | Repeatedly cycled |
| Role of decomposers | Release part as heat | Return minerals to the soil |
Part 4 of 4Exam Checklist & Tips
3.8.9 – Key Points for the Exam
- Nutrient cycling The repeated movement of mineral and element atoms between organisms and the abiotic environment.
- Saprotroph A decomposer that digests dead matter externally and releases nutrients.
- Mineral ion An inorganic ion, such as nitrate or phosphate, taken up from the soil by plants.
- Aerobic respiration Respiration that uses oxygen to release energy from organic molecules.
Common Mistakes
- Stating that energy is recycled; only matter is recycled.
- Saying plants only photosynthesise and never respire.
- Forgetting that decomposers return nutrients to the soil.
- Assuming ecosystems are closed to matter; they are open but largely self-contained.
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