Nutrient Cycles and Populations
BiologyΒ· 19.3, 19.4Β· 25 min read
1. Core: Carbon Cycleβ β ββββ± 8 min
Nutrient Cycle
The continuous recycling of essential chemical elements between the living (biotic) and non-living (abiotic) parts of an ecosystem, ensuring elements are available for use by new organisms.
The one nutrient cycle Core candidates must know is the carbon cycle (syllabus 19.3.1). The nitrogen cycle is Extended-only content, covered in a later section. The carbon cycle moves carbon between the atmosphere, living organisms and long-term fossil stores:
Carbon dioxide is removed from the atmosphere by plants via photosynthesis, to make biological molecules
Carbon is passed along food chains when animals eat plants and other animals
All organisms respire, releasing carbon dioxide back to the atmosphere
Decomposers break down dead organisms and waste, releasing carbon dioxide via respiration
Combustion of fossil fuels releases stored carbon as carbon dioxide into the atmosphere
A student states that large-scale deforestation increases atmospheric carbon dioxide levels. Explain why this is true, with reference to the carbon cycle.
- 1
- Forest trees absorb large volumes of carbon dioxide for photosynthesis. Cutting them down removes this carbon sink, so less COβ is removed from the atmosphere.
- 2
- If cut trees are burned (for fuel or land clearance), combustion releases all carbon stored in the treeβs biomass as carbon dioxide directly into the atmosphere.
- 3
Combined, these two effects raise atmospheric COβ concentrations.
Exam tip:
For Core questions, always name the processes (photosynthesis, respiration, combustion, decomposition) when describing the carbon cycle to get full marks.
2. Core: Population Dynamicsβ β ββββ± 7 min
Population
A group of organisms of the same species, living in the same area at the same time, that can breed with each other to produce fertile offspring.
Population size changes based on the balance of birth rate, death rate, immigration (individuals joining the population) and emigration (individuals leaving the population). For Core exams (syllabus 19.4.5), you need to identify all four phases of the sigmoid (S-shaped) population growth curve for an organism growing in an environment with limited resources, such as lab-grown yeast.
Lag phase: Population grows slowly as individuals adapt to their new environment, with low reproduction rates
Exponential (log) phase: Population grows very rapidly as there are no limiting factors (plenty of food, space, no predators), birth rate far exceeds death rate
Stationary phase: Population size stays roughly constant as birth rate equals death rate, limited by resource scarcity or toxic waste build-up
Death phase: Population declines as resources are exhausted and toxic waste products accumulate, so death rate exceeds birth rate
A yeast population grown in a sealed glucose flask enters the stationary phase after 48 hours. Suggest two reasons for this.
- 1
- The glucose (food source) is being used up by the growing yeast population, so there is not enough food to support more new cells.
- 2
- Yeast produce toxic ethanol waste via anaerobic respiration, which builds up in the flask and kills new cells, keeping death rate equal to birth rate.
3. Extended Only: Nitrogen Cycleβ β β β βExtended onlyβ± 7 min
Extended candidates must learn the full nitrogen cycle, including the role of specialised bacteria. Nitrogen is essential for making proteins and DNA, but atmospheric nitrogen (Nβ) is inert and cannot be used directly by most organisms.
Nitrogen fixation: Nitrogen-fixing bacteria (in legume root nodules or free soil) convert atmospheric nitrogen into ammonia, usable by plants. Lightning also fixes small amounts of nitrogen.
Nitrification: Nitrifying bacteria in soil convert ammonia into nitrites, then into nitrates, easily absorbed by plant roots.
Assimilation: Plants absorb nitrates to make proteins; animals eat plants to obtain nitrogen for their own proteins.
Decomposition: Decomposers break down dead organisms and waste, releasing ammonia back into the soil.
Denitrification: Denitrifying bacteria in waterlogged, anaerobic soil convert nitrates back into nitrogen gas, returning it to the atmosphere.
Farmers add nitrogen fertiliser to wheat fields but rarely to bean (legume) fields. Explain why.
- 1
- Leguminous plants have root nodules containing nitrogen-fixing bacteria that convert atmospheric nitrogen into usable ammonia for the plant, so they do not require added fertiliser.
- 2
- Non-leguminous crops like wheat do not have these symbiotic bacteria, so they rely on soil nitrates to grow. Fertiliser replaces nitrates removed when crops are harvested, supporting higher yields.
Exam tip:
Always pair nitrogen cycle processes with the correct bacteria name (e.g. nitrogen fixation by nitrogen-fixing bacteria, not just 'bacteria') to get full marks on extended questions.
4. Extended Only: Limiting Factors & Population Regulationβ β β ββExtended onlyβ± 5 min
Extended candidates need to distinguish between biotic (living) and abiotic (non-living) limiting factors, and explain how they regulate natural population sizes, not just lab populations.
Abiotic limiting factors: Temperature, light intensity, water availability, soil pH, mineral ion content
Biotic limiting factors: Food availability, predation, competition for mates/space, disease, parasitism
In natural ecosystems, population sizes fluctuate around a carrying capacity: the maximum population size an environment can support indefinitely.
Rabbit populations in a woodland rise one year, followed by a rise in fox (predator) populations the next year. Explain how this interaction regulates both populations.
- 1
- High rabbit numbers provide plenty of food for foxes, so fox birth rate increases and death rate decreases, leading to a rise in fox population size.
- 2
- More foxes hunt and eat more rabbits, so rabbit death rate exceeds birth rate, leading to a fall in rabbit population size the following year.
- 3
- With fewer rabbits available as food, fox death rate rises, so fox numbers fall again, completing the cycle.
5. Common Pitfalls
Wrong move:
Stating plants only remove COβ from the atmosphere, and do not release it.
Why:
All living organisms including plants respire 24 hours a day, releasing COβ; plants only remove COβ during photosynthesis when light is available.
Correct move:
Always specify that plants both take in COβ via photosynthesis and release COβ via respiration in carbon cycle explanations.
Wrong move:
Claiming plants can absorb atmospheric nitrogen gas directly.
Why:
Atmospheric nitrogen is inert and cannot be used by plants; it must first be converted into nitrates or ammonia by bacteria or lightning.
Correct move:
Mention nitrogen fixation or nitrification steps when describing how plants obtain nitrogen for protein synthesis.
Wrong move:
Confusing decomposers with specialised nitrogen cycle bacteria.
Why:
Decomposers (fungi and bacteria) break down dead organic matter to release ammonia, but do not carry out nitrification, denitrification or nitrogen fixation.
Correct move:
Distinguish decomposers from specialised nitrifying, denitrifying and nitrogen-fixing bacteria in your answers.
Wrong move:
Stating exponential population growth continues indefinitely.
Why:
All environments have limited resources, so limiting factors will eventually slow growth and lead to the stationary phase, or a population crash if resources are exhausted.
Correct move:
Specify that exponential growth only occurs in the short term under ideal lab conditions with no limiting factors.
Wrong move:
Omitting fossil fuel combustion as a source of atmospheric COβ in the carbon cycle.
Why:
Exam questions often link carbon cycle content to climate change, so naming this human impact is required for full marks in extended response questions.
Correct move:
Include both natural (respiration, decomposition) and human (combustion) sources of COβ when describing atmospheric carbon inputs.
6. Quick Reference Cheatsheet
Concept | Core Required Knowledge | Extended Additional Knowledge |
|---|---|---|
Carbon Cycle | Photosynthesis, respiration, feeding, decomposition, formation of fossil fuels, combustion | No extra content required (nitrogen cycle is the Extended-only cycle) |
Nitrogen Cycle | Not required for Core (Supplement only) | Full cycle: decomposition, nitrification, nitrogen fixation (bacteria and lightning), denitrification; roles of microorganisms |
Population Growth | Lag, exponential (log), stationary and death phases; factors: food supply, competition, predation, disease | Explain the factors leading to each phase of the sigmoid curve, with reference to limiting factors |
7. Frequently Asked
Do I need to memorise all nitrogen cycle steps for Core exams?
Core candidates only need to recall that bacteria and decomposition make nitrogen available to plants. Extended candidates must name all steps including nitrification and denitrification.
What counts as a limiting factor for population growth?
Limiting factors include food availability, predation, disease, space, and abiotic factors such as temperature, water and soil nutrient content.
Going deeper
What's Next
Now that you have mastered nutrient cycles and population dynamics for CIE IGCSE Biology 0610, you can move on to studying human impacts on ecosystems, including pollution and deforestation, which are closely linked to the content covered in this guide. Practice extended response questions on nutrient cycles to refine your exam technique, and make sure you can label unannotated cycle diagrams to test your recall. For Core candidates, focus on memorising the key processes of the carbon cycle and the four phases of the sigmoid population growth curve, while Extended candidates should spend extra time reviewing the nitrogen cycle steps and bacterial roles to maximise marks on higher-tier papers.