Nutrient cycles and populations
🎯What you need to be able to do
- Describe the carbon cycle: photosynthesis, respiration, feeding, decomposition, fossilisation and combustion.
- Describe the nitrogen cycle and the roles of microorganisms in it EXTENDED.
- Define population, community and ecosystem; state the factors affecting the rate of population growth.
- Identify the phases of a sigmoid growth curve; explain each phase in terms of limiting factors EXTENDED.
📚The biology
The carbon cycle
Energy flows through an ecosystem, but the elements in living things are recycled. Carbon is taken out of the air as carbon dioxide by photosynthesis and returned to it by respiration and combustion.
photosynthesis: carbon dioxide → carbohydrates (and then other organic compounds) in plants
feeding: carbon compounds pass along food chains
respiration: by plants, animals and decomposers, returns carbon dioxide to the air
decomposition: decomposers break down dead organisms and waste, and respire
fossilisation: dead material that does not decay is compressed over millions of years into coal, oil and gas
combustion: burning fossil fuels (and wood) releases carbon dioxide
Cutting down forests (less photosynthesis) and burning fossil fuels (more combustion) both raise the carbon dioxide concentration of the air (topic 20).
The nitrogen cycle EXTENDED
Nitrogen gas makes up about 78% of the air, but plants and animals cannot use it directly. Plants take in nitrogen as nitrate ions through their roots and use it to make amino acids and proteins; animals get theirs by eating proteins.
- Nitrogen fixation: nitrogen gas is converted to nitrogen compounds by lightning or by nitrogen-fixing bacteria (some live free in the soil, others in the root nodules of legumes such as peas and beans).
- Absorption: plants absorb nitrate ions and use them to make amino acids, then proteins.
- Feeding and digestion: animals eat plants and digest the protein to amino acids, which they build into their own proteins.
- Deamination: excess amino acids are deaminated in the liver; the nitrogen leaves in urea.
- Decomposition: decomposers break down proteins and urea in dead organisms and waste into ammonium ions.
- Nitrification: nitrifying bacteria convert ammonium ions to nitrate ions. They need oxygen, so this is fast in well-aerated soil.
- Denitrification: denitrifying bacteria convert nitrate ions back into nitrogen gas. They thrive in waterlogged soil, with little oxygen.
Populations
population: a group of organisms of one species, living in the same area at the same time
community: all the populations of different species in an ecosystem
ecosystem: a unit containing the community of organisms and their environment, interacting together (e.g. a lake, a coral reef)
The rate at which a population grows depends on the food supply, competition (for food, space, light, mates), predation and disease. A population grows when births (and immigration) exceed deaths (and emigration).
- Lag phase: few individuals; they are adjusting, growing and maturing before reproducing, so numbers rise slowly.
- Exponential (log) phase: EXTENDED plenty of food and space, little competition, predation or disease; births far exceed deaths, so the population doubles at a regular rate.
- Stationary phase: EXTENDED limiting factors take effect — food runs short, competition increases, waste builds up; birth rate equals death rate.
- Death phase: EXTENDED food is used up or toxic waste (e.g. ethanol for yeast) accumulates, so the death rate exceeds the birth rate.
✏️Worked example
A student grows duckweed in a covered tank of pond water and counts the plants every week.
(a) The count rises from 20 to 320 plants between weeks 2 and 6. How many times did the population double? [2]
(b) After week 8 the count stays at about 600. Suggest two reasons. [2]
(c) EXTENDED The tank is then left in the dark. Explain why the carbon dioxide concentration in the water rises. [2]
(a) \( 20 \to 40 \to 80 \to 160 \to 320 \): the population doubled 4 times (\( 320 / 20 = 16 = 2^4 \)), about once a week.
(b) Stationary phase: the surface is covered, so there is competition for space and light; mineral ions such as nitrate are used up. Birth rate now equals death rate.
(c) In the dark there is no photosynthesis, so no carbon dioxide is removed, while the duckweed, and decomposers acting on dead plants, still respire and release carbon dioxide.
Check it. \( 20 \times 2^4 = 320 \). Four doublings in four weeks gives the steady doubling of the exponential phase.
“The population stops growing because the plants stop reproducing.” They are still reproducing; the number stays level because the
birth rate equals the death rate.
📝Practise
In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.
1. (Multiple choice.) Which process removes carbon dioxide from the atmosphere? A: combustion. B: decomposition. C: photosynthesis. D: respiration.
C.
2. (Theory.) Explain how carbon in the body of a dead rabbit can become part of a grass plant. [3]
Decomposers (bacteria and fungi) break down the body and respire, releasing carbon dioxide into the air; the grass takes in the carbon dioxide and uses it in photosynthesis to make glucose.
3. (Theory.) Define a community and an ecosystem. [2]
Community: all the populations of different species in an ecosystem. Ecosystem: a unit containing the community of organisms and their environment, interacting together.
4. (Theory.) State four factors that affect the rate of growth of a population. [2]
Food supply; competition; predation; disease.
5. (Theory.) EXTENDED Farmers plough waterlogged fields to let air into the soil. Explain how this can increase the nitrate in the soil. [3]
More oxygen in the soil: nitrifying bacteria (which need oxygen) convert more ammonium ions to nitrate; denitrifying bacteria (which thrive without oxygen) are less active, so less nitrate is lost as nitrogen gas.
6. (Theory.) EXTENDED Explain why farmers grow bean plants in a field and then plough them into the soil before planting maize. [3]
Beans have nitrogen-fixing bacteria in their root nodules, which make nitrogen compounds; when the plants are ploughed in, decomposers release ammonium ions and nitrifying bacteria make nitrate, which the maize absorbs to make amino acids and proteins.
7. (Theory.) EXTENDED Yeast is grown in a flask of sugar solution. Explain the death phase of its growth curve. [2]
The sugar (food) is used up and the ethanol it produces builds up to toxic levels, so the death rate is greater than the birth rate.
8. (Theory.) EXTENDED Explain the lag phase and the exponential phase of a population growth curve. [4]
Lag: few individuals, still adjusting and maturing, so few are reproducing and numbers rise slowly. Exponential: plenty of food and space, little competition, predation or disease, so the birth rate greatly exceeds the death rate and numbers double regularly.
🔗Go deeper — other people’s work
These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.
- BBC Bitesize — the carbon and nitrogen cycles
- HHMI BioInteractive — population growth models