OxfordAQA International AS and A-level Biology: Nutrient cycles: carbon and nitrogen (9610) – Revision Notes

These notes condense section 3.3.5 Nutrient cycles (3.3.5.1 to 3.3.5.3) of the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1. The section is in Unit 3, which the specification lists as International A-level only. For full explanations and worked examples, read the study guide. Test yourself with the practice questions.

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3.3.5.1 Principles of nutrient cycling

• Organic substances contain elements such as carbon, nitrogen and phosphorus.
• Decomposers and saprophytic microorganisms release these elements as ions or simple inorganic molecules (carbon dioxide, ammonium ions).
• Producers take these up and build them into organic molecules.
• Digestion and assimilation pass the elements along food chains.

Method in steps: saprophytic nutrition

1. Secrete enzymes onto dead matter (extracellular digestion).
2. Enzymes hydrolyse big insoluble molecules into soluble products.
3. Absorb those products into the cell.
4. Respire part of them, which releases carbon dioxide; protein breakdown gives ammonium ions.

Key distinction: digestion and assimilation

Term	Meaning
Digestion	Large molecules hydrolysed to small soluble molecules that can be absorbed
Assimilation	Absorbed molecules become part of the organism’s own cells and tissues

3.3.5.2 The carbon cycle

Process	Effect on atmospheric carbon dioxide
Photosynthesis	Removes it (producers fix it into organic compounds)
Respiration by producers, consumers and decomposers	Adds it
Combustion of fossil fuels and wood	Adds it, from long-term stores
Deforestation	Fewer trees fix it; burning or decay releases it

• Short-term fluctuations: day and night, and the seasons. Photosynthesis only in the light; respiration all the time.
• Long-term change: a steady rise in the average, driven by human activity, with the short cycles still visible.
• Seasonal rule for a northern hemisphere station: lowest after the growing season, highest at the end of winter.

Worked reminder: a daily carbon dioxide pattern

Air above a field read 455 ppm before dawn and 361 ppm at midday. The fall is 455 − 361 = 94 ppm, and 94 ÷ 455 × 100 = 20.7%. Before dawn only respiration is acting. At midday photosynthesis runs faster than respiration. If a question also gives an evening value, say the concentration rises again as light fades and photosynthesis slows.

Greenhouse effect

1. Short-wave radiation from the Sun reaches the surface.
2. The surface emits longer-wave infrared.
3. Carbon dioxide and methane absorb some infrared and re-radiate it, including downwards.
4. The lower atmosphere stays warmer. More of these gases give an enhanced greenhouse effect and climatic change.

Gas	Main sources
Carbon dioxide	Combustion, respiration, deforestation
Methane	Anaerobic respiration by methanogenic archaea in wetlands and rice paddies, gut of ruminants, landfill, gas leaks

Climate change, crops and pests

	Possible effects
Crop yield	Longer growing season and more carbon dioxide may raise photosynthesis; heat, drought and pests may lower yield. Depends on crop and place
Insect pests	Ectotherms develop faster when warm, so shorter life cycle, more generations, better winter survival, wider range, more numbers

Reminder: analysing data

1. Quote values with units from the table or graph.
2. Calculate the change and the percentage change (change divided by the original value).
3. Describe the pattern, then explain it using processes.
4. For evaluation, note sample size, time scale, other variables and the difference between correlation and cause.

Worked reminder: a pest takes 12 days per generation in warm weather and 18 days in cool weather. In a 144-day season, that is 144 ÷ 12 = 12 generations and 144 ÷ 18 = 8 generations. The warm season gives 4 more, which is 50% more.

3.3.5.3 The nitrogen cycle

Process	Change	Conditions	Bacteria
Nitrogen fixation	N₂ to ammonium	Needs energy	Free-living in soil; mutualistic in legume root nodules
Ammonification	Proteins, urea, nucleic acids to ammonium	Dead matter and waste	Saprophytic bacteria
Nitrification	Ammonium to nitrite to nitrate	Aerobic	Two groups of nitrifying bacteria
Denitrification	Nitrate to N₂	Anaerobic, waterlogged	Denitrifying bacteria

Producers take up nitrate and ammonium, make amino acids, proteins and nucleic acids, and pass nitrogen to consumers.

Worked reminder: a nitrogen budget

A field gains 30 (fixation) and 100 (fertiliser) kg ha⁻¹ year⁻¹ of nitrogen. It loses 95 (harvest), 24 (leaching) and 6 (denitrification). Input is 130 and output is 125, so the soil store gains 5 kg ha⁻¹ year⁻¹. Leaching removes 24 ÷ 100 × 100 = 24% of the fertiliser applied. If the store is not changing, input equals output.

Fertilisers

• Harvest and removal of livestock take nitrogen out, because it does not return through death and decay.
• Natural: manure, slurry, compost. Organic, slow release, variable, adds organic matter.
• Artificial: inorganic salts such as ammonium nitrate. Soluble, quick, known composition.

Leaching and eutrophication

1. Leaching: soluble nitrate washed out of soil by draining water. Worse after heavy rain, with excess fertiliser, or on bare soil.
2. Nitrate and phosphate reach a lake or river.
3. Algae grow rapidly and form a bloom, which blocks light.
4. Plants below die. Dead material feeds saprophytic bacteria.
5. Their aerobic respiration lowers dissolved oxygen.
6. Fish and other aerobic organisms die.

Worked reminder: percentage nitrogen

Ammonium nitrate NH₄NO₃, Mr = 80, N = 28. Percentage nitrogen = 28 ÷ 80 × 100 = 35%.

Remember that the same carbon atom can pass through several of these steps in turn: fixed in photosynthesis, eaten, respired by a consumer, and fixed again by a producer. The nitrogen atoms in the nitrogen cycle follow a similar loop through proteins, ammonium, nitrate and back into plant protein.

Must-know distinctions

Pair	Difference
Ammonification and nitrification	Ammonification makes ammonium from organic nitrogen; nitrification oxidises ammonium to nitrite and then nitrate
Nitrogen fixation and denitrification	Fixation takes N₂ into ammonium; denitrification returns N₂ to the air from nitrate
Leaching and eutrophication	Leaching is the loss of ions from soil; eutrophication is the harm done in water after the ions arrive
Respiration and combustion	Both release carbon dioxide; respiration is in cells, combustion is burning
Natural and artificial fertiliser	Natural releases ions slowly after decay; artificial supplies ions at once and is more easily leached
Digestion and assimilation	Digestion breaks down and absorbs; assimilation builds the absorbed molecules into the organism

Short answer frames

• Carbon dioxide change: state the pattern with values, name the process that dominates at each stage (photosynthesis or respiration), then link to the cause.
• Nitrogen conversions: name the process, the starting form, the final form, the bacteria and the oxygen condition.
• Eutrophication: nutrient rise, algal bloom, light blocked, death of plants, decomposers multiply, oxygen falls, aerobic animals die. Write the steps in order.
• Pest numbers: warmer, so faster development, shorter life cycle, more generations, better winter survival, wider range.

Quick self-test

1. Name the type of nutrition used by a fungus that secretes enzymes onto dead wood.
2. State what ions are released from dead protein by saprophytic bacteria.
3. Distinguish digestion from assimilation.
4. Why is carbon dioxide concentration lowest in the afternoon in a crop field?
5. Name two human activities that raise global carbon dioxide concentration.
6. Name two sources of methane.
7. Explain why warmer weather may increase the damage done by an insect pest.
8. Name the process in which nitrate is converted to nitrogen gas, and the condition it needs.
9. Where do mutualistic nitrogen-fixing bacteria live in a pea plant?
10. Explain why harvest creates a need for fertiliser.
11. Carbon dioxide at a station was 421.0 ppm in one March and 423.6 ppm the next March. Calculate the percentage increase to 2 significant figures.
12. Why does eutrophication reduce oxygen in water?

Answers

1. Saprophytic nutrition (the fungus is a saprophyte).
2. Ammonium ions (ammonification).
3. Digestion hydrolyses large molecules to small soluble ones; assimilation makes absorbed molecules part of the organism’s own cells.
4. Photosynthesis is fastest in strong light and exceeds respiration, so the crop removes carbon dioxide.
5. Burning fossil fuels; deforestation.
6. Any two: cattle and sheep digestion, rice paddies, wetlands, landfill, leaks from natural gas supplies.
7. Insects are ectotherms and develop faster, so there are more generations, more survive the winter and the range widens, so numbers and damage can rise.
8. Denitrification; anaerobic conditions.
9. In root nodules.
10. Harvest removes nitrogen in the plants, which would otherwise return through decay.
11. 423.6 − 421.0 = 2.6; 2.6 ÷ 421.0 × 100 = 0.62%.
12. Algae block light and die, saprophytic bacteria multiply and use oxygen in aerobic respiration.

Where marks are usually lost

• Writing “nutrients” instead of nitrate, ammonium or carbon dioxide.
• Confusing ammonification with nitrification, or fixation with denitrification.
• Forgetting that nitrification needs oxygen and denitrification does not.
• Explaining low oxygen in a lake by algae alone, without decomposer respiration.
• Dividing by the new value in a percentage change.
• Writing that carbon dioxide “causes” a pest problem, with no mention of temperature or life cycle.
• Treating two data points as proof of cause.
• Describing a graph without quoting values and units.

Official syllabus

These notes follow section 3.3.5 Nutrient cycles in the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1, published by OxfordAQA.