B21: Nutrient Cycles
The carbon cycle and the water cycle
📋 Key Concepts
Nutrient Cycles: Materials such as carbon and water are constantly recycled through the living and non-living parts of ecosystems. Decomposers play a vital role in breaking down dead material and returning nutrients to the soil.
Key Terms
- carbon cycle
- water cycle
- photosynthesis
- respiration
- combustion
- decomposition
- decomposers
- fossil fuels
- evaporation
- condensation
- precipitation
- transpiration
- composting
- biogas generator
📝 The Carbon Cycle
The carbon cycle describes how carbon is recycled through the environment. Carbon is found in all living organisms and in the atmosphere as CO₂. The carbon cycle maintains the balance of CO₂ in the atmosphere.
Carbon cycle — key processes:
Photosynthesis: removes CO₂ from the atmosphere (plants convert CO₂ + water → glucose)
Respiration: adds CO₂ to the atmosphere (organisms convert glucose + oxygen → CO₂ + water)
Combustion: adds CO₂ to the atmosphere (burning fossil fuels and wood releases stored carbon as CO₂)
Decomposition: adds CO₂ to the atmosphere (decomposers respire as they break down dead material)
Fossil formation: removes carbon from the cycle (dead organisms form fossil fuels over millions of years)
Photosynthesis: removes CO₂ from the atmosphere (plants convert CO₂ + water → glucose)
Respiration: adds CO₂ to the atmosphere (organisms convert glucose + oxygen → CO₂ + water)
Combustion: adds CO₂ to the atmosphere (burning fossil fuels and wood releases stored carbon as CO₂)
Decomposition: adds CO₂ to the atmosphere (decomposers respire as they break down dead material)
Fossil formation: removes carbon from the cycle (dead organisms form fossil fuels over millions of years)
Carbon cycle in detail:
- Plants absorb CO₂ from the air during photosynthesis, incorporating carbon into organic molecules (glucose, starch, cellulose, etc.)
- Animals eat plants (and other animals), so carbon passes through the food chain
- All living organisms respire, releasing CO₂ back into the atmosphere
- When organisms die, decomposers (bacteria and fungi) break down their bodies. Decomposers respire during this process, releasing CO₂
- Under certain conditions, dead organisms are compressed over millions of years to form fossil fuels (coal, oil, natural gas), which lock up carbon
- Burning fossil fuels (combustion) releases the stored carbon as CO₂ back into the atmosphere
Example 1
Describe three ways CO₂ is added to the atmosphere and two ways CO₂ is removed from the atmosphere in the carbon cycle.
Solution:
CO₂ added: (1) Respiration by living organisms, (2) Combustion of fossil fuels and wood, (3) Decomposition of dead organisms by decomposers (which respire).
CO₂ removed: (1) Photosynthesis by plants (absorbs CO₂ to make glucose), (2) Formation of fossil fuels (carbon locked up underground).
Example 2
Explain how burning fossil fuels affects the carbon cycle.
Solution:
Fossil fuels contain carbon that was locked up underground over millions of years from dead organisms. When fossil fuels are burned (combustion), this stored carbon is released as CO₂ into the atmosphere. This adds CO₂ to the atmosphere faster than it can be removed by photosynthesis, causing an increase in atmospheric CO₂ concentration and contributing to global warming.
📝 The Water Cycle
The water cycle describes how water is continuously recycled between the oceans, the atmosphere and the land. The Sun provides the energy that drives the water cycle.
Water cycle processes:
- Evaporation: The Sun heats water in oceans, lakes and rivers, causing it to change from liquid to water vapour (gas)
- Transpiration: Water evaporates from the leaves of plants through stomata
- Condensation: Water vapour in the atmosphere cools and changes back into liquid water, forming clouds
- Precipitation: Water falls from clouds as rain, snow, hail or sleet
- Runoff and infiltration: Water flows over the ground (runoff) or soaks into the soil (infiltration), eventually returning to rivers and oceans
Example 3
Describe the path of a water molecule from the ocean, through the atmosphere, and back to the ocean.
Solution:
1. Evaporation: the water molecule evaporates from the ocean surface into the atmosphere as water vapour.
2. Condensation: the water vapour cools and condenses to form a cloud droplet.
3. Precipitation: the water falls as rain onto the land.
4. Runoff/infiltration: the water flows over the ground or soaks into the soil, eventually reaching a river.
5. The river carries the water back to the ocean, completing the cycle.
📝 Decomposition
Decomposition: The breakdown of dead organisms and waste by bacteria and fungi (decomposers). Decomposers secrete enzymes onto dead material and digest it externally, then absorb the nutrients. They respire during this process, releasing CO₂.
Factors affecting the rate of decomposition:
- Temperature: Higher temperatures increase the rate (more kinetic energy, faster enzyme activity) up to an optimum. Very high temperatures denature enzymes and slow/stop decomposition.
- Moisture: Decomposers need water to survive and for enzyme action. Dry conditions slow decomposition; moist conditions speed it up.
- Oxygen availability: Most decomposers are aerobic and need oxygen for respiration. Without oxygen, anaerobic decomposition occurs but it is much slower and produces methane.
Example 4
A body was found preserved in a peat bog after 2000 years. Explain why the body had not decomposed.
Solution:
Peat bogs are acidic and have very low oxygen levels (waterlogged, anaerobic conditions). Decomposers need oxygen for aerobic respiration, so they cannot function effectively in these conditions. The acidic conditions also inhibit many decomposer enzymes. Additionally, the low temperature in the bog slows enzyme activity. These factors combined mean decomposition is extremely slow, preserving organic material for thousands of years.
📝 Composting and Biogas
Composting: Decomposers break down kitchen and garden waste in a compost heap. To maximise the rate of decomposition:
- Keep the compost moist (water is needed for decomposers)
- Turn the compost regularly to introduce oxygen (aerobic decomposition is faster)
- Keep the compost warm (microbes work faster at warmer temperatures)
- Mix green (nitrogen-rich) and brown (carbon-rich) materials
Biogas generators: Use anaerobic decomposition of waste material (e.g. manure, food waste) by microorganisms to produce biogas (mainly methane). Biogas can be used as a fuel for heating or generating electricity. The remaining material (digestate) can be used as fertiliser.
| Feature | Composting | Biogas Generator |
|---|---|---|
| Type of decomposition | Aerobic (with oxygen) | Anaerobic (without oxygen) |
| Conditions | Oxygen-rich, moist, warm | Oxygen-free (sealed container), warm |
| Main product | Compost (soil conditioner/fertiliser) | Biogas (methane fuel) + digestate (fertiliser) |
| Speed | Slower (weeks to months) | Faster in warm conditions |
❓ Practice Questions
Q1: Describe the carbon cycle, explaining how carbon is added to and removed from the atmosphere.
Q2: Describe the main processes in the water cycle.
Q3: Explain three factors that affect the rate of decomposition.
Q4: Explain why a compost heap is turned regularly and kept moist.
Q5: Higher Compare aerobic and anaerobic decomposition, giving an example of where each is used.
✅ Answers
- CO₂ is removed from the atmosphere by photosynthesis (plants convert CO₂ to glucose). Carbon passes through food chains when animals eat plants. CO₂ is added to the atmosphere by: respiration (all living organisms), decomposition (decomposers break down dead material and respire), and combustion (burning fossil fuels releases stored carbon as CO₂). Carbon is also removed when dead organisms form fossil fuels over millions of years.
- Evaporation: Sun heats water in oceans/lakes, turning it to water vapour. Transpiration: water evaporates from plant leaves. Condensation: water vapour cools and forms clouds. Precipitation: water falls as rain/snow. Runoff and infiltration: water flows over or soaks into the ground, returning to rivers and oceans.
- (1) Temperature — higher temperature speeds up enzyme activity and decomposition, up to an optimum. Very high temperatures denature enzymes. (2) Moisture — decomposers need water; moist conditions speed up decomposition; dry conditions slow it. (3) Oxygen — aerobic decomposers need oxygen; lack of oxygen slows decomposition (anaerobic decomposition is slower).
- Turning the compost heap introduces oxygen, which aerobic decomposers need for respiration. This speeds up decomposition. Keeping it moist provides the water that decomposers need to survive and for enzyme action.
- Aerobic decomposition uses oxygen and happens in compost heaps. It is faster and produces CO₂ and compost. Anaerobic decomposition happens without oxygen, such as in biogas generators. It is slower and produces methane (biogas) and digestate. Biogas can be used as fuel, while digestate is used as fertiliser.
🎯 Exam Tips
- Learn the carbon cycle processes: photosynthesis REMOVES CO₂; respiration, combustion and decomposition ADD CO₂
- When drawing the carbon cycle, include arrows and label each process clearly
- Fossil fuels LOCK UP carbon — combustion of fossil fuels releases it back as CO₂
- Decomposers are bacteria AND fungi — both are needed
- Three factors affecting decomposition: temperature, moisture, oxygen — learn all three
- In the water cycle, transpiration is from PLANTS; evaporation is from water surfaces
- Composting is AEROBIC; biogas production is ANAEROBIC — don't confuse them
- Biogas is mainly methane (CH₄) — it can be burned as a renewable fuel
🔬 Required Practical
Required Practical: Decomposition
Aim: Investigate the effect of temperature on the rate of decay of milk by measuring the pH change (using lipase enzyme).
Method: 1. Set up five water baths at different temperatures (5°C, 20°C, 35°C, 45°C, 60°C). 2. Add 5 cm³ of milk and 1 cm³ of lipase solution to a test tube at each temperature. 3. Add 5 drops of cresol red indicator (turns yellow in acidic conditions). 4. Time how long it takes for the indicator to change from red to yellow (showing fatty acids produced by lipase have lowered the pH). 5. Repeat three times at each temperature and calculate a mean.
Variables: IV: temperature, DV: time for colour change (rate of decomposition), Control: volume of milk, volume of lipase, concentration of lipase, volume of indicator
🔢 Maths Skills
Mathematical Skills
You need to interpret data on atmospheric CO₂ concentrations over time, calculate rates of decomposition from experimental data, and use percentages to compare carbon stores.
Maths Example
Atmospheric CO₂ concentration was 280 ppm in 1750 and is 420 ppm today. Calculate the percentage increase.
Percentage increase = ((420 − 280) ÷ 280) × 100 = (140 ÷ 280) × 100 = 50%
Maths Example 2
In a decomposition experiment, milk at 20°C took 25 minutes for the indicator to change. At 35°C it took 10 minutes. Calculate the rate at each temperature.
Rate at 20°C = 1 ÷ 25 = 0.04 min⁻¹. Rate at 35°C = 1 ÷ 10 = 0.10 min⁻¹. The rate is 2.5 times faster at 35°C.
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Photosynthesis adds CO₂ to the atmosphere Correct: Photosynthesis REMOVES CO₂ from the atmosphere — it is respiration, combustion and decomposition that ADD CO₂
2. Wrong: Decomposers break down dead material without respiring Correct: Decomposers respire while breaking down dead material, which is why decomposition ADDS CO₂ to the atmosphere
3. Wrong: Transpiration and evaporation are the same thing Correct: Evaporation is from water surfaces (oceans, lakes); transpiration is specifically water loss from plant leaves through stomata
4. Wrong: Fossil fuels form quickly from dead organisms Correct: Fossil fuels form over millions of years under specific conditions of high pressure and temperature — this is why they are non-renewable
✍️ 6-Mark Question
Extended Answer
6 marks: Describe the carbon cycle, explaining how carbon is recycled through the living and non-living components of an ecosystem.
Carbon is constantly recycled in the carbon cycle. Plants remove CO₂ from the atmosphere during photosynthesis, converting it into glucose and other organic molecules. Carbon passes through the food chain when animals eat plants and when predators eat other animals. All living organisms respire, which releases CO₂ back into the atmosphere. When organisms die, decomposers (bacteria and fungi) break down their bodies. Decomposers respire during decomposition, releasing more CO₂ into the atmosphere. Under certain conditions over millions of years, the remains of dead organisms are compressed into fossil fuels (coal, oil, natural gas), which lock up carbon underground. When fossil fuels are burned (combustion), this stored carbon is released as CO₂ back into the atmosphere. The oceans also absorb CO₂ from the atmosphere, acting as a carbon sink. The carbon cycle maintains a balance of CO₂ in the atmosphere, but human activities such as burning fossil fuels and deforestation are disrupting this balance.
Mark scheme: 1 mark for photosynthesis removing CO₂; 1 mark for carbon passing through food chains; 1 mark for respiration releasing CO₂; 1 mark for decomposition releasing CO₂; 1 mark for fossil fuel formation and combustion; 1 mark for overall recycling / balance
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A student investigated the effect of oxygen availability on decomposition. They set up two identical containers with equal masses of vegetable peelings. Container A had air holes; Container B was sealed with no air. The mass of remaining material was measured weekly for 6 weeks.
- Container A (with air): Week 0 = 200g, Week 2 = 150g, Week 4 = 95g, Week 6 = 40g
- Container B (no air): Week 0 = 200g, Week 2 = 185g, Week 4 = 170g, Week 6 = 155g
(a) Calculate the mean rate of decomposition in g/week for each container over the 6 weeks.
(b) Explain the difference in decomposition rates.
(c) Container B produced a smell of methane. Explain why.
Answers: (a) Container A: (200 − 40) ÷ 6 = 26.7 g/week. Container B: (200 − 155) ÷ 6 = 7.5 g/week. (b) Container A had oxygen, allowing aerobic decomposers to function efficiently. Aerobic respiration releases more energy, so decomposers grow and reproduce faster. Container B had no oxygen, so only anaerobic decomposition could occur, which is much slower. (c) In the absence of oxygen, anaerobic microorganisms break down the organic material and produce methane (CH₄) as a by-product. This is the same process used in biogas generators.
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