Carbon Cycle, Decomposition, Trophic Levels & Biodiversity
All materials in the living world are continuously recycled between the biotic and abiotic components of ecosystems to provide the building blocks for future organisms. The carbon cycle relies on photosynthesis to remove carbon dioxide from the atmosphere and lock it into plant biomass, while aerobic respiration, microbial decomposition, and combustion return carbon dioxide to the air. Examiners test the carbon cycle alongside the rate of decay required practical (temperature, water, and oxygen), Triple Science pyramids of biomass and trophic efficiency calculations, and human impacts on biodiversity (deforestation, peat bog destruction, and global warming).
- The Carbon Cycle
- Photosynthesis is the only major biological process that removes CO₂ from the atmosphere, converting it into glucose and plant carbohydrates, fats, and proteins. Carbon moves up food chains as animals eat plants. CO₂ is returned to the atmosphere by: 1. Aerobic respiration of plants, animals, and decomposers. 2. Decay of dead organisms and waste by microorganisms. 3. Combustion of fossil fuels and wood.
- Decomposition & Biogas Generators (Triple)
- Decomposers (bacteria and fungi) secrete extracellular enzymes onto dead organic matter and absorb the soluble digested molecules. Rate of decay = Change in mass (or pH) / Time. Under anaerobic conditions (no oxygen), anaerobic bacteria decay plant and animal waste to produce biogas (mainly methane, CH₄), which can be burned as a renewable fuel.
- Trophic Levels & Pyramids of Biomass (Triple)
- Trophic Level 1: Producers (plants/algae that make biomass via photosynthesis). Level 2: Primary consumers (herbivores). Level 3: Secondary consumers (carnivores that eat herbivores). Level 4: Tertiary consumers (carnivores that eat other carnivores; apex predators have no predators). Pyramids of biomass are almost always pyramid-shaped because dry mass decreases at each level.
Carbon Cycle, Decomposition, Trophic Levels & Biodiversity: Enzyme Collision, Active Site & Rate Curve Lab
Drag the temperature, pH, and substrate sliders to watch how kinetic collisions, active-site binding, and denaturation change the reaction rate in real time.
Key Concepts & Exam Definitions (6)
- The Carbon Cycle
- Photosynthesis is the only major biological process that removes CO₂ from the atmosphere, converting it into glucose and plant carbohydrates, fats, and proteins. Carbon moves up food chains as animals eat plants. CO₂ is returned to the atmosphere by: 1. Aerobic respiration of plants, animals, and decomposers. 2. Decay of dead organisms and waste by microorganisms. 3. Combustion of fossil fuels and wood.
- Decomposition & Biogas Generators (Triple)
- Decomposers (bacteria and fungi) secrete extracellular enzymes onto dead organic matter and absorb the soluble digested molecules. Rate of decay = Change in mass (or pH) / Time. Under anaerobic conditions (no oxygen), anaerobic bacteria decay plant and animal waste to produce biogas (mainly methane, CH₄), which can be burned as a renewable fuel.
- Trophic Levels & Pyramids of Biomass (Triple)
- Trophic Level 1: Producers (plants/algae that make biomass via photosynthesis). Level 2: Primary consumers (herbivores). Level 3: Secondary consumers (carnivores that eat herbivores). Level 4: Tertiary consumers (carnivores that eat other carnivores; apex predators have no predators). Pyramids of biomass are almost always pyramid-shaped because dry mass decreases at each level.
- Efficiency of Biomass Transfer (Triple)
- Producers transfer only ~1% of incident light energy into biomass. Above producers, only ~10% of biomass is transferred to the next trophic level. Formula: Percentage Efficiency = (Biomass transferred to higher level / Biomass available at lower level) x 100. Losses occur via inedible parts, egestion (faeces), excretion (urea/CO₂/water), and respiration for movement and body heat.
- Human Impacts on Biodiversity
- Biodiversity is the variety of all the different species of organisms on Earth, or within an ecosystem. High biodiversity ensures ecosystem stability by reducing dependence of one species on another for food or shelter. Human population growth reduces biodiversity via: 1. Water/air/land pollution (sewage, fertiliser eutrophication, toxic chemicals, smoke/acid rain). 2. Deforestation (for cattle ranching, rice fields, biofuel crops, and timber). 3. Destruction of peat bogs. 4. Global warming (from CO₂ and methane).
- Food Security, Sustainable Fisheries & Biotechnology (Triple)
- Food security is having enough food to feed a population, threatened by birth rates, changing diets, climate change, pests, and conflict. Sustainable fishing uses fishing quotas and larger net mesh sizes so young fish escape and breed. Biotechnology includes growing the fungus Fusarium on glucose syrup in aerobic fermenters to produce protein-rich mycoprotein (Quorn).
Quick-Fire Flashcards (6)
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Which four processes are key stages in the Carbon Cycle?
In the milk decay required practical, why does the pink phenolphthalein indicator turn colourless as lipase breaks down milk fat?
State the formula for calculating percentage efficiency of biomass transfer between trophic levels.
Why do pyramids of biomass almost always have a regular pyramid shape, unlike pyramids of numbers?
Give two ways to conserve fish stocks at a sustainable level.
How is mycoprotein (Quorn) produced using biotechnology?
Exam-Style Questions & Mark Schemes (4)
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- Q1
Explain how carbon atoms in a dead oak leaf on a forest floor can eventually become part of a starch molecule in a new plant leaf. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Decomposers (bacteria and fungi) in the soil secrete enzymes that digest the dead oak leaf into soluble molecules (like glucose) which they absorb [1 mark]. 2. The decomposers carry out aerobic respiration (using the glucose and oxygen), which releases carbon dioxide gas back into the atmosphere [1 mark]. 3. A living plant absorbs this carbon dioxide from the air through the stomata in its leaves [1 mark]. 4. Inside the chloroplasts, photosynthesis converts the carbon dioxide and water into glucose using light energy, and the plant joins glucose molecules together into insoluble starch for storage [1 mark].
- Q2
In a woodland food chain, oak leaves have a dry biomass of 12,500 kg, caterpillars feeding on them have a biomass of 1,400 kg, and blue tits feeding on the caterpillars have a biomass of 112 kg. Calculate the percentage efficiency of biomass transfer from caterpillars to blue tits, and explain three reasons why biomass is lost between these two trophic levels. (Triple Science) [5 marks]
Click “Check Mark Scheme” to compare your answer, 1. Efficiency = (112 / 1,400) x 100 [1 mark]. 2. Efficiency = 8% [1 mark]. 3. Reason 1: Caterpillars use a large amount of glucose in cellular respiration to release energy for movement, losing carbon as CO₂ and water [1 mark]. 4. Reason 2: Not all of the caterpillar biomass eaten by the blue tit can be digested, so indigestible material is lost from the food chain by egestion in faeces [1 mark]. 5. Reason 3: Not all caterpillars in the population are eaten by blue tits (some die naturally or pupate), and excess nitrogenous waste is excreted in urine/uric acid [1 mark].
- Q3
Explain why tropical deforestation in the Amazon rainforest leads to an increase in atmospheric carbon dioxide concentration and a decrease in biodiversity. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Cutting down and burning trees (slash-and-burn) or allowing felled wood and soil organic matter to decay via microorganisms directly releases stored carbon as carbon dioxide into the atmosphere [1 mark]. 2. Removing millions of trees vastly reduces the rate of photosynthesis, so much less carbon dioxide is absorbed and locked away from the atmosphere [1 mark]. 3. Tropical rainforests contain a huge variety of plant and animal species; destroying the forest destroys their habitats, food sources, and nesting sites [1 mark]. 4. Many specialist species cannot adapt or migrate elsewhere and become extinct, permanently reducing global biodiversity [1 mark].
- Q4
Explain how a gardener should design a compost heap to maximise the rate of decay of dead garden plants into natural fertiliser. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Ensure the compost heap has air holes and turn the compost regularly with a fork to provide plenty of oxygen for aerobic respiration by decomposing microorganisms [1 mark]. 2. Keep the compost heap moist (add water if dry), because microorganisms need water for chemical reactions and to dissolve digested nutrients [1 mark]. 3. Insulate the compost bin or place it in a warm location so the temperature stays warm (around 35°C to 45°C), giving decomposer enzymes more kinetic energy to catalyse breakdown reactions faster [1 mark]. 4. Shred the plant waste into smaller pieces first to increase the surface area for extracellular decomposer enzymes to act on [1 mark].
Common Exam Questions (4)
Why is only about 10% of biomass transferred from one trophic level to the next?
Around 90% of biomass is lost between each trophic level because: 1. Not all of the organism is eaten (e.g., bones, roots, fur). 2. Some ingested material is indigestible and egested as faeces. 3. A large amount of glucose is broken down in cellular respiration to release energy for movement and keeping warm (especially in mammals and birds), releasing CO₂ and water as waste, plus urea in urine.
What are the three factors that affect the rate of decay by decomposing microorganisms?
Temperature, water (moisture), and oxygen availability. Warm temperatures increase the kinetic energy of decomposer enzymes (though extreme heat denatures them), moisture is needed for chemical reactions and dissolving nutrients, and oxygen is required for aerobic respiration by decomposers.
Why does destroying peat bogs contribute to both global warming and loss of biodiversity?
Peat bogs are waterlogged, acidic habitats that store vast amounts of carbon locked in partially decayed plants over thousands of years. Draining peat bogs to clear farmland or burning/using peat as garden compost exposes the peat to oxygen; microorganisms decompose it and release massive quantities of carbon dioxide (a greenhouse gas), while destroying a unique wetland habitat.
How do intensive farming methods increase efficiency of food production?
Restricting the movement of livestock (keeping animals in pens) and keeping their surroundings warm means the animals transfer much less energy through muscle contraction and thermoregulation (respiration). Therefore, a greater proportion of the food they eat is converted into new biomass (muscle/meat or milk) for human consumption.
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Related study guides
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Photosynthesis
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