What you'll learn
Ecosystem processes describe how energy and nutrients move through living communities and their physical environment. This topic is essential for Edexcel GCSE Biology as it connects photosynthesis, respiration, and the role of decomposers in maintaining stable ecosystems. Exam questions frequently test understanding of the water cycle, carbon cycle, and how disruptions affect organism survival.
Key terms and definitions
Ecosystem — a community of organisms interacting with each other and their physical environment, including biotic and abiotic factors.
Interdependence — the reliance of organisms on each other for resources such as food, shelter, and pollination; changes to one species affect others in the community.
Decomposers — microorganisms (mainly bacteria and fungi) that break down dead organic material, releasing nutrients back into the soil.
Bioaccumulation — the build-up of toxic substances in organisms at each stage of a food chain, with concentrations increasing at higher trophic levels.
Photosynthesis — the process by which plants convert carbon dioxide and water into glucose and oxygen using light energy.
Respiration — the process that releases energy from glucose in all living cells, producing carbon dioxide and water as waste products.
Trophic level — the position an organism occupies in a food chain (producer, primary consumer, secondary consumer, tertiary consumer).
Eutrophication — the excessive enrichment of water with nutrients (usually nitrates and phosphates), leading to algal blooms and oxygen depletion.
Core concepts
Energy flow through ecosystems
Energy enters ecosystems through photosynthesis in green plants and algae. These producers convert light energy from the Sun into chemical energy stored in glucose molecules. The equation for photosynthesis is:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Energy flows through ecosystems in one direction:
- Producers (plants, algae) capture approximately 1-3% of the Sun's light energy that reaches them
- Primary consumers (herbivores) eat producers and obtain roughly 10% of the energy stored in plant tissue
- Secondary consumers (carnivores) eat herbivores and gain about 10% of the energy from their prey
- Tertiary consumers (top carnivores) occupy the highest trophic level
Energy is lost at each trophic level through:
- Respiration — producing heat energy that disperses to the surroundings
- Movement — muscle contractions require energy
- Excretion — waste products contain chemical energy that is not used
- Uneaten material — bones, roots, and other parts not consumed
This explains why food chains rarely exceed four or five trophic levels. Insufficient energy remains to support organisms at higher levels. Edexcel GCSE Biology exam questions commonly ask students to calculate energy efficiency between trophic levels using the formula:
Energy efficiency = (energy available after transfer ÷ energy available before transfer) × 100
The carbon cycle
Carbon atoms cycle continuously between living organisms and the atmosphere. Understanding each process is crucial for Edexcel GCSE Biology:
Carbon dioxide is removed from the atmosphere by:
- Photosynthesis in plants and algae — carbon becomes fixed in glucose molecules and other organic compounds
- Dissolution in oceans — carbon dioxide dissolves in water, forming carbonate compounds used by marine organisms for shells
Carbon dioxide is returned to the atmosphere by:
- Respiration in all living organisms — glucose is broken down, releasing CO₂ as a waste product
- Combustion of fossil fuels (coal, oil, natural gas) — carbon stored millions of years ago is rapidly released
- Decomposition — decomposers respire while breaking down dead material, releasing carbon dioxide
The carbon cycle connects to human activity through:
- Burning fossil fuels for energy and transport
- Deforestation reducing photosynthesis rates
- Increased atmospheric CO₂ contributing to climate change
Exam questions frequently present diagrams of the carbon cycle with missing labels or ask students to explain why deforestation increases atmospheric carbon dioxide levels.
The water cycle
Water continuously moves between the Earth's surface and atmosphere:
- Evaporation — liquid water in oceans, lakes, and rivers gains energy from the Sun and becomes water vapour
- Transpiration — water evaporates from plant leaves through stomata
- Condensation — water vapour cools in the atmosphere and forms clouds (tiny liquid water droplets)
- Precipitation — water falls as rain, snow, or hail when droplets become large enough
- Percolation — water soaks into soil and rocks, replenishing groundwater supplies
- Surface runoff — water flows over land into streams and rivers, eventually returning to oceans
The water cycle connects to ecosystems because:
- All organisms require water for metabolic processes
- Plants need water for photosynthesis and transport
- Aquatic ecosystems depend on freshwater supply from precipitation
- Changes in precipitation patterns affect species distribution and survival
Edexcel GCSE Biology papers may test understanding through labelling diagrams or explaining how the water cycle maintains freshwater availability.
Decomposition and nutrient recycling
Decomposers (bacteria and fungi) are essential for ecosystem stability. They break down dead plant and animal material through extracellular digestion — secreting enzymes onto organic matter and absorbing the digested nutrients.
Decomposition rate depends on several factors:
- Temperature — higher temperatures (up to optimum enzyme temperature) increase enzyme activity and decomposer metabolism
- Oxygen availability — aerobic decomposers work faster than anaerobic bacteria
- Water availability — decomposers need moisture for enzyme function and metabolic processes
- pH — extreme pH levels denature enzymes, slowing decomposition
The decay process releases:
- Carbon dioxide — from decomposer respiration
- Mineral ions — including nitrates, phosphates, and potassium compounds that plants absorb through roots
- Heat energy — explaining why compost heaps feel warm
Without decomposers, dead material would accumulate, and essential nutrients would become locked in dead tissue rather than recycled for new growth.
The nitrogen cycle
Nitrogen makes up 78% of atmospheric air but exists as unreactive N₂ gas that most organisms cannot use directly. Plants require nitrogen in the form of nitrate ions (NO₃⁻) to make proteins and nucleic acids.
The nitrogen cycle involves several key processes:
Nitrogen fixation — converting atmospheric nitrogen into ammonia:
- Nitrogen-fixing bacteria in root nodules of legumes (peas, beans, clover) convert N₂ into ammonia
- Lightning provides energy to break nitrogen bonds, forming nitrogen oxides that dissolve in rain
Nitrification — converting ammonia into nitrates:
- Nitrifying bacteria in soil convert ammonia → nitrites → nitrates in a two-step process requiring oxygen
Absorption — plant roots actively transport nitrate ions from soil solution
Feeding — animals obtain nitrogen compounds by consuming plants or other animals
Decomposition — decomposers break down proteins in dead organisms and waste (urea, faeces), releasing ammonia
Denitrification — converting nitrates back to nitrogen gas:
- Denitrifying bacteria in waterlogged, anaerobic soil conditions convert nitrates → nitrogen gas, which returns to the atmosphere
Edexcel GCSE Biology exam questions commonly test whether students can:
- Name specific bacteria types (nitrogen-fixing, nitrifying, denitrifying)
- Explain why farmers add fertilisers containing nitrates
- Describe the role of legumes in improving soil fertility
Human impacts on ecosystem processes
Eutrophication occurs when excess fertiliser washes into waterways:
- Nitrate and phosphate concentrations increase in water
- Algae grow rapidly on the surface (algal bloom)
- Surface algae block light from reaching deeper plants
- Submerged plants cannot photosynthesise and die
- Decomposers break down dead plant material
- Decomposer respiration depletes oxygen dissolved in water
- Fish and other aquatic organisms die from oxygen shortage
This process demonstrates interdependence — changes affecting algae populations cascade through the entire aquatic ecosystem.
Bioaccumulation of pesticides illustrates energy flow problems:
- Pesticides sprayed on crops are absorbed by plant tissue in low concentrations
- Primary consumers eat many plants, accumulating higher pesticide concentrations
- Secondary consumers eat many herbivores, further concentrating pesticides
- Top predators (tertiary consumers) accumulate the highest concentrations
- Toxic effects become most severe at the top of food chains
Historical examples include DDT affecting bird reproduction by thinning eggshells, leading to population declines in birds of prey.
Worked examples
Example 1: Energy transfer calculation
A field contains grass plants that store 45,000 kJ/m² of energy. Rabbits feeding on the grass store 4,500 kJ/m² of energy.
(a) Calculate the energy transfer efficiency from grass to rabbits. (2 marks)
(b) Suggest two reasons why not all energy in grass is transferred to rabbits. (2 marks)
Answer:
(a) Energy efficiency = (4,500 ÷ 45,000) × 100 = 10% ✓✓
(b) Any two from:
- Rabbits do not eat all parts of grass plants (roots remain underground) ✓
- Energy lost through respiration as heat ✓
- Energy lost in faeces/waste ✓
- Energy used for movement ✓
Example 2: Nitrogen cycle
Explain why farmers plant clover in fields before growing wheat. (3 marks)
Answer:
Clover is a legume with root nodules ✓ containing nitrogen-fixing bacteria ✓ that convert atmospheric nitrogen into ammonia/nitrates, enriching the soil with nitrogen compounds ✓. This improves soil fertility for wheat growth without adding artificial fertiliser.
Example 3: Eutrophication
A lake near farmland shows an algal bloom covering the surface. Explain how this could lead to fish deaths. (4 marks)
Answer:
Fertiliser runoff increases nitrate/phosphate levels in water ✓. Algae grow rapidly and block light ✓. Submerged plants cannot photosynthesise and die ✓. Decomposers respire aerobically, using up dissolved oxygen ✓. Fish die from lack of oxygen.
Common mistakes and how to avoid them
Mistake: Stating energy is recycled in ecosystems. Correction: Energy flows in one direction through ecosystems and is eventually lost as heat. Only nutrients like carbon and nitrogen are recycled.
Mistake: Confusing decomposers with detritivores. Correction: Decomposers (bacteria, fungi) are microorganisms that break down material using extracellular enzymes. Detritivores (earthworms, woodlice) are larger organisms that feed on dead material.
Mistake: Writing that plants "breathe in carbon dioxide". Correction: Plants do not breathe. They take in carbon dioxide for photosynthesis and release carbon dioxide during respiration (which occurs constantly).
Mistake: Claiming nitrogen-fixing bacteria convert nitrates into nitrogen gas. Correction: Nitrogen-fixing bacteria convert nitrogen gas (N₂) into ammonia. Denitrifying bacteria convert nitrates back into nitrogen gas.
Mistake: Describing photosynthesis as "plants making food from sunlight". Correction: Plants convert light energy into chemical energy stored in glucose molecules. The raw materials are carbon dioxide and water, not sunlight itself.
Mistake: Suggesting all bacteria are decomposers. Correction: Different bacteria have different roles — some fix nitrogen, others nitrify ammonia, others cause disease. Only some bacteria act as decomposers.
Exam technique for Ecosystem Processes
"Explain" questions require cause-and-effect statements. For eutrophication questions, work through the sequence step-by-step: fertiliser runoff → algal bloom → light blocked → plants die → decomposition → oxygen depletion → fish death. Each step earns one mark.
Calculation questions on energy transfer always require you to show working. Write the formula, substitute numbers, then calculate. Even if the final answer is wrong, you can earn method marks.
Diagram questions on cycles (carbon, nitrogen, water) test specific vocabulary. Learn precise terms: "nitrifying bacteria" not "soil bacteria", "transpiration" not "evaporation from plants", "combustion" not "burning".
"Suggest" questions allow reasonable answers not directly taught. Use knowledge of limiting factors and metabolic processes to make logical predictions. Phrases like "because enzymes work faster at higher temperatures" demonstrate understanding even if the specific context is unfamiliar.
Quick revision summary
Energy flows through ecosystems from producers to consumers, with approximately 10% transferred between trophic levels and the rest lost as heat, movement, and waste. Carbon cycles between organisms and the atmosphere through photosynthesis, respiration, combustion, and decomposition. The nitrogen cycle requires bacteria to fix nitrogen gas into ammonia, which nitrifying bacteria convert to nitrates for plant absorption. Decomposers recycle nutrients by breaking down dead material. Human activities like fertiliser use cause eutrophication, while pesticides accumulate in food chains. Understanding interdependence explains how changes to one species affect entire communities.