The sun is the energy source for almost every ecosystem on Earth. Producers fix that energy into organic molecules; consumers pass it along the food chain; most of it is lost at each step as heat, faeces, excretion, and uneaten biomass. The flow is one-way.
Energy flows one way through ecosystems via producers, consumers, and decomposers.
An ecosystem is all the organisms living in a defined area (the community) together with the abiotic environmental factors of that area: light, temperature, water availability, soil mineral content. Biotic interactions and abiotic conditions together control the distribution and abundance of organisms in any habitat. The sun is the ultimate source of all energy flowing through the system.
Producers (autotrophs): green plants and algae. They fix solar energy into organic molecules through photosynthesis. Consumers (heterotrophs): obtain their energy by eating other organisms. They are classified by trophic level, with primary consumers eating producers, secondary consumers eating primary, and so on. Decomposers: bacteria and fungi. They break down dead organic matter and excreta, returning mineral ions to the soil.
Energy flows linearly through an ecosystem and cannot be recovered once lost as heat. Mineral nutrients cycle, but energy does not.
Most energy is lost between trophic levels through four distinct routes.
Only about 10% of the energy available at one trophic level reaches the next, so roughly 90% is lost at every transfer step. A complete answer about energy losses names the mechanism for each loss, not just the category. Four distinct routes account for the loss, and food chains are short because the loss compounds.
The four energy losses between trophic levels.
| Loss route | What it is | Credited phrase |
|---|---|---|
| Heat from respiration | Energy released as heat during the exothermic reactions of aerobic respiration | "heat lost via respiration" |
| Egestion (faeces) | Material ingested but never absorbed; passes through the gut | "faeces from undigested food" |
| Excretion (urea, urine) | Metabolic waste produced by cellular biochemistry; removed from body fluids | "excretion of urea" / "urine" |
| Uneaten biomass | Bones, shells, exoskeletons, bark, deep roots; biomass that left the chain rather than being transferred | "not all of the organism is eaten" |
Write
heat lost via respiration.Heat lossalone,energy used in respiration, andenergy producedare all rejected. The mechanism must be named explicitly.
Faeces is
egestion, not excretion. Faeces is undigested material that was never absorbed. Excretion is metabolic waste (urea, urine) removed from body fluids. The two are not interchangeable; 2023 P2 Q07.3 saw 5% mastery because students wroteexcretedfor faeces.
Do not write
photosynthesisordecompositionas energy losses from a consumer trophic level. Photosynthesis is producer-only. Decomposition is a separate pathway and is not a within-chain loss.
Energy transfer efficiency = (energy at next trophic level ÷ energy at previous trophic level) × 100. The working average is approximately 10%, with a range of 5–20% depending on the organisms and conditions. On multi-step chains, divide by 100 each time you apply one percentage to another. Multiplying percentages without dividing gives an answer wrong by powers of ten.
Gross and net primary productivity quantify producer energy capture.
Productivity quantifies the rate at which producers fix energy from photosynthesis into organic molecules. Two measures are used. They differ in whether the producer's own respiratory losses are subtracted from the total.
Gross primary productivity is the total rate at which producers fix light energy into organic molecules via photosynthesis. It is expressed per unit area per unit time, before any losses are subtracted. GPP is the gross figure.
Net primary productivity is the rate at which energy is incorporated into new plant biomass after the plant's own respiratory losses are subtracted. The equation is NPP = GPP − R, where R is the rate of energy loss through respiration. NPP is the energy actually available for plant growth, reproduction, and the next trophic level.
Write
NPP = GPP − Rand identify what biomass represents (dry mass, or mass of carbon) before the equation. The equation alone caps the answer at one mark; the biomass identification is the second mark. 2017 P2 Q05.3 was a high-impact failure on exactly this two-step requirement.
Mature climax communities such as established woodland can have a high GPP but a low NPP because respiration roughly equals photosynthesis: almost all of what is photosynthesised is used in the plant's own metabolism, with little surplus added to new biomass. The credited description is GPP equals respiration, or less photosynthesis is added to biomass.
For low NPP in a mature community, write
GPP equals respirationorless photosynthesis.No photosynthesisis rejected; some photosynthesis always occurs in a living community.
Consumer net production accounts for ingestion, faeces, and respiration.
The same principle that defines NPP for producers applies to consumers. Net production (N) for a consumer is the energy that becomes new biomass (growth, reproductive tissue, stored reserves) after every loss has been subtracted from what was ingested.
N = I − (F + R): net production equals ingestion minus the combined loss of faeces (F) and respiration (R) at the consumer trophic level.
I: chemical energy in ingested food. F: chemical energy lost in faeces (egestion, material that was never absorbed) and urine (excretion, metabolic waste). R: chemical energy lost as heat via respiration. N: energy left over as new biomass, available to the next trophic level. Endotherms have large R relative to I because thermoregulation is metabolically expensive; ectotherms convert a higher fraction of ingested energy into new biomass at the same trophic level.
Do not substitute the producer equation for the consumer equation.
NPP = GPP − Ris two terms; producers do not ingest.N = I − (F + R)is three terms; consumers ingest, egest, and respire. Faeces is a consumer-side loss only.
This is why fish farms and insect farms convert a higher fraction of feed into edible biomass than mammal farms at the same level of nutritional throughput. Ectotherms do not invest energy in maintaining body temperature, so a larger share of I becomes N rather than R.
Biomass is measured as dry mass; calorimetry converts dry mass to energy.
Biomass measurement sits underneath every productivity calculation. Dry mass is used rather than wet mass because the water content of biological material is highly variable (30–90% in a single leaf depending on time of day and humidity) and water does not represent stored chemical energy. Calorimetry then converts dry mass into an energy value.
Two actions, two marks. Weigh the sample to record fresh mass. Heat the sample in an oven (around 80°C) until the mass becomes constant on successive weighings, which signals that all water has been driven off. Both actions are required; specifying the temperature is not credited.
- Combust a dried sample of known mass in pure oxygen inside a sealed bomb submerged in a known volume of water. The energy released raises the temperature of the surrounding water.
- Calculate the energy released: volume of water × temperature rise × specific heat capacity (AQA provides 4.18 J cm⁻³ °C⁻¹ in calculation stems).
- Divide the energy by the sample's dry mass for J g⁻¹; divide by 1000 for kJ g⁻¹. The question's specified units determine where the chain must end.
The bomb calorimeter is insulated to minimise heat loss to the surroundings, and productivity values are expressed in the standard unit order. The credited insulation mechanism is insulation material or an air space reducing conduction and convection. Productivity units are written as kg m⁻² h⁻¹ in this exact order; reordered or inverted forms such as kg⁻¹ or m⁻² h⁻¹ kg lose the unit mark.
Write
insulation material or an air space reducing conduction and convectionfor calorimeter insulation.Vacuumis rejected; a true vacuum is not used in school-level bomb calorimeters, and writingvacuumsubstitutes a misconception for the actual mechanism.
Agricultural intervention can raise efficiency by reducing energy losses.
Because roughly 90% of energy is lost at each trophic level, food-chain length matters at population scale. Plant-based diets are more energy-efficient per unit land area than meat-based diets at the same nutritional throughput. Agricultural interventions target specific energy losses to raise the fraction of energy that reaches the human consumer.
Raise GPP at the producer level: fertiliser application (nitrogen supports amino acid and therefore protein synthesis); selective breeding for crop varieties with higher photosynthetic capacity. Reduce livestock R: warm housing and restricted movement reduce energy spent on thermoregulation and muscle activity, so a larger fraction of I becomes N. Reduce uneaten biomass: selective breeding for higher meat-yield carcasses; antiparasitic and disease-management programmes.
Compound stems such as "Explain X and the advantage of this" require both halves: the mechanism (the energy-loss route the intervention reduces) and the consequence (more biomass available, less land needed per unit food energy). The cattle-versus-arable worked example follows the same structure: the mechanism is the extra energy loss at the additional trophic level in cattle farming; the conclusion is that arable farming needs less land for the same food energy.
Key terms
- NPP
- GPP
- respiration
- biomass
- dry mass
- calorimeter
- photosynthesis
- heat
- excretion
- trophic level