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How does energy flow and matter move through the living systems of the biosphere?

The flow of energy through ecosystems via food chains and trophic levels, the inefficiency of energy transfer, productivity, and the cycling of matter that supports life in the biosphere.

A focused answer to AQA A-Level Environmental Science 3.1.5, covering energy flow through food chains, trophic levels, the inefficiency of transfer, productivity, and how matter cycles through the biosphere.

Generated by Claude Opus 4.811 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. Energy flow through trophic levels
  3. Why energy transfer is inefficient
  4. Productivity
  5. Cycling of matter
  6. Try this

What this dot point is asking

AQA wants you to explain how energy flows through ecosystems via food chains and trophic levels, why energy transfer between levels is inefficient, what productivity means, and how matter is cycled through the biosphere to sustain life. The key skill is distinguishing one-way energy flow from the recycling of matter, and handling productivity figures quantitatively.

Energy flow through trophic levels

Energy flow is one-directional. Almost all ecosystems are ultimately powered by the Sun: producers capture a small percentage of incoming solar radiation and convert it to chemical energy in glucose, which is built into biomass. This energy then passes to consumers when they feed, and finally to decomposers. At every transfer some energy is lost from the living system as heat, so unlike matter, energy is never recycled, it must be continually resupplied by the Sun. (A few ecosystems, such as deep-sea vents, are powered instead by chemosynthesis using chemical energy.)

Why energy transfer is inefficient

Endotherms (birds and mammals) that maintain a constant body temperature lose even more energy as heat, so transfer to and through them is less efficient than for ectotherms. The cumulative effect is dramatic: starting with 100 percent at producers, only about 10 percent reaches primary consumers, 1 percent secondary consumers and 0.1 percent tertiary consumers. This is why food chains rarely exceed four or five trophic levels and why ecosystems support far fewer top predators than herbivores. It also underpins ecological pyramids: a pyramid of energy is always upright, and a pyramid of biomass is usually upright too.

Productivity

  • Gross primary productivity (GPP) is the total rate at which producers fix energy in photosynthesis (units of energy per area per time, for example kJ m−2 yr−1\text{kJ m}^{-2}\text{ yr}^{-1}).
  • Net primary productivity (NPP) is GPP minus the energy the producers lose in their own respiration: NPP=GPP−R\text{NPP} = \text{GPP} - R. NPP is the energy actually stored as new plant biomass and so the energy available to consumers.
  • Secondary productivity is the rate at which consumers build biomass.

Productivity is highest in warm, wet, well-lit ecosystems such as tropical rainforests, coral reefs and estuaries, where light, water and nutrients are all plentiful, and lowest in deserts (water-limited), tundra (temperature-limited) and the open ocean (nutrient-limited). Comparing GPP and NPP across biomes is a common AQA data-handling task.

Cycling of matter

While energy flows through and is lost, matter is recycled. The atoms in carbon, nitrogen, phosphorus and other nutrients pass repeatedly between organisms and the physical environment. Decomposers break down dead organisms and waste, releasing nutrients back into the soil, water and air through the carbon, nitrogen and other biogeochemical cycles. Plants then reabsorb these nutrients, and they pass up the food chain again. This recycling keeps the biosphere continuously supplied with the limited stock of elements life needs, in contrast to the constant external input of solar energy that energy flow requires.

Try this

Q1. Explain why food chains rarely have more than four or five trophic levels. [3 marks]

  • Cue. Only about ten percent of energy passes between levels (the rest lost as heat, faeces and uneaten parts), so after a few steps too little energy remains to support another level.

Q2. Define gross and net primary productivity and give the relationship between them. [3 marks]

  • Cue. GPP is the total rate of energy fixed in photosynthesis; NPP is GPP minus producer respiration; NPP is the energy available to consumers.

Q3. Producers fix 30 000 kJ per square metre per year (GPP) and respire 11 000 kJ per square metre per year. Herbivores assimilate 1900 kJ per square metre per year. Calculate NPP and the transfer efficiency to herbivores. [3 marks]

  • Cue. NPP=30 000−11 000=19 000 kJ m−2 yr−1\text{NPP} = 30\,000 - 11\,000 = 19\,000 \text{ kJ m}^{-2}\text{ yr}^{-1}; efficiency =1900/19 000×100=10 percent= 1900/19\,000 \times 100 = 10 \text{ percent}.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20196 marksExplain why energy transfer between trophic levels is inefficient, and explain the consequences of this inefficiency for the structure of food chains.
Show worked answer →

A 6-mark explain answer needs the losses plus the structural consequences.

Why inefficient. Only about 10 percent of the energy at one trophic level is passed to the next. Energy is lost because: not all of the level below is eaten (roots, bones, some organisms escape); not all of what is eaten is digested and absorbed (energy lost in faeces); and much of the energy that is absorbed is lost as heat from respiration, used for movement, maintaining body temperature and life processes rather than being stored as new biomass. So only a small fraction becomes biomass available to the next level.

Consequences. Because so much energy is lost at each step, the energy available falls sharply along the chain, so food chains rarely exceed four or five trophic levels (too little energy remains to support another level). There is more biomass and energy at the bottom than the top, giving pyramids of energy and usually of biomass. Top predators are few and need large ranges. It also means eating lower down the chain (plant-based diets) feeds more people per unit of land because less energy is lost.

Markers reward (1) the three loss routes (uneaten, undigested, respiration or heat), (2) the 10 percent figure, (3) the limit on chain length, and (4) a consequence such as the energy pyramid or feeding efficiency.

AQA 20215 marksProducers in a grassland fix 20 000 kJ per square metre per year as gross primary productivity and lose 8000 kJ per square metre per year in respiration. Calculate the net primary productivity, then calculate the percentage efficiency of energy transfer to herbivores if they assimilate 1500 kJ per square metre per year.
Show worked answer →

A 5-mark calculation needs both results with clear working.

Net primary productivity. NPP is gross primary productivity minus producer respiration: NPP=20 000−8000=12 000 kJ m−2 yr−1\text{NPP} = 20\,000 - 8000 = 12\,000 \text{ kJ m}^{-2}\text{ yr}^{-1}.

Transfer efficiency to herbivores. Efficiency is the energy reaching the next level divided by the energy available, as a percentage. Using NPP as the energy available to herbivores: efficiency=150012 000×100=12.5 percent\text{efficiency} = \frac{1500}{12\,000} \times 100 = 12.5 \text{ percent}.

Markers reward (1) correct NPP with units, (2) using NPP (not GPP) as the denominator, and (3) the efficiency of 12.5 percent. A common error is dividing by GPP, which understates efficiency. The result, around 10 to 13 percent, is consistent with the ten percent rule of thumb.

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