Energy passes through; matter goes round: Every ecosystem needs energy flowing through it and matter used again and again. A biogeochemical cycle is how an element keeps coming back for new life.
The points to remember
- Energy flows one way: in as sunlight, out as heat lost in respiration.
- Matter cycles: the same atoms of carbon and nitrogen are used again and again.
- Earth gains almost no new matter, so elements must be recycled to stay available.
- Living things need elements such as carbon, nitrogen and phosphorus to build their bodies.
- Energy is lost at every feeding level, so food chains rarely have more than 4 or 5 levels.
Remember it as: Energy passes through; matter goes round.
Real example: in 1991 eight people were sealed inside Biosphere 2, a giant glass building in Arizona, USA, for two years. Sunlight came in, but no new air, water or food. Everything had to be recycled, and when soil microbes used up oxygen faster than plants made it, oxygen fell from 21% to about 14%.
Say which one cycles: Energy never cycles: it is lost as heat. Only matter goes round. (Open and closed systems: see 1.2.5.)
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Each element has its own cycle, with stores and flows linking them.
The points to remember
- A cycle moves an element between living (biotic) and non-living (abiotic) parts.
- Producers take elements in from air, water and soil: carbon dioxide, nitrates, phosphates.
- Elements pass along food chains when one organism eats another.
- Decomposers break down dead matter and waste and return the elements to soil, water and air.
- Without this recycling, elements would stay locked in dead matter and run out for new life.
| Cycle | Main stores | Key flows |
|---|---|---|
| Carbon | Air, living things, soil, oceans, fossil fuels | Photosynthesis, respiration, decomposition |
| Nitrogen | Air (nitrogen gas), soil, living things | Nitrogen fixation, nitrification, denitrification |
| Water | Oceans, ice, groundwater, air | Evaporation, condensation, precipitation |
| Phosphorus | Rocks, soil, living things | Weathering, uptake by plants, decomposition |
Remember it as: Taken in, passed on, broken down, taken in again.
Real example: in the Amazon rainforest, fallen leaves rot within weeks in the warm, wet air, and tree roots take the nutrients back up at once. Almost all the nutrients are in the living trees, not the thin soil, so when the forest is cleared and burned, the soil feeds crops for only a few years.
The next pages: Stores, sinks and sources are on 2.3.2. Whether a flow is a transfer or a transformation is on 2.3.4 (and 1.2.4).
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Natural cycles are roughly in balance: as much of an element enters each store as leaves it. People change the size of the flows, and the balance breaks.
The points to remember
- People speed up some flows: burning fossil fuels, clearing forests, spreading fertiliser.
- Stores then fill or empty faster than natural flows can balance them.
- More carbon in the air leads to global warming and ocean acidification.
- Extra nitrogen and phosphorus in water lead to eutrophication: algae blooms and dead zones.
- Clearing plants and soil breaks the loop, so land loses its nutrients and fertility.
- A system whose cycles are broken cannot keep itself going: it is not sustainable.
Remember it as: Speed up a flow, fill a store, break the balance.
Real example: for thousands of years the air held about 280 parts per million (ppm) of carbon dioxide. Burning coal, oil and gas since about 1750 has added carbon faster than plants and oceans take it up: by 2023 it had reached about 421 ppm, warming the planet.
Link it to sustainability: Saying 'humans burn fossil fuels' is not enough. Say what it does to the cycle (a store fills too fast) and why that makes the ecosystem less sustainable.
A cycle is also a way of thinking: a model you can use to judge whether any system, natural or human, can last.
The points to remember
- In a cycle, what flows out of a store comes back as an input: nothing is wasted.
- So stores stay steady over long periods: the root of sustainability.
- It fits natural systems: carbon, nitrogen and water cycles, ocean currents, air circulation, rocks.
- It fits human systems too: recycling and the circular economy copy nature's loops.
- It shows where a loop is broken: a one-way flow from a store to waste cannot last.
- Limit: some loops take millions of years (fossil fuels, rocks), so for people they are one-way.
- Limit: energy does not cycle, and a simple model hides detail.
Why the idea is useful
- Shows that outputs return as inputs, so stores stay steady
- Works for many systems, natural and human
- Points to the broken loop that needs fixing
Its limits
- Some loops take millions of years
- Energy flows, it does not cycle
- A simple model hides detail and is hard to measure
Real example: at Kalundborg in Denmark, firms have swapped wastes since the 1970s: the power station's spare steam heats homes and a fish farm, and gypsum from cleaning its chimney gases is made into plasterboard. A human system run like a natural cycle (circular economy, see 1.3.21).
Weigh it up: In a 'justify' or 'to what extent' answer, give the strengths with named systems, then the limits, then a clear judgement.
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How this comes up: Paper 2, Section B (c): justify how useful the idea of a cycle is for understanding sustainability [9].
Many environmental systems work as loops, from the carbon cycle in a forest to a town's recycling scheme.
With reference to named systems, justify the claim that the idea of a cycle helps us understand how a system can be sustainable.
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