The earth receives energy from the sun and radiates it back to space, but it gains almost no new matter. Every atom of water, carbon and nitrogen in use today has been used before, many times. That is what a natural cycle describes.
The water cycle
| Stage | What happens |
|---|---|
| Evaporation | the sun's heat turns liquid water into vapour, mostly from oceans |
| Transpiration | plants release water vapour from their leaves |
| Condensation | rising vapour cools and forms droplets, making clouds |
| Precipitation | droplets combine until they are heavy enough to fall as rain, snow, sleet or hail |
| Collection and runoff | water gathers in oceans, lakes and groundwater, and the cycle repeats |
The sun drives the whole thing, and gravity returns the water. There is no start and no end.
The carbon cycle
Carbon moves between the air, living things, the ocean and rock.
- Plants take carbon dioxide from the air during photosynthesis and build it into sugars.
- Animals eat plants, and both release carbon dioxide during respiration.
- Decomposers break down dead material and return carbon to the soil and the air.
- Buried remains compressed over millions of years become coal, oil and gas, which release that carbon when burned.
Photosynthesis and respiration are close to opposites. One takes in carbon dioxide and releases oxygen. The other takes in oxygen and releases carbon dioxide. Plants do both.
The nitrogen cycle
Nitrogen makes up most of the air, but plants and animals cannot use it in that form. Certain bacteria in soil and in the roots of some plants convert nitrogen gas into compounds plants can absorb, a process called nitrogen fixation. Animals get nitrogen by eating plants. Decomposers return it to the soil, and other bacteria eventually release it back to the air.
Why energy is not a cycle
Matter is reused. Energy is not. Sunlight arrives, passes through living things as they eat and move, and leaves as heat that radiates back into space. It never comes round again, which is why an ecosystem needs a constant supply and why the sun is on every food chain diagram.
Common mistakes
Thinking water gets used up
It changes state and location, not quantity. Drinking water today may have been in a cloud last month and in an ocean a century ago.
Drawing energy as a cycle
Energy flows one way through an ecosystem. Only matter cycles.
Check yourself
What supplies the energy that drives the water cycle?
The sun, which evaporates water. Gravity brings it back down.
What is transpiration?
Water vapour released from plant leaves, which adds a significant amount of moisture to the air.
Why can plants not use nitrogen straight from the air?
Because nitrogen gas is very unreactive. Bacteria must first fix it into compounds plants can absorb through their roots.
Why is energy shown as an arrow rather than a loop?
Because it passes through once and leaves as heat. Unlike matter, it is not recycled within the ecosystem.
See also
Sources
- The water, carbon and nitrogen cycles as taught in United States upper elementary science. Examples on this page are our own.