Skip Navigation

5.1: Cycling

Difficulty Level: At Grade Created by: CK-12
Turn In

Why don't natural systems run out of the materials they need?

You learned that abiotic factors are nonliving materials, such as rain, sunshine, and soil. These factors provide energy and materials that are used by living organisms in the environment. You have seen how energy flows through natural systems. Now let's take a look at how abiotic factors in the environment move through the environment and are used.

“It is always sunrise somewhere; the dew is never all dried at once; a shower is forever falling; vapor is ever rising.”

John Muir quoted in The Earth Speaks

Water is one of the most important abiotic factors in the environment. Maple trees put down roots so they can take up water. Largemouth bass absorb water across their skin and gills. Coast redwood trees trap water from fog with their needles. Desert kangaroo rats find water in the seeds they eat. And you get the water you need from what you eat and drink. Every living thing needs water in one form or another to stay alive

Water comes from rain or snow. But where does water go in the environment? Let's find out by investigating a sample water cycle where it rains and snows a lot. Meteorologists use the word precipitation (pruh-sip-ih-TAY-shun) to describe rain, sleet, hail, mist, and snow. Let's start by tracing the flow of water from a New Hampshire backyard as an example. First, it rains or snows in that backyard fairly often throughout the year. In the summer, some of the rain flows into a pond near the backyard. The pond is an example of open water. Other examples of open water include lakes, ponds, puddles, swamps, and marshes.

Did You Know?
Underground lakes and streams are called aquifers. San Antonio, Texas, is one of the largest cities in the United States to obtain much of its water from an aquifer.

Most of the precipitation that reaches the backyard soaks into the soil. This underground water is called groundwater. There is so much space between the soil particles that make up dirt that a lot of water can be stored in those spaces. In fact, 96 percent of the fresh water in North America is actually underground! Sometimes groundwater is near the surface of Earth and you can dig a well to reach it. Sometimes the water is too far under the ground to get at easily. If the ground surface dips below the level of the groundwater, it becomes open water. Look at Figure 4.1 to get an idea of how all of this water is interconnected.

Figure 4.1 How does water get to the bottom of a well? Look at this diagram of the water cycle to see if you can discover the answer.

“Saving water isn't just something to do in a drought, when the resource is scarce. Each drop of water wasted is a drop less of a wild and scenic river; a drop less of a salmon run, a drop more in a dam filling a glorious valley.”

Katrina Lutz

50 Simple Things You Can Do to Save the Earth

Once the rainwater from that New Hampshire backyard flows into the pond it can take different paths. Some of the water will evaporate or change into water vapor. Then the water vapor directly resupplies the clouds that rain on New Hampshire. Some of the pond water will form a stream that flows from the pond into a river and from the river to the ocean a few miles away. Once the water reaches the ocean, it may circulate in the ocean water for a while. But eventually, it will evaporate and resupply rain clouds that carry the water all over the world.

Let's go back to the backyard again. The water that soaks into the soil and enters the groundwater supply may stay underground for many years. But eventually the groundwater will seep into a lake, pond, or the ocean. There it will evaporate and join the water in clouds.

Did You Know?
You can use 20 liters of water or more if you leave the tap running while you brush your teeth.

You just traced water from the clouds to the ground, to a pond, to the ocean, to the clouds, and back to the ground again. This movement of water is a cycle. Have you ever heard the saying, “What goes around, comes around”? Well, that's a pretty good description of a cycle. A cycle is a process that has no distinct beginning or end but simply keeps repeating itself.

What is the source of the energy that keeps the water cycle operating? As you might suspect from what you know about food chains, the ultimate source of energy is the sun! Heat energy from the sun causes the open water to evaporate. When the amount of evaporated water in the air is more than the air can hold, clouds form. Gravity helps the clouds return the water to Earth's surface. There it can evaporate again.

Activity 4-1: A Day in the Life of a Water Molecule


You are going to investigate where water is found and how water moves through our environment. In this activity you listen to a story and use your imagination to visualize taking a journey as a water molecule. You then have a chance to act out some of the events in the day of a water molecule.


Step 1 When you start the activity, the story will be read to you. Close your eyes and listen carefully to the story. Imagine that you are the water molecule in the story. When the story is finished, you will be able to share your thoughts through the discussion questions below.

Step 2 Think about what happened in this story. Describe in your own words the importance of:

  • evaporation
  • condensation
  • precipitation
  • transpiration

Step 3 Discuss the following questions.

  • How could this water cycle vary, depending upon your location?
  • Do the same water molecules continue to cycle through the environment? Why is this important?
  • What other questions do you have about water?

Step 4 If each person in your group was a water molecule, how would you physically represent the parts of the water cycle as described in the story? Make sure you include:

  • raising up in the air
  • sticking together with other water molecules
  • falling from the sky
  • landing on the ground and
  • traveling through a plant

Step 5 Plan a role-play in your small groups. Describe the events you planned in your group to the whole class.

Step 6 Work with the whole class to create a class play using parts of each group play. Act out the events in the day of a water molecule as the story is read aloud again.

Can you describe this cycle?

What Do You Think?
Many scientists are now recommending the planting of trees on a large scale to reduce the effects of global warming. Why do you think this would help? What do you think are some ways to motivate and organize your classmates to volunteer their time to plant trees in your neighborhood?

Figure 4.2 To understand this carbon cycle diagram, pick a place to start such as Animals and Plants. Follow the arrows through the diagram until you get back to where you started.

Did You Know?
Diamonds and graphite (the “lead” inside pencils) are both forms of pure carbon. Differences in properties, such as color and hardness, result from different arrangements of the carbon atoms and the bonds that hold them together.

The Carbon Cycle

Carbon is another very important abiotic element that cycles through the environment. Carbon plays many vital roles in the environment. Carbon combines with oxygen to form the gas called carbon dioxide (CO2). Remember that carbon dioxide is one of the raw materials needed for photosynthesis. Without carbon dioxide, the process of photosynthesis could not take place. Plants could not manufacture the sugars that provide you and all other consumers the energy to live!

Look at Figure 4.2 to trace how carbon moves through the carbon cycle.

Remember that cycles have no definite beginning or ending. So let's pick a place to begin investigating the carbon cycle. Since plants are so important to the carbon cycle, let's start there. Plants use carbon in the form of carbon dioxide to photosynthesize. The carbon is transferred to animals when they eat plants. Both plants and animals carry on respiration so that their cells will have the energy they need. Respiration releases carbon dioxide to the atmosphere. Plants use carbon dioxide to photosynthesize. They also manufacture and release some carbon dioxide when they respire. Even though plants do release some carbon dioxide when they respire, they take in a lot more than they release.

Plants and animals living in the ocean also cycle carbon. However, their cycle has one additional step. They release their carbon dioxide as tiny gas bubbles into the water. The carbon dioxide gas then seeps from the water back into the atmosphere.

Did You Know?
Phosphorus is a nutrient needed for the formation of ATP as well as other important parts of a cell.

What would happen to humans if all the plants on Earth died?

The bodies of plants and animals contain many carbon compounds. So carbon passes up a food chain from producer to herbivore to carnivore. Then decomposers start their work when living organisms die and begin to decay. The decomposers consume the dead material and use the dead organism as energy. This process also releases carbon dioxide as a waste product of respiration.

A Day in the Life of a Carbon Atom Trace the path of a carbon atom. Imagine yourself as a carbon atom in a carbon dioxide molecule being exhaled from the nose of a lion.

Write a story of what happens to you for the rest of the day. Be sure to include all the major parts of the carbon cycle.

If a dead plant or animal is not completely eaten by decomposers, it may undergo carbonification (kar-bon-ih-fih-KAY-shun). The dead plants and/or animals are squished together with other dead animals and plants. Then, under huge amounts of pressure for a very long time, they eventually turn into coal, oil, or natural gas. These are fuels we use for many purposes. Coal, oil, and natural gas are called fossil fuels because they are made up of the remains of ancient plants and animals.

Our use of fossil fuels also adds to the naturally occurring carbon cycle. Fossil fuels are a good source of easy-to-transport energy. So humans uncover fossil fuels and use them to provide fuel for heat, electricity, and transportation. When coal, oil, and natural gas are burned, the carbon that was stored in them is released to the atmosphere as carbon dioxide.

Carbon dioxide is a greenhouse gas. Greenhouse gases are found in a blanket of air that surrounds Earth. These gases allow more of the sun's heat to enter Earth's atmosphere than to leave it. The greenhouse gases trap heat from the sun in the atmosphere much like glass traps the heat of the sun in a greenhouse. Some carbon dioxide has always been present in the atmosphere. But too much carbon dioxide released by burning fossil fuels can cause environmental problems.

Normally, the carbon in fossil fuels is trapped underground in a liquid or solid form. But burning fossil fuels releases large amounts of carbon to the atmosphere as carbon dioxide. The level of carbon dioxide in the atmosphere has doubled since humans have started burning fossil fuels. As the levels increase, Earth will most likely get warmer. This warming trend is called global warming.

What was the original source of energy for the plants and animals that eventually became coal, oil, or natural gas?

Other Cycles

Have you ever heard someone say, “You can't really throw anything away. There is no ‘away!' ”? They really mean that when you throw something away, it doesn't leave Earth. This is true for everything from old comic books to air pollution to carbon dioxide to water.

Create a Cycle Poster Choose one of three cycles-oxygen, nitrogen, or phosphorus-and create a poster to illustrate how the resource you chose cycles through a natural community. Use library and other references to find out about the cycle you chose.

Every drop of water in every ocean, pond, or lake evaporated from somewhere else on Earth. No matter what, every drop of water will continue to be somewhere on Earth as a part of the water cycle. Unless it is transported on a shuttle mission to another planet, a water molecule can't get to the Moon, Mars, Venus, Alpha Centauri, or anywhere outside of Earth's atmosphere. Earth is a closed system. This means that almost everything on Earth will always be on Earth in one form or another.

  • Explain if it is possible that you could be breathing an atom of carbon that was exhaled by a dinosaur.
  • Explain why you think it is or is not possible to remove elements from their natural cycles.

Imagine the journey that a carbon atom took from the moment it was exhaled by a dinosaur to the moment you exhaled it yourself in a carbon dioxide molecule. Write a story about that carbon atom's journey. Be creative. Try to think of the many plants, animals, and famous historical people of which the carbon atom could have been a part.

Almost all elements on Earth have their own cycles in Earth's closed system. Ecologists have described cycles for many of the nutrients required by organisms to live. They described an oxygen cycle because almost every living thing needs oxygen for respiration. They described a nitrogen cycle because plants need nitrogen to grow. They've even described a phosphorus cycle because plants need a tiny bit of phosphorus to grow and for ATP. And animals-including you-can get nitrogen and phosphorus from eating plants or other animals that once ate plants.

Review Questions

  1. Why don't forests in Wisconsin run out of the things such as water and carbon dioxide that they need to live and function?
  2. What provides the energy for the water cycle?
  3. Where can you find most of the fresh water in the United States?
  4. Why is the carbon cycle studied by ecologists?

Notes/Highlights Having trouble? Report an issue.

Color Highlighted Text Notes
Show More

Image Attributions

Show Hide Details
Save or share your relevant files like activites, homework and worksheet.
To add resources, you must be the owner of the section. Click Customize to make your own copy.
Please wait...
Please wait...
Image Detail
Sizes: Medium | Original