If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Biology library

Course: Biology library > Unit 28

Lesson 7: Biogeochemical cycles
Science
Biology library
Ecology
Biogeochemical cycles
© 2023 Khan AcademyTerms of usePrivacy PolicyCookie Notice

Intro to biogeochemical cycles

Google Classroom
Get an overview of how atoms are recycled through Earth's ecosystems via biogeochemical cycles.

Key points

  • Energy flows through an ecosystem and is dissipated as heat, but chemical elements are recycled.
  • The ways in which an element—or compound such as water—moves between its various living and nonliving forms and locations in the biosphere is called a biogeochemical cycle.
  • Biogeochemical cycles important to living organisms include the water, carbon, nitrogen, phosphorus, and sulfur cycles.

Introduction

What is your body made of? Not to put too fine a point on it: atoms. Lots and lots of them. About 7,000,000,000,000,000,000,000,000,000 to be precise.start superscript, 1, end superscript Where did all of those atoms come from?
If we really walk it backwards, most of the elements that make up our bodies—and those of every other living thing!—were born in dying stars billions of years ago. That's pretty cool, but it doesn't capture the whole picture. What have the atoms of your body been doing more recently, during their time on Earth?

Energy flows, but matter is recycled.

Energy flows directionally through Earth’s ecosystems, typically entering in the form of sunlight and exiting in the form of heat. However, the chemical components that make up living organisms are different: they get recycled.
What does that mean? For one thing, the atoms in your body are not brand new. Instead, they've been cycling through the biosphere for a long, long time, and they've been part of many organisms and nonliving compounds along the way. You may or may not believe in reincarnation as a spiritual concept, but there's no question that atoms in your body have been part of a huge number of living and nonliving things over the course of time!
In this image, the flow of energy is shown with yellow and red arrows. Yellow indicates usable energy and red indicates energy lost in the unusable form of heat. Green arrows show the continual recycling of chemical nutrients. Image credit: Biogeochemical cycles: Figure 1 by Eva Horne and Robert A. Bear; source article is under license CC BY 4.0
The six most common elements in organic molecules—carbon, nitrogen, hydrogen, oxygen, phosphorus, and sulfur—take a variety of chemical forms. They may be stored for long or short periods in the atmosphere, on land, in water, or beneath the Earth’s surface, as well as in the bodies of living organisms. Geologic processes—such as weathering of rocks, erosion, water drainage, and the subduction of continental plates—all play a role in this recycling of materials, as do interactions among organisms.
The ways in which an element—or, in some cases, a compound such as water—moves between its various living and nonliving forms and locations is called a biogeochemical cycle. This name reflects the importance of chemistry and geology as well as biology in helping us understand these cycles.

Which biogeochemical cycles are key to life?

Water, which contains hydrogen and oxygen, is essential for living organisms. That places the water cycle pretty high on the list of cycles we care about!
The hydrosphere—the set of places where water can be found as it cycles on Earth—is large and diverse. Water is present as a liquid on the Earth's surface and underneath the ground, as ice in the polar ice caps and glaciers, and as water vapor in the atmosphere. For more information about how water cycles among these forms, check out the water cycle article.
Water makes up more than half of our bodies, but humans cannot live by water alone. Instead, there are some other key elements that keep our bodies running and are part of biogeochemical cycles:
  • Carbon is found in all organic macromolecules and is also a key component of fossil fuels. See the carbon cycle article for more info.
  • Nitrogen is needed for our start text, D, N, A, end text, start text, R, N, A, end text, and proteins and is critical to human agriculture. See the nitrogen cycle article for more info.
  • Phosphorus is a key component of start text, D, N, A, end text and start text, R, N, A, end text and is one of the main ingredients—along with nitrogen—in artificial fertilizers used in agriculture. See phosphorus cycle article for more info.
  • Sulfur is key to protein structure and is released to the atmosphere by the burning of fossil fuels.
These cycles don't happen in isolation, and the water cycle is a particularly important driver of other biogeochemical cycles. For example, the movement of water is critical for the leaching of nitrogen and phosphate into rivers, lakes, and oceans. The ocean is also a major reservoir—holding tank—for carbon.
Though each element or compound takes its own route, all of these key chemical nutrients cycle through the biosphere, moving between the biotic—living—and abiotic—nonliving—worlds and from one living organism to another.

Want to join the conversation?

Log in