Organisms in biological ecosystems connect to one another through their need for matter as well as energy. Every organism needs molecules like proteins, carbohydrates, and fats to provide the raw building materials for their cells.

One of the most fascinating facts about Earth is that almost all the matter on the planet today has been here since Earth first formed. That means all the carbon, hydrogen, oxygen, nitrogen, and other elements that make up the molecules of living things have been recycled over and over throughout time. Consequently, ecologists say that matter cycles through ecosystems.

Scientists track the recycling of atoms through cycles called biogeochemical cycles.

Because the element carbon forms the backbone of the molecules that make up cells, one of the most important biogeochemical cycles to life on Earth is the carbon cycle.

The major components of the carbon cycle are

  • Plants take in carbon dioxide from the atmosphere, using it to build carbohydrates via photosynthesis.

  • Carbon moves through food chains as organisms eat other organisms, incorporating the carbon-containing molecules from their food into the organic molecules that make up their own bodies.

  • Carbon moves from living things back to the environment as all types of organisms use some of their food molecules as a source of energy. To transfer energy from their food to their cells, organisms break down the food molecules into carbon dioxide and water in the process of cellular respiration.

  • In addition to being animals that do cellular respiration, humans also contribute to the carbon cycle through industrial processes. The combustion of fossil fuels produces carbon dioxide and water, contributing to the amount of carbon dioxide in the atmosphere.

  • The fossil fuels that humans use today formed hundreds of millions of years ago when dead plants were deposited in the Earth in a way that slowed down the activity of the bacteria and fungi that normally decompose dead matter. Because the organic molecules in the dead plants weren't broken down by these decomposers, the organic molecules could be converted by geologic forces into oil, coal, and natural gas.

  • Carbon dioxide diffuses from the air into the oceans, where it exists in a dissolved form, creating another large reservoir of carbon.

Take out a sheet of paper to try the practice questions. For questions 1–9, use the terms that follow to label the events of the carbon cycle shown in the figure.

a. Cellular respiration by animals

b. Cellular respiration by plants

c. Cellular respiration by decomposers (decomposition)

d. Atmospheric carbon dioxide

e. Photosynthesis

f. Dissolved carbon dioxide

g. Organic carbon in living things

h. Combustion of fossil fuels

i. Conversion of organic carbon to fossil fuels


The following is how the figure should be labeled:

  1. d. Atmospheric carbon dioxide

  2. b. Cellular respiration by plants

  3. g. Organic carbon in living things

  4. a. Cellular respiration by animals

  5. e. Photosynthesis

  6. h. Combustion of fossil fuels

  7. f. Dissolved carbon dioxide

  8. c. Cellular respiration by decomposers (decomposition)

  9. i. Conversion of organic carbon to fossil fuels.