Cycles in Nature

as you read

You'll Learn

■ Explain the importance of Earth's water cycle.

■ Diagram the carbon cycle.

■ Recognize the role of nitrogen in life on Earth.

It's Important

The recycling of matter on Earth demonstrates natural processes.

Q Review Vocabulary biosphere: the part of the world in which life can exist

New Vocabulary

• evaporation

• condensation

• nitrogen fixation

• nitrogen cycle

The Cycles of Matter

Imagine an aquarium containing water, fish, snails, plants, algae, and bacteria. The tank is sealed so that only light can enter. Food, water, and air cannot be added. Will the organisms in this environment survive? Through photosynthesis, plants and algae produce their own food. They also supply oxygen to the tank. Fish and snails take in oxygen and eat plants and algae. Wastes from fish and snails fertilize plants and algae. Organisms that die are decomposed by the bacteria. The organisms in this closed environment can survive because the materials are recycled. A constant supply of light energy is the only requirement. Earth's biosphere also contains a fixed amount of water, carbon, nitrogen, oxygen, and other materials required for life. These materials cycle through the environment and are reused by different organisms.

The Water Cycle

If you leave a glass of water on a sunny windowsill, the water will evaporate. Evaporation takes place when liquid water changes into water vapor, which is a gas, and enters the atmosphere, shown in Figure 9. Water evaporates from the surfaces of lakes, streams, puddles, and oceans. Water vapor enters the atmosphere from plant leaves in a process known as transpiration (trans puh RAY shun). Animals release water vapor into the air when they exhale. Water also returns to the environment from animal wastes.

Figure 9 Water vapor is a gas that is present in the atmosphere.

Figure 9 Water vapor is a gas that is present in the atmosphere.

44 ♦ E CHAPTER 2 The Nonliving Environment

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