Chapter 22, section 2: Cycles of matter page 746 Key concepts: What processes are involved in the water cycle? How are carbon and oxygen recycled in ecosystems? What is the nitrogen cycle? Key terms: water cycle, evaporation, condensation, precipitation, nitrogen fixation

The water cycle The water cycle is the continuous process by which water moves from Earth’s surface to the atmosphere and back. Evaporation, condensation, and precipitation are the main parts of the water cycle

Evaporation Evaporation is the process by which molecules of liquid water ABSORB ENERGY and change into a gas. In the water cycle, liquid water evaporates from oceans, lakes, and other surfaces and forms water vapor, a gas, in the atmosphere. Living things also give off water. Plants release water vapor from their leaves.

Condensation The process by which a gas changes into a liquid. After evaporating, the water vapor rises into the atmosphere, cools down, and condenses into clouds.

Precipitation Rain, snow, sleet, or hail. Most of it falls back into oceans or lakes. Some of it falls on land and goes into the ground (groundwater) and some runs back into the oceans or lakes.

The carbon and oxygen cycles In ecosystems, the processes by which carbon and oxygen are recycled are linked. Producers, consumers, and decomposers all play roles in recycling carbon and oxygen.

The carbon cycle Producers take in carbon dioxide gas during photosynthesis. They use carbon to make food for themselves. When consumers eat producers, they take in the carbon. Consumers break down the food and release carbon dioxide and water. When producers and consumers die, decomposers break down their remains and return carbon compounds to the soil. Some decomposers also release carbon dioxide as a waste product.

Oxygen cycle Like carbon, oxygen gets cycled through ecosystems. Producers release oxygen as a result of photosynthesis. Most organisms take in oxygen from the air or water and use it to carry out their life processes.

Humans Human activities also affect the levels of carbon and oxygen in the air. Burning fossil fuels releases carbon, and killing large forests reduces the amount of carbon that can be taken out of the atmosphere by plants.

Nitrogen cycle In the nitrogen cycle, nitrogen moves from the air to the soil, into living things, and back into the air. The air is about 78% nitrogen gas. Nitrogen fixation – Most organisms can only use nitrogen once it has been “fixed” or combined with other elements. It is done by some kinds of bacteria. The bacteria usually live in bumps called nodules on the roots of certain plants. This is an example of mutualism. Bacteria get the plants’ sugars, and the plant is supplied with nitrogen.

Return of nitrogen to the environment Once nitrogen has been fixed, producers can use it to build proteins and other complex compounds. Decomposers break down these complex compounds and returns simple nitrogen to the soil. Nitrogen can then cycle from the soil to the producers and then to consumers again.