In this topic you will learn about how animals play various roles in the cycles of life.

Water evaporates, condenses, and moves from one part of the environment to another. In other words water is recycled. The process of naturally recycling water on Earth is called the water cycle. Heat energy from the Sun makes water from oceans and lakes evaporate. As moist air rises, it cools. When it is cooled enough, the water vapor condenses into water droplets. These droplets gather to form clouds. When the droplets become large and heavy they fall to Earth's surface as rain or snow. Some of the water seeps into the ground. The water finds its way to oceans and lakes. The cycle starts over again.

Nature recycles carbon, which is used by all organisms. The continuous transfer of carbon between the atmosphere and living things is called the carbon cycle. Carbon dioxide enters the air when animals breathe out. It enters the air when fossil fuels such as coal or oil are burned. During photosynthesis plants use the carbon from carbon dioxide to make sugars, starches, and proteins. Animals eat the plants. The carbon in the plant substances is used by the animals. When living things die and decay, the carbon compounds in their bodies are released. Some of it is turned into carbon dioxide by decomposers. Over millions of years some of it turns into fossil fuels.

Proteins are a part of your muscles and many cell structures. Proteins are rich in the element nitrogen. You need nitrogen to make parts of your body such as muscles, nerves, skin, bones, blood, and digestive tissues. Animals and plants cannot use the nitrogen that is in air. Animals get nitrogen by eating proteins. Plants get nitrogen by absorbing it from the soil. Some plants, like peas and beans, get nitrogen with the help of a special group of bacteria. The transfer of nitrogen from the atmosphere to plants and back to the atmosphere and directly into plants again is called the nitrogen cycle.

Air is about 78 percent nitrogen gas. Certain bacteria can use nitrogen from the air to make nitrogen-containing substances called nitrates. Plants absorb nitrates dissolved in water through their roots. The nitrogen is used by plants to make proteins. Animals eat plant proteins or they eat animals that eat plant proteins. Animal wastes contain nitrogen compounds. Some soil bacteria turn nitrates back into nitrogen gas and the cycle repeats.