Ecosystem Unit 1: Lesson 4

Nutrient Cycles Carbon Cycle Nitrogen Cycle Water Cycle

Carbon Cycle All living organisms need carbon to make carbohydrate, fat, and protein molecules Plants take in carbon in the form of carbon dioxide for photosynthesis. Plants convert the carbon into sugar as food. Animals consume plants and take in the carbon as food. When animals and plants die, decomposers take in the carbon as food. All living organisms including decomposers breathe out carbon as carbon dioxide into the air. Combustion of fossil fuels releases a lot of carbon dioxide.

Nitrogen Cycle All living organisms need nitrogen to make protein molecules. Nitrogen-fixing bacteria convert the nitrogen in the air into nitrates. Plants absorb these nitrates and convert them into plant proteins. Animals eat plants and convert the nitrogen into animal proteins and then into ammonium compounds. Dead organisms are broken down by decomposers into ammonium compounds. Nitrifying bacteria change these ammonium compounds into nitrates. Then, some of the denitrifying bacteria convert the nitrates in the soil into nitrogen.

Water Cycle Water is evaporated from any exposed sources such as rivers, oceans, and lakes into the air. Water vapor is also released during the process of transpiration and respiration of living things. Then, the water vapor condenses into water droplets and clouds are formed. As the water droplets get bigger, they fall to Earth as rain and return back to the rivers and oceans.

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