Trace the carbon cycle (with the nitrogen and water cycles) through living and nonliving parts of an ecosystem, and explain why matter cycles while energy does not.
Unlike energy, matter does not flow through an ecosystem and disappear -- it cycles, passing repeatedly between living things and the nonliving environment. The carbon cycle is a clear example: carbon dioxide in the air is pulled into plants during photosynthesis and built into sugars and other molecules. Animals eat those plants and incorporate that carbon into their own bodies. When any organism respires, some carbon returns to the air as carbon dioxide; when an organism dies, decomposers break its body down and release the remaining carbon back to the soil and air, ready to be used again.
Nitrogen and water cycle in similar loops. Nitrogen gas makes up about 78% of the air, but plants and animals cannot use it directly in that form; certain bacteria in soil "fix" nitrogen gas into a form plants can absorb through their roots, animals get nitrogen by eating plants (or other animals), and decomposers and other bacteria eventually return nitrogen to the soil and air. Water evaporates from oceans and lakes, falls as precipitation, is taken up by plant roots and released back to the air through transpiration, and flows back to the ocean -- the same cycle studied in earlier grades, now understood as one more example of matter looping through an ecosystem rather than flowing through it once.
Mastery looks like: They can trace carbon through at least four steps of its cycle and clearly explain why matter cycles while energy flows one way.
Common stumbles: Confusing the carbon cycle with the energy pyramid from last week; assuming nitrogen gas in the air is already in a form living things can use directly.