Energy Flows, Matter Cycles

By the end of this lesson, you will be able to explain how energy moves through a system and how matter cycles through it, while both stay conserved.

Think about a cattle ranch in the Texas Hill Country. That ranch is a system, which means a group of parts that work together and can be studied as one whole. Sunlight shines on the grass, the grass uses that light to build food, the cattle eat the grass, and later the cattle's waste and dead plant material break down into the soil. Almost every science topic you study has parts that connect like this.

Watch the energy first. Energy flows in a one-way path: it enters the pasture as sunlight, moves into the grass, then into the cattle, and along the way much of it leaves as heat into the air. Energy is never used up or destroyed, but once it spreads out as heat it is no longer useful to living things. That is why the ranch needs new sunlight every single day.

Now watch the matter. Matter cycles, meaning the same atoms are used over and over in loops rather than moving in a straight line. Carbon atoms in the grass become carbon atoms in the cow, then carbon dioxide the cow breathes out, then grass again. Water and nitrogen travel in similar loops. No new atoms are delivered to the ranch, and none disappear.

Both patterns follow conservation, the rule that the total amount of energy and the total amount of matter stay the same, even when they change form or location. If you could weigh everything and measure every joule before and after, the totals would match.

Try it: If a cow eats fifty pounds of grass in a day, where does all that matter go, since the cow does not gain fifty pounds?