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 keeps cycling without ever disappearing.

Picture a pasture in Texas. The sun shines on the grass. A cow eats the grass. Later, a hawk circles over the field hunting a mouse that nibbles seeds. All of these parts working together make a system, which is a group of parts that interact with each other. This pasture is one big system, and you can trace both energy and matter moving through it.

Energy flows in one direction, like water sliding down a hill. It starts at the sun, moves into the grass, then into the cow when the cow eats. Every time energy moves to a new living thing, some of it escapes as heat. That is why energy has to keep arriving from the sun. It does not come back around for another turn.

Matter is different. Matter is anything that takes up space and has mass, like grass, water, air, and soil. Matter cycles, which means it moves in a loop and gets used again and again. The cow eats grass, and its waste falls to the ground. Tiny decomposers break that waste down into nutrients in the soil. New grass grows using those same nutrients. The matter went in a circle and came back.

Here is the big idea. Matter is conserved, which means the total amount of matter stays the same, even when it changes form. A rotting apple does not vanish. Its matter becomes soil, gas in the air, and food for worms. If you could weigh everything, the amount would match.

Try it: If a log burns in a campfire and only a little ash is left, where did the rest of the log's matter go?