Explain the role of plants in the carbon cycle, describing how photosynthesis, respiration, and decomposition move carbon between the atmosphere and living things.
Carbon atoms move in a continuous loop between the atmosphere and living things called the carbon cycle, and plants sit at the center of it. Through photosynthesis, plants pull carbon dioxide out of the air and use its carbon atoms to build sugars and, eventually, the rest of the plant's body: leaves, wood, roots. That carbon stays locked in the plant's tissue, called biomass, for as long as the plant lives, sometimes for centuries in a large tree.
Carbon returns to the atmosphere in several ways: when a plant, or an animal that ate the plant, respires, breaking down sugar for energy, it releases carbon dioxide back out; when a plant dies and decomposes, decomposers break down its tissue and release the stored carbon as carbon dioxide again; and when wood or fossil fuels, ancient plant and animal matter, burn, their stored carbon is released quickly. Forests, because they hold so much carbon in their biomass, act as a carbon sink, a place where more carbon is stored than released, which is why they partly offset the extra carbon dioxide humans add by burning fossil fuels.
Mastery looks like: They can trace carbon's path into a plant through photosynthesis and back out through respiration, decomposition, or burning, and explain what a carbon sink is.
Common stumbles: Thinking carbon only moves one direction, into plants, without a return path; not connecting this week's carbon cycle back to the greenhouse effect and evidence discussed in unit 2.