Develop a model to illustrate the cycling of matter, including the role of photosynthesis and cellular respiration in the carbon cycle (Louisiana Student Standards for Science, High School Biology, HS-LS2-5).

What this topic is asking

Louisiana's LS2 standards (HS-LS2-5) ask you to model how matter cycles through an ecosystem, especially the carbon cycle and the roles of photosynthesis and cellular respiration. For LEAP 2025 Biology you should be able to trace carbon (and nitrogen) through living and nonliving parts of an ecosystem, explain the role of decomposers, and contrast recycled matter with one-way energy flow. Because this is a modeling standard, the test often shows a carbon-cycle diagram and asks you to identify a process or fill in an arrow.

Matter cycles, energy flows

This is the key contrast with energy: energy enters as sunlight, flows one way through organisms, and is lost as heat, so it must be constantly resupplied. Matter, in contrast, is recycled, the carbon atoms in a leaf today may be in an animal next month and in the air after that.

The carbon cycle

This is exactly why photosynthesis and respiration are studied as a linked pair: together they are the engine that moves carbon between organisms and the atmosphere.

Decomposers and recycling

Decomposers (bacteria and fungi) are essential to the cycling of matter. They break down dead organisms and waste, and in doing so they release the nutrients (carbon, nitrogen, and minerals) locked in that material back into the soil and air, where producers can take them up again. Without decomposers, nutrients would stay trapped in dead matter and the cycle would stall.

The nitrogen cycle

Nitrogen is needed to build proteins and DNA, but most organisms cannot use the nitrogen gas that makes up most of the air. The nitrogen cycle solves this: nitrogen-fixing bacteria (often in soil and on plant roots) convert nitrogen gas into compounds plants can absorb; the nitrogen then passes through the food web as organisms eat one another; and decomposers and other bacteria eventually return nitrogen to the soil and air. The main idea for the test is that bacteria make atmospheric nitrogen usable.

Try this

Q1. Name the two processes that move carbon between living things and the atmosphere, and state which way each moves it. [2]

• Cue. Photosynthesis removes carbon dioxide from the air (into glucose); cellular respiration returns carbon dioxide to the air.

Q2. Explain why matter is described as cycling while energy flows one way. [2]

• Cue. The same atoms of matter are reused over and over within the ecosystem (recycled), but energy enters as sunlight, is lost as heat at each step, and must be constantly resupplied.