Develop a model of how matter (especially carbon) cycles through an ecosystem via photosynthesis, feeding, respiration, and decomposition, and contrast the cycling of matter with the one-way flow of energy (MA STE HS-LS2-4, HS-LS2-5, energy and matter).

What this topic is asking

The Massachusetts STE framework (HS-LS2-4 and HS-LS2-5) asks you to develop a model of how matter and energy move through an ecosystem, with a specific focus on the carbon cycle. On the High School Biology MCAS, this topic builds on the cell-level carbon cycle from carbon cycling and matter in organisms and scales it up to a whole ecosystem. You are usually given a cycle diagram and asked to name the processes, explain the role of decomposers, and contrast cycling matter with one-way energy flow. The crosscutting concept is energy and matter.

Matter cycles, energy flows: the central contrast

The biggest idea in this topic, and the one the MCAS rewards most, is the contrast between matter and energy in an ecosystem:

• Matter cycles. Atoms (carbon, nitrogen, oxygen, and others) are passed between organisms and the nonliving environment and reused over and over. They are never created or destroyed, only rearranged.
• Energy flows one way. Energy enters as sunlight, passes along the food chain, and is lost as heat (mainly through respiration). Heat cannot be recaptured into food, so energy cannot cycle and must be continually resupplied by the Sun.

This is why an ecosystem can recycle its matter indefinitely but needs a constant energy input. The energy side is covered in energy flow in ecosystems; here the focus is the cycling of matter.

The carbon cycle in an ecosystem

Trace a carbon atom around the cycle:

1. Photosynthesis. Producers take carbon dioxide from the air and fix the carbon into glucose, then into their tissues. Carbon enters the living part of the ecosystem.
2. Feeding. When a consumer eats a producer (or another consumer), the carbon passes along the food chain into the consumer's body.
3. Respiration. All organisms, producers and consumers alike, carry out cellular respiration, releasing carbon dioxide back into the air.
4. Decomposition. When organisms die or produce waste, decomposers break down the material, releasing the carbon (and other nutrients) back to the environment.

So carbon moves continuously between the air and living things, and the same atoms are reused. This is the ecosystem-scale version of the photosynthesis-respiration loop.

The crucial role of decomposers

Decomposers are easy to overlook but essential. Without them, dead organisms and waste would pile up, and the carbon and nutrients trapped in them would be lost to the living community. By breaking this material down, decomposers recycle nutrients, returning carbon to the air (through their own respiration) and minerals to the soil, where producers can take them up again. The MCAS often asks specifically about the decomposer's role, so always include nutrient recycling.

Try this

Q1. Name the four processes that move carbon through an ecosystem. [2]

• Cue. Photosynthesis (air to producer), feeding (producer to consumer), respiration (organism to air), and decomposition (dead matter to environment).

Q2. Explain why matter cycles in an ecosystem but energy does not. [2]

• Cue. Atoms are conserved and reused, so matter cycles; energy is lost as heat at each step and cannot be recaptured, so it flows one way and must be resupplied by the Sun.