Describe how matter cycles through ecosystems, including the carbon, nitrogen, and water cycles, and explain the role of decomposers in returning nutrients (TEKS Biology, Reporting Category 5; energy and matter; systems and system models).

What this topic is asking

The Biology TEKS ask you to describe how matter cycles through ecosystems, the carbon, nitrogen, and water cycles, and the role of decomposers. For STAAR Reporting Category 5 you need to explain how atoms are reused (in contrast to energy, which flows one way) and the key processes that move carbon, nitrogen, and water. This is an energy and matter and systems and system models topic.

Why matter cycles

This contrast, matter cycling while energy flows one way, is the single most tested idea linking this topic to energy flow and food webs. Because the same atoms are reused, an ecosystem does not run out of carbon or nitrogen the way it would run out of energy without sunlight.

The carbon cycle

Carbon moves between the air and living things through two main processes:

• Photosynthesis removes carbon dioxide from the air, building carbon into glucose and other molecules in producers.
• Cellular respiration returns carbon dioxide to the air as organisms release energy.

Carbon also returns to the air when decomposers break down dead matter and when fuels are burned (combustion). The balance between photosynthesis (removing carbon dioxide) and respiration plus combustion (returning it) is why human burning of fossil fuels raises atmospheric carbon dioxide (see ecological succession and human impact).

The nitrogen cycle

The key idea for STAAR is the role of bacteria in making nitrogen usable and the role of decomposers in returning it. Nitrogen is needed to build proteins and nucleic acids, so its cycle is essential to life.

The water cycle and decomposers

The water cycle moves water through evaporation (from oceans, lakes, and organisms), condensation (forming clouds), and precipitation (rain and snow), with water taken up by organisms and returned to the environment. Across all the cycles, decomposers (bacteria and fungi) play a crucial role: by breaking down dead organisms and wastes, they release the locked-up nutrients back into the soil, water, and air so that producers can reuse them. Without decomposers, nutrients would stay trapped in dead matter and the cycles would stall.

Try this

Q1. Explain why matter can be recycled in an ecosystem but energy cannot. [2]

• Cue. Matter is conserved and cycles (returned by decomposers and respiration); energy is lost as heat at each transfer and cannot be reused, so it must keep entering from the Sun.

Q2. State the role of decomposers in the cycling of matter. [2]

• Cue. They break down dead organisms and wastes, returning nutrients (carbon, nitrogen) to the soil, water, and air so producers can reuse them.