Explain how matter cycles through ecosystems in the carbon, nitrogen, and water cycles (North Carolina Standard Course of Study, Biology, LS.Bio.4).

What this topic is asking

North Carolina LS.Bio.4 asks how matter cycles through ecosystems, focusing on the carbon, nitrogen, and water cycles. For the Biology EOC you need to know the key processes in each cycle, the special role of bacteria in the nitrogen cycle, and that decomposers recycle matter back to the environment. The big idea, from bioenergetics, is that matter cycles while energy flows. Items often ask you to identify a process in a cycle.

The carbon cycle

The two central processes are the ones you met in bioenergetics:

• Photosynthesis removes carbon dioxide from the atmosphere and fixes the carbon into glucose in producers. This moves carbon into living things.
• Cellular respiration breaks down glucose and returns carbon dioxide to the atmosphere. This moves carbon out of living things.

Carbon also enters the atmosphere when decomposers break down dead matter and when fossil fuels are burned (combustion), the latter being a major human addition. The carbon cycle is the clearest example of matter cycling, and it links directly to the photosynthesis-and-respiration relationship.

The nitrogen cycle and the role of bacteria

Nitrogen matters because it is needed for proteins and nucleic acids, linking this topic back to the macromolecules. The key EOC point is that bacteria make nitrogen usable, because organisms cannot use nitrogen gas straight from the air.

The water cycle and decomposers

The water cycle moves water between the land, oceans, atmosphere, and living things through three main processes:

• Evaporation turns liquid water into vapor (with transpiration adding water vapor released by plants).
• Condensation turns water vapor into clouds.
• Precipitation returns water to the surface as rain or snow, where it can run off, soak into the ground, or be taken up by organisms.

Across all the cycles, decomposers (bacteria and fungi) play a shared role: they break down dead organisms and waste, releasing the carbon, nitrogen, and other atoms back into the environment so they can be reused. Without decomposers, matter would stay locked in dead bodies and the cycles would stall.

Try this

Q1. Name the two processes that cycle carbon between organisms and the atmosphere. [2]

• Cue. Photosynthesis (takes carbon dioxide in) and cellular respiration (releases carbon dioxide).

Q2. Explain why nitrogen-fixing bacteria are essential in the nitrogen cycle. [2]

• Cue. Plants cannot use nitrogen gas directly; the bacteria convert it into usable forms (ammonia, nitrates) that plants can absorb.