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How do carbon, nitrogen, and water cycle through living things and the environment?

Explain how matter cycles through ecosystems, including the carbon, nitrogen, and water cycles, and the roles organisms play in them (NGSSS SC.912.L.17; Reporting Category 3, Organisms, Populations, and Ecosystems).

A benchmark-level answer on biogeochemical cycles for the Florida Biology 1 EOC: the carbon cycle (photosynthesis and respiration), the nitrogen cycle and nitrogen-fixing bacteria, the water cycle, and how matter cycles while energy flows.

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  1. What this topic is asking
  2. Matter cycles, energy flows
  3. The carbon cycle
  4. The nitrogen cycle
  5. The water cycle
  6. Try this

What this topic is asking

This Reporting Category 3 content asks you to explain how matter cycles through ecosystems, focusing on the carbon, nitrogen, and water cycles, and the roles organisms play. For the Florida Biology 1 EOC you need to know which processes move each substance, the special role of bacteria in the nitrogen cycle, and the big contrast that matter cycles while energy flows. Items often test the carbon cycle (photosynthesis and respiration) and nitrogen-fixing bacteria.

Matter cycles, energy flows

The cycles that move matter are called biogeochemical cycles because they pass through living things (bio), the Earth (geo), and chemical forms.

The carbon cycle

Carbon moves between the atmosphere (as carbon dioxide) and living things:

  • Photosynthesis removes carbon dioxide from the air and fixes the carbon into glucose (and then other molecules) in producers.
  • Cellular respiration (by all organisms) breaks glucose back down and releases carbon dioxide to the air.
  • Decomposition of dead matter and combustion (burning) of wood and fossil fuels also return carbon dioxide to the atmosphere.

So the carbon cycle runs mainly on the paired processes of photosynthesis (carbon in) and respiration (carbon out), the same pair from the photosynthesis and respiration connection. Burning fossil fuels adds extra carbon dioxide, contributing to climate change.

The nitrogen cycle

The most-tested point: nitrogen-fixing bacteria make atmospheric nitrogen usable. Without them, plants could not obtain nitrogen even though it surrounds them in the air.

The water cycle

Water cycles through the environment by physical processes and through organisms:

  • Evaporation (and transpiration from plants) turns liquid water into water vapor.
  • Condensation forms clouds.
  • Precipitation (rain, snow) returns water to the surface.
  • Water moves through runoff into rivers, lakes, and oceans, and through the ground.

Organisms take up water (roots, drinking) and release it (transpiration, respiration, excretion), linking the water cycle to life.

Try this

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

  • Cue. Photosynthesis (removes carbon dioxide and fixes carbon) and cellular respiration (releases carbon dioxide).

Q2. Explain the role of nitrogen-fixing bacteria in the nitrogen cycle. [2]

  • Cue. They convert nitrogen gas (which plants cannot use directly) into compounds such as ammonia and nitrates that plants can absorb to make proteins and DNA.

Exam-style practice questions

Practice questions written in the style of FLDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

FL Biology 1 EOC (2023 released style)1 marksWhich two processes are most directly responsible for cycling carbon between living things and the atmosphere? (A) Photosynthesis and cellular respiration. (B) Transcription and translation. (C) Mitosis and meiosis. (D) Diffusion and osmosis.
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A 1-point multiple-choice item on the carbon cycle.

The correct answer is A. Photosynthesis removes carbon dioxide from the air and fixes the carbon into glucose; cellular respiration breaks glucose down and returns carbon dioxide to the air. Together they cycle carbon between organisms and the atmosphere. The other pairs are unrelated to carbon cycling.

The carbon cycle runs on photosynthesis (carbon in) and respiration (carbon out).

FL Biology 1 EOC (2024 released style)1 marksPlants need nitrogen to make proteins and DNA, but they cannot use nitrogen gas from the air directly. What allows nitrogen to become available to plants? (A) Plants breathe in nitrogen gas. (B) Nitrogen-fixing bacteria convert nitrogen gas into compounds plants can absorb. (C) Nitrogen is made by photosynthesis. (D) Nitrogen comes only from rainfall as pure gas.
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A 1-point item on the nitrogen cycle.

The correct answer is B. Nitrogen gas makes up most of the air but is unusable by plants directly. Nitrogen-fixing bacteria (in the soil and in some plant roots) convert nitrogen gas into compounds such as ammonia and nitrates that plants can absorb and use to build proteins and nucleic acids. The other options misstate how plants obtain nitrogen.

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