How do carbon, nitrogen, and water cycle through an ecosystem so that matter is reused?
Analyze the cycling of matter through ecosystems, including the carbon, nitrogen, and water cycles, and the roles of photosynthesis, respiration, and decomposers (GSE SB5.b).
A Georgia Milestones Biology EOC answer on the cycling of matter: the carbon cycle (photosynthesis and respiration), the nitrogen cycle (fixation by bacteria), and the water cycle, and how decomposers recycle nutrients, contrasted with the one-way flow of energy.
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What this topic is asking
Standard SB5.b pairs energy flow with the cycling of matter. For the Georgia Milestones Biology EOC you must explain how carbon, nitrogen, and water cycle through ecosystems, the roles of photosynthesis, respiration, and decomposers, and the key contrast: matter cycles (it is reused), while energy flows one way (and is lost as heat). Items often ask which processes move carbon or what decomposers and nitrogen-fixing bacteria do.
Matter cycles, energy flows
The carbon cycle
Carbon moves between the atmosphere (as carbon dioxide) and living things mainly through two opposite processes:
- Photosynthesis removes carbon dioxide from the air and fixes the carbon into glucose and other compounds in producers.
- Cellular respiration breaks those compounds down for energy and releases carbon dioxide back into the air. Decomposition and combustion (burning, including fossil fuels) also return carbon dioxide.
So photosynthesis and respiration are the engine of the carbon cycle, the same reverse-complement pair seen at the cell level, now scaled up to the whole ecosystem.
The nitrogen cycle
Nitrogen is needed to build proteins and nucleic acids, but most organisms cannot use the nitrogen gas that makes up most of the atmosphere. The cycle solves this:
- Nitrogen-fixing bacteria (many living in soil or plant roots) convert atmospheric nitrogen gas into usable compounds (ammonia, then nitrates) that plants can absorb.
- Plants use these to make proteins; animals get nitrogen by eating plants.
- Decomposers break down dead organisms and waste, releasing nitrogen compounds back into the soil, and other bacteria return nitrogen gas to the air.
The takeaway the EOC tests is the role of nitrogen-fixing bacteria (making nitrogen usable) and decomposers (recycling it).
The water cycle
Water cycles through evaporation (liquid water to vapor, driven by the sun), transpiration (water vapor released by plants), condensation (vapor to clouds), and precipitation (rain and snow returning water to the surface). Water then flows through soil, rivers, and organisms and evaporates again. The water cycle links the living and nonliving parts of an ecosystem.
Try this
Q1. Name the two processes that cycle carbon between the air and living things. [2 points]
- Cue. Photosynthesis (removes carbon dioxide) and cellular respiration (releases it); decomposition and combustion also release it.
Q2. State the role of nitrogen-fixing bacteria. [1 point]
- Cue. They convert atmospheric nitrogen gas into usable compounds (ammonia, nitrates) that plants can absorb.
Exam-style practice questions
Practice questions written in the style of GaDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Milestones (style)1 marksWhich two processes are most directly responsible for cycling carbon between the atmosphere and living things? (A) photosynthesis and cellular respiration (B) transcription and translation (C) diffusion and osmosis (D) mitosis and meiosisShow worked answer →
A 1-point selected-response item on the carbon cycle.
The correct answer is A. Photosynthesis removes carbon dioxide from the atmosphere and fixes the carbon into glucose (and other compounds) in producers, while cellular respiration releases carbon dioxide back into the atmosphere when organisms break down those compounds for energy. Together they cycle carbon between the air and living things. The other options are genetic or cellular processes unrelated to the carbon cycle.
Milestones (style)2 marksExplain the role of decomposers and nitrogen-fixing bacteria in the cycling of matter.Show worked answer →
A 2-point item on the nitrogen cycle and decomposers.
Decomposers (bacteria and fungi) break down dead organisms and waste, releasing the nutrients they contain (including nitrogen compounds) back into the soil so producers can reuse them. Nitrogen-fixing bacteria convert nitrogen gas from the atmosphere, which most organisms cannot use directly, into nitrogen compounds (such as ammonia and nitrates) that plants can absorb and use to build proteins and nucleic acids. Both roles return usable matter to the ecosystem, keeping the nitrogen cycle going. Full points need the decomposer role (releasing nutrients from dead matter) and the nitrogen-fixing role (making atmospheric nitrogen usable).
Related dot points
- Analyze the flow of energy through ecosystems using food chains, food webs, and energy pyramids, including the roles of producers, consumers, and decomposers and the ten percent rule (GSE SB5.b).
A Georgia Milestones Biology EOC answer on energy flow: producers, consumers, and decomposers, trophic levels, food chains and food webs, the ten percent rule, and why energy pyramids narrow toward the top.
- Explain the roles of photosynthesis and cellular respiration in the cycling of matter and the flow of energy, including their reactants, products, and how the two processes connect (GSE SB1.e).
A Georgia Milestones Biology EOC answer on photosynthesis and cellular respiration: the reactants and products of each, where they occur, how energy flows and matter cycles, and why the two processes are reverse complements that link plants and animals.
- Evaluate the factors that affect biodiversity and the stability of ecosystems, including keystone species, the effects of removing species, and symbiotic relationships (GSE SB5.c).
A Georgia Milestones Biology EOC answer on biodiversity and stability: why diverse ecosystems are more stable, the role of keystone species, the effects of removing a species, and the three types of symbiosis (mutualism, commensalism, parasitism).
- Analyze data on population growth, including exponential and logistic growth, carrying capacity, and limiting factors (density-dependent and density-independent) (GSE SB5.a).
A Georgia Milestones Biology EOC answer on population growth: exponential versus logistic growth, carrying capacity, density-dependent and density-independent limiting factors, and how to read a population growth curve.
- Evaluate the impact of human activities on ecosystems (habitat destruction, pollution, invasive species, climate change) and design solutions to reduce that impact (GSE SB5.c, SB5.e).
A Georgia Milestones Biology EOC answer on human impact: habitat destruction, pollution, invasive species, and climate change, their effects on biodiversity and ecosystem stability, and conservation solutions to reduce the impact.
Sources & how we know this
- Biology Georgia Standards of Excellence (GSE) — Georgia Department of Education (2024)
- Georgia Milestones Biology EOC Assessment Guide — Georgia Department of Education (2024)