Describe how matter cycles through ecosystems in the carbon, nitrogen, and water cycles, and how living processes and human activity move and store it (Ohio's Learning Standards for Science, Biology, B.DI.2).

What this topic is asking

Ohio standard B.DI.2 (Ecosystems) pairs the one-way flow of energy with the cycling of matter: the same atoms are used again and again. The Ohio Biology EOC turns this into items where you trace an element (carbon, nitrogen) through its cycle, name the processes that move it, or explain how human activity disrupts it (which links to B.DI.3). The crosscutting idea is energy and matter: matter is conserved and cycled, unlike energy. This builds directly on photosynthesis and cellular respiration, the two processes at the heart of the carbon cycle.

Matter cycles; energy flows

The single most important contrast in this part of the standards: matter cycles, energy flows. Atoms of carbon, nitrogen, oxygen, and hydrogen are never used up; they move between living things and the non-living environment and are reused indefinitely. Energy, by contrast, enters as sunlight and is lost as heat, so it must be constantly resupplied. The cycles of matter are called biogeochemical cycles because they pass through living things (bio), the rocks and soil (geo), and chemical forms.

The carbon cycle

Carbon is the backbone of all organic molecules, and it moves between the atmosphere (as carbon dioxide) and living things.

• Photosynthesis removes carbon dioxide from the air: producers convert it into glucose (organic carbon), fixing carbon into the living world.
• Cellular respiration returns carbon dioxide to the air: organisms break down glucose for energy and release carbon dioxide.
• Decomposition releases carbon dioxide as decomposers break down dead matter.
• Combustion (burning) releases carbon dioxide, including from wood and from fossil fuels.

Carbon is stored in living organisms, in soil and dead matter, dissolved in the oceans, and underground as fossil fuels (coal, oil, gas) formed from ancient organisms. Burning fossil fuels releases carbon that was locked away for millions of years, adding carbon dioxide to the atmosphere faster than photosynthesis and the oceans remove it, which raises atmospheric carbon dioxide and drives climate change.

The nitrogen cycle

Nitrogen is needed to build proteins and nucleic acids (DNA, RNA). The atmosphere is about 78% nitrogen gas, but most organisms cannot use nitrogen gas directly, so it must be converted. Bacteria are central to every step.

• Nitrogen fixation. Nitrogen-fixing bacteria (in the soil and in the root nodules of legumes) convert nitrogen gas into ammonia and related compounds that plants can absorb.
• Uptake and transfer. Plants absorb these nitrogen compounds to make proteins; animals get nitrogen by eating plants or other animals.
• Decomposition. Decomposers return nitrogen from dead matter and waste to the soil as ammonia (and other bacteria convert it to nitrates).
• Denitrification. Denitrifying bacteria convert nitrogen compounds back into nitrogen gas, returning it to the atmosphere.

The takeaway for the EOC: plants depend on bacteria to make atmospheric nitrogen usable, because they cannot fix it themselves.

The water cycle

Water moves continuously between the atmosphere, the land, and the oceans by physical processes.

• Evaporation. Heat turns liquid water (from oceans, lakes, soil) into water vapor.
• Transpiration. Plants release water vapor from their leaves, adding to evaporation.
• Condensation. Water vapor cools and forms clouds.
• Precipitation. Water falls as rain, snow, or hail.
• Runoff and infiltration. Water flows over the surface to rivers and oceans or soaks into the ground.

Living things both depend on and drive the water cycle (transpiration moves large amounts of water), tying the cycle to ecosystems.

Try this

Q1. State the key difference between how energy and matter move through an ecosystem. [1]

• Cue. Energy flows one way through an ecosystem and is lost as heat; matter (atoms) cycles and is reused.

Q2. Name the process by which nitrogen-fixing bacteria make atmospheric nitrogen available to plants. [1]

• Cue. Nitrogen fixation.