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9th grade · Science · Unit 14 of 14

Carbon & Nitrogen Cycles

How carbon and nitrogen move through living things, air, water and soil, and what happens when the cycles are disrupted.

What your child practices

Practices the steps of the carbon cycle (photosynthesis, respiration, decomposition, combustion) and the nitrogen cycle (fixation, nitrification, assimilation, denitrification), why they matter to ecosystem stability, and the consequences of disrupting them, such as fertilizer runoff and burning fossil fuels.

Texas Essential Knowledge and Skills (TEKS) teks student expectation: B.13(C) · the carbon and nitrogen cycles and the consequences of disrupting them

How we explain it

Children can open this short lesson before they practice.

  1. Carbon cycle: photosynthesis takes carbon dioxide in; respiration, decomposition and combustion release it.
  2. Fossil fuels are remains of ancient organisms buried for millions of years.
  3. Nitrogen cycle: bacteria fix nitrogen, plants assimilate it, animals eat plants, denitrification returns it to the air.

Example: Which process releases carbon dioxide from living cells? Photosynthesis takes it in. Cells give it off when they release energy. Answer: cellular respiration

Carbon & Nitrogen Cycles in Texas

In the Texas Essential Knowledge and Skills (TEKS), this is unit 14 of 14 in 9th grade Science, after Ecosystem Stability.

Gradelings describes a child's practice on this unit as Beginning, Approaching, Meeting or Exceeding (Four practice steps). That reflects practice, not a report-card mark: the teacher decides the mark.

Source: Texas Education Agency (TEA)

Sample questions

A taste of this unit. In the app, questions change every time and get easier or harder to match your child.

  1. Question 1

    Decomposers return carbon to the air and soil by…

    • photosynthesizing
    • breaking down dead organisms
    • fixing nitrogen
    • making fossil fuels quickly

  2. Question 2

    Cutting and burning forests disrupts the carbon cycle by…

    • fixing nitrogen
    • releasing stored carbon and reducing photosynthesis
    • storing more carbon
    • cooling the planet

  3. Question 3

    Lightning helps the nitrogen cycle by…

    • removing all nitrogen
    • making sugar
    • freezing the soil
    • turning N₂ into compounds that rain carries to the soil

  4. Question 4

    Bacteria in the root nodules of beans and peanuts…

    • release only carbon dioxide
    • fix nitrogen from the air
    • turn sugar into starch
    • make fossil fuels

  5. Question 5

    Which process releases carbon dioxide from living cells?

    • Cellular respiration
    • Transpiration
    • Nitrogen fixation
    • Photosynthesis

  6. Question 6

    A lake gets far more nitrogen and phosphorus than usual. This overload is called…

    • photosynthesis
    • denitrification
    • succession
    • eutrophication

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