Cycles of Matter — Science

  1. Grade 6 — Cycles of Matter in Ecosystems
  2. Grade 7 — Cycles of Matter: Carbon & Water
  3. Grade 10 — Biogeochemical Cycles

    ↑ Deepens: matter cycles become the biogeochemical cycles.

Apply it in a new setting

Problem 1

A single carbon atom starts out in a CO_2 molecule in the air. Which sequence correctly traces it INTO a food web and back to the atmosphere?
  1. Cellular respiration builds it into a plant sugar → a rabbit eats the plant → photosynthesis releases it back to the air as CO_2
  2. Photosynthesis locks it into a plant sugar → a rabbit eats the plant → cellular respiration releases it back to the air as CO_2
  3. Condensation traps it inside a leaf → transpiration → it falls to Earth as precipitation
  4. Combustion stores it inside a plant → decomposition → it leaves the plant as nitrogen gas
Hints

Which process pulls CO_2 OUT of the air and into a plant — photosynthesis or respiration?

Which process breaks sugar back down and puts CO_2 back INTO the air?

Photosynthesis and respiration are mirror images; condensation, transpiration, and combustion belong to other cycles.

Answer and explanation

Photosynthesis locks it into a plant sugar → a rabbit eats the plant → cellular respiration releases it back to the air as CO_2

Photosynthesis absorbs CO_2 and locks the carbon into plant sugar; that carbon travels up the food web when the rabbit eats the plant.

Cellular respiration (in the rabbit, or a decomposer later) breaks the sugar down and exhales the carbon back as CO_2.

The photosynthesis → feeding → respiration path is correct — B.

Problem 2

A sealed, balanced terrarium sits in a sunny window with plants, animals, and decomposers inside. Which statement about matter and energy in it is correct?
  1. Both carbon and energy are recycled endlessly, so the terrarium would keep going even in total darkness
  2. The energy is recycled inside, but new carbon must constantly be added from outside
  3. Carbon atoms are recycled indefinitely, but energy must keep arriving as light because it flows through once and is lost as heat
  4. Neither the carbon nor the energy can change form once the container is sealed
Hints

Think about the big contrast: which one cycles, and which one flows through only once?

Matter (carbon) is trapped in the sealed box and gets reused; energy is not reused.

That is exactly why the terrarium still needs sunlight.

Answer and explanation

Carbon atoms are recycled indefinitely, but energy must keep arriving as light because it flows through once and is lost as heat

Matter is conserved and recycled: the same carbon atoms cycle between air, producers, consumers, and decomposers inside the sealed box.

Energy is different — it flows through once and leaves as heat, so a steady light supply is required.

Carbon recycled but energy resupplied is correct — C.

Problem 3

Burning coal raises atmospheric CO_2 and throws the carbon cycle out of balance. Which explanation of WHY is scientifically correct?
  1. It releases carbon that sat in a slow, geologic reservoir for millions of years, faster than photosynthesis and the oceans can reabsorb it
  2. Combustion creates brand-new carbon atoms that did not exist before the coal was burned
  3. Rising CO_2 makes the world's plants stop performing photosynthesis
  4. The carbon in coal can never again be absorbed by any plant or ocean
Hints

Matter is conserved — combustion rearranges atoms, it does not create carbon.

Fossil-fuel carbon had been locked in a SLOW reservoir; burning moves it to the air in a geologic instant.

The imbalance is about RATE: added faster than the fast cycle can pull it back.

Answer and explanation

It releases carbon that sat in a slow, geologic reservoir for millions of years, faster than photosynthesis and the oceans can reabsorb it

Carbon atoms are conserved, so combustion cannot create new carbon (ruling that out), and CO_2 can still be reabsorbed, just slowly.

The real issue is rate: coal holds carbon from a slow, million-year reservoir, and burning injects it into the air far faster than photosynthesis and the oceans remove it.

The rate-mismatch explanation is correct — A.

Problem 4

A farmer grows corn in the same field year after year, and the soil's usable nitrogen keeps dropping — even though about 78% of the air above the field is nitrogen gas. Which explanation and fix are both correct?
  1. The air's nitrogen is the wrong isotope, so installing lightning rods would restore the soil
  2. Corn absorbs nitrogen gas straight through its leaves, so only erosion could be lowering the soil nitrogen
  3. Nitrogen gas is too heavy to reach the roots; tilling the field deeper would bring it down
  4. N_2's strong triple bond keeps corn from using it directly; planting legumes, whose root bacteria fix nitrogen, would replenish the soil
Hints

Why can't plants use the abundant N_2 in the air directly? Think about its triple bond.

Nitrogen becomes usable only after nitrogen fixation converts N_2 into nitrates.

Which crops host nitrogen-fixing bacteria in their roots?

Answer and explanation

N_2's strong triple bond keeps corn from using it directly; planting legumes, whose root bacteria fix nitrogen, would replenish the soil

Plants cannot break N_2's strong triple bond, so they can't use atmospheric nitrogen directly — it must be 'fixed' into nitrates first.

Legumes (beans, clover) host nitrogen-fixing bacteria in root nodules, so rotating them in naturally replenishes soil nitrogen.

The triple-bond explanation plus the legume fix is correct — D.

Problem 5

In one year a young tree pulls in 12 kg of carbon through photosynthesis, but its own cellular respiration sends 5 kg of that carbon back to the air as CO_2. How many kilograms of carbon are left locked into new wood and leaves that year?
Hints

Photosynthesis takes carbon IN; the tree's respiration puts some back OUT.

The carbon stored in new growth is the amount taken in minus the amount released.

12 kg in − 5 kg back out.

Answer and explanation

7 kg

Carbon captured by photosynthesis = 12 kg; carbon returned by the tree's respiration = 5 kg.

Net carbon stored in new tissue = 12 − 5.

12 − 5 = 7 kg locked into new wood and leaves.

Problem 6

A large forest is cut down and the land is paved over for a parking lot. Which prediction about the LOCAL water cycle is most accurate?
  1. More transpiration, because pavement releases large amounts of water vapor into the air
  2. Less transpiration and more surface runoff, so less water vapor enters the local air and less water soaks into the ground
  3. The water cycle stops completely, since trees are the only thing on Earth that evaporates water
  4. Evaporation from the oceans shuts down
Hints

Trees release water vapor through their leaves — what is that process called, and what happens when the trees are gone?

Rain that once soaked into forest soil now hits hard, impervious pavement.

Expect less water vapor added locally and more fast runoff.

Answer and explanation

Less transpiration and more surface runoff, so less water vapor enters the local air and less water soaks into the ground

Removing the trees removes their transpiration, cutting the water vapor added to the local air; only living plants transpire, so pavement cannot increase it.

Rain now runs off impervious pavement instead of soaking in, raising runoff and lowering groundwater recharge — while ocean evaporation elsewhere continues normally.

Less transpiration and more runoff is the accurate prediction — B.

Problem 7

Imagine a soil treatment wiped out the decomposers (bacteria and fungi) in a meadow. Predict the long-term effect on the carbon and nitrogen cycles there.
  1. Nothing changes, because only living plants and animals take part in the cycles
  2. Carbon cycles faster, because more dead material piles up to burn
  3. Carbon and nitrogen stay locked in dead remains instead of returning to the air and soil, so producers eventually run short of nutrients
  4. The cycles speed up, because plants can now absorb nitrogen gas directly from the air
Hints

What job do decomposers do in every matter cycle?

Without them, dead plants and animals are not broken down.

Think about where the trapped carbon and nitrogen fail to go.

Answer and explanation

Carbon and nitrogen stay locked in dead remains instead of returning to the air and soil, so producers eventually run short of nutrients

Decomposers are the recyclers that return carbon and nitrogen from dead matter back to the air and soil.

Remove them and dead remains pile up with their nutrients locked inside, so producers are eventually starved of the recycled nutrients they depend on.

Nutrients trapped in dead remains, starving producers, is correct — C.

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