Carbon, Water & Cycling Matter · Grade 7 Science
Chapter: Ecosystems, Matter & EnergyCarbon, Water & Cycling Matter
Learning goals
- I can explain carbon, water & cycling matter.
- I can use evidence, a model, and stated assumptions to support a biological explanation.
Before this unit
Use the supplied vocabulary, distinguish an observation from an explanation, and identify any units or model assumptions. Revisit a connected earlier unit if needed; new terms are explained here.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
The carbon in the breath you just exhaled was, a few hours ago, part of something you ate; before that it was in a plant; before that it was in the air, possibly having been exhaled by someone else. Atoms are not consumed. They are moved between stores and rearranged into different molecules, over and over, and the same atoms have been doing this for as long as there has been life. A reservoir is one of those stores — the atmosphere, the oceans, living organisms, soil, rock — and a cycle is the set of processes moving matter between them.
The carbon cycle runs on processes this course has already covered. Photosynthesis moves carbon from the air into living tissue. Respiration, in plants and animals alike, moves it back out. Feeding moves it from organism to organism. Decomposition releases it from dead material. Burning moves it from fuels to the air quickly, and over long periods carbon accumulates in rock and sediment and returns slowly. The cycle is one loop, and the thing worth noticing is that respiration and photosynthesis are the two main arrows running in opposite directions.
Water cycles too, by change of state rather than by chemical reaction. Evaporation lifts it from surfaces to the air; transpiration does the same through the pores of plant leaves, and in a forest that is a large fraction of the total; condensation turns vapour back to liquid as air cools, forming cloud; precipitation returns it; and runoff carries it across the surface to rivers and the sea, while some soaks into the ground. To follow matter through any of this you draw a boundary, count what crosses in and out, and work out the storage change — which is what the pond model at the end of this unit does.
Build your understanding
Where this fits
Energy flows through and leaves. Matter does not: it cycles, which is why a decomposer's work matters as much as a producer's. This unit follows two substances round their loops, and brings back the balance-sheet method from the kidney unit as the way to check any claim about where matter has gone.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Part 1 · Carbon through living processes
Start with the two opposing arrows. Photosynthesis takes carbon dioxide from the air and builds it into glucose, so carbon moves from the atmosphere into living tissue. Respiration breaks glucose down and releases carbon dioxide, so carbon moves back. Plants do both; animals do only the second. Over a day in good light a plant takes in more than it releases, which is how it grows.
Then feeding moves carbon between organisms. The carbon in a rabbit was in grass; the carbon in a fox was in the rabbit. No new carbon is created at any step; it is relocated and rearranged, which is why an ecosystem can keep running on the same atoms indefinitely while needing a continuous supply of energy.
Decomposition is the return path that completes the loop. Dead organisms and waste are broken down by fungi and bacteria, which respire and release carbon dioxide, returning carbon to the air and nutrients to the soil. Without it, carbon would accumulate in dead material and producers would eventually be starved of it — which is roughly what happened over long stretches of geological time, producing coal.
Pause and explain
Name the two processes that move carbon in opposite directions between air and living things.
Check your reasoning after answering
Photosynthesis moves carbon from the atmosphere into living tissue, building carbon dioxide into glucose. Respiration moves it back, breaking glucose down and releasing carbon dioxide. Plants carry out both and animals only the second, and decomposition is the further return path releasing carbon from dead material.
Part 2 · Reservoirs, and how fast matter moves between them
A reservoir is a store, and the useful questions about one are how much it holds and how quickly matter enters and leaves it. The atmosphere holds a modest amount of carbon and exchanges it rapidly. Living organisms hold more and exchange over years to centuries. Soil and sediment hold a great deal and exchange slowly. Rock holds vastly more and exchanges over geological time.
Rates matter as much as amounts, and this is where the distinction becomes practically important. Burning a fossil fuel moves carbon from a slow reservoir — rock and sediment, where it accumulated over millions of years — into a fast one, the atmosphere, within seconds. The carbon is not new, and the total is unchanged; what has changed is which reservoir holds it and how quickly the transfer happened.
That reframing is worth holding onto, because it explains why conservation of matter and a changing atmosphere are not in conflict. Matter being conserved says nothing about where it is. A cycle can be entirely closed and still have one of its reservoirs grow at the expense of another.
Pause and explain
If matter is conserved, how can the amount of carbon in the atmosphere change?
Check your reasoning after answering
Because conservation says the total is unchanged, not that its distribution is. Carbon moves between reservoirs — atmosphere, organisms, soil, ocean, rock — and one can grow while another shrinks. Burning fuel transfers carbon from a slow reservoir where it accumulated over millions of years into the atmosphere in seconds, without creating any.
Part 3 · Water, states and the plant pathway
Water cycles by changing state rather than by reacting. Liquid water at a surface gains enough energy for molecules to escape as vapour — evaporation. Vapour cooling in rising air loses energy and returns to liquid droplets — condensation, which is cloud. Those droplets grow and fall as precipitation. The energy driving all of it comes from the Sun.
Plants add a pathway that is easy to overlook. Transpiration is the loss of water vapour through pores in leaves, and it is not incidental: it is what pulls water up from the roots through the whole plant. In a forested region, transpiration can account for a large share of the water returning to the air, which is why removing forest changes local water movement.
On the ground, water that does not soak in becomes runoff, flowing across the surface to streams and rivers. How much soaks in depends on the surface — soil with good pore space takes up a great deal, compacted or paved ground almost none — which is the link to the soil unit next and to the human impact unit in the following chapter.
Pause and explain
What is transpiration, and why does it matter beyond the individual plant?
Check your reasoning after answering
It is the loss of water vapour through pores in leaves, and it is what draws water up from the roots through the plant. Beyond the individual, it returns a large share of water to the air in vegetated regions, so removing forest reduces that return and changes local water movement — which is why land cover affects more than the plants on it.
Part 4 · Balancing a store within a stated boundary
To track matter you need a boundary, as the kidney unit established. Decide what is inside — here, a pond — then list every pathway crossing it, sort them into inputs and outputs, and calculate the change in storage as inputs minus outputs. The arithmetic is simple and the discipline is what catches errors.
Work the model. Over an interval, rain adds 40 L and runoff adds 20 L, so inputs total 60 L. Evaporation removes 25 L and outflow removes 15 L, so outputs total 40 L. The storage change is 60 − 40 = +20 L: the pond gains 20 L over the interval.
Two things that look contradictory are both true here. The pond's contents changed, and matter was conserved throughout — every litre is accounted for by a pathway crossing the boundary. A store growing or shrinking is not a violation of conservation; it is what conservation predicts when inputs and outputs differ. The model also states that other pathways are excluded, and a real pond would have groundwater exchange and uptake by organisms, so the 20 L is the answer for the stated boundary rather than for a real pond.
Pause and explain
A pond gains 40 L of rain and 20 L of runoff, and loses 25 L to evaporation and 15 L to outflow. What is the storage change, and does it break conservation of matter?
Check your reasoning after answering
Inputs total 60 L and outputs total 40 L, so storage increases by 20 L. It does not break conservation. Every litre is accounted for by a pathway crossing the stated boundary; conservation says the total is unchanged, and a store growing simply means more entered than left over that interval.
Apply the whole unit · Balance an illustrative water store
Illustrative pond interval, not measured data: rain adds 40 L; runoff adds 20 L; evaporation removes 25 L; outflow removes 15 L. All other pathways are excluded by the model.
Your task
Calculate the change in storage, label each pathway as an input or an output, and explain how a changing amount of water is consistent with matter being conserved.
What your response should show
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Response guidance — after your attempt
Sort the pathways: rain and runoff cross inward as inputs; evaporation and outflow cross outward as outputs.
Total the inputs: 40 L + 20 L = 60 L entering over the interval.
Total the outputs: 25 L + 15 L = 40 L leaving over the interval.
Calculate the storage change: 60 L − 40 L = +20 L, so the pond gains 20 L.
The pond gains 20 L over the interval. Rain at 40 L and runoff at 20 L are the inputs, crossing the boundary inward and totalling 60 L. Evaporation at 25 L and outflow at 15 L are the outputs, crossing outward and totalling 40 L. The change in storage is inputs minus outputs: 60 − 40 = +20 L. That increase is entirely consistent with matter being conserved, and the two ideas are often confused. Conservation says that water is not created or destroyed, not that any particular store stays the same size. Every litre in this model is accounted for by a pathway across a stated boundary: the 20 L the pond gained came from the rain and runoff that entered and did not leave. A store grows whenever inputs exceed outputs and shrinks when they do not, and in both cases the total water in the wider system is unchanged — it has simply moved between reservoirs, exactly as carbon does when it moves from rock to atmosphere. Two limits are worth stating. The model explicitly excludes other pathways, and a real pond would exchange water with groundwater through its bed, lose some to organisms and gain some from them, and might overflow differently at different levels. And the figures are for one stated interval, so they cannot be extended to a season without knowing how rainfall and evaporation vary. The method, though, transfers directly: name the boundary, list the pathways, sort them, and subtract.
Scientific references for this unit
Parent / teacher teaching notes
Ask for a because-statement and a labeled relationship, not just a memorized word. Work through one calculation step at a time, permit oral or diagram responses, and use flexible sessions for difficult reading. Compare the learner’s actual evidence with the supplied model; do not reward copying or agreement with a personal preference. No private family, genetic, or medical records are required.
The key idea
Matter moves among organisms, air, water, and soil. Carbon dioxide can supply carbon for photosynthetic food production; feeding transfers organic matter; respiration and decomposition return carbon to other forms. Some reservoirs store carbon for long periods, so a cycle does not mean every atom completes the same path quickly.
Worked examples
Example 1 · Balance a store and find a missing pathway
- Calculate the change the stated pathways predict: 30 − 12 = +18 L.
- Compare with the measurement: the pond gained only 5 L, which is 13 L less than predicted.
- Interpret: a further output of about 13 L must exist that the account omitted — outflow, seepage into the ground, or uptake by organisms.
When inputs minus outputs does not match the observed change, suspect a missing pathway first.
Example 2 · Reconcile conservation with a rising reservoir
- State what conservation claims: the total amount of carbon is unchanged by ordinary chemical and biological processes.
- Distinguish total from distribution: carbon sits in several reservoirs and moves between them.
- Apply it: moving carbon from rock and sediment into the atmosphere by burning increases one reservoir and decreases another, leaving the total the same.
Conservation constrains the total. It says nothing about where the matter is.
Avoid these mistakes
- Mistake: conservation of matter means a store cannot change. It means the total is unchanged; a pond gaining 20 L or an atmosphere gaining carbon is matter moving between reservoirs.
- Mistake: only animals respire, so plants only add oxygen. Plants respire continuously too; over a day in good light photosynthesis exceeds it, which is how they grow.
- Mistake: the water cycle is evaporation and rain. Transpiration through leaves returns a large share of water in vegetated regions, and how much soaks in versus runs off depends on the surface.
- Mistake: a balance that does not close means conservation failed. It means a pathway is missing from the account, which is exactly what the method is for finding.
Check your understanding
Does sunlight supply the carbon atoms in sugar?
Discuss the answer
No. Carbon dioxide supplies carbon matter in the plant model; light supplies energy.
Does a cycle mean no reservoir can change?
Discuss the answer
No. Inputs and outputs can differ over an interval, changing a reservoir’s stored amount.
Words to know
- reservoir
- A store of matter — atmosphere, ocean, organisms, soil, rock — characterised by how much it holds and how fast matter enters and leaves.
- transpiration
- Loss of water vapour through pores in leaves, which draws water up from the roots and returns much of a region's water to the air.
- condensation
- Vapour returning to liquid as air cools, forming cloud; a change of state rather than a chemical reaction.
- runoff
- Water flowing across the surface to streams and rivers instead of soaking in; how much depends on the surface.
- storage change
- Inputs minus outputs across a stated boundary; a store can grow or shrink while matter is conserved overall.
Extend your scientific explanation
Compare a second supplied example or the linked educational reference. Identify a connection, a model limit, and an additional observation that would test the explanation.
In the real world
Use a clear question, reliable observations, and a biological mechanism to evaluate a claim. State what remains uncertain before applying a classroom model to a different setting.
Exit check
Why can a pond lose water while matter is conserved?
Check your explanation after answering
Water can leave its chosen boundary through evaporation, flow, or other transfers and remain elsewhere in the larger system.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.