Cycling Matter Through Organisms & Environment · Grade 6 Science
Chapter: Ecosystems, Watersheds & ConservationCycling Matter Through Organisms & Environment
Learning goals
- I can trace a carbon atom through at least four stages of the carbon cycle.
- I can explain why nitrogen is scarce for plants when the air is 78 per cent nitrogen.
- I can describe how human activity has changed the carbon and nitrogen cycles.
Readings for this unit
Before this unit
Read the chapter opening and the named observation or diagram. Clarify one word or unit that blocks understanding. Recall the relevant earlier measurement, matter, or energy idea.
Study the unit across three 35–45-minute class sessions. Use the worked examples after an attempt and the supplied records when actual equipment is not available.
Conservation of mass, from chapter 2, is what makes cycles necessary. Atoms cannot be made or destroyed in any ordinary process, so every carbon atom in a leaf was somewhere else before and will be somewhere else again. Earth receives a continuous supply of energy and essentially no new matter.
The carbon cycle has a fast loop and a slow one. The fast loop runs through photosynthesis, respiration and decomposition in days to decades. The slow loop buries carbon in rock and fossil fuels for tens or hundreds of millions of years, and the two run at wildly different speeds.
Nitrogen shows the pattern that matters most. The air is 78 per cent nitrogen gas, an enormous supply, and almost no organism can use it, because the two atoms in a nitrogen molecule are held together so strongly. Plants depend entirely on a few kinds of bacteria that can break that bond.
Build your understanding
Where this fits
The last lesson followed energy through an ecosystem and found it flowed one way. This one follows matter and finds it goes round. The nitrogen section explains why fertiliser exists, which the next lesson needs when it deals with the consequences of using it.
Use three flexible 40-45-minute sessions: why matter must cycle; the carbon cycle fast and slow; the nitrogen problem and how humans changed both cycles, then the practice set, revision and the exit check. Workshop, review, quiz and exams have separate time.
Part 1 · The same atoms, over and over
Because atoms are conserved, every ecosystem is working with a fixed stock and recycling it. The carbon in the sugar a plant makes today was in the air yesterday and in some other organism before that, and the oxygen you are breathing was released by a plant somewhere. Decomposers are the step that makes this possible: fungi and bacteria break down dead tissue and return its atoms to the soil and the air in forms other organisms can take up. Remove them and the cycle jams, with nutrients locked in material nothing can use.
This also explains why a sealed terrarium can run for years, a point chapter 1 used when defining systems. Light crosses the glass continuously, supplying energy, and almost no matter crosses at all, so the same water, carbon and nitrogen go round inside indefinitely. The arrangement fails only if something breaks the cycle, such as all the decomposers dying, and when it does, it fails because matter has become stuck rather than because anything has run out. Scale up the same idea and you have the problem facing any long space mission. A crew cannot carry years of food, water and oxygen, so a closed life-support system has to recycle all three, and the engineering difficulty is almost entirely about closing the loops rather than about supplying energy, since sunlight is freely available. Experiments such as the Biosphere 2 project in the early 1990s found closing a matter cycle completely to be far harder than expected.
Pause and explain
Why can a sealed terrarium survive for years when a sealed box of food cannot?
Check your reasoning after answering
Because the terrarium cycles its matter while light keeps crossing the glass to supply energy. Plants rebuild sugars from the carbon dioxide that organisms release, and decomposers return nutrients from dead material to the soil, so the same atoms are used repeatedly. A sealed box has no producers and no light, so nothing rebuilds what is consumed.
Part 2 · Carbon on two clocks
The fast carbon cycle moves roughly 120 billion tonnes of carbon from the air into plants each year through photosynthesis, and returns almost exactly the same amount through respiration by plants, animals and decomposers. The ocean exchanges a further 90 billion tonnes or so each year, dissolving carbon dioxide at the surface and releasing it again. These flows are enormous and nearly balanced, which is why atmospheric carbon dioxide stayed between about 180 and 300 parts per million for hundreds of thousands of years.
The slow cycle moves far less and lasts far longer. A small fraction of dead material is buried before decomposers reach it, in swamps and on sea floors, and over millions of years pressure and heat turn it into coal, oil, gas and limestone. Weathering of rock and volcanic eruptions return it, but the whole slow loop handles well under one billion tonnes a year. Burning fossil fuels takes carbon out of the slow cycle and injects it into the fast one at about 36 billion tonnes of carbon dioxide a year, which is why a small flow can shift a large store: the fast cycle's balance was never designed to absorb a slow-cycle contribution. The ocean is the part of this that most people underestimate. Sea water has absorbed roughly a quarter of all the carbon dioxide released by burning fossil fuels, which has slowed the rise in the atmosphere considerably and come at a cost: dissolved carbon dioxide forms a weak acid, and ocean surface water has become measurably more acidic, which makes it harder for corals, shellfish and some plankton to build shells of calcium carbonate.
Pause and explain
The fast carbon cycle moves far more carbon each year than humans emit. Why does the human contribution still change the atmosphere?
Check your reasoning after answering
Because the large natural flows nearly cancel while the human flow does not. Photosynthesis removes roughly as much as respiration returns, so the net change from those is close to zero. Fossil fuel burning adds carbon from the slow cycle with no matching removal, so the whole of it accumulates rather than being offset.
Part 3 · An ocean of nitrogen nobody can drink
Nitrogen is required for proteins and for DNA, so no organism can grow without it, and the air above every field is 78 per cent nitrogen gas. Almost none of it is available. The two atoms in a nitrogen molecule are bound by a triple bond that takes an enormous amount of energy to break, and only a few kinds of bacteria can manage it, some living free in soil and some in nodules on the roots of peas, beans and clover. Lightning breaks a small amount as well. Everything else in the living world depends on what those bacteria release.
That bottleneck shaped agriculture for thousands of years. Farmers discovered empirically that growing clover or beans restored a field's fertility, and rotated crops accordingly, long before anyone knew bacteria existed. The bottleneck was broken in 1909 when Fritz Haber found a way to combine nitrogen and hydrogen industrially under high pressure and temperature, which made artificial fertiliser possible. Something like half the nitrogen in the body of an average person today passed through that industrial process, and the world's population could not be fed without it. The process is also a good illustration of a cost that is easy to miss. Breaking that triple bond industrially requires temperatures around 450 degrees Celsius and pressures around 200 times atmospheric, so fertiliser manufacture consumes roughly 1 to 2 per cent of all the energy used worldwide. The nitrogen in food arrives with a large energy bill attached, and almost all of that energy currently comes from burning gas.
Pause and explain
Why is nitrogen often the factor limiting plant growth when the air is 78 per cent nitrogen?
Check your reasoning after answering
Because plants cannot use nitrogen gas. The two atoms in a nitrogen molecule are held by a triple bond requiring enormous energy to break, and only certain bacteria and lightning can break it. Plants depend entirely on what those bacteria release into the soil, so the supply is limited by biology rather than by how much nitrogen is present.
Part 4 · Cycles running faster than they used to
Human activity has not created new elements; it has changed how fast they move and where they end up. For carbon, the change is the transfer from the slow cycle to the fast one, raising atmospheric carbon dioxide by about 50 per cent since 1800 and acidifying the oceans as some of it dissolves. Nothing was created, and a store that took a hundred million years to fill is being emptied in a few centuries.
For nitrogen the change is larger in proportion and less discussed. Industrial fertiliser production plus the planting of nitrogen-fixing crops now adds roughly as much usable nitrogen to the land each year as all natural processes combined, so the cycle runs at about twice its natural rate. The surplus does not stay in the field: rain washes nitrates into streams and groundwater, where they fertilise algae instead of crops. That is the process the next lesson deals with, and it is a good example of a problem created entirely by solving a different one. The phosphorus cycle has the opposite shape and is worth a sentence for contrast. Phosphorus has no significant gas phase at all, so it moves only through rock, soil, water and organisms, which means that unlike carbon and nitrogen it cannot be drawn from the air. Supplies come from mined rock, those deposits are finite and concentrated in a few countries, and phosphorus washed into the sea is effectively lost for geological time.
Pause and explain
In what sense has human activity changed the nitrogen cycle more than the carbon cycle?
Check your reasoning after answering
In proportion to the natural rate. Industrial fertiliser and nitrogen-fixing crops now add roughly as much usable nitrogen to land each year as every natural process together, so the cycle runs at about double its natural speed. The carbon change is enormous in absolute terms but smaller relative to the very large natural flows already moving carbon each year.
Apply the whole unit · Trace the atom
A carbon atom is in a molecule of carbon dioxide in the air above a field in spring. A year later it is in the body of a fox.
Your task
Describe a route the atom could have taken, and give a second route that would have taken a hundred million years instead.
What your response should show
- Names photosynthesis and feeding as distinct steps in the fast route
- Describes burial without decomposition as the entry to the slow cycle
- Identifies the ways slow-cycle carbon can return
- Draws the conclusion about rate rather than about quantity
Response guidance — after your attempt
Describe a plausible one-year path from air to fox, naming each process.
Describe the slow-cycle alternative and say what would have to happen for the atom to return to the air.
Explain what the contrast between the two routes shows about burning fossil fuels.
In the fast route, the atom is taken from the air by a grass plant during photosynthesis and built into a sugar and then into plant tissue. A rabbit eats the grass and incorporates the atom into its own body. A fox eats the rabbit, and the atom becomes part of the fox. At any of those steps it might instead have been released by respiration and returned to the air within hours, or passed to a decomposer if the organism died. The slow route begins the same way, but the plant dies in a waterlogged swamp where decomposers cannot reach it, so the carbon is buried under sediment and, over tens of millions of years, becomes coal. Returning it to the air would require that coal to be exposed by uplift and weathering, or to be erupted from a volcano, or to be dug up and burned. The contrast shows what burning fossil fuels actually does: it takes carbon that the slow cycle removed over a hundred million years and returns it to the fast cycle in a few centuries, at a rate the fast cycle has no mechanism to balance.
Scientific references for this unit
- NASA Earth Observatory: the fast and slow carbon cycles
- US Geological Survey: the nitrogen cycle and nitrogen in water
- National Oceanic and Atmospheric Administration: ocean carbon uptake and acidification
- US Environmental Protection Agency: nutrient pollution and its sources
Parent / teacher teaching notes
Begin with the literal observation, quantity, or diagram relationship. Ask the learner to explain why the evidence supports the claim, check a relevant unit or boundary, then discuss an alternative. The supplied numbers are illustrative, not real observations. A parent can read aloud and allow a spoken rehearsal before writing. Judge science and evidence rather than political, faith, or family agreement. Use the complete data option whenever an actual procedure is unavailable or unsuitable.
The key idea
The atoms in your body have all been used before, many times. Matter cycles endlessly between organisms and their surroundings because it cannot be created or destroyed, and the limit on life in most places is not the amount of an element present but whether it is in a form anything can use.
Worked examples
Example 1 · A nutrient-store model
- Track each input and output once.
- Calculate soil, plant, and animal stores.
- Separate nutrient accounting from energy and a real nutrient cycle.
Example 2 · Transfer to a different case
- Identify the distinction and conditions in this new case.
- Use the relevant mechanism or quantity; preserve units and the chosen boundary.
- Explain why the answer fits and what further evidence would be needed.
Example 3 · Include respiration in plants and animals · a focused example
- State the literal condition or observation in the example.
- Apply the distinction explained in this teaching part.
- Check whether the claim stays within the given evidence.
Avoid these mistakes
- Misleading claim: Decomposers make matter disappear.
- Misleading claim: Recycling matter automatically recycles all usable energy.
- Omitting units, measurement timing, or a model boundary makes the evidence hard to interpret.
- Treating illustrative classroom data as real field measurements or copying a model is not an independent explanation.
Check your understanding
Does the light itself become carbon matter?
Discuss the answer
No. Light supplies energy; carbon atoms come from matter such as carbon dioxide in the stated model.
Do plants respire only at night?
Discuss the answer
No. Respiration occurs in plants in daytime as well; photosynthesis depends on light under the familiar green-plant model.
Why should decomposer biomass be included?
Discuss the answer
Some material enters those organisms rather than all immediately becoming soil; the account needs actual pathways.
What is recycled in that arrow?
Discuss the answer
Specified matter or nutrient units, not the complete original usable energy supply.
Sustained scientific explanation and revision
These activities are scheduled in the parent plan. Save a first response, a correction, and the reason for the change in the science notebook.
Activity 1 · Cycling Matter Through Organisms & Environment · Model and explain
Read A Clearer Stream Is Still a Question and the linked unit teaching; use its complete supplied case and diagrams.
Your task
For Cycling Matter Through Organisms & Environment, study Parts 3–4 and attempt the worked case: Find the final stores and explain what is conserved and simplified. Explain the steps before opening the model.
Success criteria
- An accurate scientific claim answering the actual question.
- Relevant evidence or a labeled model with quantities and units when applicable.
- Reasoning that connects evidence with the mechanism rather than only repeats it.
- A specific method, boundary, scale, or alternative-explanation limit.
Hint
Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.
Response guidance — discuss after your attempt
Soil: 20 − 5 + 3 = 18 units. Plants: 8 + 5 − 2 = 11. Animals: 2 + 2 − 3 = 1. The total remains 30 units. These are simplified nutrient-account units, not a complete real decomposition pathway or energy total; actual stores and processes would be more detailed.
Activity 2 · Cycling Matter Through Organisms & Environment · Practice, revise and exit
Read A Clearer Stream Is Still a Question and the linked unit teaching; use its complete supplied case and diagrams.
Your task
For Cycling Matter Through Organisms & Environment, complete practice questions 1–6 and both choice checks, then answer independently: A student says decomposers return all the original food energy to plants. What distinction is missing? Save a correction explaining one changed claim or method.
Success criteria
- An accurate scientific claim answering the actual question.
- Relevant evidence or a labeled model with quantities and units when applicable.
- Reasoning that connects evidence with the mechanism rather than only repeats it.
- A specific method, boundary, scale, or alternative-explanation limit.
Hint
Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.
Response guidance — discuss after your attempt
Decomposition can return matter to usable nutrient pathways, but energy spreads through processes and thermal transfers. Plants still require an energy source such as sunlight.
Words to know
- decomposition
- Biological breakdown and transformation of organic material into other matter stores and forms.
- respiration
- Cellular processes releasing usable energy from organic material, with associated matter changes.
- evidence
- A relevant observation, measurement, or record used to evaluate a claim.
- system boundary
- The chosen limit specifying what is included and which exchanges cross it.
- uncertainty
- A stated limit in measurement or knowledge, distinct from a careless mistake.
In the real world
Composting is the carbon and nutrient cycle put to work in a backyard: decomposers turn food scraps and leaves into rich soil, returning their matter to grow new plants instead of sending it to a landfill.
Exit check
A student says decomposers return all the original food energy to plants. What distinction is missing?
Check your explanation after answering
Decomposition can return matter to usable nutrient pathways, but energy spreads through processes and thermal transfers. Plants still require an energy source such as sunlight.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.