The shape of the lesson
By the end of the lesson, learners can
- The carbon cycle: Carbon atoms (the black dots) travel along every arrow.
- The nitrogen cycle: Extended.
- Carbon out of the air: photosynthesis only. Back in: respiration (all organisms) and combustion.
- Decomposers feed on dead material and waste, then respire; no oxygen means fossil fuels may form.
- Extended, nitrogen: fixation (N2 → ammonium), decomposition (protein → ammonium), nitrification (ammonium → nitrate), denitrification (nitrate → N2).
- Plants absorb nitrate ions to make amino acids and proteins; animals deaminate excess amino acids to urea.
One picture every idea returns to
In the carbon cycle, photosynthesis removes carbon dioxide from the air; feeding passes carbon compounds along food chains; respiration by plants, animals and decomposers and combustion of fossil fuels return carbon dioxide to the air; dead material that does not decompose can form fossil fuels over millions of years. Extended: in the nitrogen cycle microorganisms carry out decomposition, nitrification, nitrogen fixation and denitrification, while plants absorb nitrate ions to make amino acids and proteins.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, The same atoms, again and again.. Energy flows through an ecosystem and is lost, but the atoms that build living things are used over and over. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: The carbon cycle | Carbon atoms (the black dots) travel along every arrow. Project tab 5 of the lab (The carbon cycle) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: The nitrogen cycle | Extended. Project tab 6 of the lab (The nitrogen cycle) and let a student drive it while the class predicts. | Mix-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 21 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in The nitrogen cycle, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Plants respire all the time, day and night, as well as photosynthesising. | Do not draw respiration arrows only from animals; plants and decomposers release carbon dioxide too. |
| Nitrification and nitrogen fixation are different processes. | Fixation starts from nitrogen gas; nitrification starts from ammonium ions already in the soil. Denitrification is the one that returns nitrogen gas to the air. |
The carbon and nitrogen cycles: worksheet
6 questions, 21 marks. Written in the style of the papers; no past paper question is reproduced.
Describe how a carbon atom in a carbon dioxide molecule in the air could become part of a rabbit and then return to the air.
Explain how coal was formed from plants that lived millions of years ago, and why burning coal increases the carbon dioxide in the air.
The carbon dioxide concentration inside a closed glasshouse full of plants was measured over 24 hours. It fell during the day and rose during the night. Explain these changes.
Extended. State the role of microorganisms in each of these processes: (a) decomposition, (b) nitrification, (c) nitrogen fixation, (d) denitrification.
Extended. Describe how a nitrogen atom in a protein in a dead leaf can become part of a protein in a cow.
Extended. Crops grow poorly in a field that is often waterlogged. Use your knowledge of the nitrogen cycle to explain why, and suggest how draining the field helps.
Answer key and marking notes
- Carbon dioxide is taken in by grass and used in photosynthesis to make glucose (and other carbon compounds).
- The rabbit eats the grass (feeding) and digests and absorbs the carbon compounds.
- The rabbit respires, releasing the carbon as carbon dioxide into the air.
- Or: when the rabbit dies, decomposers feed on its body and release the carbon dioxide by respiration.
- Dead plants were buried in conditions with little oxygen (for example waterlogged swamps), so decomposers could not break them down fully.
- Over millions of years, heat and pressure turned the material into coal, locking the carbon away.
- Combustion of coal releases this stored carbon as carbon dioxide, faster than photosynthesis can remove it.
- During the day the rate of photosynthesis is greater than the rate of respiration, so carbon dioxide is removed from the air.
- At night there is no light, so no photosynthesis takes place.
- Plants (and soil decomposers) continue to respire, so carbon dioxide is released and the concentration rises.
- (a) Decomposers break down proteins in dead organisms and urea in waste, releasing ammonium ions.
- (b) Nitrifying bacteria convert ammonium ions into nitrate ions.
- (c) Nitrogen-fixing bacteria (in the soil or in root nodules) convert nitrogen gas into ammonium compounds.
- (d) Denitrifying bacteria convert nitrate ions into nitrogen gas.
- Decomposers break down the protein in the leaf, releasing ammonium ions into the soil.
- Nitrifying bacteria convert the ammonium ions into nitrate ions.
- Grass absorbs nitrate ions through its root hairs and uses them to make amino acids and then proteins.
- The cow eats the grass, digests the protein into amino acids, absorbs them and builds its own proteins.
- Waterlogged soil has little oxygen, so denitrifying bacteria thrive and convert nitrates into nitrogen gas, which is lost to the air.
- Nitrifying bacteria need oxygen, so less ammonium is converted into nitrate.
- Draining the field lets air into the soil, so nitrification increases, denitrification decreases and more nitrate is available to the crop (roots can also respire for active uptake).
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which process in the carbon cycle removes carbon dioxide from the atmosphere?
- APhotosynthesis by green plants
- BRespiration by decomposers
- CCombustion of fossil fuels
- DFeeding by herbivores
A carbon atom is in a starch molecule in a potato. A pig eats the potato. Which process returns this carbon atom most directly to the air?
- ARespiration in the pig's cells
- BPhotosynthesis in the pig's cells
- CFormation of fossil fuels in the pig
- DCombustion inside the pig's stomach
Under which conditions are fossil fuels most likely to form from dead organisms?
- ABuried with little oxygen, so decay is incomplete
- BLeft on the surface in warm, moist air
- CEaten by animals and digested quickly
- DBurned soon after the organisms die
Large areas of forest are cut down and burned. Why does the carbon dioxide concentration in the air increase?
- ACombustion releases carbon dioxide and fewer trees photosynthesise
- BCombustion removes carbon dioxide and more trees respire
- CBurning produces oxygen, which turns into carbon dioxide
- DDecomposers stop respiring once the trees are burned
Which organisms are the main decomposers in the carbon cycle?
- ABacteria and fungi
- BPlants and algae
- CHerbivores and carnivores
- DViruses and protoctists
The diagram shows part of the nitrogen cycle. Which letter shows nitrification?
- AC
- BA
- CB
- DD
In the diagram, which process is carried out by bacteria that work best in waterlogged soil with little oxygen?
- AD
- BC
- CB
- DA
Excess amino acids in a human are deaminated. What happens to the nitrogen they contained?
- AIt is made into urea in the liver and excreted
- BIt is released as nitrogen gas from the lungs
- CIt is stored as protein in the liver for later
- DIt is converted directly into nitrate ions in the blood
Quiz key
Photosynthesis takes in carbon dioxide and uses it to make glucose. Respiration by decomposers and combustion both release carbon dioxide into the air. Feeding moves carbon compounds from plants to animals but does not change the carbon dioxide in the air directly.
The pig digests the starch to glucose and its cells respire, releasing the carbon as carbon dioxide. Animal cells do not photosynthesise. Fossil fuels form over millions of years from undecayed dead material, not in a living pig. Combustion is burning, which does not happen in the stomach.
When dead material is buried without oxygen (for example in waterlogged swamps or seabed mud), decomposers cannot break it down fully, and over millions of years it becomes coal, oil or gas. Warm, moist, airy conditions speed decay, so the carbon is released. Eating or burning the material releases its carbon rather than storing it.
Burning the wood (combustion) releases carbon dioxide, and with fewer trees less carbon dioxide is removed by photosynthesis. Combustion adds, not removes, carbon dioxide. Burning uses oxygen; oxygen does not turn into carbon dioxide on its own. Decomposers keep respiring, and they would add carbon dioxide, not explain an increase by stopping.
Bacteria and fungi feed on dead organisms and waste and release carbon dioxide by respiration. Plants and algae are producers. Herbivores and carnivores are consumers that feed on living organisms. Viruses are not decomposers, and protoctists are not the main decomposers.
C converts ammonium ions into nitrate ions: nitrification by nitrifying bacteria. A turns nitrogen gas into plant protein, which involves nitrogen fixation. B is decomposition of dead organisms to ammonium ions. D converts nitrate ions to nitrogen gas, which is denitrification.
D is denitrification: denitrifying bacteria thrive in waterlogged, poorly aerated soil and turn nitrate ions into nitrogen gas. C (nitrification) needs oxygen, so it is slowed by waterlogging. B (decomposition) and A (nitrogen fixation) are not the processes that are favoured by low oxygen in this way.
In the liver the nitrogen-containing part of each excess amino acid is removed and converted to urea, which the kidneys excrete. Humans do not release nitrogen gas from amino acids. Deamination happens because excess amino acids cannot be stored. Nitrate ions are made in the soil by nitrifying bacteria, not in the blood.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.