IB MYP Sciences – Cycles in science Revision Notes

These revision notes condense the carbon, water, nitrogen and rock cycles for IB MYP Sciences. They are aligned to the International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014, and suit MYP years 4 and 5, including the final weeks before the on-screen examination. There is no SL/HL split in the MYP. For full explanations and worked examples, read the cycles study guide first.

MYP has no prescribed content list; schools design their own units. “Cycles” is one of the topics the IB’s brief lists for the sciences on-screen examinations, and these notes cover standard science for it. Your teacher will share the task-specific clarifications for any assessed work.

Links: practice questions · course hub · printable checklist · assessment revision notes · criteria in practice revision notes.

Big idea in one paragraph

A cycle is a system: matter moves between stores by processes, and the total amount is conserved. Only the form and location change (transformation). Each cycle needs an energy source: the Sun (water cycle, photosynthesis, weathering) or the Earth’s internal heat (melting, metamorphism, uplift). A cycle diagram is a model: it simplifies, and usually hides rates and store sizes. The brief names systems as a key concept and energy, transformation and models as example related concepts, so use those words.

Key definitions

Term	Definition
Store (reservoir)	A place where a substance is held, e.g. atmosphere, ocean, fossil fuels
Sink	A store that takes in more of a substance than it releases
Photosynthesis	carbon dioxide + water → glucose + oxygen, using light energy
Respiration	glucose + oxygen → carbon dioxide + water, releasing energy
Combustion	Burning a fuel in oxygen, releasing carbon dioxide
Transpiration	Loss of water vapour from leaves, mainly through stomata
Infiltration	Water soaking into the soil
Surface runoff	Water flowing over the ground surface into rivers
Nitrogen fixation	N₂ converted to ammonium compounds
Nitrification	Ammonium → nitrite → nitrate, by nitrifying bacteria (needs oxygen)
Denitrification	Nitrate → N₂ gas, by denitrifying bacteria (oxygen-poor soil)
Weathering	Breakdown of rock in place
Erosion	Removal and transport of broken rock
Metamorphism	Change of rock by heat and pressure without melting
Eutrophication	Nutrient enrichment of water leading to algal growth and oxygen loss

The four cycles at a glance

Cycle	Main stores	Key processes	Energy source
Carbon	Air, oceans, organisms, soil, fossil fuels, limestone	Out of the air: photosynthesis, dissolving in oceans. Into the air: respiration, decomposition, combustion. Long-term: fossilisation	Sun (photosynthesis)
Water	Oceans, ice, groundwater, rivers, air	Evaporation, transpiration, condensation, precipitation, infiltration, runoff	Sun; gravity for flow
Nitrogen	Air (about 78% N₂), soil, organisms	Fixation, decomposition, nitrification, uptake, denitrification	Bacteria’s own respiration; lightning; industrial energy for the Haber process
Rock	Igneous, sedimentary and metamorphic rock, magma	Weathering, erosion, deposition, compaction, cementation, metamorphism, melting, uplift	Earth’s internal heat; Sun and gravity

Method in steps

Tracing an atom round a cycle

1. Name the starting store and the chemical form (e.g. carbon in CO₂ in the air).
2. Name each process and the organism or condition that drives it.
3. Say the new form at each store (glucose, protein, nitrate).
4. Close the loop back to the starting store.

Water budget

1. Precipitation = evapotranspiration + runoff + groundwater recharge + change in storage.
2. Rearrange for the unknown. Keep all values in mm (or all in the same volume unit).
3. For a percentage: part ÷ total precipitation × 100.

Sediment or rate question

1. Convert to matching units (m → mm: × 1,000).
2. Time = thickness ÷ rate; rate = change ÷ time.
3. Give units with the answer.

Criterion D extended response

1. State the problem in cycle terms.
2. Explain how each solution works (which store or process it changes).
3. Strengths and limitations of each.
4. At least one other factor: economic, environmental, social, ethical.
5. Justified final judgement.
6. Cite sources you used.

Small worked reminders

• A fictional forest takes in 12.0 t C/ha by photosynthesis; plants respire 6.0 t, decomposers 5.0 t. Net = 12.0 − 11.0 = +1.0 t C/ha/yr: a sink.
• A catchment gets 1,100 mm rain, loses 650 mm by evapotranspiration and 300 mm as runoff. Recharge = 150 mm.
• 3.6 m of sediment at 0.3 mm/yr: 3,600 ÷ 0.3 = 12,000 years.

Reading a cycle diagram or data table

• Arrows are processes; boxes are stores. Label every arrow with a process name, not a vague verb such as “goes to”.
• Check the direction. Photosynthesis points from the air into plants; respiration, decomposition and combustion point back to the air.
• Look for a balance. If inputs to a store exceed outputs, the store grows (a sink); if outputs exceed inputs, it shrinks (a source).
• Watch the time scale. Photosynthesis and respiration move carbon in days to years; fossil fuel and rock stores change over millions of years. Burning fossil fuels moves slow-cycle carbon into the fast cycle.
• Separate trend from fluctuation. In a long CO₂ record, the year-to-year rise is the trend; the up-and-down pattern within each year is a seasonal fluctuation linked to photosynthesis.
• Say what the model leaves out. A simple diagram rarely shows store sizes, rates or where on Earth each process happens.

Human impacts

• Carbon: burning fossil fuels and deforestation add CO₂ faster than it is removed → enhanced greenhouse effect → global warming; more CO₂ dissolving makes oceans more acidic, harming shell-building organisms.
• Water: deforestation and paved surfaces reduce infiltration and transpiration, so more runoff and faster flooding; over-abstraction lowers rivers and groundwater.
• Nitrogen: excess fertiliser washes into water → eutrophication; the Haber process fixes huge amounts of nitrogen for fertiliser.
• Rock: quarrying and mining remove rock quickly and can load rivers with sediment.

Eutrophication chain: nitrate runoff → algal bloom → light blocked → water plants die → decomposers respire and use up oxygen → fish die.

Must-know distinctions

• Nitrogen fixation vs nitrification vs denitrification: N₂ → ammonium; ammonium → nitrate; nitrate → N₂.
• Respiration vs combustion: both release CO₂; respiration happens in cells with enzymes, combustion is burning.
• Photosynthesis vs respiration in plants: plants respire all the time; they photosynthesise only in light.
• Evaporation vs transpiration: from any wet surface vs from leaves.
• Weathering vs erosion: breakdown in place vs removal and transport.
• Intrusive vs extrusive igneous: slow cooling underground, large crystals (granite) vs fast cooling at the surface, small crystals (basalt).
• Metamorphism vs melting: metamorphism changes solid rock; melting makes magma.
• Greenhouse effect vs enhanced greenhouse effect: the natural warming that keeps Earth habitable vs the extra warming from added greenhouse gases.

Quick self-test

1. Name the process that removes carbon dioxide from the atmosphere in the carbon cycle.
2. Write the word equation for aerobic respiration.
3. Which group of bacteria converts ammonium to nitrate?
4. Why do waterlogged soils lose nitrate?
5. Granite has larger crystals than basalt. Explain why.
6. Sediment builds up at 0.4 mm per year. How long does a 2.0 m layer take?
7. A catchment receives 750 mm of precipitation; 30% leaves as surface runoff. What depth of water is runoff?
8. A bag of leaf litter falls from 25.0 g to 18.0 g. Calculate the percentage mass loss.
9. Name two water cycle processes powered by energy from the Sun.
10. How do legumes increase the nitrate in soil?
11. Name the three key concepts the IB’s sciences brief uses to frame the curriculum.
12. A fictional grassland takes in 9.0 t of carbon per hectare per year by photosynthesis and releases 9.5 t by respiration and decomposition. Is it a carbon sink or a carbon source? Show the net change.

Answers

1. Photosynthesis (dissolving in the oceans also removes some).
2. glucose + oxygen → carbon dioxide + water (energy released).
3. Nitrifying bacteria.
4. The soil lacks oxygen, so denitrifying bacteria convert nitrate to nitrogen gas, which escapes to the air.
5. Granite is intrusive: it cooled slowly underground, giving crystals time to grow. Basalt cooled quickly at the surface.
6. 2,000 ÷ 0.4 = 5,000 years.
7. 0.30 × 750 = 225 mm.
8. (25.0 − 18.0) ÷ 25.0 × 100 = 28%.
9. Evaporation and transpiration.
10. Nitrogen-fixing bacteria in their root nodules convert N₂ into ammonium compounds; when the plants decay, nitrifying bacteria turn the ammonium into nitrate.
11. Change, relationships and systems.
12. Net change = 9.0 − 9.5 = −0.5 t C per hectare per year. More carbon leaves than enters, so it is a carbon source.

Where marks are usually lost

• Writing “plants breathe in CO₂” instead of naming photosynthesis.
• Swapping nitrification and nitrogen fixation, or forgetting which one needs oxygen.
• Saying decomposers “make” nutrients rather than releasing CO₂ by respiration and ammonium from proteins.
• Describing metamorphic rock as forming from magma.
• Giving a time or rate answer with no units, or forgetting to convert metres to millimetres.
• Calculating a percentage of the wrong total (use precipitation, or starting mass, as the denominator).
• Linking deforestation only to carbon and forgetting its effect on transpiration and runoff.
• In criterion D, describing a solution without evaluating it, or giving no final judgement.
• Using everyday words (“pollution”, “gets used up”) where a scientific term (eutrophication, dissolved oxygen, respiration) is expected.

Official syllabus

International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014. The brief lists “cycles” among the topics explored in the MYP sciences on-screen examinations and names the key and related concepts quoted above.