IB MYP Sciences – Cycles in science Study Guide

This study guide teaches the carbon, water, nitrogen and rock cycles for IB MYP Sciences. It is aligned to the International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014, and suits MYP years 4 and 5, including students preparing for the sciences on-screen examination. There is no SL/HL split in the MYP.

MYP has no prescribed content list: schools design their own science units. This page covers “cycles”, one of the topics the IB’s brief lists as explored in the MYP sciences on-screen examinations. Your school’s unit may use different examples; your teacher will share the task-specific clarifications for any assessed work.

Use it with the cycles revision notes and the cycles practice questions. The course hub is at /boards/ib/ib-myp/myp-sciences/ and the printable checklist is at /checklists/ib/ib-myp/myp-sciences/.

What this unit covers

Area	What you should be able to do	Criteria it trains
Carbon cycle	Name the stores and the processes that move carbon between them; write the word equations for photosynthesis and respiration	A, C
Water cycle	Describe evaporation, transpiration, condensation, precipitation, infiltration and runoff; use a water budget	A, C
Nitrogen cycle	Explain fixation, decomposition, nitrification, uptake and denitrification, and which organisms carry them out	A
Rock cycle	Explain how igneous, sedimentary and metamorphic rocks form and change into one another	A, C
Human impacts	Explain how fuel use, land clearing, fertilisers and extraction change each cycle	A, D
Extended response	Evaluate a scientific solution to a cycle-related problem	D

The brief says the on-screen examination has three tasks: knowing and understanding (criterion A, 25 marks), investigation skills (criteria B and C, 50 marks) and applying science (criterion D, 25 marks). Cycles content can appear in any of them. For more on the exam structure, read the assessment revision notes and the investigation skills exam preparation page.

Why cycles are a “systems” topic

The brief names change, relationships and systems as key concepts, and gives energy, movement, transformation and models as examples of related concepts. Every cycle in this unit is a system: matter moves between stores (also called reservoirs) by processes (also called flows or fluxes). The total amount of the element or substance stays the same; only its location and form change. That is transformation in action.

Each cycle also needs an energy source. The Sun drives the water cycle and photosynthesis. The Earth’s internal heat drives melting and metamorphism in the rock cycle. When you describe a cycle, name the energy source as well as the steps.

A cycle diagram is a model. It simplifies: it hides how fast each process runs and how much each store holds. Saying what a model leaves out is a criterion A skill. (For how schools organise MYP science courses, a different meaning of “model”, see the course models guide.)

The carbon cycle

Stores

• the atmosphere (as carbon dioxide)
• living organisms (as carbohydrates, proteins and fats)
• dead organic matter in soil
• the oceans (dissolved carbon dioxide, and carbonate in shells)
• fossil fuels (coal, oil and natural gas)
• sedimentary rocks such as limestone (calcium carbonate)

Processes

Process	Direction	What happens
Photosynthesis	atmosphere → plants	Plants and algae use light energy to make glucose
Respiration	organisms → atmosphere	All living things release energy from glucose
Feeding	plants → animals	Carbon compounds pass along food chains
Decomposition	dead matter → atmosphere	Bacteria and fungi respire as they break down waste and dead bodies
Combustion	fuels → atmosphere	Burning releases carbon dioxide
Fossilisation	dead matter → fossil fuels	Over millions of years, remains that do not fully decay form coal, oil and gas
Dissolving	atmosphere ↔ oceans	Carbon dioxide dissolves in seawater and can return to the air

The two word equations to learn:

Photosynthesis:  carbon dioxide + water → glucose + oxygen   (light energy, chlorophyll)
Respiration:     glucose + oxygen → carbon dioxide + water   (energy released)

Worked example: is this forest a carbon sink?

A fictional forest plot is studied for one year. Per hectare, photosynthesis takes in 12.0 tonnes of carbon. Plant respiration releases 6.0 tonnes. Decomposers release 5.0 tonnes. Is the forest a sink or a source?

1. Carbon in = 12.0 t (photosynthesis).
2. Carbon out = 6.0 + 5.0 = 11.0 t (respiration of plants and decomposers).
3. Net change = 12.0 − 11.0 = +1.0 t of carbon per hectare per year.
4. More carbon enters than leaves, so the forest is a carbon sink this year.

If the forest were burned, combustion would release stored carbon quickly and photosynthesis would fall: the sink becomes a source.

The water cycle

Water moves between the oceans, the atmosphere, ice, lakes and rivers, soil water and groundwater.

• Evaporation: liquid water at the surface becomes water vapour, using energy from the Sun.
• Transpiration: water evaporates from leaves, mostly through the stomata, pulling water up from the roots. Evaporation plus transpiration is often called evapotranspiration.
• Condensation: rising air cools; water vapour condenses into droplets, forming clouds.
• Precipitation: rain, snow or hail falls when droplets or ice crystals grow heavy enough.
• Infiltration: water soaks into the soil.
• Surface runoff: water flows over the ground to rivers when it cannot infiltrate fast enough.
• Groundwater flow: water moves slowly through permeable rock back to rivers and the sea.

The Sun supplies the energy for evaporation. Gravity pulls precipitation down and drives runoff and river flow.

Worked example: a water budget

A fictional river catchment receives 1,100 mm of precipitation in a year. Evapotranspiration is 650 mm and surface runoff is 300 mm. Assuming no change in soil water storage, how much water recharges the groundwater?

1. Inputs must equal outputs plus storage change.
2. Recharge = precipitation − evapotranspiration − runoff.
3. Recharge = 1,100 − 650 − 300 = 150 mm.
4. Evapotranspiration is 650 ÷ 1,100 × 100 = 59.1% of the precipitation.

If the catchment’s forest were cleared, transpiration and interception by leaves would fall and runoff would rise, so rivers would flood more quickly after heavy rain.

The nitrogen cycle

Nitrogen gas makes up about 78% of the air, but most organisms cannot use it directly because the triple bond in the N₂ molecule is very strong. Plants need nitrogen in the form of nitrate (or ammonium) ions to make amino acids, proteins and DNA.

Stage	What happens	Carried out by
Nitrogen fixation	N₂ → ammonium compounds (lightning instead forms nitrogen oxides, which become nitrate)	Nitrogen-fixing bacteria, some free in soil and some in root nodules of legumes such as peas, beans and clover; also lightning, and the Haber process in industry
Uptake and feeding	Roots absorb nitrate; plants build proteins; animals eat plants	Plants and animals
Decomposition (ammonification)	Proteins in dead matter and urea in waste → ammonium	Decomposers (bacteria and fungi)
Nitrification	Ammonium → nitrite → nitrate	Nitrifying bacteria; needs oxygen
Denitrification	Nitrate → N₂ gas returned to the air	Denitrifying bacteria in waterlogged, oxygen-poor soil

Worked example: why farmers rotate crops

A farmer grows beans in one year and wheat in the next. Explain why the wheat grows better than it would after another wheat crop.

1. Beans are legumes with root nodules containing nitrogen-fixing bacteria.
2. These bacteria convert N₂ from the air into ammonium compounds the plant uses.
3. When the bean roots and stems decay, decomposers release ammonium; nitrifying bacteria convert it to nitrate.
4. The soil now holds more nitrate, which the wheat roots absorb to make proteins, so it grows better with less added fertiliser.

The rock cycle

The rock cycle is much slower than the other three. It is driven by the Earth’s internal heat (melting, uplift) and by the Sun and gravity (weathering and erosion).

• Igneous rock forms when magma or lava cools and crystallises. Intrusive rocks, such as granite, cool slowly underground and have large crystals. Extrusive rocks, such as basalt, cool quickly at the surface and have small crystals.
• Sedimentary rock forms from sediments. Rocks are broken down by weathering (in place) and carried away by erosion and transport. Sediments are deposited in layers, then compacted by the weight above and cemented together. Examples: sandstone, limestone, shale.
• Metamorphic rock forms when existing rock is changed by high temperature and pressure without melting. Examples: limestone becomes marble; shale becomes slate.
• If rock melts fully it becomes magma, and the cycle can start again. Uplift brings buried rock back to the surface, where it is weathered.

Worked example: how long did a layer take?

In a fictional lake, sediment builds up at an average of 0.3 mm per year. How long would a 3.6 m layer take to form?

1. Convert to the same units: 3.6 m = 3,600 mm.
2. Time = thickness ÷ rate = 3,600 ÷ 0.3.
3. Time = 12,000 years.
4. Compaction would later squeeze the layer thinner, so the final rock would be less than 3.6 m thick.

Human impacts on the cycles

Cycle	Human activity	Effect
Carbon	Burning fossil fuels	Adds carbon dioxide faster than photosynthesis and the oceans remove it; strengthens the greenhouse effect (the enhanced greenhouse effect) and warms the climate
Carbon	Deforestation	Less photosynthesis; burning or decay of cleared trees releases stored carbon
Carbon	More CO₂ dissolving in oceans	Seawater becomes more acidic, making it harder for corals and shellfish to build calcium carbonate shells
Water	Deforestation and paving	Less infiltration and transpiration, more runoff, more flooding
Water	Abstraction for farms and cities	Rivers and groundwater stores fall
Nitrogen	Excess fertiliser	Nitrate is washed into rivers and lakes, causing eutrophication
Rock	Quarrying and mining	Speeds up removal of rock; the waste can release sediment into rivers

Eutrophication in steps: nitrate runs off fields → algae grow fast and cover the water surface → light cannot reach water plants, which die → decomposers break down the dead matter and use up dissolved oxygen as they respire → fish and other animals die from lack of oxygen.

Writing a criterion D response on cycles

Criterion D asks you to evaluate the implications of science applied to a specific problem, using varied scientific language, and to acknowledge the work of others. The applying science task in the exam carries 25 marks. A reliable structure:

1. State the problem using cycle language (for example, rising atmospheric carbon dioxide from combustion).
2. Explain how each solution works scientifically: which store or process it changes.
3. Give strengths and limitations of each: cost, time scale, scale of effect, side effects, who benefits.
4. Consider at least one other factor: economic, environmental, social, ethical or political.
5. Reach a justified judgement that weighs the evidence, not just a list.
6. Cite your sources when you use data or ideas that are not your own.

The criteria in practice guide shows what top-band evidence looks like for each criterion in general. The practice set for this unit includes a full cycles-based criterion D question with a model plan.

Common errors

• Saying plants “breathe in carbon dioxide”. Plants respire all the time and photosynthesise only in light; name the correct process.
• Mixing up nitrogen fixation (N₂ → ammonium) with nitrification (ammonium → nitrate).
• Forgetting that denitrification needs oxygen-poor conditions, and nitrification needs oxygen.
• Calling transpiration “evaporation from the soil”. It is loss of water vapour from leaves.
• Saying metamorphic rock forms from melted rock. Melting makes magma; metamorphism happens without melting.
• Confusing weathering (breakdown in place) with erosion (removal and transport).
• Treating the greenhouse effect as harmful in itself. Without it the Earth would be far colder; the problem is the enhanced effect.
• In criterion D answers, listing only benefits, or giving no final judgement.

Next steps

Condense this page with the revision notes, then test yourself with the practice questions. For the wider course, see the subject guide and the syllabus guide.

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; the cycles content on this page is standard science chosen by Marlbridge to teach that topic.