IB Biology HLEnergy & Matter in EcosystemsPaper 1 & 2~11 min read
The Carbon Cycle
Energy passes through an ecosystem once and leaves. Carbon atoms do not. The same atom can sit in a leaf, then a caterpillar, then the air, then the ocean – and a carbon cycle diagram is simply a map of every route it can take.
📚 What you need to know
The carbon cycle is all the processes that transfer carbon from one store to another.
Carbon exists in organic form (biomass, e.g. carbohydrates and proteins) and inorganic form (atmospheric carbon dioxide, hydrogen carbonate ions in the oceans).
Diagrams show pools or sinks (the stores) and fluxes (the transfers).
A carbon sink takes up and stores carbon; a carbon source releases it.
Net uptake happens when photosynthesis exceeds respiration; net release when respiration exceeds photosynthesis.
Combustion of fossil fuels, peat and biomass returns carbon to the atmosphere.
The Keeling curve from Mauna Loa shows a seasonal zig-zag from photosynthesis and a rising trend from burning fossil fuels.
Pools and fluxes
Every carbon cycle diagram, however it is drawn, shows the same two things:
Term
What it is
Examples
Pool (or sink)
A store of carbon
The ocean, fossil fuels, living organisms, the atmosphere
Diagrams can be illustrated with drawings, or kept simple with just text boxes and arrows. They can show terrestrial carbon cycling, marine cycling, or both combined in one diagram.
Notice how many arrows point up into the atmosphere and how few point down. Photosynthesis and dissolving are doing all the removing.
Carbon sinks and carbon sources
Type
Definition
Examples
Carbon sink
A part of the carbon cycle that takes up and stores carbon
Plants storing carbon compounds in tissues; fossil fuels and peat over very long periods; carbon dioxide dissolving in the oceans
Carbon source
A part of the carbon cycle that releases carbon
Burning plant material; the decay of dead or waste material
Net uptake and net release
The word net means the overall direction of movement once you have added up everything going both ways.
If an organism carries out photosynthesis at a higher rate than respiration – a plant, for example – there is a net uptake of carbon dioxide, and that organism functions as a carbon sink.
If an organism carries out respiration at a higher rate than photosynthesis – an animal, for example – there is a net release of carbon dioxide, and it functions as a carbon source.
This is why the answer “plants take in carbon dioxide and animals give it out” only gets half a mark. Plants do both. What makes a plant a sink is that photosynthesis outpaces its own respiration.
Releasing carbon dioxide
Carbon returns to the atmosphere when fossil fuels and organic material are burned, a process known as combustion. Complete combustion releases carbon dioxide and water as by-products.
Fuel
What it is
How long the carbon was locked away
Coal, oil, natural gas
Fossil fuels, formed over millions of years from the bodies of dead plants and animals
Millions of years
Peat
Forms when plant matter does not fully decompose, because conditions are waterlogged and acidic
Thousands of years
Biomass
Plant matter such as wood, burned in fires or biomass boilers
Within the lifetime of the plant
Peat also releases its carbon when it is allowed to dry out and decompose, not only when it is burned.
Why burning biomass is treated differently
Burning biomass is considered to have a less significant impact on atmospheric carbon dioxide than burning peat or fossil fuels. The reason is timing: the carbon in plant tissues was removed from the atmosphere relatively recently, within the lifetime of the plant. The carbon in peat has been out of circulation for potentially thousands of years, and the carbon in fossil fuels for millions.
Fires are not all natural. Organic material burns when fires occur in forests or grasslands. These can start naturally, e.g. lightning hitting hot dry ground, but can also be set by humans, e.g. when clearing land for farming. Climate change has increased the occurrence of wildfires, and human activity has increased the burning of biomass overall.
Reading the Keeling curve
The Mauna Loa Observatory in Hawaii has recorded carbon dioxide levels since 1958.
Data collection was started by the American scientist Charles Keeling, and the dataset is now named after him.
Scientists from the World Meteorological Organisation and other research stations have taken quantitative measurements of atmospheric carbon dioxide for many years.
The zig-zag: seasonal photosynthesis
Photosynthesis removes carbon dioxide from the atmosphere, so levels decrease in whichever hemisphere is experiencing spring and summer.
The decrease is reversed in autumn and winter, when photosynthesis rates fall and are overtaken by respiration, decomposition and combustion.
The rise: human activity
The overall upward trend is the result of human activities. Combustion of fossil fuels releases carbon dioxide faster than photosynthesis can remove it, so carbon dioxide levels are slightly higher every year.
The zig-zag and the climb have completely different causes: one is the seasons breathing, the other is us.
Worked examples
WE 1
Explain the seasonal fluctuations
Explain the yearly fluctuations shown on the Keeling curve. (3 marks)
Point 1: the fall
In spring and summer, rates of photosynthesis increase, removing carbon dioxide from the atmosphere, so the concentration falls.
Point 2: the rise
In autumn and winter photosynthesis rates decrease and are overtaken by respiration, decomposition and combustion, so the concentration rises again.
Point 3: the detail
The pattern follows the hemisphere experiencing spring and summer, which is why the cycle repeats once a year.
Photosynthesis down, respiration up – the curve follows the seasonsname the processes that put carbon back: respiration, decomposition and combustion, not just “plants stop”
WE 2
Sink or source?
Explain why a growing forest is described as a carbon sink, while a herd of cattle is a carbon source. (3 marks)
Point 1: the forest
Trees carry out photosynthesis at a higher rate than respiration, so there is a net uptake of carbon dioxide.
Point 2: what happens to it
That carbon is stored as carbon compounds in their tissues, so the forest takes up and stores carbon.
Point 3: the cattle
Cattle only respire, so there is a net release of carbon dioxide and they function as a carbon source.
Sink = net uptake. Source = net release.use the word “net” and say which process is faster – that is where the mark sits
WE 3
Compare two fuels
Suggest why burning wood is considered to have a smaller impact on atmospheric carbon dioxide than burning coal. (2 marks)
Point 1: wood
The carbon in wood was removed from the atmosphere relatively recently, within the lifetime of the tree.
Point 2: coal
The carbon in coal has been locked away for millions of years, so burning it adds carbon that had left the cycle long ago.
It is about how long the carbon was out of circulationboth release carbon dioxide – the comparison is about timescale, not about which one is “clean”
💡 Exam tips
Use the terms pool (or sink) and flux when describing a cycle diagram.
Learn three sinks and two sources, with examples.
Define net as the overall direction once both processes are added up.
For the Keeling curve, give two separate explanations: one for the zig-zag, one for the trend.
Quote the timescales: biomass = a lifetime, peat = thousands of years, fossil fuels = millions.
Remember the oceans – dissolving is reversible, so the ocean is both sink and source.
⚠ Common mistakes
Saying plants do not respire. They do. A sink is about which process is faster.
Explaining the whole Keeling curve with fossil fuels. That explains the trend only, not the yearly zig-zag.
Calling the yearly dip “pollution falling”. It is seasonal photosynthesis.
Saying burning wood releases no carbon dioxide. It does; the carbon was simply removed from the air recently.
Forgetting peat. It releases carbon when burned and when it dries out and decomposes.
Mixing pools with fluxes. The ocean is a pool; dissolving is a flux.
Up next: Nutrient Cycling – the last page of this unit, where carbon joins nitrogen, phosphorus and the rest, and the whole system finally closes the loop.
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