IB Biology SLTopic 3 — Energy & MatterPaper 1 & 2Core idea~10 min read
The Carbon Cycle
Carbon does not get used up. The same atoms move between the air, the oceans, living bodies and rocks, over and over. Learn the diagram as a set of stores joined by processes and it stops being something to memorise and becomes something you can reason about.
📘 What you need to know
Carbon exists in organic form (in biomass) and inorganic form (carbon dioxide, hydrogen carbonate ions).
Carbon pools or sinks are stores; fluxes are the processes that move carbon between them.
A carbon sink takes up and stores more carbon than it releases; a carbon source releases more than it takes up.
Photosynthesis, respiration, decomposition, combustion and dissolving are the fluxes you must know.
The Keeling curve shows both a seasonal wobble and a long-term rise in atmospheric carbon dioxide.
The seasonal wobble is caused by photosynthesis; the long-term rise is caused by human combustion of fossil fuels.
Pools and fluxes
Two words do most of the work on this page.
A pool (or sink) is a place carbon sits. The atmosphere, the oceans, living organisms, soil, fossil fuels and limestone are all pools.
A flux is a process that moves carbon from one pool to another. Photosynthesis, respiration, feeding, decomposition, combustion and dissolving are all fluxes.
In diagrams, pools are usually boxes and fluxes are usually arrows. If you can label a diagram with those two categories, you can answer most questions on it.
Boxes are pools. Numbered arrows are fluxes — see the key below.
📝 Key to the numbered fluxes
Photosynthesis — producers fix carbon dioxide into carbon compounds.
Respiration by producers — carbon dioxide released back to the air.
Feeding — carbon compounds pass into consumers.
Respiration by consumers — carbon dioxide released.
Death of producers, plus leaf fall and other dead plant parts.
Death and egestion by consumers.
Respiration of saprotrophs and detritivores during decomposition.
Incomplete decomposition and fossilisation under waterlogged or anaerobic conditions.
Combustion of fossil fuels and peat.
Dissolving of carbon dioxide into the oceans.
Release of carbon dioxide from the oceans back to the atmosphere.
Notice fluxes 10 and 11 point in opposite directions. Dissolving is a reversible process, so the ocean can act as a sink or a source depending on conditions such as temperature. That is a favourite Paper 2 question.
Sinks and sources
Whether something counts as a sink or a source depends on the net direction of movement — the overall result once you subtract the opposite process.
Carbon sink
Carbon source
What it does
Takes up and stores more carbon than it releases
Releases more carbon than it takes up
Balance of processes
Photosynthesis exceeds respiration
Respiration or combustion exceeds photosynthesis
Examples
Growing forests, peat bogs, the oceans
Animals, burning fossil fuels, a drained peat bog
So a plant is a carbon sink, because it photosynthesises faster than it respires and locks the surplus away as biomass. An animal is a carbon source, because it respires but cannot photosynthesise at all.
The word “net” is doing all the work. A tree respires every second of every day. It is still a sink, because over a year its photosynthesis takes in far more carbon than its respiration gives back.
Long-term storage: fossil fuels and peat
Normally, dead organisms are decomposed and their carbon is released within months or years. Sometimes decomposition is blocked.
Peat forms where ground is waterlogged and acidic. Low oxygen slows saprotrophs down, so partly decomposed plant material builds up. Peat holds carbon for thousands of years.
Coal, oil and natural gas formed from organisms buried under sediment millions of years ago, under heat and pressure. This carbon has been out of circulation for a very long time.
Both are carbon sinks — until we burn them. Combustion releases carbon dioxide and water and returns that ancient carbon to the atmosphere in seconds.
Burning wood and burning coal are not equivalent, even though both release carbon dioxide. The carbon in wood left the atmosphere within the tree’s lifetime; the carbon in coal left it millions of years ago. That is why the two are treated so differently in climate discussions.
The Keeling curve
Since 1958, the Mauna Loa Observatory in Hawaii has measured atmospheric carbon dioxide continuously. The dataset, named after Charles Keeling, is one of the most important in environmental science.
Two patterns in one graph: an annual zig-zag, and a rise that never stops.
The annual zig-zag
Carbon dioxide dips every northern spring and summer and rises again every autumn and winter. The northern hemisphere holds most of the world’s land and therefore most of its vegetation, so when northern plants are photosynthesising hard they pull measurable amounts of carbon dioxide out of the air.
In autumn and winter, photosynthesis slows down while respiration, decomposition and combustion carry on, so the concentration climbs back up.
The long-term rise
Underneath the wobble, the line climbs year after year. Humans are releasing carbon dioxide by burning fossil fuels faster than photosynthesis and the oceans can absorb it, so a little more stays in the atmosphere every year.
WORKED EXAMPLE
The Keeling curve shows a concentration of about 315 ppm in 1958 and about 420 ppm in 2022. Calculate the mean annual increase, and explain the annual fluctuation superimposed on this trend. [4]
Step 1: find the total change420 − 315 = 105 ppmStep 2: find the number of years2022 − 1958 = 64 yearsStep 3: divide105 ÷ 64 = 1.64 ppm per yearStep 4: explain the fluctuationin northern spring and summer, high rates of photosynthesis remove carbon dioxide, so levels fall; in autumn and winter photosynthesis slows and respiration and decomposition dominate, so levels rise
💡 Exam tip
Label diagrams using the words pool (or sink) and flux. Examiners use them, so you should too.
Name specific processes: photosynthesis, respiration, decomposition, combustion, dissolving. “Carbon moves” scores nothing.
For sink-versus-source questions, always use the word net.
Explain the Keeling zig-zag with northern hemisphere seasons, not just “seasons”.
Explain the rise with combustion of fossil fuels exceeding removal.
When reading values off a graph, quote the units, e.g. ppm, and use a ruler.
⚠ Common mix-up
Carbon is not created or destroyed. It is only moved between pools.
Plants do respire. They are sinks because photosynthesis exceeds respiration, not because they never respire.
The oceans are not only a sink. Dissolving is reversible, so they can release carbon dioxide too.
Decomposition does not always happen. Waterlogged, acidic or anaerobic conditions block it, which is how peat and fossil fuels form.
The seasonal dip is not caused by fewer people driving. It is photosynthesis.
Do not confuse a pool with a flux. Oceans are a pool; dissolving is a flux.
Up next: Nutrient Cycling — carbon is only one element that goes round. On the last page of this topic we widen the lens to nitrogen, phosphorus and the rest, and tie the whole unit together.
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