ACARA v9 CONTENT DESCRIPTION “represent the carbon cycle and examine how key processes including combustion, photosynthesis and respiration rely on interactions between Earth’s spheres (the geosphere, biosphere, hydrosphere and atmosphere)”
Builds on earlier work picturing Earth as a set of connected systems and on the idea that matter is conserved. Here those ideas come together in one cycle: the same carbon atoms move between the air, living things, water and rock, driven by a few key processes.
One planet, four spheres
Scientists describe the Earth as four overlapping systems, or spheres: the atmosphere (the air), the biosphere (all living things), the hydrosphere (the oceans and other water) and the geosphere (the rock and soil). Carbon is stored in every one of them, and the carbon cycle is the way that carbon moves from one sphere to another.
Earth’s four spheres
Earth’s carbon is split between four systems: the air, living things, water and rock. Pick a sphere to see how carbon is stored there.
Carbon is stored here as carbon dioxide gas (CO2).
Photosynthesis: pulling carbon from the air
Plants take carbon dioxide gas out of the atmosphere and, using energy from sunlight, build the carbon into sugars and tissue. This single process moves a huge amount of carbon out of the air and into the biosphere every day. It is the main pathway by which carbon enters living things.
Photosynthesis: air into plants
Plants pull carbon dioxide out of the air and use sunlight to build it into their tissue, moving carbon from the atmosphere into the biosphere.
Carbon dioxide (CO2) sits in the air as a gas.
Respiration and combustion: sending carbon back
Two processes return carbon to the atmosphere as carbon dioxide. Respiration happens inside living things as they release energy from their food, sending CO2 back into the air. Combustion is the burning of fuels such as wood, coal, oil and gas, which also releases CO2. Burning fossil fuels moves carbon that was stored in the geosphere for millions of years back into the atmosphere.
Respiration and combustion: back to the air
Two processes send carbon back to the atmosphere as CO2: living things releasing it as they respire, and fuels releasing it as they burn.
Respiration happens in living things every moment, turning stored carbon back into CO2 gas. Combustion burns fuels such as wood, coal, oil and gas, and also releases CO2. Both return carbon from other spheres to the atmosphere.
A loop with no beginning
Put these processes together and they form a cycle. A plant draws carbon from the air; an animal eats the plant and breathes some carbon back out; the ocean absorbs CO2 from the air; shells and skeletons settle and harden into rock; and burning fuel returns carbon to the air. No carbon is created or destroyed, so the same atoms travel round and round between the spheres.
Follow a carbon atom around the cycle
Carbon never leaves the planet; it just moves between the spheres. Step a single atom one hop at a time and trace the loop.
The same carbon atoms cycle round and round. A plant takes carbon from the air, an animal eats the plant and breathes some back out, ocean water absorbs more, shells and skeletons lock it into rock, and burning fuel releases it again. The cycle has no start and no end.
Each process links two spheres
The clearest way to read the carbon cycle is to treat every process as a bridge between two spheres. Photosynthesis links the atmosphere and the biosphere; respiration links the biosphere and the atmosphere; dissolving links the atmosphere and the hydrosphere; and combustion links the geosphere and the atmosphere. Mapping each process to the spheres it connects shows how tightly the four systems are tied together.
Which two spheres does each process link?
Every process in the carbon cycle is an interaction between two spheres. Pick a process to see which two it connects and which way carbon flows.
Links the atmosphere and the biosphere: plants take CO2 from the air.
Why this matters
The carbon cycle is one of the main ways the Earth’s systems stay connected. Tracking where carbon is stored and how it moves helps explain how the planet works as a whole, from the growth of forests to the chemistry of the oceans. Combustion releases carbon dioxide into the atmosphere, which is a measurable part of this cycle and a focus of much scientific study.
Quick self-check
1. In which form is carbon stored in the atmosphere?
2. What does photosynthesis do to carbon?
3. Which two processes return carbon to the atmosphere as CO2?
4. Combustion links which two spheres of the carbon cycle?