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Explain the steps and reservoir interactions in the carbon cycle.

The Cycling of Carbon

an bohr model of a carbon atom with 6 electrons floating around the central nucleus — ***Carbon has 6 protons, 6 neutrons, and 6 electrons and is the basis of all living things.***

Biogeochemical cycles cycle through both living and non-living parts of the environment. “Geo” is a prefix that means “earth” or “rock” and “bio” means “living”, thus biogeochemical cycle.

Carbon is crucial to life on Earth. All living organisms are carbon-based. Carbon is special because it can form an almost infinite number of molecules.

The most important carbon molecules for living things are carbohydrates, fats, nucleic acids, and proteins. The law of conservation of matter states that matter can change form, but it is never destroyed. Thus, the amount of carbon on Earth is the same now as it was millions of years ago.

As organisms are being consumed, they are leaving behind waste materials. These waste materials are recycled and put back into the environment by decomposers, which allows the producers access to the nutrients that were trapped in the consumers.

If decomposers didn’t exist, we wouldn’t be able to continually supply the producers with the nutrients they need to survive and produce glucose through photosynthesis. If the decomposers died, the producers would die when they ran out of nutrients, which would mean the rest of the ecosystem would collapse without the producers to capture the energy from the Sun. The carbon cycle allows carbon to move around the Earth, cycling between the atmosphere, hydrosphere, geosphere, and the biosphere.

Several processes transfer carbon throughout ecosystems – respiration, consumption, decomposition, photosynthesis, combustion, diffusion, and weathering. We will talk about how each of these contributes to the carbon cycle in this lesson. Watch the brief animation (from NASA) below for a quick overview of the carbon cycle:

Essential Knowledge

Biogeochemical cycles are essential for life and each cycle demonstrates the conservation of matter.

The carbon cycle is the movement of atoms and molecules containing the element carbon between sources and sinks.

Steps in the Carbon Cycle

Carbon cycles throughout the atmosphere, hydrosphere, and lithosphere. Explore each phase of the carbon cycle below to see how carbon cycles through each sphere as it moves through the carbon cycle.

Phase 1
Phase 2
Phase 3
Phase 4
Phase 5
Phase 6

Phase 1: Photosynthesis

dandelions in a sunny field — ***Plants use atmospheric carbon dioxide for photosynthesis, making glucose.***

Carbon can be found in the atmosphere as carbon dioxide and methane.

The equation below is the photosynthesis equation:

$LaTeX: 6CO_2+6H_2O\Longrightarrow C_6H_{12}O_6+6O_2$

The ocean is the second largest sink for carbon. Photosynthesis by phytoplankton and macroalgae removes carbon dioxide from the huge reservoir of inorganic carbonates in seawater. Respiration by living organisms then returns the carbon to the ocean.

Phase 2: Respiration

Respiration, which occurs in ALL living things, releases carbon dioxide back into the atmosphere. Cellular respiration also uses a small about of carbon dioxide, but most is released back to the environment when you breathe. When you breathe out, you breathe out mostly carbon dioxide. Because plants also respire and are so abundant, they can actually produce a decent amount of carbon dioxide, but it is mostly offset by the amount of carbon dioxide that they use in photosynthesis, so plants are still considered consumers of carbon dioxide, rather than releasers of carbon dioxide. You can see the cellular respiration equation is the opposite of the photosynthesis equation:

$LaTeX: C_6H_{12}O_6+6O_2\Longrightarrow6CO_2+6H_2O$

About half of the carbon in the carbon cycle is respired by plants and animals and the other half is deposited in the soil in the form of detritus. The atmosphere and the soil are both considered reservoirs in the carbon cycle, which means they store carbon.

Phase 3: Consumption and Decomposition

a parrotfish biting coral — ***Parrotfish have hard teeth and eat coral. Their waste, which is coral skeleton, makes beach sand.***

When herbivores consume plants, they are also consuming the carbon that was once in the atmosphere and is now in the glucose molecule.

When all living things die, they decompose. The process of decomposition is carried out by bacteria and returns carbon to the soil, and eventually, to the atmosphere.

Interestingly, not much decomposition occurs on the bottom of the ocean, so things that die and end up at the bottom of the ocean tend to stay there, which means the ocean is considered a carbon sink, or place where carbon is stored.

Phase 4: Weathering

limestone — ***Limestone is made from animals with calcium carbonate skeletons (shells, coral).***

Carbon is found in rocks as coal and limestone. Limestone is a type of sedimentary rock that is mostly calcium carbonate.

Limestone rock is the largest terrestrial reservoir for carbon.

Uplifting of rock, weathering, and erosion all release carbon into the environment when carbon-rich rocks like limestone wear down due to wind, temperature, and precipitation. This is a which is a very, very slow process.

This carbon is often carried by water via rivers and streams to other locations. Carbon in rocks takes millions of years to cycle.

Phase 5: Combustion

Natural

a forest burning, showing tall flames and copious amounts of smoke — ***Forest* *fires clean out dead plants, enrich soil, and make space for new growth. They are a natural part of matter cycling in many environments.***

Fire is another way carbon is returned from living organisms to the soil or atmosphere.

Forest fires are different from wildfires. Wildfires are fires that are burning out of control and need to be controlled and put out because they threaten natural ecosystems and human settlements.

We also release extra carbon into the atmosphere when we burn forests. Forests are usually considered carbon sinks because so many autotrophs are conducting photosynthesis in the forest, using carbon dioxide. When these forests are burned, not only are we adding extra carbon into the atmosphere from the act of burning the forests, but we are also reducing a critical carbon sink at the same time.

Anthropogenic

A huge component of the carbon cycle has become the combustion of fossil fuels by humans. Some organisms that died millions of years ago did not fully decompose and ended up covered by layers and layers of sediment until they were deep underground. Once these remains are deep underground, pressure and other geological processes can convert them into fossil fuels.

A graphic titled ‘COAL FORMATION’ depicts three stages: ‘Swamp’ with green grass and water, ‘Dirt Dead Plants’ showing submerged organic matter, and ‘Rocks and Dirt’ revealing a coal layer. Coal forms from buried dead plant matter under high pressure and temperature over millions of years. — ***Coal formation typically occurs when dead plants are buried for millions of years and requires high temperatures and pressure to form.***

A graphic titled ‘OIL AND NATURAL GAS FORMATION’ illustrates the process in three parts: ‘Ocean’ with fish, ‘Sand and Silt’ with organic remains, and ‘Rocks, Oil and Gas’ showing an oil well tapping into underground layers. It demonstrates how oil and natural gas form from organic matter under extreme pressure and temperature over eons. — ***Oil and natural gas are formed when organic matter (usually oceanic) is buried for millions of years and require high temperatures and pressure to form.***

The carbon in fossil fuels was active in the atmosphere many millions of years ago. This carbon has been locked away from the carbon cycle for these millions of years. During this time, it was not part of the carbon cycle.

This video explains the difference between old and young carbon. Can you think of an example of young carbon? Old carbon?

Once fossil fuels are extracted and burned, we are adding that carbon back into the carbon cycle, which can increase the carbon concentration in the atmosphere, which can intensify the greenhouse effect, causing an overall warming of the planet.

Phase 6: Diffusion

Some carbon also diffuses into the ocean. This diffusion of carbon is why you may have heard people discussing ocean acidification. As more carbon is released into the atmosphere by humans, more carbon is absorbed by the ocean.

An infographic titled ‘Ocean Acidification’ explains how changes in ocean chemistry impact marine life. It features a white background with a blue wave pattern at the top. The top section illustrates the chemical reaction of carbon dioxide with water and carbonate ions, forming bicarbonate ions. The bottom section demonstrates how this process hinders calcification in marine organisms, using the example of a shell. Black text accompanies the blue chemical reaction

The oceans absorb a large amount of carbon from the atmosphere, which can cause ocean acidification because the carbon dioxide reacts with other salts in the water, lowering the pH of the ocean water.

Ocean acidification lowers the pH of the ocean when carbon dioxide is absorbed from the atmosphere. When carbon dioxide reacts with water, and other salts in the ocean, bicarbonate is formed, which is acidic. Just one of the results of ocean acidification is the reduction in the strength and thickness of calcium carbonate shells.

Essential Knowledge

Carbon cycles between photosynthesis and cellular respiration in living things.

Plant and animal decomposition have led to the storage of carbon over millions of years.

The burning of fossil fuels quickly moves stored carbon into atmospheric carbon, in the form of carbon dioxide.

Putting it all Together

The carbon cycle moves carbon between the atmosphere, oceans, land, and living organisms. Through photosynthesis, plants absorb carbon dioxide from the air, turning it into organic compounds and acting as the primary carbon source in land ecosystems. These compounds are then consumed by herbivores and, subsequently, by carnivores, forming a complex web of interactions. Marine algae take in carbon dioxide, contributing to the creation of organic matter in the ocean. Carbon is consistently exchanged between the atmosphere and oceans through processes like respiration and dissolution.

Oceanic carbon cycles approximately every 400 years, while the carbon in the atmosphere cycles about every 5 years. Carbon in living or previously living organisms cycles about every 10 years or so.

Human activities, like burning fossil fuels, have significantly impacted the carbon cycle, resulting in increased atmospheric carbon dioxide levels and contributing to global climate change.

Essential Knowledge

Some of the reservoirs in which the carbon cycle holds carbon compounds for long periods of time, while some of them hold them for relatively short periods of time.

AP Exam Tip

You will need to know both the photosynthesis and cellular respiration equations. Memorize one - the other is the reverse of the first!

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