How Is Carbon Exchanged Between Organisms?

Carbon exchange between organisms refers to the biological processes by which carbon is transferred and cycled between living things. Carbon is a fundamental building block of life on Earth, and all organisms require carbon to grow, reproduce and carry out metabolic functions. The carbon found in organic molecules within an organism can be passed between organisms through various biological mechanisms.

The most well-known processes of carbon exchange occur during photosynthesis, cellular respiration, decomposition, and consumption within food chains and webs. During photosynthesis, plants take in carbon dioxide from the atmosphere and use energy from sunlight to convert it into energy-rich organic compounds like sugars. The plant stores this carbon in its tissues. When animals consume plants, or other animals that have eaten plants, they intake carbon-rich organic molecules. Cellular respiration releases energy from the organic compounds, converting the carbon back into carbon dioxide gas, which is exhaled. Decomposers like bacteria and fungi release carbon from dead organisms back into the environment.

These biological carbon exchanges are part of what is known as the carbon cycle. The movement of carbon between the biosphere, atmosphere, oceans and geosphere allows carbon to be continuously recycled in different forms. The exchange of carbon between organisms and the environment is crucial for sustaining life on Earth.


Plants take in carbon dioxide (CO2) from the atmosphere and use the carbon to build sugars, releasing oxygen (O2) as a byproduct of photosynthesis. Photosynthesis occurs in chloroplasts, organelles found in plant cells that contain the green pigment chlorophyll. When sunlight shines on chlorophyll, the energy from the light drives a series of chemical reactions that convert CO2 and water (H2O) into glucose and O2.

The overall chemical equation for photosynthesis is:

6CO2 + 6H2O + Light Energy → C6H12O6 + 6O2

This equation shows that for every 6 molecules of CO2 consumed, 1 molecule of glucose is produced, while 6 molecules of oxygen are released as a byproduct. The glucose provides energy for the plant and the released oxygen replenishes the atmosphere. Through photosynthesis, plants use the power of the sun to transform carbon from the air into carbohydrates they use for growth.

Cellular Respiration

Cellular respiration is the process by which organisms convert oxygen and food molecules like glucose into carbon dioxide, water, and energy. This process occurs in all living cells and is essential for organisms to obtain the energy needed to power their life processes.

Cellular respiration takes place in the cells’ mitochondria, where glucose and oxygen are used to produce ATP (adenosine triphosphate, the energy currency of cells). The overall chemical reaction of cellular respiration can be summarized as:

C6H12O6 (glucose) + 6O2 → 6CO2 + 6H2O + energy (ATP)

There are three main stages of cellular respiration:

  • Glycolysis – Glucose is broken down into pyruvate and the energy released is used to produce 2 ATP molecules.
  • Krebs Cycle – Pyruvate enters the mitochondria and is further broken down into carbon dioxide, and more energy is released to produce ATP.
  • Electron Transport Chain – Electrons from the Krebs cycle provide energy to pump hydrogen ions across the mitochondrial membrane. This creates a proton gradient that drives the synthesis of many ATP molecules.

As oxygen is consumed and carbon dioxide is produced, this exchange of gases allows organisms to take in and release carbon. The carbon from the glucose molecules ultimately gets released as CO2 through this process.

The Carbon Cycle

The carbon cycle is the biogeochemical cycle by which carbon is exchanged between the biosphere, pedosphere, geosphere, hydrosphere and atmosphere of the Earth. It describes the movement of carbon as it is recycled and reused throughout the biosphere and the Earth’s ecosystems.

Carbon is an essential element for life on Earth. It is stored in reservoirs known as ‘carbon sinks’ such as the atmosphere, oceans, soil, plants and fossil fuels. The movement of carbon between these reservoirs is known as the carbon cycle.

Carbon cycles between the Earth’s ecosystems through a series of processes such as photosynthesis, respiration, decay, combustion, weathering, fossil fuel formation and ocean-atmosphere gas exchange. These processes regulate the amount of atmospheric carbon dioxide, which plays an important role in global climate.

Human activities such as fossil fuel combustion, deforestation and land use changes have significantly impacted the global carbon cycle by altering carbon fluxes and increasing atmospheric CO2 concentrations. Understanding the carbon cycle is key to understanding and mitigating human impacts on climate change.

Carbon in the Atmosphere

carbon dioxide moving between the atmosphere and organisms
The atmosphere plays a crucial role in the carbon cycle as it facilitates the exchange of carbon dioxide between organisms and the environment. Carbon dioxide is an important greenhouse gas that helps regulate Earth’s temperature. It is produced through cellular respiration by organisms like plants, animals, fungi and microbes. Some of this CO2 is absorbed by terrestrial plants and marine phytoplankton through photosynthesis. However, excess CO2 can build up in the atmosphere.

The exchange of carbon dioxide between organisms and the atmosphere is key to maintaining balance in the carbon cycle. For example, forests act as carbon sinks, removing CO2 from the air through photosynthesis. Deforestation can disrupt this process and release stored carbon back into the atmosphere. On the other hand, the burning of fossil fuels adds substantial amounts of CO2 that cannot be easily absorbed. The two-way flow between living organisms and the atmosphere is critical for supporting life while regulating the greenhouse effect. Any disturbance can impact the delicate equilibrium of carbon in the biosphere.

Carbon in Oceans

The oceans play a major role in the global carbon cycle, absorbing vast amounts of carbon dioxide from the atmosphere. The exchange of carbon between oceans and organisms occurs through both biological and physical processes.

Marine plants like phytoplankton take up dissolved CO2 through photosynthesis, using it to build organic matter and releasing oxygen. When these organisms die, some of their biomass sinks to the ocean floor, taking carbon with it. This is known as the biological carbon pump.

The oceans also absorb CO2 directly from the atmosphere through gas exchange across the air-sea interface. Some of this dissolved inorganic carbon gets transported to the deep ocean through currents and mixing, removing it from contact with the atmosphere for centuries or more.

Animals like fish and whales obtain carbon by eating other organisms or respiratory gases dissolved in seawater. Their respiration returns some inorganic carbon back to the oceans. When marine animals die, their bodies decompose and release carbon either in shallow waters or on the seafloor.

Human activities are altering natural carbon cycling in the oceans. Rising atmospheric CO2 is increasing the amount of inorganic carbon the oceans can absorb. However, warmer ocean temperatures are reducing CO2 solubility. Fossil fuel emissions also cause ocean acidification, which can disrupt the marine food web.

Carbon in Soil

Soil is a major reservoir of carbon on Earth. Carbon is exchanged between soil and plants, animals, fungi, and microorganisms that live in the soil.

Plants take up carbon dioxide from the atmosphere through photosynthesis. Some of this fixed carbon is transferred to the soil as roots die and decay, releasing organic compounds into the soil. Plants also exude sugars and other carbon compounds through their roots, which are consumed by microbes in the rhizosphere.

Soil organisms like bacteria and fungi break down organic carbon from decaying plant matter, humus, and manure. This releases inorganic carbon like carbon dioxide. Soil animals like worms ingest carbon compounds and respire carbon dioxide.

Carbon is stored in soil organic matter. Stable forms like humus can sequester carbon in soils for decades to millennia. However, tillage and erosion can disturb soils, causing loss of soil carbon to the atmosphere. Land management practices like reduced tillage, cover cropping, and erosion control help sequester carbon in agricultural soils.

Overall, soils actively exchange carbon with plants and organisms. Management practices affect whether soils are a net source or sink of atmospheric carbon.

Food Chains

Carbon moves through food chains as organisms consume other organisms. Green plants, algae, and some bacteria make their own food through photosynthesis, taking carbon dioxide from the air and incorporating the carbon into carbohydrates. Herbivores get carbon when they eat plants. Carnivores get carbon when they eat other animals. Omnivores get carbon from eating both plants and animals.

Each link in the food chain passes biomass and energy to the next. When an organism dies, its carbon-containing compounds are eaten by detritivores or decomposed by microbes, recycling the carbon back into the ecosystem.

Longer food chains have less energy available at each successive link, as energy is lost along the chain. A greater fraction of the energy and biomass carbon ends up sequestered in the environment or emitted back into the atmosphere as carbon dioxide through respiration at each step.

Humans impact carbon cycling through food chains by removing large predators, overfishing, pollution, land use changes, and other disturbances to ecosystems. Sustainable practices are needed to maintain balanced food chains and healthy carbon exchange.

Carbon Sequestration

Carbon sequestration refers to the long-term storage of carbon in plants, soils, geologic formations, and the ocean. This process removes carbon dioxide from the atmosphere and stores it in “carbon sinks” such as:

Forests: Trees and plants absorb carbon dioxide through photosynthesis. Expanding forests helps remove carbon from the atmosphere.

Soils: Carbon can be stored in soil organic matter. Farming practices like cover crops, no-till agriculture, and crop rotations can increase carbon storage in farmland.

Geologic formations: Carbon dioxide can be injected into depleted oil and gas reservoirs, coal seams, or deep saline aquifers. This is known as carbon capture and storage.

The ocean: The ocean absorbs roughly a quarter of human-caused carbon emissions. Increasing ocean alkalinity can boost carbon storage.

Carbon sequestration provides a natural way to remove carbon dioxide from the atmosphere. This helps mitigate global climate change. Sequestering carbon also allows time for society to transition from fossil fuels to renewable energy sources.


In summary, carbon is exchanged between organisms through various biological processes and environmental systems. Photosynthesis absorbs carbon dioxide from the atmosphere to produce carbohydrates, while cellular respiration releases carbon dioxide back into the air. The carbon cycle circulates carbon through the biosphere, driven by these metabolic processes.

Carbon dioxide naturally cycles between the atmosphere, oceans, soils and living things. Plants and phytoplankton take in CO2 while respiring organisms give off CO2 as a byproduct. Carbon is incorporated into biomass through food chains and webs. Human activities have disrupted the balance of the carbon cycle through burning fossil fuels and deforestation, leading to a buildup of atmospheric CO2. Methods like reforestation and carbon capture aim to sequester excess carbon and mitigate climate change.

Understanding how carbon moves through ecosystems provides important insight into biological interactions and environmental impacts. This overview of carbon exchange elucidates the close interconnection between the biosphere, atmosphere and climate.

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