Carbon is the most important element in the biological system and constitutes about 50% of all living organisms. It occurs through atmosphere, oceans and through living and dead organisms.

1. from the atmosphere to plants: plants take in carbon dioxide from the atmosphere and release oxygen during the process of photosynthesis. The reaction is:
6 CO₂ + 6 H₂O + solar energy → C₆H₁₂O₆ + 6 O₂

2. From plants to animals: Consumption of plants by animals directly or indirectly transfers the carbon molecules as glucose through the food chains.

3. From plants to animals: From living things to the environment: When plants and animals die, they decompose into the soil. During this decomposition, the carbon compounds in their bodies re-join the soil.

4. From living things into the atmosphere: When we exhale, we give out carbon dioxide into the atmosphere. Through the process of respiration, carbon is cycled through the atmosphere from living things.

5. From fossil fuels into the atmosphere when fuels are burned: Our species relies on fossil fuels for a large portion f our energy needs even though the trend is gradually changing. When we burn fossil fuels, we are releasing carbon compounds like carbon dioxide and carbon monoxide into the atmosphere. Every year, around five and a half billion tons of carbon is released by the burning of fossil fuels. Out of the fossil fuels burned, around three billion tons of it goes into the atmosphere whereas most of the rest get mixed with the oceans or settle down on the ground.

6. Carbon moves from the atmosphere to the oceans: The oceans soak up carbon through the surface and deposits from rivers and estuaries.