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Living organisms release carbon back into the environment primarily through the process of respiration. During respiration, organisms convert glucose and oxygen into energy, carbon dioxide, and water. This process occurs in cells, where organic molecules are broken down, releasing energy for various cellular functions.

As glucose is metabolized, carbon atoms from the glucose molecule are released in the form of carbon dioxide. This carbon dioxide then diffuses out of the organism and enters the atmosphere, contributing to the carbon cycle. This cycle is crucial for maintaining ecological balance, as carbon dioxide is a key component utilized in photosynthesis by plants, which absorb the gas and use it to create organic matter.

The other processes mentioned do not primarily serve to release carbon back into the environment in the same manner. For instance, photosynthesis involves the uptake of carbon dioxide to produce glucose, while transpiration relates to the movement of water through a plant, and digestion refers to the breakdown of food for energy and nutrient absorption, rather than the direct release of carbon into the atmosphere.