Final answer:
Carbon is oxidized to CO₂ during cellular respiration, and it can re-enter the organic carbon cycle through photosynthesis. These processes are part of a biological cycle that allows life to sustain itself with energy from the sun. The correct option is a).
Step-by-step explanation:
Oxidation Process in Aerobic Environment
Once carbon has been oxidized to CO₂, its most stable form in an aerobic environment, it can only cycle back into the organic portion of the carbon cycle through photosynthesis. This is because oxidation is a highly favorable process in which macromolecules are broken down to release energy.
During cellular respiration, carbohydrate molecules are oxidized, leading to the loss of hydrogens and forming CO₂, while oxygen is reduced forming water (H₂O). This interplay of oxidation and reduction underscores the biological cycles that sustain life on Earth.
Photosynthesis is the process that converts carbon dioxide back into organic compounds using the energy from sunlight. In this cyclic process, energy from a distant star is harnessed by chloroplasts within green plants to build carbohydrates, thereby pulling CO₂ out of the atmosphere and supporting life.
The correct answer to the question provided would be (a) CO₂, photosynthesis. This encapsulates the essence of the global carbon cycle, where CO₂ is neither a waste product of respiration nor oxygen is a waste product of photosynthesis. Both are essential for sustaining the biological cycles on Earth.