Carbon dioxide

Explanation

In biological energy chemistry, carbon dioxide (CO2) is important because it represents an "energetically exhausted" carbon molecule, or the end product of energy harvesting of sugars and fats. Briefly, electrons in C-H and C-C bonds (common in sugars and fats) are in relatively high energy orbitals, whereas those in C-O bonds are relatively low energy. The processes of respiration and the electron transport chain 'harvest' high energy electrons and convert their energy to other forms. The rearrangements involved result in products CO2 and H2O--all electrons are 'spent'.

CO2 is also biologically important because it allows sunlight to pass through readily (visible wavelengths of light), but it reflects warmth (infrared wavelengths). More CO2 in the atmosphere therefore traps more heat while not decreasing the amount of sunlight energy coming to the planet.