The organisms in a pond and the physical factors influencing them best describes an ecosystem.
The right answer is CO2.
The degradation of glucose is either by respiration or by fermentation. In breathing we have a release of CO2.
Respiration refers to both gaseous exchange resulting from inhalation and expiration of the air (CO2 carbon dioxide release and oxygen O2 absorption) and cellular respiration that allows, by degrading glucose through oxygen , to obtain energy.
