Answer:
1. Carbon dioxide
Explanation:
During photosynthesis, autotrophs such as green plants, algae, etc. remove carbon dioxide from the air and incorporate it into complex organic compounds such as glucose. Plants use much of the glucose to produce their own biomass. Heterotrophs feed on autotrophs and obtain these organic compounds from them. Heterotrophs perform cellular respiration during which oxidation of organic compounds produces ATP molecules. The process of cellular respiration returns carbon dioxide to the atmosphere. Autotrophs obtain this CO2 from the air for photosynthesis and the cycle continues.
