I understand that plant cells have both Chloroplasts for Photosynthesis and Mitochondria for cellular respiration (breaking down the glucose that photosynthesis creates?). Does this mean that plants use both oxygen and carbon dioxide? Most non-science people tend to think that we use oxygen, and plants use CO2. However, plants are really using both for those two processes, right? And because autotrophs create their own food from the sun's energy, does that mean that autotrophs were the first living things? It seems as if heterotrophs consume autotrophs and so on, so wouldn't there have to first be autotrophs before heterotrophs could consume them?