Carbon cycling Photosynthetic organisms use CO2 as a source of carbon for synthesizing organic compounds. Photosynthesis produces O2 and aerobic respiration produces CO2 as a byproduct. Carbon cycled from atmosphere to photosynthesis, to heterotrophs that ate plants, then released as aerobic respiration waste. Ozone, O3, forms from molecular O2 in upper atmosphere. Filters out ultraviolet radiation, which can cause mutations.
Organisms can live closer to water surface and eventually move onto land.
Chloroplasts evolved from photosynthetic bacteria Mitochondria evolved from aerobic bacteria. Could have been ingested, but not digested, then reproduced along with the host. Endosymbiont eventually lost the ability to survive outside the host. Host couldn t survive without endosymbiont. Mitochondria and chloroplasts have some of their own genetic material and translation.