ok so more c02 goes out than comes in with mammals because of various biological processes at the cellular level that makes sense. im sure i learned this in high school but forgot thanks. plants on the other hand basically break the co2 bond and use the carbon for there tissues and release the o2 into the atmosphere correct more or less? so basically a plant assimilates its tissue from atmosphere amazing. where as us mammals have to consume other organisms to get our carbon to grow. so in a way plants are made out of air it sounds odd but basically true in laymans terms. very fascinating