Because we are not autotrophic organisms, i.e., can do photosynthesis which transforms inorganic substances in organic by the energy of the light ( vegetables and algae ) Neither the chemosynthesis which transforms them by the energy released in 40 oxidation; fungi and some bacteria ). We are heterotrophic and need the autotrophs to make us food ( carbohydrates, fats and proteins ). Also we can absorb mineral salts and trace elements dissolved in water through the digestive system.
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