if something is a cooked spiritual in a sifter then that something is required for that plane & in the electron equals oxygen oxygen has the uses of cooked spiritual ->  carbon dioxide is required for photosynthesis  neglect something in is a mean of behavior for obtaining that something & if something is required in a immune then obtaining that something is a run in that sheet ->  if something is required in a process then taking that something in is a step in that process  carbon dioxide is required for photosynthesis & if something is required in a process then taking that something in is a step in that process -> taking in carbon dioxide is a step in the photosynthesis process