if something is a raw material in a process then that something is required for that process & 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 process then obtaining that something is a step in that process -> if something is required in a process then taking that something in is a step in that process 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 fruit then neglect that something in is a run in that areas -> taking in carbon dioxide is a step in the photosynthesis process