if something is a raw material in a process then that something is required for that process & in the photosynthesis process carbon dioxide has the role of raw material ->  carbon dioxide is required for photosynthesis  taking something in is a kind of method 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  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