The sentence is discussing the conditions under which a substance (referred to as "it") is necessary for a plant. Specifically, the sentence outlines two possibilities: The substance is a component of an essential plant constituent, such as an enzyme. This means that the substance is a necessary part of a larger structure or system within the plant, and its absence would disrupt the plant's normal functioning. The substance is required for a distinct metabolic step, such as an enzyme reaction. This means that the substance plays a specific role in a chemical process within the plant, and its absence would prevent that process from occurring. Overall, the sentence is discussing the importance of certain substances to a plant's growth and functioning, and the specific ways in which they are necessary.