Final answer:
Nitrogen gas must be chemically transformed through nitrogen fixation to become usable by plants. This process turns atmospheric nitrogen into ammonia or related compounds that plants need to synthesize important biological molecules.
Step-by-step explanation:
The substance that must be chemically transformed before it can be used by plants is nitrogen gas. Although nitrogen is an essential macronutrient for plant growth, atmospheric nitrogen (N₂) is not directly usable by most plants. Instead, it must undergo a process known as nitrogen fixation, where nitrogen gas is converted into ammonia (NH₃) or related compounds that plants can assimilate. This biological nitrogen fixation is usually performed by symbiotic bacteria found in the root nodules of legumes or by free-living bacteria in the soil. Once nitrogen has been fixed, plants can use it to synthesize amino acids, nucleotides, and chlorophyll.
For context, photosynthesis is the process in which plants convert carbon dioxide and water into sugars/carbohydrates and oxygen using sunlight. Photosynthesis takes place within chloroplasts and is facilitated by the pigment chlorophyll. Inorganic compounds like nitrate and ammonium derived from nitrogen fixation are then used by plants to build biomolecules essential for their growth and development.