Final answer:
Photosynthesis is an energy-producing process that uses sunlight to make ATP and NADPH, whereas biological nitrogen fixation is a process that consumes ATP to fix nitrogen into ammonia.
Step-by-step explanation:
The main difference, from an energy point of view, between photosynthesis and biological nitrogen fixation is that photosynthesis uses the sun's energy to produce ATP and NADPH, which are then used to synthesize carbohydrates from carbon dioxide. In contrast, biological nitrogen fixation is an ATP-consuming process where ATP is expended to fix atmospheric nitrogen into ammonia, which is then incorporated into organic compounds such as protein. Therefore, while photosynthesis is an energy-producing process, biological nitrogen fixation is an energy-consuming process.