To answer this correctly, I would need more specific context about the diagram you're referring to. However, based on common biological concepts related to processes like photosynthesis, the most logical pair of terms that could be missing (for components often discussed in this context) would be:
A, chlorophyll; B, ATP.
Chlorophyll is essential for capturing sunlight during photosynthesis and ATP is one of the main products of this process. If the diagram relates to photosynthesis in plants, this response would fit well.
If the question pertains to another biological process or context, please provide additional details!