Final answer:
A cell that requires more 5-phosphate without the accompanying need for NADPH can utilize the pentose phosphate pathway or engage in cyclic photophosphorylation, a process of photosynthesis that produces ATP without NADPH.
Step-by-step explanation:
When a cell needs more 5-phosphate but not NADPH, it can use the pentose phosphate pathway (PPP). This metabolic pathway operates parallel to glycolysis and its primary function is to generate NADPH and five-carbon sugars, including ribose 5-phosphate, which is essential for nucleotide synthesis. However, if the cell requires more ATP relative to NADPH, it may opt for cyclic photophosphorylation during photosynthesis.
In cyclic photophosphorylation, only Photosystem I (PSI) is involved. The high-energy electron from the PSI reaction center is transferred to the electron transport system (ETS), circulates back to PSI, and in the process, generates ATP without producing NADPH. This is beneficial for cells with a higher demand for ATP to power other processes like the Calvin Cycle.
It's important to note that in oxygenic photosynthesis, noncyclic photophosphorylation typically generates both ATP and NADPH. However, if ATP requirements exceed those for NADPH, cyclic photophosphorylation becomes a more efficient alternative to satisfy the cell's energy demands.