The conversion of 3-phosphoglycerate (PGA) to glyceraldehyde 3-phosphate (PGAl) in the Calvin cycle is an endergonic reaction, meaning it requires energy input.

The source of this energy is ATP (adenosine triphosphate).

Here's how it works:

1. PGA is phosphorylated: ATP provides a phosphate group to PGA, converting it to 1,3-bisphosphoglycerate (1,3-BPG). This reaction is catalyzed by the enzyme phosphoglycerate kinase.

2. 1,3-BPG is reduced: NADPH (nicotinamide adenine dinucleotide phosphate), a reducing agent, donates electrons to 1,3-BPG, converting it into PGAl. This reaction is catalyzed by the enzyme glyceraldehyde 3-phosphate dehydrogenase.

Therefore, both ATP and NADPH are crucial for the conversion of PGA to PGAl in the Calvin cycle.