Step 1: The 2003 Nobel Prize in Chemistry was jointly given to Peter Agre and Roderick MacKinnon for work on how substances cross cell membranes.
Agre's specific contribution was identifying and characterising aquaporins, dedicated water-channel proteins that explain how water moves through membranes far faster than by simple diffusion alone.
None of nucleosome (a chromatin unit), photosynthetic pigment, or ATPase relate to this discovery, which singles out aquaporins as the correct answer.
Answer: Aquaporins.