in Measurement Of Chlorophyll of the fractionation of the spinach leaf cells, why is PSII activity measured and not PSI? Solution Photosynthetic electron transport in chloroplasts involves the transfer of electrons from water to NADP+. This process is carried out by two photosystems namely photosystem I and II. PSII transfers electrons from water to plastoquinone and PSI transfers electrons from plastocyanin to NADP+. During the subcellular fraction of chlorophyll from spinach leaves, the measurement of PSII activity is relatively easy to measure in vitro due to artificial electron acceptor such as dichlorophenol indophenol (DCPIP). This can be used to measure the activity of PSII by the change in the absorbance of a reaction mixture..