Chloroplasts in spinach have different regions for photosystem I and II. To understand how electrons move between these regions, scientists studied the number of reaction centers and electron carriers. They found that the ratio of photosystem I, cytochrome b6/f complexes, and photosystem II is almost equal. The pool of fast electron donors to P700 includes electrons from cytochrome b6/f and plastocyanin. Slower electrons come from plastoquinol.