The diffusion of the protein plastocyanine and complex formation between plastocyanine and cytochrome f (a subunit of a cytochrome b6/f complex) in the chloroplast thylakoid lumen has been studied. A 3D computer simulation model of diffusion and binding of plastocyanine and cytochrome f has been constructed, which considers their electrostatic interaction. Based on the experimental data, the parameters of the model for complex formation between plastocyanine and cytochrome f in solution have been estimated. The dependence of the rate of plastocyanine-cytochrome f reaction on the size of the luminal space has been studied. It was shown that the contraction of the luminal space leads to a decrease in the reaction rate, which is in agreement with the experimental data on the inhibition of the reaction under hyperosmotic stress.