  Photosystem I is a protein-pigment complex that performs photosynthesis in
plants, green algae, and cyanobacteria. It contains an aggregate of
chlorophylls that absorbs light and delivers the resulting electronic
excitation to the special pair of chlorophylls where the excitation energy is
used for producing charge separation across the cell membrane. The seemingly
random arrangement of chlorophylls in photosystem I poses the question which
pathways excitation migration follows towards the special pair after absorption
of light at any of its chlorophylls. We employ a master equation to describe
the process of excitation migration in photosystem I, and find representative
paths of excitation migration based on the mean first-passage time from each
chlorophyll to the special pair. The resulting paths, beginning at each of the
chlorophylls and ending at the special pair, provide a complete yet distilled
picture of the excitation migration towards the special pair.
