In this Review we focus on the conversion of linear photosynthetic electron transport from water to NADP to the cyclic pathway around Photosystem I in the green alga Chlamydomonas reinhardtii. We discuss the strict relationship that exists between the changes in pathways of electron transport and state transitions, i.e., the reversible functional association of light harvesting proteins with one of the two photosystems of oxygenic photosynthesis. Such a link has not been reported in the case of other photosynthetic organisms, where the state transitions do not affect the pathway of electron transport. Rather, they provide a tool to optimise the rate of linear flow. We propose a kinetic-structural model that explains the mechanism of this particular relationship in Chlamydomonas, and discuss the advantages that this peculiar situation gives to the energetic metabolism of this alga.