A green plant can be viewed as a control volume in which exergy from solar radiation is convened into the exergy of photosynthetic products. Even today a portion of the world's exergy requirements come from biomass. Thus, after some considerations about the exergy value of black radiation and of solar energy, the main purpose of this paper is an analysis of the thermodynamic efficiency of photosynthesis, subdividing the whole process in various phases, of everyone of which the relative entropy production and exergy efficiency are examined. In addition, with reference to some recent papers, remarks are reported about the difference of photosynthetic conversion between cultivated crops and natural ecosystems.