In this paper a flux-coupling model of photosynthesis is presented. By requiring minimum entropy production, it is found that the photosynthetic efficiency is essentially given by the square root of D/lambda. D and lambda are the diffusion coefficient and thermal conductivity of the rate-limiting processes in the chloroplast, respectively. For experimental values of D and lambda, the efficiency is found to be 2.4-7.5%, with a likely value of 6.1%, whereas C4-plants are known to have an efficiency of 6.2%. We conclude that the process of photosynthesis is in quantitative agreement with the principle of minimum entropy production.