An analysis of the mental model on the photosynthesis concept of a professor who teaches Natural Sciences, named for the present study Guillermo, is presented, being necessary the realization of a graphic representation, a verbal explanation and the completion of a conceptual questionnaire, all related to the model in mention. The results are described from a qualitative methodology, using the case study as a design, which makes possible two aspects of the teacher's model: the ontological constituent and the epistemological one through the application of the ONEPSI model. From the obtained information the possible conceptual errors about the photosynthesis are evidenced, when the contrast with the School Scientific Model is made, being found mainly the absence of necessary entities to explain the concept in mention, besides the assignment of inadequate properties to Some entities, for example for Guillermo, glucose is a reagent for the photosynthetic process and not a product.