Great portions of the world's exergy requirements come from solar energy. In addition, green plants can be viewed as biological energy conversion processes in which exergy from solar energy is converted into chemical energy by chlorophyll using water and carbon dioxide. In this article, exergy analysis of biological energy conversion and a comparative study of four photosynthetic pathways are presented. This comparison is made by using the thermal efficiency of light to chemical energy conversion and derived simple thermal efficiency. Non sulfur purple bacteria and non sulfur green bacteria from the others have the best thermodynamic performance. The rate of lost work consists of photochemical reaction, respiration, and thermal absorption of radiation. According to calculated lost work components or equivalently, the available work or exergy lost, the photochemical process accounting for the work lost has the highest rate. Apparently, chemical reactions taking place inside the system are higher entropy producers than material or energy transport between the leaf and its environment.