In principle, heat energy will be released from every chemical bond formation of molecules, and contrary, to break down every chemical bond require heat energy. Accordingly, for your case, you need to know the molecular structure of the biomolecules involved in the photocatalytic reactions. Based on this structure, you can predict which one of the chemical bond will be formed and which one will be broken down. The heat will be released from the reaction if the energy released to form new chemical bonds is higher than the energy absorbed to break down the original chemical bonds, and vice versa.