In this lesson, Dr. Randy Gauci explains the process of photosynthesis, highlighting that the leaf, specifically the chloroplast, is the site where this occurs. Key adaptations of the leaf, such as the wax cuticle and placement of stomata mostly on the underside, help optimize photosynthesis by minimizing water loss while allowing carbon dioxide entry. Dr. Gauci touches on the complexity of the photosynthesis process, stating it involves reactants like carbon dioxide and water resulting in the production of glucose within chloroplasts.