Abstract <p>The article addresses a formal paradox related to the formation of molecular oxygen during photosynthesis. Following the studies of van Niel in the early 1930s, it has become clear that in the oxygenic photosynthesis, molecular oxygen originates from water rather than carbon dioxide. However, the overall equation of photosynthesis, <i>n</i>CO₂ + <i>n</i>H₂O → (CH₂O)_n + <i>n</i>O₂, suggests that the amount of oxygen produced exceeds what could be derived from the water molecules involved. This paradox can be resolved by analyzing the light and dark reactions of photosynthesis, which ultimately result in the incorporation of carbon from CO₂ into carbohydrates and production of molecular oxygen. Despite its simplicity, the solution is not immediately obvious. One reason is that in the scientific and educational literature, the dark reactions of photosynthesis are often depicted schematically, without precise specification of all components involved. The author argues that analyzing this paradox and underlying physicochemical principles of photosynthesis can be valuable for students specializing in biochemistry.</p>