Variations in the apparent photon requirement for photosynthesis (Φ−1co2) or (Φ−1co2) in the data of Warburg & Burk (1950) and Yuan, Evans & Daniels (1955) can be ascribed to changes in O2 uptake and energy-dependent processes which result in aberrant photon requirements in organisms subjected to non-optimal conditions. The increase in Φ−1co2 with increases in the gas exchange quotient (γ) in the observations of Yuan et al. (1955) is consistent with increases in photorespiratory production of glycollate, whilst changes in Φ−1co2 and Φ−1co2 in the results of Warburg & Burk (1950) can be explained by a variable Kok effect associated with nitrate assimilation at low light levels. When these O2 and energy-dependent processes are minimal, the lowest values should be observed. The minimum value obtained when Chlorella is photosynthesizing under optimal conditions is 6 mol photons mol−1O2. These results provide direct independent evidence for a photon requirement for photosynthesis of less than 8 mol photons mol−1O2. Such a value is not consistent with the Z scheme of photosynthesis.