Emerson's Experiments and Hill's Reaction

Three classic pieces of experimental evidence, all covered in this section, together forced photosynthesis researchers to abandon a single-photosystem model of the light reaction and adopt the two-photosystem model still used today. Emerson's red-drop observation (13.7.1) revealed an unexplained loss of efficiency at the far-red end of the spectrum. Emerson's enhancement effect (13.7.2) then showed that supplementing far-red light with shorter-wavelength red light more than restored - it actually boosted - photosynthetic output beyond what either wavelength achieved alone, direct evidence that two distinct light-absorbing systems, tuned to different wavelength ranges, must be cooperating. Separately, Robert Hill's isolated-chloroplast experiment (13.7.3) demonstrated that the light reaction's oxygen-evolving, electron-donating …