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4.2 outline the classic experiments of Emerson and Arnold and their interpretation by Gaffron and Wahl that led to the hypothesis of a photosynthetic unit consisting of chlorophyll and photoenzyme molecules
outline the classic experiments of Emerson and Arnold and their interpretation by Gaffron and Wahl that led to the hypothesis of a photosynthetic unit consisting of chlorophyll and photoenzyme molecules
- Emerson and Arnold illuminated algal cells with very brief flashes of light that could excite every chlorophyll molecule at least once. The importance of their work was that it allowed Emerson and Arnold to study the relationship between the amount of incident light energy, the amount of chlorophyll present and the amount of oxygen evolved in algal cells. By exposing algal cells to repetitive flashes of light, they found that only one O2(g) molecule was given off by the algae for every 2400 chlorophyll molecules. This result implied that not all chlorophyll molecules are photochemically active.
- Gaffron and Wahl interpreted this information to propose that light is absorbed by hundreds of chlorophyll molecules that transfer their energy to a single reaction centre. The hundreds of molecules described by Gaffron and Wahl form the antenna complex of a photosystem. The antenna complex also includes accessory pigments, such as the carotenoids, to collect light of other wavelengths. When a chlorophyll molecule in the antenna complex is excited, the energy is transferred from one molecule to another by resonance transfer energy (kinetic energy in vibrations) until it reaches a pair of chlorophyll-a molecules in the photochemical reaction centre. Each antenna complex effectively acts as a funnel, sending energy to the photochemical reaction centre.
- The two chlorophyll molecules at the heart of the photochemical reaction centre are held in a transmembrane protein-pigment complex next to a chain of electron acceptors. The pair of chlorophyll molecules acts as an irreversible trap by passing their excited electron to the chain of electron acceptors and moving it quickly to a more stable environment where it is used in further reactions.