Photosynthetic Unit (Quantasome)

A quantasome is the physically observable structure that corresponds to the photosynthetic unit - the minimum functional pool of chlorophyll and accessory pigment molecules that must act cooperatively to release one molecule of oxygen or reduce one molecule of CO2. Quantasomes sit on the inner surface of the thylakoid lamellar membrane, and each one measures roughly 180 x 160 Angstrom and about 100 Angstrom thick. They were first observed, though not yet understood, by Steinmann in 1952, who noticed granular structures studding chloroplast lamellae under the electron microscope; Park and Biggins confirmed in 1964 that these granules are the physiological units of photosynthesis and coined the term "quantasome" for them, estimating that a single quantasome contains roughly 230 chlorophyll molecules. Emerson and Arnold's flashing-light experiments (1932) arrived at a compatible estimate from a completely different, purely functional angle: they found that about 2500 chlorophyll molecules are needed to fix one CO2 molecule, and since fixing one CO2 requires 10 quanta of light, each functional unit works out to about 2500 divided by 10, or roughly 250 chlorophyll molecules - close to Park and Biggins's structural estimate. Taken together, these lines of evidence converge on 200 to 300 chlorophyll molecules as the accepted size of one physiological photosynthetic unit. Emerson additionally established that 8 quanta of light are needed …