Dioxide Reduction in the Dark

The subcellular unit responsible for photosynthesis in green plants is a membrane-enclosed entity which is called the chloro-plast. The chloroplast contains a complex structure. Prominent features of this structure are the lipid and protein layers which are called lamellae. These lamellae are large thin sheets which may extend the length and width of the chloroplast. They were revealed originally by techniques of electron microscopy. 

Grana appear not to be essential for photosynthesis, since they are not found in unicellular algae such as Chlorella. Probably the grana are useful to some functional specialization in higher plants. For example, leaves of higher plants may convert a large fraction of carbon taken up to sucrose for transport to nonphotosynthetic parts of the plant. 

Trebst et al. (1958), as well as Park and Pon (1961), fragmented chloroplasts from spinach and obtained mixtures of particulate green matter and soluble enzymes, which together were capable of slowly reducing carbon dioxide to sugar in the light. When this 

The soluble proteins left in the supernatant after the centrifugation appeared to include most of the enzymes required for CO2 fixation during photosynthesis. By themselves, they were capable of very little fixation and reduction of carbon dioxide. However, when the particulate green material obtained by sedimentation was recombined with the soluble proteins and illuminated in the presence of radioactive carbon dioxide, a significant amount of fixation of carbon dioxide and form.ation of reduced carbon compounds was observed. Thus, the separation of light and dark phases of photosynthesis (predicted by Van Niel and confirmed by the experiments of Hill and Ruben) was demonstrated in terms of the physical separation of the light and dark biochemical machinery. 

Under certain conditions, the green particulate material is capable of rates of energy conversion and oxidation-reduction reactions comparable to the rates exhibited by whole cells. However, in the combined system, the rates of carbon dioxide fixation and reduction are, at best, only 1 to 5% of the rates of the intact organism on a unit chlorophyll basis. It is important that this difference be kept in mind when one considers the probable organization and relationships of the component machinery of photosynthesis as it is performed in vivo (Bassham, 1963). 

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