Effects Of Glycerol And Sucrose On Structure And Function Of Photosystem I

It has been shown that a large number of water molecules coordinate with the pigments and subunits of photosystem I (PSI); however, the function of these water molecules remains to be clarified. In this study, the photosynthetic properties of PSI from spinach were investigated using different spectroscopic and activity measurements under conditions of decreasing water content caused by increasing concentrations of co-solutes glycerol or sucrose. The results show that:1. Addition of glycerol or sucrose caused pronounced changes in the photochemical activity of PSI particles. At low concentrations (<60%, v/v), glycerol stimulated the rate of oxygen uptake in PSI particles, while higher concentrations of glycerol cause inhibition of PSI activity. However, sucrose stimulated the oxygen uptake rate at all concentrations in the experiment.2. The effect of glycerol or sucrose also brought about the alteration of the capacity of P700 photooxidation, which increased with glycerol concentrations lower than 60%. In contrast, this capacity decreased at higher glycerol concentrations. Similar phenomenon was observed during sucrose treatments.3. On the other hand, glycerol or sucrose considerably affected the distribution of the exicted energy between the bulk and red antenna chlorophyll (Chl) forms or states. More energy was trapped in the red forms following an increase in the concentrations of glycerol or sucrose.4. In addition, the glycerol or sucrose effect led to the alteration of the...