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The following points highlight the three main products of photosynthetic cell. The products are: 1. Starch 2. Sucrose 3. Amino Acids.
Product # 1. Starch:
The synthesis of starch and its regulation through ADP-glucose pyrophosphorylase has been vividly worked out and described in one of the earlier chapters. Recent studies have brought out that ratio of the activator glycerate-3-phosphate and inhibitor Pi is the critical factor in the regulation of ADP-glucose pyrophosphorylase.
The enzyme from blue green algae and higher plants has been shown to be similar. Starch is tightly bound in the chloroplast till enzyme solubilizes it and hydrolyzes it to soluble sugar and in this state it leaves the chloroplast.
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Much variations exist on the nature of this process in general thinking. In any case in the initial degradation both phosphorylase and amylase are involved since maltose, glucose and sugar phosphates have been isolated as products of starch synthesis in chloroplast.
Phosphorylase activity has also been observed in leaves and chloroplasts. Before their exclusion from starch the three mentioned substances are metabolized to triose-p and/or glycerate-3-p in the chloroplast. In fact glycolytic pathway could accommodate this metabolism.
The fate of glucose which was possibly formed from starch is still obscure and could be directed into glycolysis. Incidentally chloroplasts are shown to have hexokinase. Pi seems to exert considerable influence on the starch metabolism.
At low concentrations this anion in light promotes starch synthesis. This is possibly due to the fact that chloroplasts are unable to export triose-p in the absence of Pi. On the contrary starch degradation in the dark needs sufficient amount of Pi for the phosphorylase.
Product # 2. Sucrose:
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The biochemical events associated with the sucrose synthesis in the photosynthetic cells have received much attention in recent years. Recent reports show that photosynthesizing chloroplast was dependent upon cytoplasmic sucrose formation.
Thus Pi released during this process must be brought back to chloroplast so that exclusion of triose-p to the cytoplasm can continue. Under in vivo conditions, sucrose-p synthetase and FBPase were likely to have regulatory role. Recently some workers have purified FBPase from spinach leaves and it differed from the chloroplast enzyme as regards allosteric inhibition by AMP.
Product # 3. Amino Acids:
Leaf discs of some plants including alfalfa when given NH4+ during photosynthesis produced amino acids at the expense of sucrose formation. In maize also starch and sucrose synthesis decreased in the presence of NO3– or NH4+.
Apparently photosynthetic tissues given nitrate or ammonia incorporated these into amino acids. Ammonium ions may also uncouple photosynthetic phosphorylation whereas nitrate reduces the chloroplast stromal pH.
