Plant Physiology - Musienko M. M. 2001

Photosynthesis: Physiological, Biochemical, and Ecological Aspects
End products of the dark phase of photosynthesis

In photosynthetic Cells, fixed CO₂ is converted into two primary CARBOHYDRATES: sucrose and starch. Sucrose is subsequently transported to regions of rapid growth as well as to developing seeds and tubers. Starch, on the other hand, serves essentially as a temporary reserve of fixed carbon. It accumulates within METABOLISM/14.html">Chloroplasts in the form of granules before being converted back into sucrose and exported from The Cell.

Starch synthesis utilizes fructose-6-phosphate, which is withdrawn from The Calvin Cycle and sequentially converted into glucose-6-phosphate, glucose-1-phosphate, and ultimately ADP-glucose:

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ADP-glucose is then converted into amylose. This reaction is initiated using short-chain maltose or dextrin as a primer, which was formed during the preceding dark period through the action of amylase on starch granules synthesized during the previous light phase. A significant portion of this amylase is subsequently converted into amylopectin.

Sucrose is synthesized in the Cytoplasm. Fixed carbon is exported from the chloroplast predominantly as dihydroxyacetone phosphate (DHAP), which is then utilized for sucrose synthesis. Located in the inner chloroplast membrane is the so-called "phosphate translocator"; through its operation, the export of one phosphate molecule in the form of DHAP from the chloroplast to the cytoplasm is coupled with the import of one orthophosphate ion from the cytoplasm into the chloroplast stroma. Subsequently, under The Influence of cytoplasmic glycolytic Enzymes, DHAP is converted into glucose-1-phosphate. The latter gives rise to UDP-glucose in a reaction catalyzed by the enzyme glucose-1-phosphate uridylyltransferase. Sucrose is then synthesized through the action of the enzymes sucrose-phosphate synthase and sucrose-phosphatase.

It should be noted that DHAP can also serve as a precursor for the synthesis of Lipids and Amino Acids.

The breakdown of chloroplast starch granules occurs when Photosynthesis slows down or ceases entirely. During this period, starch is converted into glucose-1-phosphate by phosphorylase, and into glucose via Reactions Catalyzed by amylases. Subsequently, glucose and glucose-1-phosphate are converted by chloroplast enzymes into DHAP, which is either exported to the cytoplasm for sucrose synthesis or utilized for The Biosynthesis of lipids and Proteins within the cell itself.

Carbohydrate Metabolism in C₄ plants differs from that of C₃ plants in only a few specific aspects. As is well known, in C₄ plants, substantial quantities of starch granules are formed within mesophyll chloroplasts, even though CO₂ fixation yielding carbohydrates occurs in bundle-sheath chloroplasts. It is believed that the carbon source for starch synthesis in mesophyll chloroplasts is 3-PGA, which is produced in bundle-sheath chloroplasts and subsequently transported to the mesophyll chloroplasts. There, 3-PGA is converted into triose phosphates and further into starch. It is hypothesized that starch degradation yielding triose phosphates takes place in both mesophyll and vascular bundle-sheath chloroplasts.

These processes are less thoroughly understood in CAM plants. It appears that starch is synthesized in their chloroplasts, whereas sucrose is formed in the cytoplasm. These reactions take place during daylight hours.

In the dark, starch degradation yielding phosphoenolpyruvate—which is required for the dark carboxylation reaction—is initiated by phosphorylase, amylase, and glucokinase. Subsequently, through The Glycolytic Pathway, the resulting glucose phosphates are converted into phosphoenolpyruvate.

Among the primary products of photosynthesis are such Amino acids as Alanine, Serine, glutamic acid, and Glycine. It is likely that phosphoglyceric acid, formed during the initial stage of the Calvin cycle, can be converted into pyruvic acid. This process proceeds particularly intensely under conditions of NADP×H₂ deficiency, which impedes The conversion of phosphoglyceric acid into phosphoglyceraldehyde (the standard pathway in the Calvin cycle). In the presence of NH₃, pyruvic acid yields The amino acid alanine. Pyruvic acid can also serve as a precursor for a series of organic acids via the Krebs cycle.

Alongside carbohydrates and amino acids, fats and other products can also be synthesized from photosynthetic intermediates.

Thus, substances synthesized within the chloroplast must first enter the cytoplasm, then travel via the parenchyma or intercellular spaces to the sieve tubes of the phloem, and subsequently to various plant Organs. A.L. Kursanov distinguishes intracellular, intercellular, parenchymal, and phloem Transport of substances.