Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Photosynthesis
Glucose serves as a precursor for typical plant carbohydrates, such as sucrose, starch, and cellulose.

Glucose-6-phosphate, produced during Photosynthesis, serves as the precursor to three typical plant CARBOHYDRATES—sucrose, starch, and Cellulose—which are not synthesized in animal organisms. Sucrose is formed by transferring D-glucose residues from UDP-glucose (Section 15.9) to D-fructose-6-phosphate. The product of this reaction, sucrose-6-phosphate, is subsequently hydrolyzed by a phosphatase to yield sucrose, as shown in Fig. 23-24. Sucrose is the primary form in which carbohydrates are transported throughout the plant (Fig. 23-25). It is produced in the leaves during photosynthesis and enters in large amounts into the sieve tubes, the unique "capillaries" of the leaves. This role likely evolved for sucrose because the unusual bond in its molecule between the anomeric carbon atom of D-glucose at position 1 and the anomeric carbon atom of D-fructose at position 2 is not hydrolyzed by amylases and other common carbohydrate-cleaving Enzymes.

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Fig. 23-24. Synthesis of typical plant carbohydrates from glucose-6-phosphate produced during photosynthesis.

Fig. 23-25. The Role of sucrose in The transport of phloem sap to the roots. Sieve tubes contain sucrose, which draws Water into them via osmotic forces. This generates the driving force that propels the phloem sap downward to the roots.

Cellulose, the major extracellular structural polymer in most plants (Section 11.9), is likewise synthesized in plants from D-glucose. Depending on the plant species, the direct precursor for the glucose monomer units of cellulose—linked within the polymer chain by ß (1→4) linkages—is ADP-glucose, CDP-glucose, or GDP-glucose. These nucleoside diphosphate glucoses are structurally and functionally similar to UDP-glucose (Section 20.13), the precursor of Glycogen in animal Tissues. Thus, we encounter yet another example illustrating the role of various NUCLEOTIDES in directing metabolic intermediates along specific biosynthetic pathways (Section 14.18).

Starch, whose monomer units in the main chains are linked by a (1→4) bonds (Section 11.8), is synthesized in most plants via a similar pathway from ADP-glucose (Fig. 23-24).