Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

5. Plant Respiration
5.5 Apotomic Pathway

The apotomic pathway of hexose Catabolism (the Pentose Phosphate Pathway of glucose oxidation, hexose monophosphate shunt, pentose shunt) takes place in the Cytoplasm and, in the absence of light, in METABOLISM/14.html">Chloroplasts. Glucose is phosphorylated by hexokinase to glucose-6-phosphate, which is then oxidized by glucose-6-phosphate dehydrogenase. This reaction yields reduced NADPH and phosphogluconic acid lactone. The lactone is hydrolyzed spontaneously or with the participation of gluconolactase to 6-phosphogluconic acid. Under the action of phosphogluconate dehydrogenase with the coenzyme NADP, it undergoes decarboxylation to form reduced NADPH and the five-carbon sugar ribulose-5-phosphate. This is the Water/144.html">Origin of the term "apotomic pathway" (apotomy meaning truncation). The subsequent reactions constitute the regeneration cycle of the initial substrate, glucose-6-phosphate. A complete cycle requires three molecules of glucose-6-phosphate. As can be seen in Figure 5.4, six molecules of glucose-6-phosphate yield six molecules of CO₂ and six molecules of ribulose-5-phosphate, from which five molecules of glucose-6-phosphate are regenerated. In addition, this process produces twelve molecules of NADPH, which upon oxidation in the Respiratory Electron Transport chain can yield thirty-six ATP molecules, matching the energy yield of The Glycolytic Pathway. The products of the apotomic pathway are also involved in cellular metabolism.