Glucose sink

Hayes (1998) proposed that the disposal of glucose by the twin processes of diffusion and fermentation can be viewed as a glucose sink and that whilst the sink is operating there is little glucose available on... 

Figure 6.10 Factors affecting the fermentative capacity of the bed and its relationship to the glucose sink. Reproduced from Hayes (1998) with permission.

Figure 21.24 An overview of amino acid metabolism, particularly amino acid metabolism, in a patient suffering from cancer. The tumour acts as a sink for glucose, amino acids and glutamine. As tumour grows in size, the sink is exaggerated and cachexia develops. This diagram can be considered with that in Figure 21.22 in order to include fatty acids in tumour metabolism. Note the thicker line to indicate magnitude of release of glutamine by muscle.

Most instructive is what happens to the same processes in 02-limited turtle liver cells. Under conditions of total anoxia, the ATP demand of protein turnover drops to less than 10% of normoxic rates urea biosynthesis drops to essentially zero, as does the biosynthesis of glucose (not unexpectedly, because a major role of the liver under anoxic conditions is to supply glucose for the rest of the body). Although the ATP requirements of the Na+-K+-ATPase are also drastically reduced, the suppression in percentage terms is less than for overall ATP turnover. As a result, under anoxic conditions, the Na+ pump becomes the cell s dominant energy sink, accounting for up to 75% of the ATP demand of the cell (Buck and Hochachka, 1993). In turtle brain these adjustments occur along with a measurable decrease in Na+-K+-... 

Incubation of spinach leaf starch granules with extrachloroplastic phosphorylase resulted in the formation of glucose 1-phosphate (Steup et al., 1983). Hammond and Preiss (1983) reported a large increase in cytosolic phosphorylase from spinach leaf in a time course that approximates the time of leaf senescence (i.e., when starch chloroplast must be hydrolyzed and exported to active sinks). 

Under aerobic conditions, pyruvate can be oxidatively decarboxylated via the pyruvate dehydrogenase multienzyme complex to yield acetyl-CoA, which can then be completely oxidised via the citric acid cycle (Fig. 2). In eubacteria growing anaerobically, pyruvate is metabolised fermentatively, thus serving as an electron sink for reducing equivalents generated in its formation from glucose. The diverse array of possible fermentative reactions from pyruvate is reviewed in [5]. 

Tuber specific expression has been achieved by using the patatin B33 promotor. The transformation has been carried out using Agrobacterium tumefaciens. Due to the activity of the inserted invertase, sucrose is cleaved into glucose and fructose. As sucrose is the major transport form for photosynthesis products in the plant, this causes changes in the source (leaves) / sink (tubers) interactions, which eventually lead to increased yield and/or increased tuber size... 

---