Glycogen granules

Glycogen and its enzymes are compartmentalized. Glycogen granules are only present in astrocytes of adult animals but are found in both astrocytes and neurons of immature animals. Of the enzymes involved in glycogen metabolism, glycogen phosphorylase is found in astrocytes only. Under steady-state conditions, it is probable that less than 10% of phosphorylase in brain is in the unphosphorylated b form (requiring AMP). This form is probably not very active at the low AMP concentrations present when intracellular glucose is sufficient to maintain ATP synthesis. Brain phosphorylase b kinase is activated indirectly by cAMP and by the molar concentrations... 

Glycogen is the major storage polysaccharide in animals. This tiny portion of a tangled glycogen granule shows the highly branched polymer, made of glucose units. 

Fig. 2. Macroscopic and microscopic structure of muscle (a) Entire muscle and its cross-section with fatty septa, (b) Fascicle with several muscle fibres (cells). A layer of fat along the fascicle is indicated, (c) Striated myofibre corresponding with one single muscle cell containing several nuclei. The lengths of a myofibre can be several tens of centimetres, (d) Myofibril inside a myocyte. It is one contractile element and contains actin and myosin and further proteins important for the muscular function, (e) Electron myograph of human skeletal muscle showing the band structure caused by the contractile myofilaments in the sarcomeres. One nucleus (Nu) and small glycogen granules (arrow, size <0.1 pm) are indicated.

At the level of the superficial layer, itself made of two or three layers, the cells become more and more flattened and are 45-pm long and 4-prn thick. Their nucleus is stretched out along the long axis and is 25-prn long. Their cytoplasm contains three specific structures contractile proteins, glycogen granules, and a very developed Golgi s apparatus. 

FIGURE 7-14 Electron micrographs of starch and glycogen granules. 

GI, glycogen granules Gap, space -10-20 nm thick between adjacent cells... 

Figure 21.2. Electron Micrograph of a Liver Cell. The dense particles in the cytoplasm are glycogen granules. [Courtesy of Dr. George Palade.]...

In eucaryotic cells, glycolysis, gluconeogenesis and fatty acid synthesis takes place in the cytosol, while the Krebs cycle is isolated within mitochondria glycogen is made in glycogen granules, lipid is synthesized in the endoplasmic reticulum and lysosomes carry on a variety of hydrolytic activities. As in procaryotic cells, ribosomes in the cytosol are the site of protein synthesis. 

Fig. 2.17 Hyaloplasm of the hepatocyte glycogen granules (G), mitochondria (M), rough endoplasmic reticulmn with ribosomes (R) edge of the cell nucleus (CN) x 32,000 (s. fig. 2.9)...

Glycogen Granules Enzymes of glycogen synthesis and breakdown including branching and debranching. 

---