Cell structure Golgi complex

Fig. 1 Images of the cell interior, (a) Illustration of a cross-section oiaa Escherichia coli cell generated based on available biochemical, structural, and microscopic data. A flagellar motor is shown at the upper right (From The Machinery of Life, 2nd ed, scheduled for release in late 2008 through Springer-Verlag). (b) Three-dimensional model of pancreatic p cells derived by electron tomography from thin sections of plastic-embedded freeze-substituted cells. The Golgi complex with seven cistema (C1-C7) is at the center (From [2])...

The eukaryotic cell cytoplasm consists of the cytosol, which is the matrix that presents no visible structure even when observed by electron microscopy. Organelles and inclusion bodies are immersed in the cytosol. The organelles are responsible for intense metabolic activity and include the mitochondria, Golgi complex, lysosome, and endoplasmic reticulum Figure 2.1). Inclusion bodies are less frequent and consist of lipid, carbohydrate, and pigment-storing structures. 

Many secretory and membrane proteins are modified in the lumen of the ER. Protein glycosylation, the addition of covalently bound oligosaccharide chains, is a common reaction in the ER lumen and the Golgi complex. Most proteins in the ER lumen destined for secretion from the cell or for transport to other intracellular sites are glycoproteins. The carbohydrate content of glycoproteins can vary from 0.5 to 80% or more of the glycoprotein mass. Glycoprotein structures were described in Chapter 9, and their biosynthesis are covered in Chapter 20. 

Fig. 15.14. Fate of proteins synthesized on the RER. Proteins synthesized on ribosomes attached to the ER travel in vesicles to the cis face of the Golgi complex. After the membranes fuse, the proteins enter the Golgi complex. Structural features of the proteins determine their fate. Some remain in the Golgi complex, and some return to the RER. Others bud from the trans face of the Golgi complex in vesicles. These vesicles can become lyso-somes or secretory vesicles, depending on their contents. Secretory proteins are released from the cell when secretory vesicles fuse with the cell membrane (exocytosis). Proteins with hydrophobic regions embedded in the membrane of secretory vesicles become cell membrane proteins. See Chapter 10 for descriptions of the endoplasmic reticulum, Golgi complex, lysosomes, and the cell membrane, and also for an explanation of the process of exocytosis.

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