FIGURE 2.36 • Summary of events in protein traffcking from the TGN. The tubulovesicular array of the TGN serves as the sorting station for transporting vesicles that deliver proteins to the following destinations: (1) apical plasma membrane (i.e., epithelial cells); (2) apical region of the cell cytoplasm where proteins are stored in secretory vesicles (i.e., secretory cells); (3) early or late endosomal compartment; (4) selected proteins containing lysosomal signals, which are targeted to lysosomes; (5) lateral plasma membrane (i.e., epithelial cells); (6) basal plasma membrane (i.e., epithelial cells); (7) proteins destined for apical, basal, and lateral surfaces of plasma membrane, which are delivered to the basal plasma membrane (i.e., in hepatocytes); (8) all proteins endocytosed and sorted in early endosomes; (9) apical plasma membrane from early endosomes; (10) lateral plasma membrane; and (11) basal plasma membrane. Note two targeting mechanisms of proteins to different surfaces of plasma membrane. In epithelial cells, proteins are directly targeted from the TGN into the appropriate cell surface as shown in steps (1), (5), and (6). In hepatocytes, all proteins are secreted frst to the basal cell surface, and then they are distributed to the appropriate cell surface via the endosomal compartment as shown in steps (7) to (11).
(see Fig. 2.35). Mature secretory vesicles eventually fuse with the plasma membrane to release the secretory product by exocytosis. This type of secretion is characteristic of highly specialized secretory cells found in exocrine glands.
Sorting and packaging of proteins into transport vesicles occurs in the trans-Golgi network.
Proteins that arrive in the TGN are distributed to different intercellular locations within transport vesicles. The intercellular destination of each protein depends on the sorting signals that are incorporated within the polypeptide chain of the protein.
The actual sorting and packaging of proteins in TGN is primarily based on sorting signals and physical properties.  Sorting signals are represented by the linear array of amino acid or associated carbohydrate molecules. This type of signal is recognized by the sorting machinery, which directs the protein into the appropriately coated transport vesicle.  Physical properties are important for packaging func tionally associated protein complexes. These groups of proteins are first partitioned into separate lipid rafts that targeted organelle.
Mitochondria are abundant in cells that generate and expend large amounts of energy.
Mitochondria were also known to early cytologists who observed them in cells vitally stained with Janus green B. It is now evident that mitochondria increase in number by division throughout interphase, and their divisions are not synchronized with the cell cycle. Videomicroscopy confirms that mitochondria can both change their location and undergo transient changes in shape. They may therefore be compared to mobile power generators as they migrate from one area of the cell to another to supply needed energy.
Because mitochondria generate ATP, they are more numerous in cells that use large amounts of energy such as striated muscle cells and cells engaged in ﬂuid and electrolyte transport. Mitochondria also localize at sites in the cell where energy is needed, as in the middle piece of the sperm, the intermyofibrillar spaces in striated muscle cells, and adjacent to the basolateral plasma-membrane infoldings in the cells of the proximal convoluted tubule of the kidney.
Mitochondria evolved from aerobic bacteria that were engulfed by eukaryotic cells.