Figure 234

Summary of events in protein trafficking from the frans-Golgi network (TGN). The tubulovesicular array of the TGN serves as the sorting station for transporting vesicles that deliver proteins to four major locations in the cell. The constitutive secretory pathway to the basolateral plasma membrane (red arrows) uses clathrin-coated vesicles. The endosomal pathway (green arrows) is an organelle-specific protein delivery system. Another constitutive secretory pathway to the apical plasma membrane (purple arrows) also uses clathrin-coated vesicles. The regulated secretory pathway (orange arrows) directs protein to the apical region of the cell, where proteins are stored in large secretory vesicles. These vesicles eventually fuse with the plasma membrane and release the secretory product by exocytosis.

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 in foldings in the cells of the proximal convoluted tubule of the kidney.

plasma membrane early endosome late endosome lysosome constitutive transport vesicle secretory storage vesicle

Mitochondria

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

Mitochondria evolved from aerobic bacteria that were engulfed by eukaryotic cells

Mitochondria are believed to have evolved from an aerobic prokaryote (bacterium) that lived symbiotically within primitive eukaryotic cells. This hypothesis received support with the demonstration that mitochondria possess their own genome, increase their numbers by division, and synthesize some of their structural (constituent) proteins. Mitochondrial DNA is a closed circular molecule that encodes 13 enzymes involved in the oxidative phosphorylation pathway, 2 rRNAs, and 22 transfer RNAs (tRNAs)

used in the translation of the mitochondrial mRNA. Mitochondria possess a complete system for protein synthesis including the synthesis of their own ribosomes. The remainder of the mitochondrial proteins are encoded by nuclear DNA; new polypeptides are synthesized by free ribosomes in the cytoplasm and then imported into mitochondria with the help of protein chaperones.

Mitochondria are present in all cells except red blood cells and terminal keratinocytes

The number, shape, and internal structure of mitochondria are often characteristic for specific cell types. When present in large numbers, mitochondria contribute to the acidophilia of the cytoplasm because of the large amount of membrane they contain. Mitochondria may be stained specifically by histochemical procedures that demonstrate some of their constituent enzymes, such as those involved in ATP synthesis and electron transport.

Mitochondria possess two membranes that delineate distinct compartments

Mitochondria display a variety of shapes, including spheres, rods, elongated filaments, and even coiled structures. All mitochondria, unlike other organelles described above, possess two membranes (Fig. 2.35). The inner mitochondrial membrane surrounds a space called the ma-

inner mitochondrial membrane

• cytochromes

• dehydrogenases

• flavoproteins intermembrane space outer mitochondrial membrane

membrane of crista

(10nm)

ÎT

iUUii

elementary particles • ATP synthase crista matrix matrix granules elementary particles • ATP synthase crista

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