Intracellular Organization and Protein Sorting PDF

the nucleolus is not enclosed by a membrane and represents one example of a biomolecular condensate. In liver and pancreatic cells, for example, the endoplasmic reticulum has a total membrane surface area that is, respectively, 25 times and 12 times that of the plasma membrane. The membrane-encl...

the nucleolus is not enclosed by a membrane and represents one example of a biomolecular condensate. In liver and pancreatic cells, for example, the endoplasmic reticulum has a total membrane surface area that is, respectively, 25 times and 12 times that of the plasma membrane. The membrane-enclosed organelles are packed tightly in the cytoplasm, and, in terms of area and mass, the plasma membrane is only a minor membrane in most eukaryotic cells There is evidence that the first eukaryotic cells arose when an ancient anaerobic archaeon joined forces with an aerobic bacterium roughly 1.6 billion years ago. An early step in this process was cell migrations , probably through protrusions and blebs. The highly curved membrane at the necks of these protrusions might have been stabilized by proteins that eventually became part of the nuclear pore. The added surface area of these protrusions facilitated metabolite exchange with the environment and with neighboring cells. A fruitful symbiotic relationship with an aerobic bacterium might have allowed the archaeon to increase in volume. These protrusions eventually fused with each other to pinch off internal membrane-enclosed compartments, some of which contained the symbiotic bacteria. This intermediate now begins to resemble modern-day eukaryotes, with a primordial nucleus and nuclear pores, internal compartments, and an endosymbiont destined to become the mitochondrion. The lumen of the internal compartments is topologically equivalent to the extracellular space. The membrane- enclosed endosymbiont subsequently escaped the enclosing membrane into the cytosol where it evolved into modern-day mitochondria. The internal compartments expanded and became progressively specialized to form the major intracellular compartments of a eukaryotic cell. Their common origin from a primordial intracellular compartment explains why all of these compartments can exchange material with each other through vesicular transport. The nucleus was formerly the cytosol in the ancient archaeon, explaining why the cytosol and nucleus are topologically equivalent compartments that can intermix during mitosis.  plasma m e m b r a n e rough R E lysosome cargo molecule nucleu s compartment 1 transport vesicle compartment 2 inner nuclear membrane outer nuclear membrane apparatus secretory vesicle nuclear envelope endosome (A) (B) Topologically equivalent compartments in the secretory and endocytic pathways in a eukaryotic cell. Topologically equivalent spaces are shown in red. (A) Molecules can be carried from one compartment to another topologically equivalent compartment by transport vesicles that bud from one and fuse with the other. (B) In principle, cycles of membrane budding and fusion permit the lumen of any of the organelles shown o t communicate with any other and with the cell exterior by means of transport vesicles. Blue arrows indicate the extensive outbound and inbound vesicular traffic Some organelles, most notably mitochondria and (in plant cells) plastids, do not take part in this communication and are isolated from the vesicular traffic between organelles shown here. NUCLEOLUS