The abundance and shape of membrane-enclosed organelles are regulated to meet the needs of the cell. This is particularly apparent in cells that are highly specialized and therefore disproportionately rely on specific organelles. Plasma cells, for example, which secrete their own weight every day in antibody molecules into the bloodstream, contain vastly amplified amounts of rough ER, which is found in large, flat sheets. Cells that specialize in lipid synthesis also expand their ER, but in this case the organelle forms a network of convoluted tubules. Moreover, membrane-enclosed organelles are often found in characteristic positions in the cytoplasm. In most cells, for example, the Golgi apparatus is located close to the nucleus, whereas the network of ER tubules extends from the nucleus throughout the entire cytosol. These characteristic distributions depend on interactions of the organelles with the cytoskeleton. The localization of both the ER and the Golgi apparatus, for instance, depends on an intact microtubule array; if the microtubules are experimentally depolymerized with a drug, the Golgi apparatus fragments and disperses throughout the cell, and the ER network collapses toward the cell center (discussed in Chapter 16). The size, shape, composition, and location are all important and regulated features of these organelles that ultimately contribute to the organelle’s function.
evolutionary origins may help explain the Topological relationships of organelles
To understand the relationships between the compartments of the cell, it is helpful to consider how they might have evolved. The precursors of the first eukaryotic cells are thought to have been relatively simple cells that—like most bacterial and archaeal cells—have a plasma membrane but no internal membranes. The plasma membrane in such cells provides all membrane-dependent functions, including the pumping of ions, ATP synthesis, protein secretion, and lipid synthesis. Typical present-day eukaryotic cells are 10–30 times larger in linear dimension and 1000–10,000 times greater in volume than a typical bacterium such as E. coli. The profusion of internal membranes can be regarded, in part, as an adaptation to this increase in size: the eukaryotic cell has a much smaller ratio of surface area to volume, and its plasma membrane therefore presumably has too small an area to sustain the many vital functions that membranes perform. The extensive internal membrane systems of a eukaryotic cell alleviate this problem.
The evolution of internal membranes evidently went hand-in-hand with the specialization of membrane function. A hypothetical scheme for how the first eukaryotic cells, with a nucleus and ER, might have evolved by the invagination and pinching off of the plasma membrane of an ancestral cell is illustrated in Figure 12–3.This process would create membrane-enclosed organelles with an interior or lumen that is topologically equivalent to the exterior of the cell. We shall see that this topological relationship holds for all of the organelles involved in the secretory and endocytic pathways, including the ER, Golgi apparatus, endosomes, lysosomes, and peroxisomes. We can therefore think of all of these organelles as members of the same topologically equivalent compartment. As we discuss in detail in the next chapter, their interiors communicate extensively with one another and with the outside of the cell via transport vesicles, which bud off from one organelle and fuse with another (Figure 12–4).