Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

644 Chapter 12: Intracellular Compartments and Protein Sorting
rough endoplasmic
reticulum
nucleus
lysosomes
Figure 12–2 An electron micrograph
of part of a liver cell seen in cross
section. Examples of most of the major
intracellular organelles are indicated.
(Courtesy of Daniel S. Friend.)
mitochondrion
peroxisome
5 µm
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 MBoC6 create m12.02/12.02
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).
As described in Chapter 14, mitochondria and plastids differ from the other
membrane-enclosed organelles because they contain their own genomes. The
nature of these genomes, and the close resemblance of the proteins in these
organelles to those in some present-day bacteria, strongly suggest that mitochondria
and plastids evolved from bacteria that were engulfed by other cells
with which they initially lived in symbiosis (see Figures 1–29 and 1–31): the inner
membrane of mitochondria and plastids presumably corresponds to the original
plasma membrane of the bacterium, while the lumen of these organelles evolved
from the bacterial cytosol. Like the bacteria from which they were derived,
both mitochondria and plastids are enclosed by a double membrane and they
remain isolated from the extensive vesicular traffic that connects the interiors of
most of the other membrane-enclosed organelles to each other and to the outside
of the cell.