Essential Cell Biology 5th edition

Membrane-Enclosed Organelles
499
DNA
membranebound
ribosomes
plasma
membrane
anaerobic
archaeon
inner
nuclear
membrane
outer nuclear
membrane
nuclear
pore
nucleus
anaerobic
eukaryotic cell
endoplasmic
reticulum
cytosol
Membrane-enclosed Organelles Evolved in Different Ways
In trying to understand the relationships between the different compartments
of a modern eukaryotic cell, it is helpful to consider how they
evolved. The precursors of the first eukaryotic cells are thought to have
been simple microorganisms, resembling present-day archaea, which had
a plasma membrane but no internal membranes. The plasma membrane
in such cells would have provided all membrane-dependent functions,
including ATP synthesis and lipid synthesis, as does the plasma membrane
in most modern prokaryotes. Archaea and bacteria can get by with
this arrangement because of their small size, which gives them a high
surface-to-volume ratio: ECB5 their e15.03/15.03
plasma membrane area is thus sufficient to
sustain all the vital functions for which membranes are required. Presentday
eukaryotic cells, by contrast, have volumes that are 1000 to 10,000
times greater. Such a large cell has a small surface-to-volume ratio and
presumably could not survive with a plasma membrane as its only membrane.
Thus, the increase in size typical of eukaryotic cells probably could
not have occurred without the development of internal membranes.
Membrane-enclosed organelles are thought to have arisen in evolution
in stages. The nuclear membranes and the membranes of the ER,
Golgi apparatus, endosomes, and lysosomes most likely originated by
invagination of the plasma membrane, as illustrated for the nuclear and
ER membranes in Figure 15−3. The ER, Golgi apparatus, peroxisomes,
endosomes, and lysosomes are all part of what is collectively called the
endomembrane system. As we discuss later, the interiors of these organelles
communicate extensively with one another and with the outside
of the cell by means of small vesicles that bud off from one of these
organelles and fuse with another. Consistent with this proposed evolutionary
origin, the interiors of these organelles are treated by the cell in
many ways as “extracellular,” as we will see. The hypothetical scheme
shown in Figure 15–3 also explains why the nucleus is surrounded by two
membranes.
Mitochondria and chloroplasts are thought to have originated in a different
way. They differ from all other organelles in that they possess their
own small genomes and can make some of their own proteins, as discussed
in Chapter 14. The similarity of their genomes to those of bacteria
and the close resemblance of some of their proteins to bacterial proteins
strongly suggest that both these organelles evolved from bacteria that
were engulfed by primitive eukaryotic cells with which they initially lived
in symbiosis (Figure 15–4). As might be expected from their origins, mitochondria
and chloroplasts remain isolated from the extensive vesicular
Figure 15–3 Nuclear membranes and
the ER may have evolved through
invagination of the plasma membrane.
In modern bacteria and archaea, a single
DNA molecule is typically attached to the
plasma membrane. It is possible that, in
a very ancient anaerobic archaeon, the
plasma membrane, with its attached DNA,
could have invaginated and, in subsequent
generations, formed a two-layered envelope
of membrane completely surrounding the
DNA. This envelope is presumed to have
eventually pinched off completely from the
plasma membrane, ultimately producing
a nuclear compartment penetrated by
channels called nuclear pores, which enable
communication with the cytosol. Other
portions of the invaginated membrane may
have formed the ER, which would explain
why the space between the inner and outer
nuclear membranes is continuous with the
ER lumen.
QUESTION 15–1
As shown in the drawings in
Figure 15–3, the lipid bilayer
of the inner and outer nuclear
membranes forms a continuous
sheet, joined around the nuclear
pores. As membranes are twodimensional
fluids, this would
imply that membrane proteins can
diffuse freely between the two
nuclear membranes. Yet each of
these two nuclear membranes has
a different protein composition,
reflecting different functions. How
could you reconcile this apparent
contradiction?