Essential Cell Biology 5th edition

A:38 Answers
this mixture, so that electrons can flow in the energetically
favored direction from reduced ubiquinone to the
cytochrome c reductase complex to cytochrome c. Although
energetically favorable, the transfer in (A) cannot occur
spontaneously in the absence of the cytochrome c reductase
complex to catalyze this reaction. No electron flow occurs in
the other experiments, whether the cytochrome c reductase
complex is present or not: in experiments (B) and (F), both
ubiquinone and cytochrome c are oxidized; in experiments
(C) and (G), both are reduced; and in experiments (D) and
(H), electron flow is energetically disfavored because an
electron in reduced cytochrome c has a lower free energy
than an electron added to oxidized ubiquinone.
Chapter 15
ANSWER 15–1 Although the nuclear envelope forms
one continuous membrane, it has specialized regions
that contain special proteins and have a characteristic
appearance. One such specialized region is the inner nuclear
membrane. Membrane proteins can indeed diffuse between
the inner and outer nuclear membranes, at the connections
formed around the nuclear pores. Those proteins with
particular functions in the inner membrane, however, are
usually anchored there by their interaction with other
components such as chromosomes and the nuclear lamina (a
protein meshwork underlying the inner nuclear membrane
that helps give structural integrity to the nuclear envelope).
ANSWER 15–2 Eukaryotic gene expression is more
complicated than prokaryotic gene expression. In particular,
prokaryotic cells do not have introns that interrupt the
coding sequences of their genes, so that an mRNA can
be translated immediately after it is transcribed, without
a need for further processing (discussed in Chapter 7). In
fact, in prokaryotic cells, ribosomes start translating most
mRNAs before transcription is finished. This would have
disastrous consequences in eukaryotic cells, because most
RNA transcripts have to be spliced before they can be
translated. The nuclear envelope separates the transcription
and translation processes in space and time: a primary RNA
transcript is held in the nucleus until it is properly processed
to form an mRNA, and only then is it allowed to leave the
nucleus so that ribosomes can translate it.
ANSWER 15–3 An mRNA molecule is attached to the ER
membrane by the ribosomes translating it. This ribosome
population, however, is not static; the mRNA is continuously
moved through the ribosome. Those ribosomes that
have finished translation dissociate from the 3ʹ end of the
mRNA and from the ER membrane, but the mRNA itself
remains bound by other ribosomes, newly recruited from
the cytosolic pool, that have attached to the 5ʹ end of the
mRNA and are still translating the mRNA. Depending on its
length, there are about 10–20 ribosomes attached to each
membrane-bound mRNA molecule.
ANSWER 15–4
A. The internal signal sequence functions as a membrane
anchor, as shown in Figure 15–17. Because there is
no stop-transfer sequence, however, the C-terminal
end of the protein continues to be translocated into
the ER lumen. The resulting protein therefore has its
N-terminal domain in the cytosol, followed by a single
(A)
(B)
(C)
N
N
Figure A15–4
C
C
N
n n–1
N
transmembrane segment, and a C-terminal domain in
the ER lumen
ECB5
(Figure
eA15.04-A15.04
A15–4A).
B. The N-terminal signal sequence initiates translocation of
the N-terminal domain of the protein until translocation
is stopped by the stop-transfer sequence. A cytosolic
domain is synthesized until the start-transfer sequence
initiates translocation again. The situation now resembles
that described in (A), and the C-terminal domain of
the protein is translocated into the lumen of the ER.
The resulting protein therefore spans the membrane
twice. Both its N-terminal and C-terminal domains are
in the ER lumen, and a loop domain between the two
transmembrane regions is exposed in the cytosol
(Figure A15–4B).
C. It would need a cleaved signal sequence, followed by
an internal stop-transfer sequence, followed by pairs of
start- and stop-transfer sequences (Figure A15–4C).
These examples demonstrate that complex protein
topologies can be achieved by simple variations and
combinations of the two basic mechanisms shown in Figures
15–16 and 15–17.
ANSWER 15–5
A. Clathrin coats cannot assemble in the absence of
adaptins that link the clathrin to the membrane. At high
clathrin concentrations and under the appropriate ionic
conditions, clathrin cages assemble in solution, but
they are empty shells, lacking other proteins, and they
contain no membrane. This shows that the information
to form clathrin baskets is contained in the clathrin
molecules themselves, which are therefore able to selfassemble.
B. Without clathrin, adaptins still bind to receptors in the
membrane, but no clathrin coat can form and thus no
clathrin-coated pits or vesicles are produced.
C. Deeply invaginated clathrin-coated pits form on the
membrane, but they do not pinch off to form closed
vesicles (see Figure A15–21B).
N
N
signal
peptidase
cleavage
C
C
C
n
N
N