Essential Cell Biology 5th edition

530 CHAPTER 15 Intracellular Compartments and Protein Transport
• Cells ingest fluid, molecules, and sometimes even particles by endocytosis,
in which regions of plasma membrane invaginate and pinch
off to form endocytic vesicles.
• Much of the material that is endocytosed is delivered to endosomes,
which mature into lysosomes, in which the material is degraded by
hydrolytic enzymes; most of the components of the endocytic vesicle
membrane, however, are recycled in transport vesicles back to the
plasma membrane for reuse.
KEY TERMS
autophagy
clathrin
coated vesicle
endocytosis
endomembrane system
endoplasmic reticulum (ER)
endosome
exocytosis
Golgi apparatus
lysosome
membrane-enclosed organelle
nuclear envelope
nuclear pore
peroxisome
phagocytic cell
phagocytosis
pinocytosis
Rab protein
receptor-mediated endocytosis
rough endoplasmic reticulum
secretion
secretory vesicle
signal sequence
SNARE
tethering protein
transport vesicle
unfolded protein response (UPR)
vesicular transport
QUESTIONS
QUESTION 15–9
Which of the following statements are correct? Explain your
answers.
A. Ribosomes are cytoplasmic structures that, during
protein synthesis, become linked by an mRNA molecule to
form polyribosomes.
B. The amino acid sequence Leu-His-Arg-Leu-Asp-Ala-Gln-
Ser-Lys-Leu-Ser-Ser is a signal sequence that directs proteins
to the ER.
C. All transport vesicles in the cell must have a v-SNARE
protein in their membrane.
D. Transport vesicles deliver proteins and lipids to the cell
surface.
E. If the delivery of prospective lysosomal proteins from the
trans Golgi network to the late endosomes were blocked,
lysosomal proteins would be secreted by the constitutive
secretion pathways shown in Figure 15−30.
F. Lysosomes digest only substances that have been taken
up by cells by endocytosis.
G. N-linked sugar chains are found on glycoproteins that
face the cell surface, as well as on glycoproteins that face
the lumen of the ER, trans Golgi network, and mitochondria.
H. Ribosomes bound to the outer nuclear membrane make
proteins that are translocated co-translationally into the
membrane.
QUESTION 15–10
Some proteins shuttle back and forth between the nucleus
and the cytosol. They need a nuclear export signal to get
out of the nucleus. How do you suppose they get into the
nucleus?
QUESTION 15–11
Influenza viruses enter the cell by receptor-mediated
endocytosis. The viruses are surrounded by a membrane
that contains a fusion protein, which is activated by the
acidic pH in the endosome. Upon activation, the protein
causes the viral membrane to fuse with cell membranes. An
old folk remedy against flu recommends that one should
spend a night in a horse’s stable. Odd as it may sound,
there is a rational explanation for this advice. Air in stables
contains ammonia (NH 3 ) generated by bacteria in the
horse’s urine. Sketch a diagram showing the pathway (in
detail) by which flu virus enters cells, and speculate how
NH 3 may protect cells from virus infection.
(Hint: NH 3 can neutralize acidic solutions by the reaction
NH 3 + H + → NH 4 + .)
QUESTION 15–12
Consider the v-SNAREs that direct transport vesicles
from the trans Golgi network to the plasma membrane.
They, like all other v-SNAREs, are membrane proteins that
are integrated into the membrane of the ER during their
biosynthesis and are then carried by transport vesicles to