by the small subunit of the ribosome (answer d).Ribosomes are composed
of both protein and RNA (predominantly rRNA, but also mRNA and
tRNA). Single ribosomes are involved in synthesis of cytosolic proteins.
Polyribosomes, linked by mRNA (answer e),synthesize proteins that are
translocated into the cisternal space of the rough endoplasmic reticulum
(RER) and destined for export or specific organelles.
39.The answer is d.(Alberts, pp 923, 936, 940–944.)The stability, arrange-
ment, and functions of actin filaments depend on the actin-binding proteins.
The fundamental structure of the actin molecule is the same no matter what
the function or arrangement in a cell. Actin-binding proteins have a variety of
functions: (1) tropomyosin strengthens actin filaments, (2) fibrin and villin
are actin-bundling proteins, (3) filamin and gelsolin regulate transformation
from the sol to the gel state, (4) members of the myosin II family are respon-
sible for sliding filaments, (5) myosin I (minimyosin) is responsible for move-
ment of vesicles on filaments, and (6) spectrin cross-links the sides of actin
filaments to the plasma membrane.
40.The answer is d.(Alberts, pp 736–738. Ross and Pawlina, pp 48–51.
Junqueira, pp 35–37.)The biochemically compartmentalized organelle,
labeled with the arrows in the electron micrograph, is the Golgi apparatus.
There is a specific organization to the Golgi stacks related to the function of
specific enzymes. Histochemical stains, such as acid phosphatase and
nucleoside diphosphates, show that the Golgi apparatus is topologically
compartmentalized(answer a).It presents two faces: a cisface, which is
the point of entry of transport vesicles (COP-II-coated, see question 50
feedback) in transit from the rough endoplasmic reticulum (RER) to the
Golgi(answer b),and a transface, which is the exit point associated with
granule formation and the maturation of proteins (answer c).Both pro-
teins and lipids are transported from the transitional elements of the ER to
the Golgi apparatus (answer e).Packaging is not the sole function of the
Golgi. This organelle is also involved in the processing of proteins (e.g.,
addition and trimming of oligosaccharide chains) that was initiated in the
RER as well as sulfation.
41.The answer is b.(Alberts, pp 801–803, 952–953. Ross and Pawlina, pp
57–58.)There are differences in the way that intermediate filaments interact
with microtubules and microfilaments within the cytoplasm; however, their
Cell Biology: Cytoplasm Answers 117