Essential Cell Biology 5th edition

Answers A:55
ANSWER 20–9
A. False. Gap junctions are not connected to the
cytoskeleton; they form cell–cell channels that allow
small molecules to pass from one cell to another.
B. True. Upon wilting, the turgor pressure in the plant cell is
reduced, and consequently the cell walls, having tensile
but little compressive strength, like a deflated rubber
tire, no longer provide rigidity.
C. False. Proteoglycans can withstand a large amount of
compressive force but do not have a rigid structure.
Their space-filling properties and ability to resist
compression result from their tendency to absorb large
amounts of water.
D. True.
E. True.
F. True. Stem cells stably express control genes that ensure
that their daughter cells can only develop into certain
differentiated cell types.
ANSWER 20–10 Small cytosolic molecules, such as glutamic
acid, cyclic AMP, and Ca 2+ ions, pass readily through both
gap junctions and plasmodesmata. Some proteins and
mRNAs can pass through plasmodesmata, but all such
macromolecules are excluded from gap junctions. Plasma
membrane phospholipids diffuse in the plane of the
membrane through plasmodesmata because the plasma
membranes and smooth ER membranes of adjacent cells
are continuous through these junctions. This traffic is not
possible through gap junctions, because the membranes of
the connected cells remain separate.
ANSWER 20–11 Plants are exposed to extreme changes
in the environment, which often are accompanied by huge
fluctuations in the osmotic properties of their surroundings.
An intermediate-filament network as we know it from animal
cells would not be able to provide full osmotic support for
cells: the sparse, rivetlike attachment points would not be
able to prevent the membrane from bursting in response to
a huge osmotic pressure applied from the inside of the cell.
ANSWER 20–12 Action potentials can, in fact, be passed
from cell to cell through gap junctions. Indeed, heart muscle
cells contract synchronously by this mechanism. This way
of passing the signal from cell to cell is rather limited,
however. As we discuss in Chapter 12, synapses are far
more sophisticated and allow signals to be modulated and
integrated with other signals received by the cell. Thus, gap
junctions are like simple soldered joints between electrical
components, while synapses are like complex relay devices,
enabling systems of neurons to perform computations.
ANSWER 20–13 To make jello, gelatin is boiled in water,
which denatures the collagen fibers. Upon cooling, the
disordered fibers form a tangled mess that solidifies into a
gel. This gel actually resembles the collagen as it is initially
secreted by fibroblasts. It is not until the fibers have been
aligned, bundled, and cross-linked that they acquire their
ability to resist tensile forces.
ANSWER 20–14 The evidence that DNA is the blueprint
that specifies all the structural characteristics of an organism
is based on observations that small changes in the DNA
by mutation can result in large changes in the organism.
Although DNA provides the plans that specify structure,
these plans need to be executed during development.
This requires a suitable environment (a human baby would
not fit into a stork’s egg shell), suitable nourishment,
suitable molecular tools present in the egg (such as the
appropriate transcription regulators required for early
embryo development), suitable spatial organization (such
as the asymmetries in the egg cell required to allow
for appropriate cell differentiation during the early cell
divisions), and so on. Thus inheritance is not restricted
to the passing on of the organism’s DNA, because
development requires appropriate conditions to be set
up by the parent. Nevertheless, when all these conditions
are met, the plans that are archived in the genome will
determine the structure of the organism to be built.
ANSWER 20–15 White blood cells circulate in the
bloodstream and migrate into and out of tissues in
performance of their normal function of defending the body
against infection: they are therefore naturally invasive. Once
mutations have occurred to upset the normal controls on
production of these cells, there is no need for additional
mutations to enable the cells to spread through the body.
Thus, the number of mutations that have to accumulate
to give rise to leukemia is smaller than for other types of
cancer.
ANSWER 20–16 The shape of the curve reflects the need
for multiple driver mutations to accumulate in a cell before a
cancer results. If a single driver mutation were sufficient, the
graph would be a straight horizontal line: the likelihood of
occurrence of a particular mutation, and therefore of cancer,
would be the same at any age. If two driver mutations were
required, the graph would be a straight line sloping upward
from the origin: the second mutation has an equal chance
of occurring at any time, but will tip the cell into cancerous
behavior only if the first mutation has already occurred
in the same cell lineage; and the likelihood that the first
mutation has already occurred will be proportional to the
age of the individual. The steeply curved graph shown in the
figure goes up approximately as the fifth power of the age,
and this indicates that far more than two driver mutations
have to accumulate before cancer sets in. It is not easy
to say precisely how many, because of the complex ways
in which cancers develop. Successive mutations can alter
cell numbers and cell behavior, and thereby change both
the probability of subsequent mutations and the selection
pressures that drive the evolution of a cancer.
ANSWER 20–17 During exposure to the carcinogen,
mutations are induced, but the number of relevant (driver)
mutations in any one cell is usually not enough to convert
it directly into a cancer cell. Over the years, the cells that
have become predisposed to cancer through the induced
mutations accumulate progressively more mutations.
Eventually, one of the mutant cells will turn into a cancer
cell. The long delay between exposure and cancer has made
it extremely difficult to hold cigarette manufacturers or
producers of industrial carcinogens legally responsible for
the damage that is caused by their products.
ANSWER 20–18 By definition, a carcinogen is any substance
that promotes the occurrence of one or more types of
cancer. The sex hormones can therefore be classified as
naturally occurring carcinogens. Although most carcinogens
act by directly causing mutations, carcinogenic effects
are also often exerted in other ways. The sex hormones
increase both the rate of cell division and the survival of