Essential Cell Biology 5th edition

Answers A:43
ANSWER 16–14 Activation in both cases depends
on proteins that catalyze GDP–GTP exchange on the
G protein or Ras protein. Whereas activated GPCRs
perform this function directly for G proteins, enzymelinked
receptors assemble multiple signaling proteins into
a signaling complex when the receptors are activated by
phosphorylation; one of these proteins is an adaptor protein
that recruits a guanine nucleotide exchange factor that
fulfills this function for Ras.
ANSWER 16–15 Because the cytosolic concentration of
Ca 2+ is so low, an influx of relatively few Ca 2+ ions leads
to large changes in its cytosolic concentration. Thus, a
tenfold increase in cytosolic Ca 2+ can be achieved by
raising its concentration into the micromolar range, which
would require far fewer ions than would be required to
change significantly the cytosolic concentration of a more
abundant ion such as Na + . In muscle, a greater than tenfold
change in cytosolic Ca 2+ concentration can be achieved
in microseconds by releasing Ca 2+ from the sarcoplasmic
reticulum, a task that would be difficult to accomplish if
changes in the millimolar range were required.
ANSWER 16–16 In a multicellular organism such as an
animal, it is important that cells survive only when and
where they are needed. Having cells depend on signals
from other cells may be a simple way of ensuring this. A
misplaced cell, for example, would probably fail to get
the survival signals it needs (as its neighbors would be
inappropriate) and would therefore kill itself. This strategy
can also help regulate cell numbers: if cell type A depends
on a survival signal from cell type B, the number of B cells
could control the number of A cells by making a limited
amount of the survival signal, so that only a certain number
of A cells could survive. There is indeed evidence that such a
mechanism does operate to help regulate cell numbers—in
both developing and adult tissues (see Figure 18–41).
ANSWER 16–17 Ca 2+ -activated Ca 2+ channels create
a positive feedback loop: the more Ca 2+ that is released,
the more Ca 2+ channels that open. The Ca 2+ signal in the
cytosol is therefore propagated explosively throughout the
cardiac muscle cell, thereby ensuring that all myosin–actin
filaments contract almost synchronously.
ANSWER 16–18 K2 activates K1. If K1 is permanently
activated, a response is observed regardless of the status of
K2. If the order were reversed, K1 would need to activate
K2, which cannot occur because in our example K2 contains
an inactivating mutation.
ANSWER 16–19
A. Three examples of extended signaling pathways to the
nucleus are: (1) extracellular signal → RTK → adaptor
protein → Ras-activating protein → MAP kinase
kinase kinase → MAP kinase kinase → MAP kinase →
transcription regulator; (2) extracellular signal → GPCR
→ G protein → phospholipase C → IP 3 → Ca 2+ →
calmodulin → CaM-kinase → transcription regulator;
(3) extracellular signal → GPCR → G protein → adenylyl
cyclase → cyclic AMP → PKA → transcription regulator.
B. An example of a direct signaling pathway to the nucleus
is Delta → Notch → cleaved Notch tail → transcription.
ANSWER 16–20 When PI 3-kinase is activated by
an activated RTK, it phosphorylates a specific inositol
phospholipid in the plasma membrane. The resulting
phosphorylated inositol phospholipid then recruits to the
plasma membrane both Akt and another protein kinase that
helps phosphorylate and activate Akt. A third kinase that
is permanently associated with the membrane also helps
activate Akt (see Figure 16−32).
ANSWER 16–21 Polar groups are hydrophilic, so
cholesterol, with only one polar –OH group, would be too
hydrophobic to be an effective hormone by itself. Because
it is virtually insoluble in water, it could not move readily as
a messenger from one cell to another via the extracellular
fluid, unless carried by specific proteins.
ANSWER 16–22 In the case of the steroid-hormone
receptor, a one-to-one complex of steroid and receptor
binds to DNA to activate or inactivate gene transcription;
there is thus no amplification between ligand binding
and transcriptional regulation. Amplification occurs later,
because transcription of a gene gives rise to many mRNAs,
each of which is translated to give many copies of the
protein it encodes (discussed in Chapter 7). For the ionchannel-coupled
receptor, a single ion channel will let
through thousands of ions in the time it remains open; this
serves as the amplification step in this type of signaling
system.
ANSWER 16–23 The more steps there are in an
intracellular signaling pathway, the more places the cell
has to regulate the pathway, amplify the signal, integrate
signals from different pathways, and spread the signal along
divergent paths (see Figure 16−9).
ANSWER 16–24 Animals and plants are thought to
have evolved multicellularity independently, and therefore
will be expected to have evolved some distinct signaling
mechanisms for their cells to communicate with one another.
On the other hand, animal and plant cells are thought to
have evolved from a common eukaryotic ancestor cell,
and so plants and animals would be expected to share
some intracellular signaling mechanisms that the common
ancestor cell used to respond to its environment.
Chapter 17
ANSWER 17–1 Cells that migrate rapidly from one place
to another, such as amoebae (A) and sperm cells (F), do not
in general need intermediate filaments in their cytoplasm,
since they do not develop or sustain large tensile forces.
Plant cells (G) are pushed and pulled by the forces of wind
and water, but they resist these forces by means of their
rigid cell walls rather than by their cytoskeleton. Epithelial
cells (B), smooth muscle cells (C), and the long axons of
nerve cells (E) are all rich in cytoplasmic intermediate
filaments, which prevent them from rupturing as they are
stretched and compressed by the movements of their
surrounding tissues. All of the above eukaryotic cells
possess intermediate filaments in their nuclear lamina.
Bacteria, such as Escherichia coli (D), have none whatsoever.
ANSWER 17–2 Two tubulin dimers have a lower affinity for
each other (because of a more limited number of interaction
sites) than a tubulin dimer has for the end of a microtubule
(where there are multiple possible interaction sites, both
end-to-end for tubulin dimers adding to a protofilament,
and side-to-side for the tubulin dimers interacting with