Essential Cell Biology 5th edition

Answers A:3
proteins, nucleic acids, and other chemicals together in a
test tube, for example, and make a cell. The cell functions
by virtue of its organized structure, and this is a product of
its evolutionary history. Cells always come from preexisting
cells, and the division of a mother cell passes both chemical
constituents and structures to its daughters. The plasma
membrane, for example, never has to form de novo, but
grows by expansion of a preexisting membrane; there will
always be a ribosome, in part made up of proteins, whose
function it is to make more proteins, including those that
build more ribosomes.
ANSWER 1–16 In a multicellular organism, different cells
take on specialized functions and cooperate with one
another, so that any one cell type does not have to perform
all activities for itself. Through such division of labor,
multicellular organisms are able to exploit food sources
that are inaccessible to single-celled organisms. A plant, for
example, can reach the soil with its roots to take up water
and nutrients, while at the same time, its leaves above
ground can harvest light energy and CO 2 from the air. By
protecting its reproductive cells with other specialized cells,
the multicellular organism can develop new ways to survive
in harsh environments or to fight off predators. When food
runs out, it may be able to preserve its reproductive cells
by allowing them to draw upon resources stored by their
companions—or even to cannibalize relatives (a common
process, in fact).
ANSWER 1–17 The volume and the surface area are
5.24 × 10 –19 m 3 and 3.14 × 10 –12 m 2 for the bacterial cell,
and 1.77 × 10 –15 m 3 and 7.07 × 10 –10 m 2 for the animal cell,
respectively. From these numbers, the surface-to-volume
ratios are 6 × 10 6 m –1 and 4 × 10 5 m –1 , respectively. In
other words, although the animal cell has a 3375-fold larger
volume, its membrane surface is increased only 225-fold. If
internal membranes are included in the calculation, however,
the surface-to-volume ratios of both cells are about equal.
Thus, because of their internal membranes, eukaryotic cells
can grow bigger and still maintain a sufficiently large area
of membrane, which—as we discuss in more detail in later
chapters—is required for many essential cell functions.
ANSWER 1–18 There are many lines of evidence for a
common ancestor cell. Analyses of modern-day living
cells show an amazing degree of similarity in the basic
components that make up the inner workings of otherwise
vastly different cells. Many metabolic pathways, for
example, are conserved from one cell type to another,
and the organic compounds that make up polynucleotides
(DNA and RNA) and proteins are the same in all living
cells, even though it is easy to imagine that a different
choice of compounds (e.g., amino acids with different side
chains) would have worked just as well. Similarly, it is not
uncommon to find that important proteins have closely
similar detailed structures in prokaryotic and eukaryotic
cells. Theoretically, there would be many different ways
to build proteins that could perform the same functions.
The evidence overwhelmingly shows that most important
processes were “invented” only once and then became
fine-tuned during evolution to suit the particular needs of
specialized cells and specific organisms.
It seems highly unlikely, however, that the first cell
survived to become the primordial founder cell of today’s
living world. As evolution is not a directed process with
purposeful progression, it is more likely that there were a
vast number of unsuccessful trial cells that replicated for
a while and then became extinct because they could not
adapt to changes in the environment or could not survive
in competition with other trial cells. We can therefore
speculate that the primordial ancestor cell was a “lucky” cell
that ended up in a relatively stable environment in which it
had a chance to replicate and evolve.
ANSWER 1–19 A quick inspection might reveal the
characteristic beating of cilia on the cell surface; their
presence would tell you that the cell was eukaryotic
(prokaryote flagella have entirely different structures and
motions compared to eukaryote cilia and flagella). If you
don’t see them—and you are quite likely not to—you will
have to look for other distinguishing features. If you are
lucky, you might see the cell divide. Watch it then with
the right optics, and you might be able to see condensed
mitotic chromosomes, which again would tell you that it was
a eukaryote. Fix the cell and stain it with a dye for DNA: if
the DNA is contained in a well-defined nucleus, the cell is a
eukaryote; if you cannot see a well-defined nucleus, the cell
may be a prokaryote. Alternatively, stain it with fluorescent
antibodies that bind actin or tubulin (proteins that are highly
conserved in eukaryotes but absent in bacteria). Embed it,
section it, and look with an electron microscope: can you
see organelles such as mitochondria inside your cell? Try
staining it with Gram stain, which is specific for molecules in
the cell wall of some classes of bacteria. But all these tests
might fail, leaving you still uncertain. For a definitive answer,
you could attempt to analyze the sequences of the DNA
and RNA molecules that it contains, using the sophisticated
methods we describe more fully in Chapter 10. If the nucleic
acid sequences encode molecules that are highly conserved
in eukaryotes, such as those that form the core components
of the nuclear pore, you can be sure your cell is a eukaryote.
If there are no eukaryote-specific sequences, you should
still be able to distinguish whether you are looking at a
bacterium or an archaeon. If you can’t detect any DNA or
RNA, you are probably looking not at a cell but at a
piece of dirt.
Chapter 2
ANSWER 2–1 The chances are excellent because of
the enormous size of Avogadro’s number. The original
cup contained one mole of water, or 6 × 10 23 molecules,
and the volume of the world’s oceans, converted to cubic
centimeters, is 1.5 × 10 24 cm 3 . After mixing, there should be
on average 0.4 of a “Greek” water molecule per cm 3
(6 × 10 23 /1.5 × 10 24 ), or 7.2 molecules in 18 g of Pacific
Ocean.
ANSWER 2–2
A. The atomic number is 6; the atomic weight is 12
(= 6 protons + 6 neutrons).
B. The number of electrons is 6 (= the number of protons).
C. The first shell can accommodate two and the second
shell eight electrons. Carbon therefore needs four
additional electrons (or would have to give up four
electrons) to obtain a full outermost shell. Carbon is
most stable when it shares four additional electrons with
other atoms (including other carbon atoms) by forming
four covalent bonds.