Essential Cell Biology 5th edition

A:2 Answers
nondestructive and passes readily through water, making
it possible to observe living cells. Electron microscopy, on
the other hand, is much more complicated, both in the
nature of the instrument and in the preparation of the
sample (which needs to be extremely thinly sliced, stained
with an electron-dense heavy metal, and completely
dehydrated). Living cells cannot be observed in an electron
microscope. The resolution of electron microscopy is much
higher, however, and biological objects as small as 1 nm
can be resolved. To see any structural detail, microtubules,
mitochondria, and bacteria would need to be analyzed
either by electron microscopy or by using specific dyes
to make them visible by confocal or super-resolution
fluorescence microscopy (although no form of fluorescence
microscopy can match the resolution of an electron
microscope).
ANSWER 1–7 Because the basic workings of all cells
are so similar, a great deal has been learned from studying
model systems. Brewer’s yeast is a good model for
eukaryotic cells because yeast cells are much simpler than
human cancer cells. We can grow them inexpensively and
in vast quantities, and we can manipulate them genetically
and biochemically much more easily than human cells.
This allows us to use yeast to decipher the ground rules
governing how cells grow and divide. Cancer cells grow
and divide when they should not (and therefore give rise
to tumors), and a basic understanding of how cell growth
and division are normally controlled is therefore directly
relevant to the cancer problem. Indeed, the National Cancer
Institute, the American Cancer Society, and many other
institutions that are devoted to finding a cure for cancer
strongly support basic research on various aspects of cell
growth and division in different model systems, including
yeast.
ANSWER 1–8 Check your answers using the Glossary and
Panel 1–2 (p. 25).
ANSWER 1–9
A. False. The hereditary information is encoded in the cell’s
DNA, which in turn specifies its proteins (via RNA).
B. True. Bacteria do not have a nucleus.
C. False. Plants, like animals, are composed of eukaryotic
cells, but unlike animal cells, they contain chloroplasts as
cytoplasmic organelles. The chloroplasts are thought to
be evolutionarily derived from engulfed photosynthetic
bacteria.
D. True. The number of chromosomes varies from one
organism to another, but is constant in all nucleated
cells (except germ cells) within the same multicellular
organism.
E. False. The cytosol is the cytoplasm excluding all
membrane-enclosed organelles.
F. True. The nuclear envelope is a double membrane, and
mitochondria are surrounded by both an inner and an
outer membrane.
G. False. Protozoans are single-celled organisms and
therefore do not have different tissues or cell types.
They have a complex structure, however, that has highly
specialized parts.
H. Somewhat true. Peroxisomes and lysosomes contain
enzymes that catalyze the breakdown of substances
produced in the cytosol or taken up by the cell. One
can argue, however, that many of these substances are
degraded to generate food molecules, and as such are
certainly not “unwanted.”
ANSWER 1–10 In this plant cell, A is the nucleus, B is a
vacuole, C is the cell wall, and D is a chloroplast. The scale
bar is about 10 μm, the width of the nucleus.
ANSWER 1–11 The three major filaments are actin
filaments, intermediate filaments, and microtubules.
Actin filaments are involved in rapid cell movement,
and are the most abundant filaments in a muscle cell;
intermediate filaments provide mechanical stability and
are the most abundant filaments in epidermal cells of the
skin; and microtubules function as “railroad tracks” for
many intracellular movements and are responsible for the
separation of chromosomes during cell division. Other
functions of all these filaments are discussed in Chapter 17.
ANSWER 1–12 It takes only 20 hours (i.e., less than a day)
before mutant cells become more abundant in the culture.
Using the equation provided in the question, we see that
the number of the original (“wild-type”) bacterial cells at
time t minutes after the mutation occurred is 10 6 × 2 t/20 .
The number of mutant cells at time t is 1 × 2 t/15 . To find out
when the mutant cells “overtake” the wild-type cells, we
simply have to make these two numbers equal to each other
(i.e., 10 6 × 2 t/20 = 2 t/15 ). Taking the logarithm to base 10 of
both sides of this equation and solving it for t results in
t = 1200 minutes (or 20 hours). At this time, the culture
contains 2 × 10 24 cells (10 6 × 2 60 + 1 × 2 80 ). Incidentally,
2 × 10 24 bacterial cells, each weighing 10 –12 g, would weigh
2 × 10 12 g (= 2 × 10 9 kg, or 2 million tons!). This can only
have been a thought experiment.
ANSWER 1–13 Bacteria continually acquire mutations in
their DNA. In the population of cells exposed to the poison,
one or a few cells may already harbor a mutation that makes
them resistant to the action of the poison. Antibiotics that
are poisonous to bacteria because they bind to certain
bacterial proteins, for example, would not work if the
proteins have a slightly changed surface so that binding
occurs more weakly or not at all. These mutant bacteria
would continue dividing rapidly while their cousins are
slowed down. The antibiotic-resistant bacteria would soon
become the predominant species in the culture.
ANSWER 1–14 10 13 = 2 (t/1) . Therefore, it would take only
43 days [t = 13/log(2)]. This explains why some cancers
can progress extremely rapidly. Many cancer cells divide
much more slowly, however, and many die because of
their internal abnormalities or because they do not have
a sufficient blood supply, and so the actual progression of
cancer is usually slower.
ANSWER 1–15 Living cells evolved from nonliving
matter, but they grow and replicate. Like the material they
originated from, they are governed by the laws of physics,
thermodynamics, and chemistry. Thus, for example, they
cannot create energy de novo or build ordered structures
without the expenditure of free energy. We can understand
virtually all cellular events, such as metabolism, catalysis,
membrane assembly, and DNA replication, as complicated
chemical reactions that can be experimentally reproduced,
manipulated, and studied in test tubes.
Despite this fundamental reducibility, a living cell is
more than the sum of its parts. We cannot randomly mix