Essential Cell Biology 5th edition

Answers
Chapter 1
ANSWER 1–1 Trying to define life in terms of properties
is an elusive business, as suggested by this scoring exercise
(Table A1–1). Vacuum cleaners are highly organized objects,
and take matter and energy from the environment and
transform the energy into motion, responding to stimuli
from the operator as they do so. On the other hand, they
cannot reproduce themselves, or grow and develop—but
then neither can old animals. Potatoes are not particularly
responsive to stimuli, and so on. It is curious that standard
definitions of life usually do not mention that living
organisms on Earth are largely made of organic molecules—
that is, life is carbon based. As we now know, the key
types of “informational macromolecules”—DNA, RNA, and
protein—are the same in every living species.
containing a nutrient broth that could support microbial
growth never grew anything if properly heat-sterilized first.
He overcame the objections of those who pointed out the
lack of oxygen, or who suggested that his heat sterilization
killed the life-generating principle, by using a special flask
with a slender “swan’s neck,” which was designed to
prevent spores carried in the air from contaminating the
culture (Figure A1–3). The heat-sterilized nutrient broth in
these flasks never showed any signs of life; however, it was
capable of supporting life, as could be demonstrated by
washing some of the “dust” from the neck of the flask into
the broth.
TABLE A1–1 PLAUSIBLE “LIFE” SCORES FOR A
VACUUM CLEANER, A POTATO, AND A HUMAN
Characteristic
Vacuum
Cleaner
Potato
1. Organization Yes Yes Yes
Human
original flask
Figure A1–3
swan’s-neck flask
2. Homeostasis Yes Yes Yes
3. Reproduction No Yes Yes
4. Development No Yes Yes
5. Energy Yes Yes Yes
6. Responsiveness Yes No Yes
7. Adaptation No Yes Yes
ANSWER 1–2 Most random changes to the shoe design
would result in objectionable defects: shoes with multiple
heels, with no soles, or with awkward sizes would obviously
not sell and would therefore be selected against by market
forces. Other changes would be neutral, such as minor
variations in color or in size. A minority of changes, however,
might result in more desirable shoes: deep scratches in a
previously flat sole, for example, might create shoes that
would perform better in wet conditions; the loss of high
heels might produce shoes that are more comfortable
(and less dangerous). The example illustrates that random
changes can lead to significant improvements if the
number of trials is large enough and selective pressures
are imposed.
ANSWER 1–3 It is extremely unlikely that you created
a new organism in this experiment. Far more probably,
a spore from the air landed in your broth, germinated,
and gave rise to the cells you observed. In the middle of
the nineteenth century, Louis Pasteur invented a clever
apparatus to disprove the then widely accepted belief that
life could arise spontaneously. He showed that sealed flasks
ANSWER 1–4 6 × 10 39 (= 6 × 10 27 g/10 –12 g) bacteria
would have ECB5 the same eA1.3/A1.3
mass as the Earth. And 6 × 10 39 =
2 t/20 , according to the equation describing exponential
growth. Solving this equation for t results in t = 2642
minutes (or 44 hours). This represents only 132 generation
times(!), whereas 5 × 10 14 bacterial generation times have
passed during the last 3.5 billion years. Obviously, the total
mass of bacteria on this planet is nowhere close to the mass
of the Earth. This illustrates that exponential growth can
occur only for very few generations—that is, for minuscule
periods of time compared with evolution. In any realistic
scenario, food supplies very quickly become limiting.
This simple calculation shows us that the ability to grow
and divide quickly when food is ample is only one factor
in the survival of a species. Food is generally scarce, and
individuals of the same species have to compete with one
another for the limited resources. Natural selection favors
mutants that either win the competition or find ways to
exploit food sources that their neighbors are unable to use.
ANSWER 1–5 By engulfing substances such as food
particles, eukaryotic cells can sequester them and feed
on them efficiently. Bacteria, in contrast, have no way of
capturing lumps of food; they can export substances that
help break down food substances in the environment, but
the products of this labor must then be shared with other
organisms in the same neighborhood.
ANSWER 1–6 Conventional light microscopy is much
easier to use and requires much simpler instruments.
Objects that are 1 μm in size can easily be resolved; the
lower limit of resolution is 0.2 μm, which is a theoretical limit
imposed by the wavelength of visible light. Visible light is