CELLS
THE BUILDING
BLOCKS OF LIFE
The Evolution
of Cells
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Cells: The Building Blocks of Life
Cell Structure, Processes, and Reproduction
Cells and Human Health
The Evolution of Cells
How Scientists Research Cells
Plant Cells
Stem Cell Research and Other Cell-Related Controversies
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TERRY L. SMITH
CELLS
THE BUILDING
BLOCKS OF LIFE
The Evolution
of Cells
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THE EVOLUTION OF CELLS
Copyright © 2012 by Infobase Learning

All rights reserved. No part of this book may be reproduced or utilized in any form or
by any means, electronic or mechanical, including photocopying, recording, or by any
information storage or retrieval systems, without permission in writing from the
publisher. For information, contact:
Chelsea House
An imprint of Infobase Learning

1 The Beginnings of Life 7
2 The Chemistry of Life 19
3 Prokaryotes: the Simplest Cells 30
4 Eukaryotes: the Cells of Complex Life 40
5 Cells in Action 51
6 Genetics and Cell Evolution 66
7 Plant Cells and Evolution 78
8 The Diversity of Complex Cells 89
9 Cells: Key to the Future 99
Glossary 109
Bibliography 113
Further Resources 115
Picture Credits 116
Index 117
About the Author 122
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7
The BeginningsThe Beginnings
of Lifeof Life
 ere is something about being human that instills in us a sense of won-
der. When we stop to think about it, the very idea of life seems such a mys-
tery. Where did we come from? How did life begin? When we look at the
sky, we wonder about the vastness of the universe and whether other life
may exist there. If we look through a microscope at a drop of pond water,
we are amazed at the variety of tiny creatures we see.
Since cells form the very basis of life, it is only natural that our sense
of wonder extends to the cell. Where did the fi rst cells come from? How is
it possible that cells with the same basic components can form creatures
as simple as bacteria or as complex as a human being? How do brain cells

agree on a defi nition that includes all possible life forms! Allowing for the
possibility that entire new life forms may exist elsewhere in the universe,
life as we know it on Earth shares certain features in common: the need
to take in energy; production of waste that must be eliminated; growth;
replication into similar life forms; and response to the environment, both
as individual organisms and through evolution across generations.
Early Beliefs
 ere is evidence that even very ancient people were concerned about the
nature of their existence and origins of life. Paintings of animal images
from 10,000 to 30,000 years ago have been found in caves in Altimira,
Spain, and the Vézère Valley of France.  ey suggest that the humans who
painted them were even then grappling with the nature of existence and
their place in nature. Stories about the beginning of life developed in most
cultures, as ancient people struggled to understand their origins. Many of
these beliefs about life’s origin remain alive today in the poetry and reli-
gion of various cultures around the world.
Recorded history of the Dark Ages in Europe (from about 500 to
1100 ..) tells us that mankind also came to depend on their obser-
vations to form their belief systems about the origin of life. If a piece
of meat was le to rot, maggots soon appeared; to a person unfamiliar
with science, it was easy to conclude that the maggots had spontaneously
appeared in the rotten fl esh.  is gave rise to the commonly held belief
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The Beginnings of Life 9
in “spontaneous generation” as a source of many life forms. In fact, there
was even a seventeenth-century recipe for the creation of mice: store
dirty underwear with grains of wheat in an open jar and a er 21 days
mice will spontaneously appear. Although the real source of the mice
seems obvious to us today, this belief was consistent with the knowledge
of that era.

heated to a high temperature to kill any living matter in the broth.  e
fl asks were le at room temperature and exposed to the air for some
time. Microorganisms present in the air could fall into the straight-
neck fl ask but not into the fl ask with the curved neck. At the end of the
experiment, the broth in the straight-neck fl ask was dark and cloudy,
and microorganisms could be observed in the broth. No evidence of
organisms was found in the curve-necked fl ask, thereby demonstrating
that organisms were not spontaneously generated in the broth but had
fallen in from the air.
To convince the French Academy of the truth of his discovery, Pas-
teur fi rst formed a hypothesis based on his previous observations of the
growth of microorganisms. It stated that microorganisms would not grow
in the sterilized broth if they could not fall into the fl ask from the air.
His next step was to conduct an experiment consisting of a control case
(straight-neck fl ask) and a test case (curved-neck fl ask). A er observing
the results of the experiment, Pasteur concluded, “Never will the doctrine
of spontaneous generation recover from the mortal blow of this simple
experiment.” Today’s scientists who seek evidence of the fi rst life forms on
Earth—whether analyzing fossils or conducting experiments in chemistry
or genetics—continue to adhere to these 150-year-old steps of the scien-
tifi c method.
EARLY EARTH
No one will ever know for certain exactly how the fi rst life began on
Earth. Yet, to think about how life could have begun, how that very fi rst
cell developed, it is essential to understand the physical conditions that
existed on early Earth.
Our solar system is thought to have formed some 4.6 billion years ago
from a giant rotating cloud of gas and dust. As much of the rotating mate-
rial collapsed toward the center, a new star, our planet’s Sun, was formed.
Other large chunks of material collided and eventually attracted addi-

hot, not too cold, but juuust right,” as the famous fairy tale goes. How-
ever, if the existence of this planet, known as Gliese 581g, is confi rmed,
it may have the ideal temperature that would allow for the presence
of water in liquid form. In addition, its size suggests it could have the
proper gravitational forces and atmosphere to sustain life. Although the
planet is relatively close to Earth, given the vastness of the universe, no
one will be visiting to check for signs of life anytime soon. Its distance
of 120 trillion miles (193 trillion kilometers) away would require several
generations of humans for a spaceship to reach there.
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The Beginnings of Life 13
used to search for evidence of how this fi rst life may have come into being:
fossil analysis, radioactive dating of rocks, evolutionary relationships
among organisms, chemistry experiments, study of present-day organ-
isms living in extreme conditions, and analysis of genomes. Scientists
involved in this search come from the fi elds of biology, physics, chemistry,
geology, paleontology, and astrobiology.
Much of the research into the origins of life involves learning more
about organisms that exist on today’s Earth. In their search for where
the fi rst life may have originated, scientists have focused attention on
extremophiles.  ese are organisms that survive in extreme condi-
tions, such as the high pressure that occurs on the ocean fl oor, or the
high temperatures of hot springs, such as those in Yellowstone National
Park. Much evidence now suggests that cells may have fi rst formed near
Figure 1.3 This view of a geothermal pool in Yellowstone National Park
in Wyoming shows the colors caused by deposited minerals and colonies
of heat-loving bacteria and algae called extremophiles. The pool is heated
because of magma (molten rock) at the bottom.
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14 THE EVOLUTION OF CELLS

a project for a graduate student and would delay his academic degree.
Still, Miller persisted, and it was fi nally agreed that he would work on
the project for a year.
In just a short time, Miller succeeded in producing the results that
his professor had proposed. He assembled an apparatus containing a
fl ask of water to simulate the ocean, then heated it to produce water
vapor. A mixture of methane, hydrogen, and ammonia gases, which were
then believed to be the components of early Earth’s atmosphere, was
circulated through the apparatus to mix with the water vapor. Electrical
charges then pelted the gaseous mixture to simulate an energy source in
the form of lightning. Chemical reactions in the mixture soon produced
compounds that colored the waters of the “ocean.” Analysis revealed
that Miller’s experiment had yielded many amino acids, the building
blocks of proteins.
The scientifi c community greeted Miller’s work with surprise and
disbelief. Resistance to publishing the write-up of his experiment was
so strong that it might have gone unpublished had it not been for the
backing of his eminent professor. Soon, however, others were able to
reproduce the now-famous experiment and Miller’s work became widely
accepted. Its radical departure from anything that had gone before
changed the course of scientifi c thinking about the origins of life.
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16 THE EVOLUTION OF CELLS
Fossil hunts are exciting and have provided critical evidence about
the oldest life on Earth. Yet important origin-of-life research also takes
place in the laboratory. Experiments can serve as “proofs of concept” to
form hypotheses about how life may have originated. In other words, if
certain chemical reactions can be observed in the laboratory, it is possible
that similar reactions took place under the conditions present on early
Earth. One of the earliest experiments that searched for how life began

molecules to form something resembling a strand of RNA. At the Scripps
Research Institute, molecular biologist Gerald F. Joyce and colleagues
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The Beginnings of Life 17
have produced forms of RNA molecules that can promote each other’s
synthesis, as they attempt to recreate a key element of life: genetic material
that can reproduce itself.
THE PARADOX OF THE FIRST CELL
Cells of present-day complex organisms rely on three essential com-
pounds: DNA for replication and storage of genetic information, RNA for
various functions, and proteins that serve as the workhorses of the cell.
Certain proteins, called enzymes, have the important function of serving
as catalysts for essential chemical reactions. Since RNA molecules have
much simpler chemical forms than DNA, scientists assume that the fi rst
cells depended on RNA for preservation of genetic material. Still, how
could cells produce new RNA molecules, an essential function for living
things, without proteins to catalyze the chemical reactions?  is question
gave rise to one of the great paradoxes of origin-of-life studies—which
came fi rst, RNA or protein? Progress in the fi eld was slow until a key dis-
covery was made in the 1980s.  omas Cech of the University of Colorado
discovered an RNA molecule within a protozoa that could chemically
manipulate itself without the assistance of a protein. In other words, the
RNA molecules were playing the chemical role of an enzyme. Sidney Alt-
man of Yale University independently discovered a similar RNA molecule,
and the two researchers shared the 1989 Nobel Prize in Chemistry for
their discoveries.  ese RNA molecules were given the name ribozymes
for their ability to fold themselves into biologically active molecules that
were able to play the role of an enzyme in chemical reactions.  us, the
fi rst cell would have depended on a simple RNA molecule, or ribozyme,
that could both transmit genetic material and act in place of proteins to

tions.  is knowledge involves an understanding of organic chemistry,
the branch of chemistry associated with carbon-containing compounds.
 ese compounds are, for the most part, those associated with living
organisms.
Carbon was a common element in the universe that produced our
solar system. It would have been in plentiful supply as the crust of Earth
was developing billions of years ago. Volcanic eruptions circulated ele-
ments from Earth’s interior to its surface. Gravitational forces pulled in
debris from space, contributing to the variety of elements available for
the fi rst organic chemical reactions. In addition to carbon, other major
components of organic compounds, such as nitrogen, oxygen, and hydro-
gen, all existed on Earth’s surface. Eruptions spewed steam and gases into
the atmosphere, creating shi s in how much heat from the sun reached
Earth’s surface. Water varied in form from atmospheric vapor to liquid
oceans to ice as Earth rotated through cycles of heat and cold. At some
point, the right chemicals got together under just the right conditions for
life to begin.
The Chemistry The Chemistry
of Lifeof Life
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20 THE EVOLUTION OF CELLS
Whenever we picnic under the shade of a tree on a hot summer day,
we sit surrounded by organic chemistry and never give it a thought.  e
wood of the picnic table, the corn chips we eat, the leaves providing our
shade, the ants crawling about, our very selves—they are all containers of
organic chemicals that form the core of our existence. All living things are
composed of cells, and the business of a cell is chemistry. Our human cells
contain the same chemical compounds and undergo similar chemical pro-
cesses as the cells in the corn we eat and the bacteria that live all around us.
 e next time you eat a good meal, consider that your body’s billions of cells

fatty acids, the ring structures that form the basis of sugar compounds,
and even gaseous molecules such as carbon dioxide.  e covalent (mean-
ing that atoms share electrons) chemical bonds that carbon atoms form
with other atoms are relatively strong, making the organic molecules more
stable.
Given the abundance of carbon available on early Earth, and its ideal
chemical properties, it is not surprising that life developed as a carbon-
based system. Another property of carbon molecules that works to the
advantage of essential chemical reactions is their “handedness,” or chiral-
ity. Organic carbon molecules can exist in either a “right-handed” or a
“le -handed” form, which are mirror opposites of each other. Life forms
have developed in a way that incorporates only the right-handed forms of
sugars and the le -handed forms of amino acids. Biochemical reactions
FIGURE 1.7 During a news conference at NASA headquarters in Washington, DC,
in December 2010, NASA astrobiology research fellow Felisa Wolfe-Simon
announces finding a potential new form of life. Wolfe-Simon said that after a two-
year study at Mono Lake in California, she found a bacterium that could eat and
grow on arsenic instead of phosphorus, one of the basic building blocks of life.
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22 THE EVOLUTION OF CELLS
are highly precise in the way that complex molecules must fi t together
and are dependent on the particular right- or le -handed forms of these
molecules being available.
SHAPE MAKES ALL THE DIFFERENCE
Every cellular molecule, no matter how large and complex, is made up of
a series of smaller chemical modules that follow an orderly arrangement
of atoms. A small number of relatively simple chemical building blocks
forms these modules, yet they are capable of arranging themselves in an
almost endless variety of complex three-dimensional forms. Biochemi-
cal reactions take place by means of chemical bonds that form or break

Figure 2.2 Note the base pairs of nucleotides that make up the “rungs” of
the ladder in the structure of DNA.
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24 THE EVOLUTION OF CELLS
(R) serves as the sugar unit in RNA.  e base units, all of which con-
tain nitrogen, may take on one of fi ve chemical structures: cytosine (C),
thymine (T), uracil (U), guanine (G), or adenine (A).  ymine bases are
found only in DNA molecules, uracil bases are found only in RNA mol-
ecules, and the others occur in both DNA and RNA. Although these base
molecules are similar in size, they take on quite diff erent chemical shapes,
and their pattern along the length of the NA molecule provides for the
wide diversity of coded information that these molecules contain.
RNA has a much simpler, and chemically less stable, form than DNA.
Early forms of cells likely relied on RNA both for conveying genetic infor-
mation and for cell maintenance. In the modular structure of RNA, the
ribose sugar unit is linked to a phosphate unit. One of the four bases—
either C, U, G, or A—chemically bonds to each sugar-phosphate unit,
forming a nucleotide. Many sugar-phosphate units then bond together to
form the backbone of a nucleic acid, with the base units projecting to the
side. Despite the relatively simple single-chain structure of RNA, these
chains can be quite long and take various looped and twisted forms, allow-
ing them to perform diff erent roles within a cell. Complex cells depend on
three major forms of RNA to transport information and to manufacture
proteins: messenger RNA, transfer RNA, and ribosomal RNA.
DNA molecules can be thought of as two chains of nucleotides that are
lined up to form a ladder.  e sugar-phosphate units of the chains form
the sides of the ladder, while the projecting base pairs link in the middle
to form the rungs. However, this is a very long ladder; DNA molecules
may contain hundreds of thousands of base-pair rungs. What’s more, the
form of the ladder is twisted, forming the well-known double helix struc-