The Evolution of Cells potx

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?. Allowing for the possibility that entire new life f

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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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THE EVOLUTION OF CELLS

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

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Library of Congress Cataloging-in-Publication Data

Smith, Terry L (Terry Lane),

The evolution of cells / by Terry L Smith.

p cm — (Cells, the building blocks of life)

Includes bibliographical references and index.

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• • •

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The Beginnings

of Life

Th ere is something about being human that instills in us a sense of 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,

won-we are amazed at the variety of tiny creatures won-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 allow us to think, and how do cells of the hand work together to allow us

to play the piano? Video animations of cell interiors let us see the ing molecular “machines” that move around inside every cell as they copy genetic molecules, shuttle nutrients, relay messages, and build or repair membranes

amaz-What follows is an exploration of these subjects, beginning with the

ﬁ rst cell that started the cascade of events leading to life on Earth as we know it Th e exact nature of the very ﬁ rst life on our planet and how it came into being may be forever unknown Yet research into the nature

of life can help us understand what is in the realm of possibility tists have vastly increased our knowledge about the connectedness of all life forms through their study of contemporary organisms and their genetic structures Investigations into the inner workings of cells now

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Scien-8 THE EVOLUTION OF CELLS

hold great promise in the ﬁ elds of evolutionary biology, medicine, and

bioengineering

THE NATURE OF LIFE AND ITS ORIGINS

Defining Life

Th e existence of life might seem to be an obvious fact, but coming up with

a deﬁ nition of life is not so simple Even biologists have diﬃ culty agreeing

on a description that can include all possible life forms How can

microbi-ologists know if they have found life on Mars if they can’t ﬁ rst agree what

deﬁ nes life? Furthermore, how is it possible to look for the ﬁ rst signs of

life on Earth without this deﬁ nition? A chemist may see life as a

self-sus-taining chemical system that can evolve, while a physicist may see life as

an ordered system, in contrast to nature’s tendency toward disorder

Ger-ald Joyce, a prominent researcher in the origin-of-life ﬁ eld, was quoted in

Smithsonian magazine as joking that life could be deﬁ ned as “that which

is squishy.” Some scientists have even concluded that it is not possible to

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

Th 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 Th 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 a.d.) 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 left 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 Th 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 aft 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

Era of Science

Th e idea of spontaneous generation of life held sway for several ries Beginning in the sixteenth century, the principles of modern science were also gradually coming into existence All over Europe, educated men began to systematically investigate their observations of the natural world

centu-Tools such as the microscope were invented, and for the ﬁ rst time, it was possible to view a hidden world unavailable to the naked eye Even if they did not understand what they were observing, these early scientists real-

ized that plants were made of tiny partitions they called cells, for their

resemblance to the tiny residential cells of monks Th ey also observed

organisms they called animalcules, which they believed arose

spontane-ously in water aft er it sat for a few days in the laboratory

Figure 1.1 Cave paintings found in the Vézère Valley in southwestern France feature images of animals Discovered in 1940, they are believed to be about 17,000 years old

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10 THE EVOLUTION OF CELLS

Th en, in 1859, the French Academy of Science sponsored a

competi-tion for the best experiment to prove or disprove the idea of

spontane-ous generation Louis Pasteur, a prominent scientist of the time, set out

to disprove the idea His experiment has become a classic example of

what we know as the scientiﬁ c method, which forms the basis of all

modern science

Figure 1.2 Louis Pasteur used uniquely designed swan-necked ﬂ asks to

dispel the theory of spontaneous generation

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Pasteur started with two glass fl asks into each of which he poured a meat broth One fl ask had a straight neck; Pasteur bent the neck of the second fl ask into a curved shape Th e contents of both fl asks were then heated to a high temperature to kill any living matter in the broth Th e

fl asks were left 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) Aft 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 tiﬁ c method

scien-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 tional matter until rotating planets formed Shortly aft er Earth’s forma-tion, in an event known as the Giant Impact, Earth collided with another large object, resulting in the expulsion of a chunk of vaporized rock that gave rise to our moon Earth would have been a violent place over the next

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addi-12 THE EVOLUTION OF CELLS

billion or so years as it underwent frequent collisions with asteroids and

comets, periods of extreme heat, and volcanic eruptions

Gradually, the Earth cooled, and the oceans, a surface crust, and rounding atmosphere formed, although the timing of these events remains

sur-controversial Th e chemical content of the oceans, rocks, and atmosphere

would have been critical for the ﬁ rst formation of life, but these also

remain uncertain It is likely the oceans were highly acidic, with elements

such as iron and calcium dissolved in them Th e atmosphere would have

contained carbon dioxide and hydrogen-containing compounds such as

methane and ammonia, but little oxygen

SEARCHING FOR THE FIRST CELLS

However or whenever the ﬁ rst life may have originated, many scientists

agree that life existed on Earth by about 3.8 billion years ago, relatively

early in the violent life of a new planet Several scientiﬁ c approaches are

IS THERE LIFE OUT THERE?

The presence of life somewhere else in the universe is not just the stuff of science ﬁ ction Astrobiology is the branch of science that stud-ies the possibility of life elsewhere in the universe Astrobiologists also explore the chemistry of interstellar space, the habitability of planets, and evolutionary biology of Earth’s organisms

After much searching through astronomical data, astronomers in

2010 found what may be a “Goldilocks” planet, so named for its ence of conditions suitable for the existence of life Until this planet’s discovery, only Earth was regarded as having ideal conditions—“not too

pres-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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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 Th ese are organisms that survive in extreme 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

condi-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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a deep sea hydrothermal vent Essential chemicals could have been

pres-ent at such a location, and the vpres-ent would have provided energy to fuel

chemical reactions DNA sequencing information indicates that a

com-mon ancestor of various life forms was likely a microorganism living in

extremely high temperatures

Th e oldest fossils that record the ﬁ rst evidence of life were discovered

in western Australia in formations called stromatolites Th ese layered

for-mations were produced by the actions of early cyanobacteria and consist

of calcium carbonate which precipitated over the growing mats of

bacte-rial ﬁ laments Stromatolites continue to form oﬀ the Australian coast, and

the bacteria that form them are being closely studied by scientists of the

Australian Centre for Astrobiology Th e great diversity of bacteria found

Figure 1.4 Stromatolites dot a nature reserve in Shark Bay in western

Australia This mineralized microbial community formed from cyanobacteria

build up over the last 4,000 years

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in these present-day stromatolites suggests that the fossil stromatolites may have been formed by organisms suﬃ ciently evolved to develop such diversity If this is the case, it puts the ﬁ rst appearance of life on Earth earlier than previously thought

STANLEY L MILLER,

A PIONEERING GRADUATE STUDENT

In 1951, Stanley L Miller, a new graduate student in chemistry at the University of Chicago, attended a lecture that ﬁ red his imagination The lecturer, Professor Harold C Urey, proposed an outlandish idea that it might be possible to produce building blocks of organic molecules in the laboratory by simulating conditions of early Earth Miller was ready

to take him up on the idea At ﬁ rst, Professor Urey discouraged him from undertaking such an experiment, claiming it was much too risky

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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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

was conducted by Stanley L Miller and Harold C Urey, researchers at the

University of Chicago in 1951 Th eir experiment, which attempted to

cre-ate amino acids—a basic component of cells—from compounds known

to exist on early Earth, changed the scientiﬁ c approach to the search for

the origin of life As more has been learned about the environmental

con-ditions on early Earth, scientists have conducted experiments that more

accurately replicate those conditions

Nobel-laureate Jack Szostak, geneticist and co-director of the gins of Life Initiative at Harvard Medical School, heads a team of scien-

Ori-tists who seek to understand how life could have arisen spontaneously

from the chemicals and conditions of early Earth By exploring novel

chemical systems in laboratory conditions, they can learn about possible

pathways that could have led to the formation of a primitive cell One

by one, Szostak and his fellow scientists have tackled tough questions

about how life may have formed from non-living compounds: Which

came ﬁ rst, proteins to carry out cellular functions or the genetic

mate-rial to produce the proteins? Where did the essential element

phospho-rus come from, since it was thought to be unavailable in a water-soluble

form? What sources of energy could have set oﬀ the chemical reactions

required to form complex cellular compounds from simpler molecules?

Which came ﬁ rst, a reproducible genetic system or the membrane to

enclose it? Furthermore, if a membrane existed, how were essential

nutrients able to pass through it?

Using chemical compounds that are thought to have been present on

early Earth, Szostak’s team has assembled what they call a protocell Th ese

protocells have a double-layered spherical membrane made up of fatty

acids, resembling membranes of present-day organisms Since fatty acids

are hydrophobic (unable to mix with water), the membrane maintains its

identity, separate from the surrounding water-soluble compounds Small

nucleotide molecules can enter the sphere, where they join with other

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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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 pounds: DNA for replication and storage of genetic information, RNA for various functions, and proteins that serve as the workhorses of the cell

com-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? Th 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 Th 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 Th 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 Th 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 carry out essential cell functions Th is discovery of the primary role of RNA led scientists to refer to the world in which life ﬁ rst developed as the

RNA world.

LIFE IN AN RNA WORLD

Scientists have found another reason to think that life evolved in an RNA world through their study of the structure of modern cells Proteins are

assembled within our cells by large molecular complexes called

ribo-somes Th ese complexes contain both RNA and protein components

Recent biochemical analysis indicates that the mechanism for protein assembly is catalyzed by the RNA portion of the complex, not protein

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Th us, ribosomes within each of our cells carry this “fossil” evidence that

life developed in an RNA world

Th e other essential role of RNA in primitive cells was to replicate itself

in order to produce new cells Without this ability of RNA to replicate

itself, life could not exist As more and more cells were produced through

this replication process, some variation would have existed in how well

the molecules could copy themselves Th is variation opened the door for

the process of natural selection—better replicators would produce more

oﬀ spring cells Over time, RNA molecules with superior copying ability

would have dominated, leading to a population with stable and eﬃ cient

replicating capability

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What was it about the conditions on early Earth that made life possible?

To be sure, life might have developed under other conditions, and sisted of diﬀ erent chemical elements Yet the forms of life that did develop

con-on Earth are dependent con-on the element carbcon-on, which happens to have chemical properties that are ideal for the requirements of cellular func-tions Th is knowledge involves an understanding of organic chemistry, the branch of chemistry associated with carbon-containing compounds

Th 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 shift 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

of Life

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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 Th 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

Th e next time you eat a good meal, consider that your body’s billions of cells

count on the energy your food provides them What follows is an

explora-tion of some basic principles of organic chemistry and an explanaexplora-tion how

they have contributed to the development of cells of increasing complexity

CARBON, THE BACKBONE OF LIFE

Cells are composed of a huge variety of complex molecules containing

carbon Why carbon? Th e elemental structure of carbon makes it ideal for

ARSENIC:

COMPATIBLE WITH LIFE?

Six chemical elements provide the basis for the molecules of life: bon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus Can arsenic be added to that list? It is common knowledge that arsenic is toxic to life In fact, it has been a favorite murder weapon for centuries

car-Astrobiologist Felisa Wolfe-Simon, working with the U.S Geological Survey, noted the chemical similarity between the elements phosphorus and arsenic She wondered if there might not be an organism able to substitute arsenic for the phosphorus that is contained within DNA of all known life forms

She set out to look for such an organism in the arsenic-rich waters

of Mono Lake in California Among the microorganisms she found there was one that goes by the unglamorous name of GFAJ-1 The organism grew in the laboratory, even when the concentration of arsenic was gradually increased If Dr Wolfe-Simon’s evidence that the bacteria are incorporating arsenic into their DNA is conﬁ rmed by other researchers, her discovery would represent a milestone in our knowledge about con-ditions that make life possible

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The Chemistry of Life 21

forming strong chemical bonds with other carbon atoms and also with other chemical elements Th e versatility of carbon bonds allows organic molecules to take on such diverse forms as the long chains that make up fatty acids, the ring structures that form the basis of sugar compounds, and even gaseous molecules such as carbon dioxide Th 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

“left -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 left -handed forms of amino acids Biochemical reactions

F IGURE 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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are highly precise in the way that complex molecules must ﬁ t together

and are dependent on the particular right- or left -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 chemiBiochemi-cal bonds that form or break

between electrons of nearby atoms For two complex molecules to interact,

their shapes must ﬁ t together precisely in order to bring electrons of the

appropriate atoms close enough to interact

THE FOUR MOLECULES OF LIFE

Every cell can be thought of as a miniature factory that takes in raw

mate-rials and processes them to manufacture some product needed by the cell

or for the larger organism of which it forms a part Th e “loading dock”

of the factory, or cell membrane, is highly selective, taking in only the

required raw materials Th e “command center,” or cell nucleus, issues

orders regarding what products the cell will manufacture, and various

transport systems convey materials around to the cell’s “assembly lines,”

or organelles Th e products are then sent out through the cell membrane,

along with waste accumulated during processing Th ese diverse cell

func-tions are carried out by four primary types of organic molecules: nucleic

acids, proteins, carbohydrates, and lipids Th ese are oft en referred to as

macromolecules because of their large size, and because they are typically

composed of many smaller molecules chemically bonded together

Nucleic acids

If cellular molecules had an “all-star” list, DNA and RNA would be at the

top Th ese molecules have become so familiar to those who study science

that we rarely take the time to call them by their full chemical names,

deoxyribonucleic acid and ribonucleic acid Th ese highly important

nucleic acids (NAs) direct a cell’s production of proteins and also store

its genetic code Although the molecules form long, complex chains, their

chemical structures are fairly easy to understand because they consist of

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repeated modules called nucleotides Each nucleotide molecule is formed from three building blocks: a sugar unit, a phosphate unit, and a base unit

In DNA molecules, the sugar unit is named deoxyribose (D), while ribose

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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(R) serves as the sugar unit in RNA Th 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) Th 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 Th 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-ture of DNA Chemical bonding limits the combinations of base pairs to

either adenine + thymine or guanine + cytosine (or thymine + adenine

and cytosine + guanine) Despite this limited number of base pair types,

the coded information they can convey is almost unlimited because there

are so many of them and because of the stable helical structure in which

they are bound

Proteins

Proteins form another of the macromolecule groups that take a variety of

complex forms, but they are made up of simple modular structures that

are bonded tightly together Th ese chemical modules are amino acids

Th e basic structure of an amino acid consists of a carboxyl group of

car-bon, oxygen, and hydrogen that is linked to an amino group consisting of

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nitrogen and hydrogen Attached to the carboxyl-amino group is another group, which is oft en referred to as a “side group” or an “R group.” Th is side group can take several forms and is what makes each amino acid distinctive Human cells require 20 amino acids for the manufacture of essential proteins Some of these amino acids can be synthesized by adult human cells and others must be obtained from diet

Proteins are produced in special organelles within cells Th is tion process is based on coded information transmitted by RNA mol-ecules Initially, they take the shape of long, straight chains of amino acids Once a chain is formed, it begins to twist and fold on itself, forming chemical bonds as certain parts of the chain come into close proximity

produc-Proteins take thousands of roles in the human body, for example, to port the skeleton, control the senses, digest food, move muscles, and pro-tect against disease Th e three-dimensional structure of proteins and how they perform so many cellular functions is an important area of biological research

sup-Enzymes are a large group of proteins that act as catalysts for chemical reactions Catalysts function by coming in close contact with other molecules and accelerating a biochemical reaction, but they are not changed in the process Enzymes are chemically shaped to have active sites that ﬁ t precisely into the chemical molecules with which they are designed to interact, rather like a lock and key arrangement

bio-Carbohydrates

Carbohydrates provide an energy source for cellular functions, and also make up the stiﬀ cell walls of plants in the form of cellulose Sugar mol-ecules, with their ring structures of carbon, hydrogen, and oxygen atoms, form the chemical building blocks for carbohydrates Some complex carbohydrates consist of hundreds of sugar molecules, which are linked either into a chain or branched structure Carbohydrates store energy in their many carbon-hydrogen bonds

Lipids

Lipids are fats and fat-like substances Th e most important quality of lipids that distinguishes them from other organic molecules is that they are not water soluble Th is makes them ideal materials for cell membranes, which separate the watery cell interior from its environment Lipid molecules are made up of chains of fatty acids, and they are chemically very eﬃ cient at storing energy Lipid molecules are capable of storing about twice as much

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energy as carbohydrate molecules, on a weight basis Long-term energy

reserves in animals are stored throughout the body in adipose tissues

Th ese tissues consist of specialized cells capable of storing large amounts

of lipid molecules

POWERING THE CELL’S CHEMICAL

REACTIONS

Cell Energy on Early Earth

Everyone knows that humans require food to fuel their bodies Even so, it

is quite a stretch to think about how a peanut butter sandwich one ate for

lunch provides the energy for biochemical reactions taking place within

our cells Yet that is exactly what happens Every cell requires energy,

and the ultimate source of that energy is the sun Plant cells, through

the process of photosynthesis, are able to convert energy from the sun

into stored chemical energy Animal cells do not have this capability and

must acquire biochemical energy by either eating plants or eating animals

that eat plants Th e body’s multi-step digestive process uses enzymes that

can break food molecules apart in order to extract energy stored in their

chemical bonds Th is process of extracting energy through the breakdown

of a complex molecule is referred to as catabolism In a reverse process,

called anabolism, cells use energy to synthesize new molecules

Metabo-lism refers to all the chemical reactions taking place within cells by which

complex molecules are broken down in order to release energy, or by

which energy is consumed to build up complex molecules

Th e complex energy pathways used by present-day cells were not able to the ﬁ rst cells on early Earth Yet those simple cells also would have

avail-required an energy source Most likely, the ﬁ rst cells ﬂ oated in an ocean

“soup” that contained assorted bits of organic matter Th e cells would have

taken in small glucose molecules and extracted energy from them Since

oxygen was not available in the early atmosphere, it is thought that early

cells used an ineﬃ cient anaerobic method (meaning it did not require

oxygen) to extract energy from the glucose molecules Th is process of

obtaining energy from glucose in the absence of oxygen is referred to as

glycolysis, which is a form of fermentation Many present-day organisms

still use this primitive method to provide for their energy needs Some

examples are the anaerobic bacteria that live in compost piles and the yeast

that live oﬀ the sugars in grape juice while converting it into wine Cells

that have developed more eﬃ cient metabolic processes retain the

capabil-ity to switch to this primitive method of deriving energy under certain

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THE GREAT OXIDATION EVENT

No one knows just when it occurred or what caused it, but time about 2.4 billion years ago, Earth underwent what geologists call the Great Oxidation Event At that time, the amount of oxygen in the atmosphere underwent a rapid buildup That availability of oxygen in the oceans and atmosphere played a key role in the further develop-ment of life forms The atmosphere of very early Earth contained little oxygen, and could certainly not have supported many present-day life forms that depend on oxygen Studies of some of Earth’s oldest rocks, obtained from rock core samples, reveal that a signiﬁ cant amount of oxygen began to appear in either Earth’s oceans or atmosphere from

some-50 to 100 million years before the Great Oxidation Event It is believed that early photosynthetic organisms began to release oxygen into the atmosphere at an even earlier time If so, why then was there not a grad-ual buildup in atmospheric oxygen rather than a sudden increase? One possibility is that the Great Oxidation Event coincided with a sudden shift from undersea volcanic eruptions to terrestrial volcanoes Gases from underwater volcanoes may have reacted with oxygen emitted from single-celled photosynthetic organisms fl oating in the oceans, prevent-ing a buildup of oxygen When volcanoes shifted to land forms, their escaping gases would not have tied up oxygen in the same way, allow-ing oxygen to accumulate in the atmosphere Some evidence suggests that geologic changes resulted in a rapid decrease in the ocean’s nickel content, leading to a decrease in organisms whose waste products had prevented the accumulation of oxygen Another possible explanation is that dissolved iron in the oceans reacted with oxygen to produce miner-als that sank to the ocean’s fl oor At some point, all the available iron was used up and oxygen fl oated free up into the atmosphere

However the sudden build-up of oxygen occurred, some of it verted to an alternate oxygen form called ozone Ozone absorbs ultravio-let rays Over time, a layer of ozone collected and began to shield Earth’s surface from the ultraviolet radiation of the sun This protective layer paved the way for the advent of more diverse life forms Whatever caused the shift in Earth’s atmospheric oxygen, one thing is certain Its presence was crucial for the development of more efﬁ cient metabolic processes and the switch to complex life forms

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con-28 THE EVOLUTION OF CELLS

conditions For example, when human muscle cells are stressed, they can

switch for a short time to glycolysis to provide an extra boost of energy

New Cell Energy Sources

With so many organisms gobbling up organic matter that made up the

ocean “soup” of early Earth, food became scarce Fortunately for the

continuation of life, two major changes in cells’ ability to obtain energy

paved the way for new forms of life First, some cells invented a new way

to essentially produce their own food, by using energy from the sun Th e

second major advance occurred as oxygen became more plentiful Some

cells devised a much more eﬃ cient metabolic process that depended on

the presence of oxygen

As long ago as 3 billion years ago, some cells developed the ability to derive energy from the light of the sun by a process we know as photo-

synthesis Aft er another few million years, some of these cells devised a

metabolic process that resembled that of today’s plants Using the sun’s

energy, plus carbon dioxide and water from the environment, these

primi-tive cells could produce glucose and oxygen Oxygen was a waste

prod-uct discharged into the atmosphere, while glucose remained in the cell to

provide stored energy Eventually, other cells that lacked photosynthetic

capability devised alternate chemical pathways that incorporated oxygen

from the atmosphere along with a wider variety of organic energy sources

Th is led to the eventual development of the cellular respiration

methods on which much of present-day life depends Respiration can be

thought of as a controlled burning process, really just an extension of the

process that keeps us warm when we burn wood in a ﬁ replace Organic

molecules, whether stored in the molecular structure of wood or in the

bread we eat, combine with oxygen, releasing the energy tied up in their

chemical bonds Carbon dioxide and water are waste products of the

res-piration process

Whatever a cell’s primary energy source, the cell also requires a anism to store and transport that energy for its essential chemical reac-

mech-tions Almost all cells use a molecule called adenosine triphosphate, or

ATP, for this purpose ATP, while central to the biochemistry of life as

we know it today, has been nicknamed a “molecular fossil” because it is

thought to have arisen very early in the history of life Biologists oft en refer

to ATP as the energy “currency” of the cell for its ability to move about the

cell, collecting and dispensing energy A cell can store energy by hooking

electrons onto the molecule, aft er which the ATP molecule hauls its load

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of chemical energy to a site in the cell that requires it Th ere, the electrons are unhooked, releasing (or “spending”) their stored energy Th e triphos-phate portion of the ATP molecule consists of a chain of three phosphate groups; the bonds joining them are rich in potential energy Remember that peanut butter sandwich? Th e energy it contains ends up being stored

in chemical bonds of ATP

Sometimes the process of cellular respiration is confused with the process of breathing that takes place in our lungs How are they related?

Cellular respiration is the process of breaking down sugar molecules to produce energy-rich ATP molecules, releasing carbon dioxide and water

in the process Th e simplest animals rely on a process of exchanging gases with the outside environment through their cell surfaces As animals evolved over millions of years, specialized cells and tissues developed to facilitate the internal exchange of gases, a process that takes place in the gills of ﬁ sh and the lungs of mammals Human lungs play an essential role

in the process of cellular respiration by providing the required oxygen and removing the waste carbon dioxide

Respiration is a complementary process to photosynthesis, and together, they are the basis of Earth’s ecosystems: Plants require carbon molecules in the form of carbon dioxide to produce carbohydrates, and animals require the energy incorporated into the structure of carbon mol-ecules in carbohydrates to provide their energy needs Th is carbon cycle keeps carbon atoms endlessly on the move as they cycle through every form of life on Earth

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Th e impact of human beings is so dominant everywhere on Earth that

it is diﬃ cult to imagine their absence Yet for the ﬁ rst few billion years,

much diﬀ erent organisms “ruled” and were responsible for huge changes

on our planet Similar organisms still exist on Earth in simple cell forms

called prokaryotes Th ey evolved from the ﬁ rst cells that appeared on

Earth, and in the blink of a cosmic eye, they popped up everywhere

Early prokaryotes appeared in the tidal pools and shallow oceans of

those geologically turbulent times With their simple unicellular

struc-tures, they proved amazingly adaptable to any environmental

condi-tion As Earth’s climate and surface changed over eons of geologic time,

prokaryotes evolved to ﬁ ll every niche, from the ocean depths to dry

mountaintops

Prokaryotes have persisted since their ﬁ rst emergence billions of years ago, and today these single-celled organisms are found just about

everywhere Th ey make up two of the three domains into which

sci-entists classify all of life (the third domain is eukaryotes.) Bacteria

comprise the largest and most familiar group of prokaryotes A second

group of less familiar, but fascinating, prokaryotes is called archaea, so

named because they stem from an archaic form of life

Prokaryotes:

The Simplest Cells

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Prokaryotes: The Simplest Cells 31

PROKARYOTE CELL STRUCTURE:

SIMPLE BUT CAPABLE Prokaryote Evolution

No one knows exactly how cells evolved from their ancient “RNA world”

to the double-helix DNA that serves as the control structure for all current life forms Th e best evidence indicates that those ﬁ rst cells survived with the help of multitasking, RNA-like molecules Th ese molecules would have carried the cell’s genetic information and performed routine cell func-tions, in addition to controlling replication of the cell By chance, some of

Figure 3.1 All organisms living on Earth today are divided into three broad domains Scientist are still working to understand the three domains and how they relate

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the molecules would have been better replicators than others, and the ones

that did a better job of copying themselves would have produced more

oﬀ spring Over countless generations of cells, those with small

molecu-lar variations that improved their ability to replicate themselves would

have predominated, until eventually one of them happened to give rise

to a double-stranded RNA molecule resembling what we know as DNA

Th ese “super-RNA” molecules were likely larger, allowing more genetic

information to be transmitted, and their two-stranded form would have

provided greater chemical stability Cells governed by these “super”

mol-ecules would have had a huge evolutionary advantage, and it is no wonder

that they soon took over all life forms on Earth

Basic Cell Structure

However the ﬁ rst DNA molecule evolved, DNA makes up the genetic

mate-rial of all present-day prokaryote cells Within their simple structures,

prokaryotes possess all the biochemical machinery required for growth,

reproduction, and use of energy An outer cell wall provides structure

and protection from the environment Next comes a plasma membrane,

which serves as a critical selective barrier for the cell, allowing passage of

essential food molecules and gases Th is membrane surrounds cytoplasm,

a semi-liquid material that holds all the molecules necessary for

carry-ing out metabolic reactions Th e deﬁ ning characteristic of prokaryotes is

the absence of any interior membranes to separate working units Genetic

information is on a single circular DNA molecule, which is attached to

the outer membrane but is in direct contact with the cytoplasm (the name

prokaryote comes from Greek meaning “before nucleus”) Proteins are

manufactured within the cytoplasm by means of a structure called a

ribo-some, which is a molecular complex of RNA and proteins

Specialized Structures

Some prokaryotes contain additional DNA in the form of small circular

molecules called plasmids Genetic material can be shared from cell to cell

by the movement of plasmids through cell walls Th is “sharing” of

infor-mation conveys a survival advantage to the cells by, for example,

transfer-ring a cell’s capability to resist antibiotics Genetic engineetransfer-ring makes use

of these plasmids and their ability to pass through cell walls to introduce

new genetic material into a cell (see Chapter 9) Many prokaryotes also

contain specialized structures that allow them to live in a wide variety

of environments Photosynthetic prokaryotes contain pigments that allow

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them to capture energy from the sun; gas bubbles allow some prokaryotes

to ﬂ oat in the ocean environment; others have tiny iron deposits that help

them orient to the Earth’s magnetic ﬁ eld; a few have appendages called ﬂ

a-gella that allow them to navigate within a liquid environment Some cells

have molecular capsules around their walls that act as protective coating

Figure 3.2 Unlike eukaryotic cells, prokaryotic cells have no nucleus

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Th e human immune system is thwarted by the capsules that surround

cer-tain infectious bacteria Prokaryotic cells are quite small, and their shape

may be rodlike, spherical, or spiral

ARCHAEA: OLD LIFE, NEW DISCOVERY

Th e archaea comprise a surprising group of prokaryotes from which

sci-entists are learning much about the evolution of cells and organisms Why

is this surprising? First, because even though archaea have been present

on Earth for billions of years, they were unrecognized by science until the

1970s Second, because scientists continue to be surprised by discoveries

of their existence in extreme life conditions

Biologists have been aware of the existence of archaea for many years but always regarded them as bacteria, which they closely resemble By

the 1970s, however, molecular biology had advanced to the point that

researchers were able to sequence the nucleic acids within organisms

Bac-teriologists used these techniques to sequence bacterial RNA in an eﬀ ort

to better understand the relationships among the many varieties of

bac-teria Professor Carl Woese and his colleagues, working at the University

of Illinois, were surprised to ﬁ nd that the RNA of one group of organisms

did not at all resemble that of other bacteria Th ese organisms were diﬀ

er-ent in their physical characteristics as well, suggesting that they had arisen

by a much diﬀ erent evolutionary pathway than had given rise to the

bac-teria Woese originally suggested the name archaebacteria for this newly

recognized group, but later shortened it to archaea in order to clarify their

distinction from true bacteria

For an organism whose existence has only been known for less than half a century, and one that many people have never heard of, it is also sur-

prising to learn that archaea exist everywhere! Many of them are

anaero-bic; in other words, they do not require oxygen and may even ﬁ nd it toxic

Th ey are common inhabitants of swamps, sewage, and the intestines of

cows Archaea species have been found living in conditions that are too

acidic or too alkaline, or too contaminated with toxic metals, to sustain

other life forms In view of their tolerance for extreme conditions, it is

quite believable that their ancestors would have thrived in the much

dif-ferent environmental conditions of early Earth

BACTERIA SHAPED OUR WORLD

From our vantage point at the top of the tree of life on Earth, it seems only

natural that the tree has grown and branched until the diversity of life

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reached its present form However, evolutionary biologists point out that there were many junctures in the tree’s growth at which life could just as easily have been snuﬀ ed out As organisms proliferated in the local eco-systems that existed billions of years ago, the dominant life forms used

up most of the raw materials on which they depended for survival Th ey also contaminated the environment with their waste products Fortu-nately for the continuation of life, new organisms evolved that could adapt to this ﬁ rst “food crisis,” extracting energy from whatever organic molecules prevailed Th is led to interdependent ecosystems of bacteria,

in which the metabolic waste products of some organisms served as food for others Th e ability of bacteria to swap genes with each other allowed them to adapt quickly to changing environmental conditions It did not

THE HIDDEN TREASURES OF YELLOWSTONE NATIONAL PARK

Most people visit Yellowstone National Park to see the geysers, hot springs, or maybe a grizzly bear However, microbiologists ﬂ ock there because they hope to unlock the secrets of microorganisms that live

in the extreme conditions of the park’s hot springs The water in some

of those springs is near boiling temperature and so acidic that it could dissolve nails Certainly no one would ever be tempted to go for a swim

in the springwater pools! In fact, most organisms could not survive in such a hostile environment, yet these waters are teeming with microbial life that not only survives, but thrives there Visitors who stop to admire the vivid and otherworldly colors of the springs are often unaware that they are looking at colorful microorganisms Microbiologists are study-ing an assortment of archaea and bacteria that have adapted to these conditions Usually, these organisms have developed resistant cell walls and unique enzymes that can function under such extreme conditions

Research in this area is difﬁ cult because it is nearly impossible to vide the necessary environment to grow the organisms in a laboratory

pro-Still, learning more about these organisms may provide insights into how life developed on Earth, or whether it may exist on other planets

Further ﬁ ndings may also have applications in medicine, industrial cesses, and environmental controls

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pro-36 THE EVOLUTION OF CELLS

take long for an amazing variety of bacteria to colonize every corner of

the planet

Th e oldest fossils yet discovered were left by cyanobacteria that

lived perhaps 3.5 billion years ago Th ese ﬁ rst cyanobacteria evolved to

use carbon dioxide and hydrogen as their food source, plus energy they

derived from sunlight Th ey gave oﬀ oxygen as a waste product As this

oxygen built up in the ecosystem, it opened the way for bacteria whose

LIFE IN AN UNDERWATER FANTASY WORLD

Black “smoke” belches from chimneys towering high overhead Clumps

of colorful giant tubeworms rise from the sea ﬂ oor Fantastical rock formations surround you Your grip on reality begins to slip Have you time-travelled back billions of years? Are you on an alien planet? Surely, this sight cannot exist on twenty-ﬁ rst century Earth Nevertheless, to a scientist who has descended deep under the ocean in a tiny submarine

to study new forms of life, the sight is both real and truly amazing

Two hundred miles (321.8 kilometers) off the coast of ton and Oregon, a ridge of undersea volcanic mountains rises along

Washing-an area where tectonic plates intersect To reach the area, scientists must climb into a 7-foot (2.1-meter) submarine and descend 1.4 miles (2.2 km) beneath the Pacifi c Ocean surface There, they can view this exotic environment and collect biological specimens The underwater mountain scenery is like nothing else on Earth One scientist describes the experience as like being in a time machine, witnessing what life may have been like 3 billion years ago Giant thermal vents, sometimes referred to as “black smokers” for the black fumes that spew from their tops, rise up from the sea fl oor When hot fl uids rising through the vents meet the cold, salty water, metal compounds solidify to form porous rocky deposits around the vents Bacteria thrive in this environment by synthesizing organic nutrients, using energy derived from chemicals such as hydrogen sulfi de These bacteria in turn form the basis of a food chain for an unusual community of organisms, including giant tube-worms, deep-sea crabs, and mussels

Back in their laboratories, scientists study the specimens collected

at the site and seek to understand the metabolic processes of these

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metabolism depended on respiration Th e early era in the evolution of bacteria also saw the development of organisms that could convert to alternate forms with hard shells Th ese protective capsules allowed an organism to remain inactive for some time until, by chance, it blew or

ﬂ oated to a place with a more hospitable environment

Th e land and water ecosystems of present-day Earth are highly dent on the constant activity of bacteria Th e atmospheric carbon dioxide

depen-unusual organisms In addition to providing insights into how life may have developed on Earth or on other planets, some of the bacteria may eventually prove useful as sustainable sources of chemicals or biofuels

F IGURE 3.3 A view from a deep-submergence vehicle shows black “smoke” being emitted by the Saracen’s Head hydrothermal vent on the ocean floor Known as

a “black smoker,” Saracen’s Head pours out a sulfurous mineral-rich fluid from its chimney mound The volcanic fluid bubbles up at a temperature of 360 degrees Celsius due to geothermal energy in the Earth’s crust Deep sea vents provide an unusual habitat to some primitive forms of extremophile bacteria

and deep sea crabs (at lower center) Saracen’s Head vent is located in the

Mid-Atlantic Ridge about 10,170 feet (3,100 meters) below sea level.

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on which plants depend would quickly be bound up as unavailable

car-bon if it were not for the action performed by bacteria to decompose dead

organisms Plants are dependent on nitrogen compounds in the soil and

cannot derive them from atmospheric nitrogen Certain bacteria (known

as nitrogen ﬁ xers) use gaseous nitrogen in their metabolic processes,

releasing it to soils in forms that plants can use Th e constant action of

unseen bacteria keeps carbon and nitrogen available for the synthesis of

proteins and nucleic acids on which all life depends

CLASSIFICATION OF BACTERIA

How many bacteria are there? Certainly too many to count Despite

their small size, there are so many of them that they make up much

of the biomass on Earth Th ey even outnumber us, in terms of cells, in

our very own bodies! How does one begin to get a handle on a group

of organisms that exists in such great number and variety? For many of

us, if we even think about bacteria, it may only be in terms of whether

they can make us sick (pathogenic bacteria) or not (non-pathogenic

bacteria) Yet it is essential that we have a more systematic way to

orga-nize and identify all of them because they play such an important role

in almost every aspect of human life Several methods of classifying

bacteria exist, depending on which aspect of bacterial activity is being

studied

Shape

Th e shape of a cell aﬀ ects its ability to survive in its environment Bacteria

with a variety of shapes have evolved to meet the challenges of the many

diﬀ erent environments they inhabit For example, having less surface area

allows an organism to better withstand dry conditions Many bacteria

have a spherical shape, which provides the least amount of surface area

in proportion to volume Th ese organisms are called cocci (singular,

coc-cus) However, having a greater surface area allows more nutrients to be

taken in through the cell wall Th erefore, other bacteria evolved with an

elongated rod shape to provide additional surface area to take in less

plen-tiful nutrients Bacteria with this shape are called bacilli Some bacteria,

called spirillum, acquired a corkscrew shape to give them less resistance

as they moved through a liquid environment A few bacteria, known as

ﬁ lamentous forms, take on elongated shapes and may grow into complex,

branched ﬁ laments

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Cell Wall Structure

A rapid laboratory method used for identifying bacteria takes advantage

of characteristics of an organism’s cell wall Hans C Gram, a century Danish physician, recognized that bacteria absorbed diﬀ erent amounts of dye depending on the thickness and chemical composition of their cell walls Types of bacteria with thick cell walls take up a lot of dye and look dark purple under a microscope; they are identiﬁ ed as Gram-positive Th inner-walled bacteria take up less dye and are Gram-negative

nineteenth-Th e combined classiﬁ cation of Gram stain and shape is useful in guishing several common infectious organisms

distin-Respiration

Bacteria that evolved in environments with available oxygen use it as a basis for their metabolism Th ese are referred to as aerobic bacteria Other

organisms, which do not depend on the presence of oxygen, are called

anaerobic Th ese organisms vary in the amount of oxygen that they can tolerate; some of them ﬁ nd even small amounts of oxygen toxic

Growth Factors

Scientists who study the role of bacteria in our natural environment are oft en most concerned with how a particular variety of bacteria obtains its energy and food Bacteria that depend on sources outside their own

bodies for organic compounds are called heterotrophs Most bacteria are

heterotrophs, such as those that break down dead organic matter in the

soil Autotrophs are able to provide their own food by changing inorganic

compounds into organic compounds Examples of autotrophs are the photosynthetic bacteria that use energy from the sun to convert carbon dioxide and water into a food source Bacteria that live near ocean vents are also autotrophs Th ey convert inorganic compounds discharged from the vents to use as a food source

Tiêu đề	The Evolution of Cells
Tác giả	Terry L. Smith
Trường học	Chelsea House, an imprint of Infobase Learning
Chuyên ngành	Cells and Life Science
Thể loại	Book
Năm xuất bản	2012
Thành phố	New York

Định dạng
Số trang	123
Dung lượng	46,05 MB