Are We Alone in the Universe? - Columbia Southern … · 30 Chapter 2 Are We Alone in the Universe? Figure 2.1 Homeostasis. Black-capped chickadees can maintain a core body temperature

Are We AlOne in the Universe? Are We AlOne in tH

Are We Alone inthe Universe?Water, Biochemistry, and Cells

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Does life existon Mars?

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Biology: Science for Life, Third Edition, by Colleen Belk and Virginia Borden Maier. Published by Benjamin Cummings. Copyright © 2010 by Pearson Education, Inc.

2.1 What Does Life Require? 30A Definition of Life • The Properties of Water • Organic Chemistry • Structure and Function of Macromolecules

2.2 Life on Earth 40Prokaryotic and Eukaryotic Cells • Cell Structure • The Tree of Life and Evolutionary Theory

Is there life on other planets? Scientists have found persuasive, if not

conclusive, evidence of life on Mars. When NASA scientist David McKay first pro-

posed that this cold, dry, rather harsh planet could harbor life, people were astounded.

The evidence of life found by Dr. McKay and his team did not in any way resemble

the cartoon images often used to depict Martians. Instead, what these scientists found

was evidence of life in a 3.6-billion-year-old, potato-size rock. They believe that the

rock had been ejected from the surface of Mars around 15 million years ago and

had traveled through space for nearly that entire time. Ultimately the rock

crashed to Earth, landing in Antarctica about 13,000 years ago and remaining

there until discovered by scientists in 1984. This meteorite, drably named

ALH84001, appeared to contain the same features that scientists use to

demonstrate the existence of life in 3.6-billion-year-old Earth rocks—there

were fossils, various minerals that are characteristic of life, and evidence of com-

plex chemicals typically produced by living organisms.

While many scientists debate the assertion that this rock provides evidence of life

on Mars, the announcement served to inject new energy into Mars exploration. Since

then, multiple robotic rovers and mapping satellites have been sent to the planet, and the

U.S. government announced an initiative to send astronauts to the red planet by the

2020s.While there are many reasons to explore Mars, the question that remains most

intriguing—and is a significant focus of several of these missions—is whether life

ever existed there.

The fascination about potential Martian life speaks to a fundamental

question that many humans share: Are the creatures on Earth the only living

organisms in the universe? Our galaxy is filled with countless stars and planets,

and the universe teems with galaxies. Even if we find no convincing evidence of

life on Mars, there are a seemingly infinite number of places to look for other liv-

ing beings. In this chapter, we discuss the characteristics and requirements of life

and examine techniques that scientists use to search the universe for other living

creatures.

Universe? Are We AlOne in the Universe? Are we alOne

A meteorite ejectedfrom Mars andfound in Antarcticamay contain lifelikeforms.

Will the Mars roverfind evidence of lifeoutside Earth?

Popular imagesaside, we know ofno other intelligentlife in our solarsystem.

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Chapter 2 Are We Alone in the Universe?30

Figure 2.1 Homeostasis. Black-cappedchickadees can maintain a core bodytemperature of 108°F (~42°C) duringthe day, even when the air tempera-ture is well below zero.

2.1 What Does Life Require?Because the galaxy likely contains billions of planets, scientists looking for lifeelsewhere seek to identify the range of conditions under which they would ex-pect life to arise.What is it that scientists look for when identifying a planet (ormoon) as a candidate for hosting life?

A Definition of LifeIn science-fiction movies, alien life-forms are often obviously alive and even some-what familiar looking. But in reality, living organisms may be truly alien; that is,they may look nothing like organisms we are familiar with on Earth. So howwould we determine whether an entity found on another planet was actually alive?

Surprisingly, biologists do not have a simple definition for a “living organ-ism.” A list of the attributes found in most earthly life-forms includes growth,movement, reproduction, response to external environmental stimuli, andmetabolism (all of the chemical processes that occur in cells, including thebreakdown of substances to produce energy, the synthesis of substances neces-sary for life, and the excretion of wastes generated by these processes). However,this definition could apply to things that no one considers to be living. For exam-ple, fire can grow, consume energy, give off waste, move, reproduce by sendingoff sparks, and change in response to environmental conditions. And some or-ganisms that are clearly living do not conform to this definition. Male mulesgrow, metabolize, move, and respond to stimuli, but they are sterile (unable to re-produce).

If we examine more closely the characteristics of living organisms onEarth, we will see that all organisms contain a common set of biological mol-ecules, are composed of cells, and can maintain homeostasis, that is, aroughly constant internal environment despite an ever-changing externalenvironment (Figure 2.1). The ability to maintain homeostasis requirescomplex feedback mechanisms between multiple sensory and physiologicalsystems and is possible only in living organisms. In addition, populations ofliving organisms can evolve, that is, change in average physical characteris-tics over time. If we search the universe for planets that could support lifesimilar to that found on Earth—and thus organisms that we would clearlyidentify as “living”—the list of planetary requirements becomes more strin-gent. In particular, an Earth-like planet should have abundant liquid wateravailable.

The Properties of WaterWater is a requirement for life. Al-though Mars does not currently appearto have any liquid water, ice is found atits poles (Figure 2.2a), and features ofits surface indicate that it once con-tained salty seas and flowing water(Figure 2.2b). The presence of liquidwater on Mars would fulfill an essen-tial prerequisite for the appearance oflife. But why is water such an impor-tant feature?

Water is made up of two elements:hydrogen and oxygen. Elements arethe fundamental forms of matter and

(a) Frozen water (b) Running water

Chemistry and Waterweb animation 2 1

Nucleic Acidsweb animation 2 2

Figure 2.2 Water on Mars. (a) This im-age from the Mars rover indicates thatfrozen water exists on Mars. (b) Thisphotograph, taken by the EuropeanMars Express orbiter, shows a channelon Mars that may have been formedby running water.

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Chapter Section 2.1 What Does Life Require? 31

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Figure 2.4 Polarity in a water molecule. Water is a polar molecule. Its atoms do not share electronsequally. Visualize This: Toward whichatom are the electrons of a water mole-cule pulled?

are composed of atoms that cannot be broken down bynormal physical means such as boiling.

Atoms are the smallest units that have the propertiesof any given element. Ninety-two natural elementalatoms have been described by chemists, and several morehave been created in laboratories. Hydrogen, oxygen, andcalcium are examples of elements commonly found inliving organisms. Each element has a one- or two-lettersymbol: H for hydrogen, O for oxygen, and Ca for cal-cium, for example.

Atoms are composed of subatomic particles calledprotons, neutrons, and electrons. Protons have a positiveelectric charge; these particles and the uncharged neutronsmake up the nucleus of an atom. All atoms of a particularelement have the same number of protons, giving the ele-ment its atomic number. The negatively charged electrons are found outsidethe nucleus in an “electron cloud.” Electrons are attracted to the positivelycharged nucleus (Figure 2.3). A neutral atom has equal numbers of protons andelectrons. Electrically charged ions do not have an equal number of protons andelectrons. In this case, the atom is not neutral and is instead charged.

The Structure of Water. The chemical formula for water is H2O,indicating that it contains two hydrogen atoms for every one oxygen.Water, like other molecules, consists of two or more atoms joined bychemical bonds. A molecule can be composed of the same or differentatoms. For example, a molecule of oxygen consists of two oxygen atomsjoined to each other, while a molecule of carbon dioxide consists of carbonand oxygen atoms.

Water Is a Good Solvent. Water has the ability to dissolve a widevariety of substances. A substance that dissolves when mixed with anothersubstance is called a solute. When a solute is dissolved in a liquid, such aswater, the liquid is called a solvent. Once dissolved, components of aparticular solute can pass freely throughout the water, making a chemicalmixture or solution.

Water is a good solvent because it is polar, meaning that different regions,or poles, of the molecule have different charges. The polarity arises becauseoxygen is more attractive to electrons, that is, it is more electronegative, thanmost other atoms, including hydrogen. As a result of oxygen’s electronegativ-ity, electrons in a water molecule spend more time near the nucleus of the oxy-gen atom than near the nuclei of the hydrogen atoms. With more negativelycharged electrons near it, the oxygen in water carries a partial negative charge,symbolized by the Greek letter delta, d2.The hydrogen atoms thus have a par-tial positive charge, symbolized by d+ (Figure 2.4).When atoms of a moleculecarry no partial charge, they are said to be nonpolar.

Water Facilitates Chemical Reactions. Because it is such apowerful solvent, water can facilitate chemical reactions, which are changesin the chemical composition of substances. Solutes in a mixture, calledreactants, can come in contact with each other, permitting the modificationof chemical bonds that occur during a reaction. The molecules formed as aresult of a chemical reaction are known as products.

Water molecules tend to orient themselves so that the hydrogen atom(with its partial positive charge) of one molecule is near the oxygen atom(with its partial negative charge) of another molecule (Figure 2.5a).The weakattraction between hydrogen atoms and oxygen atoms in adjacent moleculesforms a hydrogen bond. Hydrogen bonding is a type of weak chemical bond

NeutronAtomicnucleus

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Figure 2.3 Atomic structure. An oxygenatom contains a nucleus made up of 8protons and 8 neutrons. Orbiting elec-trons surround the nucleus. Althoughthe number of particles within eachatom differs, all atoms have the samebasic structure.

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that forms when a partially positive hydrogen atom is attracted to a partiallynegative atom. Hydrogen bonds can be intramolecular, involving different re-gions within the same molecule, or they can be intermolecular, between differ-ent molecules, as is the case in hydrogen bonding between different watermolecules. Figure 2.5b shows the hydrogen bonding that occurs between wa-ter molecules in liquid form.

Water Is Cohesive. The tendency of like molecules to stick together iscalled cohesion. Cohesion is much stronger in water than in most liquids as aresult of hydrogen bonding and is an important property of many biologicalsystems. For instance, many plants depend on cohesion to help transport acontinuous column of water from the roots to the leaves.

When heat energy is added to water, its initial effect is to disrupt the hy-drogen bonding among water molecules. Therefore, this heat energy can beabsorbed without changing the temperature of water. Only after the hydrogenbonds have been broken can added heat increase the temperature. In otherwords, the initial input of energy is absorbed.

The flowing water that was once found on Mars is now only in the form ofice. Until scientists can land on Mars and collect ice samples for analysis, its ac-tual composition is a matter of conjecture. However, images taken by a NASArover have led scientists to believe that some of the rocks on Mars were proba-bly produced from deposits at the bottom of a body of salt water. Salt water onMars is likely to be the same as salt water on Earth, a solution of water andsodium chloride.We know from surveying Earth’s oceans that salt water is hos-pitable to millions of different life-forms. In fact, most hypotheses about theorigin of life on Earth presume that our ancestors first arose in the salty oceans.

The ability of water to dissolve substances such as sodium chloride is a di-rect result of its polarity. Each molecule of sodium chloride is composed ofone sodium ion (Na+) and one chloride ion (Cl2). In the case of sodium chlo-ride, the negative pole of water molecules will be attracted to a positivelycharged sodium ion and separate it from a negatively charged chloride ion(Figure 2.6). Water can also dissolve other polar molecules, such as alcohol,in a similar manner. Polar molecules are called hydrophilic (“water loving”)because of their ability to dissolve in water.

Salts are produced by the reaction of an acid (a substance that donatesH+ ions to a solution) with a base (a substance that accepts H+ ions). Watercan break apart or dissociate into H+ and OH2 ions.The pH scale is a meas-ure of the relative amounts of these ions in a solution.The more acidic a solu-tion is, the higher the H+ concentration is relative to the OH2 ions. Hydrogenion concentration is inversely related to pH, so the higher the H+ concentra-tion, the lower the pH. Basic solutions have fewer H+ ions relative to OH2

ions and thus a higher pH (Figure 2.7). These ions can react with other

Hydrogen bond

Oxygen Hydrogen

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(a) Bonds between two water molecules (b) Bonds between many water molecules

Figure 2.5 Hydrogen bonding.Hydrogen bonding can occur whenthere is a weak attraction between thehydrogen and oxygen atoms between(a) two or (b) many different watermolecules.

NaCl (Salt) H2O (Water)

Saltwater

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Figure 2.6 Water as a solvent. Whensalt is placed in water, the negativelycharged regions of the water mole-cules surround the positively chargedsodium ion, and the positively chargedregions of the water molecules sur-round the negatively charged chlorineion, breaking the bond holding sodiumand chloride together and dissolvingthe salt.

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charged molecules and help to bring them into the water solution. At anygiven time, a small percentage of water molecules in a pure solution will bedissociated. There are equal numbers of these ions in pure water, and so it isneutral, which on the pH scale is 7.The pH of most cells is very close to 7.

Nonpolar molecules, such as oil, do not contain charged atoms and are re-ferred to as hydrophobic (“water fearing”) because they do not easily mix withwater.

When a European Space Agency probe landed on Titan, one of Saturn’smoons and a place where the chemical composition of the atmosphere may besimilar to that found on early Earth, the photos transmitted by the probe indi-cated that liquid was present on the surface of this bitterly cold place. Atatmospheric temperatures of approximately 2292°F (2180°C) the liquid isobviously not water; instead, it is most likely a mixture of ethane and methane,both nonpolar molecules. As a result, oceans on Titan are much poorer sol-vents than are oceans on Earth, and conditions in these oceans are probablynot suitable for the evolution of life.

Organic ChemistryThe Martian meteorite ALH84001 had one characteristic that provided someevidence that the rock once contained living organisms—the presence of com-plex molecules containing the element carbon.

All life on Earth is based on the chemistry of the element carbon. Thebranch of chemistry that is concerned with complex carbon-containing mole-cules is called organic chemistry.

Overview: Chemical Bonds. Chemical bonds between atoms andmolecules involve attractions that help stabilize various configurations. Ingeneral, this involves the sharing or transfer of electrons. One of the mostimportant elements in biology is carbon, which is often involved in chemicalbonding because of its ability to make bonds with up to four other elements.Like a Tinkertoy™ connector, carbon has multiple sites for connections thatallow carbon-containing molecules to take an almost infinite variety of shapes(Figure 2.8).

Figure 2.7 The pH scale. The pH scale is a measure of hydrogen ion concentration rang-ing from 0 (most acidic) to 14 (most basic). Each pH unit actually represents a 10-fold(103) difference in the concentration of H+ ions. Visualize This: A substance with a pH of5 would have how many times more H+ ions than a substance with a pH of 7?

Figure 2.8 Carbon, the chemical Tinkertoy™ connector. Because carbon forms fourcovalent bonds at a time, carbon-containing compounds can have diverse shapes.

Methane (CH4)

Carbon dioxide (CO2) Glucose (C6H12O6)

Carbon:The key chemicalTinkertoy® connector

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H

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Figure 2.9 Covalent bonding. (a) Car-bon has 4 unpaired electrons in its valence shell and is thus able to makeup to 4 chemical bonds. (b) Methaneconsists of carbon covalently bondedto 4 hydrogens.

Stop and Stretch 2.1: What does not make sense (chemically) aboutthe structure below?

H2C5C2H

A Closer Look: Chemical BondsThe ability of elements to make chemical bonds depends on the atom’s electron configuration.The electrons in the electron cloud that surrounds the atom’s nucleus have different energy levels based on their distance from the nucleus. The first energy level, or electron shell, isclosest to the nucleus, and the electrons located there have the lowest energy. The secondenergy level is a little farther away, and the electrons located in the second shell have a littlemore energy. The third energy level is even farther away, and its electrons have even more en-ergy, and so on.

Each energy level can hold a specific maximum number of electrons. The first shell holds 2electrons, and the second and third shells each hold a maximum of 8. Electrons fill the lowest en-ergy shell before advancing to fill a higher energy-level shell. For example, hydrogen with its 1electron needs only one more electron to fill its first shell.

Atoms with the same number of electrons in their outermost energy shell, called the valenceshell, exhibit similar chemical behaviors.When the valence shell is full, the atom will not normallyform chemical bonds with other atoms. Atoms whose valence shells are not full of electrons oftencombine via chemical bonds.

Atoms with 4 or 5 electrons in the outermost valence shell tend to share electrons to com-plete their valence shells. When atoms share electrons, a type of bond called a covalent bondis formed.

Carbon, with its 4 valence electrons is said to be tetravalent (Figure 2.9a). In other words,it can form up to four bonds. Carbon can form 4 single bonds, 2 double bonds, 1 double bondand 2 single bonds, and so on, depending on the number of electrons needed by the atomthat is its partner. Figure 2.9b shows carbon covalently bonded to 4 hydrogens to producemethane, an organic compound that is common in the atmosphere of Titan. Covalent bondsare symbolized by a short line indicating a shared pair of electrons (Figure 2.10a). When anelement such as carbon enters into bonds involving two pairs of shared electrons, this is calleda double bond. A carbon-to-carbon double bond is symbolized by two horizontal lines (Figure 2.10b).

Atoms with 1, 2, or 3 electrons in their valence shell tend to lose electrons and therefore be-come positively charged ions, while atoms with 6 or 7 electrons in the valence shell tend to gainelectrons and become negatively charged ions. Positively and negatively charged ions associate intoa type of bond called the ionic bond.

Ionic bonds form between charged atoms attracted to each other by similar, opposite charges.For example, the sodium atom forms an ionic bond with a chlorine atom to produce table salt(sodium chloride) when the sodium atom gives up an electron and the chlorine atom gains one

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(Figure 2.11). More than 2 atoms can be involved in an ionic bond. For in-stance, calcium will react with 2 chlorine atoms to produce calcium chloride(CaCl2). This is because calcium has 2 electrons in its valence shell—when itloses these it has 2 more protons than neutrons, giving it a double-positivecharge. Each chlorine atom, with 7 electrons, picks only 1 more electron tohave a stable outer shell and a single negative charge.Thus, 2 chlorine ions willbe attracted to a single calcium ion.

Ionic bonds are about as strong as covalent bonds.They can be more eas-ily disrupted, however, when mixed with certain liquids containing electricalcharges. Water is one liquid that causes ions in molecules to dissociate or fallapart.

Figure 2.10 Single and double bonds.(a) Covalent bonds are symbolized by a short line indicating a shared pair ofelectrons. (b) Double covalent bondsinvolve two pairs of shared electrons,symbolized by two horizontal lines.

(a) Methane (b) Ethylene

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Figure 2.11 Ionic bonding. Ionic bonds form when electrons are transferredbetween charged atoms.

The simple organic molecules found in the Martian meteorite that appearto have formed on Mars are carbonates, molecules containing carbon and oxy-gen, and hydrocarbons, made up of chains and rings of carbon and hydrogen.Carbonates and hydrocarbons can form under certain natural conditions evenwithout the presence of life. However, the meteorite lacked convincingevidence of macromolecules—organic molecules that are known to be pro-duced only by living organisms.

Structure and Function of MacromoleculesThe macromolecules present in living organisms are carbohydrates, proteins,lipids, and nucleic acids.To date, every living Earth organism, whether bacte-ria, plant, or animal, has been found to contain these same macromolecules.

Carbohydrates. Sugars, or carbohydrates, provide the major source ofenergy for daily activities. Carbohydrates also play important structural rolesin cells. The simplest carbohydrates are composed of carbon, hydrogen, andoxygen in the ratio (CH2O). For example, the carbohydrate glucose issymbolized as 6(CH2O) or C6H12O6. Glucose is a simple sugar, or mono-saccharide, that consists of a single ring-shape structure. Disaccharides aretwo rings joined together. Table sugar, called sucrose, is a disaccharidecomposed of glucose and fructose, a sugar found in fruits.

(A Closer Look continued)

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Glucose monomer

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Cellulose—a polysaccharide

Figure 2.12 Carbohydrates.Monosaccharides, which tend to formring structures in aqueous solutions,are individual sugar molecules. Disac-charides are 2 monosaccharides joinedtogether, and polysaccharides are longchains of sugars joined together. Themonosaccharide glucose and the disac-charide sucrose are important sourcesof energy, and cellulose plays a struc-tural role in plant cell walls.

Joining many individual subunits, or monomers, together produces poly-mers. Polymers of sugar monomers are called polysaccharides (Figure 2.12).Plants use tough polysaccharides in their cell walls as a sort of structural skele-ton.The polysaccharide cellulose, found in plant cell walls, is the most abundantcarbohydrate on Earth. The external skeletons of insects, spiders, and lobstersare composed of the polysaccharide chitin, and the cell walls that surround bac-terial cells are rich in structural polysaccharides.

According to David McKay and his colleagues, the particular set of hy-drocarbons found in the Martian meteorite is identical to the set formed whencarbohydrates in certain bacteria on Earth break down.These trace remains ofpossible Martian carbohydrates are an important piece of evidence that scien-tists use to argue that Mars once harbored Earth-like life. Evidence of thepresence of proteins on the meteorite is less convincing.

Proteins. Living organisms require proteins for a wide variety ofprocesses. Proteins are important structural components of cells; they areintegral to the structure of cell membranes and make up half the dry weight ofmost cells. Some cells, such as animal muscle cells, are largely composed ofproteins. Proteins called enzymes accelerate and help regulate all thechemical reactions that build up and break down molecules inside cells. Thecatalytic power of enzymes (their ability to drastically increase reaction rates)

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allows metabolism to occur under normal cellular conditions. Proteins canalso serve as channels through which substances are brought into cells, andthey can function as hormones that send chemical messages throughout anorganism’s body.

Proteins are large molecules made of monomer subunits called aminoacids. There are 20 commonly occurring amino acids. Like carbohydrates,amino acids are made of carbons, hydrogens, and oxygens; these form the amino acid’s carboxyl group. In addition, amino acids have nitrogen aspart of an amino (2NH2

+) group along with various side groups. Side groupsare chemical groups that give amino acids different chemical properties(Figure 2.13a).

Polymers of amino acids can be joined together in various sequencescalled polypeptides. The chemical bond joining adjacent amino acids is apeptide bond. Figure 2.13b shows three amino acids—valine, alanine, andphenylalanine—joined by peptide bonds. Precisely folded polypeptides pro-duce specific proteins in much the same manner that children can use differ-ently shaped beads to produce a wide variety of structures (Figure 2.13c).Each amino acid side group has unique chemical properties, including beingpolar or nonpolar. Since each protein is composed of a particular sequence ofamino acids, each protein has a unique shape and therefore specialized chem-ical properties.

Scientists have found no evidence of proteins in the Martian meteorite, al-though one group of investigators did report the presence of tiny amounts ofthree amino acids within the rock. However, it may be the case that these aminoacids are contaminants; that is, they are present in the meteor because the me-teor has been on protein-rich Earth for several thousand years. In addition,some amino acids are known to form under conditions where life is not pres-ent, so the presence of amino acids isnot necessarily evidence of life.

Lipids. One type of organic mol-ecule, abundant in living organisms,that has not been found in theMartian meteorite is lipids. Lipidsare partially or entirely hydropho-bic organic molecules made prima-rily of hydrocarbons. Important lipids include fats, steroids, andphospholipids.

Fat. The structure of a fat is that of a 3-carbon glycerol molecule with up to 3 long hydrocarbon chainsattached to it (Figure 2.14a). Like thehydrocarbons present in gasoline, thesecan be burned to produce energy.Thelong hydrocarbon chains are calledfatty acid tails of the fat. Fats arehydrophobic and function in energystorage within living organisms.

Steroids. Steroids are composedof 4 fused carbon-containing rings.Cholesterol (Figure 2.14b) is onesteroid that is probably familiar; itsprimary function in animal cells (plant

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Figure 2.13 Amino acids, peptidebonds, and proteins. (a) All amino acidshave the same backbone but differentside groups. (b) Amino acids are joinedtogether by peptide bonds. Longchains of these are called polypep-tides. (c) Polypeptide chains fold uponthemselves to produce proteins.

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cells do not contain cholesterol) is to help maintain the fluidity of membranes.Other steroids include the sex hormones testosterone, estrogen, and progesterone,which are produced by the sex organs and have effects throughout the body.

Phospholipids. Phospholipids are similar to fats except that eachglycerol molecule is attached to 2 fatty acid tails (not 3, as you would find ina dietary fat). The third bond in a phospholipid is to a phosphate headgroup. The phosphate head group is hydrophilic, and the two tails arehydrophobic (Figure 2.14c). Phospholipids often have an additional headgroup, attached to the phosphate, that also confers unique chemicalproperties on the individual phospholipid. Phospholipids are importantconstituents of the membranes that surround cells and that designatecompartments within cells.

Even if the Martian meteorite contained unambiguous traces of carbohy-drates, proteins, and lipids, the source of these molecules would not clearly beliving organisms without a mechanism for passing information about theirtraits to the next generation. The hereditary, or genetic, information commonto all life on Earth is in the form of nucleic acids.

Nucleic Acids. Nucleic acids are composed of long strings of monomerscalled nucleotides. A nucleotide is made up of a sugar, a phosphate, and anitrogen-containing base. There are two classes of nucleic acids in livingorganisms. Ribonucleic acid (RNA) plays a key role in helping cells synthesizeproteins and is discussed in detail in later chapters.The nucleic acid that servesas the primary storage of genetic information in nearly all living organisms isdeoxyribonucleic acid (DNA). Figure 2.15 shows the three-dimensionalstructure of a DNA molecule and zooms inward to the chemical structure.Youcan see that DNA is composed of two curving strands that wind around eachother to form a double helix. The sugar in DNA is the 5-carbon sugardeoxyribose.The nitrogen-containing bases, or nitrogenous bases, of DNAhave one of four different chemical structures, each with a different name:

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Figure 2.14 Three types of lipids. (a)Fats are composed of a glycerol mole-cule with 3 hydrocarbon-rich fatty acidtails attached. (b) Cholesterol is asteroid common in animal cell mem-branes. (c) Phospholipids are com-posed of a glycerol backbone with 2fatty acids attached and 1 phosphatehead group. The cartoon drawing tothe right shows how phospholipids areoften depicted.

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(a) DNA double helix is made of two strands. (b) Each strand is a chain of of antiparallel nucleotides.

“Backbones” made of sugars and phosphates

“Rungs” made of nitrogenous bases

Sug

ar–p

ho

sph

ate

“bac

kbo

ne”

Sug

ar–ph

osp

hate “b

ackbo

ne”

“Rung”

The two strands are connected by hydrogen bonds between the nucleotides.

(c) Each nucleotide is composed of a phosphate, a sugar, and a nitrogenous base.

Phosphate (P) Sugar (S)

Deoxyribose

Nitrogenous bases

Purines Pyrimidines

Adenine (A) Thymine (T)

Guanine (G) Cytosine (C)

A always pairs with T (see part b)

G always pairs with C (see part b)

P

P

P

P

P

P

S

S

S

A

G

C

T

C

G

S

S

S

A

G

T

C

OH

H

H

COOH

H

H

O

O

O OH H

OH

P

NH2N

H

H

NN H

N

ON

N N NH2

H

N

NH2

O

N

N

CH3

N

O

H H

N HH

HH

H HO

C

O

C

C C

Nucleotides within strand are connected by covalent bonds.

Figure 2.15 DNA structure. (a) DNA is adouble-helical structure composed ofsugars, phosphates, and nitrogenousbases. Visualize This: Point out thesugar-phosphate backbone and the ni-trogenous bases in this image. Why arethere two colors and sizes of nitrogenousbases? (b) Each strand of the helix iscomposed of repeating units of sugarsand phosphates, making the sugar-phos-phate backbone, and of nitrogenousbases. (c) A phosphate, a sugar, and anitrogenous base comprise the structureof a nucleotide. Adenine and guanine arepurines, which have a double-ringstructure; cytosine and thymine arepyrimidines, which have a single-ringstructure.

adenine (A), guanine (G), thymine (T), and cytosine (C). Nucleotides arejoined to each other along the length of the helix by covalent bonds.

Nitrogenous bases form hydrogen bonds with each other across thewidth of the helix. On a DNA molecule, an adenine (A) on one strand al-ways pairs with a thymine (T) on the opposite strand. Likewise, guanine(G) always pairs with cytosine (C). The term complementary is used todescribe these pairings. For example, A is complementary to T, and C is

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complementary to G. Therefore, the order of nucleotides on one strand ofthe DNA helix predicts the order of nucleotides on the other strand.Thus, ifone strand of the DNA molecule is composed of nucleotides AAC-GATCCG, then we know that the order of nucleotides on the other strand isTTGCTAGGC.

As a result of this base-pairing rule (A pairs with T; G pairs with C), thewidth of the DNA helix is uniform. There are no bulges or dimples in thestructure of the DNA helix because A and G, called purines, are structurescomposed of two rings; C and T are single-ring structures called pyrimidines.A purine always pairs with a pyrimidine and vice versa, so there are always 3rings across the width of the helix. A-to-T base pairs have 2 hydrogen bonds holding them together. G-to-C pairs have 3 hydrogen bonds holdingthem together.

Each strand of the helix thus consists of a series of sugars and phosphatesalternating along the length of the helix, the sugar-phosphate backbone.The strands of the helix align so that the nucleotides face “up” on one side ofthe helix and “down” on the other side of the helix. For this reason, the twostrands of the helix are said to be antiparallel.

The overall structure of a DNA molecule can be likened to a rope ladderthat is twisted, with the sides of the ladder composed of sugars and phos-phates (the sugar-phosphate backbone) and the rungs of the ladder composedof the nitrogenous-base sequences A, C, G, and T.The structure of DNA wasdetermined by a group of scientists in the 1950s, most notably James Watsonand Francis Crick (Figure 2.16).

How Might Macromolecules on Other Planets Differ? Manyscientists argue that the fundamental constituents described here—car-bohydrates, proteins, lipids, and nucleic acids—will be essentially similarwherever life is found. They will readily admit that the finer details are verylikely to differ, however. For example, all proteins known on Earth contain only20 different amino acids, despite an infinite number of possibilities. Presumably,proteins on other planets could contain completely different amino acids andmany more than 20.

Not all scientists agree with this position, which they call “carbon chau-vinism.” Carbon is not the only chemical Tinkertoy connector; other ele-ments, including silicon, can also make connections with four other atoms.Silicon is also relatively abundant in the universe and could theoretically formthe backbone of an alternative organic chemistry. The basic constituents ofsilicon-based life may be very different from the chemical building blocks oflife on Earth.

Even if all life in the universe is based on carbon chemistry, it is very un-likely that the suite of organisms found on another planet will look much likelife on our planet. However, understanding the history of life on Earth alsoprovides insight into the possible nature of life elsewhere in the universe.

2.2 Life on EarthOne of the most dramatic features of the Martian meteorite is the presence offossils that look remarkably like the tiniest living organisms known from Earth.The largest of these fossils is less than 1/100th of the diameter of a humanhair, and most are about 1/1000th of the diameter of a human hair—smallenough that it would take about 1000 laid end to end to span the dot at theend of this sentence. Some are egg shaped, while others are tubular.These fos-sils appear similar to the simplest and most ancient of known organisms and

Figure 2.16 The DNA model. AmericanJames Watson (left) and EnglishmanFrancis Crick are shown with thethree-dimensional model of DNA theydevised while working at the Univer-sity of Cambridge in England.


A Comparison of Prokaryotic and Eukaryotic Cells

web animation 2 3

Tour of a Plant Cell

Tour of an Animal Cell

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Chapter Section 2.2 Life on Earth41

are the strongest piece of evidence supporting the hypothesis that Mars oncewas home to living organisms.

Prokaryotic and Eukaryotic CellsDavid McKay and his colleagues argue that the fossil structures in the Mart-ian meteorite are the remains of tiny cells. A cell is the fundamental structuralunit of life on Earth, separated from its environment by a membrane andsometimes an external wall. Bacteria are composed of single cells, which per-form all of the activities required for life. More complex organisms can becomposed of trillions of cells working together and do not have any cells thatcould survive and reproduce independently.

All cells can be placed into one of two categories, prokaryotic or eukary-otic, based on the presence or absence of certain cellular structures. Bacteriaare prokaryotic cells. Prokaryotes do not have a nucleus, a separate mem-brane-bound compartment that contains genetic material in the form ofDNA. They also do not contain any membrane-bound internal structures.Prokaryotic cells are much smaller than eukaryotic cells (Figure 2.17a), andaccording to the fossil record, they pre-date eukaryotic cells.The fossils in theMartian meteorite resemble modern prokaryotic cells known as nanobacteria.However, bacterial cells also have a cell wall that helps them maintain theirshape (Figure 2.17b). The fossils in the Martian meteorite show no evidenceof similar walls, which does not support the hypothesis that the fossils arerelics of once-living organisms.

Eukaryotic cells have a nucleus and other internal structures withspecialized functions, called organelles, that are surrounded by membranes.Eukaryotic organisms include single-celled organisms such as amoebas andyeast as well as multicellular plants, fungi, and animals, to name a few. As you will learn in Chapter 12, scientists believe that the first prokaryotic cells

NucleoidChromosomeCytoplasm Ribosome

CapsuleCell membraneCell wall Flagellae

(a) Different sizes: eukaryotic vs. prokaryotic cells

(b) Prokaryotic cell features

Figure 2.17 Prokaryotic and eukaryotic cells. (a) Prokaryotic cells are typicallyabout 1/10 the diameter of a eukaryotic cell as evidenced by the size of thetwo bacterial cells and a white blood cell. (b) Prokaryotic cells are structurallyless complex than eukaryotic cells.

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Component Function

(continued on the next page)

TA B L E 2.1 Cell components. Illustrations and descriptions of cell components and their functions

Plasma membrane All cells are surrounded by a plasma membrane.It is composed of a bilayer of phospholipids per-forated by proteins. Proteins in the bilayer helptransport substances across the hydrophobiccore of the membrane. Cholesterol in the mem-branes of animal cells helps maintain the fluidityof the membrane.

Nucleus Eukaryotic cells contain a nucleus. The nucleus is a spherical structure surrounded by two mem-branes, together called the nuclear envelope. The nuclear envelope is studded with nuclearpores that regulate traffic into and out of the nu-cleus. Inside the nucleus is chromatin, com-posed of DNA and proteins. The nucleolus iswhere ribosomes are produced.

appeared on Earth over 3.5 billion years ago, and that the first eukaryotes ap-peared about 1.7 billion years later.

Many scientists dispute David McKay’s interpretation of the tubularstructures in ALH84001. In fact, similar structures can be formed in theabsence of life by certain minerals under extremes of heat and pressure. If theMartian fossils are indeed cells, they likely contained features found insideearthly cells, some of which should be visible in the fossils.

Cell StructureEach living cell can be considered a veritable factory working to break downnutrients and to recycle its components. We start from the outside of the celland examine the structure and function of various cell components as wework our way into the cell (Table 2.1).

Lysosome A lysosome is a membrane-enclosed sac of diges-tive enzymes that degrade proteins, carbohy-drates, and fats. Lysosomes roam around the celland engulf targeted molecules and organelles forrecycling.

Membrane

Digestive enzymes and digested material

Sugarchains

Cholesterol

Phospholipidbilayer

Tail

Head

Protein

Chromatin

Nucleolus

Nuclearenvelope

Nuclear pore

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Chapter Section 2.2 Life on Earth 43

OutermembraneInnermembraneStroma

Thylakoids

Granum

TABLE 2.1 Cell components. Illustrations and descriptions of cell components and their functions. (Continued)

Component Function

Nuclear envelope

Rough endoplasmic reticulum

RibosomesVesicleSmooth endoplasmic reticulum

(continued on the next page)

Ribosomes

Chloroplast An important organelle present in plant cells, thechloroplast uses the sun’s energy to convert car-bon dioxide and water into sugars. Each chloro-plast has an outer membrane, an innermembrane, a liquid interior called the stroma, anda network of membranous sacs called thylakoidsthat stack on one another to form structurescalled grana (singular: granum). Chloroplasts alsocontain pigment molecules that give green partsof plants their color.

Ribosomes Ribosomes are found in eukaryotic and prokary-otic cells. Ribosomes are built in the nucleolus andshipped out of the nucleus through nuclear poresto the cytoplasm, where they are used as work-benches for protein synthesis. They can be foundfloating in the cytoplasm or tethered to the ER.

Endoplasmic reticulum (ER) The ER is a large network of membranes that be-gins at the nuclear envelope and extends into thecytoplasm of a eukaryotic cell. ER with ribosomesattached is called rough ER. Proteins synthesized onrough ER will be secreted from the cell or will be-come part of the plasma membrane. ER without ri-bosomes attached is called smooth ER. The functionof the smooth ER depends on cell type but includestasks such as detoxifying harmful substances andsynthesizing lipids. Vesicles are pinched-off piecesof membrane that transport substances to the Golgiapparatus or plasma membrane.

Golgi apparatus The Golgi apparatus is a stack of membranoussacs. Vesicles from the ER fuse with the Golgiapparatus and empty their protein contents. Theproteins are then modified, sorted, and sent tothe correct destination in new transport vesiclesthat bud off from the sacs.

Centrioles Centrioles are barrel-shaped rings composed ofmicrotubules that help move chromosomesaround when a cell divides. Centrioles are in-volved in microtubule formation during cell divi-sion and the formation of cilia and flagella.

Vesicle from ER arriving at Golgi apparatus

Vesicle departing Golgi apparatus

Microtubuletriplet

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TABLE 2.1 Cell components. Illustrations and descriptions of cell components and their functions. (Continued)

Component Function

Cytoskeletal elements Cytoskeletal elements are protein fibers in the cy-toplasm that give shape to a cell, hold and moveorganelles (including transport vesicles), and areinvolved in cell movement.

Cell wall The cell wall is found outside the plasma mem-brane of plant and bacterial cells. The cell wall inplants is rich in the polysaccharide cellulose. Cellu-lose is assembled into strong fibrils and embed-ded in a matrix.

Central vacuole Plant cells also have large membrane-bound, fluid-filled vacuoles that can occupy as much as 90% ofa cell’s total volume. The plant vacuole contains avariety of dissolved molecules, including sugarsand pigments that give color to flowers andleaves. Vacuoles also function to maintain pres-sure inside individual cells, which helps supportthe upright plant.

MicrofilamentsIntermediate filaments

Microtubules

Plant Cellulose fibrils Cellulose

O O

O

O

O

O

O O

O

O

O

O

O

O

Mitochondrion Plant and animal cells contain mitochondria,energy-producing organelles surrounded by twomembranes. The inner and outer mitochondrialmembranes are separated by the intermem-brane space. The highly convoluted inner mem-brane carries many of the proteins involved inproducing ATP. The matrix of the mitochondrionis the location of many of the reactions ofcellular respiration.

Outer membrane

Intermembrane space

Matrix

Innermembrane

Plasma Membrane. All cells are enclosed by a structure called a plasmamembrane. The plasma membrane defines the outer boundary of each cell,isolates the cell’s contents from the environment, and serves as a semipermeablebarrier that determines which nutrients are allowed into and out of the cell.Membranes that enclose structures inside the cell are usually referred to as cellmembranes, while the outer boundary is the plasma membrane.

Internal and external cell membranes are composed, in part, of phos-pholipids. The chemical properties of these lipids make membranes flexi-ble and self-sealing. When phospholipid molecules are placed in a waterysolution, such as in a cell, they orient themselves so that their hydrophilicheads are exposed to the water and their hydrophobic tails are away from

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the water. They cluster into a form called a phospholipid bilayer, inwhich the tails of the phospholipids interact with themselves and excludewater, while the heads maximize their exposure to the surrounding waterboth inside and outside the membrane. The bilayer of phospholipids isstuffed with proteins that carry out enzymatic functions, serve as recep-tors, and help transport substances.

A Fluid Mosaic of Lipids and Proteins. All of the lipids andmost of the proteins in the plasma membrane are free to bob about, slidingfrom one location in the membrane to another. Because lipids and proteinsmove about laterally within the membrane, the membrane is a fluidmosaic of lipids and proteins. The membrane is fluid since thecomposition of any one location on the membrane can change. In the samemanner that a patchwork quilt is a mosaic (different fabrics making up thewhole quilt), so, too, is the membrane a mosaic with different regions ofmembrane being composed of different types of phospholipids andproteins.

Cell membranes are semipermeable in the sense that they allow somesubstances to cross and prevent others from crossing. This characteristic al-lows cells to maintain a different internal composition from the surroundingsolution.

Nucleus. All eukaryotic cells contain a nucleus surrounded by a doublenuclear membrane, which houses the DNA. Inside the nucleus is thenucleolus, which is where ribosomes are assembled.

Cytosol. Between the nucleus and the plasma membrane lies thecytosol, a watery matrix containing water, salts, and many of the enzymesrequired for cellular reactions. The cytosol houses the subcellularstructures called organelles. The term cytoplasm includes the cytosol andorganelles.

Organelles. Organelles are to cells as organs are to the body. Eachorganelle performs a specific job required by the cell, and all organelles worktogether to keep an individual cell healthy and to produce the raw materialsthat the cell needs to survive. Each organelle is enclosed in its own lipidbilayer. Some organelles are involved in metabolism. For example, organellescalled mitochondria help the cells convert food energy into a form usable bycells, called ATP (adenosine triphosphate), while chloroplasts in plant cellsuse energy from sunlight to make sugars. Lysosomes help break downsubstances that are ingested before they are sent to the mitochondria. Otherorganelles are involved in producing proteins. The endoplasmic reticulum(ER) is an extensive membranous organelle that can be studded withribosomes (rough endoplasmic reticulum) and involved in proteinsynthesis or tubular in shape and involved in lipid synthesis (smoothendoplasmic reticulum). Proteins that are assembled on the membranes ofthe rough ER can be modified and sorted in a membranous structure calledthe Golgi apparatus.

There are other important subcellular structures that are not consid-ered organelles because they are not bounded by membranes. Ribosomesare workbenches where proteins are assembled. Centrioles are involved inmoving genetic material around when a cell divides, and many fibers thatcompose the cytoskeleton help maintain the cell shape. Some organellesand subcellular structures are found in certain cell types only. For in-stance, in addition to having chloroplasts and a cell wall, the plant cell alsohas a vacuole to store water, sugars, and pigments. Table 2.1 describes the

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Plasmamembrane

Intermediatefilament

Plasmamembrane

Nucleus

Nucleolus NucleusNuclear envelope

Ribosomes

NucleolusRough endoplasmic reticulum

Cytosol

Cytoplasm

Cellwall

Golgi apparatus

Mitochondrion

MitochondrionMicrofilamentMicrotubule

Rough endoplasmicreticulum

Smooth endoplasmicreticulum

Smoothendoplasmicreticulum

CentrioleRibosomesCytoskeleton:

Microfilament

Intermediatefilament

Microtubule

Cytoskeleton:

Lysosome

Lysosome

Golgiapparatus

Centralvacuole

Chloroplast

(a) Animal cell (b) Plant cell

Figure 2.18 Animal and plant cells.These drawing of a generalized (a)animal cell and (b) plant cell show thelocations and sizes of organelles andother structures. Visualize This:What three structures are present inplant but not animal cells? Stop and Stretch2.2: Would you expect prokaryotic cells to contain

ribosomes? Why or why not?

The Tree of Life and Evolutionary TheoryBiologists disagree about the total number of different species, or types ofliving organisms, that are present on Earth today. This uncertainty stemsfrom lack of knowledge. Although scientists likely have identified most ofthe larger organisms—such as land plants, mammals, birds, reptiles, andfish—millions of species of insects, fungi, bacteria, and other microscopicorganisms remain unknown to science. Amazingly, credible estimates of thenumber of species on Earth range from 5 million to 100 million. Given thislevel of uncertainty, most biologists think that the likeliest number is near10 million.

Theory of Evolution. While the diversity of living organisms istremendous, there exist remarkable similarities among all known species. Allhave the same basic biochemistry, including carbohydrates, lipids, proteins,and nucleic acids. All consist of cells surrounded by a plasma membrane. Alleukaryotic organisms (including fungi, animals, and plants) contain nearly the same suite of cellular organelles. The best explanation for the sharedcharacteristics of all species, what biologists refer to as “the unity of life,” isthat all living organisms share a common ancestor that arose on Earth nearly4 billion years ago. The divergence and differences among modern speciesarose as a result of changes in the characteristics of populations, both inresponse to environmental change (a process called natural selection) and dueto chance.These ideas underlie the entire science of biology and are known asthe theory of evolution.

structures and functions of most cellular organelles in greater detail.Figure 2.18 shows an animal cell and a plant cell complete with their com-plement of organelles.

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A major process in the diversification of life from a single ancestor wasnatural selection. The theory of natural selection is discussed in detail inChapter 10, but the basic principle is simple: Individual organisms vary fromeach other, and some of these variations increase their chances of survival andreproduction. A genetic trait that increases survival and reproduction shouldbecome more prevalent over time. In contrast, less-successful variants shouldeventually be lost from the population.

The common ancestor can be thought of as the starting place for life onEarth, and the continual divergence among species and groups of speciescan be thought of as life’s branching. Modern organisms can therefore bearranged on a tree of life that reflects their basic unity and relationships. Ac-cording to current understanding, living organisms can be grouped intothree large groups: two that are prokaryotic and one containing all eukary-otes. Eukaryotes can be further grouped into several categories made up pri-marily of free-living, single-celled organisms (such as amoebas and algae)and the three major multicellular groups—plants, fungi, and animals(Figure 2.19). Chapter 12 provides a deeper exploration of the diversity oflife on Earth.

Because evolutionary change results from chance events and environmen-tal changes (including the appearance of other species), the group of speciespresent on Earth today represents only one set of an infinite number of possi-bilities. In other words, life on other planets need not look identical to life onEarth. For example, instead of the common body form found in animals,called bilateral symmetry, by which bodies can be visually divided into twomirror-image halves, life on other planets could be primarily radially symmet-ric and thus look very different (Figure 2.20). In fact, it is possible that life onother planets might not even be based on carbon. Scientists have no examplesof what living organisms would look like on a planet where the organic mole-cules were based on silicon or bathed in liquid methane.

Life in the Universe. Do other living organisms exist in the universe?Given the universe’s sheer size and complexity, most scientists who study thisquestion think that the existence of life on other planets is nearly certain.

Bacteria AmoebaArchaea Brown algae Green algae Plants Fungi Animals

Common ancestor

Prokaryotes Eukaryotes

Figure 2.19 Tree of life. All life onEarth shares basic characteristics andcan be arranged into a tree of lifebased on more specific similarities. Inthis illustration, many groups are omit-ted for simplicity.

Figure 2.20 Diversity of body form. Notall animals are bilaterally symmetriclike us, with two eyes, two ears, twoarms, two legs, an obvious head and“tail,” and one central axis. A sea staris radially symmetric—it can be di-vided into two equal halves in anydirection. Visualize This: Is an earth-worm bilaterally symmetric?

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While the evidence of life in the Martian meteorite is unconvincing to manyscientists, we may find out in our lifetimes that life exists, or once existed, onour planetary neighbor.

What about the existence of intelligent life that could communicate withus? Some scientists argue that as a result of natural selection, the evolution ofintelligence is inevitable wherever life arises. Others point to the history of lifeon Earth—consisting of at least 2.5 billion years, during which all life wasmade up of single-celled organisms—to argue that most life in the universemust be “simple and dumb.” It is clear from our explorations of the solar sys-tem that none of the sun’s other planets host intelligent life. The nearest sun-like stars that could host an Earth-like planet, Alpha Centauri A and B, areover 4 light years away—nearly 40 trillion miles. With current technologies, itwould take nearly 50,000 years to reach the Alpha Centauri stars, and there iscertainly no guarantee that intelligent life would be found on any planets thatcircle them. For all practical purposes, at this time in human history, we arestill alone in the universe.

Savvy Reader Detox Drinks

A C L E A R W I N N E R I N T H E F E E L - G O O D S TA K E S | B Y C A R O L I N E S TA C E Y |T H E I N D E P E N D E N T ( L O N D O N ) | J A N U A RY 3 , 2 0 0 4

So you thought water was just adrink? Think again. It’s a lifestylechoice. We can all safely drink our litre or more a day straightfrom the tap. But where’s the ca-chet or the profit in that? It’s al-most as free as air. And wonderfuland hydrating though tap water is,the latest bottled waters offer somuch more—to make you sportier,healthier, and less hungover.

With Oxygizer you pay forair and water together. It’s oxy-

genated, but not fizzy. Bottled inthe Tyrolean mountains by acompany based in Innsbruck,Austria, it describes itself as “a sip of fresh air.” Already big inthe Middle East—where water’sa more precious commoditythan it is here—it has beenlaunched in Europe and now inthe UK.

Oxygizer doesn’t just slake athirst, it provides the body withextra oxygen too. A litre contains

150 mg of oxygen, around 25times more than what’s in a litreof tap water. This apparentlyhelps remove toxins and ensuresa stronger immune system, aswell as assisting the respiratorysystem so you recover betterfrom exercise. Some claim detoxbenefits, it helps hangovers, andeven enhances flavours to makefood taste better.

1 List the claims made by this article. Is there enough information presented in this article to back upthe claims made?

2 Use the appropriate questions in the checklist provided in Chapter 1, Table 1.2, to evaluate this news-paper article. What types of information are missing from this article?

3 Is any data presented to substantiate the claim that oxygenated water improves health?

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Chapter Review 49

• Living organisms must be able to grow, metabolizesubstances, reproduce, and respond to externalstimuli (p. 30).

• Living organisms contain a common set of biologi-cal molecules, are composed of cells, and canmaintain homeostasis and evolve (p. 30).

• Water is a good solvent in part because of its po-larity (p. 31).

• Hydrogen bonding occurs when a weak attractiondevelops between hydrogen and other atoms (pp.31–32).

• The polarity of water also facilitates the dissolvingof salts. Salts are produced by the reaction of anacid with a base (p. 32).

• The pH scale is a measure of the relative percent-ages of and ions in a solution and ranges from 0(acidic or rich in ions) to 14 (basic or rich in ions)(pp. 32–33).

• Chemical bonding depends on an element’s elec-tron configuration. Electrons closer to the nucleushave less energy than those that are farther awayfrom the nucleus. The first energy level can hold 2electrons. The next 2 levels each hold 8 electrons.Atoms that have space in their valence shell formchemical bonds (pp. 33–34).

• Covalent bonds form when atoms share electrons.These tend to be strong bonds (pp. 34–35).

• Ionic bonds form between positively and negativelycharged ions. These tend to be weak bonds (p. 35).

• Life on Earth is based on the chemistry of the ele-ment carbon, which can make bonds with up tofour other elements (p. 35).

• Carbohydrates function in energy storage and playstructural roles. They can be single-unit monosac-charides or multiple-unit polysaccharides with sugarmonomers arranged in different orders (p. 36).

• Proteins play structural, enzymatic, and transportroles in cells. They are composed of amino acidmonomers arranged in different orders (p. 37).

• Lipids are partially or entirely hydrophobic andcome in three different forms. Fats are composedof glycerol and three fatty acids. Fats store energy.Phospholipids are composed of glycerol, 2 fattyacids, and a phosphate group. They are importantstructural components of cell membranes. Steroidsare composed of 4 fused rings. Cholesterol is asteroid found in some animal cell membranes andhelps maintain fluidity. Other steroids function ashormones (pp. 37–38).

• Nucleic acids are polymers of nucleotides, eachof which is composed of a sugar, a phosphate,and a nitrogen-containing base (pp. 38–40).

Life on Earth• There are two main categories of cells: Those with

nuclei and membrane-bound organelles are eu-karyotes; those lacking a nucleus and membrane-bound organelles are prokaryotes (pp. 41–42).

• The plasma membrane that surrounds cells is asemipermeable boundary composed of a phospho-lipid bilayer that has embedded proteins and cho-lesterol (p. 45).

• Lipids and proteins can move about the membrane.This fluidity of the membrane allows changes in the

protein and lipid composition (p. 45).

• Some organisms, such as plants and bacterialcells, have a cell wall outside the plasma mem-brane that helps protect these cells and maintaintheir shape (p. 44).

• Subcellular organelles and structures performmany different functions within the cell. Mitochon-dria and chloroplasts are involved in energy con-versions. Lysosomes are involved in breakdown ofmacromolecules. Ribosomes serve as sites for pro-tein synthesis. Proteins can be synthesized on ribo-somes attached to rough endoplasmic reticulum.

Summary

What Does Life Require?

CHAPTER Review For study help, animations, and more quizquestions go to www.mybiology.com.

Chemistry and Waterweb animation 2 1

Nucleic Acidsweb animation 2 2

2.1

2.2

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Roots to RememberThe following roots of words come mainly from Latin and Greek and will help you to decipher terms:

Smooth endoplasmic reticulum synthesizes lipids.The Golgi apparatus sorts proteins and sends themto their cellular destination. Centrioles help cellsdivide. The plant cell vacuole stores water andother substances (pp. 42, 45–46).

• There may be nearly 10 million unique life-formson Earth. Despite all of this diversity, all life onEarth shares the same organic chemistry, geneticmaterial, and basic cellular structures (p. 46).

• The similarities among living organisms on Earth pro-vide support for the theory of evolution, which statesthat all life on Earth derives from a common ances-

tor. The process of evolutionary change since the ori-gin of that ancestor led to the modern relationshipsamong organisms, called the tree of life (pp. 46–47).

1. List the four biological molecules commonly found in liv-ing organisms.2. Describe the structure and function of the subcellular or-ganelles.3. Describe the structure of the plasma membrane.4. Water .A. is a good solute; B. dissociates into H+ and OH2 ions; C.serves as an enzyme; D. makes strong covalent bonds with othermolecules; E. has an acidic pH5. Electrons .A. are negatively charged; B. along with neutrons comprise thenucleus; C. are attracted to the negatively charged nucleus; D.located closest to the nucleus have the most energy; E. all of theabove are true6. Which of the following terms is least like the others?A. monosaccharide; B. phospholipid; C. fat; D. steroid; E. lipid

7. Different proteins are composed of different sequences of.

A. sugars; B. glycerols; C. fats; D. amino acids8. Proteins may function as .A. the genetic material; B. cholesterol molecules; C. fat reserves;D. enzymes; E. all of the above.9. A fat molecule consists of .A. carbohydrates and proteins; B. complex carbohydrates only; C.saturated oxygen atoms; D. glycerol and fatty acids10. Eukaryotic cells differ from prokaryotic cells in that

.A. only eukaryotic cells contain DNA; B. only eukaryotic cellshave a plasma membrane; C. only eukaryotic cells are consid-ered to be alive; D. only eukaryotic cells have a nucleus; E. onlyeukaryotic cells are found on Earth

Summary (continued)

A Comparison of Prokaryoticand Eukaryotic Cells

web animation 2 3

Tour of an Animal Cell

Tour of a Plant Cell

cyto- and -cyte mean cell or a kind of cell.hydro- means water.-mer means subunit.macro- means large.micro- means small.

mono- means one.-philic means to love.-phobic means to fear.-plasm means a fluid.poly- means many.

Learning the Basics


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Chapter Review 51

Analyzing and Applying the Basics

Connecting the Science

1. A virus is made up of a protein coat surrounding a smallsegment of genetic material (either DNA or RNA) and a fewproteins. Some viruses are also enveloped in membranes de-rived from the virus’s host cell. Viruses cannot reproduce with-out taking over the genetic “machinery” of their host cell. Basedon this description and biologists’ definition of life, should avirus be considered a living organism?2. Any molecule containing oxygen can be polar. The struc-ture of methanol (CH3OH) is drawn in Figure 2.21. Whichpart of this molecule will have a partial negative charge, andwhich will have a partial positive charge?3. Some scientists have argued that silicon (Si) could also bean appropriate basis for organic chemistry because it is abun-dant and can form bonds with many other atoms. Carbon con-tains 6 electrons, and silicon contains 14. Recalling that thelowest electron shell contains 2 electrons, and the next 2 shells

1. Water’s characteristic as an excellent solvent means thatmany human-created chemicals (including some that are quitetoxic) can be found in water bodies around the globe. Howwould our use and manufacture of toxic chemicals be differentif most of these chemicals could not be dissolved and diluted in

water but instead accumulated where they were produced andused?2. Do you believe that humans should expend considerableenergy and resources looking for life, even intelligent life, else-where in the universe? Why or why not?

OC

H

H

H

H

Figure 2.21 Methanol.

can contain a maximum of 8, how many “spaces” does siliconhave in its valence shell? How does this compare to carbon?

ISB

N 0

-558

-712

11-8


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Are We Alone in the Universe? - Columbia Southern … · 30 Chapter 2 Are We Alone in the Universe? Figure 2.1 Homeostasis. Black-capped chickadees can maintain a core body temperature