cellular adaptations 98 - planetarybiology.com · cellular adaptations 14.1 introduction...

172 Chapter 14

Copyright© 1999 by Tom E. Morris. http://www.planetarybiology.com

This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.

Chapter 14

Cellular Adaptations

14.1 Introduction

Biosynthesis happens inside living cells. The moleculeswe talked about in the previous chapter are assembledand disassembled inside cells. As a result of thebiosynthetic chemistry that happens inside living cells,the cell is a biological unit with powerful potential tochange the environment in three ways:1) by processing materials from the surrounding

environment2) by contributing to the development of resource

competition, which is an important component inthe diversification and spreading of life

3) by supporting the development of very large,multicelled organisms that have the hardware tomore aggressively find and exploit planetaryresources.

In support of their internal biosynthetic demands, cellstake in materials from the surrounding environment,change them (as a result of biosynthesis), then releasethem back into the environment in different forms. Ifyou cover a planet with cells, they are bound to changethe environment by virtue of their materials processingactivities.

The living cell is an important adaptation that hasgreatly extended the scope and scale of life’s influenceon Earth. This chapter will describe the two basic kindsof cells that make up life on Earth, the prokaryotic celland the eukaryotic cell. The prokaryotic cell is thesimpler of the two. Despite its simplicity, this cell typehas members that process materials in such uniqueways that, without them, multicelled life probably wouldbe impossible. The eukaryotic cell is more complex. It isparticularly interesting because of itscompartmentalized design and large size. Thesequalities make it a potent materials processor and asuccessful candidate for making large, multicelledorganisms. Finally, I want to describe how some of thecomponents of eukaryotic cells work, and how they cancontribute to the diversification and spreading of life,and to the transformation of the planet.

14.2 Cell theory describes living cells as discrete, self-contained units of life

The molecules of life and all the living things you seeare the creations of the internal chemistry of cells. Theidea of living cells seems so abstract to most of us. Theyare so small. Cells were first described by Robert Hookewho had observed them with a microscope of his owndesign. When, in 1665, Hooke published his landmarkbook, Microphagia, he created quite a stir with hisdrawings of tiny objects. Hooke is famous for hisdrawings that show cork to be constructed of tiny box-like objects he called “cells” (fig.14.1). His observationssuggested a whole new world for biologists to explore,and they did. Using the microscope, biologists began tosee the biological world as a composition of tiny, andfine objects. Apart from Hooke, other scientists alsowere probing the microscopic realm. Antoni VanLeeuwenhoek, who invented and refined the microscopeand inspired much of Hooke’s work, was a prolificinvestigator. But unlike Hooke, who peered atconventional objects like needles and razors,Leeuwenhoek sought truly novel observations includingcrystals, plants, animals, and even scrapings from histeeth. In 1674, Leeuwenhoek proclaimed theremarkable discovery of microorganisms, as he hadobserved tiny moving things with his microscope.Today, we call these things, bacteria and protozoans.Leeuwenhoek’s drawings were so astonishing at thetime that his contemporary scientists did not believethem. He responded to such criticism by engagingdoctors, judges, and ministers as reliable witnesses.Thus it was then asit is now, hard tograsp that the truebusiness of lifehappens in suchminiature places.

The study of cellsevolved following thework ofmicroscopists likeHooke,Leeuwenhoek,Marcello Malpighi,who studied thehuman body, andNehemiah Grew,

Figure 14.1. Robert Hooke'sdrawings of cork cells.

Cellular Adaptations 173Copyright© 1999 by Tom E. Morris. http://www.planetarybiology.com

This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.who focused (literally) on plants and animals. But whatdid these observations mean? By themselves, they wereinteresting, and some thought sacrilegious probingsinto the construction of nature, but nothing more.However, the accumulations of newer observations andideas eventually led to new insights about what this allmeant — a cell theory.

The initiation of the modern cell theory usually isattributed to botanist Matthias Jacob Schleiden, andmedical scientist Theodor Schwann. In the mid 1800s,Schleiden made his stunning contribution to the theoryof cells. He proposed that all plants were assemblages ofcells, which he described as individual entities, yet partof a larger whole. According to Schleiden, the cells inplants had two jobs, one serving themselves, and oneserving the whole organism of which they were a part.Schleiden saw all aspects of plant construction andoperation basically as products of cellular activity.Theodor Schwann extended Schleiden’s ideas to includeanimals as well as plants. That is, all the different partsof plants and animals are constructed from cells. AndSchwann contributed a more important insight havingto do with what goes on inside a cell. He hadexperimented widely with yeasts (single-celledorganisms) in the conversion of grape juice to wine. Inthe mid 1800s, Schwann had discovered that yeast“particles” are actually living cells and that alcoholfermentation happens inside the yeast cells. He wasseverely ridiculed for this claim (which we know today tobe true) by other scientists who thought it absolutelypreposterous. Still, Schwann is credited for establishingthat metabolic activity, such as processing food andgenerating body heat, happens inside the cell. Butwhere do cells come from?

Robert Remake had studied embryology, which is thefield of science concerned with how things grow anddevelop from a single egg cell. In the mid 1800s, hedetermined that embryos grow by increasing thenumber of cells, and that the number of cells increasesby the division of existing cells into new cells. Remake’s

ideas were later confirmed by medical scientist RudolfVirchov. In 1854, Virchov wrote that, “there is no lifewithout direct succession.” In other words, all cellscome from pre-existing cells. According to thishypothesis, all the cells that make you up aredescendants (by cell division) of the first, progenitor cellmore than 3.8 billion years ago. Virchov also saw thatcells were the fundamental unit of life, and that theyconstructed ever more complex systems eventuallyleading to a whole organism. This hierarchy consists ofcells, tissues, organs, organ systems, and finally thewhole living thing.

The last major plank in the cell theory established thatcells contain the stuff of heredity. In the late 1800s,four German biologists (Oskar Hertwig, EdouardStrasburger, Rudolf Kolliker, and August Weisman)independently proposed that the chromosomes (whichwe now know contain the DNA) inside the cell nucleuswere the basis for heredity. Their ideas arose fromdetailed studies of cell division, and in fertilization ofegg cells by sperm cells.

In the 1900s, better staining techniques and morepowerful microscopes revealed that cells are verycomplex places. They are filled with numerous kinds ofstructures of different shapes and sizes, such as thenucleus, mitochondria and chloroplasts. Today, themodern field of cellular biology is devoted to betterunderstanding the functions of these structures. Briefdescriptions of some of these structures are presentedlater on in this chapter when I discuss the componentsof eukaryotic cells.

So, to conclude, the living cell is the fundamental unitof life in that it builds all living things, carries out thebiosynthetic chemistry of life, and is the guardian andtransmitter of the means of inheritance. When studyinglife, I cannot help admiring the quiet power possessedby these diminutive machines who together and forbillions of years have silently transformed an entireplanet.

14.3 A cell provides a confined place that organizesmolecular activities

Cells provide an organized place for biosynthesis tohappen. In the previous chapter, we saw that life prettymuch is an enterprise that involves the activities ofmolecules — thousands of molecules. These moleculesare assembled and disassembled inside cells. Some staythere. Some leave the cell. While inside the cell, themolecules of life engage in very deliberate reactions tomake new molecules. This is the biosynthesis weexplored in chapter 7. Some of the molecules are usedfor energy, some for information storage. Whatever theirusefulness, the reactions of molecules drive the

Panel 14.1 The major elements of modern cell theory

1) All living things are composed of cells.2) The chemistry of life happens inside cells. This includes

processing food, consuming oxygen, generating carbondioxide, and making products such as sugar, starch, fats,enzymes, and DNA.

3) All cells are derived from pre-existing cells in the processof cell division.

4) Cells contain the means of heredity, otherwise known asgenetic information. This genetic information is passed onto new cells during cell division.

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This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.machinery of life. To the extent that these reactionsoccur more efficiently, life becomes more vigorous. Butin order for molecules to react, they must encounter oneanother.

Cells help increase the efficiency of molecular reactionsmostly by providing a confined, chemically secure placefor molecular encounters. Confinement helps organizelife by keeping molecules close by, preventing themfrom wandering off, and by preventing intrusion fromdisruptive molecules. Cells are very busy places, andbusy places on all levels are possible because of theorganizing influence of confinement. For example, arestaurant is a busy place. Inside the ingredients andcooks are confined to the kitchen. This speeds up the

process of preparing meals. It doesn’t make sense forthe cooks to be wandering through the mall and thepizza dough being run over on the freeway. How longwould you have to wait for your meal? (and who wouldwant to eat it?) Cells, like restaurant buildings andfootball stadiums (fig. 14.2), bring reactive thingstogether so they may carry out their activities in aconvenient and focused way.

I will talk later in this chapter about how the insides ofcells can be further confined, or compartmentalized. Itturns out that the overall capabilities of cells can beincreased by partitioning them up into special anddedicated work areas, like mitochondria andchloroplasts. The point is that confinement is a goodway to organize and speed up the biosynthetic activitiesof life.

14.4 Cells contribute to life�s ability to change the planetin three ways

Cells are the physical agents of life whose busy activitieshave helped change the planet in three major ways: 1)by exchanging materials with the environment; 2) bycontributing to resource competition; and 3) bysupporting the development of very large, multicelledorganisms.

14.5 Cells exchange materials with the environment

First, in order to maintain themselves cells mustactively take in materials from their surroundingenvironment, and put out materials to the surroundingenvironment. For example, all plant cells take inmolecular oxygen (O2 ) from the atmosphere during theprocess of aerobic respiration. Aerobic respiration is partof the biosynthetic process where cells extract energyfrom sugar molecules. Those same cells put out carbondioxide as a wasteproduct of aerobicrespiration.

Animal cells havemore aggressiveoxygen requirementsthan plants.Animals have anelaborate respiratorysystem (lungs [fig.14.3], or gills) andcirculatory system(heart, arteries,veins) that serve theoxygen needs oftheir cells. Whenyou breathe in, eachoxygen moleculeabsorbed by yourlungs ends up inone of your cells. Itis your collection ofcells demandingoxygen thatstimulates you to inhale. Your lungs are a slave to yourbody’s union of cells. So, in order for cells to stay alive,they must exchange materials with the environment.

Some of these materials have contributed to planetarychanges. For example, since living things have builtthemselves out of carbon, they have helped to reduceatmospheric carbon dioxide. Removing carbon dioxidehas helped cool the planet. Photosynthetic cells put outatmospheric oxygen. The point is that if there areenough of these cells on a planet, they can influence thecomposition of the atmosphere.

14.6 Cells contribute to resource competition

Competition is created by cells seeking resources fromtheir surrounding environment. You seek food in themorning because your cells are “hungry” for resources.The mouths, stomachs and blood vessels of animals arestructures that acquire and deliver resources to thecells of their bodies. In plants, it’s roots, leaves andvascular tissue.

Figure 14.3. Human lungs are theorgans that exchange oxygen andcarbon dioxide with the atmosphere.The oxygen is needed by the body'scells. The carbon dioxide is generatedin the body's cells.

Figure 14.2. A football stadium provides a place for footballplayers to meet and react. Cells provide a place for biologicalactivity to happen.


This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.Competition for resources is animportant component ofenvironmental stress. Environmental

stresses of all kinds, includingcompetition, are basic elements

of Charles Darwin’s theory ofevolution by naturalselection. Please indulge me

for a minute as I repeatmyself. Theoretically,

competition takeseffect when there is ahigh demand forresources that are in

short supply. The cells of organisms, either in single-cellform (like an amoeba) or multicelled form (like asycamore tree) demand resources from theirenvironment. As they multiply in their local area, theyeventually can create a situation where there is greaterdemand than there are resources. Hence, competitionbecomes established and the environment is morestressful. According to Darwin’s theory, in a stressfulworld, some individuals of a kind of living thing (aparticular, individual sycamore tree, for example) maybe better able to survive the stressful circumstancesthan other individuals (for example, because thisparticular tree has bigger roots). Those that survive longenough to reproduce, pass on their attributes to theiroffspring. As innovative features appear in each newgeneration, they are tested in the competitive worldcreated by life itself. Sometimes innovations aresuccessful, and living things become slightly morediverse. Sometimes these innovations help living thingsto spread and live in new environments where otherscannot follow — partially because cells hungrilydemand to be fed.

14.7 Cells support the development of very large,multicelled organisms

I have a special interest in how large, multicelledorganisms contributed to planetary changes. This isbecause the colonization of the continents by landplants (starting about 360 million years ago), verysignificantly increased the overall presence of life andits influence on the environment. Big living thingsdeveloped big structures with which to “scoop” biggerportions of resources from the planet. Before landplants, the continents were occupied by a veneer ofsingle-celled bacteria, some filamentous fungi and somesmall lichens (a neighborly combination of a fungi and acyanobacteria). Plants are large, multicelled organismsmade from trillions and trillions of cells. Because plantsuse atmospheric carbon to build themselves, plant-based continental ecosystems probably speeded up theremoval of atmospheric carbon dioxide. Theirphotosynthetic activities also probably increased theproduction of oxygen. David Schwartzman and TylerVolk have pointed out that land plants also transformed

the land by creating and holding on to soil. This had theeffect of increasing the rate of rock weathering and theremoval of carbon dioxide from the atmosphere. AndMark McMenamin, in his sweeping Hypersea theory,argues that the large roots of land plants aggressivelypenetrated deep into the earth to pump up the chemicalsubstances needed for life on the dry continentallandscape. According to Hypersea theory, by bringingwater and other earthly substances to the surface, lifewas able to spread onto the continents in a very bigway. Urban sprawl invaded the continents as densecities of cells sprang up in their tree-shaped high-risecondominiums (fig. 14.4).

The big living things on land are made possible by thedevelopment of big structures — namely roots, treetrunks, branches and leaves. These structures cannotbe constructed as a single cell since cells must remainsmall (see the sidebar on the next page). Instead, theymust be built by clusters of cells. Later in this chapter Idiscuss why one kind of cell (the eukaryotic cell) mayhave been a better candidate for multicelled life thansimpler cell designs.

For now, I want to describe the two basic kinds of cells,how they are constructed and how they process theEarth’s materials.

Figure 14.4. A human city scene is blended with a grove of treesto make a point. The trees are great congregations of cells, likecities are accumulations of offices and apartments.

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There are two main benefits to beingsmall. First, the smaller the cell, the moreefficiently diffusion works. Second, thesmaller the cell, the more surface areathe cell has relative to its internal volume.

Diffusion and Cell Size

The benefits of diffusion have helpedkeep cells small. Diffusion is a physicalprocess that causes materials to spreadout. Basically, heat energy makes particlesbounce around, sort of like kernels ofcorn in a popcorn maker. The more heat,the more bouncing. Diffusion helps movestuff in and out of the cell, and within thecell at no expense to the cell. Therefore,cells can conserve their energy resources.But diffusion has its limitations. It worksbest over extremely short distances. Itseffectiveness decays exponentially withdistance. For example, if it takes onesecond to travel one millimeter, it willtake four seconds to travel 2 millimeters,and 10,000 seconds to travel 100millimeters. So, by staying small, cells canbenefit from diffusion and stay vigorous. Ifthey get too big, diffusion becomesineffective, and the cell will becomesluggish and less competitive. The smalleryou are, the better diffusion works. Whycan�t cells keep on getting smaller andsmaller?

Surface Area and Cell Size

Cell to increase their opportunities toexchange materials with its surroundingenvironment by having a large surfacearea relative to their internal volume.They achieve this by being small. Thespace inside the cell is f illed withequipment that needs raw materials.Also, during the production process,many wastes are produced. The quickerthe cell can obtain needed materials andget rid of wastes the better. Materials andwastes are exchanged with the cell�ssurrounding environment through thecell membrane. The more entryways andexits a cell has, relative to its internalvolume, the more materials can moveinto and out of the cell. One way toevaluate a cell�s materials exchangeefficiency is to examine the surface areaof the cell membrane relative to the cell�sinternal volume.

A very desirable situation would be a cellwith an extremely large surface areacompared to the volume of the cell. Thiswould mean abundant access to theoutside. No waiting to get in or out. Anundesirable situation would be a cell thathas a very small surface area comparedto the volume of the cell. In this case, thecell�s interior activities would starvebecause the materials would not be ableto get into the cell fast enough. Andwastes would build up inside because

they would not be able to get out of thecell fast enough. Clearly, it is best to havea large surface area relative to volume.Which brings us to the topic of cell size.

There is an interesting geometricrelationship between surface area andvolume. What happens is that the ratio ofsurface area to volume gets larger as thesize of an object gets smaller. Forexample, if we take a cube that is 4 incheson a side (f ig. 5.9), we can calculate itssurface area as 4 x 4 x 6 = 96 squareinches. The volume of this cube can becalculated very simply. It�s 4 x 4 x 4 = 64cubic inches. The ratio of surface area tovolume is 96/64 = 1.5 : 1. Now lets dividethe cube into one-inch cubes and do thecalculation again. The one-inch cube has asurface area of 1 x 1 x 6 = 6 squareinches. The volume is 1 x 1 x 1 = 1 cubicinch. For the smaller cube, the surfacearea to volume ratio is 6/1 = 6 : 1. So wecan see from this simple demonstrationthat even a modest reduction in size canlead to large increases in the surface areato volume ratio. When we do the samecalculation for an object the size of a cell,the surface area to volume ratio worksout to be about 9000 : 1. There is nodoubt that the smaller a cell is, thegreater is its ability to exchange materialswith its surrounding environment. In themovie, Honey, I Expanded the Kids (orsomething), why would the little kid nothave survived after he turned into a giant?

Sur fa c e a re a fo r o ne c ub e = ( 1 "x 1 " ) x 6 = 6 s q . in.V o lum e = 1 "x 1 "x 1 " = 1 c u. in .Sur fa c e a re a / V o lum e = 6 / 1 = 6 : 1

4.0 inches

Sur fa c e a re a = ( 4 "x 4 " ) x 6 = 9 6 s q . in.V o lum e = 4 "x 4 "x 4 " = 6 4 c u. in .Sur fa c e a re a / V o lum e = 9 6 / 6 4 = 1 . 5 : 1

Panel 14.2 Why are cells so small?



14.8 There are two main cell designs, prokaryotic andeukaryotic

There are two major designs for cells on Earth. First,were cells of much simpler arrangement, theprokaryotic cells. Later, the more complicatedeukaryotic cells appeared. Prokaryotes have no nucleus,and lack discrete internal organelles (specialcompartments that perform specific functions, likemitochondria and chloroplasts). Despite their simplerdesign, prokaryotic cells are still powerful life forms.Without them, advanced life as we know it might not bepossible.

14.9 Prokaryotic cells were the first kinds of cells onEarth

Today, organisms with prokaryotic cells are the bacteria,and the cyanobacteria. The modern prokaryotic cellembodies the simpler of the two major cell designstrategies (fig. 14.5). In general, “Prokaryotic”, means“before nucleus”. As the name implies, prokaryotic cellshave no nucleus. In addition to the absence of anucleus, prokaryotic cells differ from eukaryotic cells ina number of ways. For example, nearly all prokaryoticcells are surrounded by a rugged cell wall. Just insidethe cell wall lies the cell membrane that, as witheukaryotes, regulates the border crossings of materialsin and out of the cell.

The interior of a prokaryotic cell is notcompartmentalized like eukaryotic cells are. Still, thereare special regions. The cell’s DNA is clustered togetherin an interior region of the cell. The arrangement of the

DNA in prokaryotic cells is differentfrom that of eukaryotic cells. Inprokaryotic cells, the DNA consists ofa single strand that, when stretchedout, is connected at both ends, in asort of circle.

Special kinds of prokaryoticorganisms called a cyanobacteria arephotosynthetic. Cyanobacteria haveelaborate internal membranes towhich chlorophyll and othercomponents needed forphotosynthesis attach.

Prokaryotic cells are simpler thaneukaryotic cells, but they appear inmore diverse forms than eukaryoticcells. The high variety of prokaryoticcells has enabled them to live innearly every imaginable environmenton Earth. And some prokaryotic cellsperform certain biological tasks thateukaryotic cells depend on. For

example, soil bacteria remove nitrogen gas from theatmosphere and convert it into a form that can be usedby plants. Plants can’t remove nitrogen in this way andare utterly dependent upon bacteria to do it for them. Ifwe instantly removed prokaryotes from the planet,complex eukaryotic life would fail soon after. I willprovide a more detailed comparison of prokaryotic cellsand eukaryotic cells in the next chapter. For now, thinkof prokaryotic cells as being present on Earth in manydiverse forms. Yet individual prokaryotic cells are not asversatile as eukaryotic cells.

14.10 Eukaryotic cells are much larger than prokaryoticcells and have special compartments that divide upthe cell�s business

Eukaryotic cells make up most of the living things onthe planet. Protozoans, algae, mushrooms, plants andanimals are made up of eukaryotic cells. The eukaryoticcell isolates its internal functions into diverse andhighly organized compartments called organelles.Rather than have the possibility of different cellularactivities “running into one another”, the organellesprovide special places where the necessary equipmentand molecules can get the cell’s work done with as littledistraction as possible.

Eukaryotic cells are much larger and more complexthan prokaryotic cells, and this added complexity mayaccount for their greater amount of DNA. The addedamount of DNA presents two problems to eukaryoticcells: 1)getting at the information quickly and efficiently;and 2) copying the DNA promptly in preparation for cell

Nucleoid (with the DNA)

Cell wall

Cell membrane

Figure 14.5. A drawing of a bacterium. Bacteria are single-celled, prokaryotic organisms.

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Cell membrane

NucleusLysosome

Mitochondria

Endoplasmicreticulum

Golgiapparatus

Cytoplasm

Ribosomes(b)

Cytoplasm

Ribosomes

Cell membraneNucleus

Lysosome

Mitochondria

Endoplasmicreticulum

Golgiapparatus

Cell Wall

ChloroplastsVacuole

(a)

Figure 14.6. (a), A plant cell. Note the cell wall and chloroplasts. (b) An animal cell. Note theabsence of the cell wall and chloroplasts. I have purposely left out many other cellcomponents


This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.division. The solution? Instead of keeping all of the DNAon a single strand (like in prokaryotes), eukaryotic cellsdistribute the DNA onto several new kinds of structurescalled chromosomes. Add more lanes to the freeway.

The eukaryotic cell is finely organized into discretecomponents, each with a special cellular function (fig.14.6). Below is a summary of the cell parts that arerelevant to the main theme of the book. I havepurposely left out many cellular components and termsbecause they are not important to us right now. If youwant a more complete description of cells and all theircomponents, please consult almost any other biologytextbook or any good encyclopedia.

14.11 (Security and confinement)The cell membrane helps confine the cell�s contentsand separates the inside from the outside

The cell’s contents are enveloped within a very thin, andvery fluid membrane. The cell membrane confines thecell’s activities and helps the cell to control the comingsand goings of molecules. All of the gases, enzymes, foodmolecules and wastes pass into and out of the cellthrough the membrane. In essence, the cell membraneprotects the internal chemistry of the cell.

In eukaryotic cells, the cell membrane mainly iscomposed of a double layer of opposing fat moleculescalled phospholipids (fig. 14.7). The fatty (lipid) part ofthis molecule repels water. Yet the phospho end isattracted to water. These two extremely oppositepreferences exist at opposite ends of the same molecule.This is very good for making a membrane because whenmixed together they organize themselves based upontheir opposite preferences. The phospho ends align withone another. The lipid ends avoid the phospho ends andline up in their own group. In other words, the cell

membrane self-assembles from existing phospholipidmolecules. The result is a double row of phospholipidmolecules that forms a cell membrane. Soap bubblesform in very much the same way.

In addition to the phospholipids, there are specialprotein molecules in the membrane. They help the cellcommunicate with the outside world, regulate the flowof materials across its borders, connect the cell to anadjacent cell, or simply use the membrane as a space toset up shop to conduct enzyme activities. Membraneproteins occupy as much as half of the cell membranefor many cell types. And they are distributedasymmetrically inside, within, and outside themembrane.

Finally, the whole construction of the cell membrane isunderlain with a meshwork of protein fibers for addedstructural support.

I want to stress here that the membrane is a very fluidstructure in which the membrane molecules jostlearound very much like tightly packed Cheerios floatingon milk. It is not a rigid structure by any means. Also,we know the membrane is highly folded and has deeppockets. Although the cell membrane provides chemicalprotection, it provides little mechanical protection. It isabout as flimsy as a soap bubble. Added structuralsupport is provided by cell walls, in plants.

Phospho end(likes water, hates fat)

Phospho end(likes water,hates fat)

Lipid middle(like fat, hates water) Phospholipid

molecules

Membraneproteins

UnderlyingproteinfibersFigure 14.7. Section of the eukaryotic cell membrane.

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14.12 (Building strong walls)In plants, the cell wall provides strong structuralsupport

Most plants have a rigid cell wall that is secreted by thecell itself (fig. 6.6a). Of the eukaryotic organisms, onlyplants have a cell wall. Cell walls provide structuralsupport for the cell, and collectively, the whole plant(fig. 14.8). The wood of trees is nothing less than cellwalls. It is very strong and durable. The cell wallactually is a combination of two wall types, but for ourpurposes we will think of them as a single unit. Itsconstruction is similar to a wall of reinforced concrete(except that materials move freely through the cell wall).Cellulose is a long, stringy molecule made by chainingindividual sugar molecules together. The cell wall isreinforced by bundles of cellulose molecules, like aconcrete wall is reinforced with rebar. The cellulosestrands are set in a matrix (cement) of three kinds ofproteins and cellulose-like molecules.

14.13 (Library and copying service)The nucleus houses and protects the eukaryotic cell�sDNA

DNA occurs in the cellin two main places: 1)the nucleus; and 2)the chloroplasts andmitochondria. TheDNA in the nucleus iscalled nuclear DNAand it is the mainDNA we are concernedwith right now. Thecell’s nuclear DNA isbound up in large,complex moleculescalled chromosomes.These are held insidethe cell nucleus. The

nucleus is shielded from the rest of the cell by a double-layered membrane of its own, similar in design to thecell membrane. In general, all of the information neededfor the operation and replication of the cell is containedin the chromosomes.

Multicelled organisms grow,maintain themselves and repairthemselves by adding newcells. The new cells comefrom the division of existingcells. When a cell divides, itsplits into two smaller cells.The DNA in the original cellgets copied so that each newcell has an exact copy of theDNA. This process isrepeated thousands of timesin plants, animals, algae,and fungi. So large,multicellular, resource-hungry organisms arepossible by DNA’s self-copying abilities.

14.14 (Stirring it up in a mixing bowl)The cytoplasm is a turbulent mixing arena, speedingup the cell�s chemistry

The cytoplasm is all the material inside the cellmembrane, and outside the nucleus. It is a watery baththat contains all of the cell’s non-nuclear internalcomponents (such as chloroplasts and mitochondria).This is a turbulent world where the watery plasma ofthe cell circulates somewhat like a circular stream inwhat is called cytoplasmic streaming. Biologists thinkthe streaming is caused by movements of thecytoskeleton. The cytoskeleton is a three-dimensional

Books store information in humancode (the alphabet and humanlanguage). DNA stores chemicalinformation. Books are stored in alibrary. DNA is stored in the cellnucleus.

Humans use a copyingmachine to reproduceinformation. DNA can makecopies of itself.

Figure 14.8. (a) A construction crew builds awall reinforced with strips of steel. (b) Atree's cell builds a wall reinfoced withcellulose.

(a)

(b)


This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.lattice of interconnecting filaments. The filaments havethe ability to contract like tiny muscles. As thecytoplasm circulates, organelles rapidly spin and danceabout, increasing their materials exchangeopportunities with the cytoplasm. It is sort of like a tinymixing bowl. Cytoplasmic streaming is roughlyequivalent to stirring your coffee after you put in thesugar. It helps distribute the sugar much faster, than ifthe coffee remained unstirred. We will see later thatcytoplasmic streaming may be very important in makingeukaryotic cells, and multicelled life possible.

14.15 (Energy capture and storage)Chloroplasts help make sugar and convert nitrate toammonia

The chloroplast is the place where photosynthesishappens in green plants and other photosyntheticeukaryotes, like algae (fig 14.6a). Not all plant cellscontain chloroplasts. Which parts of a tree do you thinkdo and do not have chloroplasts? The chloroplast has aspecial design that supports the ‘capture’ of sunlightenergy which is used to support carbon fixation and thesynthesis of sugar. In order to make sugars, thechloroplasts need building materials. So they consumeH2O, and CO2. In the process, they release O2 as a wasteproduct. The O2 exits the chloroplast and, if the O2

doesn’t immediately flow toward mitochondria in thesame cell, it is eventually released to the atmosphere,available now for your mitochondria.

Chloroplasts also are the place where assimilativenitrate reduction happens. This is a biosyntheticprocess where nitrate is ‘transformed’ into ammonia.Ammonia is a vehicle that makes nitrogen available forother biosynthetic reactions that use it.

14.16 (Making energy available � right now!)The mitochondria help consume sugar and oxygen, anddistribute energy for all the cell�s activities

All eukaryotic cells havemitochondria (includingplant cells). Themitochondria are veryspecialized organellesthat play an importantrole in extracting energyfrom sugar. In theprocess, they use upmolecular oxygen (O2).

Let’s follow O2 and CO2

through your body for amoment. When youinhale, the O2 taken in byyour lungs makes it wayinto the blood, and then

to the cells of the body. Once inside the cell, the O2

enters the mitochondria, combines with waste hydrogenatoms to become H2O. When you exhale, your breathhas an elevated amount of CO2 in it. The CO2 comesfrom the final breakdown of sugar in the mitochondria.The CO2 makes its way out of the mitochondria, out ofthe cell and into your blood where it is released to theatmosphere by your lungs. This is how exercise helpsyou lose weight, by releasing more CO2 to theatmosphere.

Biologist, Lynn Margulis of the University ofMassachusetts has proposed a theory arguing thatchloroplasts and mitochondria were one time free-livingbacteria. At one point in their history, they becameengulfed and enslaved by eukaryotic cells. According toher serial endosymbiosis theory, eukaryotic cells camefrom bacteria by the development of cooperative livingarrangements. That is, certain kinds of bacteria took uphousekeeping inside the larger, eukaryotic cell. It is areasonable idea since chloroplasts and mitochondriaare very much like bacteria. Chloroplasts andmitochondria possess their own unique DNA, and theydivide on their own, independent of their host cell. Theorganization of such cooperative living conditions was avery significant step in the development of eukaryoticcells, and multicelled life. I will elaborate on this issuein the next chapter.

14.17 (Home improvement and manufacturing for export)Proteins and fats are assembled in the endoplasmicreticulum

The endoplasmic reticulum is a complex system ofmembranes that is the manufacturing center of the cell.This is where most of the deep biosynthesis happens ineukaryotic cells. Cells make things mainly for internaluse. They also make substances for export. The types ofmolecules that are made here are proteins (usuallyenzymes), and fats.Proteins areassembled in therough endoplasmicreticulum and fatsare assembled in thesmooth endoplasmicreticulum. Enzymesexert a powerfulcontrolling influenceover the cell’sbusiness activities.Sugar reserves toolow? Theendoplasmicreticulum willsynthesize anenzyme that

A cook prepares a meal on a kitchencounter. The kitchen to humans ismuch like an endoplasmic reticulum tocells. It's a place where the ingredientsof life come together and areassembled into something new.

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This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.increases the uptake of sugar across the cellmembrane. Where do the plans for making this enzymecome from? The DNA in the nucleus.

Note that DNA carries information for the constructionof important regulatory molecules like enzymes. Thatcoded information is copied in the nucleus, then thismessage is delivered to the endoplasmic reticulum.There, the code is used code to assemble amino acids inthe correct order making an enzyme. Enzymes greatlyspeed up chemical reactions (therefore resourcesconsumption) inside the cell. The endoplasmicreticulum provides a workspace for enzymes to besynthesized. That is, the endoplasmic reticulum playsan integral role in increasing the cell’s level of activityand demand for resources.

14.18 (Shipping department)The golgi apparatus packages and ships products likeenzymes, hormones, and mucus

The cell needs anefficient way topackage and ship itsproducts. The golgiapparatus is a foldedsystem ofmembranes that arearranged like abunch of flattenedsacs. Theendoplasmicreticulum feedsproducts to thesesacs where they arepackaged andshipped outside ofthe cell membrane.Sometimes the golgiapparatus packagesproducts for internaluse. Such anexample would be the packaging of powerful protein-eating enzymes into a new internal cell componentcalled a lysosome.

Cell products that are shipped by golgi apparatusinclude such substances as saliva, mucus, tears,digestive enzymes in the gut, bile from the liver, skinoils and skin pigments (fig. 14.9).

14.19 (Housekeeping and recycling)Lysosomes digest engulfed resources and recycleworn out parts

The lysosome helps recycle aged or unused cellularcomponents. The lysosome itself is a package ofenzymes that has the potential of dismantling everykind of molecule in the cell. But the cell at large isprotected from the destructive chemicals of thelysosome because it has a double-layered membrane.Sometimes certain organelles fall prey to the lysosome’srecycling tendencies. For example, when the cell hasmore mitochondria than it needs, the lysosomes engulfthem and digest them. The digested remains are thenreleased back into the cytoplasm where they are usedagain to make new molecules and new organelles.

A restaurant worker delivers foodprepared by the cook. In our humanworld, products we consume arepackaged. This protects the productand makes it easier to move. In cellsthe golgi apparatus packages cellularproducts and ships them out.

Figure 14.9. The saliva glands of the human body. The glands aremade up of cells whose endoplasmic reticula make lots ofdigestive enzymes. The enzymes are packaged in the golgiapparatus, shipped out of the cell where they enter a saliva duct.When you eat something the saliva flows and its enzymes startthe digestion process.



14.20 Considering the abilities of eukaryotic cells in thesupport of large living things

The remaining part of this chapter is devoted tounderstanding the beauty of the eukaryotic cellarchitecture as it relates to the requirements ofconstructing macroorganisms (large living things).

The world’s continents are covered with largeorganisms, especially plants. This observation isimportant because large living things greatly contributeto life’s overall influence on the environment. Theyconsume huge amounts of resources to build, operate,and maintain themselves. Macroorganisms also supportlarge numbers of single-celled organisms in a complexecological network.

Eukaryotic cells have played a crucial role in thedevelopment of large organisms. Vastly greaterdemands for resources and territory accompanied theappearance of eukaryotic cells as they consumed thesmaller prokaryotic cells. In theory eukaryotic cellsactually are a community of prokaryotic cells that haveevolved to lived together inside the cell — thus forminga new kind of cell altogether, the eukaryotic cell. Thepresence of macroorganisms, made from trillions ofeukaryotic cells, stepped up competition for resourcesin a big way. The versatile eukaryotic cell architecturehelped support the development of newer kinds ofmacroorganisms capable of surviving in the dryfrontiers of the continents.

But why are macroorganisms constructed of eukaryoticcells and not prokaryotic cells? The answer pursued inthis chapter has a lot to do with versatility and size.Biologists have observed thatmacroorganisms generally containhundreds and even thousands oftimes more DNA than do single-celledorganisms like bacteria. Based uponthis evidence it may be that lots ofDNA is required to make amacroorganism. And a very large cellis needed in order to store the DNA.Prokaryotic cells are not goodcandidates for multicelled life largelybecause they are too small to housethe large amounts of DNA needed.But eukaryotic cells are thousands oftimes larger than most prokaryoticcells. How they are able to achievethis feat has much to do with theirbasic architecture, compartmentaldesign features, and unique ways ofhandling their large amounts of DNA.Nonetheless, being small has itsadvantages.

14.21 Prokaryotic cells have exploited the advantages ofbeing small

Prokaryotic cells and eukaryotic cells both are highlyevolved biological structures. However, they haveachieved success in very different ways. Prokaryoticcells are much smaller and simpler than eukaryoticcells. They are only about a tenth as long as eukaryoticcells and fill only about 1/1000 of the volume of aeukaryotic cell (fig. 7.3). Also, they don’t have thecomplex interior that eukaryotic cells have. There areadvantages to being small and simple. For example,diffusion works better on smaller cells. Their simpledesign speeds up the cell division process and hurriesthe growth of new cells. So, prokaryotic cells grow anddivide much faster than eukaryotic cells. The muchstudied bacteria, E. coli (it lives in your intestines)divides about once every 20 minutes. SomeStaphylococcus bacteria (that cause staph infections)divide about every 25 minutes. This kind of growth canbe sustained as long as there is food, and as long asbacterial wastes don’t build up too high to become toxic.If we follow a staph infection in an ideal situation, wecan see how you can get a sore throat in such a shortperiod of time. Figure 14.10 shows how fast bacteriacan grow. The curve goes up quickly. If a singleStaphylococcus cell becomes established, you will haveover 16 million Staphylococcus bacteria in your throat inabout 10 hours (hypothetically). With this kind ofreproductive speed, bacteria can quickly dominate anew environment.

0.00 0.83 1.67 2.50 3.33 4.17 5.00 5.83 6.67 7.50 8.33 9.17 10.00

0

2,000,000

4,000,000

6,000,000

8,000,000

10,000,000

12,000,000

14,000,000

16,000,000

18,000,000

0.00 0.83 1.67 2.50 3.33 4.17 5.00 5.83 6.67 7.50 8.33 9.17 10.00

Hours

Staph Bacteria Population Growth

Figure 14.10. A drawing of a microscope view of cells, superimposed on a graph showinghow fast bacteria poulations can grow. Notice the Amoeba (a kind of eukaryotic cell) ismuch larger than the many bacteria. On the graph, can you see how long it took to reach2 million? How many bacteria will be present after 10 hours and 25 minutes?

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This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.Being small and relatively uncomplicated helps theprokaryotes to deploy rapidly when nutrientsperiodically become available. They quickly colonizesurfaces where nutrients may be sparse and short-lived. And producing such large numbers helps reducetheir risk to extinction. Prokaryotic cells do producewastes such as acids and toxins, but no real productsfor export. Since they do not build macroorganisms,they rarely manufacture products for use elsewhere. So,prokaryotic cells spend the bulk of their energy simplygrowing, then dividing and growing some more.Prokaryotic cells are simple in design and “single-minded” in their lifestyles. They are very efficient, butthey are not individually versatile.

14.22 Prokaryotic cells are more efficient and morebiochemically diverse than eukaryotic cells

There is a myth amongst some biologists thatcompartmentalization makes eukaryotic cells moreefficient than prokaryotes. This view is probably wrong.Prokaryotic cells are far more efficient than eukaryoticcells. They grow faster, they divide faster, they consumeresources faster and they produce wastes faster thaneukaryotic cells. The small size of prokaryotic cellshelps them achieve this efficiency. Materials moveacross short spaces inside, making diffusion moreefficient. They have a large surface area-to-volume ratiothat increases their relative contact with theenvironment. Therefore, materials are exchangedefficiently. So, despite their compartmentalization,eukaryotic cells are not as efficient as prokaryotic cells.Still, there is value in the compartmental approach. Ithelps the cell to be larger and more versatile.

14.23 As a whole, prokaryotic cells are more biochemicallydiverse than eukaryotic cells, but individually theyhave a narrow repertoire

Prokaryotic cells are a very diverse group of highlyspecialized individuals. If we compare the whole groupof prokaryotic cells with the whole group of eukaryoticcells, the prokaryotic cells are more biochemicallydiverse. We discussed their diverse biosynthetic abilitiesin chapter 7.

As a whole, prokaryotic cells do some things eukaryoticcells cannot. For example, some kinds of prokaryoticcells can remove nitrogen from the atmosphere. Otherprokaryotes return nitrogen to the atmosphere. Specialkinds of prokaryotic cells can use sulfur as an energysource. And they can ferment sugar into more kinds ofchemicals (like methane) than eukaryotic cells can. Yetindividual prokaryotic cells have very narrowrepertoires. They are dedicated primarily to exploitingtheir current environmental conditions in such a way asto maximize their rate of growth and reproduction.There are very few, if any, additional internal activities

of significance. This miniaturization, specialization,rapid growth, and reproduction is the road prokaryoticcells have followed. Their ability to narrowly specializehas enabled them to occupy even the most extremelyhot, cold, dry, and acid habitats. They cover the Earth,but not as complex, macroorganisms. Althoughspecializaton does have its benefits, it also comes at theexpense of individual versatility. In a cell-to-cellcomparison, individual eukaryotic cells are far moreversatile and capable than individual prokaryotic cells.And this may have better supported the development ofmacroorganisms.

14.24 Increased internal versatility and larger cell sizesupported the development of macroorganisms

Apparently, macroorganisms are better supported bycells that have the internal ability to do many differentthings simultaneously.

Perhaps it has to do with the dual life they lead, as waspointed out by Matthias Jacob Schleiden about 150years ago. He recognized that plant cells mustcontribute to the growth, operation and maintenance ofthe whole plant, as well as themselves. Individual cellsin macroorganisms have a much more complicated lifethan bacteria cells do. In addition to keeping themselvesalive, they must be “productive members of society”.The society being the whole organism of which they area part. The society of cells in macroorganisms is not“laid back.” It is a fast-paced world that places hugedemands on its members. It may be that the complexitywe see in eukaryotic cells supported the development ofcells that could engage in this connected, networkedand busy life. This high complexity may have beenbetter supported in cells larger than the prokaryotickind. In addition, one of the prices of macroorganismallife is the high overhead of carrying around hugeamounts of DNA.

Organism

Number of Base Pairs

in the DNA

(times 1 million)

Prokaryotes

Small mycoplasma 1.6

E. coli 4

Large cyanobacterium 16

Eukaryotic organelle

Baker�s yeast mitochondrian 2-3

Eukaryotes

Small yeast species 5

Baker�s yeast 12-20

Range in mammals 2800-5300

Range in protozoa 55-320,000

Table 14.1. A comparison of the amounts of DNA in different kindsof cells. Can you see a general trend?


This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.Eukaryotic cells generally have much more DNA thando prokaryotic cells. Table 14.1 lists some examples ofprokaryotes, eukaryotes, and a mitochondrion(remember mitochondria have their own DNA). The tableindicates that mammals (fur-bearing animals) havehundreds and even thousands times more DNA thanprokaryotes. There is a striking difference in the obviousfunctionality of the DNA when comparing prokaryotesand eukaryotes. In prokaryotes, nearly all of the DNAhas a coding function. That is, their DNA is filled withuseful information without excess nonsense. Noting thischaracteristic, biologist, Walter Gilbert describedprokaryotic DNA as “streamlined.” However, ineukaryotes, most of the DNA is non-coding. In otherwords, the nucleus of eukaryotes is filled mostly with“junk DNA” that contains no obvious information.Biologists debate why more complex organisms have somuch non-coding DNA, and there is no solid answer.For now, I will adopt the notion that the extra DNA wassomehow important in the evolutionary development ofeukaryotic cells and the macroorganisms they build.

So, building a macroorganism was probably a veryunlikely possibility for prokaryotes. Their small size istoo cramped for the more massive internal hardwareand DNA we see in macroorganisms. The developmentof macroorganisms became the province of eukaryotes.Still, the prokaryotes were not entirely left behind in themarch toward more complex life forms. We will see laterthat prokaryotic cells contributed to the development ofeukaryotic cells. Today, eukaryotic cells are thebeneficiaries of prokaryotic efficiency in more ways thanone.

14.25 How eukaryotic cell architecture supports thedemands of macro-life

Being larger has two main benefits:1) having the space for the equipment that lets the cell

do more things simultaneously2) having the space for the much greater amounts of

DNA that accompany macroorganisms

All macroorganisms are composed of eukaryotic cells.One of the more satisfying benefits of being bigger isthat eukaryotic cells can engulf and consumeprokaryotic cells outright. Thus, eukaryotic cells canobtain concentrated supplies of nutrients with littleexpenditure of energy. Fungus biologist, Michael Carlile,made the enlightening comparison of prokaryotes andeukaryotes by depicting them as aircraft with distinctlydifferent missions. Figure 14.11 (next page) and muchof this chapter, for that matter is inspired by Carlile’swork. Study this drawing and its caption. Also, LynnMargulis proposed that the development of theeukaryotic cell actually got started by prokaryotic cellsengulfing other prokaryotic cells. Her idea is called theSerial Endosymbiosis Theory. More about it later.

The greater versatility of eukaryotic cells may have beena prerequisite for the construction of the greatcommunity of cells in macroorganisms. Internalversatility is achieved by specialized cellular “hardware”such as chloroplasts, the endoplasmic reticulum andother internal structures I discussed earlier. However,this added equipment requires lots of space. Also,coming together to form multicellular macroorganismswas accompanied by much more DNA. Both the internalhardware and the greater amounts of DNA take up lotsof cell space. Eukaryotic cells provide the space forthese features.

While there are benefits to being larger, there are alsopenalties, such as:1) longer growth and development times, and slower

reproduction cycles2) greater resource demand, but slower diffusion rates

because of greater distances and lower surface-to-volume ratio

3) greater quantities of DNA, which present specialcopying problems at the time of cell division

Eukaryotic cells have incorporated special innovationsthat help them overcome some of these potentialshortcomings. The following sections describe theseunique eukaryotic features.

14.26 Eukaryotic cells speed up resource uptake anddiffusion in the cell with the use of a highly folded andpocketed membrane, the ability to engulf, andcytoplasmic streaming

The biggest disadvantage eukaryotic cells havecompared to prokaryotic cells is a much smallersurface-to-volume ratio. This is simply a function ofgeometry. The larger the cell, the smaller the surfacearea-to-volume ratio, and the harder it is for the cell toget the things it needs from the surroundingenvironment. Also, the larger the cell, the longer it takesfor materials to travel from the cell membrane to cellcenter. This is because diffusion loses its efficiency asdistances increase. The bigger cells clearly are at adisadvantage in this area when compared to prokaryoticcells. But eukaryotic cells have a few tricks that helpthem improve the movement of materials.

One way to improve the uptake of materials (and thedisposal of wastes) is to increase the cell surface area-to-volume ratio. Prokaryotic cells have a relativelysmooth cell membrane, but the membrane is highlyfolded in eukaryotes. The folds significantly increase thecell’s overall surface area while not increasing itsvolume. Lately, biologists have learned that theeukaryotic cell membrane is still more complicated. Forexample, Thomas Landh of the State University of NewYork at Buffalo has described pockets in the membranethat fold in such complex ways that it took him twoyears and some sophisticated mathematics before he

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This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.could visualize them. He calls the pockets “cubicmembranes” which are structures made from whatmathematicians call “periodic minimal surfaces” (fig.14.12). The interesting property of cubic membranestructures is that they partition space with themaximum surface area-to-volume ratio. Landh’sobservations appear to be consistent with those madeby biologists Richard Anderson and Michael Lisanti.

They have discovered what appear to be “caves” in thecell membrane. They have found that these tinystructures can open, take in materials, then close. Thiscould be a way to transport materials into the cell. Thepoint is that the cell membrane in eukaryotes is highlycomplicated. Still, there are more aggressive ways ofgetting materials inside.

Figure 14.11This drawing depicts a large “mother ship” in a struggle with smaller aircraft. The mother ship has a broad diversity of internal capability, andrepresents a eukaryotic cell. The planes flying around it are smaller, have far less individual capability but occur in many different forms. Theyrepresent the prokaryotic cells. Notice that one of the planes has been captured by the mother ship. This is a major advantage of being big.Eukaryotic cells can eat prokaryotic cells. See the pizza chefs running around? He represents the many mitochondria that provide immediateenergy for the cell’s activities. According to the Serial Endosymbiosis theory, the mitochondria are descended from bacteria that were oncecaptured by the larger cell. They took up refuge inside the cell and continue to provide this useful service. So, the mitochondria, like the pizzachef is a tireless captive. Notice also that while one part of the ship is capturing planes, other parts of the cell are simultaneously engaged inother activities such as photosynthesis, manufactiring, shipping, and security. (Inspired by Michael Carlile)


This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.Eukaryotic cellsdon’t have to waitaround for nutrientsto slowly diffuse intothem. They cansimply engulf them(fig. 14.13). You maybe familiar with thesingle-celledAmoeba. Watchingthis organism givesa gooddemonstration ofhow the engulfingprocess works.(Note, biologists haveabout three or four different terms to describe thisprocess, but we will just refer to it as “engulfing.”)During this process the cytoplasm and cell wallsurround the food item. Once surrounded, it’s no longeroutside the cell, it’s inside. The act of engulfing isperformed by re-shaping the cell. Reshaping isaccomplished by moving the cytoplasm around. This is

a mechanical process accomplished by movements inthe cell’s delicate and flexible cytoskeleton. The abilityto move cytoplasm around also helps stir things upinside the cell in another important cellular processcalled cytoplasmic streaming.

Cytoplasmicstreaming verysimply is thesomewhat vigorousand circularmovement of thecytoplasm (fig14.14). It isespecially visible inplant cells.Biologists thinkcytoplasmicstreaming is causedby contractilemovements of thecell’s fine

cytoskeleton. As the cytoskeleton moves, it carriescytoplasm components with it, such as mitochondriaand chloroplasts. The advantage of cytoplasmicstreaming is that it acts to stir up the cytoplasm whichincreases the speed in which molecules encounter andreact with each other inside the cell. Although thecytoplasm is a turbulent place, it is not necessarilychaotic. The reason is compartmentation.

14.27 Compartments of specialized activity help improveefficiency inside eukaryotic cells.

The internal activities of eukaryotic cells have specialplaces for them, as we saw in the previous chapter. Myfavorite examples of such compartments are themitochondria and the chloroplast. But there are manymore. Compartmentation helps the cell do two mainthings:1) counteract losses in cell efficiency due to greater

size2) organize and segregate the complex inner workings

of cells that are engaged in a variety of activitiessimultaneously

Compartmentation does not make the eukaryotic cellmore efficient than the smaller prokaryotic cell. Butcompartmentation does make the large eukaryotic cellmore efficient than it would be withoutcompartmentation. In other words, compartmentationimproves cellular efficiency such that the large cell cansurvive. For example, the chloroplast establishes aplace for photosynthesis to happen. Inside thechloroplast are all the enzymes in just the rightamounts that are needed to keep the process going.These enzymes are not simply floating around in thecytoplasm. They are concentrated inside thechloroplast. Compartmentation improves the efficiencyof cellular processes by confining the chemicals of lifesuch that they are more likely to react at optimumlevels.

Compartmentation also is important in segregating thedifferent activities. Eukaryotic cells have so much goingon. There are thousands of reactions happening eachsecond. If there was not some way to keep the cell’schemicals from randomly moving about, they would endup interfering with the cell’s overall chemistry. In otherwords, the product of one reaction could be a poison toanother reaction. If these two are not separated, the cellcould fail. One shouldn’t pour the dish water into thesoup. So, compartmentation supports the high varietyof simultaneous activities in eukaryotic cells by keepingthem separate.

Two of the special “compartments” inside the cell arethe chloroplasts and the mitochondria. In addition, thefine, contractile filaments make up the cytoskeleton,and flagella (whip-like structures that propel the cell).How they came to occupy eukaryotic cells is aninteresting story, covered next.

Figure 14.12. A drawing that tries torepresent a cross-section of theeukaryotic cell membrane asenvisioned by Thomas Landh's workon cubic membranes. Notice that thefolds substantially increase the surfacearea while the increase in volume isnegligible. Why is this good for a cell?

a. b. c.

Figure 14.13. Prokaryotic cells can't do this. An Amoeba engulfs abacterium. Flexing of the cytoskeleton directs the movement of thecytoplasm around the bacterium. Your white blood cells work likethis to fight infection.

Figure 14.14. A cross-section of thecell showing cytoplasmic streaming.This movement is expensive becauseit requires energy. Wouldn't the cell bebetter off conserving its energy insteadof using it to stir up the cytoplasm?

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Panel 14.3 The Serial Endosymbiosis Theory

PlantsAnimals Fungi

KingdomProtoctista(Algae,Protozoans)

Ancestral, free-livingCyanobacteria.Source of modernchloroplasts.

Ancestral, free-livingspirochete bacteria.Source of contractilefilaments used incytoskeleton.

Ancestral, free-livingThermoplasmabacteria.Source of nucleusand cytoplasm.

Ancestral, free-living,oxygen-respiringbacteria.Source of modernmitochondria.

Intermediate stage

The Serial Endosymbiosis Theory (SET)was contributed in the 1970s by LynnMargulis of the University ofMassachusetts. It attempts to explain howeukaryotic cells came about at a time inthe Earth�s history when the world wasdominated by prokaryotic cells. BeforeSET, the prevailing view was thatprokaryotic cells somehow evolved intoeukaryotic cells. Margulis was unsatisfiedwith this view. Her studies onmicroorganisms led her to see theremarkable similarities of bacteria andcertain eukaryotic cell components. Oneof the most inexplicable things about thechloroplasts and mitochondria ineukaryotic cells is that they have theirown DNA, and it is separate from thenuclear DNA. They move veryindependently, and divide veryindependently. It�s as if they are, well,independent. Despite the conventionalwisdom to the contrary, Margulis couldnot ignore this situation. So, she madethe startling prediction that mitochondriaand chloroplasts in modern eukaryoticcells are really the descendants of oncefree-living bacteria that were engulfed bya larger cell, survived digestive assaults,and took up permanent residence insidethe cell. As a result, the larger cellprospered by the energy managementabilities of these new inhabitants. And so,they lived together (symbiosis). Thedrawing at right shows the paths themitochondria, nucleus, cytoplasm,

cytoskeleton, and chloroplasts may havetaken to result in modern eukaryoticcells. SET is now gaining widespreadacceptance by biologists.

So, eukaryotic cells are not entirely newkinds of cells. Instead, they are acommunity of diverse prokaryotic cellsliving together and achieving remarkablebiological success. The cells inside youand I are filled with mitochondria that aredescended from free-living bacteria.Prokaryotic cells were not left out of themacroorganism game. They built it.

There are two lessons from this story: 1)a person with a vision, backed bypowerful evidence can overturnconventional ways of looking at theworld; and 2) life has produced amazinglycomplex organisms and wholeecosystems by seasoning competition-based evolution with sprinkles of spicycooperation.

Drawing below adapted from Margulis, Lynn, 1992,Biodiversity: molecular biological domains,symbiosis and kingdom origins. Biosystems ( 27),39-51.



14.28 Eukaryotic cells are big houses in which specialprokaryotic cells have taken up residence

Microsoft Corporation is the world’s largest computersoftware company. It struggled for years to develop acomputer program that would help people like you andme manage our checkbook and do simple financialtasks. Their best attempt was called Microsoft Moneyand it never caught on. This was partly due to the factthat its program had been preceded by one produced bya competitor. The competitor was called IntuitCorporation and its product was called Quicken. Oneday in 1994, Microsoft decided they would be better offif they simply bought Intuit Corporation. This would begood for Microsoft since it then would own Quicken andcould enjoy the benefits of the revenue it generates. Italso would be good for Intuit Corporation since it wouldmean greater security and the availability of moreresources for the future development of its programs,like Quicken. So, what does this little story have to dowith cells?

The purpose of the above story is to demonstrate thatsometimes the best way to get more of what you want isto form alliances. We see this often in biology. Forexample, your whole body is a collective of cells whohave ‘agreed’ not to compete with one another but tocooperate. Cooperation can lead to greater things. Also,this story points out the advantages of large entitiesacquiring smaller entities. Because this is howbiologists think the nucleus, cytoskeleton,mitochondria, and chloroplasts came to be incorporatedinto eukaryotic cells.

The origin of the nucleus, cytoskeleton, mitochondria,and chloroplasts, in eukaryotic cells long has puzzledbiologists. Chloroplasts and mitochondria move and actvery independently inside the cell. They have their ownDNA that is far different from the DNA inside the cell’snucleus. In the late 1970s biologist Lynn Marguliswondered about these structures. She proposed that allof these components were not developed “in-house” bythe evolving eukaryotic cell. Instead they were acquired.In her initially controversial, and now widely accepted‘Serial Endosymbiosis Theory’, Margulis has outlinedhow bacteria with beneficial specialties have come toreside inside eukaryotic cells. According to her theory

(Panel 14.3), all eukaryotic cells and theliving beings they construct harbor and arethus dependent upon the internal activitiesof prokaryotes. In other words, eukaryoticcells are principally a cooperativecommunity of prokaryotic cells that benefitby living together (symbiosis).

The primary consequence of the activitiesof endosymbionts like mitochondria andchloroplasts is that they maintainthemselves. The secondary consequence ofthe activities of endosymbionts is that theyprovide services and products to the cell atlarge. The ultimate consequence is that theendosymbionts and the whole cell thrive.

14.29 Eukaryotic cells use mitosis as away of reliably duplicating their huge quantitiesof nuclear DNA

As I mentioned earlier, eukaryotic cellshave much more DNA than do prokaryoticcells. This represents a huge amount ofmolecular code. The DNA of prokaryotesand the nuclear DNA of eukaryotes arepart of larger molecules calledchromosomes. Recall that the DNA holdsthe coded information that helps build,operate, and maintain the cell. It also hasthe ability make copies of its code either for

1. Cell in growing phase.DNA is spread out.

2. Cell in preparation forcell division. DNA hasbeen copied, and beginscoil up into compactchromosomes.

3. Copied chromosomesline up at cell equator.Getting ready to separate.

4. Each half of the copiedchromosome is pulledin opposite directions.

5. The cell membranestarts to pinch in andform two new cells.The chromosomes startto unravel.

6. Two smaller cells result.Each one is an exact copyof the original cell.

Figure 14.15. Some of the essentials of mitosis. This process is not found inprokaryotic cells. It is important in eukaryotic cells since they have so much DNA. Itcould become hopelessly tangled during cell division. Mitosis makes the processorderly and neat so that the DNA is reliably copied, separated and moved to the newcells.

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This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.the operation of the cell or for cell reproduction. This isall fine except that there is so much of it in eukaryoticcells. Some mammals have as many as five billion basepairs.

The problem is how to move such huge amounts oftangled DNA during cell division without breakage. Asthe size of the chromosome increases, it is more difficultto move during cell reproduction. One feature that helpswith this problem is that the DNA in eukaryotic cells isplaced not on a single huge chromosome, but onseparate chromosomes. For example, the DNA insideeach of your cells exists on 23 pairs of chromosomes.Your dog has 39 pairs of chromosomes. Prokaryoticcells have a single chromosome. It is easier and morereliable to move several smaller chromosomes than asingle gigantic chromosome. But putting the DNA onseveral chromosomes is the first step. The next questionis how to move these chromosomes without getting alltangled up.

It is simple to visualize the difficulty in untangling aplate of spaghetti 5 miles across, while not breaking asingle noodle. Mitosis is a cellular process in which thechromosomes are reliably copied and moved during celldivision (fig. 14.15, previous page). One of the keyfeatures of mitosis is very similar to eating a plate ofspaghetti. Most of us spin the noodles around our fork,then eat it. Mitosis handles the long, stringy DNA inmuch the same way. Instead of a fork, the DNA iswrapped around spherical protein molecules. These arefurther coiled into tight packages that are easily andreliably moved without breakage. Mitosis only happensin eukaryotes. Because it helps in the reproduction andmovement of large amounts of DNA, it is one morefeature that has supported the colonization of thecontinents by large organisms.

14.30 DNA replication occurs at many points simultaneouslyin eukaryotic cells

Before chromosomes and their DNA can be movedduring cell reproduction, the DNA must be copied.Prokaryotic cells start the DNA copying process at asingle point on the inside of the cell membrane. This isefficient enough since prokaryotes have such a smallamount of DNA. However, the DNA molecules ineukaryotic cells have many points in which thereplication process can start. So in preparation for celldivision, the DNA of eukaryotic cells is busy copyingitself at many different locations simultaneously. Thishelps speed up the replication process a great deal. Itreduces time between cell division, thereby increasingthe rate of growth of macroorganisms. If DNA did notreplicate itself at many different locations, the processwould be so slow that eukaryotic cells andmacroorganisms might not be possible.

14.31 Summary Remarks

The main purpose of the second half of this chapter wasto help you to understand how the eukaryotic cellarchitecture is relevant to life’s greater influence on theplanet. Without eukaryotic cells, it is uncertain if Earthwould have forests filled with trees, flowers, insects andjaguars. Although animals existed in the sea for at least200 million years before there were any plants, it wasthe plants that moved life onto dry land in greatmeasure. As they spread, plants increased life’s overallinfluence on the global atmosphere twofold. But thesuccess of plants depended upon a series of priorbiological developments that all began with the way thecell was put together.

In Panel 14.4, I have summarized the main steps in celldesign that led to the colonization of the continents. Usethis drawing to help you better visualize therelationships between the topics I presented in thischapter.



The colonization of the continents byplants is a good example of life�stendency to exploit the Earth in moreaggressive ways. The continentspresented special opportunities andobstacles to life. Althoughmicroorganisms (single-celled organismslike bacteria and protozoans) wereextremely successful in the sea, theyachieved only limited success on thecontinents. Life more fully exploitedopportunities on the land by thedevelopment of plants. But this tookseveral steps. This drawing summarizes

these steps as a daisy chain ofevolutionary changes that f inally enabledlife to maximize its colonization of thecontinents.

I have arranged the chain so it can beused to trace the steps forwards orbackwards. The point is to understandthe significance of eukaryotic cells in thecontext of the bigger picture of life�soverall influence on the planet.

Panel 14.4 Daisy chain of steps leading to the colonization of the continents and life's greater planetary infuence

Maximization of life's infuence on the atmosphere

Maximization of life's presence on the continents

Simultaneous access to sun & atmosphere, and underground water and nutrients

The development of large living things

The development of multicellularity. Living things made from many cells

Cells that are more internally versatile and can hold more DNA

The development of larger cells

Internal compartmentation for greater internal efficiency

Improvements in ways to get materials in and out of the cell

Improvements in organization, replication, and movement of DNA

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cellular adaptations 98 - planetarybiology.com · cellular adaptations 14.1 introduction...

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