• Describe characteristics ofrepresentative organisms.

• Compare the structure andfunction of prokaryotic andeukaryotic cells.

• Classify organisms living inyour neighbourhood.

Figure 11.1 Is this an animalor a plant? How would youdecide?

Everyone classifies. You classify your books bygrouping them in a particular order on a shelf. Youclassify your clothes by putting them in particulardrawers or hanging them up in the closet. Thegroups we make and the names we give the groupsreflect patterns in the world around us. Somegroups are clearly defined. For example, "peopleunder 18 years of age" and "people 18 years of ageor older." Other groups are more fuzzy. Forexample, "people who speak French" and "peoplewho don't speak French." In which group wouldyou place people who know a few French phrasesbut are not fluent? One of the most interestingchallenges for biologists is finding a way to classifyEarth's millions of species.

If you were given the task of separating allorganisms into categories, how could you begin?What characteristics would you use to classifyliving things? Among the most obvious are thecharacteristics that separate animals and plants.Over two thousand years ago, the Greek philosopherAristotle divided all living things into these twolarge groups, Plantae and Animalia. He called eachgroup a kingdom, a term still used today.

Classifying organisms is not alwaysstraightforward. You could start with the fact thatmost animals are motile (have a method oflocomotion) and ingest the food they have obtained.Most plants, on the other hand, are non-motile andobtain their food through photosynthesis. However,some organisms, such as the ones shown in

Figure 11.1, appear to have characteristics of bothplants and animals. For example, sponges andcorals, like those in Figure 11.2, spend most oftheir lives fixed in one place like plants. Unlikeplants, however, sponges and corals do not maketheir own food through photosynthesis.

Figure 11.2 Some organisms, such as these sponges andthe coral reef that they live on, can be difficult to classifyusing obvious characteristics such as the ability to move.

JJ PAUSE RECORD

Classification can be much more difficult than you mightthink at first glance. Think about organisms you have seenor read about. Make a list of ten organisms that you feelwould be easy to classify, and a list of ten organisms thatwould be more difficult. Consider your lists, and write aparagraph summarizing your thoughts.

The small tropical country of Costa Rica (in CentralAmerica) has 200 known mammal species and more than850 known species of birds. By comparison, Canada-which is 185 times larger - is home to only 147 knownspecies of land mammals and 394 known species of birds.

Micro-organismsClassifying organisms became more difficult afterthe microscope was invented in the seventeenthcentury. The new magnifying tool revealed apreviously unknown world of living things. Somemicroscopic organisms have methods oflocomotion and capture their food, as animals do.Some carry out photosynthesis, as plants. do.However, some micro-organisms havecharacteristics of both animals and plants. Forexample, euglena are common single-celledorganisms that swim by means of a flagellum butalso carry out photosynthesis.

In 1866, the German biologist Ernst Haeckelproposed classifying micro-organisms that are

neither animals nor plants in a third kingdom,which he named Protista. As biologists learnedmore about the structure and way of life ofdifferent organisms, they added more kingdoms tothe classification system. Fungi, such asmushrooms and moulds, were originally includedin the plant kingdom. However, fungi do not carryout photosynthesis. They obtain food by absorbingmaterials into their bodies. They are classified in afourth kingdom, named Fungi.

Bacteria consist of very small cells that differ inappearance from the cells of animals, plants,protists, and fungi. Bacteria lack a nucleus andother organelles, and are able to grow and obtainenergy in a wide range of environments. They areclassified in a fifth kingdom, named Bacteria. Sometextbooks refer to this kingdom as Eubacteria,Prokaryotae, or Monera.

During the 1990s, there was growing interest incertain types of bacteria found living in extremeenvironments, such as salt lakes or hot, acidicsprings. Detailed studies showed that theseorganisms have unique structures and mechanismsthat allow them to live in conditions where noother organisms - including other bacteria - cansurvive. The new data led scientists to renamethese micro-organisms and to reclassify them in asixth kingdom, named Archaea. Some sources mayrefer to these organisms as Archaebacteria. Thesesix kingdoms are shown in Figure 11.3 on thenext page.

IIIHow many different organisms live in your neighbourhood?In this activity, you will make an inventory of and classifylocal species. Make a list of all the different organisms youobserve during a 15-minute walk around your school orhome. Be as specific as possible (for example, robin, notbird; earthworm, not worm). You should have at least20 species in your list. Organize your list into groups basedon the similarities and differences you observe among thevarious types of organisms. Begin by choosing acharacteristic that lets you divide your list of organisms intotwo groups: one group that has the characteristic and onegroup that does not (for example, wings and no wings). Listthe organisms in each group on a separate sheet of paper.Record the group characteristic in your notebook. Next,decide if you can subdivide one or both of your groups

using another characteristic. If so, list the organisms in yournew subgroups on separate sheets of paper. Continuesubdividing your lists until you cannot see another way todo so. Record the classification characteristics you use inyour notebook.

Analyze1. What characteristics did you use to define your groups

of organisms? How many different subgroups did youmake? Suggest a way in which you could improve theclassification of your groups.

2. Exchange your collection of groups with a partner. Canyou discover the characteristics used by your partner tocreate each group? Record the characteristics youthink your partner used. How accurately did yourpartner identify the characteristics of each group youcreated? Explain.

66 245Eras: shown in millions of years ago

Figure 11.3 This fan diagram represents the six kingdomsof life extending out over the geologic time scale. The originof life is shown at the base during the Precambrian Era, andpresent-day species are represented along the outer edgeof the fan. Each kingdom is represented by a different colour.

, PLAY

The incredible diversity of living things can be difficult toappreciate with only a few examples. Go to your ElectronicLearning Partner for videos showing the biodiversity presentin different ecosystems.

Kingdoms describe very large groups of organisms. Usually,you need more than one single characteristic in order toidentify which kingdom an organism belongs to. The bestway to classify organisms is to study as many of theirdifferent characteristics as possible.

Your teacher will provide you with samples of twoorganisms you have probably never seen before. Your taskis to classify each by kingdom. Observe the samples bothwith the naked eye and under a microscope. Record asmany characteristics of each specimen as possible. Reviewyour list and determine the kingdom to which eachorganism belongs.

Analyze1. What characteristics of each organism helped you

make your decision? Explain why these characteristicswere helpful.

2. Your teacher will provide you with other examples oforganisms in different kingdoms. List characteristicsthat you could use to classify these organisms just bylooking at them. Would you need more information?Explain your answer.

3. What characteristics on your list are present in morethan one kingdom? Are there any characteristics thatwere not helpful in your classification scheme? List thecharacteristics that you did not find helpful. Discusswhy you feel they are not helpful.

Prokaryotes and EukaryotesThe study of cells is an important first step inunderstanding the diversity of life. Biologistsrecognize two basic types of cells based ondifferences in their size, structure, and othercharacteristics, shown in Figure 11.4. Recall fromChapter 2 that cells lacking a true nucleus andmost other types of organelles are calledprokaryotic cells (pro means before, karyon meansnucleus). Bacteria and archaea, the smallest andsimplest type of cells, are referred to asprokaryotes. The cells of other organisms are largerand have a more complex internal structure. Theycontain nuclei and other types of membrane-boundorganelles. Such cells are called eukaryotic cells(eu means true), and organisms with these cells arecalled eukaryotes. Table 11.1 summarizes some ofthe differences between these cells.

Draw a graph or pie chart illustrating the proportions of thefollowing relationships:• First prokaryotes appear 3.5 billion years ago.• First eukaryotes appear 1.5 billion years ago.• First multicellular organisms appear 700 million years ago.For what percentage of time that life has existed on Earth did itconsist only of prokaryotes?

For what percentage of time that life has existed on Earth did itconsist only of unicellular organisms?

Table 11.1Two types of cells

EukaryotesI

protists, plants, fungi, animals

large (100-1000 /o-lm)

DNA in nucleus bounded bymembrane

small (1-10 /o-lm)

DNA circular, not bounded bymembrane

genome made up of a singlechromosome

genome made up of severalchromosomes

cell division not by mitosis andmeiosis

cell division by mitosis andmeiosis

asexual reproduction common

multicellular forms rare

sexual reproduction common

most forms are multicellular

mitochondria and othermembrane-bound organelles absent

many are anaerobic(do not require oxygen)

mitochondria and other organellespresent

most are aerobic (require oxygen)

Fossil evidence shows that the first forms of lifewere prokaryotic organisms similar in appearanceto bacteria. Remains of these single-celledorganisms have been found in rocks dated at3.5 billion years old. The first eukaryotes didnot appear until about 2 billion years later(1.5 billion years ago). The first multicellular formsof life appeared only 700 million years ago. Inother words, unicellular organisms have a muchlonger history than do multicellular organismssuch as plants, animals, and fungi.

Figure 11.4 Plants, animal, fungi, and protists are referred to as eukaryotes.They contain eukaryotic cells like the one shown in (A). Bacteria and archaea arereferred to as prokaryotes. They are prokaryotic cells like the one shown in (B).

Although today's prokaryotes look little differentfrom the earliest forms of life, recent research hasshown that living prokaryotes are far more diversethan anyone had previously suspected. Details ofthe molecular biology of bacteria and archaea,especially studies of their RNA structure, show thatthey are as different from each other as either isfrom eukaryotes. This led to these three groupsbeing organized into a new level of classificationabove kingdoms, known as domains. The threedomains - Bacteria, Archaea, and Eukarya - areshown in Figure 11.5.

Domain BacteriaKingdomBacteria

Domain ArchaeaKingdomArchaea

Within the Domain Eukarya, the greatestbiological diversity occurs within the KingdomProtista. For example, there is a much greatergenetic difference between some species of protiststhan there is between, for example, corn andhumans. Protists have lived on Earth for a muchlonger time than plants and animals, which hasgiven this group more opportunity to change anddiversify. Many scientists believe the KingdomProtista should be divided into several differentkingdoms, but no one new system of classificationfor it has been generally accepted.

Domain EukaryaKingdoms

Protista, Fungi,Plantae, Animalia

Figure 11.5 Kingdoms are grouped into three domains based on the types of cellsthe organisms in each kingdom have. Note that Eukarya are linked more closely toArchaea than to Bacteria. .

I

SECTION REVIEW

1. «:m) List examples of classification systems you useevery day.

2. «:m) Would a group identified as "flying animals" beuseful to a biologist? Explain why or why not.

3. «:m) Why should all scientists use the same schemeto classify living things?

4. e Create a concept organizer to show the differentdomains and kingdoms. Include the appropriatecharacteristics that define each grouping in list form.

5. 0 You have discovered an unknown organism whileon a field trip. You think it is a new species of protist.How could you test to identify this species as aprotist? What data would you need to have toclassify it in the Kingdom Protista?

6. «:m) Plants and fungi have been classified in thesame kingdom in the past. Most scientists todayclassify fungi in their own kingdom. Explain howthese two different classification schemes aresupported by data.

7. 0 You discover an unusual organism growing on thebark of a dying tree. Later, in the laboratory, you lookat some of its cells under a microscope. It i$ amulticellular organism with eukaryotic cells, but nochloroplasts. To what kingdom does it belong?

8. e Using the information contained in Table 11.1 andFigure 11 .4, create a handout contrasting prokaryoticand eukaryotic cells. If you were to teach this materialto students in an earlier grade, what informationwould be the most important to teach them thebasic differences between the two cell types?

9. aiB When a new development is being proposed,the developer usually has to submit an EnvironmentalImpact Assessment. This is a scientific study thatoutlines the effect of the development on the localenvironment. Explain how a knowledge ofclassification would be necessary to prepare yourEnvironmental Impact Assessment. Why would youneed to know about the different kingdoms?