Cells, Tissues, and Organ Systems

CH AP T E R S EC TIO NS3- 1 The Cell Theory3-2 St ructure and Functi on

of Cells3-3 Cell Processes3-4 Organization of li ving

Things

CH A PT ER O BJ ECT IV ESAfter completing this chapter, you will be able to

3- 1 Explain the cell theory.3-2 Describe the structures

and functions of a cell.3- 2 Compare a plant cell and an

animal cell.3-3 Describe the activities of a

cell.3-3 Describe the process of

mitosis.3- 4 Explain division of labor.3-4 Describe the five levels of

organization of living things.

Mo re than one hund red trillion of these tiny str uc tu res make up your bod y. Some are in the shape of a rectangle, ot hers in the sha pe of a sphere, and still ot he1·s are sp iral sha ped . So me have tails. Ochers a re star shaped. What are these structures? If you have not al ready guessed, they are cells.

Cells are the basic units of life. All living things oak trees, spiders, elephants , and people- are made up of cells. Some living things are made up of only one cell. Others are made up of many cells. In some living things, cells perform all the life pro cesses need ed for the living thing's surv iva l. In o ther living things, cells perform very specialized tasks, Which type of living thing are you?

Most cells are so small that they can be seen only unde r a microscop e. T he photograph on the o pp osite page was take n thro ugh an electron microscope , which magn ified the human cells 10,000 times! To help you see some of the tinier stru ctures inside th e cells, the cells were sliced in a way that is similar to slicing bread.

As you may have a lread y discov ered, cells a re fas cinating and in ma ny ways very myste rio us objects. Scien tists continue to probe the secrets of cells like explorers journ eying th rough parts of a n uncharted world . If you read on, you will become an ex plorer a nd travel on a fantastic jou rney th rough parts of a ce ll.

Human skin cells magnified 10,000 rimes 55

'

Section ObjectiveTo describe the cell theory

3-1 The Cell TheoryT he basic units of struc tur e and function of liv

ing things are cells. As you read in the beginning of this chapte r, most cells are hard to see with the un aided eye . As a result, many of the tiny cell st ruc ture s that make up the cell remained a mystery for hund reds of yea rs . These structures are called organelles (or-guh- NE HLZ), or tin y organs. The in\'ention of the microscope in the seventeenth cen tur y, howeve r, enabled people to see some of these o rga nelles.

In 1665, while loo king al a thin slice of co rk through a compou nd microscope Robert Hooke, an Englis h scient ist, observed tin y roomlike struc tur es. He called t hese struc tures cells. But the cells that Hooke saw in th e slice of cork were not alive . What Hoo ke saw were actually the o ute r walls of dead plant cells.

ma lcules are know n today as bacter ia. Bacteria are o ne-celled o rga nisms. T hese discove ries made van

Leeuwenhoek wor ld-famous.Duri ng the next two h und red yea rs, new and

b e tte r microscopes we re develo ped. Because of these improved micros copes, Ma thias Schleiden, a German botanist, was able to view d ifferent types of plant parts. Schleiden discovered that all of the plantparts he examined were made of cells. For this rea son, Schleiden pro posed that all plants ar e made of cells. O ne year later, Theodor Schwann, a German zoologist, mad e the same observations using animal parts. About twenty yea r s lat er, Rudolph Virchow, a German physician, discove red t hat all living cells come o nly from other living cells.

T he work of Schleide n, Schwann, Virchow, andother biologists led to the developme nt o f the cell theory. T h e cell theory states that

All li ving thi ngs are made up of ce lls.Cells are the basic uni ts of structure and func·

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Spec imen holder

Thumbscrews

At about the same time, Anton van Leeuwen hoek (LAY-vuhn -hook). a Dutch scient ist, was using a simp le microscope to exam ine materials such as

tion in li ving things.Living cells come only from othe r living cells.

SECTION REVIEW1. What is the ce ll theory?2. Who was the first person to use the wo rd cell?

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blood, rainwater, and scrapings from his teeth. In each mate rial, van Leeuwen ho e k was able to observe living cells. He even found tiny living things in a drop of rainwater . Van Leeuwenhoe k called these

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3·living things "animalcules." The smallest of the ani-

Figure 3- 1 All living things, such as the red onion {left) and Acetabularia. a one-celled green algae (right), are made up of cells. How do these cells differ from one another?

3. Wh at th ree scientists co ntribu ted to the cell the ory?

4. Explain the relations hip between tech nology a ndthe development of the cell theo ry.

3-2 Structure and Functionof Cells

Yo u are about to take an imaginary journey. It will be qu ite an unusua l tr ip because you will betr ave lin g inside a livin g organis m, visitin g its cells.

Most ce lls are too small to be seen without the aid of a microsco pe . In fact, most cells are sma ller

than the period at the end of this sentence. Yet wit hin these tiny cells are even smalle r struc tu res

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=======-Figure 3- 2 In the mid 1600s, Anton van Leeuwenhoek made a series of simple microscopes /top) through which he observed and recorded several different types of tiny living things {bottom). How many tenses did van Leeuwenhoek's microsco pe have?

Section ObjectiveTo list and describe the various functions of cell structures

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Vacuole- - - ---t- - ¼ +- - - -

Mitochondrion,- - - - -

Figure 3-3 This dia gram shows the structures of a typical plant

IIH - - - - Cell wall

Ill/ii' > --< U Ribosomes

Endoplasmic reticulum

ide , an d cer tain dissolve d materials to pass throug h it. So sail on through .

T he rigid cell wall is found only in pla nt cells.T his structure helps give prot ect io n and su ppo rt so the plant can grow tall. T hin k for a moment of grasses a nd flower s t hat su p po rt themselves u pright. No doubt yo u can ap preciate the im po rta nt ro le the cell wall plays for th e indi vid ual ce ll and for the en tire plan t.

Cell MembraneOnc e past the cell wall, you pre pare to e nter the

living mat er ia l o f the ce ll. Bio logists call all the liv ing ma te r ial in both plant and animal cells protoplasm. Pro to plasm is not a sin gle str uctur e or substance. It is a term used to describe all the living materia ls in a cell.

T he first stru cture you encounter is the cell membrane. The cell memb rane is a thin, flexible envelo pe tha t sur rou nds the cell. I n a plant cell, th e cell membr an e is j ust insid e the cell wall. ln an a ni

cell. What are the tiny structures within the cell called?

that carr y o ut the activities of the cell. The struc tures within the cell function in storing and releas ing energy, building and repairing cell parts, get ting rid of waste materials, responding to the environment, and reproducing.

There are ma ny types of cells. For example ,

mal cell, the cell membrane forms the outer covering of the cell . Loo k again at Figure 3-3 and at 3-4.

Figure3-4 This diagram shows the structures of a typical animal cell. How does an animal cell differ from a plant cell?

- - - - Vacuole

human cells include muscle cells, bone cells, and nerve ce lls. In plants , there are leaf cells, stem cells, Ri.bos omes(a ttache d ) :z : . ·- .,(!:".°"\and root cells. However, whe ther in an animal or a plant, most cells share certain similar characteristics. So hop aboard yo ur imaginar y ship and prepare to ent er a typica l pla nt cell. You will begin by sailing up thro ug h the trunk of an oak tree. Yo ur des tina tion is that box-shaped structure di rectly ahead. See Figure 3- 3.

Cell WallEntering the cell of an oak tree is a bit di fficult.

First yo u must pass through the cell wall. The cell wall is made of cellulose, a non living mat e rial. Cel lulose is a long chain of sugar molecules that the cell manufactures . Althou gh the cell wall is stro ng and stiff, it does allow wate r , oxygen, carbon diox-

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Ribosomes(free) , _ '•

Nucleolus I INucleus \ ·, Y -

Nuclear - - - .- - -membrane

Cell membran e

il' l , Lysosome

= - -'o - - Centnoles

Endoplasmicreticulum

{' :,;.ft; ;/ M itochondrion

Cytoplasm

Figure 3-5 The cell wall gives support and protection to plant cells. What is the name of thenonliving material that makes up the cell wall?

Sharpen Your :ikllls

Observing Cells1. Obtain a thin piece of

cork and a few drops of rain water.

2. Place each of these on a di fferent glass sl ide and cover with a cover slip.

3. Obtain a prepared sli de of human blood.

4. Observe each of these slides under the low and high power of your microscope.

5. Make a labeled diagram of what you see on each sli de.

Using reference books, find diagrams of these materials as seen by Hooke and van Leeu wenhoek. How do your dia grams compare with theirs?

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T he cell membrane has several importa nt jobs. You can discover one of its job s on yo ur own. Push d own on yo ur skin with yo ur thumb . Your skin does not break, does it? Now lift your thumb. The skin bounces back to its o rigi nal position. Your skin can do this because the cell membrane around each skin cell is elast ic and flexible. T he cell membrane allows the cell to change shape under pressure. T he flexi ble cell memb rane also keeps the protoplasm of the cell separated from the outside environment.

As your ship nears the edge of the cell mem brane, yo u notice that there are tiny openings in the membrane. You steer toward an o pen ing. Sudde nly, your ship narro wly misses being struck by a chunk of floating material passing out of the cell. You have discovered another job of the cell membrane. The cell membrane helps control th e movemen t of mate rials into and out of the cell.

Ever ything the cell needs, from food to oxygen, enters the cell thr oug h this membran e. Harmful waste products exit through the cell membrane. In this way, the cell stays in smooth-running order, keeping conditions inside the cell the same even though condition s omside the cell may cha nge. T he ab ility of a ce ll to maintain a stable intern al e nviron ment is called homeostasis (hoh-me e-oh-sn v-sihs).

NucleusAs you sail inside the cell, a large oval stru cture

comes into view. This structure is the control center of the cell, or the nucleus (N0o - klee-uh s), which acts

as the "brain" of the cell. See Figure 3- 6. T he nu cleus co ntrols all cell activit ies.

The la rge, co mp lex molecules that are found in the nucleus are comp o unds called nucleic (noo KLEE-ihk) acids . Nucleic acids store the information that he lps a cell make the substances it needs.

There are two types of nucleic acids. O ne of the nucleic acids is DNA, o r d eoxyribo nucleic (dee -ahk sih- righ-boh-noo-KLEE-ih k) acid. DNA stores the in formation needed to build proteins. Proteins are used co build and re pair ce lls. Some proteins, for example, are used to for m structura l parts of the cell, such as the cell membrane. DNA also car ries" messages" a bout an organ ism that are passed fromparent to o ffsp ring.

The other nucleic acid is RNA , or r ibonucleic (rig h-boh-noo-KLEE-ihk) acid. RNA "rea ds" the DNA messages and guides the protein-making process.Unlike DN A, RNA, carr ying its protein-building in structions, leaves the nucleus through a nuclea r pore . Now let's look at so me of the structu res that make u p the nucleus.

NUCLEAR MEMBRANE The th in membrane that sep arates the nucleus from the rest of the cell is called the nucl ear membrane. T his membrane is simila r co

Sharpen Your Skills

Word Clues

The defin ition of a word can often be determined by know ing the meaning of its prefix . Choose five of the followi ng words, look up the prefix for each, and tell how it relates to the definition.

chloroplast protop lasm mitochondr ia vacuole chromosome lysosome cytop lasm

Figure 3-6 This diagram of the cell nucleus shows the nuclear membrane. chromosomes, and nucleolus. The nucleus in the photograph is magnified 20,000times. Which structure containsRNA?

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th e cell membrane in that it allows mater ials to pass into and ou t o f the nucle us. Small o pen ings, or pores, are sp aced regula rly around the n uclea rme mbr a ne. Each pore acts as a passageway into and out of the nu cleus. Set your sights for that po reahead and carefu lly glide into the nucle us.

Figure 3- 7 You can clearly seethe threadlike shape of a fruit fly'schromosomes in this pho tograph. What nucleic acid is found in the chromosomes?

Sharpen Your Skills

Plant and Animal Cells1. To view a plant cell, re

move a very thin transparent piece of tissue from an oni on.

2. Place the on ion ti ssue on a glass slide.

3. Add a drop of iodine stain to the ti ssue and cover with a cover slip.

4. Observe the oni on und er the low and high power of your microscope. Draw a dia gram of one onion cell and label its parts.

5. To view an animal cell, gently scrape the in si de of your cheek with a toothpick .

6. Gently tap the toothpick on to the center of another glass slide.

7. Add a drop of methylene blue and cover with a cover slip.

8. Observe the cheek cells under the high and low power of your microscope. Draw adi agram of one cell and label its parts.

Compare the two cells.

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CHROMOSOMES T he large, ir re gu lar mass of thin threads that take up most of the space in the nu cle us is the chromos omes. Steer ver y car efu lly toavoid colliding with and da mag ing the delicate c hro moso mes. T he chrom osomes are made of DNA.Chro mosomes dir ect the activities of the cell and pass on the traits of the cell to new cells .

NUCLEOLUS As you pre pare to leave the nucleus, you see a small, d ense object float past. It is the nucleolus (noo-KLEE- u h-luhs ), or "little nucleus."T he nucleolus is made up of RNA and protein.T his tiny struct ur e plays an importa nt ro le in mak in g some of the cell's RNA. Now hitch a ride o n the RNA leav ing the nucleus and continue you r exploration o f the cell.

Endoplasmic ReticulumO utside the nucleus, float ing in a clear , thick

fluid, you r s hip needs no pro pulsio n. For here the jellylike material called cytop lasm is cons tantly mov ing. Cyto plas m is the term for all the protoplasm, or living material o f the cell, outside the nucleus.

Stee ring will be a bit difficult here because many particles a nd tubelike structur es are scatteredth rou ghout the cytop lasm. T he maze of tubula r passagewa ys that leads out fro m the nuclear mem bra ne forms the endopl asmic re ticul um (ehn-d uh PLAZ-mihk rih-TIH K- yuh-luhm) . In some cells, t hese passage ways conn ect with the cell mem brane . See Figure 3- 8.

T he end op lasmic re ticulum is in volv ed in thetransport of proteins. Proteins made in one par t of th e cell can pass thro ugh the endo plasmic reticulum to other parts of the cell. Other ma ter ials can be tran sported to the outsi de of the cell through th is syste m. How is the endopla smic re ticulu m su ited for its job?

ENDOPLASM IC RETI CUL UM

Riboso mesLook closely at the inner surface of one of the

passageways in the endo plasmic ret iculum. Attached to the surface are grainlike bod ies called ribosome s, which are made up main ly o f th e nucleic ac id RNA. Ribosomes are the pro te in-making sites of the cell. The RNA in the ribosomes , alon g with the RNA sent out from the nucleus, directs the production of proteins. Ribosomes are well posit ioned as they not only he lp make proteins, they "d rop" them d irectly into the endoplasm ic reti culum. From there the pro teins go to any part of the cell that needs them.

Not all ribosomes are attached to the e ndo plas mic reticu lum. Some float freely in the cytoplasm. Watch out! There go a few passing by. The cell you are in has many riboso mes. What might this tell you about its protein -making activity?

MitochondriaAs you pass by the ribosomes, you see ot her

structures loo ming ahead. These structures are called the mit ochondria (might-uh-KAHN-dree -uh; singular; mitochond rion). Mitoc hondria supply most of the e nergy for the cell. Somewha t larger than the ribosomes, these rod-shaped str uctu res are often referred to as the "powerhouses" of the cell. See Figure 3-9 on page 64.

Figure 3- 8 The endoplasmic reticulum transports materials throughout the cell. The dark dots lining the tubelike passageways are ribosomes. What is the function of the ribosomes?

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Vacuoles act like sto rage tanks. Food and other materials needed by the cell are stored insid e the vacuo les. Vacuo les ca n also store waste products. I n plant cells, vacuoles are the main water -storag e areas. When water vacuoles in plant cells are full, they swell and make the cell plump . T h is plumpness keeps a plant firm .

Figure 3-9 The mitochondrion is the "powerhouse" of the ceff. What food substances provide energy for the ceff?

64

Inside the mitochondria, simple food substances such as sugars are broken down to water and car bon dioxide gas. Large amounts of energy are re leased during the breakdown of sugars. The mito chond ria gather this energy and store it in special energy-rich molecules. These molecules are conven ient energy packages that the cell uses to do all its work. The more active a cell is, the more mitochon dria it has. Some cells, such as a human liver ce ll, contain more than 1000 mitochondria. Would you expect your muscle cells or your bone cells to have mo re mitochondr ia?

Because mitochondria have a small amount of their own DNA, some scientists believe that they were once tiny organisms that invade d living cells millions of years ago. The DNA molecules in the mitochondr ia were passed from one generation of cells to the next as simp le organ isms changed into complicated ones. Now all living cells contain mito chond ria. No longer invaders, mito chondria a re an important part of living cells.

VacuolesThe large, round, water-filled sac tha t is floating

in the cytoplasm of this cell is called a vacuole (vA kyoo-wohl). Most plant cells and some animal cells have vacuoles. Plant cells often have one very large vacuole and animal cells have a few small vacuoles.

LysosomesIf you .carefu lly swing yo ur ship around the

lakelike vacuole, yo u may be lucky eno ugh to see a lysosome (LIGH -suh -sohm). Lysosomes are common in animal cells but not often observed in plant cells.

Lysosomes are sma ll, round structures involved with the digestive activities of the cell. See Figure 3- 11. Lysosomes contai n enzymes that break down large food molecules into smaller o nes. Then these smalle r food molecules are passed on to the mito chondria, where they are "burned" to provide en ergy for th e cell.

Although lysosomes digest substances in the cytoplasm, you need not wo rr y about your ship's safet y! The membrane surrou nding a lysosome keeps the enzymes from escaping and digesting the entire cell. However, lysosomes do d igest whole cells when the cells are injur ed or dead . In an interesting process in the growth of a tad pole into a frog, lysosomes in the tadpole's tail cells digest the tail. Then this pro toplasmic mate rial is reused to make new body pa rts for the frog.

ChloroplastsYour journey is jus t about over, and you will

soon be leaving the cell. But first look around you.Floating in the cytoplasm are large, irregularlyshape d green structures called chloroplasts. Chloro plasts contai n a g re en pigment called chlorophyll. Chlorophyll captures the energy of sunligh t and uses it to make food for the plant cell. In whichpart of the plant do you think cells with chlorop lasts are located?

Chloroplasts are found only in plant cells. How ever, the sea slug, an animal, often eats plants that contain chloro plas ts. After the sea slug d igests the

Figure 3-10 The large , roundish empty spaces in these plant ceffs are vacuoles. Whal materials do vacuoles store?

Figure 3- 11 The dark do ts in this photograph are lysosomes. What is the function of lysosomes?

Figure 3-12 The cells of green plants contain special food-making structures called chloroplasts. Why are chloroplasts green?

!I-

plant, some of the chlo rop lasts get in to the cells of its digestive system . There the chloro p lasts continue to make food just as they do in a plant. The process goes on for a week or so and provides the sea slug's cells with food for energy.

SECTION REVIEW1. What are five functions of the cell?2. Why is it important that the cell memb rane a llow

mate ria ls to enter and leave the cell?3. Name the two stru ctur es that are found with in

the nucleus.4. Exp lain how you would distingu ish betwee n a

plant ce ll and an anima l cell.

converts, stores, and uses energy . Ob taining and using energy is one of the most important activities of a living cell.

MetabolismEven while yo u sleep, yo u need energy to ke ep

you alive. But where does this energy come from? Cells provide it. Although cells d o not make e nergy, they do change energy from one form to another. Cells obtain energy from their environment and cha nge it into a usable form .

This conversion process is very complex . And itinvolves many chemical reactions. Some reactions brea k down molecu les. O ther reactions build new molecules. The sum of all the building-up and breaking-down activities that occur in a living cell is called metaboli sm.

Figure 3-13 A cell, such as the plant cell , is like a miniature factory that carries out all the activities necessary to life. Which structure controls all the activities ot the cell?

Section ObjectiveTo li st and de s cri be the life processes perf orm ed by cells

3-3 CelI ProcessesThe ce ll ca rries out a variety of chem ical proc

esses necessary to life. Life processes performed by cells include metabolism, respiration, diffusion, osmosis, and reproduction. And the cell as a whole carr ies out th e chem ical proce sses necessary to life.T his tiny unit, basic to a ll fo rm s o f life, is like amin iatu re factory that produces many kinds of chemicals. Like any facto ry, a cell must have energ y to do wo rk. Working day and night, a cell traps, 1 Nuc leus

2 Chromosomes3 Mitochondria

4 Ribosomes5 Chloroplasts6 Vacuoles

7 Endoplasmic Reticulum8 Cell Membrane

concentrtaion of substance on

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Sharpen Your Skllls

The Big Egg1. Prepare a hardboiled egg.2. Peel it very carefully.3. Using a piece of strin g,

measure the egg's circumfer ence in millimeters. Record this information in a chart.

4. Place the egg in a beaker of water for five days. M ea sure the circumference of the egg every day and record thisin the chart.

Think for a mome nt of all the things cells do: grow, repair structures, absorb food, manu facture proteins, get rid of wastes, a nd re prod uce. T he en ergy for these activities is locked up in the molecules in food. As a result of metabolism, the stored en ergy in food is set free so it can be used to do work.

Cell RespirationEa rlier you lea rn ed that ene rgy is released when

simple food substances such as sugars are broken down inside the mitochond ria. The process by which a cell r eleases energy from food is called

How do you get the oxygen needed for respirat ion? How do you get rid of the carb on d iox ide tha t is produced during respiration?

In ana ero bic (an-ehr -o tt-bihk) resp iration, e nergyis released and AT P is produced with out oxygen. For example, in yeast cells, glucose is broken down to alcohol through anae robic resp iration. Du ring strenuous exercise, your mu scle cells ma y lack oxy gen . At this time, energy is released by a naero bic respira tion. Which type of respiration would pro duce more energy, aerobic or anaerobic?

Diffusion

How does the circumference of the egg vary? Can you ob serve any other changes in the egg's appearance? What proc ess is demonstrated in thisactivity?

Figure 3-14 Respiration is performed by all living things, suchas yeast cells (left) and the

respiration. Respiration is per formed consta ntly by a ll living things.

There are two types of respiration. In aerobic (ehr-o tt-bihk) respiration, food, such as the sugar glucose, is broken down with the help of oxygen . In this reacti o n, water and carbon dioxide are pro duced. The energy that is released is sto red in an energy-rich molecu le called AT P. The energy in ATP can then be used directly by t he cell to dowor k when it is needed. Although aerobic resp ira tion occurs in a series of complicated ste ps, the ove rall react ion can be shown in the following chemica l equat ion.CsH120s + 6 02 --" 6 CO2 + 6 H20 + ATP

Remember how you sailed th rough the cell mem bran e to enter the cell? Well, th e subs tances tha t get in to and ou t of the cell do the same thing. Food molecules, oxygen, water, and other mater ials enter and leave the ce ll through openings in the cell memb ra ne by a process called diffus ion.

Why does diffusion occur? Molecules of all substances are in constant motion, continuously collid ing with one another. This motion causes the mole cu les to sp read out. The molecules move fro m an area where there are more of them to an area where there are fewer of them. See Figure 3-15.

If there are many food mo le c ules outside thecell, for examp le , some diffuse through the mem

Figure 3- 15 Diffusion is the movement of molecules of a substance into a cell or out of acell. Substances move from places where they are more concentrated to places where they are less concentrated. What structure controls the movement of

mountain lion (right). What type of respiration occurs in each of these organisms?

glucose oxygen carbon dioxide

water energy brane into the ce ll. At the same time, waste materi als built up in the cell pass out of the cell by th e process of diffus ion .

If substances can move into and out of the cell through the membrane, what kee ps the protoplasm from oozing out? What keeps harmfu l mater ials from moving in? The cell membrane safeguards the conte nts of the cell because it is selectively permea ble. That is, it permits only ce rtain substances,mainly oxygen and food molecules, to d iffuse into the cell. On ly waste produ cts such as carbon dioxide are allowed to diffuse out of the cell.

substances into and out of thecell?

OsmosisWater is the most import ant substance that

passes th rough the cell mem brane . In fact, about 80 percent of protop lasm is water. Water passes

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H igh er

o! substance both sides o! inside the cell cell membrane

IWi wJH i gh er Equal amountsconcentraiot n o! substance on of substance both sides of outsidethe cell cell membrane

-Equa·l arr1

Figure 3- 16 Normal red blood cells (left) will shrink if too much water leaves the cells (center). If too much water enters the cells, the cells will swell (right). What is the movement of water into and out of cells called?

through the cell membrane by a special type of dif fusion called osmosis . Dur ing osmosis, water mole cules move from a place of grea te r conce ntratio n to a place of lesser concentration. This movement keeps th e cell from drying out.

Suppose you pu t a cell in a glass of salt water. T he concentration of water outside the cell is lower than the concentratio n of water inside the cell. Sowater le aves the ce ll, and the cell starts to shrink. If too

muc h water leav es the cell, it dries up and d ies. Now, if the cell was put in a gla ss of fresh water

ide ntical to the other and to the parent cell. How d oes th is pro cess occur?

If a parent cell-a skin cell, lea f ce ll, or bo necell, for exa mple- is to pr odu ce two ident icaldau gh ter cells, then the exact co ntents of its nucleus must go into the nucleus of each new cell. In other words, the "blu eprints " of life in the parent cellmust be passed on to each daughte r cell. Now, if the parent cell simply s plits in half, each daugh ter cell wou ld get only half the contents of the nu cleus-only half the "blue p rints." It woul d nolonger be the same kind of cell- a skin cell, leaf cell, or bone cell.

For tu nately, this is not what hap pens. J us t be fore a cell divides, all the mate rial in the nucleus is du plicated, or copied . The division of the nucleus and the cytoplas m a nd the d uplicatio n of the chromo somes is called mit os is (migh-TOH-sihs).

Mitosis is a conti nuou s process. To form two cells, the nucleus of the dividing cell goes through four stages: prophase, metaphase, anaphase, and

Sharpen Your Skills

How Many Cells?

Suppose a cell splits in half every five minutes. How many cell s will there be after 2 hours?

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instead of salt water, jus t the opposite would occur. Wate r enters the cell, and the cell swells. This hap pens because the concentration of water is lower in side the cell than outside the cell. As you mightima gine , if too much water enters the cell, the cell bur sts.

Reprodu ctionDo you remember how many cells there are in

the bod y of an adult human? T here are approxi mately on e hundred trillion cells. I nte restingly th ese trillions of cells came from just one original cell.How is th is poss ible?

In order for the total n umber of cells to increase and for the organism to grow, the cells must un dergo reproduction. Re prod uction is the process by which living things give rise to the same type of liv ing thing. T he cells re prod uce by d ividing into two new ce lls. Each new cell, called a daugh ter cell, is

HELP WANTE D: MEDICAL ARTISTAdvanced degree in medical illustration required. College background in biology or zoology desirable. Needed to prepare acc urate and creative d raw ings for medi· cal textbooks.

Have you ever looked at a biology book or a magazin e about biology that did not have dia grams? Probably not. because many concepts and objects related to biology cannot be easily understood by reading only words. For exam ple. cell structures and functions, body parts, and organism development can best be " de scribed" through the use of drawings. Making such drawings is the job of a special group of artists.

Artists who create visual materials dealing with health and medicine are medical artists . Sometimes their work involves view ing a speci men through various kinds of microscopes in order to draw it. Or a medical artist may dissect and study theparts of animals and plants. Some medical artists work closely with surgeons or

other kinds of doctors in order to prepare accurate drawings of medical conditions.

Most medical artists are employed by pub lishers, medical. veterinary, or dental schools, or by hospitals with programs in teaching and research. To receive more information about this field, write to t he Association of Medi cal Illustrators, Route 5, Box 311F, Midlothia n, VA 23113.