Many people have compared the cell to a busrlingcity, like the one shown in Figure 7.16. From agreat distance, a city may seem a quiet place withlittle activity. At its core, however, a city is inconstant motion. People with different skills takecare of the city's needs. These include as generatingpower, bringing in food, disposing of garbage, andmoving people and supplies from place to place.

To run smoothly, a city relies on teamworkand organization. Even its shape and structurehelp it to function successfully. What wouldhappen if cities had no streets, sidewalks, buses,water and sewer lines, or power lines? Theywould not be very efficient. Transportation ofpeople, food, wastes, and energy would occurmuch more slowly, if at all. Well-designed citiesare structured to enable communication withintheir borders, as well as with other cities. Ports,harbours, and highways provide connections withthe outside world.

Cells are just as complex, busy, and organized ascities. In plant and animal cells, specific functionsare carried out by specific internal cell parts calledorganelles. Various organelles are illustrated inFigures 7.17 A and B on the next four pages.Boundaries create compartments for specificcell functions and also protect the cell. Vitalconnections run among organelles and connectorganelles with the cell membrane. Co-ordinationof the organelles enables cells to survive andfunction in a variety of environments. Just asa city is always bustling with different activities,all of the cell functions described in Figure 7.17 Aand B take place at the same time.

IiIm[i;Ul;I How is a cell like a busy city? As you read the followingsection, compare the roles of different cell structures to the functionsof people, vehicles, and structures in a city.

Plant and Animal CellsPlants and animals are made up of many different tissues, each of which ismade up of specialized cells. Specialized cells take on different forms. Forexample, under a microscope, a muscle cell looks different from a bloodcell. Plant and animal cells have many of the same parts. They also havesome features that distinguish one from the other. All cells, different thoughthey may be, share several basic structural and functional similarities. Learnmore about plant and animal cell structure by examining the diagrams onthe next pages.

Chapter 7 The Basis of Life. MHR 277

~

A Cell membraneEvery cell has a cell membrane. This boundary around the cell separates the cell interiorfrom the environment. The cell membrane (also called the plasma membrane) holds in fluids,chemicals, and structures that the cell needs to survive. These contents are suspended withinthe jelly-like cytoplasm that fills the cell. The cell membrane keeps the cell's contents fromleaking out. It also prevents unwanted materials from getting inside the cell while allowing

needed materials to enter.The cell membrane and membranes surrounding the organelles are composed mainly

of lipids, the basic units of fats and oils. Lipids give the membranes a fluid-like structure.Like a drop of oil floating on water, the cell membrane can change shape. Small sectionsmay pinch off, forming small membrane sacs called vesicles. These structures store ortransport materials in and out of the cell. Protein molecules embedded in the membraneact as tiny doors that let certain molecules cross the boundary. Some other materials, suchas oxygen and carbon dioxide, simply diffuse across the membrane.

The cell membrane is also the cell's front line of communication. Proteins in the membranecan interact with other cells and detect chemicals in the environment.

C LysosomesInside special vesicles called Iysosomes various materials aredegraded with the help of digestive enzymes, a type of protein.Inside the Iysosomes, the pH is generally low, which helps thedigestive enzymes to function. Lysosomes can fuse with vesiclescontaining food particles taken up by animal cells or single-celled organisms. The digestive enzymes break down particles,just as enzymes in our digestive system break down the foodwe eat. Some Iysosomes protect the body by killing infectiousmicro-organisms. In various cell types, Iysosomes also devourold, worn-out cell parts. These components may be recycledor used to provide energy.

B NucleusLike the mayor of a city, the nucleus is the manager thatdirects all of the cell's activities. Within the nucleus are thecell's genes. Information encoded in the DNA is copied inthe form of messenger molecules, which leave the nucleusthrough nuclear pores in its membrane, called the nuclearenvelope. In the cytoplasm, the messenger molecules attachto larger molecules called ribosomes. Ribosomes are producedin an area of the nucleus called the nucleolus. Ribosomestranslate information on the messenger molecules into proteins,which carry out cellular functions and are a part of manycell structures.

E Goigi apparatusMolecules transported in vesicles from the endoplasmic reticulumoften make their first stop at the Golgi apparatus. The vesiclemembrane fuses with the membrane of the Goigi apparatus. Themolecules delivered from the endoplasmic reticulum are modifiedby enzymes and made fully functional. Then, like a post office, theGoigi apparatus sorts and repackages the molecules into vesicles,which are distributed to other locations in the cell. Vesicles thatcontain materials needed outside the cell pinch off from the Goigiapparatus and travel to the cell membrane. The Goigi apparatusis also where Iysosomes (see C), are produced.

D Endoplasmic reticulumThe endoplasmic reticulum is a network of folded membranesand tubes connected with the nucleus. The endoplasmicreticulum has two types of membranes: rough and smooth.The rough endoplasmic reticulum is studded with ribosomes.Ribosomes build proteins. These are then packaged in vesiclesfor transport. The smooth endoplasmic reticulum has noribosomes. The smooth endoplasmic reticulum synthesizeslipids and packages large molecules (proteins or lipids) invesicles. These vesicles are transported to other parts ofthe cell, usually the cell membrane or the Golgi apparatus.

278 MHR . Unit 3 Cycling of Matter in r"iving Systems

~

Animal Cell

Gnucleus

nuclear pores

nuclear.envelope

/

cytoplasm

CD

mitochondrion

4) vacuole(see H onnext page)

lysosome

eGoigi apparatus

vesicle

G CentriolesWhen a cell divides, it must reproduce all of its geneticmaterial and then divide this material between the two newcells. Centrioles are cylindrical structures located just outsidethe nucleus. In animal cells, these structures play an importantrole in cell division by helping to move genetic material correctlyinto each new cell. Most plant cells do not have centrioles.

F MitochondriaWhen you turn on a light in your house, the energy needed tooperate it flows in from a power plant. When your cells do work,they also need a source of energy. Mitochondria (singular,"mitochondrion") provide that energy. Cells that need a lot ofenergy, such as muscle cells, tend to have many mitochondria.

Within the mitochondria, chemical reactions convert the energyin food molecules into a form that the cell can use to carry outother activities. The reactions occur on folded membranes inside

the mitochondria.

Animal cell

Chapter 7 The Basis of Life. MHR 279

.. I endoplasmic reticulum Irough endoplasmic smooth endoplasmic

reticulum ' reticulum

/

H VacuolesYou keep your extra clothing, food, and other materials in storage, in a cabinet or closet.In many cells, balloon-like vesicles called vacuoles provide storage space for water, food, andminerals. Plant cells also use vacuoles to regulate the amount of water in the cells. Digestion ofold cell parts occurs in certain vacuoles. Some vacuoles contain wastes. Some plant cells evencontain poisons to prevent animals from eating the plant. Plant cells usually have a large centralvacuole that may occupy nearly the entire cell volume. Animal cells have many smaller vacuoles.

I Cell wallPlant cells, fungi, and some bacteria have cell walls. Like the walls of a house, cell walls arerigid and supportive, giving cells their shape. When Robert Hooke observed cork cells, what heactually saw were the cell walls of dead plant cells. The cell wall, which is composed of a fibrousmaterial called cellulose, completely encloses the cell membrane.

J ChloroplastsGreen plants (and some single-celled organisms) manufacture food from water and carbondioxide by trapping the energy of sunlight. This process is called photosynthesis. Chloroplasts,the organelles of photosynthesis, have two layers of membranes: an inner layer of flattenedmembrane sacs enclosed by a second outer membrane. The inner membranes contain the greenpigment molecule chlorophyll. This molecule absorbs energy from the Sun and converts it tochemical energy. This chemical energy is used to power the chemical reactions that synthesizesugars from carbon dioxide gas and water.

?Wood products, paper, and cotton are all made fromthe cellulose walls of plants. Cellulose is an importantpart of our diets, too. When we eat fruits andvegetables, the cellulose in their cell walls providesfibre that keeps the digestive tract healthy. However,because people lack digestive enzymes to break downcellulose, it has no nutritional value.

280 MHR . Unit 3 Cycling of Matter in Living Systems

4

Plant Cell

Gnucleus

nuclear pore

nucleolus0

central vacuole

nuclear

envelope

8chloroplast

ribosome

cell wall

cell membrane

0mitochondrion

0cytoplasm

Goigi apparatus

smooth rough

endoplasmic endoplasmicreticulum reticulum

l 4~ ~endoplasmic reticulumcell wall of

Plant cell

Chapter 7 The Basis of Life . MHR 281

If you were to compare a plant with an animal, you would fInd the two to be verydifferent. However, if you were to examine individual cells of these organisms, wouldthey look as different? In this investigation, you will compare plant and animal cells.

For tips on makingscientific drawings,turn to Skill Focus 9.Question

How do plant and animal cells differ? How are iliey similar?

Safety Precautions ~. Be careful when using sharp objects such as tweezers.

. Handle glass slides and cover slips carefully. If broken,they are sharp and dangerous.

MaterialsVallisneriaprepared slide of

animal cells

8 Focus on one cell. Draw what you see, labellingyour diagram. Increase the magnification tomedium power and then to high power tomake more detailed observations of individualorganelles. What organelles and other structuresare visible? Do you see any movement? Recordyour observations in the data table.

" Return to low power and remove the slide.. Place a prepared slide of animal cells underthe microscopes. Repeat steps 4 and 5.

Apparatusmedicine droppermicroscope slidescissorstweezerscover slip

compound light microscope

Procedure0 Copy dte data table below into your notebook.

. Use the medicinedropper to put a dropof water in the centreof a clean slide. Cuta small portion of theVallisneria leaf. Usingtweezers, place it on top of the drop of water.Cover it with a cover slip, making sure thatno bubbles are visible.. Observe the leaf on low power. Focus on the toplayer of cells. Then, carefully focus down throughthe leaf to <;>bserve the layers of cells below.

282 MHR . Unit 3 Cycling of Matter in Living Systems

Think About It

This section opens with a comparison between a celland a city. Many people also describe cells as miniaturefactories with separate areas for production, transport,and other tasks. Analogies like this are an importanttool for communicating scientific ideas. In thisinvestigation, you will develop your own analogyto describe a cell. You will then use this analogyto share information about cellular structures.

What to Do0 Your teacher will divide the class into small

groups. With your group, gather and organizethe data you will need. For example, collectinformation about the function of each organelle,its size, and its location in the cell. Visit the libraryor search the Internet to find the data you need.

. Brainstorm possible analogies you could use tocommunicate information about the cell and itsorganelles. Then decide on the one analogy thatwill be the basis for your project. Discuss withyour group how to make your analogy fit wellwith the information you have gathered.

. Prepare a model or a presentation to presentyour information using the analogy your grouphas chosen. For example, if you have chosen tocompare the cell to a factory, you could do oneof the following:

(a) create a physical model of a factoryshowing how different areas representdifferent organelles;

(b) prepare a "virtual tour" of the factory ona computer;

(c) perform a skit live or on videotape, in whichyou show your audience around the factory.

rh"nt"r 7 The B""j" of Life. MHR 283

this new technology. These organizations assumed thatU.S. companies were probably doing similar researchwith more lucrative funding.

Because of the many delays, a foreign company wasable to develop the same technology and file patents

, for it. Those patents were the ones that eventuallyturned the technology into the success story it is today.

Recognizing that research such as Zhang and Dovichi'smust be financially supported, the University of Alberta

has formed an Industrial Liaison Office, partly to ensurethat its star researchers will not go unrecognized.

Acro~; .CanadaDrs. Jianzhong Zhang and Norm Dovichi, of the Universityof Alberta, developed a technology that accelerated byseveral years the mapping of the human genome. Theirtechnology involved a DNA sequencer that could analyzeDNA ten times faster than previous instruments. I

Zhang and Dovichi began work on the sequencer Iin the late 1980s, and spent several years trying to 1

secure funding and patents. However, they encountered \frustration whenever they tried to obtain funding for theirwork. None of the funding organizations was sufficientlyknowledgeable at that time to appreciate the importance of

Section 7.3 SummaryIn this section, you learned about the structure and function of cell organelles.You compared the organelles in plant and animal cells and learned that the twotypes of cells share many structures and functions. Structures shared by both plantand animal cells include the cell membrane, the nucleus, cytoplasm, mitochon-dria, the endoplasmic reticulum, the Golgi apparatus, and the vacuoles. Plantcells usually have one large vacuole, while animal cells contain many smallervacuoles. Structures that appear in plant cells but not in animal cells includechloroplasts and the rigid cell wall that supports a plant cell and gives it its shape.

Some scientists think that bothmitochondria and chloroplastsevolved from bacteria. Accordingto one hypothesis, ancient cellshad nuclei and a few otherorganelles, but no mitochondriaor chloroplasts. Some of thesecells engulfed bacteria, whichcontinued to survive. Eventually,the two grew to depend on eachother and their relationshipbecame permanent. One pieceof evidence for this hypothesisis that both mitochondria andchloroplasts contain a smallamount of their own DNA, whichlooks similar to bacterial DNA.

Check Your Understanding

1. Name two cellular structures that are present in plant cells, but not inanimal cells.

Pause&-- Reflect I

2. Which structure within the cell carries out protein synthesis?

3. How is the Golgi apparatus like a post office?

4. Organelles divide the cell interior into many different compartments.Explain how having compartments benefits the cell.

5. Describe the functions of vacuoles in plant cells.

6. How are mitochondria and chloroplasts distinct from other organelles?

7. Thinking Critically What would happen inside a cell if its lysosomeshad leaky membranes?

8. Thinking Critically During the development of red blood cells, the cellnucleus is lost. How do you think this affects a red blood cell's longevityand ability to divide?

9. Thinking Critically The endoplasmic reticulum is attached to thenucleus. How is this close association important to the cell's activities?

10. Thinking Critically Assume you could tag the atoms in a food moleculewith a visible marker. Using a flow diagram, trace the path of one suchatom in the cell, starting with the digestion of the food molecule. Followthe atom up to the point when it is used in cell structures. Name theorganelles involved at each leg of the journey.

How are mitochondria andchloroplasts similar? Answerthe following questions aboutmitochondria and chloroplastsand organize your responses ina table. You may use drawingsto illustrate your answers.. In which kind of cell is each of

these organelles found?. What is its function?. What are its shape and

structure?. In what part of the organelle

do the energy-producingreactions occur?

284 MHR . Unit 3 Cycling of Matter in Living Systems

4

Now that you have completed this chapter, try to do the following. If youcannot, go back to the sections indicated in parentheses after each part.

(f) Describe how microscopy stains work and whythey a~e used. (7.2)

(a) Describe how the observations made by Hooke,van Leeuwenhoek, Schleiden, and Schwann ledscientists to conclude that all living things aremade up of cells. (7.1) (g) Describe some of the ways that cancer cells are

being studied in the laboratory. (7.2)(b) Explain the difference between spontaneous

generation and biogenesis. (7.1) (h) Explain why physicians and scientists are inter-ested in research on stem cells. (7.2)

(c) Explain how Louis Pasteur's experimentsupported the hypothesis of biogenesis. (7.1) (i) Define what an organelle is, and explain how

organelles contribute to cell functions. (7.3)(d) State which type of rnicroscope(s) would be

most appropriate for studying the following:(i) living cells, (ii) the shape of a molecule,(iii) the shape of a large or thick specimen.Explain your choice. (7.2)

(j) Explain how animal and plant cellsniffer. (7.3)

(k) Describe the functions of the followingorganelles and cell structures and find themon the diagram shown here: (i) nucleus,(ii) endoplasmic reticulum, (iii) cell membrane,(iv) mitochondria. (7.3)

(e) Explain how computers have made microscopyeasier and more precise. (7.2)

Chapter 7 The Basis of Life. MHR 285

Key Terms

spontaneous generation

abiogenesis

biogenesiscell

cell theory

simple microscope

compound light microscopeelectron microscope

electron micrograph

transmission electronmicroscope (TEM)

scanning electronmicroscope (SEM)

confocal laser scanningmicroscope (CLSM)

scanning tunnellingmicroscope (STM)

base

gene sequencingcell culturecell linestem cell

organelle

rough endoplasmicreticulum

smooth endoplasmicreticulum

Golgi apparatusmitochondriacentriolevacuolecell wallcellulose

chloroplast-endoplasmic reticulum (ER)

10. What are some of the techniques beingused to study genes? (7.2)

Understanding Key ConceptsSection numbers are provided if you need to review.

vi Who was Francesco Redi, and how did hecontribute to the theory of biogenesis? (7.1)

11. Explain how a cell wall differs from a cellmembrane in both structure and function. (7.3)

12. Match each organelle with the correct function.

(7.3)(a) ribosome (i) transports materials(b) mitochondrion throughout cell(c) vacuole (ii) converts energy in(d) vesicle food molecules into

a form cells can use(ill) makes proteins(iv) stores water in

plant cells

.£ How have modern microscopes improved on earlymicroscopes? Explain your response using threeexamples of improvements. (7.1)

/' Explain why certain plant cells have chloroplasts,whereas animal cells do not. (7.3)

~ Describe the role of technology in the developmentof the cell theory. (7.1)

S. Compare the ideas of Rudolf Virchow with thoseof Louis Pasteur. How were they similar? (7.1)

6. Explain the difference between adult stem cellsand embryonic stem cells. (7.2)

7. What kind of cell is picturedhere - an animal cell ora plant cell? Explain yourreasoning. (7.3)

Developing Skills1..-13. Design an experiment to test the hypothesis

that the formation of algae in a fish tank isdue to biogenesis.

(a) Based on your hypothesis, predict theoutcome of the experiment.

(b) What experimental results would suggestthat your hypothesis was not valid, and thatalgae form spontaneously in the fish tank?

8. Describe the similarities anddifferences between vesicles,lysosomes, and vacuoles. (7.3)

14. Make a diagram of a cell illustrating the effectthat opening a hole in the cell membrane wouldhave on the cell.

9. Explain why much of cancer researchfocusses on genes. (7.2)

286 MHR . Unit 3 Cycling of Matter in Living Systen

cell membrane

cytoplasmlipidvesiclenucleusnuclear poresnuclear enveloperibosomenucleolusIvsosome

15. Refer to the photographs shown here. He wants to make observations of the cells' nuclei,but is having trouble identifying them. Suggesthow your classmate could improve his ability toobserve the nuclei.

20. Type 1 diabetes occurs when the pancreatic cellsare damaged. As a result, these cells are unableto make insulin, a cherni'cal messenger thatregulates the level of sugar in the blood. Howmight researchers develop a cure for Type 1diabetes using stem cells?mitochondrion

21. How would a mutation in a gene encodinga ribosomal protein affect cell function?

Critical Thinking.;I Mature red blood cells have no mitochondria.Explain how this affects the function of these cells.

M: Explain why Hooke's observations alone would,/ probably not have led scitfntists to develop the

cell theory.

24. Explain how the cell theory has influenced theway scientists study conditions such as cancer,Alzheimer's disease, and spinal cord injury.

25. If Antony van Leeuwenhoek had not published hisfIndings, would the course of the developmentof the cell theory have been different? Why orwhy not? Explain how the communication ofideas and data are important for the progressof scientific discovery.

chloroplast

(a) If 2 cm represents 265 nm on the electronmicrograph of a mitochondrion, determinethe approximate length of this organelle.

(b) Use the scale with the electron micrographof a chloroplast to determine the approximatesize of this organelle.

(c) Make a diagram showing the relative sizeof each organelle. Include the scale.

26. One criticism of science-based explanations fornatural phenomena is mat mese explanationsoften change. For example, Hooke initiated medevelopment of me cell meory by claiming matcork was made of cells. Later, Schleiden andSchwann revised Hooke's argument and statedmat attliving mings are made of cells. Presenta counter-argument to me criticism mat revisingme "facts" is a weakness of me scientific memod.

Pause& .

Reflect!

Problem Solving! Applying16. Suppose you are doing an investigation of

mitochondria and want to be able to see themmagnified in clear detail. Describe some of themethods you might use in your research.

17. A classmate has given you three test tubescontaining dividing cells. One of the test tubescontains stem cells, and the other two containspecialized cells. However, she has forgottento label the tubes. How could you determinewhich tube contains stem cells?

18. Design a procedure to identify whether a sample ofcells was collected from an animal or from a plant.

19. A classmate is studying the root cells of a radishseedling under a compound light microscope.

Chapter 7 The Basis of Life. MHR 287