LESSON i

Exploring Cells

X^H^KTiv;<* "U •» V. a

When Robert Hooke first looked at a slice of cork

through a microscope, the "tiny cavities" he

described reminded him of a bee's honeycomb.

It prompted him to call the tiny cavities cells.

INTRODUCTION

In Lessons 1 through 6, you looked briefly atmany of the organisms that you will explore inmore depth in the rest of this module. To learnmore about organisms, you must first under-stand the nature of the cell, which is the basieunit of life. In this lesson, you will observe algal,plant, and animal cells through a microscope.You will draw, label, and measure the cells, fol-lowing the guidelines for scientific drawings. Youalso will compare the structures of the cells anddiscuss whether their structures are suited totheir functions.

OBJECTIVES FOR THIS LESSON

Observe, draw, label, and measure cells

based on specific guidelines.

Observe and identify certain organelles

of plant and animal cells.

Observe the effect of salt solution on

Elodea leaf cells.

Compare the structure of various cells

for evidence that they are suited to their

functions.

Update the organism photo cards for

Elodea and Spirogyra.

82 STC/MH1

PLANT AND ANIMAL CELLS: THE SAME, BUT DIFFERENT

Almost all living things on Earth are made up ofcells. Cells are the basic units of life. The sim-plest organisms—amoebae, for example—con-sist of only one cell. Complex organisms, suchas humans, have trillions of cells that aredivided into about 200 different types. Eachcell type has a different function.

As the building blocks of living matter, plantand animal cells have many things in common.They also differ in some ways. Those differ-ences are important because they point tosome of the factors that distinguish one form oflife from another. To understand this better,

let's take a cross-sectional look at an animalcell and a plant cell.

Inside an Animal CellAlthough there is no typical animal cell, mostanimal cells have three basic parts that scien-tists call "cellular organelles." Organellemeans "little organ." The first organelle is thecell membrane, sometimes referred to as the"plasma membrane." This living membrane sep-arates the cell from the rest of its environmentand helps control the passage of substancesinto and out of the cell.

(continued)

Golgi body Mitochondrion

Vacuole

Cytoplasm

Nuclear envelope

Nucleus

Nucleolus

Animal cell

Ribosome

Endoplasmic reticulum

Lysosome

STC/MS1 83

LESSON 7 EXPLORING GKLI .S

(continued from pg. 83)

The second cellular organelle is the "nucle-us," which usually occupies the central portionof the animal cell. Think of the nucleus as"command central." The nucleus regulates allthe activities that take place in the cell.Instructions for the cell's activities are storedin the chromosomes, which are found in thenucleus. The chromosomes, which almostalways occur in pairs, are composed of a sub-stance called "DNA" (deoxyribonucleic acid).DNA carries the hereditary traits that arepassed from parent to offspring. The nucleus issurrounded by a double membrane called the"nuclear envelope."

The third basic cellular organelle is a jelly-like substance called "cytoplasm," which liesbetween the cell membrane and the nuclearenvelope. In addition to the three basic cellu-lar organelles, there are additional organellesin the cytoplasm. Each organelle carries out a

specific cell function. For example, nutrientsare broken down in the sausage-shapedorganelles called "mitochondria." The energyproduced in these organelles is either releasedto support the cell's activities or stored in thecell for future use. That is why mitochondriaare often referred to as the "powerhouses" ofthe cell.

Ribosomes are organelles that help make theproteins that the cell needs to perform its lifeactivities. Many ribosomes are located alongthe endoplasmic reticulum, or ER for short. TheER is a series of cavities that is connected tothe nuclear envelope. Some substances travelbetween the nucleus and cytoplasm throughthese cavities. Golgi bodies package the pro-teins made by the ribosomes so that they canbe sent out of the cell. Organelles called "lyso-somes" help the cell digest proteins. The cyto-plasm also contains organelles called vacuoles.Filled with water, food, or waste, they are thecell's "storage tanks."

One rung of this coiled ladderis called a base pair.

One gene can be hundreds orthousands of base pairs in length.

This is a model of a portion of a DNA molecule that makes up a chromosome.

Hereditary structures called genes are made up of varying numbers of base pairs.

84 HTC/M.S™ O R G A N I S M S -

LESSON 7 EXPLORING C E L L S

Inside a Plant CellPlant and animal cells have the same basiccell parts—cell membrane, nucleus, and cyto-plasm. But there are some differences.

First, the plant eel! is surrounded by arigid, outer layer called the "cell wall." Thecell wall contains cellulose, a tough sub-stance that supports and protects the cell.Like the cell membrane that lies within, thecell wall allows materials to pass into andout of the cell. Unlike the cell membrane, thecell wall is nonliving.

The nucleus is much the same in plant andanimal cells. But some of the organelles in theplant cell's cytoplasm are different. For exam-ple, some plant cells have organelles called"plastids," which contain pigments. Pigmentsgive parts of plants their characteristic colors—

red for tomatoes, orange for carrots, and greenfor spinach.

A chloroplast is a special plastid in aplant's leaf and stem cells. Chloroplasts con-tain a green pigment called chlorophyll.Chlorophyll traps energy from the sun. Plantcells use this energy to produce glucose, asimple sugar, during a process called photo-synthesis. The vacuoles in plant cells aremuch larger than those in animal cells. Mostplants have a large central vacuole thathelps support the plant cell and also servesas a storage place for water, sugar, starch,and protein.

Although plant and animal cells have manyorganelles in common, each has organellesthat the other does not, making them thesame, but different!

Chloroplast

Golgi body

Cell wallCell membrane

Cytoplasm

Endoplasmic reticulum

MitochondrionRibosome

Nucleolus Vacuole

NucleusNuclear envelope

Plant cell

8TG/MS1 85

LESSON 7 E X P L O R I N G

Getting Started

•i Work with your group to draw on a pieceof newsprint what you think a typical culllooks like. Label any parts with which youare familiar.

n Share your group's drawing with the class.

MATERIALS FOR

LESSON 7

For you1 copy of Student

Sheet 7.1:Template forSpirogyra CellDrawing

1 copy of StudentSheet 7.2:Template forOnion Leaf CellDrawing

1 copy of StudentSheet 7.3:Template forElodea Leaf CellDrawings

1 copy of StudentSheet 7.4:Template forAnimal CellDrawings

1 box of coloredpencils

For your group1 set of organism

photo cards1 piece of newsprint

Several strands ofSpirogyra

2 pieces of slicedonion

2 Elodea leavessoaked in freshwater

2 Elodea leavessoaked in saltsolution

1 prepared slide ofhuman cheekcells

1 prepared slide ofmammalian nervecells

2 compound lightmicroscopes

2 plastic slides2 coverslips2 metric rulers,

30 cm (12")2 transparent rulers1 dropper bottle of

Lugol solution1 plastic pipette

86 STC/MS™ O R f i A M S M S — F R O M M A C R O TO M I C R O

LESSON7 EXPLORING C E L L S

Inquiry 7.1Observing, Drawing, andMeasuring an Algal Cell

PROCEDURE

•| Read "Plant and Animal Cells: The Same," but Different," at the beginning of this les-

son. Discuss the reading selection withthe class and ask questions to clarify any-thing you do not understand.

^ Use a plastic pipette to obtain a smallsample of water from the containermarked "Spirogyra." Spirogyra is a typeof common pond alga whose cells arejoined in chains. Make sure that the sam-ple includes from two to four of the greenstrands that are floating in the water.

O Put a drop of the sample on the middle ofthe slide and add a coverslip.

A Focus on a chain of Spirogyra cells underlOOx. Alter observing Spirogyra throughthe microscope, discuss with your partnerhow you think it got its name.

C Switch to 400x and focus on one cell (seeFigure 7.1). Draw the cell in the circle onStudent Sheet 7.1: Template for SpirogyraDrawing. Title your drawing, "SpirogyraCell." Label at least two organelles. Referto "Plant and Animal Cells: The Same,But Different" to help you identify thestructures. Follow the guidelines for scien-tific drawings 011 Student Sheet 2.3A,which you used in Lesson 2.

Figure 7.1 With a good microscope and a little fine tuning, you can even see the nucleus in a Spirogyra cell.

STC/MS O R G A N I S M S — F R O M M A C R O TO M I C R O 87

LESSON 7 EXPLORING C E L L S

C Using your transparent ruler as a cover-slip, measure the length of one Spirogyracell, as seen in Figure 7.2. Record themeasurement in parentheses to the rightof the title below the circle.

Review your drawing to ensure that youhave followed each guideline. Follow yourteacher's directions for turning in yourdrawing.

Figure 7.2 Measuring the length of the cell using the transparent ruler

88 STCAIS™ O R G A N I S M S -

LESSON 7 EXPLORING G K I . L S

Inquiry 7.2Observing, Drawing, andMeasuring an Onion LeafCell

PROCEDURE

•1 Follow these steps to prepare a wet-mount slide of a leaf cell from the bulb ofan onion plant:

A. Obtain a piece of sliced onion from thecontainer provided by your teacher.

B. Follow the steps in Figure 7.3 to prepare awet-mount slide of an onion leaf.

SAFETY TIP

Wear splash-

proof safety gog-gles wheneveryou use chemi-cals such asLugol solution.Lugol solutionwill stain skinand clothing andcan be harmfulwhen it comes incontact with youreyes or mouth.

1. Put one drop of Lugol solution in the middle of a plastic slide.

2. Bend the piece of onion against the curve until itsnaps. Push one side under the other to peel off a thinmembrane of leaf epidermis.

3. Spread the membrane out in the Lugol solutionso that it is flat on the slide. Add a coverslip.

Figure 7.3 How to prepare onion leaf membrane for viewing under the microscope

STC/MS™ O R T . A X I S M S — F R O M M A C R O TO M I C R O 89

LESSON? E X P L O R I N G C K L U S

O Take turns with your partner to prepare adrawing of one onion leaf cell under highmagnification. Draw the cell in the circleon Student Sheet 7.2: Template for OnionLeaf Cell Drawing. Follow the guidelinesfor scientific drawings. Title your drawing"Onion Leaf Cell." Label the cell wall,nucleus, and cytoplasm. Discuss with

your partner why you cannot see chloro-plasts in these cells even though these areplant cells.

0 Use the transparent ruler to measure thelength of the cell. Record the measure-ment in the appropriate place on yourdrawing.

A When you and your partner have com-pleted your drawings, rinse and dry yourslide, coverslip, and transparent ruler, andproceed to the next inquiry.

Inquiry 7.3Observing, Drawing, andMeasuring Elodea Leaf Cells

PROCEDURE

1 Follow your teacher's directions to obtainone Elodea leaf that is soaking in freshwater. Elodea is a common, freshwaterplant whose leaves are good specimensfor observing typical plant leaf cells.

^ Place the leaf on the slide and add a cov-erslip. Focus on a layer of cells underIGOx. Switch to 400x and focus on asmaller group of cells. With your partner,discuss which structure you can see inthese cells that were not present in theonion leaf cells.

O Draw one cell in the upper circle on

Student Sheet 7.3: Template for ElodeaLeaf Cell Drawings. Title your drawing"Elodea Leaf Cell." Label three of itsorganelles. Use the transparent ruler to

measure the length of one cell. Recordthe length in the appropriate place onyour drawing.

A Glean your slide and then obtain a secondElodea leaf that has been soaking in saltwater. Set up your slide as you did for theElodea that had been soaking in freshwater. Move the slide around to find a cellwhose contents have shrunk into a roundor oval shape. Draw one cell under 400xin the lower circle on Student Sheet 7.3.Label the same three organelles as youdid in Procedure Step 3. Also label afourth organelle that has become visible

because water has been forced out of thecell by the salt solution.

5 Rinse and dry your slide, coverslip, and• ' '

transparent ruler.

90 STC/MS™ O R G A N I S M S — F R O M M A C R O TO M I C R O

LESSON 7 E X P L O R I N G C E L L S

Inquiry 7.4Exploring Animal Cells

PROCEDURE

•I Have one member of your group obtainone prepared slide of mammalian epithe-lial tissue (cheek cells) and one of mam-malian nerve tissue.

^ Take turns with your partner to observe,draw, and measure one cell from one ofthe prepared slides. When each pair inyour group is finished with its drawing,trade slides. Draw one slide in each of thecircles on Student Sheet 7.4: Template forAnimal Cell Drawings.

O Title the appropriate drawing "CheekCell." Label the cell membrane, cyto-plasm, and nucleus.

A Title the other drawing "Nerve Cell."Label the cell membrane, cytoplasm, andnucleus. Refer to Figure 7.4 if you havedifficulty identifying a cell within thenerve tissue.

C Look at the cells in your drawings andthose in Figures 7.5 and 7.6. Discuss withyour partner why these cells are so differ-ent in size and shape.

C Work with others in your group to updateyour organism photo cards for Spirogyra,Elodea, and humans. Return them to yourteacher.

Figure 7.4 Animal cells are often difficult to distinguish on a slide. This photo should help you with your identification.

The cell membrane, cytoplasm, and nucleus in these two nerve cells are clearly visible.

STC/MS™ O R G A N I S M S — F R O M M A C R O TO M I C R O 91

LESSON 7 E X P L O R I N G

Figure 7.5 The individual cells in this photo of skeletal muscle—taken through a microscope at approximately 400x—

are long, narrow, and so tightly packed that they are difficult to identify.

Figure 7.6 This photo of highly magnified blood tissue contains many red blood cells and one white blood cell, right in

the center.

92 STC/MS™ O R G A N I S M S — F R O M M A C R O TO M I C R O

LESSON 7 EXPLORING C E L L S

REFLECTING ON WHAT YOU'VE DONE

Answer the following questions on the studentsheets indicated.

A. Based on the algal Spirogyra that youobserved, would you consider Spirogyrato be more plant-like or animal-like?Defend your answer. (Student Sheet 7.1)

B. Why do you think the bulb of theonion plant is so big? What function doesit serve for the plant? (Student Sheet 7.2)

G. What happened to the Elodea leaf cellswhen they were soaked in salt solution?How do you think this relates to whathappens when you eat salty foods?(Student Sheet 7.3)

D. Use the Venn diagram to show the cellstructures and organelles that you

observed in cells from the onion bulb,Elodea leaf, and epithelial tissue. (StudentSheet 7.4)

E. If animal cells do not have cell walls,what gives animals such as mammalsshape and support? (Student Sheet 7.4)

F. Give one example from among the cellsyou observed in this lesson of how thesize and shape of a cell is well suited forits particular function. (Student Sheet7.4)

G. Compare the cells you drew in theinquiries with the one your groupsketched during "Getting Started." Basedon what you have learned, discuss withthe class what you should do to makeyour sketch more accurate. (StudentSheet 7.4)

STC/MS' 93

COURTESY OF HENRY MILNE/Q [JSRC

PART Continuingthe Cycle

LESSON 8

LESSON 9

I

Cell Division: Multiplying by Dividing 96

Inquiry 8.1 Simulating Interphase, Mitosis,and Cytokinesis 97

Inquiry 8.2 Creating a Model of Interphaseand the Stages of Mitosis 98

Sexual Reproduction in Flowering Plants 106

Inquiry 9.1 Dissecting a Perfect Flower 108Inquiry 9.2 Pollinating a Fast Plants Flower 111

LESSON 10 Leaf Structure and Transpiration 120

Inquiry 10.1 Observing f ind Drawing a StomalalUnit From the Epidermis of aLettuce Leaf 122

Inquiry 10.2 Preparing a Model of a StomatalUnit 124

Inquiry 10.3 Exploring Transpiration inWisconsin Fast Plants 126

LESSON 11 Exploring Microorganisms 132

Inquiry 11.1 Exploring Living Protists 136Inquiry 11.2 Observing and Drawing Protists

From Prepared Slides 138Inquiry 11.3 Creating a Protist Cartoon 139

LESSON 12 Revisiting Your Pond

Inquiry 12.1 Observing and Drawing My Pondand Its Microbes

Inquiry 12.2 Determining the Average DailyIncrease in the Number ot'Lcmna Fronds 150