Download Chapter 7: Cell Structure and Function

Chapter 7: Cell Structure and Function
Instructional Sequence
Section 7.1:
What are
Cells?
Four 45minute
class
periods
Section 7.2:
Cells, a Look
Inside
Four 45minute
class
periods
112
1. Complete Chapter 7
Pretest.
2. Complete Investigation 7A.
3. Read Section 7.1, pp. 136
to 140.
4. Complete Section Review,
p. 141.
1. Read Section 7.2, pp. 142
to 148.
2. Complete Section Review,
p. 149.
3. Complete Investigation 7B.
4. Complete Chapter
Assessment, pp. 153 to
154.
UNIT 3: CELL BIOLOGY
Learning Goals
Activities and Resources
• Explain that cells are the basic
unit of structure and function in
all living organisms.
• Explore cell theory and analyze
similarities and differences among
cells.
• Differentiate between cell
structure and function.
• Classify living cells as prokaryotic
or eukaryotic.
Laboratory Investigation 7A: Examining Onion Tissue
• Identify and describe parts of a
cell and their functions.
• Compare and contrast animal and
plant cells.
Laboratory Investigation 7B: Animal and Plant Cells
Connection: Organ Transplants
Chapter Activity: Building a Scale Model of a Cell
Chapter Project: Cellular Song
Teacher’s Resource CD:
• Chapter 7 Pretest
• Graphic Organizer: Prokaryotic and Eukaryotic Cells
• Skill and Practice: Robert Hooke, Antonie van
Leeuwenhoek
Teaching Illustrations CD:
• Cell Theory
Teacher’s Resource CD:
• Graphic Organizer: Plant vs. Animal Cells
• Skill and Practice: Animal Cell Parts, Plant Cell Parts
Teaching Illustrations CD:
• Diagram of an Animal Cell, Diagram of a Plant Cell
National
Standards
INQ.1.2
INQ.1.7
LS.C.1.2
LS.C.1.2
LS.C.1.3
LS.C.1.4
LS.C.2.1
LS.C.2.4
LS.C.3.1
LS.C.3.4
INQ.1.2
INQ.1.3
INQ.1.4
INQ.1.7
INQ.1.8
LS.C.1.1
LS.C.1.2
LS.C.1.3
LS.C.1.4
LS.C.1.5
LS.C.2.3
LS.C.2.4
CHAPTER 7 RESOURCES
Language Tools
Differentiated Strategies
Literary Picks
ELL Strategies: Listed in the ELL Ancillary
Vocabulary: cell theory, cell
membrane, organelle, cytoplasm,
prokaryotic cell, eukaryotic cell
Learning Strategies:
• Active Learning - Motivate
• Cooperative Learning - Investigation 5A
• Active Research - Extend
Word Origins: cytoplasm
Materials
Active Learning - Motivate: dropper bottles, pond water,
distilled water, microscope, slides, coverslips
Investigation 7A: microscope, slides, coverslips, tweezers,
piece of onion, iodine stain
Teaching Tip: Viewing Water Samples
Vocabulary: mitochondria,
vacuole, endoplasmic reticulum,
ribosome, golgi body, lysosome,
cytoskeleton, chloroplast, cell wall
Word Origins: endoplasmic
reticulum
RAFT Writing Strategy
Learning Strategies:
• Active Learning - Motivate
• Cooperative Learning - Investigation 5B
• Graphic Organizer - Explain: Venn Diagram
• Active Learning and Active Research - Extend
Teaching Tip: Cell Concentration
Cell Concentration: 10 microscopes, 5 pairs of prepared
slides
Investigation 7B: microscope, slides, coverslips, tweezers,
flat toothpicks, prepared slides of muscle tissue, live
Ulothrix, methylene blue stain, colored pencils, sanitizing
beaker (bleach water) for disposal of toothpicks
Chapter Activity: large sheets of construction paper, metric
ruler, calculator
Closure Cards
113
7.1 What Are Cells?
Literary Picks...
Reading
Level
About the Lesson
In this lesson, students learn that cells are the basic units of structure and function in living
organisms. They discover how the invention of the microscope helped scientists identify
and learn more about cells. Students explore the cell theory and analyze similarities and
differences among cells. They also differentiate prokaryotic and eukaryotic cells.
Divide students into small groups. Provide each group with two dropper bottles: one with
distilled water (labeled A) and one that contains pond water (labeled B). Do not tell
students anything about the composition of either sample. Instead, have them make
qualitative observations about the contents in each bottle before viewing them under the
microscope. Then have them prepare a slide to observe each sample. Ask students to
describe what they see. Without identifying the specific organisms, lead a discussion to
help students conclude that these tiny organisms are made of cells. You may even want to
have students create names for these “mystery creatures” based on their shape or patterns
of movement.
Students complete Investigation 7A before reading Section 7.1. The focus of this
investigation is to familiarize students with general characteristics of cells; and how to
prepare and view a microscope slide.
1. Allow one class period to complete the
investigation.
2. Students work in groups of three to five.
3. Prepare dropper bottles of iodine and
containers of sliced onion in advance.
4. Have students wear gloves, goggles, and an
apron when preparing slides.
114
UNIT 3: CELL BIOLOGY
Materials
•
•
•
•
•
•
microscope
slides
coverslips
tweezers
piece of onion
iodine stain
The Cell Works: Microexplorers by
Patrick Baeuerle
An expedition into the world of cells. The
author explores the different parts of
cells, how they are made, and how they
work together.
6-8
The Double Helix by James Watson and
Lawrence Bragg
A true story about the discovery of the
DNA structure. This book explains the
science process as it applies to the
overall history of DNA.
9-12
Microbe Hunters by Paul de Kruif
Explores the discovery of germs by
providing a short biography of various
scientists. Includes a chapter on Anton
van Leeuwenhoek and the discovery of
microbes.
Teaching Tip . . .
INVESTIGATION 7A: EXAMINING ONION TISSUE
Setup
3-5
Title
Vocabulary
cell
tissue
cell wall
nucleus
cytoplasm
Viewing Water Samples
Be sure that you view the pond sample in advance, as
it may not contain enough organisms. You can always
add organisms to the pond water to make the viewing
more interesting. Revisit this activity when teaching
about protists so that students can observe variations
in how these microorganisms move.
7.1 WHAT ARE CELLS?
Word Origins . . .
Prokaryotes vs. Eukaryotes
Students often question how a prokaryotic cell is able to function since it does not have a
nucleus. The answer lies in how students differentiate prokaryotic and eukaryotic cells.
Although prokaryotic cells have no nucleus, they do have DNA. Prokaryotic DNA is much
simpler than DNA found in eukaryotes. For example, prokaryotes usually have one large,
round DNA molecule that has none of the proteins common to eukaryotic DNA. However,
prokaryotes (like eukaryotes) do have ribosomes for protein synthesis and genes that
function in heredity.
Prokaryotes lack membrane-covered organelles, like vacuoles, lysosomes, and endoplasmic
reticulum. They also have no mitochondria; but, they are able to use the inner surface of
the cell membrane to make ATP. Many prokaryotes also have a cell wall. Unlike plant cell
walls (made of cellulose), prokaryote cell walls get their shape and strength from
peptidoglycan (or murein), a molecule made of amino acids and sugars.
Cytoplasm (cyto-, from Greek kutos or kytos,
meaning “hollow vessel”; -plasm, from Greek
plasma, meaning to “form or shape”)
The term cytoplasm is formed from the union of a
prefix, cyto- and a suffix, -plasm. Cyto- means “cell”
and -plasm means “substance or material that
forms.”
1. Have students use the prefix, cyto- and suffix -plasm
to define cytoplasm in their own words.
2. Ask students, What do you think a cytologist does?
Note: Write the term on the board. Remind students
that the suffix -ology means the “study of.”
Prokaryotes have no microtubules. Therefore the structure of a prokaryotic flagellum is
very different from that of a eukaryote. Prokaryotes move as the flagellum rotates.
Eukaryotes are propelled by the whip-like motion of their flagella.
1.
Have students guess how many microorganisms are present in one drop of pond water.
Instruct small groups of students to prepare a slide and view it under low, medium,
and high power. Each time, have students count the number of organisms they can
see. Create a table (Figure 7.1) on the board for students to record their data and find
the average number of organisms.
2.
Students create closure cards and reflect on at least two new things they learned
about cells.
3.
Students use the Internet, encyclopedia, or other resources to research one
specialized cell found in the human body. Draw the cell and describe its function.
Label different organelles within the cell, like the cell membrane, cytoplasm, and
nucleus. Display students’ work in your classroom.
Number of Creatures
medium
low power
high power
power
Group A
Group B
Average
Figure 7.1: Sample table for counting the number of
organisms in a drop of pond water.
Students complete Section 7.1 review questions.
7.1 WHAT ARE CELLS?
115
Investigation 7A: Examining Onion Tissue
Hold up an onion before students.
What are some characteristics of this onion?
Students make qualitative observations about the onion. For example, students may describe
the shape or color of the onion. Divide students into their lab groups. Provide each group
with a container of onion pieces.
Look at the onion pieces in your container. How are these pieces different in appearance from
the whole onion you viewed?
Students offer comparisons of the onion pieces to the whole, such as differences in texture
or color. Accept all reasonable responses.
Let’s examine the onion in another way. Onions have many layers. Suppose you peel off a layer of
onion and view it under a microscope. What do you think you will see?
Gradually peel away thinner layers of onion. Then use tweezers to peel away a layer that is
thin enough to examine under the microscope. Some practice is needed to get a layer that is
thin enough to view. Encourage students to make predictions about what the onion tissue will
look like when viewed under the microscope.
Prepare a slide in order to view the tissue under the microscope. Place a drop of water on the
slide. Use tweezers to remove a thin piece of onion tissue from the inside of a small piece of
onion. Gently lower the onion tissue onto the slide. Be careful to not to crease the skin. Use the
tweezers to place the coverslip over the onion skin.
Tell students to place the coverslip over the onion skin slowly in order to avoid trapping air
bubbles between the coverslip and the slide.
Draw a picture to describe what you see.
Students prepare an unstained slide of the onion tissue. Students should be able to see cells
in orderly patterns, which may resemble white floor tiles or a brick wall. They may even be
able to detect some motion in the cytoplasm. Doing this step before adding a stain gives students practice in preparing a slide and obtaining a thin layer of onion tissue. It also helps students to see why staining the tissue enhances viewing the components of the cell.
A
Preparing a slide of onion tissue
Prepare another slide of onion tissue. This time, you will apply a stain. The stain that you will
use is iodine. Be sure that you use only one drop of the iodine. Use the pictures shown in part
one of your investigation handout as a guide.
Inform students to be cautious as iodine can stain skin and clothing. It may also be harmful if
ingested. Students prepare the slide of onion tissue with stain. Walk around and observe as
students prepare the slides. Assist students as needed. Remind students to lower the coverslip slowly in order to reduce the probability of trapping air bubbles.
116
UNIT 3: CELL BIOLOGY
7A Examining Onion Tissue
What is onion tissue made of?
Do you think peeling an onion will make you
cry? Well, here’s your chance to test your
hypothesis. In this investigation, you will peel a
thin layer from a piece of onion. You will stain
that layer of tissue on a microscope slide. Then
you will examine your slide under a microscope
and make sketches of what you see.
Materials
•
•
•
•
•
•
Microscope
Slides
Coverslips
Tweezers
Piece of onion
Iodine stain
Safety tip: Wear gloves, goggles, and an apron while
preparing the slides.
A
Preparing a slide of onion tissue
Onions have many layers. The inner surface of each layer has a thin layer of tissue that’s easy
to peel off. Since it is almost transparent, you will need to apply a stain so you can see things
under a microscope. Follow the procedures below to make a slide of onion tissue.
1. Place a drop of iodine stain onto a slide.
2. Using the tweezers, gently peel the thin layer of tissue off the inside of a small piece
of onion.
3. Using the tweezers, gently lower the onion skin onto the slide. Be careful not to
crease the skin.
4. Use the tweezers to place a cover slip over the onion skin.
B
Stop and think
a.
What is the purpose of the iodine stain?
b.
What do you think you will see under the microscope? In your notebook, sketch what you
think you will see.
c.
What is a tissue? What is the next level of organization above tissues? What is the level of
organization below tissues?
38
7.1 INVESTIGATION 7A: EXAMINING ONION TISSUE
Stop and think
B
Take a moment to think about the slides you have prepared. Then answer each question in part
two of the investigation.
Discuss students’ answers to each question. Walk around and check students’ sketches of
what they predict they will see when viewing the stained onion tissue. Talk about the levels
of organization of living organisms. This is a good time to revisit the vocabulary terms, cell,
tissue, and organ.
Observing onion tissue under a microscope
C
Now it is time to view the stained onion tissue. What should you do first?
Students ensure that the stage is lowered to its lowest point.
Good answer. Check your objectives. Are you at low power? Place your slide on the stage and
secure it with the stage clips.
Students set up the microscope and prepare to view the sample under low power.
View the onion tissue at low, medium, and high power. Make a detailed sketch of what you see at
each magnification. Also record any other observations beneath each sketch.
Students view the slide at each magnification and record what they observe. Remind students to look at the microscope from the side when changing objectives to ensure that the
slide is not hit, shifted, or damaged. Also, remind students not to use the coarse adjustment
knob when viewing the slide at high power as this could also damage the slide.
Thinking about what you observed
D
Examine each of your sketches closely. Do you notice any trends in what you observed as the
power was changed?
Students should note that they were able to see more detail of the onion tissue as the power
was increased. Encourage students to discuss what they saw.
Background Information...
Why Onions Make You Cry
Students know that onions have cells. Onion cells
consist of enzymes and other sulfur-containing
compounds. The membranes in whole onion cells
keep these substances apart. When an onion is sliced,
this barrier is broken and a chemical reaction
between sulfur and enzymes is initiated. One of the
products of this reaction is a gas.
As this gas is released, it further interacts with the
water that is already present in our eyes. The
product of this interaction, sulfuric acid, irritates the
nerve endings in the eye and creates a stinging
sensation. The stinging is like a message being sent to
the brain, telling it to “do something.” The brain’s
response is to protect the eyes by attempting to
dilute the acid. The brain directs the tear ducts to
produce more water, which makes the eyes water, or
“cry.”
Enzymes are affected by temperature. If an onion is
cooled before slicing, the effects are minimized. This
is why it is often recommended to either peel and
refrigerate an onion before cutting it, or to slice it
under running water.
Use your observations to help you answer each of the questions in part four.
Discuss the answers to each question with the class. Review the visible organelles with the
students and talk about their characteristics. As a result of completing this investigation,
many students often question why onions make the eyes water. Use the information provided
in Why Onions Make You Cry to explain the reasoning to your students.
7.1 WHAT ARE CELLS?
117
7.2 Cells: A Look Inside
Teaching Tip . . .
About the Lesson
In this lesson students take an in-depth look at plant and animal cells. Students learn to
identify organelles and describe how they function. Students also compare and contrast
plant and animal cells.
Show students pictures of different types of cells. For each picture, have students describe
and identify familiar organelles, like the nucleus. Be sure to have a variety of cell types.
Then have students group the cells based on their similarities. Students may begin to
categorize cells by shape or size, or even by the presence (or absence) of certain
organelles. See Teaching Tip (right) for an alternate activity.
Students complete Investigation 7B after reading Section 5.2. Students will view and
identify organelles that are common to plant and animal cells, and describe those that are
unique to each cell type. It is important that students realize that not all animal cells (or
plant cells) look and function exactly alike.
INVESTIGATION 7B: ANIMAL AND PLANT CELLS
Setup
1. Allow class period to complete the
investigation.
2. Students work in groups of three to five.
3. Store the Ulothrix according the supplier’s
recommendations.
4. Check the condition of prepared muscle
tissue slides before the investigation.
5. Prepare a waste container for used
toothpicks.
6. Caution students that methylene blue may
stain skin and clothing and is harmful if
ingested.
120
UNIT 3: CELL BIOLOGY
Materials
•
•
•
•
•
•
•
•
•
•
microscope
slides
coverslips
tweezers
flat toothpicks
prepared slides of muscle
tissue
live Ulothrix
methylene blue stain
colored pencils
Sanitizing beaker (bleach
water) for disposal of
toothpicks
Vocabulary
organelle
epithelial cell
chloroplast
multicellular
Cell Concentration
Set up ten microscopes and number them one to 1en.
Prepare ten slides, two each of five different cell
types. Put one slide under each microscope. Have
students work in pairs to view each slide. Don’t tell
students that they are playing a game just yet.
Simply instruct them to pay close attention to the
details of each cell. Then have students try to
remember which pairs of microscopes had similar
cells. They can identify the matching microscopes by
their numbers. Be sure to include at least one plant
cell pair, as students should easily identify its unique
cell wall.
Word Origins . . .
Endoplasmic reticulum (endo-, from Greek
meaning “inside”; reticulum, from Latin
meaning “little net”)
The endoplasmic reticulum is “the little network
located inside the cytoplasm of the cell.”
Many organelles share common prefixes, suffixes, or
roots. Here are a few to discuss with your students.
1. the suffix -some (from Greek, soma, meaning
“body”)
Examples: lysosomes, ribosomes
2. the prefix chloro- (from Greek khloros, meaning
“green”)
Examples: chlorophyll, chloroplast
7.2 CELLS: A LOOK INSIDE
Using a Venn Diagram
Students sometimes experience difficulty differentiating the cell wall and the cell
membrane. In many instances, students are confused because both organelles have names
beginning with the word cell. Others may forget that the plant cell has both a cell wall and
a cell membrane, while animal cells have no cell wall. Instruct students to create a Venn
diagram (Figure 7.2) to compare and contrast these structures.
1.
Play a game of “Cell Charades.” Students work in groups to act out the role of
organelles in cells while their classmates guess what they are. Encourage students to
create visuals or other effects to make their acting more interesting. For example, the
group representing the nucleus may create a sign for one of its members that says “I’m
the boss!” Then that person could walk around the classroom telling other organelles
(group members) what to do.
2.
Plan a field trip for students to visit a local factory. Have students compare jobs of
various factory workers with parts of the cell.
3.
Students write a RAFT to demonstrate their knowledge of the structure, function, and
other characteristics of organelles in the cell (see example, right).
Students complete Section 7.2 review questions.
7.2 CELLS: A LOOK INSIDE
121
Investigation 7B: Animal and Plant Cells
Guided Practice . . .
What do you have in common with a rose, a tiger, an ant, a fungus, and a bacterium?
Display pictures (or transparencies) of each of these objects before students. Students may
make a variety of connections among these items. For example, like humans, these are all
examples of living organisms.
RAFT Writing
Very good. Human beings, like each of these things, are examples of living organisms. What are
some of the qualities of living things?
Encourage students to recall what they learned about the characteristics of living things in
Chapter 2. Acceptable responses are that all living things are able to grow, develop, respond
to their surroundings, reproduce, use energy, and are made up of cells. Write these characteristics on the board as students name them.
Audience: The cell membrane or cell wall
You are correct. These are all qualities of living things. In today’s investigation, you will focus on
one of the characteristics of living organisms: cells. All living things are made of cells. Do you
think that your cells are like those of a rose?
Students may have differences in opinion about whether human and rose cells are alike in
any way. Encourage students to discuss how cells in the human body may be similar to or different from a rose plant’s cells.
One feature that you have in common with the rose is that both you and the rose are multicellular. What does the term multicellular mean?
Multicellular means having many cells. Talk about the prefix multi- and contrast it with the
prefix uni- or the term unicellular.
Good answer. Both plants and animals have different types of cells that allow them to perform
different functions. Each of these functions, or jobs, is important to the cell’s survival. However,
the types of structures found in plant and animal cells are different. A big factor in these differences is the way cells store materials and create the energy they need to stay alive.
A
Observing animal cells
Today you will compare animal cells to plant cells. You will view two types of animal cells: epithelial cells and muscle cells. Epithelial cells are the cells that line your mouth.
Write the term epithelial and its meaning on the board.
The epithelial cells are your own cheek cells. You will prepare a slide of the cheek cells first.
Students gather the materials they need to prepare the slides. Demonstrate the proper technique for scraping the cheek and preparing the slides. Instruct students to follow the visual
steps provided to prepare the slide. Assist students as needed. Verify that each group has
correctly prepared each slide.
Follow each step outlined in part 1 of the investigation. Look at the cheek cells under low,
medium, and high power. What do you see? Draw a picture of what you see at each magnification
in Table 1. Make note of any structures that you are able to identify.
Students view the stained cheek cells under low, medium, and high power. Some structures
that students may identify include the cell membrane, nucleus, and cytoplasm. Direct
122
UNIT 3: CELL BIOLOGY
Role: Any organelle in a cell, except the cell
membrane or cell wall.
Format: Student’s choice of a letter or advertisement
placed in The Cellular Scoop
Topic: You have not been performing your organelle
duties up to the standards set by the cell wall or cell
membrane (also known as the “Gatekeeper”). The
local newspaper, The Cellular Scoop, has reported
that you are about to be evicted from the cell.
Convince the Gatekeeper that you are essential to
the cell’s survival and should be allowed to stay.
Ordering live organisms
Be sure to order the Ulothrix 3 to 4 weeks in advance of
the investigation. If the organisms arrive early, you may
keep them in an aquarium with a bubbler.
7.2 INVESTIGATION 7B: ANIMAL AND PLANT CELLS
students to properly dispose of the toothpicks (see Teaching Tip). It may also be helpful to
have colored pencils available for students to complete their sketches.
Look at the prepared slide of the muscle tissue cells under low, medium, and high power. Sketch
what you see and record your observations in Table 1.
Students record their observations. Students should note that they are able to see more
details of the cells as the power is increased.
Thinking about what you observed
B
Use your sketches and observations to help you answer each question in part 2.
7B Animal and Plant Cells
What are the differences between animal and plant cells?
In this investigation you will compare
animal cells (your own epithelial cells
and prepared slides) and plant cells
(live Ulothrix - an algae that is closely
related to plants, and the onion cells
you observed in the last investigation).
Materials
•
•
•
•
•
Microscope
Slides
Coverslips
Tweezers
Colored pencils
• Flat toothpicks
• Prepared slide of muscle
tissue
• Live Ulothrix
• Methylene blue stain
Safety tip: Wear gloves, goggles, and an apron when
preparing slides.
Allow time for students to complete the questions. Discuss their observations.
Observing Ulothrix cells
C Ulothrix
is a filamentous and photosynthetic green algae. Therefore it is actually a protist,
not a plant. However, it is beneficial to observe Ulothrix under the microscope because it has
clearly defined organelles, like cell walls and chloroplasts, in common with plants.
Prepare a wet mount of Ulothrix filaments. View this slide under the same magnifications that
you used to look at the two animal cell samples. Sketch what you observe.
Tell students not to clump too many filaments on the slide. Emphasize to students that this is
not a case where more is better. Students record their observations of Ulothrix and create
sketches. Students should clearly see organelles like the cell wall, nucleus, and chloroplasts.
Thinking about what you observed
D
Think about the last investigation. Do you remember viewing the onion cells? What was different
A
Observing animal cells
The cells that line the inside of your mouth are called epithelial cells. These cells are easy to
collect and observe. Follow the procedures below.
1. Place a small drop of methylene blue stain onto a clean slide.
2. Remove a clean flat toothpick. Wet the wide end of the toothpick using tap water.
3. Gently scrape the inside of your cheek with the wide end of the toothpick. DO NOT
USE FORCE!
4. Place the toothpick into the stain on the slide and gently swirl to mix the cheek cells
with the stain. Dispose of the toothpick as directed by your teacher. DO NOT REUSE
THE TOOTHPICK.
5. Using tweezers, gently place a coverslip on top of the methylene blue solution as
shown.
6. Place the slide on the microscope stage and observe under low power, medium power,
and high power. Sketch what you see and record your observations in Table 1.
7. Dispose of the cheek cell slide as directed by your teacher.
8. Obtain a prepared slide of muscle tissue cells. Place the slide on the microscope stage
and observe under low power, medium power and high power. Sketch what you see
and record your observations in Table 1.
about the method you used to look at the onion cells compared to how you looked at the
Ulothrix?
Students needed a stain to see details of the onion cells but did not for the Ulothrix.
How are onion and Ulothrix cells alike? How are they different?
Students answer each question in part 4. Set up a few microscopes with prepared slides of
stained onion cells, or you may have students prepare slides of onion tissue once more. Allow
students to look at them again to make comparisons.
Applying your knowledge
E
You have viewed both animal and plant cells under the microscope. Which structures are common to both plant and animal cells? Which are unique to plant cells?
Students answer questions with members of their investigation group. Engage the class in a
discussion to review their responses to the questions. Questions a and b require students to
identify specific structures within the cell. Use this time to review the name, identifying
characteristics, and function of each organelle. Point out each of these organelles to students as you name them.
40
Teaching Tip . . .
Disposal of Used Toothpicks
Prepare a beaker of water with a teaspoon of bleach
added. Label the beaker Used Toothpicks. Instruct
students to place their used toothpicks in the beaker.
Dispose of the toothpicks at the end of the class
period.
7.2 CELLS: A LOOK INSIDE
123