CHAPTER 9: CELL-CELL INTERACTIONS
WHERE DOES IT ALL FIT IN?
Chapter 9 combines the information in Chapters 4 and 5 to provide a story about cellular interactions.
These interactions are essential for understanding the signaling that controls gene expression. In
addition, cell-cell interactions determine the success of multicellularity and promote the development
of a fertilized egg into a normal embryo. This information should be reviewed when covering later
chapters that cover cell signaling, embryology, endocrine system, and nervous system.
SYNOPSIS
Cells of multicellular organisms must communicate with one another so that they behave as a
coordinated group of cells rather than just a bunch of independent ones. Recall the cliche “the
left hand doesn’t know what the right hand is doing.” The signals to which a cell responds are
dependent on the kinds of receptor proteins associated with that cell. If the signal is just the right
molecular shape, it and the receptor bind eliciting a response somewhere in the cell. There are
four main types of cell signaling. Direct contact and paracrine signaling are important in early
organismal development. Endocrine signaling via hormones provides widespread response in
both plants and animals. Synaptic signaling found in animal nervous systems produces more
localized responses at the chemical synapse between the neuron and the receptor cell.
Intracellular receptors are small molecules that are able to pass through the plasma membrane of
the target cell. Nitrous oxide is one example. It activates the enzyme that catalyzes synthesis of
cyclic GMP. A superfamily of steroid hormone receptors have specific DNA binding sites
normally occupied by an inhibitory protein. When the signal molecule binds to another site on
the receptor the inhibitor is released. The receptor then binds to DNA to activate or suppress a
certain gene.
There are three superfamilies of cell surface receptors. The signals associated with these
receptors bind to receptor proteins on the cell surface. Thus an extracellular signal is converted
to an intracellular signal. In chemically-gated ion channels the receptor is a transmembrane
protein that winds back and forth through the membrane several times (called a multi-pass
protein). The center of this protein forms a channel through which specific ions can pass.
Enzymatic receptors are linked to enzymes or themselves act as enzymes. Most of these are
protein kinases; they add phosphate groups to proteins. They are single pass transmembrane
proteins where the region that binds with the signal molecule is located outside the cell and the
portion that initiates enzymatic activity is found within it. G protein-linked receptors are sevenpass transmembrane proteins that become activated when they bind to GTP. These receptors are
important because they provide the mechanism of action for over half of the therapeutic drugs
currently in use.
Second messengers are small molecules or ions that alter the shape and behavior of receptor
proteins to relay the signal message to enzymes or genes within a cell. cAMP is the most widely
used second messenger in animal cells. Ultimately the cAMP binds to A-kinase, activating it to
phosphorylate certain cell proteins. Calcium is normally sequestered outside a cell or within its
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endoplasmic reticulum. With the proper G protein signal inositol triphosphate is eventually
produced, which opens calcium channels in the ER membrane. This influx of calcium triggers
many activities. In most cases, a cellular signal is too insignificant to result in an adequate
cellular response. Protein kinase cascades amplify the signal. In vision for example, a single
light-activated rhodopsin molecule activates many transducin molecules that further split 105
cyclic GMP molecules.
The expression of cell identity results from the presence of unique cell surface markers, some of
these, like those associated with ABO blood types, are glycolipids. Others are proteins anchored
in the plasma membrane — the major histocompatibility complex (MHC) proteins, for example.
Cells in a multicellular organism establish physical connections whereby sheets are created to
form walls and partitions. Tight junctions are an example of these within an animal’s digestive
tract. Anchoring junctions are common in tissues exposed to stress. In cadherin junctions,
desmosomes for example, a cell’s cytoskeleton is attached to that of other cells or to the
extracellular matrix. Adherens junctions connect actin filaments of cells via linking proteins
called integrins. Communicating junctions, gap junctions in animals, and plasmodesmata in
plants, allow the passage of ions or molecules from one cell to another.
LEARNING OUTCOMES
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Know what receptor proteins are, where they are located, and what they do.
Describe the four types of cell signaling.
Differentiate between intracellular receptors and cell surface receptors in terms of function.
Know the three cell surface receptor superfamilies, the basic structures of each and how each
functions to convert an extracellular signal to an internal one.
Understand how cAMP and calcium function as second messengers and why they are necessary.
Explain the amplification process associated with protein kinase cascades.
Know what cell surface markers are, where they are located, and give examples.
Describe the three primary kinds of junctions that exist between cells.
Compare and contrast communicating junctions, gap junctions, and plasmodesmata.
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COMMON STUDENT MISCONCEPTIONS
There is ample evidence in the educational literature that student misconceptions of information
will inhibit the learning of concepts related to the misinformation. The following concepts
covered in Chapter 9 are commonly the subject of student misconceptions. This information on
“bioliteracy” was collected from faculty and the science education literature.
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The lipid layer is a solid surface
Cell membrane proteins are immoveable in the bilayer
Only hormones are cell signals
All hormones are related to steroids
Desmosomes are the same as plasmodesmata
Plant cells are completely isolated from each other and make no cell contact
Plants do not produce hormones
Cells make direct contact with each other’s lipid bilayer
Neurotransmitter receptors are not the same as hormone receptors
All hormones act on cell surface receptors
Cell surface receptors are not associated with changes in gene expression
INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE
Students need to be secure in the material from the previous chapter to progress in this one.
There is a lot of new terminology. Flash cards will help the students learn this information —
making them as much as using them. Table9.2 is an invaluable summary of the chapter, but too
many students will use it as a crutch and just memorize it. It’s much better if they make their
own table(s). The best part of the learning process is in the organization and gathering of
information.
Re-emphasize the importance of molecular binding to induce three-dimensional shape changes to
effect a variety of changes within the cell.
Stress the importance of protein kinases. The topic will come up again in gene expression and
regulation. It’s amazing how a simple addition of a PO4 group can do so much in a cell!
Don’t let your students confuse desmosomes and plasmodesmata. They sound similar, but are
very different structures.
HIGHER LEVEL ASSESSMENT
Higher level assessment measures a student’s ability to use terms and concepts learned from the
lecture and the textbook. A complete understanding of biology content provides students with
the tools to synthesize new hypotheses and knowledge using the facts they have learned. The
following table provides examples of assessing a student’s ability to apply, analyze, synthesize,
and evaluate information from Chapter 9.
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Application
Analysis
Synthesis
Evaluation
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Have students predict would happen if a chemical that blocks cAMP
were given to a culture of cells.
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Ask students to determine the chemical nature of a hormone that affects a
cell in spite of the presence of cell surface receptors for the hormone..
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Ask students to explain why insufficient calcium in the tissues affects
cell interactions.
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Ask students describe the properties of a chemical that blocks an
intracellular receptor.
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Have students explain how cell to cell communication will be affected by
bathing cells in a solution of enzymes that degrade proteins.
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Ask students explain why certain organisms that enter cells can only
invade specific cells in an organism.
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Ask students come up with uses of a drug that inhibits the action of
protein kinases in targeted cells.
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Ask students to come up with commercial and food preparation uses for
fungi known to degrade cell junction proteins.
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Ask students to explain how genetic modification techniques that alter Gprotein function can be used to produce new types of agricultural
animalss.
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Ask students to compare and contrast the cell surface receptors to
proteins involved in active transport.
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Ask students to evaluate a claim about a drug that increases alertness
because it supplies phosphorus that fuels kinases of the brain cells.
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Have students explain the validity of using oral ATP supplements to
enhance cell signaling.
VISUAL RESOURCES
1. The amplification analogy in the book is very straight forward, but students will quickly
identify with the shampoo commercial where “you tell two friends and they tell two
friends ... while they see on TV one person, then two, then four, and so on. You could
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also do a class demo on this, distributing note cards, rubber bands, or other small,
inexpensive objects.
2. Make models of various cell–cell interactions using plastic, paper, string, straws, etc.
Better yet, have your students make them, collect them, choose the best, and you will
have models to show in class next semester!
IN-CLASS CONCEPTUAL DEMONSTRATIONS
A. Cell Signaling Role Playing
Introduction
Molecular role playing is a fun and effective way to demonstrate complex concepts to
students. This activity asks students to demonstrate the differences between cell communication
systems. Use this activity to help students summarize and review the events of cell signaling.
Materials
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Colored markers
Sheets of 8 ½” X 11” white paper
Students volunteers
2 15’ sections of rope of twine
Scissors
10 feet of thick tubing
Ample space to move around
Procedure and Inquiry
1. Tell students that they are to plan a play in which they will be role modeling the
mechanisms of cell to cell communication.
2. Ask them to use the rope to represent the cell membrane of an individual cell, the tubing
to depict a blood vessel, and each student will represent a cell structure involved in cell
signaling. They must use the marker and paper to label the role of each individual who is
demonstrating a cell signaling structure.
3. First, instruct the students to role play the four basic mechanisms of cell communication:
direct contact, paracrine signaling, endocrine signaling, or synaptic signaling.
4. Have the students discuss the accuracy of their role model.
5. Next, ask the students to role play intracellular receptor cell signaling.
6. Have the students discuss the accuracy of their role model.
7. Then, ask the students to role play cell surface receptor cell signaling.
8. Have the students discuss the accuracy of their role model.
9. Recap the activity by asking how such demonstrations may be useful for researchers
investigating cell signaling mechanisms.
B. Building Cell Junction Models
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Introduction
Molecular modeling using household materials was one of the techniques used by
Watson and Crick as they were deciphering the structure of DNA. This activity asks students to
models of cell junctions using common household materials provided to them. Use this activity
to help students reinforce and review the structure and function of cell junctions.
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Materials
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Colored markers
Sheets of 8 ½” X 11” white paper
Students volunteers divided into teams or work groups
Color pipe cleaners
Modeling or craft clay
Scissors
Fish tank tubing or laboratory hosing
Ample table space to do the construction
Double sided cellophane or carpet tape
Clear cellophane tape
Procedure and Inquiry
1. Tell students they will work in teams to use the materials given to them to represent the
different types of cell junctions taught in class.
2. They cannot use the book, Internet, or other resources to design the junctions.
3. Have students divided into teams that will construct one type of cell junction. Large
classes can have multiple groups produce junctions so the class can compare the models.
4. Students should be asked to model the following junctions:
a. Desmosome
b. Hemidesmosome
c. Tight junction
d. Gap junction
e. Plasmodesmata
f. Adherens junctions
5. Have the students discuss the accuracy of their models.
6. Recap the activity by asking how models may be useful for researchers investigating cell
structure.
USEFUL INTERNET RESOURCES
1. Case studies are excellent for reinforcing scientific concepts. The University of Buffalo
produced a study called “Taking It on the Chin: A Case Study on the Nervous System”
addresses cell communication related to illness. It has students investigate signaling
programs related to nervous system function. This case study can be done in class or be
given as a take-home. The case study can be found at
http://www.sciencecases.org/chin/chin_notes.asp.
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2. Case studies introduce real-world applications of biological principles. This case study
developed for the University of Buffalo called “Gender: In the Genes or in the Jeans?” to
give students an understanding of cell signaling and other factors related to gender
development. This is an excellent case study for uncovering the myths associated with
the biology of gender. The case study is available at
http://www.sciencecases.org/gender/gender.asp.
3. Undergraduate students are rarely exposed to the specilized research involved in building
the conceptual foundations of biology textbooks. Students can be show the different types
of cell signaling research being done by visiting the “Signaling Gateway” website hosted
by the Association of Learned and Professional Society Publishers. The website can be
found at http://www.signaling-gateway.org/.
4. Animations used during lecture are great for reinforcing abstract concepts such as cell
signaling. Supplement a cell signaling lecture using Flash animations available on the
Gallery of Cell Signaling website. The website can be found at
http://www.celanphy.science.ru.nl/Bruce%20web/Flash%20Movies.htm.
LABORATORY IDEAS
A. Response to Cell Signals
a. Have students investigate the signaling response of an animal model to adrenalin.
b. This laboratory activity looks at the effect of adrenalin as a signal that affects the
metabolic rate of particular cells.
c. Provide students with the following materials:
i. Daphnia kept in a container of clean fresh water
ii. A large container of clean water for used daphnia
iii. Droppers for collecting and transporting the daphnia
iv. Microscope
v. Microscope slides per observation
vi. Droppers for transporting chemicals
vii. Over-the-counter epinephrine preparation (asthma pills) soaked in 100ml
of a 50% ethyl alchohol/50% water mixture
viii. Surgical gloves and goggles
d. Have the students carry out the following procedure:
i. Ask the students to place some daphnia on the microscope slide and
observe their normal behavior
ii. Make sure they keep adding water to the slide to keep the daphnia from
drying out and dying
iii. Have them keep track of the speed at which they are moving around how
fast they move their feet.
iv. Then have the students add one drop of the epinephrine solution to the
daphnia
v. The students should note what happens to movement of the daphnia
vi. Have the students repeat steps iv and v several times
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vii. Students should then be asked to investigate how adrenalin works as a cell
signal and whether their observations were consistent with findings on
other organisms
viii. Ask students to investigate the types of cells and tissues in the daphnia
that are response to the adrenalin
B. Comparative Response to Cell Signals
a. Have students investigate the specificity of cell signaling responses of an animal
and a plant.
b. This laboratory activity looks at the differential effects of caffeine and adrenalin
as a signal that may or may not affect the metabolic rate of cells.
c. Provide students with the following materials:
i. Daphnia kept in a container of clean fresh water
ii. Elodea kept in a container of clean fresh water
iii. Droppers for collecting and transporting the daphnia
iv. Forceps for transporting the elodea
v. Microscope
vi. Droppers for collecting samples of epinephrine and caffeine
vii. 2 clean microscope slides per observation
viii. Over-the-counter epinephrine preparation (asthma pills) soaked in 100ml
of a 50% ethyl alchohol/50% water mixture
ix. 100mg caffeine pill soaked in 100ml of a 50% ethyl alchohol/50% water
mixture
x. Surgical gloves and goggles
d. Have the students carry out the following procedure:
i. Ask the students to place some daphnia and elodea on the same
microscope slide
ii. Observe their normal behavior and the cytoplasmic streaming of the
elodea
iii. Make sure the students keep adding water to the slide to keep the daphnia
and elodea from drying out and dying
iv. Have them keep track of the speed at which the daphnia and the
cytoplasmic are moving.
v. Then have the students add two drops of the epinephrine solution to the
slide
vi. The students should note what happens to the daphnia and the elodea
vii. Next, ask the students to place some daphnia and elodea together on a new
microscope slide
viii. Then have the students add two drops of the caffeine solution to the slide
ix. The students should note what happens to the daphnia and the elodea
x. Students should then be asked to investigate the effects of adrenalin and
caffeine on the cells. They should use the Internet or other resources to see
how each works as a cell signal.
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LEARNING THROUGH SERVICE
Service learning is a strategy of teaching, learning and reflective assessment that merges the
academic curriculum with meaningful community service. As a teaching methodology, it falls
under the category of experiential education. It is a way students can carry out volunteer projects
in the community for public agencies, nonprofit agencies, civic groups, charitable organizations,
and governmental organizations. It encourages critical thinking and reinforces many of the
concepts learned in a course.
Students who have successfully mastered the content of Chapter 9 can apply their knowledge for
service learning activities in the following ways:
1. Have students talk to youth sports groups about how steroids affect cell function.
2. Have students design an electronic animated presentation on cell signaling for teachers at
local schools.
3. Have students tutor middle school or high school biology students studying cell function.
4. Have students talk to the elderly about “youth drugs” that rely on cell signaling such as
growth hormone or estrogens.
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