CHAPTER 5: MEMBRANES
WHERE DOES IT ALL FIT IN?
Chapter 5 takes a more detailed look at the cell by investigating the fine structure and functions of the
cell membrane. The discussion of information on the cell membrane provided in Chapter 4 should be
briefly reviewed before going into a lecture on Chapter 5. It is also important to stress that the
information discussed in this chapter is needed later to understand organismic adaptations and
evolution.
SYNOPSIS
Phospholipids are the foundation of all known biological membranes. The characteristic lipid bilayer
forms as a result of the interactions among nonpolar phospholipid tails, polar phospholipid heads,
and the surrounding water. The nonpolar tails face inward toward each other while the polar heads
face outward toward the water. The arrangement of the lipid bilayer is stable, yet fluid. The
membranes of living organisms are assembled from four components. The phospholipid bilayer
provides an impermeable flexible matrix in which the other components are arranged.
Transmembrane proteins that float within the bilayer are channels through which various molecules
pass. A supporting protein network, anchored to the actin filament cytoskeleton, prevents these
channels from moving. The glycocalyx consisting of sugars and membrane proteins provide a cell’s
identity.
All of the cell’s activities are in one way or another tied to the membrane that separates its
interior from the environment. Net diffusion occurs when the materials on one side of the
membrane have a different concentration than the materials on the other side. Facilitated
transport of materials is necessary to control the entrance and exit of particular molecules.
Facilitated diffusion is a simple process that utilizes protein carriers that are specific to certain
molecules. It is a passive process driven by the concentration of molecules on the inside and the
outside of the membrane. Osmosis is a specialized form of diffusion associated specifically with
the movement of water molecules. Many cells are isosmotic to the environment to avoid
excessive inward or outward movement of water. Other cells must constantly export water from
their interior to accommodate the natural inward movement. Most plant cells, on the other hand,
are hyperosmotic with respect to their immediate environment. The resulting turgor pressure
within the cell pushes the cytoplasm against the cell wall and makes a plant cell rigid.
Large molecules enter the cell by endocytosis, a nonselective process. Endocytosis of particulate
material is called phagocytosis while endocytosis of liquid material is called pinocytosis. Exocytosis
is the reverse mechanism and is used by plants to construct the cell wall and by animals to secrete
various internally produced chemicals. Receptor-mediated endocytosis is a complicated mechanism
that involves the transport of materials via coated vesicles. Some molecules are transported into or
out of the cell independent of concentration. This process requires the expenditure of energy in the
form of ATP and is called active transport. Such transport channels are coupled to a sodiumpotassium pump. The proton pump produces ATP through two special transmembrane protein
channels through a process called chemiosmosis.
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LEARNING OUTCOMES
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Understand the biochemistry of phospholipids and how they are organized into membranes.
Know the function of each of the four components of a cell membrane.
Differentiate among diffusion, facilitated transport, facilitated diffusion, osmosis, and active
transport.
Describe the six classes of membrane proteins and how each interacts with the membrane.
Describe solution and solute movement into and out of a cell under hyperosmotic, hypoosmotic,
or isosmotic conditions.
Explain and give examples of endocytosis, phagocytosis, pinocytosis, receptor-mediated
endocytosis, and exocytosis.
Understand the importance of selective permeability in biological systems.
Describe the operation of the sodium potassium pump and the proton pump.
Understand the importance of coupled channels, cotransport, and countertransport.
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 6 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
Diffusion only occurs through pores in the membrane
Any molecule can diffuse across a membrane
Diffusion and osmosis do not occur together
Osmosis moves any substance
Osmosis works by the opposite principles of diffusion.
Particles are not moving back and back during isosmotic conditions
Diffusion is not temperature dependent
Active transport only moves against a diffusion gradient
Gases do not diffuse
Ions easily pass through a membrane
All transport proteins require ATP
Carriers are highly specific to one molecule
Pinocytosis takes in pure liquids and not solutes
Turgor is due to osmosis and not due to a diffusion gradient
INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE
One could develop a lengthy comparison of a cell and its organelles to a community with respect
to energy, transportation, communication, growth, and so forth.
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Stress the fluidity of the plasma membrane and the regular replacement of its components to
ensure constant integrity. It is as though one could remove a chipped brick from a house and
replace it with a new one. A completely fluid house would have doors, windows, and walls that
moved with respect to the current needs of the occupants and the appearance of the environment.
Membrane fluidity is related to the presence of saturated and unsaturated fatty acids in the
phospholipid tails. Saturated fats are like books stacked tightly on a shelf. They can’t readily be
moved from shelf to shelf and are very rigidly organized. Warped books (unsaturated fats), on
the other hand, can’t be as closely packed. It is easier to get one’s fingers between the books and
move them around.
Do not be confused that a 3-D surface view of a membrane is viewed in a transmission electron
microscope rather than a scanning one. The resolution of the TEM is far better (down to 2m), but
only a very thin section can be viewed, unlike the SEM which can be used to examine very thick
specimens and even whole objects like protozoa, insects, leaves, and flowers. To examine cell
membranes and still satisfy the thin layer requirement, it is the cast of the surface that is viewed
under the TEM, not the cell surface itself — as would occur if one examined the coated surface
of a cryofractured cell with the SEM.
Be careful to present hypoosmotic/hyperosmotic as being relative to one another. A solution
cannot simply be hypoosmotic unless it is compared to another solution. A cell may be
hypoosmotic to its solution, but the solution is also hyperosmotic to the cell.
The types of movement of molecules through a membrane are more readily remembered when
the students understand what the names mean. Simple diffusion is just that, nothing else assists
it. Facilitated diffusion requires the presence of channels that aid in the passage of molecules.
Active transport is like any active versus passive mechanism, it requires the expenditure of
energy.
It is important that students begin to understand how ATP is generated as it will come up again
(most notably in the next set of chapters regarding metabolism). Similarly, cell surface receptors
and cell surface markers will be discussed in greater depth in a later chapter entitled The Immune
System.
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 5.
Application
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Have students predict the direction of diffusion and osmosis given a cell
placed in a concentrated urea solution.
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Have students predict the direction of diffusion and osmosis given a cell
placed in a dilute sodium chloride solution.
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Analysis
Synthesis
Evaluation
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Ask students explain why patients with severe dehydration are given
hypoosmotic drinks.
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Ask students to explain how cell transport is affected if a cell is immersed
on a solution that degrades proteins.
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Ask students to explain the effects of a toxin that binds up sodium so that
it does not pass through the sodium-potassium pump.
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Have students determine nature of a toxin that blocks the facilitated
diffusion of glucose.
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Ask students hypothesize the role of an enzyme in the cytoplasm that
converts glucose to glycogen upon glucose entering the cell.
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Have students to find a use for a large artificial cell membrane capable of
pumping sodium and chloride ions in one direction..
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Ask students to explain why human brain cells must rely on the transport
capabilities of surrounding cells for obtaining nutrients.
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Ask students to evaluate the transport properties that a kidney dialysis
machine must have to keep a person’s body cells alive.
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Ask students to evaluate a claim that drinking too much water can cause
swelling of brain cells.
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Have students discuss the benefits and risks of drugs that affect the
function of the sodium-potassium pump.
VISUAL RESOURCES
1. Diffusion can be demonstrated by placing mothballs or perfume in one corner of the
lecture hall before lecture begins. As time progresses, have students raise their hands (or
colored flags) when they smell the odor.
2. Gortex® is a material that is unidirectionally permeable to water. It allows moisture to
pass from the inside (of a jacket or tent) outward, but does not allow rain to penetrate
inward.
IN-CLASS CONCEPTUAL DEMONSTRATIONS
A. Wine from Raisins
Introduction
Students enjoy and gain educational value from simple classroom demonstrations that
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show concrete examples of complex topics. Osmosis and diffusion can be boring topics to
students without some break in the lecture to observe a simple example. This demonstration uses
raisins as a model for understanding osmosis and barriers to osmosis.
Materials
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Raisins
Acetone
Small beaker
Forceps
40oC distilled water
2 Petri plates
Overhead projector
Procedure & Inquiry
1. At the beginning of the lecture, tell the class you will be demonstrating a lesson on
osmosis.
2. The immediately, place the two Petri plates on an overhead projector and fill to capacity
with the 40oC distilled water. Do not tell the students that the solution is distilled water.
3. Rinse one raisin in acetone telling the class that you are washing the raisin in a solvent.
4. Place that raisin in the one Petri plate.
5. Then place the other raisin in the other Petri plate without doing an acetone rinse.
6. Have the students observe the results. They will notice the acetone-washed raisin swell
during the class period as the other does not.
7. Have the students explain the nature of the solution in the Petri dish compared to the
raisin.
8. Ask them to explain the role of the solvent in permitting osmosis to occur freely.
9. Have the students hypothesis the unwashed raisin was similar to the protective value of
human skin.
B. Effect of Temperature on Diffusion
Introduction
It is important to stress the effects of temperature on diffusion rate. Many organisms
maintain a particular body temperature that is favorable to a particular diffusion rate. It also
reinforces the principle that diffusion is driven by molecular vibrations that is measured
indirectly as temperature. This demonstration is a simple depiction of the influence of
temperature on diffusion rate.
Materials
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Overhead projector
4 labeled Petri plates
o One labeled 90
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o One labeled 40
o One labeled RT
o One labeled Ice
Blue dye in a dropper bottle
100 ml of 90oC in a small beaker
100 ml of water in a small beaker chilled in an ice bath
100 ml of 370C - 400C water in a small beaker
100 ml of room temperature water in a small beaker
Procedure & Inquiry
1. Explain that you will be demonstrating the relationship between temperature and
diffusion.
2. Place the four Petri plates on the overhead projector.
3. Add the appropriate water samples in the labeled Petri plates.
4. Tell the students to watch carefully as you slowly add one drop of blue dye to each Petri
plate.
5. Ask the students to discus the importance of the temperature-diffusion relationship to an
organism’s survival.
USEFUL INTERNET RESOURCES
1. Case studies are excellent for reinforcing scientific concepts. The University of Buffalo
produced a study using puffer fish or tetradotoxin poisoning as a means of teaching the
function of ion channels. 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/badfish/badfish_genbio1.asp.
2. Animations used during lecture are great for reinforcing abstract concepts such as cell
transport. Supplement a lesson on active transport with the on-line animations available
on Cell Biology Animations by John Kyrk. The website is
http://www.johnkyrk.com/cellmembrane.html.
3. Researchers used detailed graphic representations of cells and molecules as a wayl of
better understanding cell transport. Virtual Cell is an educational website the provides
accurate and detailed depictions of membrane proteins involved in cell transport. The
website can be found at http://vcell.ndsu.nodak.edu/animations/.
4. Medical cases studies are good tools for stimulating student interest in a biological
concept. A case study provided by the University of Buffalo uses cystic fibrosis for a
model of diseases caused by disruption to normal cell transport mechanisms. The website
is http://www.sciencecases.org/cf/cf.asp.
LABORATORY IDEAS
A. Osmolarity of Plant Cells
a. Have students investigate the osmolarity of living tissues using plants cells as a
model.
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b. Tell the class that they will use circular sections of apples to test the isosmolar
point of plant tissues. Students should be able to calculate the approximate
concentration of solutes in the cytoplasm of apple cells in this investigation.
c. Provide students with the following materials:
i. Fresh apple
ii. 4mm diameter cork borer
iii. Small ruler
iv. Forceps
v. 7 small test tubes
vi. Test tube rack
vii. Marker
viii. A gradient of salt solutions in containers with large droppers:
1. 10% sodium chloride
2. 5 % sodium chloride
3. 2 % sodium chloride
4. 5 % sodium chloride
5. 1 % sodium chloride
6. 0.2% sodium chloride
7. Distilled water
d. Have students cut 4mm circles of potato wedges measuring each one before the
experiment begins.
e. The students should then set up the test tubes so that they are labeled:
i. 10% sodium chloride
ii. 5 % sodium chloride
iii. 2 % sodium chloride
iv. 5 % sodium chloride
v. 1 % sodium chloride
vi. 0.2% sodium chloride
vii. Distilled water
f. Students should then add 5 ml of the appropriate solution into the test tubes.
g. Then have them place one apple circle in each of the tubes and let sit for 15
minutes.
h. Next, have the students measure the size of each apple circle.
i. They should be asked to conclude the approximate osmolarity of the apple cells
based on their findings. They should be encouraged to examine the data of other
students to see if there was consistency in the findings.
B. Plant model for Diabetes-Induced Dehydration
a. Have students use plant cells to understand the dehydrating effects of high blood
sugar as found in diabetes.
b. Diabetes is indicated by higher than average blood glucose levels for a period
after a sugary meal is taken in the body. A typical interpretation of blood glucose
levels is given below:
mmol/l mg/dl
2
35
3
55
4
70
Interpretation
extremely low, danger of unconsciousness
low
slightly low
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6
8
10
15
20
22
33
110
150
180
270
360
400
600
normal before eating
normal after eating
maximum after eating
slightly high
very high
dangerously high
severely high, electrolyte imbalance
c. Tell students that you will be using plants as a model for understanding the effects
of high blood sugar on human cells.
d. Provide students with the following materials:
i. Healthy elodea
ii. Microscope
iii. 4 microscope slides and cover slips
iv. Glucose solutions (each represents a blood glucose condition):
1. 0.8 grams in 400cc of distilled water (normal level)
2. 1.0 grams in 400cc of distilled water (slightly high level)
3. 2.0 grams in 400cc of distilled water (dangerously high level)
4. 4.0 grams in 400cc of distilled water (severely high level)
v. Instruct students to prepare four slides labeling them with a glucose level
amount from the samples provided above.
vi. They should then place several drops of the appropriate solutions on a
slide.
vii. Have the students place one elodea leaf on each slide to observe under the
microscope.
viii. They should record their results and determine effects high sugar
concentrations in blood would have on cells and tissues.
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 5 can apply their knowledge for
service learning activities in the following ways:
1. Have students talk to youth sports groups about the benefits of proper hydration.
2. Have students design an electronic presentation on cell transport for use by teachers at
local schools.
3. Have students tutor middle school or high school biology students studying cell transport.
4. Have students judge science fair projects related to cell transport.
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This project is funded by a grant awarded under the President’s Community Based Job Training Grant as implemented by the U.S.
Department of Labor’s Employment and Training Administration (CB-15-162-06-60). NCC is an equal opportunity employer and
does not discriminate on the following basis:
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against any individual in the United States, on the basis of race, color, religion, sex, national origin, age disability, political
affiliation or belief; and
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against any beneficiary of programs financially assisted under Title I of the Workforce Investment Act of 1998 (WIA), on
the basis of the beneficiary’s citizenship/status as a lawfully admitted immigrant authorized to work in the United States, or
his or her participation in any WIA Title I-financially assisted program or activity.
This workforce solution was funded by a grant awarded under the President’s CommunityBased Job Training Grants as implemented by the U.S. Department of Labor’s Employment
and Training Administration. The solution was created by the grantee and does not
necessarily reflect the official position of the U.S. Department of Labor. The Department of
Labor makes no guarantees, warranties, or assurances of any kind, express or implied, with
respect to such information, including any information on linked sites and including, but not
limited to, accuracy of the information or its completeness, timeliness, usefulness, adequacy,
continued availability, or ownership. This solution is copyrighted by the institution that
created it. Internal use by an organization and/or personal use by an individual for noncommercial purposes is permissible. All other uses require the prior authorization of the
copyright owner.
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