Chapter 9. Cellular Transport Mechanisms: Understanding Diffusion and Osmosis.
Student Learning Outcomes
At the completion of this exercise, the student will be able to:
1)
2)
3)
4)
5)
6)
7)
8)
Compare and contrast passive and active transport mechanisms.
Describe several means of passive and active transport.
Provide specific examples of passive and active transport.
Compare and contrast endocytosis and exocytosis.
Define equilibrium
Define solution, solvent, and solute.
Compare and contrast hypotonic, hypertonic, and isotonic solutions.
Describe cytolysis and plasmolysis.
Overview:
One of the major functions of the plasma membrane is to regulate the movement of
substances into and out of the cell. This process is essential in maintaining the homeostatic state
of the cell. If you recall, the plasma membrane is composed primarily of a phospholipid bilayer
and specialized proteins. The unique structure of the plasma membrane allows it to be selectively
permeable to certain substances. The permeability of the membrane is regulated by several
variables, including size of the molecule, polarity of the molecule, and external conditions such
as concentration, temperature, and pressure.
For molecules to enter or exit a cell, they must overcome the concentration gradient,
which involves the movement of molecules from regions of greater concentration to regions of
lesser concentration. Living systems have two primary mechanisms for moving substances in
and out of the cell – passive and active transport. In passive transport the cell uses no energy
(ATP) as essential substances are moved across the plasma membrane. Examples of molecules
moved by the various means of passive transport are oxygen, water, and glucose.
The most fundamental means of passive transport is diffusion, the random movement of
molecules from regions of greater concentration to regions of lesser concentration. This random
movement also is known as Brownian motion. A state of equilibrium is attained when an equal
distribution of molecules exists throughout the system. The movement of water across the
plasma membrane in living systems is called osmosis.
Other mechanisms of passive transport include facilitated diffusion and filtration.
1) In facilitated diffusion, carrier proteins along the cell membrane are required to ferry
specific molecules such as glucose across the membrane into the cell.
2) Filtration involves hydrostatic pressure (water pressure) forcing molecules through a cell
membrane. Filtration is an essential mechanism that takes place in the kidneys in the
formation of urine.
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In active transport, energy in the form of ATP is required for the movement of substances
across the concentration gradient. Biological pumps essential in this process are the following:
1) The proton or hydrogen pump is necessary to maintain the normal pH of the stomach.
2) The calcium pump is important in nerve and muscle function, and
3) The sodium – potassium pump is integral in cellular metabolism.
Macromolecules such as polypeptides and polysaccharides are too large to traverse the cell
membrane by either passive of pump systems. Instead, they must be transported into the cell by
endocytosis and out of the cell by exocytosis. Both endocytosis and exocytosis employ the
formation of vesicles in transporting substances.
In endocytosis, solid substances can be engulfed by a cell through phagocytosis.
Everyday examples in biological systems include the ingestion of food particles by an amoeba
and the engulfing of foreign particles by a macrophage. Another example of endocytosis is
pinocytosis, or cell drinking, in which liquid droplets that may contain salts and other molecules
are taken into the cell through vesicle formation. Specialized cells in the roots of plants use
pinocytosis to ingest a variety of nutrients.
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TAKE A WHIFF
The instructor will spray perfume or room deodorizer in a front corner of the classroom. The molecules
will diffuse throughout the room in an attempt to attain a state of equilibrium. Timing devices are used to
determine the rate of diffusion of the molecules across the room.
PROCEDURE 9.1
SMELL
1) Students should be equally dispersed throughout the classroom.
2) The instructor sprays a small amount of a scent in the front corner of the room and this will serve
as time zero.
3) Students will record the time and raise their hands when they detect the odor.
4) Students discuss their results.
Q1. How long did it take for the scent to reach the farthest point?
_____________________________________________________________________________________
Q2. What variables can affect the rate of diffusion of the scent?
_____________________________________________________________________________________
WHAT IS IN THE BAG?
In this activity, dialysis tubing will serve as the selectively permeable membrane. After making a bag with
the dialysis tubing and filling it with colorless cornstarch solution, the bag will be immersed in a beaker
containing an iodine solution that is caramel in color. Movement of the iodine molecules across the
membrane can be detected by a change in the cornstarch solution to a purplish – brown color.
Materials
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250 mL beaker
25 mL graduated cylinder
Cornstarch solution
Iodine solution
Water
Glass rod
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Dialysis tubing
String
Timing device
Ruler
Paper towels
PROCEDURE 9.2
MEMBRANE
1)
2)
3)
4)
5)
6)
7)
Obtain glassware and needed accessory materials.
Measure and cut a 15 cm length of dialysis tubing.
Place the tubing in water until it becomes soft and pliable.
Using string, form a bag with the dialysis tubing closing one end.
Fill the dialysis tubing bag halfway with the cornstarch solution.
Using string, tie off the top of the bag.
Immerse the bad containing the cornstarch solution into a beaker containing 200 mL of iodine
solution and record the time and the color of the solution.
3
Figure 1 Method for filling dialysis tubing.
1. What color is the iodine solution?
__________________________________________________________________________
2. What is the color of the cornstarch solution?
__________________________________________________________________________
8) Leave the experiment undisturbed for 15 minutes.
9) After the elapsed time, remove the bag from the solution and place it on a paper towel.
10) Observe the color changes in the dialysis bag.
3. What color changes did you observe in the bad and in the solution.
__________________________________________________________________________
4. Explain the reason for such color changes in both the bag and solution.
In the space provided below, graphically illustrate the results of the activity.
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OSMOSIS ACTIVITIES
Recall that osmosis is the diffusion of water across a selectively permeable membrane, and
diffusion always occurs from regions of greater concentration to regions of lesser concentrations.
A typical solution consists of two components – the solvent as the dissolving medium and the
solute as the substance dissolved in the solvent. In a saltwater solution, the water serves as a
solvent and the salt as the solute.
Tonicity refers to the concentration of solute in the solvent.
1) In a hypotonic solution, there is a lower concentration of solute relative to the inside of
the cell. If a cell such as a red blood cell or a potato cell is placed in a hypotonic solution,
water will rush into the cell in attempt to reach a state of equilibrium. An ideal hypotonic
solution is distilled water, because it devoid of solutes. As the cell begins to fill with
solvent, the cell will swell, and perhaps burst. The bursting of cells in a hypotonic
solution is called cytolysis.
2) In a hypertonic solution, there is a higher concentration of solute relative to the inside of
the cell. If a cell such as a red blood cell or a potato cell is placed in a hypertonic
solution, water will be drawn out of the cell into the outside solution in an attempt to
teach a state of equilibrium. This is called crenation in red blood cells and plasmolysis
in plant cells.
In plants, the swelling of cells placed in a hypotonic solution results in turgor pressure. The
framework cell wall protects the cell from bursting. Turgor pressure keeps the plant erect. If the
turgor pressure is lost in a plant, the plant will wilt. Just think of the plants in your yard on a hot
summer’s day.
Many cells exist in an isotonic solution, which has the same concentration of solute outside and
inside of the cell. Normally, red blood cells exist in an isotonic state in plasma. Many organisms,
such as some species of sharks, can undergo osmoregulation, in which they can alter their
internal solutions to equal that of the environment to maintain homeostasis. This explains why
some species of sharks can be found living in freshwater rivers near coasts.
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PLANT TISSUE: HYPOTONIC VS. HYPERTONIC
Testing Tissues in Hypotonic and Hypertonic Solutions
Plant tissues
In this activity, strips of potato tissue will be placed in hypotonic and hypertonic solutions, and
observations will be made and recorded.
Materials
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Potato strips
3 test tubes
Test tube rack
Sharpie
Water
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Distilled water
10% sodium chloride
solution
Paper towels
Forceps
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Scalpel
Microscope slides
and cover slips
Microscope
PROCEDURE 9.3
PLANT TISSUE
1)
2)
3)
4)
5)
6)
Obtain the needed equipment and bring it to your lab station.
With a marker, label the tubes “Control”, “Hypotonic Solution”, and “Hypertonic Solution”.
Cut three strips of potato, each about 4 cm in length and 1 cm wide.
Place each strip in a labeled test tube.
Add tap water to the test tube labeled “Hypotonic Solution”, totally immersing the potato strips.
Add 10% sodium chloride solution to the test tube labeled “Hypertonic Solution”, totally
immersing the potato strip.
7) Add distilled water to the test tube labeled “Hypotonic Solution”, totally immersing the potato
strip.
8) Let the solutions sit quietly for 30 minutes before making your observations.
9) Remove the potato strips and place them on a paper towel. Record your observations.
10) Using the scalpel carefully cut a thin piece of tissue from each strip.
11) Prepare a wet mount of each strip.
12) Using the microscope, observe each slide at 400X and record your results.
13) Clean your lab station
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Sketch and Label your microscopic observations:
1) Potato strip in control (tap water) solution.
2) Potato strip in Hypotonic (ddH2O) solution
3) Potato strip in Hypertonic (10% NaCl) solution
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1. Which strip was most limp? Why?
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2. Which strip was most stiff? Why?
___________________________________________________________________________
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CHICKEN EGG: HYPOTONIC vs. HYPERTONIC
This activity is designed for you to do in your home or dormitory. Before engaging in this
activity, review the definitions of hypertonic and hypotonic solutions. Acquire the materials and
follow the procedure below.
Materials
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Raw egg
White vinegar
Green food coloring
Light corn syrup
2 – quart Mason jars or similar containers
PROCEDURE 9.5
VINEGAR
1)
2)
3)
4)
5)
6)
7)
Obtain the needed materials.
Fill a Mason jar or container approximately halfway with white vinegar.
Add 3 – 5 drops of green food coloring to the vinegar.
Carefully place the entire egg into the vinegar solution.
Set the experiment aside at least 8 hours
Observe the egg
Using a digital camera, video camera, or phone camera, or sketch (draw) to record the
egg
1. Describe the egg
______________________________________________________________________________
______________________________________________________________________________
2. Using your knowledge of solutions, describe what has happened to the egg.
______________________________________________________________________________
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PROCEDURE 9.6
CORN SYRUP
1)
2)
3)
4)
5)
6)
Obtain the needed materials
Fill a Mason jar or container approximately halfway with light corn syrup.
Carefully place the entire egg into the corn syrup.
Set the experiment aside for at least 8 hours
Observe the egg
Using a digital camera, video camera, or phone camera, record the egg.
1. Describe the egg
______________________________________________________________________________
______________________________________________________________________________
2. Using your knowledge of solutions, describe what has happened to the egg.
______________________________________________________________________________
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Why do Sea Turtles Cry???
Have you ever watched a nature show and seen sea turtles crying as they lay their eggs on some
lonesome beach? Why do turtles cry? Is it because they are anticipating the future of their young,
or perhaps they are sentimental, or maybe it hurts? No, it is not any of these reasons. Marine
turtles have glands in the regions of their eyes that help them remove excess salt consumed from
the hypertonic solution in which they live. That is why these turtles appear to cry!
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Selective Permeability of Membranes
Osmosis occurs when different concentrations of water are separated by a selectively
permeable membrane. One example of a selectively permeable membrane within a living cell is the
plasma membrane. Dialysis tubing is a selectively permeable material that provides a means to
demonstrate the movement of substance (ions or large molecules?) through cellular membranes. The
dialysis tubing is a selectively permeable cellulose sheet that permits the passage of water and ion
molecules but obstructs the passage of larger molecules such as starch and proteins. If you examined this
membrane (dialysis tubing) with a scanning electron microscope, you would see that is POROUS; it thus
prevents molecules larger than the pores from passing (diffusing) through the membrane.
Materials
Per Group:
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One 15 cm length of dialysis tubing,
soaking in ddH2O
Two 10 cm pieces of string
Dishpan half-filled with ddH2O
400 mL beaker
Ring stand and funnel apparatus
8 test tube
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Test tube rack
Sharpie
25 mL graduated cylinder
Lugol’s (I2KI) solution
2% barium chloride (BaCl2) solution
2% silver nitrate (AgNO3) solution
Biuret reagent
Scissors
Per Lab:
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Bottle of 1% soluble starch in 1% sodium sulfate
Bottle of 1% albumin in 1% sodium chloride
4 test tubes demonstrating positive test for starch, sulfate ion (SO4-2), chloride ion (Cl-), and
protein (Egg albumin).
Procedure
1) Obtain a 15 cm section of dialysis tubing that has been soaking in ddH2O.
2) Tie one end of the bag with a string or dental floss to form a leak proof bag.
3) Slip the open end of the bag over the stem of a funnel and fill the bag approximately half full with
15 ml of a solution of 1% soluble starch in 1% sodium sulfate (Na2SO4).
4) Remove the bag from the funnel and tie the open end of the bag. (Make sure there is no air in the
bag)
5) Rinse the tied bag in a dishpan filled with ddH2O.
6) Pour 200 mL of a solution of 1% albumin in 1% sodium chloride (NaCl) into a 400 mL beaker.
7) Place the bag into the beaker.
8) Record the time. ___________________
9) With a sharpie, label eight test tubes, numbers 1-8
10) 75 minutes after the start of the experiment, pipet 3 mL of the solution that is in the beaker to test
tube 1-4.
11) Perform the following tests. Your instructor will provide the positive controls. Record your
result on table 9.1.
a.
b.
c.
d.
Test for starch. Add 2-3 drops of Lugol’s solution (I2KI) to test tube 1.
Test for sulfate ions. Add 10 drops of 2% barium chloride solution (BaCl2) to test tube 2.
Test for chloride ions. Add 10 drops of 2% silver nitrate solution (AgNO3) to test tube 3.
Test for protein. Add 10 drops of Biuret reagent to test tube 4.
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12) Remove the bag from the beaker and rinse the bag in the dishpan with ddH2O.
13) Using scissors cut open and empty the solution in a graduate cylinder.
14) Pipet 3 mL of the solution that is in the gradate cylinder to test tube 5-8.
15) Perform the following tests. Your instructor will provide the positive controls. Record
your result on table 9.2.
a.
b.
c.
d.
Test for starch. Add 2-3 drops of Lugol’s solution (I 2KI) to test tube 5.
Test for sulfate ions. Add 10 drops of 2% barium chloride solution (BaCl2) to test tube 6.
Test for chloride ions. Add 10 drops of 2% silver nitrate solution (AgNO3) to test tube 7.
Test for protein. Add 10 drops of Biuret reagent to test tube 8.
16) Discard content of test tubes, graduated cylinder, and beaker down the sink drain. Wash
glassware and return them to your station.
Table 9.1 Result of tests for substances in the beaker
Substances
At start of experiment
After 75 min.
Starch
Sulfate ions
Chloride ions
+
Protein (egg albumin)
+
Content of beaker: (+) = Presence. (-) = Absence
Table 9.2 Result of tests for substances in the Dialysis bag
Substances
At start of experiment
After 75 min.
Starch
+
Sulfate ions
+
Chloride ions
Protein (egg albumin)
Content of beaker: (+) = Presence. (-) = Absence
1. To which substances was the dialysis tubing permeable?
_________________________________________________________________________
_________________________________________________________________________
2. What physical property of the dialysis tubing might explain its differential
permeability?
_________________________________________________________________________
3. To which substances (molecules) was the dialysis tubing impermeable?
________________________________________________________________________
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Plasmolysis in Plant Cells
Plant cells are surrounded by a rigid cell wall composed primarily of the glucose polymer,
cellulose. Many plant cells have a large central vacuole surrounded by the vacuolar membrane.
The vacuolar membrane is selectively permeable. Normally, the solute concentration within the
cell’s central vacuole is greater than that of the external environment. Consequently, water
moves into cell, creating turgor pressure, which presses the cytoplasm against the cell wall.
Such cells are said to be turgid. Many non-woody plants (like beans and peas) rely on turgor
pressure to maintain their rigidity and erect stance.
In this experiment, you will discover the effect of external solute concentration on the
structure of plant cells.
Materials
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Compound Microscope
Forceps
Microscope slides and coverslips
Elodea leaf
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Bottle of ddH2O
Bottle of 20% Sodium Chloride (NaCl)
Paper Towel
Procedures:
1) With a forceps, obtain two healthy elodea leaves.
2) Place one piece of leaf on a microscope slide and add one drop of ddH2O and place a
coverslip on top of the leaf. Place a paper towel on top of the slide and tap around the
coverslip to extract excess water (See “wicking” technique below).
The technique of "wicking" is used to draw a solution across the specimen on a slide. By placing
a piece of tissue or paper towel at one edge (right) and dropping the solution at the edge of the
other side (left), the solution is drawn or "wicked" across the specimen.
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3) View the slide of the elodea leaf in ddH2O through a compound microscope. Focus
the slide with the low-power objective. Once focus add a drop of oil immersion and
switch the objective to oil immersion (1000X). Label the pointing structures below.
Figure 1 above shows normal Elodea. The chloroplasts are spread throughout the cell before the
salt solution, after the distilled water is put onto the slide. Distilled water represents a
hypotonic solution, yet the cells do not burst because of the cell wall.
4) Once observed switch to low-power objective and remove the slide.
5) After completion of the activity, clean up your work area and return or dispose
of the materials as instructed.
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6) Place the other piece of leaf on a microscope slide and add one drop of 20% Sodium
Chloride (NaCl) and place a coverslip on top of the leaf. Place a paper towel on top of
the slide and tap around the coverslip to extract excess water. Then, view the slide of
the elodea leaf through a compound microscope. Focus the slide with the low-power
objective. Once focus add a drop of oil immersion and switch the objective to oil
immersion (1000X). After a few minutes, the cell will lose water this process is called
plasmolysis (See figure 2 below). Label the pointing structures below.
7) Once observed switch to low-power objective and remove the slide.
8) After completion of the activity, clean up your work area and return or dispose
of the materials as instructed.
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Tonicity illustrates one solution’s solute concentration evaluates to that of another
solution. The solution containing the lower concentration of solute molecules than another is
hypotonic relative to the second solution. Solutions containing equal concentrations of solute are
isotonic to each other, while one containing a greater concentration of solute relative to a second
one is hypertonic (See picture below for tonicity in a plant cell).
1. Was the inside of the central vacuole in the elodea leaf hypotonic, isotonic, or hypertonic
compared to the drop of ddH2O that was added (outside of the cell or environment)?
______________________________________________________________________________
2. Was the 20% NaCl solution added (outside of the cell or environment) hypertonic, isotonic, or
hypotonic relative to the cytoplasm?
______________________________________________________________________________
3. In what direction will the water move if a hypotonic and a hypertonic solution are divided by a
selectively permeable membrane. ___________________________________________________
4. Name three selectively permeable membranes that are present within the elodea cells and that
were involved in the plasmolysis process (see figure 3 on page 14)?
a) _____________________________________________________________________
b) _____________________________________________________________________
c) _____________________________________________________________________
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___________________________________________
Last Name, First Name [lab partner N0. 1]
____________________________________________
Last Name, First Name [lab partner N0. 2]
_______________________________
_______________________________
Last Name, First Name [lab
partner N0. 3]
___________________________
Section
Last Name, First Name [lab
_______________
partner N0. 4]
____________________
group #
Date
Review Questions Chapter 9: Cellular Transport Mechanisms: Diffusion and Osmosis
1. Why does the cell membrane have to be selectively permeable?
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______________________________________________________________________________
______________________________________________________________________________
2. Compare and contrast active and passive transport?
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3. What variables affect the rate of diffusion in biological systems?
______________________________________________________________________________
______________________________________________________________________________
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4. Describe osmosis in your own words.
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5. What is the difference between pinocytosis and phagocytosis?
______________________________________________________________________________
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6. Explain why a sailor set adrift cannot drink seawater?
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7. If a medical technologist notes that many red blood cells are crenate, what are possible
sources that could have caused this phenomenon to occur?
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8. List and describe three active transport pumps that function in biological systems.
______________________________________________________________________________
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9. Draw and then describe what happens to a red blood cell when placed in a hypotonic
solution, a hypertonic solution and an isotonic solution.
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LEADING TO RESEARCH
1. Discuss specific adaptations of marine fishes that allow them to survive in saline
environments. Why ate marine organisms said to literally live in a desert?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
2. Discuss specific adaptations of marine fishes that allow them to survive in saline
environments. Why are marine organisms said to literally live in a desert?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
3. What is the difference between an osmoregulator and an osmoconformer? Give
examples of each.
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