Movement Through the Membrane
Section 7-3
Learning objectives
1. Explain the role of cell membranes as a highly selective barrier (diffusion, osmosis and active transport).
2. Predict the result of cells placed in isotonic, hypertonic and hypotonic solutions.
Cornell Notes
• Lecture; students use the template to take notes– Abbreviations, don’t copy slides word for
word, don’t use complete sentences, develop “shorthand” of your own
• At conclusion of lecture topic, students compare notes with a partner for one minute
• Write reflection in bottom space• Possible open-notes quiz• Cornell Notes must be attached to Chapter
Notes and turned in at time of chapter test
Assignments
• Complete the Chapter Notes through Section 7-3
• Complete Worksheet Diffusion/Osmosis Problem Set
• Read ahead through Section 7-4
Assessment
• Quiz, Section 7-3
Transport
• All cells must move – transport – substances across the cell membrane – both into the cell and out of the cell
• Two types of transport mechanisms exist:– Passive– Active
Passive Transport
• Passive transport does not require the cell expend energy
• Relies on natural tendency of substances to move from where they are highly concentrated to where they are in low concentration
• Passive transport– Diffusion of a substance across a membrane
with no energy investment
Passive Transport
• Three kinds of passive transport– Diffusion– Facilitated diffusion– Osmosis
Diffusion
• Cytoplasm is really a solution– Mixture of two or more substances in which the
molecules of the substances are evenly distributed
• Cytoplasm is really an aqueous solution– Solution in which water is the solvent– Solvent – substance in which the solute is
dissolved– Solute – substance(s) that is dissolved in the
solvent to form a solution
Diffusion
• Diffusion is– When molecules in solution move from areas
where there are more of them (higher concentration) to areas where there are fewer of them (lower concentration).
• Can diffusion happen in a solid?– NO! Diffusion can happen in a liquid or in a
gas.
Demonstrate Diffusion
Diffusion
http://faculty.southwest.tn.edu/rburkett/GB1-osmosis.htm
Diffusion
• When the drops of ink first enter the water, that place has a high concentration of ink molecules.
• But given time, the ink molecules spread out until they are equally concentrated throughout the water.
Diffusion
• To understand how diffusion works, you first need to understand the concept of concentration
Diffusion
• Cytoplasm in a cell is essentially a rich solution of many different substances – water is still the solvent.
• Concentration:– Mass of the solute in a given volume of
solvent– Mass (grams) / Volume (liters)– 12 grams of salt in 3 liters of water,
concentration is 12 g / 3 L = 4 g / 1 L
Diffusion
• Concentration can be expressed as a percentage of solute in the solvent– Example: a 50% glucose solution means half
the molecules in the solution are glucose; the other half water.
– Example: 40 grams of solute in 120 grams of solvent is a 33.3% solution (40g/120g)• (Solute quantity / Solvent quantity) x 100 =
Percent solution
Diffusion
Mass of Solute
Volume of Solvent
MSolute
VSolvent
Diffusion
• What happens to the concentration if…– The amount of solute increases, but the
volume remains the same?• Concentration increases
MSolute
VSolvent
Diffusion
• What happens to the concentration if…– The amount of solute remains the same, but
the volume is decreased?• Concentration increases
MSolute
VSolvent
Diffusion
• What happens to the concentration if…– The amount of solute decreases, but the
volume decreases proportionally?• Concentration remains the same
MSolute
VSolvent
Diffusion
• What happens to the concentration if…– The amount of solute remains the same, but
the volume is increased?• Concentration decreases
MSolute
VSolvent
Diffusion
• Remember, in diffusion– Solutes move from an area of high
concentration to an area of low concentration– A form of passive transport
• Solutes move down a concentration gradient
Concentration Gradient
Look at the blue arrow . It shows that a dissolved substance – a solute – will leave the area of greatest concentration and go to the area of lowest concentration.
So we say the solute moves down a concentration gradient.
Diffusion
• Why do molecules or particles move down the concentration gradient?
• Remember the kinetic theory of matter?– In solids, liquids and gases, molecules
constantly and randomly move.• Diffusion is simply the random movement of
molecules or particles.• Eventually, this random movement causes the
molecules to spread out.
Diffusion
• What if there are two solutes?
Diffusion
• Eventually, the random movement of the solute molecules ends with..– an equal concentration of the solute
molecules everywhere• When this happens, we have reached
equlibrium
Diffusion
• When all the ink molecules are evenly spread out, when there is no place of higher or lower concentration, there is equilibrium.
• Equilibrium – state in which molecules of a substance are evenly distributed throughout another substance.
• May or may not involve a membrane.
Equilibrium
See…no membrane in this case.
Diffusion
• At equilibrium, do molecules stop moving?• Again, think back to the Kinetic Theory of
Matter…– As long as there is energy in the molecules,
they will continue to move randomly.• The result is their random movement maintains
equilibrium.
Diffusion
Passive Transport
Animation – How Diffusion Works
• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html
OK, I understand diffusion.
What does diffusion have to do with cells?
What if a membrane separates high and low concentrations?
• All cells have a membrane, right?
• It’s simple. Diffusion can take over.
• Molecules from the high concentration move to the low concentration if they can get through the membrane.
What can diffuse directly across a cell membrane?
• Some molecules are small enough to easily pass through the phospholipid bi-layer of cell membranes.
• Which molecules? Oxygen (O2) and Carbon dioxide (CO2)• Why is this an advantage? Breathing!! Quick exchange of gasses is
vital!
Facilitated Diffusion
• In facilitated diffusion– Transport proteins speed the movement of
molecules across the plasma membrane– Still a form of passive transport
Facilitated Diffusion• Channel proteins
– Provide corridors that allow a specific molecule or ion to cross the membrane
Figure 7.15
EXTRACELLULARFLUID
Channel proteinSolute
CYTOPLASM
A channel protein (purple) has a channel through which water molecules or a specific solute can pass.
(a)
Facilitated Diffusion• Carrier proteins
– Undergo a subtle change in shape that moves the solute-binding site across the membrane
Figure 7.15
Carrier proteinSolute
A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute.
(b)
Animation – Facilitated Diffusion
http://www.youtube.com/watch?v=s0p1ztrbXPY
Animation – How Facilitated Diffusion Works
• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html
Osmosis
• A special kind of diffusion and passive transport
• Osmosis– is the movement of water - free water
molecules - down a concentration gradient and across a semi-permeable membrane
– is passive transport, and– a form of diffusion
Osmosis
• Free water molecules are just that – free and unbound to dissolved molecules of solute
Osmosis
• Bound water molecules are connected to dissolved molecules of solute, forming a hydration shell
Osmosis
• Bound water molecules are not free move; sort of creating a much larger particle of water molecules and dissolved solute
• Likely unable to pass through a membrane
Osmosis
Osmosis
• Osmosis– Movement of free water molecules from an
area of high concentration to an area of low concentration
– Solutions with:• low or zero solute concentration have high
free water molecule concentration• high solute concentration have low free
water molecule concentration
Osmosis
• On the left of the membrane, high free water molecule concentration/low solute concentration
• On the right, low free water molecule concentration/high solute concentration.
Osmosis
• The result: water moved by osmosis across the membrane from left to right
• Water moved from places of high water/low solute concentration to places of low water/high solute concentration.
Osmosis
Continues until equilibrium – the concentration of solute is the same on both sides of the membrane.
Remember! It is the water that moves across the membrane – not the solute!
Osmosis
Osmosis
Osmosis
• Once again, water moves..
High Free Water Molecule / Low Solute concentration
Low Free Water Molecule/ High Solute concentration
Video – U-Tube Osmosis
http://www.youtube.com/watch?v=GbudKs-49jo
Animation – Effects of Osmosis on Water Balance
http://www.youtube.com/watch?v=sdiJtDRJQEc
Osmosis Tutorial
http://www.youtube.com/watch?feature=endscreen&v=eQsAzXr0UCU&NR=1
How Osmosis Works
• Direction of osmosis is determined only by a difference in total solute concentration.– The kinds of solutes in the solutions do not
matter.– The number of solutes in the solutions does
not matter.
How Osmosis Works
• This makes sense because– the total solute concentration is an indicator of
the abundance of bound water molecules (and therefore of free water molecules).
• It is the free water molecules that move down the concentration gradient and across the membrane
How Osmosis Works
Continues until equilibrium – the concentration of solute is the same on both sides of the membrane.
Remember! It is the water that moves across the membrane – not the solute!
How Osmosis Works
• Water will continue to move across the membrane until equilibrium is reached– When the concentrations of water and solute
will be equal
How Osmosis Works
• Tonicity– Is the ability of a solution to cause a cell to
gain or lose water– Has a great impact on cells without walls
How Osmosis Works
• If a solution is isotonic– The concentration of solutes is the same as it
is inside the cell– There will be no net movement of water– Concentration of solutes and concentration of
water is equal.• [Solute] = [Solvent]
• When two solutions are isotonic, water molecules move at equal rates from one to the other, with no net osmosis - equilibrium
How Osmosis Works
• If a solution is hypertonic– The concentration of solutes is greater than it
is inside the cell– The cell will lose water– A higher concentration of solutes also means
a lower concentration of water• [Solute] > [Solvent]
How Osmosis Works
• If a solution is hypotonic– The concentration of solutes is less than it is
inside the cell– A lower concentration of solutes means a
higher concentration of water• [Solute] < [Solvent]
– The cell will gain water
How Osmosis Works
• These are comparative terms.– Tap water is hypertonic compared to distilled
water but hypotonic when compared to sea water.
How Osmosis Works
Ever have a saline IV?
Suppose this IV bag contained only pure, distilled water?
Osmotic Pressure
High water concentration – water moves into cell
Low water concentration (very salty water) – water moves out of cell
Goldilocks would say, “Just right.”
Concentrations of water and solute are equal inside and outside the cell.
No net movement of water
Hypotonic, Hypertonic or Isotonic?
Isotonic
Hypertonic
Hypotonic
Osmotic Pressure
http://www.youtube.com/watch?v=EA_ss8ZkjAM&feature=PlayList&p=BDF73FDD707BDDDA&playnext=1&playnext_from=PL&index=8
Osmotic Pressure
http://www.youtube.com/watch?v=IRQLRO3dIp8&feature=PlayList&p=BDF73FDD707BDDDA&playnext=1&playnext_from=PL&index=7
Osmotic Pressure
http://www.youtube.com/watch?v=plen79Fgmz0&feature=PlayList&p=BDF73FDD707BDDDA&index=9&playnext=2&playnext_from=PL
Osmotic Pressure
• Animal cell. An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water.
Figure 7.13
Hypotonic solution Isotonic solution Hypertonic solution
(a)
H2O H2O H2O H2O
Lysed Normal Shriveled
Osmotic Pressure
• Animals and other organisms without rigid cell walls living in hypertonic or hypotonic environments
H2O
Figure 7.13
Hypotonic solution Isotonic solution Hypertonic solution
(a)
H2O H2O H2O
Lysed Normal Shriveled
• Example, Paramecium, a protist, is hypertonic when compared to the pond water in which it lives.– Water continually enters the Paramecium cell.– Paramecium have a
specialized organelle, the contractile vacuole, that functions as a bilge pump to force water out of the cell.
Osmotic Pressure
Osmotic Pressure
Osmotic Pressure
• Cell walls of plant cells– help maintain water balance
Osmotic Pressure
• If a plant cell is turgid– It is in a hypotonic environment– It is very firm, a healthy state in most plants
(b)
H2OH2OH2OH2O
Turgid (normal) Flaccid Plasmolyzed
Osmotic Pressure
• If a plant cell is flaccid– It is in an isotonic or somewhat hypertonic
environment
(b)
H2OH2OH2OH2O
Turgid (normal) Flaccid Plasmolyzed
• If a plant cell is plasmolyzed– It is in a hypertonic solution, a cell wall has
no advantages.(b)
H2OH2OH2OH2O
Turgid (normal) Flaccid Plasmolyzed
Osmotic Pressure
• As the plant cell loses water, its volume shrinks.• Eventually, the plasma membrane pulls away
from the wall – plasmolysis is usually lethal.
(b)
H2OH2OH2OH2O
Turgid (normal) Flaccid Plasmolyzed
Osmotic Pressure
• Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environment – uptake of water is eventually balanced by the elastic wall pushing back on the cell.
(b)
H2OH2OH2OH2O
Turgid (normal) Flaccid Plasmolyzed
Osmotic Pressure
Osmotic Pressure
• Elodea cells were placed in a hypertonic (salty) solution.
• The arrows point to cytoplasm which shriveled away from cell wall. Why?
• Water moved out of the cell.
Animation – Plasmolysis
http://www.youtube.com/watch?v=GOxouJUtEhE&feature=related
Animation – Osmosis and Salad
http://www.youtube.com/watch?v=H6N1IiJTmnc&feature=fvw
Osmotic Pressure
Why would a supermarket shower water over its fresh fruit and
vegetables?
• Yes, to keep the produce fresh, but answer in terms of cell transport.
• The water is in drops on the fruit. Where will it go?
• Yes! Osmosis! Osmosis is the diffusion of water across a membrane.
• The water moves by osmosis into the cells of the fruit and vegetables, keeping them from drying out.
Physiological effects of osmosis - drinking seawater
Drinking seawater promotes dehydration. As the seawater flows through the stomach and intestine it draws water out of bodily tissues.
Solutes always flow from a region of higher to region of lower concentration.
Seawater has lower concentration of water than pure water has (because of the salt!).
Water molecules migrate toward a sample of seawater. With seawater in a person's stomach or intestines, water moves toward the seawater from the body's tissues, resulting in dehydration.
Osmosis
What will happen?
What will happen?
What will happen?
What will happen?
What will happen?
0.5 M glucose
1.5 M sucrose
1.5 M sucrose
1.0 M glucose
In terms of solute concentration, is solution A hypertonic, isotonic or hypotonic with respect to solution B?
What will happen?
0.5 M glucose
1.5 M sucrose
1.5 M sucrose
1.0 M glucose
In terms of solute concentration, is solution B hypertonic, isotonic or hypotonic with respect to solution A?
What will happen?
0.5 M glucose
2.0 M sucrose
1.5 M sucrose
1.0 M glucose
Now looking at the solute concentrations, is solution A hypertonic, isotonic or hypotonic with respect to solution B?
What will happen?
0.5 M glucose
1.5 M sucrose
1.5 M glucose
1.0 M glucose
What will be observed once the system reaches equilibrium?
The water level on side B will rise as water diffuses across the membrane from an area of high concentration A to the area of lower concentration B.
Summary: Passive Transport
• Movement of substances across a membrane.
• Diffusion and Osmosis.• Substance moves through a
simple channel built into a protein molecule in the cell membrane.
• No energy is needed.• Substance naturally moves
from an area of high concentration to an area of low concentration.
Click on the image to hyperlink to a website video and narration about passive transport.
Animation – Passive Transport
http://www.youtube.com/watch?v=STzOiRqzzL4&feature=related
Animation – How Osmosis Works
• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html
Simulation – Under What Conditions Do Cells Lose or Gain Water?
• Computer Lab• http://www.glencoe.com/sites/common_assets/
science/virtual_labs/LS03/LS03.html• Scroll down to Procedures• Document results in Journal and Table/Graphs,
print results and hand in with name and period.
Assignment
• Distribute Worksheet, Chapter 7 Diffusion and Osmosis Problem Set
Online Tutorial
• Go to this online tutorial:
http://www.mun.ca/biology/Osmosis_Diffusion/tutor2.html
Lab
• Lab, Elodea and Saltwater
That was passive transport.
What if a cell needs a substance from the outside, but must work against
the concentration gradient?
Welcome to Active Transport.
Active Transport
• When pushing an object up a hill, you need to work hard…you need to use energy!
• Active transport is like that…the cell must work against a hill – the concentration gradient.
Active Transport
• Situation:– A substance required by the cell is found
outside the cell, but• In a concentration less than found inside
the cell• Therefore, the cell cannot rely on diffusion
of the substance into the cell
Active Transport
• Somehow the cell must move substances from an area of low concentration to an area of high concentration
Active Transport
High concentration
Low concentration
Inside the cell Outside the cell
Active Transport
• To move solutes across a membrane against the concentration gradient, cell must expend energy– Not until pushing an object up a hill
• The energy is provided by the molecule ATP.• Also requires use of transport proteins in the cell
membrane
Animation – Active Transport
http://www.youtube.com/watch?v=STzOiRqzzL4&feature=related
Active Transport
• Active transport– Moves substances against their concentration
gradient– Allows cells to keep a different or higher
concentration of small molecules inside– Requires energy, usually in the form of ATP
• ATP often transfers its terminal phosphate group to a membrane protein
• This transfer may cause the protein to change conformation
Comparing Passive and Active Transport
Active Transport
• Now suppose you need to move really big molecules across a cell membrane.
• Diffusion, Osmosis and Active Transport won’t do. This is a job for…
Endocytosis
• Sometimes large protein molecules or bacteria may come upon the cell membrane.
• The cell membrane may fold in on itself, creating a sphere of membrane around the protein molecule or bacteria or whatever.
• The sphere pinches off and moves to the interior of the cell.
• This is Endocytosis.
EndocytosisEndo means In
It is a process in which a cell takes up water. Tiny pockets along the cell membrane, and then fill with liquid. Those tiny pockets then break off into the cell to form tiny vacuoles filled with water.
http://www.ri.net/schools/Narragansett/NHS/PerEwebpage/cell_mem1.html
Pinocytosis – a form of endocytosis
Phagocytosis, a type of endocytosis
T-lymphocyte cell attacking invading E. coli
Phagocytosis applies when solid substances are taken in.
Exocytosis
• There are many cells that make molecules that do work outside the cell.– Hormones, enzymes, acids, etc.
• Vesicles and vacuoles in the cytoplasm transport these products to the membrane.
• The products are expelled from the cell by exocytosis.
ExocytosisExo mean Out
Key point – vesicle membrane made of same phospholipids as cell membrane
Review Section 7.3
Animation – Endocytosis
http://www.youtube.com/watch?v=4gLtk8Yc1Zc&feature=related
Animation – Endocytosis and Exocytosis
http://highered.mcgraw-hill.com/olc/dl/120068/bio02.swf
Transport Graphic Organizer
Transport
Passive Active
Diffusion
Osmosis
Passive and Active Transport
Passive Transport• Requires no energy from
the cell.• Relies on a simple fact –
all molecules will automatically move from a place of high concentration to a place of low concentration.
• Diffusion and osmosis fall under passive transport.
Active Transport• Needs energy from the
cell to work.• Used when the cell must
take in or force out molecules, working against the concentration gradient.
• Includes molecular transport, exocytosis and endocytosis.
Before we look at diffusion, we need to master measuring concentration
Passive and Active Transport Review
1. In what way are cell membranes selectively permeable?
– They allow some things to pass through but not others.
2. Compare osmosis and diffusion.– Molecules move from where there are many to
where there are fewer – a concentration gradient.– Requires no energy.– Kinetic Theory of Matter – the constant, random
motion of particles of matter – is responsible.– Osmosis is a type of diffusion.
Passive and Active Transport Review
3. Identify how diffusion and osmosis are different:
– Osmosis involves the movement of water molecules across a selectively permeable membrane.
– Diffusion is the movement of solute particles which may or may not involve a membrane.
4. Identify the molecule that helps substances move through the cell membrane.
– Transport proteins.
Passive and Active Transport Review
5. What is it called when energy is used to move molecules?
A. DiffusionB. OsmosisC. Active TransportD. Passive TransportC. Active Transport
6. How may bacteria be taken into cells?A. OsmosisB. EndocytosisC. ExocytosisD. DiffusionB. Endocytosis
Passive and Active Transport Review
7. What occurs when molecules are evenly distributed through a liquid or gas?
A. Equilibrium
B. Metabolism
C. Fermentation
D. Cellular respiration
A. Equilibrium
Assignments
• Complete the chapter notes through Section 7-3• Complete the worksheet Section Review 7-3• Complete Worksheet Diffusion/Osmosis
Problem Set• Read ahead through Section 7-4
Cornell Notes
• Lecture; students use the template to take notes– Abbreviations, don’t copy slides word for
word, don’t use complete sentences, develop “shorthand” of your own
• At conclusion of lecture topic, students compare notes with a partner for one minute
• Write reflection in bottom space• Possible open-notes quiz• Cornell Notes must be attached to Chapter
Notes and turned in at time of chapter test
Cornell Notes
• Tonight– Reread your Cornell Notes in the right column– Review the ideas in the left column– Study your summary/reflection
Assessment
• Quiz, Section 7-3
Lab
• Lab, Diffusion and Osmosis
Animation - Diffusion
• Click on the image at the left to view an applet of diffusion.
• You’ll see how the random movement of molecules explains why the smell of perfume travels across a room.
Animation – DiffusionAnimation – Diffusion
Diffusion is the movement of particles to an area of high concentration to an area of low concentration through a selectively permeable membrane.
http://www.indiana.edu/~phys215/lecture/lecnotes/diff.html