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