Water Kit© Osmosis Lesson

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Osmosis Teacher Key - Page 1

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Water Kit

©

Osmosis Lesson

Objectives

Students will:

• Dene osmosis as the diffusion of water through a membrane.

• Construct and explain a physical representation of osmosis in hypertonic, hypotonic and isotonic

environments.

• Compare the movement of water molecules through a membrane in hypertonic, hypotonic and

isotonic environments.

• Recognize and account for the necessity of aquaporins in water transport across a membrane.

• Conceptualize the scaling factor for the water molecule models.

• Quantify the relative size of a water molecule in relation to a typical human cheek cell.

Materials

• 1 Water Kit

©

cup per small group

• 1 copy of this packet per person

Osmosis

Living things must perform vital activities in order to maintain their existence including

exchanging gases like CO

2

and O

2

; taking in water, minerals and food, and eliminating wastes. These tasks

occur at the cellular level and require that molecules move through a membrane that surrounds the cell.

The cell membrane is a complex structure that is responsible for separating the contents of the cell from its

surrounding environment and for controlling the movement of materials into and out of the cell.

It is important to understand how water ows in and out of a cell through the membrane as it will

directly impact a cell’s ability to survive. The passive transport of water across a selectively permeable

membrane is called osmosis. The net ow of water is in the direction toward the highest

concentration of solute.

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Osmosis Teacher Key - Page 2

Directions

You will explore osmosis by making models of the hypertonic, hypotonic and isotonic

states of osmosis and predicting the ow of water in each state.

You will use the water molecule and ion models in the Water Kit

©

and the graphic image of a cell on

page 10 to make your models. After exploring each state, you will document your ndings by drawing

your model on the smaller cheek cell image of a cell and answering the questions in the blue boxes.

1. Note that the water molecules and ions are at a different scale than the image of a cell on page

10. Answer the questions below to explain the differences in scale.

Water Kit

©

Osmosis

Questions

1. Based on the size of the water molecule models, how large would the image of the cell

be, if they were at the same scale?

_____________________________________________________________________

2. Explain your process in determining what the size the cell image would be, if it was at

the same scale as the water molecular models.

_____________________________________________________________________

3. What source(s) did you use to determine the relative proportion of a water molecule

and a cheek cell?

_____________________________________________________________________

4. Are all cells the same size? _______________________________________________

5. What does this imply about your calculations?

_____________________________________________________________________

Answers will vary based on sources used. Average cheek cell = 50 microns. Water

Molecule = .1nm (1x10

-10

m). Model water = 2.7cm (.027m).

0.027/(1x10

-10

) = 2,700,000,000 x’s bigger.

50 microns x 2,700,000,000 = 135,000,000,000 microns (divide by 1,000,000) =

135,000 meters in lengh.

Answers will vary.

No.

The calculations are an approximation for an average sized cell.

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Osmosis Teacher Key - Page 3

Questions

1. Identify the solute. Where is the solute located?_______________________________

2. Water may pass through the membrane but the solute may not. Predict the direction

of the net ow of the water by drawing arrows to indicate this on your diagram.

Explain why the water would ow in this direction.

_____________________________________________________________________

3. When water ows in the direction you predicted, what happens to the volume of the cell?

_____________________________________________________________________

When the concentration of solutes outside the cell is higher than the concentration of

solutes inside the cell, the net ow of water will be out of the cell. This type of a solution is

referred to as hypertonic.

Hypertonic

Water Kit

©

Osmosis

Cheek Cell Phospholipid Bilayer

2. Place your sodium (Na

+

) and chloride (Cl

-

) ion models on the outside of the cell image

(page 10). Place four water molecules (H

2

O) on the inside of the cell and four water

molecules on the outside of the cell.

In the image below, draw how you placed the molecules and ions on the large image. Use H

2

O to indicate

water, Na to indicate sodium and Cl to indicate chloride. Draw a circle around the solute.

The solute is sodium chloride (Na

+

, Cl

-

).

See diagram for arrow. The highest concentration of solute is outside the cell. Net flow

of water is towards the hightest concentration of solute.

The volume of the cell will decrease and the cell will shrink.

H

2

O

H

2

O

H

2

O

H

2

O

H

2

O

H

2

O

H

2

O

Na

+

Cl

-

H

2

O

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Osmosis Teacher Key - Page 4

Water Kit

©

Osmosis

3. Again, using the molecules and image on page 10, set up a physical representation

where the concentration of solutes is higher inside the cell than outside. This type

of solution is referred to as hypotonic.

Sketch your placement of the water and solute molecules in the diagram below. Indicate the net

ow of water in this system.

Hypotonic

Questions

1. Where is the initial concentration of solute molecules higher?

_____________________________________________________________________

2. Predict the direction of the net ow of the water by drawing arrows to indicate

this on your diagram. Explain why the water would ow in this direction.

_____________________________________________________________________

3. What happens to the volume of the cell in this system?

_____________________________________________________________________

Cheek Cell Phospholipid Bilayer

The solute molecules have a higher concentration inside the cell.

See diagram for arrow. Because the highest concentration of solute is inside the cell,

water flows into the cell

The volume of the cell will increase and the cell will expand.

H

2

O

H

2

O

H

2

O

H

2

O

H

2

O

H

2

O

H

2

O

H

2

O

Na

+

Cl

-

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Osmosis Teacher Key - Page 5

Water Kit

©

Osmosis

Equilibrium

4. Next, with the models, create a model of a system where equilibrium has been reached.

You will have to work with another group in order to use two sodium and chloride models.

Place one Na

+

both inside and outside the cell. Place one Cl

-

both inside and outside of the cell.

Place an equal amount of water molecules inside and outside of the cell. Sketch the placement of

the water and solute molecules in the diagram below. Indicate the direction of the net ow of water.

When the concentration of solutes is equal on either side of the cell membrane, a state of equilibrium

has been reached. Water still continues to ow through the membrane but at an equal rate in and

out of the cell. This type of solution is said to be isotonic.

Questions

1. Explain what happens to the ow of water in an isotonic solution.

_____________________________________________________________________

Questions Continued on Next page

Cheek Cell Phospholipid Bilayer

Water will flow inside and out at an equal rate when equilibrium has been reached.

H

2

O

H

2

O

H

2

O

H

2

O

H

2

O

H

2

O

H

2

O

H

2

O

Na

+

Na

+

Cl

-

Cl

-

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Osmosis Teacher Key - Page 6

Water Kit

©

Osmosis

Questions

2. Using the vocabulary of osmosis, explain what may happen to the vegetation along the

side of a road when excessive amounts of salt are used during the winter.

_____________________________________________________________________

3. Thinking osmotically, explain why grocery stores spray water on their fresh vegetables.

_____________________________________________________________________

4. Explain what will happen to a blood cell if it is placed in a 1.5% salt solution when

normal blood has a salt concentration of 0.9%. Sketch a model of this system in the

space below.

_____________________________________________________________________

The high concentration of salt would create a hypertonic environment for the plant cells

causing the water to flow out of the cells.

The water would flow into the vegetables due to a higher concentration of solutes. The

cells would expand, giving the vegetables a plump look to consumers.

Water wull flow out of the cell in this hypertonic solution to dilute the higher

concentration of the salt outside the cell. The blood cell will shrink in volume.

Normal Isotonic 1.5% Salt Solution

Blood Cell Blood Cell

0.9% Salt

1.5% Salt

0.9% Salt

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Phospholipid Bilayer

Water molecules are small enough to diffuse

across the phospholipid bilayer (left photo), but the middle zone

of the cell membrane (bottom photo) is highly hydrophobic, since

it consists of compact carbon atoms. Given the nature of water,

the hydrophobia of the middle zone impedes the passage of water

across the phosphilipid bilayer.

Discovery of Aquaporin

The movement of the water molecules through cell membranes is too rapid to be explained by unaided

diffusion alone. Transport proteins called aquaporins facilitate the diffusion of water across the cell

membrane. While studying Rh factors in red blood cells, Peter Agre made the serendipitous discovery

of a protein that later became known as aquaporin 1. The 1992 discovery was considered so important

that Agre was awarded the 2003 Noble Prize in Chemistry. To date, 13 variants of aquaporins have

been discovered in humans.

Osmosis Teacher Key - Page 7

Aquaporin

This space lled model of a

phospholipid bilayer is printed

on a 3-D ZCorp Printer by 3D

Molecular Designs.

Passage of the water molecules.

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Osmosis Teacher Key - Page 8

Aquaporin

This alpha carbon backbone model of aquaporin is printed on a 3-D ZCorp

Printer by 3D Molecular Designs. It is based on 1J4N.pdb and features the

six alpha helices and two half-alpha helices of the structure and the two

asparagine involved in selectively moving water through the channel. From

this perspective you can see portions of the six alpha helices (red, orange,

dark green, light green, blue and yellow), two half-alpha helices (magenta and

purple) and one of the two asparagines.

Asparagine

Side Chain

Color Key

oxygen

nitrogen

carbon

Aquaporin Structure

Aquaporin consists of six alpha helices and two

half-alpha helices.

Two asparagine (ASN) amino acids – ASN 78 and ASN

194 – are found at the turns of the two half alpha helices

(colored magenta and purple in the photo). These are

located at the narrowest part of the hour-glass shaped

channel and form the lter that allows water to pass

through aquaporin.

Asparagine

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Function

Water molecules rapidly ow in single le through the aquaporin channel. The ability

of aquaporin to selectively bind water molecules and prevent other molecules from

entering the channel is referred to as the aromatic /arginine selectivity lter.

While the process is not fully understood, many researchers

1

believe that water molecules

roll over as they reach the narrowest part of the channel, where the arginine are located.

In computer simulations the oxygen (red) atom of each

water molecule points down as it moves through the

channel toward the two asparagine. To pass through the

narrow opening each water molecule binds rst to one

asparagine and then to the second. In this process each

water molecule rolls over so that the oxygen points up

toward the asparagine — now from the opposite side of the

passageway — and passes through the remaining portion

of the channel. (See illustration right.)

Note: Water molecules form hydrogen bonds with

asparagine. The partially negative oxygen atom forms a

hydrogen bond with the partially positive nitrogen (blue)

atom of the asparagine amino acid.

Most of the amino acids in the aquaporin channel are

hydrophobic, which enables water molecules to move

freely within the channel until binding with asparagine.

For an animation and explanation from the National

Institutes of Health (NIH) Center for Macromolecular

Modeling & Bioinformatics and the University of Illinois at

Urbana-Champaign, go to http://www.ks.uiuc.edu/Gallery/

Movies/aquaporin-movie-explanation.html

Osmosis Teacher Key - Page 9

Aquaporin

1

Tajkhorshid E, Nollert P, Jensen MØ, Miercke LJ, O’Connell J, Stroud RM, Schulten K (2002). “Control of the selectivity of the aquaporin

water channel family by global orientational tuning”. Science 296 (5567): 525–30. doi:10.1126/science.1067778. PMID 11964478.

Water Channel

Questions

1. What factors may inuence the passage of water through a membrane?

___________________________________________________________________

2. Water is reabsorbed in the cells of the kidneys. What would happen to the rate of diffusion

of water if the number of aquaporin protiens decreased? Explain your answer

___________________________________________________________________

Answers may include the type of solution (hypotonic, hypertonic, isotonic) into which

the cell is placed and the number of aquaporin protiens present in the cell membrane.

The rate would greatly decrease as the water would have difficulty passing though

the lipid bilayer of the cell.

Osmosis Teacher Key - Page 10

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Cheek Cell Phospholipid Bilayer

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Osmosis Teacher Key - Page 11

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National Framework

Connections to: A Framework for K-12 Science Education

Practices, Crosscutting Concepts, and Core Ideas*

*The NSTA Reader’s Guide to A Framework for K-12 Science Education, National Research Council (NRC), 2011. A Framework for K-12

Science Education: Practices, Crosscutting Concepts, and Core Ideas. Washington, D.C.: National Academies Press.

Dimension 1: Scientic and Engineering Practices

2. Developing and Using Models

6. Constructing Explanations and Designing Solutions

Dimension 2: Cross Cutting Concepts

1. Patterns

3. Scale, Proportion and Quantity

4. Systems and System Models

5. Energy and Matter: Flows, Cycles, and Conservation

6. Structure and Function

7. Stability and Change

Dimension 3: Disciplinary Core Ideas

Physical Sciences

HS-PS1 Matter and Its Interactions

HS-PS1-2. Construct and revise an explanation for the outcome of a simple chemical

reaction based on the outermost electron states of atoms, trends in the periodic table,

and knowledge of the patterns of chemical properties.

Life Sciences

HS-LS1 From Molecules to Organisms: Structures and Processes

HS-LS1-2. Develop and use a model to illustrate the hierarchical organization of

interacting systems that provide specic functions within multicellular organisms.