Diffusion and OsmosisLab 2 Background

Membranes

Phospholipid bilayer containing embedded proteins

Fatty acids limit the movement of water because of their _______________.

Water

Passes freely through the membrane by osmosis or through aquaporins.

Moves from areas of high potential/high [water] to low potential of low [water].

* In walled cells, osmosis is affected by [solute] & resistance to water movement in the cell by the cell wall (known as Turgor Pressure).

Ions

Move through protein channels

Larger Molecules

Ex. Carbohydrates

Use transport proteins

Diffusion

Simplest form of movement

Solute move from areas of high [solute] to areas of low [solute]

Does not require energy input

*ATP and protein carriers called Pumps are required to move solute from areas of low [] to areas of high [].

Diffusion

Molecular weight is inversely related to the rate of diffusion

Non-directional

Increases as kinetic energy increases (for example via increase in temperature)

At equilibrium, diffusion reaches a dynamic equilibrium not static equilibrium.

Solute concentrations

Hypertonic- high [solute]; low water potential

Hypotonic- low [solute]; high water potential

Isotonic- equal water potentials

The above 3 terms are used to describe solutions separated by selectively permeable membranes

These terms are used to describe solute concentration.

How does solute reduce free water in a system?

Which way will water move through a selectively permeable membrane in the above solutions?

Homeostasis

Cells must maintain their internal environments and control solute movement

Why?

Water Potential

As water moves out of the cell, the cell shrinks or undergoes crenation.

As water moves into the cell, it may eventually burst or lyse.

In walled cells (fungal & plant), the cell wall prevents the cell from bursting as water enters but pressure builds up inside the cell and affects the rate of osmosis.

Water Potential

Measures the tendency of water to move from 1 compartment to another

Is the free energy/mole of water

Represented by the Greek letter psi ____.

= pressure potential (+ve or –ve) + solute potential (aka osmotic potential)

*pressure potential = exertion of pressure

* Solute potential = [solute]

Calculating Solute Potential

Solute potential ____= -iCRT

i=ionization content

C= molar concentration

R= pressure constant = 0.0831

liter bars/mole-K

T= temp in K (273 + degree Celcius)

* Bar- a metric measure of pressure and is the same as 1 atmosphere at sea level

Hints

Water potential ____of pure water =0

Water potential ____of a solution = -ve value

Why?

*Summary videos:

http://www.youtube.com/watch?v=nDZud2g1RVY

http://www.youtube.com/watch?v=Ez1oeY0L0oQ

Practice

Calculate the solute potential of a 0.1M NaCl solution at 25 degree Celcius. If the concentration of NaCl inside the plant cell is 0.15M, which way will the water diffuse if the cell is placed into the 0.1M NaCl solution?

What must turgor pressure equal if there is no net diffusion between the solution and the cell?

Part 1

Think-pair-share: discuss with your lab partners the answers to the questions under getting started on pgs. S54-S55.

Conduct Step 1

Conduct Step 2 & answer the questions

* Cut 3 square blocks of agar (1cm, 2cm & 3cm)

*Don’t forget to record your rate of diffusion.

Part 2

Think-pair-share: discuss with your lab partners the answers to the questions under getting started on pgs. S56-S57.

Set up your experiment- you should 5 cups, don’t forget to label your cups with what solution is inside tube and what soln is outside

Get initial weight; after 30 mins and after 24hrs.

Dialysis Videos

http://www.youtube.com/watch?v=2Th0PuORsWY&feature=related

http://www.youtube.com/watch?v=DRHKq0piN0M&feature=related

Elodea Video

http://www.youtube.com/watch?v=VK-_YHakvho

Part 3

Think-pair-share: answer question in Step 1 S58-S59. Skip Step 2 Design and Conduct your investigation (use potato

pieces of the same size however you may vary the type of potato)

Leave overnight Answer questions on S59 Rank solutions from lowest to highest solute

concentrations Rank solutions from lowest to highest water

potential.