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Diffusion and Osmosis Lab

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Tom Gitchell

November 6, 2012

AP Biology

Mrs. Dahle

Formal Lab Report

October 29, 2012; Joe Pagliuco

TITLE: Diffusion and Osmosis Lab

INTRODUCTION: The movement of molecules in and out of cell membranes is crucial to life. The movement must be regulated though. Membranes are phospholipid bilayers with hydrophilic heads and hydrophobic tails. This makes the membrane selectively permeable. The simplest form of movement through the membrane is diffusion where solutes move from an area of high concentration to low concentration (this is called moving down the concentration gradient). Diffusion is simple because it does not require energy. The kinetic molecular theory says that molecules are constantly in motion. This causes molecules to bump into each other and spin off in different directions resulting in diffusion. This is also why increasing temperature speeds up diffusion. But the semi permeable membrane restricts diffusion. For instance, some molecules are too big to diffuse quickly enough for cell use. Cells must use transport proteins or ion channels to pass these molecules along. Though slightly more complicated, the use of transport proteins and ion channels is still passive transport. When a solute moves from an area of low concentration to high concentration (moving against the concentration gradient), it requires energy in the form of ATP and protein carriers called pumps. This active transport, though more complicated than diffusion, is still crucial to cell survival.

The transport of water is so important that it gets its own name. Osmosis is the diffusion of water from an area of high concentration of water (high water potential), to an area of low concentration of water (low water potential). When the external and internal solute concentrations separated by a permeable membrane are the same (isotonic), no net osmosis occurs. When the solute concentration of one solution is lower than the other, the less concentrated solution is called hypotonic and has a higher water potential. If a cell is put into a hypotonic solution, osmosis will cause water to move into the cell. The solution that has higher concentration is called hypertonic and has a lower water potential. If a cell is put into a hypertonic solution, water will move out of the cell. In some cells, a cell wall resists osmosis. This resistance is called turgor pressure.

Since osmosis flows from high to low water potential, water potential is a good indicator of which way water till diffuse across a membrane. Water potential is the free energy per mole of water and is calculated by the solute potential and the pressure potential.

Ψ = ΨP + ΨS

Water Potential = Pressure Potential + Solute Potential

The topics that have just been discussed will be put into practice in the lab. In part 1, one can observe how variance in cube or "cell" size affects diffusion. In part 2, the purpose is to see how hypertonic, hypotonic, and isotonic environments affect a solution in dialysis tubing as a model to how they affect cells. The purpose of part 3 is to observe osmosis in actual living plant cells with real cell membranes, instead of dialysis tubing.

HYPOTHESIS:

1B: If a large agar cube with .01% sodium hydroxide is submerged in acetic acid for 10 minutes, then more diffusion of acetic acid will occur than in a small cube because the larger surface area of the large cube will allow for more diffusion of acetic acid to take place.

Part 2A: If the solutions in dialysis tubing are placed in a hypertonic solution, then the weight of the tubing solutions will decrease. If the tubing solutions are placed in a hypotonic solution, then the weight of the solutions will increase. This is because water will flow to the area with less water and more solute particles (osmosis) until the ratio between the inside and outside the tubing has reached equilibrium. If the tubing solution is placed in an isotonic solution, the weight will remain relatively stagnant because the solutions are already at equilibrium.

Part 3A: If a mystery glucose solution is put on a leaf, then the cells of that leaf will become bigger or smaller depending on the molarity of the glucose solution because some lowly concentrated solutions may be hypotonic to the plant cells and some highly concentrated solutions may be hypertonic causing uneven osmosis.

Part 3B: (Same as 2A only replace dialysis tubing with potato cores)

MATERIALS:

Part 1B:

Phenolphthalein agar cubes (3)

100 mL of white vinegar

Plastic knife (1)

Plastic spoon (1)

Plastic Cup (1)

Metric ruler

Timer (1)

Part 2A:

20cm Dialysis Tubing (5)

Balance (1)

Graduated Cylinder (1)

Roll of string (1)

500 mL of Distilled or tap water

250 mL of 1 M Sucrose solution

250 mL of 1 M Sodium Chloride

250 mL of 1 M Glucose Solution

250 mL of 5% Albumin Solution (egg white protein)

Part 3:

150 mL Red Mystery solution

150 mL Orange Mystery solution

150 mL Yellow Mystery solution

150 mL Green Mystery solution

150 mL Clear Mystery solution

175 mL Blue Mystery solution

Microscope slide (1)

Ruler (1)

Balance (1)

Coverslip (1)

Cork Borer (1)

Compound Microscope (1)

Potatoes

Elodea

...

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