Modelling Diffusion and Osmosis Through Experimentation

DIFFUSION AND OSMOSIS

Introduction

It is important for the cell to have a way to separate the cytoplasm (inside of the cell) from the extra cellular materials (outside of the cell). This is accomplished by the cellular, or plasma, membrane. The plasma membrane is composed of a phospholipid bilayer with embedded proteins. Because of the hydrophobic nature of the phospholipid bilayer, cellular membranes are semi- or selectively permeable. In other words, some materials will be able to pass through the membrane and some will not. This allows the cell to regulate precisely the components of the cytoplasm.

Materials will be able to pass through the membrane by diffusion. Diffusion is the movement of particles from an area of high particle concentration to an area of low particle concentration, until concentrations are equal. The energy driving diffusion comes from the kinetic energy of the molecules themselves and the concentration differences of those molecules.

Osmosis is a special case of diffusion. Osmosis is the diffusion of water across membranes. Water will diffuse from a region of its higher concentration to a region of its lower concentration. However, this is not a very useful definition. Rarely will someone describe a solution by the concentration of water. Instead, they will describe the concentration of the solute in the solution. Another, more useful, way to put osmosis is the movement of water from an area of low molecule concentration to an area of high molecule concentration (across a semi-permeable membrane) until concentrations are equal.

There are three ways to describe comparative concentrations of solutions separated by a semipermeable membrane.

A hypertonic solution has a greater molecule concentration than the solution to which it is being compared.

Isotonic solutions have equal molecule concentrations.

A hypotonic solution has a lower molecule concentration than the solution to which it is compared.

A cell placed in these solutions will either lose water, gain water, or stay the same, depending on the movement of water. Cells placed in a hypotonic solution will gain water (water will move into the cell by osmosis). Cells placed in a hypertonic solution will lose water (water will move out of the cell by osmosis). Cells placed in an isotonic solution will stay the same (water will move in and out equally).

In this lab you will be modeling both diffusion and osmosis. In Exercise 1, you will use a sandwich bag as a semipermeable membrane. Although they are plastic, sandwich bags are porous and have tiny holes in them. Some substances may be able to pass through the bag if they are small enough, but others will not be able to pass if they are too big. You will investigate the ability of starch (a polysaccharide made of glucose molecules) to pass through a sandwich bag. In order to tell if diffusion has occurred, you will need to be able to detect the presence of starch. Unfortunately, starch is a colorless molecule and will not be directly visible in the water. However, iodine will interact with starch and in a not completely understood reaction will turn from golden yellow to black. Development of this black color will indicate where the starch is located.

In Exercise 2, you will be using an egg to explore the hyper-, hypo-, or isotonic nature of different liquids. Remember that when a cell is placed in solutions of different concentrations it may gain or lose water. By soaking an egg in syrup (high sugar concentration) and distilled water (low concentration of anything) and measuring how much liquid is left, you will be able to tell if the egg absorbed (gained) or gave off (lost) water. Then you can figure out if the liquid was hypertonic, hypotonic, or isotonic. However, the shell of the egg may prevent the easy movement of water into and out of the egg. You must first remove the shell using vinegar. Vinegar contains acetic acid, which will interact with the calcium carbonate crystals of the egg shell and separate them into calcium ions and carbonate molecules, which will form carbon dioxide bubbles. The reaction is:

CaCO3+ 2H+ Ca+2 + H2O +CO2

Although an egg looks like it is only one cell, the membrane around the whites and yolks are actually made up of multiple cells. However, any water that enters or leaves the egg must pass through those cells (and their membranes) and so will behave just like it would if we were using a real cell.

Exercise 1 – Diffusion

Materials:

• 1 ml of iodine
• One teaspoon corn starch
• Plastic sandwich bag
• 250 mL Beaker
• Distilled Water

In this experiment you will be modeling the diffusion of a substance (corn starch) across a semipermeable membrane (the sandwich bag). To detect where the starch has traveled, we will use the indicator iodine. Iodine will turn black in the presence of starch, and will be your indication of where the starch is. In other words, your results will be where the color change (blackening) happens.

Procedure:

1. Develop a hypothesis and prediction about the permeability of the bag for starch. Record them in the lab report sheet.
2. Fill beaker about halfway with distilled water and place 1 ml of Iodine in beaker.
3. Prepare the starch by mixing 50 ml of distilled water with 1 tsp of corn starch. Heat this mixture (microwaving for 10-15 seconds will work) to help the starch dissolve in the water.
4. Place the corn starch mixture in the sandwich bag. Make sure sandwich bag is water tight by tying the bag closed.
5. Place sandwich bag/starch into the beaker and observe for 15-20 minutes.
6. If nothing observable occurs in the 15 minutes a slightly warming of the sandwich bag and beaker in the microwave for 10 – 15 seconds will help speed up the process.
7. Record observations in Table 1 in the lab report sheet.

Exercise 2: Osmosis

Materials:

• Fresh Egg (in shell)
• Wax Pencil (Optional)
• 250 mL Beaker (3 ideal, 1 will work)
• 200 mL White Vinegar
• 200 mL Clear Sugar Syrup (Karo)
• 200 mL Distilled Water

In this experiment you will be modeling osmosis of water across a cell membrane to determine if different liquids are hyper-, hypo-, or isotonic. The egg will represent our cell. Be careful with the eggs as you experiment - if they break as you are working they will make a big mess and you will be at risk because of salmonella. Be sure to wash everything (and yourself) with warm, soapy water. You may also want to use a disinfectant.

Procedure:

Develop a hypothesis and prediction about the tonicity of the solutions (If the solution is hyper-, hypo-, or isotonic). Record them in the lab report sheet.

Follow the instructions below to treat your egg. Make sure to record your data and observations in Table 2 in the lab report sheet.