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Diffusion and Osmosis Essay

NGSSS:

SC.912.L.14.2 Relate structure to function for the components of plant and animal cells. Explain the role of cell membranes as a highly selective barrier (passive and active transport). SC.912.L.14.3 Compare and contrast the general structures of plant and animal cells. Compare and contrast the general structures of prokaryotic and eukaryotic cells. AA

Background: (Source: www.explorelearning.com)

Diffusion is the process in which there is a net movement of molecules from a high area of concentration to a low area of concentration. Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane (Semi-permeable membranes are very thin layers of material which allow some things to pass through them but prevent other things from passing through.) to a region of low water concentration. This is seen in cell membranes. When there is a higher concentration of one type of molecule outside of a cell, water will move through a membrane out of the cell in order to make the water concentrations equal. This causes the cell to shrink (hypertonic). If the concentration of certain molecules is higher inside of the cell, then the water will move into the cell causing it to swell (hypotonic). When the molecule concentrations are equal on both sides of the membrane, water does not move (isotonic).

In the human body, many salts and enzymes help to regulate a cell’s state and the processes necessary for the human body to function such as potassium and calcium channels in the heart. These functions are carried out by having constant changes in concentration of molecules from one side of the membrane to another. Cell membranes will allow small molecules like oxygen, water, carbon dioxide, ammonia, glucose, amino acids, etc., to pass through. Cell membranes will not allow larger molecules like sucrose, starch, protein, etc., to pass through.

Problem Statement(s): What is the movement of material through a semi-permeable membrane? (Activity 1) What is the osmotic effect of varying sucrose solutions on the physical characteristics of a potato core? (Activity 2)

Vocabulary: cell, cell membrane, permeable, diffusion, semi-permeable membrane, osmosis, hypertonic, hypotonic, isotonic

Materials (per group):
Activity 1
·Ziploc bag
·liquid starch
·forceps
·beaker (500 ml)
·water
·Iodine (Lugol’s solution)

Activity 2
·razor
·potato
·ruler
·balance
·graduated cylinder
·distilled water
·sucrose solution
·dissecting needle
·beaker
·aluminum foil
Procedures:
Activity 1: Diffusion through a semi-permeable membrane.

1.Get a small plastic zip-top bag, a cup of liquid starch, a forceps and a large container of water to which a few drops of iodine solution has been added. Be careful with container and do not get the iodine solution on your skin. It will stain. 2.Place liquid starch in plastic bag and securely zip the top closed. Record observations of the color of the starch and the water in the container. 3.Place bag with starch into the iodine/water solution. Let it sit while you go on to Activity 2. 4.Observe and record the changes over time to both the iodine/water solution and the starch in the bag. You will need to carefully lift the bag part way out of the water with a forceps to observe. Then lower the bag back into the water and return the container as directed by the teacher.

Activity 2: Potato Osmosis

1.Using the razor, carefully cut each potato core into a cylinder of about three to five centimeters in length. Make sure that all of the potato cores are the same length and note this length for later use. Also measure and record the diameter of each potato core. 2.Using the balance, measure and record the mass of each potato core. 3.Fill the graduated cylinder with tap water two-thirds of the way up. Measure and record the volume of water in the graduated cylinder. Attach each potato core, one at a time, to the end of the dissecting needle and hold it so that the potato core is completely submerged in the water. Measure and record the water level in the cylinder.

The difference in your two measurements is the volume of the potato core. 4.Place one potato core in the beaker with distilled water and label this beaker “100”. Place the second core in the beaker of 90% water/10% sucrose and label the beaker “90/10”. Place the third core in the beaker with 80% water/20% sucrose and label this beaker “80/20”. 5.Cover the top of each beaker with a piece of aluminum foil. Fold the aluminum foil down along the sides of the beaker so that it cannot fall off easily. 6.Allow the beakers to sit for a day.

7.Remove the cores from each beaker using the dissecting needle and gently blot them. Measure and record the length, diameter, mass, and volume of each potato core as you did earlier. 8.Dispose of the cores in the trash, not the sink.

Observations/Data:
Table 1 – Diffusion of Starch
Initial ObservationsFinal Observations
Ziploc Bag with Starch
Iodine/Water Solution

Table 2 – Physical Changes in Potato Cores
100% water90% water
10% sucrose80% water
20% sucrose
Day 1Day 2ChangeDay 1Day 2ChangeDay 1Day 2Change
Length (mm)
Diameter (mm)
Mass (g)
Volume (ml)

Record any qualitative changes (in color, texture, etc.) you noticed in the potato cores.

Observations/Data Analysis:
Activity 2: Potato Osmosis

1.Calculate the “Change” by subtracting “Day 1” from “Day 2.Calculate the “% Change” by dividing the “Change” by “Day 1” (NOTE: Keep any calculations which are negative do not use the absolute value). 3.Graph the % change in mass on one graph and the % change in the volume on a separate graph. 4.Correctly label the Y (vertical) axis and the X (horizontal) axis of each graph.

Activity 1: Diffusion of Starch
1.Sketch the experiment, use arrows to show the way the diffusion was occurring.

Results/Conclusion:
Activity 1: Diffusion of Starch
Observing the diffusion process in real cells is difficult because they are too small to be seen easily. In this activity you created a giant model of a cell so that you can observe the effects of diffusion through a membrane. In your cell model describe the role of each of the following parts: a.Ziploc bag:

b.contents of the bag:
c.area outside the bag:

Activity 2: Potato Osmosis
1.In this experiment, why was it important that the potato cores were the same length? 2.Why was it important to cover each beaker with a piece of aluminum foil? 3.Into which of the potato cores did water flow? From which of the potato cores did water flow? How can you tell? 4.Which solutions (if any) were hypertonic, isotonic, or hypotonic? Explain how you know.


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