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Diffusion and Osmosis Lab Report
Instructions: In this lab activity, you will investigate membrane permeability to specific substances and how that affects diffusion. You will also investigate how the concentration of a solution will affect the water potential of a plant cell. Lastly, you will create your own investigation to test the effects of cell size or temperature on the exchange of materials between cells and the environment. Submit your lab report to your instructor when completed. Title: Diffusion and Osmosis Lab Report Objective(s): ● Explain concepts of diffusion and osmosis ● Investigate how the concentration of a solution will affect the water potential of a plant cell ● Investigate how membrane permeability affects diffusion ● Test for the movement of molecules using indicator chemicals Hypothesis: As described above, there are three parts to this investigation, each with different variables. Review the procedures section. Then write a prediction for each part of your investigation. Be sure to compare your independent variable to your dependent variable using an "if /then” for each hypothesis statement. Lab Safety: ● Wash your hands before and after handling lab materials. ● It is important to always protect your eyes during lab activities. Wear protective goggles whenever possible. ● When working at the kitchen counter or table, be sure to protect the workspace from spills and clean up any splashes or spills immediately. ● All materials in this lab activity are safe to dispose in the garbage (solids) and down the drain (liquids only). Procedure:
A list of materials and summary of steps is provided for you in part 1 and 2. However, you are responsible for the summary of steps in part 3. Materials: Four large plastic cups or bowls Eight small cups/bowls Two thin non-durable zip-lock plastic sandwich bags (the generic version work best) Measuring cups and spoons Iodine ¼ cup cornstarch or small potato Strainer (optional) Paper towels 100 mL distilled water 60 g or 5 tbs sugar (sucrose) One potato One of the following: sweet potato, pear, or apple Aluminum foil or plastic wrap Knife Metric ruler Summary of Steps: Part 1: Cell Permeability Controlled variables: distilled water Independent variable: whether a solution has starch
- Fill the two large cups or bowls half-full of water.
- Label each cup based on the contents of the bag it will contain: "starch" and "water."
- Add three drops of iodine into each cup of water and stir. Observe and record the initial color in your data table.
- Carefully place each bag in the appropriate cup of water/iodine solution.
- Record your hypothesis about what you expect to observe in each cup. Be sure to include a justification for your hypothesis. Hypothesis: I expect the bowl with the starch solution to have the surrounding water to be blue/black. I expect the bowl with just water to stay a clear color.
- After 30 minutes, observe the bags and surrounding water for any color change. · Iodine and starch react together to form a dark blue/black solution. · A color change to a dark blue/black indicates the presence of starch.
- Record all results in Table 1. Part 2: Water Potential Controlled variables: distilled water solution Independent variable: size of the cubes of potatoes and apples Dependent variable: % change in volume
- Prepare the following solutions, two of each, in the eight small cups or bowls. Stir each solution until the sucrose dissolves. A. Make 2 cups/bowls of Solution A: 1 cup of distilled water, no sucrose added · Label one cup "Potato A" · Label the other "Sweet Potato or Pear or Apple A." B. Make 2 cups/bowls of Solution B: 1/2 teaspoon of sucrose in 1 cup of distilled water
· Label one cup "Potato B" · Label the other "Sweet Potato or Pear or Apple B" C. Make 2 cups/bowls of Solution C: 1 teaspoon of sucrose in 1 cup of distilled water · Label one cup "Potato C" · Label the other "Sweet Potato or Pear or Apple C" D. Make 2 cups of Solution D: 2 teaspoons of sucrose in 1 cup of distilled water · Label one cup "Potato D" · Label the other "Sweet Potato or Pear or Apple D"
- Cut four cubes (approximately 1 cm × 1 cm) of the potato and four cubes of the sweet potato (or pear/apple). Be sure that all of your cubes are the same size and none of the cubes have any skin on them.
- Measure the length, width, and height of all eight cubes after they have been cut. To find the volume of each cube, multiply the three values and record the initial cube measurements in Table 2.
- Place one potato cube in each of the cups labeled Potato A, B, C, and D. Do the same for the sweet potato/pear/apple cubes in cups A, B, C, and D. Each cup or bowl should only have one cube inside.
- Cover the cups tightly with plastic wrap or aluminum foil and let sit overnight.
- Record your hypothesis about what you expect to observe on Day 2 and how you believe the two types of potatoes will compare. Be sure to include a justification for your hypothesis, based on the sugar content of each potato (or fruit). Hypothesis: I expect both the potatoes and apple slices to absorb water by osmosis and have a larger volume. The more sugar the solution has, the less water I expect the potatoes and apples to absorb because osmosis will try to make the solution and apples or potatoes isotonic to each other.
Dependent variable: % increase in volume Hypothesis: I expect the smaller cubes to have a greater percent increase in volume because of its greater surface area to volume ratio. Data: Complete the table to organize the data collected in this lab. Don’t forget to record measurements with the correct number of significant figures. Table 1 Bag Content Initial color of water/iodine solution in the cup Final color of the solution in the cup Starch and Water (^) clear blue Water (^) clear clear Table 2 Substance Potato Apple Contents in cup/bowl A: Distilled water B: ½ tsp. sucros e C: 1 tsp. sucros e D: 2 tsp. sucros e A: Distilled water B: ½ tsp. sucros e C: 1 tsp. sucros e D: 2 tsp. sucros e
Initial cube volume (cm^3 ) 1 cm^3 1 cm^3 1 cm^3 1 cm^3 1 cm^3 1 cm^3 1 cm^3 1 cm^ Final cube volume (cm^3 **)
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cm^ % change in volume 60% increase 50% increase 60% increase 60% increase 50% increase 25% increase 90% increase 90% increase Calculations:** Show your calculations of percent change for each cup/bowl here. Refer to your AP Science Skills Tutorial in the AP toolkit , practice 5, page 3 for assistance. You will need the ionization constant of sucrose ( i = 1.0). Potato A: (1.6-1/1) x100 Apple A: (1.5-1/1) x Potato B: (1.5-1/1) x100 Apple B: (1.25-1/1) x Potato C: (1.6-1/1) x100 Apple C: (1.9-1/1) x Potato D: (1.6-1/1) x100 Apple D: (1.9-1/1) x Graph: Graph the change in sucrose concentration in solution (x-axis) to the change in percent volume of your potato and fruit cubes (y-axis).
Use this table to collect your data for part 3 of your investigation. If you are testing cell size, be sure to include surface-area-to-volume ratio data. P:A A:A P:B A:B P:C A:C P:D A:D Surface-area-to -volume-ratio
Initial volume 1 cm^3 1 cm^
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cm^ 8 cm^3 8 cm^3 15. cm^
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increase Conclusion: Write a conclusion statement that addresses the following questions: · For part 1, describe the permeability of the sandwich bag to starch. Then to iodine. The sandwich bag is not permeable to starch, but is permeable to iodine. · For part 2, explain what happened to each cell type in response to its surrounding solution. Be sure to include water potential in your explanation. An increase in solute potential (sugar concentration), reduces the capacity of water to move and do work. As sugar concentration increased, water potential should decrease, causing less of an increase in volume. · For part 3, describe your observations and conclusion about your tested variable. ● Does your data support or fail to support your hypotheses (include examples for each)? My data doesn’t support my hypothesis. I hypothesized that the smaller cubes would have the greatest percent change in volume. Instead my results seem scattered and have no correlation to
the initial cube size. Potato A had a 50% increase, while Apple A had no increase. The cubes that had the midrange volume actually saw the most increase. ● Discuss any possible sources of error that could have impacted the results of this lab. Human error could have affected my measurements of the cubes. The apples also floated at the top of the water which would have affected the diffusion of water into the apple. Lab Reflection Questions Answer the reflection questions using what you have learned from the lesson and your experimental data. It will be helpful to refer to your class notes. Answer questions in complete sentences.
- Use the molecular structures of starch and iodine to explain their movement across a permeable membrane in part 1 of the investigation. Iodine has a smaller molecular size than starch which allowed it to cross the selectively permeable membrane. Because starch has a larger molecular size, it couldn’t pass through the membrane.
- Explain if each type of cell (potato, sweet potato/pear/apple) was isotonic, hypertonic, or hypotonic to each of the solutions in part 2 of the investigation. All of the cells in Part 2 were placed in an hypotonic solution which caused the net movement of water molecules to go into the cell. This means that the cells were hypertonic to the solution.
- Based on the observations, which of the vegetables/fruits used in this experiment has a greater amount of sucrose in its cells? Explain your answer. Potatoes have a greater amount of sucrose in its cells because potatoes have a greater amount of percent change in volume than apples. Whichever vegetable/fruit has more sucrose would take in more water because osmosis would try to make the cells and solution have the same concentration of sucrose.
- If you tested cell size in part 3, explain how the surface-area-to-volume ratios of your plant cells affected your results. If you tested temperature, explain how your results can be explained by kinetic molecular theory. The greater the surface-area-to-volume ratio, the greater the increase in volume. The more surface area the cell has compared to volume, the more water can enter the cell due to the greater access to a semi permeable membrane.
