Laboratory Activity

DIFFUSION AND OSMOSIS

Part I: Surface area to volume ratio on the rate of diffusion*

Part II: Effect of salt on plant cells*

Unit I: Cell Structure and Function

Section 1: Cell Structure and Function

*Teacher Notes: Based on time constraints parts I and II may be performed concurrently or as separate laboratory activities.

Time Allotment:

Part I : 40-50 minutes

Part II: 15-20 minutes

PASS Objectives:

Content Standard (CS): 1.1 The Cell – Cells are the fundamental units of life, composed of a variety of structures that perform functions necessary to maintain life.

Process Standard 1-6

EOI Item Specifications*

Emphasis: Function and structure of the nucleus, cell membrane, cell wall, cytoplasm, ribosomes, mitochondria, and chlorolplasts. Content Limits: Assessable structures are limited above. Test may include both the structures and general functions. Cell membrane will include specific functions to include osmosis, diffusion, and ratio of surface area to volume in cells. Not Tested: terms; eukaryotic, prokaryotic, hypotonic, hypertonic and isotonic. Classify cells as plant, animal, or bacterial, based on assessable structures listed. Part I: Surface area to volume on the rate of diffusion

Textbook Correlation: Biology the Dynamics of life Copyright 2005, published by Glencoe, McGraw-Hill

Part I: Chapter 6 section 2 and Chapter 8 section 2

Part II: Chapter 8 section 1

Part I: Surface Area to Volume Ratio on the Rate of Diffusion

Time Allotment: 40-50 minutes

Learner Objectives:

The student will be able to:

  • Write a testable hypothesis
  • Follow the steps of the scientific method.
  • Measure the distance the iodine moved into the different sized potato cubes
  • Calculate the rate of diffusion
  • Calculate surface area to volume ratio of the cubes
  • Compare amount of penetration for the different size cubes
  • Report the data in an appropriate manner
  • Identify the effect of volume on the rate of diffusion, and how this relates to cell size limitations.

Materials:

(Based on class of 30 working in groups of 3)

  • 5 potatoes, peeled
  • 10 cups, baggies or other suitable containers
  • Iodine solution 1 to 5%, 1.5L
  • Scalpel blades, razor blades or other cutting device
  • Forceps or Tongs
  • Plastic or Latex gloves
  • Plastic or Chemical aprons
  • Safety goggles

Procedure:

  1. Cut your slice of potato into three different sized cubes (ie. 1cm x 1cm, 2cm x 2 cm, 4cm x 4cm) CAUTION: Be careful withsharp objects. Do not cut objects while holding them inyour hand.
  2. Use forceps to carefully place the cube in a cup or baggie containing the iodine solution. The solution should coverthe cubes. Note and record the time. Let the cube stand inthe solution for between 10 and 30 minutes.
  3. Using forceps or tongs, remove the cube from the solution andnote the time. Cut the cube in half.
  4. Measure, in millimeters, how far the iodine solution hasdiffused, and divide this number by the number of minutesyou allowed your potato to remain in the solution. (This is the diffusion rate)
  5. Record data in a data table.
  6. Graph results (Bar graph/histogram recommended)

Data Analysis, Conclusions, and Questions

  1. Identify the dependent variable, independent variable, and the control for this experiment.
  2. Were your results quantitative, qualitative or both? Explain your reasoning.
  3. Were the distances of iodine penetration the same or different? Why?
  4. What is the rate of diffusion for the iodine solution? (To calculate this divide the mm penetrated by the number of minutes that the potato cube was resting in the solution).
  5. Calculate the surface area for each cube Surface area= (edge length)2
  6. Calculate the volume for each cube Volume= (edge length)3
  7. Estimate the percent of penetration of iodine from the center of the potato cube (Example if full penetration =100%, half penetration= 50% etc.)
  8. How does the surface area to cube volume ratio affect diffusion?
  9. Based on this information, infer how cell size is limited by rate of diffusion.

Part II: Effect of Salt on Plant Cells

Time Allotment: 15-20 minutes

Introduction:

When the solutions on each side of a membrane have the same concentration of dissolved material (solute), the two solutions are said to be isotonic to each other. Thus, the concentration of water in the solutions on each side of the membrane are also the same. Water moves rather freely across the plasma membrane of a cell; this process is called osmosis. If the solutions on each side of a membrane are isotonic then an equal amount of water moves out of the cell as moves into the cell, resulting in “no net movement” of water across the membrane. If the solution on one side of the plasma membrane has less solute than is present in the solution on the other side of the plasma membrane, then the solution is said to be hypotonic to the other solution. The Hypotonic solution thus has a greater water concentration in it than does the solution on the other side of the membrane. Thus there will be a net movement of water from the region of greater water concentration to the region of lesser water concentration on the other side until isotonicity (equilibrium) is attained.

Learner Objectives:

The student will be able to:

  • Follow the steps of the scientific method.
  • Observe and describe the process of osmosis
  • Identify the results of hypotonic and hypertonic solutions on the organelles of an elodea leaf.
  • Use the terms hypertonic and hypotonic when relating to the process of osmosis

Materials:

(Based on class of 30 working in groups of 3)

  • Elodea leafs 10-15
  • 10 microscopes
  • 10-15 microscope slide/coverslip
  • 10 dropper bottles of 5% salt solution and distilled water

Procedure

  1. Prepare a wet mount with one Elodea leaf on a microscope slide. (Same leaf used for all steps)
  2. View the leaf. Recordand sketch observationsin data table
  3. Add a drop of distilled water to edge of coverslip. Using a small piece of paper towel on opposite edge of coverslip to draw distilled water across the leaf.
  4. View the leaf. Record and sketch observations in data table
  5. Add a drop of 5% salt solution to edge of coverslip. Using a small piece of paper towel on on opposite edge of coverslip to draw distilled water across the leaf.
  6. View the leaf. Record and sketch observations in data table

Data Analysis, Conclusions, and Questions

  1. Identify the dependent variable, independent variable, and the control for this experiment.
  2. Were your results quantitative, qualitative or both? Explain your reasoning
  3. Describe the process being observed?
  4. If a normal Elodea plant cell is put directly into distilled water, which has no dissolved material (solutes), in which direction is the net movement of water? What would be the proper terminology for the distilled water solution compared to the plant cell?
  5. If a cell is placed into a hypertonic solution, which has more dissolved material (solutes) than in the cell’s cytoplasm, then would you expect any net movement of water? If so in what direction? What would happen to the cell overall?
  6. Compare and contrast the processes of diffusion and osmosis.

Resources:

QuickLab 6.1 pg 155, Biology the Dynamics of life Copyright 2005 published by Glencoe, McGraw-Hill

1