Investigation 4 – Lab
This Investigation is designed to:
- introduce the concept of solutions of differing concentrations,
- reinforce the operation of the microscope and preparation of a wet mount slide,
- provide a method by which you can distinguish the cell walls and cell membranes of plant cells,
- observe the cellular effects of osmosis and infer the function of the cell wall and cell membrane within plant cells, and
- infer differences in membrane permeability based on microscopic observations of plant cells.
Student Preparation for the Investigation includes having students gather the following materials. This preparation takes place on lab day after student lab groups have settled at their assigned lab tables.
Note: The materials are listed in students’ SDRs. They are also listed below for your reference.
- (1) microscope
- (4) 50 ml beakers
- (4) plastic droppers
- (4) glass slides
- (4) coverslips
- (4) pieces of tape
- (1) paper towel
- (1) forceps
- (1) scalpel
- (4) 50 ml beakers with 20 ml of one of the following: tap water, 2% salt solution, 4% salt solution, and 8% salt solution
Half of the groups in the class will need
- (1) piece of red onion
Half of the groups in the class will need
- (2) Elodea leaves
Note: Half of the student groups in the class will create wet mount slides of onion cells, and the other half will create wet mount slides of Elodea leaf.
Direct one student from each lab group to collect the materials listed in their SDRs.
- Review the concept of osmosis. Include the following in your review.
- Osmosis describes the diffusion of water across a selectively permeable membrane.
- Osmosis occurs when the concentration of solutes on two sides of a membrane is not equivalent and when the solutes cannot pass through the membrane. In such a circumstance, water diffuses through the membrane in a direction that promotes a greater balance in the solute concentrations on both sides of the membrane.
- You will observe how the structure of the cell wall and cell membrane affects the movement of molecules in and out of plant cells. You will do this by placing plant cells in solutions of various concentrations of salt and observing the behavior of the cells.
- Reflect on the PreLab video as you move through the procedural steps.
- During the Experiment, every procedural step is important. If one step is skipped, data can become invalid. To help you keep on track, read each step thoroughly, complete the step, then check it off (Read it – Do it – Check it).
- Complete all of the procedural steps in your SDR.
Note: The procedural steps are listed below for your reference.
- Prepare your plant specimen to make four wet mount slides.
- If using onion, carefully peel a thin layer when preparing the slides.
- If using Elodea leaf, obtain three or four leaves or sections of leaves. The Elodea leaf is only two or three cells thick, so you will be able to place the whole leaf or section on a slide.
Note: students may use the scalpel to cut each Elodea leaf into small pieces, approximately 2 or 3 mm wide, so that the specimens fit well under the coverslip.
- Create four wet mount slides of your plant. Prepare the slides with the following liquids:
- Slide A: tap water
- Slide B: 2% salt solution
- Slide C: 4% salt solution
- Slide D: 8% salt solution
Note: Using a thin permanent marker, you should label each slide by writing on the edge of the slide.
- Compare the concentration of salt in each solution. Which has the most salt? Which has the least salt?
Note: Since the leaves of the Elodea plant contain two layers of cells, it is important that you focus on only one layer of cells at a time to view each cell with ease.
- Record: View each of the four slides. Record your observations by writing or drawing what you observe in the correct columns of Table A.
- Switch slides with another group so that you can observe the plant that you have not yet viewed.
- Record: View each of the four slides. Record your observations by writing or drawing what you observe in the correct columns of Table A
Note: When recording, you should keep in mind the following questions:
- Are the sizes of the cells the same or different?
- Has the arrangement of the cell wall and cell membrane changed?
- Can I distinguish between the cell wall and the cell membrane?
- Has the arrangement of the cytoplasm and other organelles changed?
Be sure to clean up your bench after completing the experiments.