Name: ____________________________ Period: ______ Date: ________________
Objectives
In this laboratory experiment, students will be able to observe the process of osmosis and
distinguish between hypertonic, hypotonic, and isotonic solutions.
Purpose
You are a laboratory assistant who has been selected to be part of newly formed research team.
You have been asked to give a lecture on the process of osmosis. To show that water molecules move in and out of a cell by osmosis, you will set up the following demonstrations.
Background
Cell membranes help organisms maintain homeostasis by controlling what substances may enter or leave cells. Some substances can cross the cell membrane without any input of energy by the cell. The movement of such substances across the membrane is known as passive transport .
The simplest type of passive transport is diffusion. Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. A second type of passive transport is osmosis. Osmosis is the process by which water molecules diffuse across a cell membrane from an area of higher concentration to an area of lower concentration. Molecules must move about in a cell in order for a cell to survive.
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Plasmolysis is the shrinking of the cytoplasm of a plant cell in response to diffusion of water out of the cell and into a hypertonic solution (high solute concentration) surrounding the cell as shown in the figure below. During plasmolysis the cellular membrane pulls away from the cell wall. In the next lab exercise you will examine the details of the effects of highly concentrated solutions on diffusion and cellular contents.

Red Onion Membrane in Distilled Water and Salt Water Solutions
1.
Prepare a slide of red onion membrane. Add 2 drops of distilled water , cover with a coverslip and observe under the microscope ( high power ). Draw a picture of your observations below ( use colored pencils ) and label the cell wall, cell membrane, and cytoplasm .
2.
Using the prepared slide from above and a dropper, place a drop of salt water solution at one edge of the coverslip. Place a small piece of paper towel on the opposite edge to draw the salt water solution under the coverslip. Repeat with a second drop of salt water solution. Observe the onion cells under the microscope ( high power ) for several minutes.
Draw a picture of your observations below ( use colored pencils ) and label the cell wall, cell membrane, and cytoplasm .
3.
Explain ( in detail ) the changes you observed in the distilled water solution. Please be sure to discuss the positioning of the cell wall, cell membrane, and cytoplasm . Refer to your handout on hypotonic, hypertonic, and isotonic solutions for additional help.
4.
Explain ( in detail ) the changes you observed in the salt water solution. Please be sure to discuss the positioning of the cell wall, cell membrane, and cytoplasm . Refer to your handout on hypotonic, hypertonic, and isotonic solutions for additional help.
5.
In the winter, grass often dies near roads that have been salted to remove ice. What causes this to happen?