Red Onion Diffusion Lab
Osmosis and Water Flow Through Cell Membranes
Introduction: Water is a polar molecule - and the cell membrane also contains polar molecules in the
lipid bilayer (see Life Occurs in Water, the Water Lab and your cell membrane notes and models).
Charged particles and molecules are said not to be able to pass across a cell membrane. If Water does
flow across the membrane then osmosis is occurring (see Osmosis Demo).
Guiding questions: 1) Does water flow across the cell membrane?
2) If water does flow across the cell membrane then how will it flow (to the inside or
to the outside of the cell)?
Hypothesis: Construct a hypothesis with predictions that could test these questions.
Materials:
Red Onion
1 Glass Microscope Slide
Solution A (Distilled H2O)
Solution B (Distilled H2O and Salt)
Eye Droppers (for each Solution
1 Compound Microscope
Procedure:

Take a strip of onion and break it on the white side so that the red side is still intact and you can
peel the red layer away from the other cells

Take the thin red layer and place it flat in the center of a glass microscope slide. Do not apply a
cover slip.

Bring into focus at low power and look for a group of single cells at the edge of the section that
are unbroken. Move those cells to the center of the field of view. Increase magnification and
focus to 400X

Draw that group of cells as it appears now. Label any visible features, colors and record the
magnification and subject.

Place a few drops of salt water on the cells and wait 5 minutes

Draw and label changes that you see in the field of view

Place a few drops of distilled water on the slide and wait 5 more minutes

Draw and label and changes that you see in the field of view
Assignment Instructions.
Include the following in your (abbreviated) lab report
Title:
Question:
Hypothesis: “If…then…because” for both solutions (distilled water and saltwater). Predict which way
the water will flow in both cases (to the inside or outside of the cells).
Think of your Osmosis
Demo.
Analysis Questions
1. Describe the changes that you observed in the cells.
2. Describe the changes that occurred in the cells in terms of osmosis (for both the distilled and
freshwater solutions. (Hint: again, think of your Osmosis Demo).
3. Was your hypothesis correct? Explain why or why not supporting your conclusion with examples
from your observations. Remember to address the results of both the distilled water and
saltwater.