Membrane Function Lab Planning Center

Semipermeable membrane
Image from Wikimedia Commons

As you and your group prepare for the Membrane Function Inquiry Investigation, there are several things you need to consider before you begin. This page has been designed to help you collaborate with your fellow group members to prepare for and document your experiment.

Diffusion is the spontaneous tendency of molecules to spread apart and move from areas of high concentration to low concentration. Passive transport occurs when substances diffuse across biological membranes. Biological membranes are selectively permeable, which means that some substances can pass through the membrane and some cannot. Passive transport does not require energy, distinguishing it from active transport, which does require energy.

2625 Aquaporin Water Channel
Aquaporins allow water molecules to cross a phospholipid bilayer
Image from Wikimedia Commons

Osmosis is a special type of passive transport involving the movement of water molecules down of concentration gradient across a selectively permeable membrane. Osmosis occurs across the plasma membrane that surrounds all living cells. Most of the diffusion of water through the plasma membrane occurs through special channel proteins called “aquaporins.” Therefore, osmosis through a living membrane is actually an example of facilitated diffusion.

In this investigation, dialysis tubing membranes, living potato cells, living carrot cells, and living Anachris cells will be available to demonstrate the properties of a selectively permeable membrane in relation to osmosis. Dialysis tubing membranes are made of purified cellulose containing microscopic pores. The pore size is controlled during manufacturing and determines the membrane’s permeability to molecules of different sizes. Typically, dialysis tubing will only allow smaller molecules to pass through the membrane, while larger molecules will remain in the dialysis tubing. Potato cells, carrot cells, and Anachris cells are surrounded by selectively permeable living membranes composed of phospholipid bilayers with embedded aquaporins allowing diffusion of water through them.

Dialysis Tubing is Selectively Permeable

But how do we know that water can cross these membranes? Will osmosis occur through the dialysis tubing? Will osmosis occur through the living membranes of potato cells, carrot cells, or Anachris cells?

Planning Your Experiment

Your group will need to choose one of the provided membrane sources and develop an experiment or series of experiments that will allow you to answer the following research question:

Does osmosis occur across the membrane?

Remember that a controlled experiment should test a single independent variable. In order to draw a meaningful conclusion, you need to be able to compare the outcome of your experiment with what would happen if you did not manipulate the variable you have chosen to test.

With all of this in mind, consider the following questions to help you get started:

  • What variable will you be testing (independent variable)?
  • What variable(s) will you be measuring (dependent variable)?
  • What variables will you hold constant (constant variables)?
  • What evidence would confirm that water has diffused across the membrane?
  • How will you be confident in the validity of your results?
  • What will you use as a standard of comparison (control group)?

Navigate to your group’s page below to develop the plan for your experiment.

Select Your Page Below

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Group 1Group 1
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Group 7Group 7
Group 8Group 8

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