Membrane Function Lab Planning Center

Semipermeable membrane
Image from Wikimedia Commons

Biological membranes are selectively permeable, which means that some substances can pass through the membrane and some substances cannot. Diffusion is the spontaneous tendency of molecules to spread apart and move from areas of high concentration to low concentration. When diffusion occurs across a biological membrane, it is a form of passive transport. Passive transport occurs when substances diffuse across a biological membrane without an energy boost from the cell. Passive transport does not require energy, distinguishing it from active transport, which does require energy. Active transport occurs when a cell uses energy to force substances to move across a biological membrane.

Yeast is a unicellular organism. It can be used to make bread and alcoholic beverages such as beer and wine. Different species of yeast may cause infections. Yeast cells are smaller than animal and plant cells, but slightly larger than bacteria. Like all cells, the plasma membrane of a yeast cell is selectively permeable.

Congo red is a red dye consisting of small molecules suspended in water and ethanol. It was formerly used to dye cotton but is now used to stain biological tissues for microscopic examination. Small molecules like Congo red can freely diffuse across the plasma membrane of yeast cells. Since Congo red can disrupt yeast metabolism, healthy yeast cells carry out active transport to remove this foreign molecule from their cytoplasm. This means the cytoplasm of the active yeast cells will become colorless over time as the dye is pumped out of their cytoplasm. Unhealthy or dead yeast cells cannot carry out active transport and will be unable to remove the dye from their cytoplasm.

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 in preparation for the the lab.

Your group will need to develop an experiment or series of experiments that will allow you to answer the following research question:

Research Question: How does Congo red move across the plasma membrane of yeast cells?

As discussed in class, the answer to this question has been stated as the following hypothesis:

Hypothesis: The stain moves into the yeast cells through passive transport (diffusion) and moves out of the yeast cells through active transport.

This hypothesis represents a claim that can be evaluated. Your task in this lab is to design and conduct an experiment that will allow you to evaluate the validity of this claim. You will then use scientific reasoning to explain how the evidence you have collected supports or refutes the claim.

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 your 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 variable)?
  • What evidence would confirm that the dye has crossed 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

Hour 1Hour 4
Group 1Group 1
Group 2Group 2
Group 3Group 3
Group 4Group 4
Group 5Group 5
Group 6Group 6
Group 7Group 7
Group 8Group 8

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