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Plasma Membrane Lab
Group Leader: ____________________ Technician 1: ______________________
Recorder : ________________________ Technician 2: ______________________
Problem:
How can environmental changes affect the plasma membranes of a cell?
Background
All cells have a plasma membrane that separates the cell’s contents from its environment. This membrane is fluid and flexible. It is also selectively permeable, which means that it allows some substances to pass through, but not others. This is important because every cell has different requirements.
A cell must be able to control what molecules may enter and leave in order to maintain cellular homeostasis.
This selective permeability is a result of the composition and structure of the plasma membrane. The plasma membrane consists of a phospholipid bilayer. Phospholipids are molecules that have two distinct regions: a polar head and a pair of nonpolar tails. You will recall that the term “polar” refers to a molecule or some portion of a molecule that has a slight internal charge. Water is an example. The term “nonpolar” refers to a molecule or some portion of a molecule that does not have an internal charge. Most triglycerides (fats, oils, and waxes) belong to this group. A phospholipid molecule is simply a nonpolar triglyceride that has replaced one of its nonpolar fatty acids with a polar phosphate group, commonly called the
“head.”
It is the arrangement of the phospholipid molecules in the plasma membrane that makes it selectively permeable. The polar heads of the phospholipid molecules are hydrophilic (“waterliking”) meaning they are attracted to water. The nonpolar tails are hydrophobic (“waterfearing”) meaning they repel water. The phospholipid molecules are arranged in two layers called a bilayer. However, both layers are not the same. One layer is “upside-down” so that the tails of its phospholipid molecules touch the tails of the other layer. In this manner, the hydrophobic tails are sandwiched between two layers of hydrophilic heads. This arrangement makes it difficult for substances that are dissolved in the cell’s watery environment to pass through the membrane. To do so, they would have to travel through the hydrophobic part of the membrane surrounded by water molecules, a feat that the hydrophobic tails do not allow.
The structure of the plasma membrane was an important adaptation for the first cells. By separating their internal fluids from the outside environment, they could then alter the chemical make-up of their cells and cause certain chemical reactions to occur. However, like many barriers, the plasma membrane is not without its vulnerabilities. Changes in the environment of

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Plasma Membrane Lab the cell can cause the phospholipid molecules to separate, break down, or change shape. When these changes occur, the plasma membrane no longer functions properly. Gaps in the membrane could let unwanted substances into or out of the cell. At worst, the entire plasma membrane could fall apart spilling the cell’s internal fluids into the environment.
In this laboratory, you will investigate how a change in a cell’s environment might affect the plasma membrane. Beet root cells contain a purple-colored chemical called betacyanin. If the plasma membranes of beet root cells are altered or damaged, the purple betacyanin will escape.
You will use beet root cells as your test specimen in this guided laboratory to determine what effect some environmental change might have on the plasma membrane.
A guided laboratory is one in which a set procedure is not given, but rather some suggestions or examples for investigation are provided by the teacher. Using these suggestions and examples, students will brainstorm what environmental changes could affect the plasma membranes of beet root cells and how these changes might affect the cells. After your laboratory group has decided which environmental condition to investigate, it will then decide upon a method for studying the effect that a change in that environmental condition will have on plasma membranes.
Objectives
In this experiment, you will
 investigate how a change in some environmental condition affects a plasma membrane.
 participate in a guided inquiry laboratory as part of a group that plans, develops, and follows a procedure to test an experimental question.
Pre-Lab Questions
1.
What does it mean to say that the plasma membrane is selectively permeable?
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2.
What environmental conditions can you test, given the materials provided for this laboratory?
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3.
What effects might changes to the environmental conditions mentioned above have on the plasma membranes of beet root cells? How might you be able to tell that the beet root cells are affected?
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Plasma Membrane Lab
Materials
This is a complete list of all materials that will be available to your group. You do not have to use all of these materials in your investigation. beet roots salt distilled water pH paper plastic pipettes stirring rods liquid dish detergent plastic knife vinegar baking soda
150-mL beaker(s) isopropyl alcohol
Safety stopwatch balance hot plate
Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles and chemical-resistant gloves and apron. All plant items that have been brought into the lab are considered laboratory chemicals and are for lab use only. Do not taste or ingest any materials in the laboratory after they have been used in the lab. Be careful when working around hot plates and heated substances to avoid burns. Wash hands thoroughly with soap and water before leaving the laboratory.
Hypothesis
Choose an environmental condition to investigate. Then make a prediction about how changing this environmental condition will affect the plasma membranes of beet root cells.
If ___________________________________________________________________________, then _________________________________________________________________________ because ______________________________________________________________________.
Procedure
Explain the procedure that your laboratory group will use to test your hypothesis. Be sure to include a control setup in your experiment. Write your procedure in a step-by-step manner, like a recipe. Do not assume that the readers of your procedure will know what you mean; explain every detail of what you did in the laboratory.
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Plasma Membrane Lab
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Plasma Membrane Lab
Data & Results
Create a data table to record the results of your investigation. A sample table is included below. If possible, create a graph of your data. Data tables and graphs can be attached to this laboratory packet if necessary.
Environmental
Condition (variable)
Observations/Data
Analysis
1. Describe the environmental condition that your group changed.
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2. Using your data table(s), describe any changes you observed or measured in the beet root cells during your investigation.
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Plasma Membrane Lab
3. Compare your results to those of other groups in your class. Describe other investigations that could be made about environmental effects on plasma membranes.
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4. The investigations made by you and your class mates involved only one type of cell.
Do you think your results can be applied to other types of plasma membranes as well? Explain why or why not.
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5. Describe a way that your investigation or that of another group could be applied to a situation in everyday life. For example, orange crops may be lost when freezing temperatures cause water in the plants to form ice crystals that rupture plasma membranes.
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Plasma Membrane Lab
6. Describe any problems you may have encountered during your investigation.
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Did this problem(s) affect your results? If so, explain how.
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Conclusion
Do you agree or disagree with your hypothesis? Why or why not?
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References:
Guided Inquiry Lab Manual. Teacher Edition. Columbus, OH: Glencoe/McGraw-Hill, 2007.