AP Biology: Notes and Practice
Fluidity of Cell Membranes
The membrane lipids of a cell can be labeled with a fluorescent tag so the entire surface of the cell will glow evenly
under ultraviolet light. If a strong laser light is then shone on a tiny region of the cell, that region gets bleached (the
strong light destroys the fluorescent tag) and there is a “hole” in the cell surface fluorescence. After the laser is turned
off, the hole gradually fills in with fluorescent lipids that diffuse in from other part of the membrane. This process is
shown in the figure below.
A
Cell membrane with fluorescent
labeled (star!) phospholipids
Phospholipid is bleached (no star )
Phospholipids continue diffusing,
bleached phospholipid not visible
B
The time it takes for the hole to disappear is a measure of the membrane fluidity. The table below shows data for cells
with altered membrane compositions.
Condition
Time (sec) for “Hole” to Become
Fluorescent
No alteration
65
Decreased length of fatty acid
38
chains
Increased cholesterol
88
Increased desaturation of fatty
42
acid chains
Increased membrane protein
90
content
1. The data table indicates that under normal conditions, it takes 65 seconds for “hole” to become fluorescent.
Because the cell membrane is fluid and phospholipids can move around, this means it takes 65 seconds for another
phospholipid to move into the location that the bleached phospholipid was in initially. In the figure above, which
phospholipid (A or B) is more likely to move into the original spot of the bleached phospholipid? Explain your
reasoning.
2. What overarching question can scientists answer by analyzing the data in the table?
AP Biology: Notes and Practice
3. In addition to lipids, the cell membrane contains proteins.
a. Based on the data in the table, does protein add to a cell membrane’s fluidity, or make the membrane more
rigid? Explain your answer.
b. Based on your knowledge of protein structure, why does your response to part a make sense?
4. Which alterations to the cell membrane allow for faster repair after a hole is made? For each alteration listed,
propose an explanation as to why it allows for faster repair.