CELL MEMBRANE STRUCTURE
The cell membrane is made up mainly of proteins and lipids. The main lipids in the cell
membranes of animals, plants, protists, and bacteria are the phospholipids. A phospholipid is
made of a “water-loving” or hydrophilic head and “water-fearing” or hydrophobic tails. The cell
membrane is made of two layers of lipids. The structure of the phospholipid molecules gives
the membranes its two-layer organization. In the lipid bilayer, the hydrophilic heads of the
phospholipids face the inside and outside of the cell, where there is a water environment. The
hydrophobic tails of the phospholipids face the interior of the membrane, which is protected
from the water environment.
Labeled drawing of a bilayer
PROCEDURE:
1. Read the information, “Cell Membrane Structure”.
2. To make the model, begin by filling the tray approximately 1/3 full with bubble
solution.
3. Thread the cotton string through the straws to make a rectangle about ¾ of the size
of the tray. Knot the ends.
4. Form a film of bubble solution on the straw by submerging the rectangle into the
solution. Then, slowly draw it up at a 4.5 degree angle. Gently move the rectangle up
and down and observe the bubble film. In your science notebook, record:
a. Your observations of the bubble film.
b. What characteristics of a cell membrane do you observe?
5. Reform a bubble film if necessary. Insert a dry pencil into the bubble membrane. In
your science notebook, record:
a. Your observations.
b. What characteristics of a cell membrane do you observe?
6. Remove the pencil and dry it completely with a paper towel. Reform a bubble film if
necessary. Then insert the dry plastic tube into the bubble membrane. In your
science notebook, record:
a. Your observations.
b. What characteristics of a cell membrane do you observe?
7. Reform a bubble film if necessary. Now coat the plastic tube completely with the
solution in the tub. Insert the coated tube into the bubble membrane. While the tube
is inserted in the membrane, pass a dry pencil through the plastic tube. In your
science notebook, record:
a. Your observations.
b. What characteristics of a cell membrane do you observe?
8. Move the plastic tube slowly and carefully around the bubble membrane. Insert a
second coated object, such as a straw, into the bubble membrane and move it
around the bubble membrane also. In your science notebook, record:
a. Your observations.
b. What characteristics of a cell membrane do you observe?
9. Carefully remove the objects from the bubble membrane
10. Next, cut a piece of cotton thread 6-7 centimeters in length. Knot it to form a small
circle.
Note: be patient and gentle during steps 11 and 12. If your bubble breaks, place it back in
the bubble solution to form another bubble.
11. Form an opening in the bubble membrane by floating a circle of thread on the film
and popping the inside of with the rounded end of a paper clip. In your science
notebook, record:
a. Your observations.
b. What characteristics of a cell membrane do you observe?
12. Use the straight end of the paper clip to gently remove the circle of thread from the
bubble membrane. In your science notebook, record:
a. Your observations.
b. What characteristics of a cell membrane do you observe?
ANALYSIS:
1. What does it mean to say that the cell membrane is fluid?
2. A. What does it mean to say that the cell membrane is a selective
barrier?
3. How well did the models of the cell membrane work to simulate
the structure and function of a cell membrane?