THE CELL MEMBRANE
INTRODUCTION
All cells, from all organisms, are surrounded by a cell membrane. The cell membrane is a thin
layer of lipid, protein, and carbohydrates that separates the cell’s contents from the world
around it. A stack of 10,000 membranes is about equal to the thickness of this page.
Membranes are made mostly of phospholipid molecules, a special type of lipid molecule.
Phospholipids are a kind of fat made from glycerol, two fatty acids, and a phosphate group. The
phospholipid “head” is composed of the phosphate group and the glycerol. The “head” is polar
and hydrophylic, meaning “water loving”. The phospholipid “tail” is composed of the two fatty
acids. The “tail” is nonpolar and hydrophobic, meaning “water fearing”. Examine how the
complex structure is represented by a simple model in the diagram below.
A variety of proteins are embedded in the cell membrane. Some are on only one side of the
membrane. Other proteins go all the way through the membrane. Proteins help materials move
in and out of the cell.
Carbohydrates are attached to some of the proteins at the surface of the cell membrane. These
carbohydrates may act as chemical identification cards for other cells.
carbohydrate
phospholipids
READING QUESTIONS
1. List three types of molecules that make up the cell membrane:
_______________________, ________________________, and ______________________
2. A stack of about ____________ membranes equals the thickness of this page!
3. What is the special type of lipid that composes most of the membrane? _________________
4. List the three types of molecules that make up a phospholipid molecule:
_______________________, ________________________, and ______________________
5. Make a drawing of the simplified version of a phospholipid molecule.
6. The phospholipid “head”:
a. What two types of molecules compose the “head”?
________________________ and ________________________
b. How does the “head” behave around water? ________________________________
7. The phospholipid “tail”:
a. What type of molecule composes the “tail? ________________________
b. How does the “tail” behave around water?
________________________________
8. Proteins
a. Where are proteins found in the membrane? ________________________________
b. What is the function of the proteins? ______________________________________
9. Proteins
a. Where are carbohydrates found in the membrane?
b. What is the function of the carbohydrates?
__________________________
________________________________
MODEL
The cell membrane faces water on the exterior and on the interior. Because of their
behavior around water, cell membrane phospholipids arrange themselves in a very
unique pattern. In this part of the activity, you will have the opportunity to learn
how and why the phospholipids arrange themselves so uniquely.
1. Examine the “cell” on the back of this page.
a. Label water on the interior of the cell (cytoplasm is mostly water)
b. Label the cell membrane (the racetrack–like diagram)
c. Label water on the exterior of the cell (cells are bathed in water)
2. Arrange the phospholipid models on the cell membrane.
a. Remember which part of the phospholipid molecule is hydrophobic vs hydrophilic.
b. Fill the entire cell membrane diagram with phospholipid models.
3. Have your teacher sign off on your model: ___________________________________
4. How will you remember the structure for the TEST?
Draw phospholipids, proteins, and carbohydrates on a segment of the membrane diagram.
5. Now the challenging part: explain why you arranged the phospholipids in a unique way.
__________________________________________________________________________
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