PHOSPHOLIPID TUTORIAL

As you know, all living things are compose of a cells.  Cell are places where chemical reaction occur.  Like test tubes and beakers in a chemistry class, the chemical reactions of life require a container.  In the chime tutorial, you will become familiar with the molecule that composes the container we call the cell.  Specifically, you will learn the characteristics of this molecule in relation to water, and how and why collections of these molecules form the cell membrane.

The following tutorial requires that you have the MDL Chime plugin installed appropriately on your computer.  Read instructions on how to download and install Chime.  The following tutorial will work best if you read and click the buttons that following in chronological order.  Otherwise, the models may not appear as intended.  If you click out of order by mistake, just refresh the webpage through your browser and begin again.

Phospholipid are Schizo

• In the right-hand display, you should see a single phospholipid.  This molecule is currently being displayed in wireframe.
• To view as ball and stick, click the button that follows.  Feel free to rotate the water molecule around by left click holding and moving your mouse.
• As you rotate the phospholipid around, you should notice that one end of the molecule is entirely composed two chains of carbon and hydrogen atoms.  These chains of the phospholipid are called the tails and are known to be hydrophobic (they fear water).
• The other end of the phospholipid contains a number of oxygen atoms, a phosphorus atom, and a nitrogen atom and is referred to as the phospholipid head. The phosphorus the four oxygen atoms to which it is bound form a functional chemical group called a phosphate group.  This group is known to be hydrophilic (it loves water).
• The hydrophilic head loves water because it has a negative charge and is attracted to the slight positive charges on the polar water molecules.
• The hydrophobic tails fears water because the atoms that compose the tails do not have a charge, and thus do not interact with water. 

Phospholipids form Monolayers

• Imagine dropping phospholipids into a beaker of water.  Based on the the fact that the heads of the phospholipid molecules love water, you would expect the molecules to arrange themselves in a single layer with the heads facing down interacting with water molecules while the tails face up away from water (see image below).

Phosholipids form Micelles

• Now, imagine dropping phospholipids into a beaker of water with the added stipulation that they must be submerged below the surface.  At first, this may seem impossible.  How could the tails be satisfied under water?  The solution requires that the phospholipids act together with the water loving heads providing protection to the water fearing tails (see image below).  The phospholipids would actually form in a sphere.  The image below is of a cross-section through such an arrangement.

Phospholipids form Bilayers

• Finally, imagine how the phospholipids would arrange themselves if the resulting submerged structure had to be a container for water too.  Working on the previous resolution, this answer requires that the phospholipids form two layers, the inner layer of phospholipids protecting their tails from the contained water, while the out layer phospholipids continues to protect their tails from the water they are being submerged in.  These phospholipids would also form a sphere, a small portion of which is shown in the image below.
• Interestingly, such lipids bilayers form spontaneously in water and are the underlying structure of all known cell membranes. 

• View a section of the lipid bilayer. 

• View the lipid bilayer in spacefill.  You may be able to notice now that all the atoms at the top and bottom are actually the water molecules attracted to the negatively charged phosphate groups.

• View the previous section without water molecules. 

Summary Questions

1. Diagram and label the chemical schizo nature of a phospholipid.

2. Define the terms hydrophilic and hydrophobic.

3. Explain why the head of a phospholipid loves water and why the tails fear water.

4. Diagram and describe why phospholipids form monolayers in water.

5. Diagram and describe how phospholipids form micelles.

6. Predict how micelles may be used in the transport of particular medicinal drugs.

7. Diagram and describe how phospholipids form the cell membranes lipid bilayer.

8. Based on your knowledge of cell function, why are membrane barriers important?

9. Based on your knowledge of cell function, why is it important that such membrane structures are able to contain water?

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This chime tutorial was created by
Eric Kessler, Biology Teacher at Blue Valley North High School.
Questions or comments to ekessler@bluevalleyk12.org