Liposome

Liposome Definition

A liposome is a closed, spherical lipid bilayer, which forms an internal cavity capable of carrying aqueous solutions. A lipid bilayer is composed of two sheets of tightly arranged phospholipids. These molecules have a hydrophobic tail and a hydrophilic head region. When two single membranes come together, the hydrophobic tails attract toward each other, while the heads of both membranes are attracted to the surrounding water. This forms a double layer of phospholipid molecules, which exclude the internal solution from the outside. The solution can then be transported with the liposome where it is needed.

A liposome should not be confused with either a micelle or a lysosome. A micelle is similar to a liposome in that it is a sphere of phospholipids. However a micelle is composed of a single layer and therefore does not have an aqueous interior. A lysosome is a specialized organelle in cells which separates caustic enzymes from the interior of the cell. While it too is similar to a liposome, it has many specialized proteins embedded in its membrane which help it function as an organelle within the cell.

How Does a Liposome Form?

Liposome can be generated naturally when tissues are disturbed. When tissue is damaged, small pieces of the cell membrane may become detached. The exposed pieces of lipid bilayer folds back on itself, encapsulating a small packet of whatever solution it forms in. This happens because of the hydrophobic and hydrophilic interactions between the pieces of lipid bilayer and the surrounding aqueous solution. It forces the ends of the pieces, where the hydrophobic core is exposed, to come together and create a sealed internal pouch. This process can be replicated in the lab.

Using a sonic wave generator, scientists can use sonic waves to break apart lipid bilayer membranes into any size liposome they want. The sonic waves carry energy, which pulls apart the molecules of the bilayer and separates it into pieces. These pieces are then subject to the same forces that naturally occurring liposomes are created by, and fold into the same shape.

What is a Liposome Used For?

Liposomes have been used as models, to study cell membranes and organelles. By embedding various proteins into the lipid bilayer, scientists can test the function of those proteins by testing the internal solution compared to the external solution. Studies like this helped establish modern cell theory. In fact, liposomes were named after lysosomes because of their similarity to the organelle. By studying non-living, easy-to-watch actions in liposomes, scientists were able to predict and identify the methods cells used to move and transport various chemicals. The actions of the endoplasmic reticulum and Golgi apparatus, in packaging and processing cell products, is directly related to how liposomes interact. Cells simply add various proteins to the surface of their organelles, which direct and control the interactions of various organelles. These processes are now being studied, so that the targeted effect it gives organelles can be extended to artificially created liposomes.

On this front, drugs are being developed which have a liposome delivery method. For instance, certain cancer drugs are packaged in liposomes to be delivered specifically to cancer cells. The theory behind this method is simple. The liposomes are embedded with special proteins, which attach to receptor proteins on the target cell. Once this happens, a process is initiated and the liposome bonds with the target cell, depositing its contents into the cell. Research into liposome delivery systems is expanding into different areas including vitamins, minerals, and even gene therapy. By using targeted liposomes, even DNA can be delivered to specific tissues. If the DNA is functional, it can be read and the protein it encodes for can be produces. The cell can then begin to produce the protein and reverse the deficiency. This process may soon be used to alleviate various genetic diseases.

Other industries are developing liposomes for different uses. Because a liposome is essentially a small cell, it is biodegradable over time but can still carry an aqueous solution in a protected manner. Scientist are working on using this feature to develop liposomes which can carry out complicated tasks. Some of these application include delivering nutrients to crops using liposomes as small machines. If the right “machinery”, or DNA and related proteins, are placed in a liposome it essentially becomes a small living cell which can be programmed to preform various actions. While commercial versions of this are not in effect, much research is being done on this front.

Quiz

1. What is the difference between a micelle and a liposome?
A. A liposome is made of a single layer
B. A micelle has an internal compartment which can store water
C. A liposome is composed of a bilayer, rather than a single layer

Answer to Question #1
C is correct. A micelle is formed of a single layer and has no internal compartment. A micelle is formed from chemicals like soap, which are both hydrophobic and hydrophilic. These tend to form small micelles, which can be felt as the slipperiness of soap on your fingers.

2. True or False. A liposome contains specialized enzymes.
A. True
B. False

Answer to Question #2
B is correct. Technically speaking, a liposome contains anything it was formed around. It could contain those enzymes, but does not have to. Remember that lysosomes are specialized organelles which do contain a variety or specialized digestive enzymes. While these organelles resemble liposomes, they are much more complex because they must interact with a variety of other cellular components.

3. How could liposomes be used to clean up environmental hazards?
A. Impossible!
B. Package the liposomes with enzymes to process the hazard
C. Spray liposomes onto the ocean

Answer to Question #3
B is correct. Several research projects are being established around the idea of using specialized liposomes to clean up environmental toxins. For instance, if the DNA encoding for an enzyme which could breakdown oil were introduced into a liposome along with the machinery to produce the enzyme, the small package could survive a long time, continuously breaking down oil. An added benefit would be that the entire package would be biodegradable itself.

References

  • Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., . . . Matsudaira, P. (2008). Molecular Cell Biology (6th ed.). New York: W.H. Freeman and Company.
  • Nelson, D. L., & Cox, M. M. (2008). Principles of Biochemistry. New York: W.H. Freeman and Company.
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