Fatty acids, Soaps, Micelles, and the Cell Membrane

Electronegativity, Polarity, and Hydrophobicity

Polar molecules are formed from an inherent electrical or charge difference between the atoms in the molecule. This leads to the concept of electronegativity, Section 11.6. Polar molecules dissolve within each other where this phenomenon is known as hydrophobicity. The most common polar solvent is water and polar molecules that are hydrophilic, “water loving”, readily dissolve in water. Table salt, or Sodium chloride, is a strongly polar molecule and thus it readily dissolves in water.

Conversely, nonpolar molecules do not possess an inherent electronegativity, either because there is no difference in charge between neighboring atoms or the geometric symmetry of the atoms within the molecules causes the charges to cancel each other. Strongly nonpolar molecules include many of the organic molecules, like alkanes and alkenes, such as propane and 1,3-dioctene. Nonpolar molecules dissolve in each other and are known as hydrophobic, “water hating”, molecules. For instance, paint pigments dissolve in mineral oil, but certainly not in water.

In summary, the old saying, “like dissolves like,” comes from the polar-nature or hydrophilic molecules associating and dissolving within each other, and in like manner for the nonpolar-nature or hydrophobic molecules. Mixtures of hydrophilic solutions with hydrophobic solutions will always separate into the two respective layers once stirring ceases. The underlying understanding of this phenomena is the inherent electronegativity throughout a molecule leading to a polar or nonpolar molecule.

Fatty Acids

Fats and oils are essential organic molecules that are necessary for every living cell, and thus life in general. They derive from fatty acids that are long linear carbon chains, typically 4 to 20, with one end containing a carboxylic acid functional group. In fact, you can think of fatty acids as being basic alkanes, such as butane and octane, or alkenes, such as 1,3-dibutene and 3,4,7-tridocene, that have a carboxylic acid at one end. Your textbook shows excellent examples of oleic and linoleic acids in Section 20.3, Lipids.

The long carbon end of the fatty acid is very hydrophobic, “water hating”, and thus will not dissolve in water. Fatty acids, fats, and oils tend to avoid and separate out of water to create two layers. The bottom layer is water and its hydrophilic solutes, and alkanes, alkenes, oils and fats in the top layer, along with the other hydrophobic solutes. This basic hydrophilic/hydrophobic nature of molecules is the basis of the saying, “Oil and water don’t mix.”

All food that we eat contains fatty acids, fats, and oils. Upon digestion, nearly all fats and oils, or triglycerides, will be broken down into the constituent fatty acid chains and glycerol. The fatty acid chains are readily absorbed through the intestinal wall and into the blood stream. Since blood is primarily a water-based solution, hydrophilic in nature, fatty acids tend to separate and thus clump along the inner membranes of the blood vessels. Years of this clumping can cause obstructions and restrict blood flow through the vessels. This is the reason why cardiologists suggesting a low fat diet.

Fatty acids that resemble alkanes, those with no double or triple bonds, are long straight chains. Couple this chain geometry with the strong hydrophobic nature of alkanes and it is apparent that the clumping and density of these molecules is great. One can visualize these saturated fatty acids packing tightly like large piles of construction wood neatly stacked together. These saturated fatty acids produce the densest and hardest clumps within the blood vessels. These clumps are the hardest for the body to dissolve or remove from the lining of the blood vessels.

However, dieticians realize that fatty acids are necessary for proper cellular growth, maintenance, and health, in general. A balance has been found between a purely zero fat diet to a fat gorge. Unsaturated fats, on the other hand, resemble alkenes, in which the double bonds cause bends and kinks in the long chain. The kinks prevent the fatty acids from tightly packing and thus from forming dense hard clumps. It is much easier for your body to dissolve these less dense clumps from the lining of the blood vessels. Luckily, the medical profession has realized this more balanced and proper approach to better health through the eating of better fats, those high in polyunsaturated fatty acids.

The carboxylic acid end of the fatty acid is very hydrophilic, “water loving”, and thus dissolves readily in water. And, the long carbon end just after the hydrophilic end is very hydrophobic, “water hating”, and thus only dissolves in organic solvents or oils. From these chemical properties, it can be seen that a single fatty acid molecule is both hydrophilic and hydrophobic, each end is able to dissolve in its respective solvent, either water or oil, respectively.

For example, first mix water and decane and then allow the two to separate into the two hydrophilic/hydrophobic layers. Add some fatty acids to the container. The fatty acids will find the thin interface between the decane and water. The fatty acids will naturally align themselves such that the hydrophobic long carbon ends will point upwards into the decane layer and the hydrophilic carboxylic acid ends will point downwards into the water layer.

Soaps and Micelles

The natural alignment and orientation of the hydrophilic/hydrophobic fatty acid in a mixed water/organic solution is also indicative of all soap molecules. Soap molecules are very similar to fatty acids in that they have the same hydrophilic/hydrophobic characteristic in each molecule. The only real difference between a fatty acid and a soap is that the carboxylic acid group is replaced by a RO^-Na⁺ group, where the R is the long alkane or alkene carbon chain. In solution, the Sodium ion dissolves off into the water layer and the RO^- group is a free molecule. It will align and orient itself in the same fashion as fatty acids.

From the latter example of the water/organic solution, soap molecules will also align and orient themselves between the hydrophilic/hydrophobic layers, as did the fatty acids. If this naturally aligned and layered solution is vigorously stirred and mixed, the layers will disperse. However, the powerful hydrophilic/hydrophobic forces will still tend to align and orient the molecules. Very small droplets of organic solvent, the alkanes or alkenes, will form with the hydrophobic ends of the fatty acids and/or soaps pointing towards the organic droplet. The hydrophilic ends of the fatty acids and/or soaps will then point outward towards the water solvent. Since the organic droplets are semispherical in shape, then the fatty acids and/or soaps will also form a ball-like shell around the organic droplet, also forming a ball. This ball, with its inner organic core, the hydrophobic ends aligned internally and the hydrophilic ends aligned externally to the outer water solvent, is known as a micelle. Every time you use soap in the bathroom or kitchen, you are using micelles to carry or transport organic droplets away in a solvent of water. Therefore, fatty acids and soaps allow large nonpolar organic molecules to collidially suspend in water, which cleans your hands or dishes.

Cell Membranes

The cell membrane is the outer-most sheath of all living cells, from bacteria, to molds, to yeasts, to human liver or brain cells. It distinguishes the inside from the outside of the cell. Ninety nine percent of all chemical processes occur within the cell membrane, within the cell. The cell membrane acts as a barrier between the outside environment and the inner cell molecules and organelles. If the cell membrane ruptures, the cell dies.

The cell membrane is composed of a phospholipid bilayer. A phospholipid is a single glycerol bonded to two fatty acids and a single phosphate ion (PO₄^-3), or

where the R and R’ groups are the long hydrophobic fatty acid chains, as shown below

or

where this more complex phospholipid is similar to fatty acids and soaps in that each molecule has a polar hydrophilic end and a nonpolar hydrophobic end.

Since the cell is mostly water within and without the cell, the phospholipids align and orient themselves into a bilayer in which the polar hydrophilic phosphate group points outward from the center of the bilayer. Or, the phosphate groups form the very outer and very inner surfaces of the cell itself. The hydrophobic fatty acid chains point to the inner center of the bilayer or towards each other. The cell membrane, therefore, looks like

where the hydrophobic tails are the stringy lines pointed towards each other and the outer ball-like phosphate groups are aligned top and bottom. And, with the greatest detail

and finally,

As can be seen from the above diagrams, the cell membrane is a sea of phospholipids arranged in a bilayer configuration to fulfill the powerful hydrophilic/hydrophobic forces and yet provide a viable interface between the outside and the inside of all living cells. You can also infer from the diagrams that many substances, such as proteins and carbohydrates, permeate the cell membrane to help facilitate transport of materials in and out of the cell or chemically or hormonally communicate with other cells throughout the organism.

References

Baum, S., Introduction to Organic and Biological Chemistry, 3^rd Edition, MacMillan Publishing Company, Inc., New York, (1982).

Darnell, J., Lodish, H., Baltimore, D., Molecular Cell Biology, Scientific American Books, New York, (1986).

Lehninger, A.L., Principles of Biochemistry, Worth Publishers, Inc., New York, (1982).

Trefil, J., Hazen, R.M., The Sciences, An Integrated Approach, 3^rd Edition, John Wiley and Sons, Inc., New York, (2001).