Today I want to talk about a very special set of chemical bonds that enable the actions of living things; that enable life to do something beyond existing. These bonds are called hydrophilic and hydrophobic bonds.
TEMPERATURE AND LIFE
In an earlier post we pointed out that the stability of life, that is its ability to last a while, is a result of the strength of covalent bonds, the kind of bonds that attach hydrogen to oxygen in water. Molecules, held together by covalent bonds, are very stable at biological temperatures. But we noted that Life must do more than exist; it must also do something. This something is called metabolism. We must have weaker bonds that break more easily to support metabolism.
There are other types of bonds that do have energies similar to the energies of thermal water molecules. These bonds are typically of an electrical nature. Because the energy in these bonds is closer to thermal energy of water molecules, collisions break them apart quite regularly. In fact, when observing these bonds, it is quite difficult to tell that bonding has occurred at all. Yet the effect of these bonds is biologically significant. Let’s take a closer look at this type of bonding.
What happens when a material dissolves in water? Let’s take the example of salt, which is composed of one atom of sodium, , and one atom of chlorine, . When sodium chloride, salt, is placed in water the molecule dissociates into a positive sodium ion, , and a negative chlorine ion, . Quantum effects cause the chlorine nucleus to grab an electron from the sodium nucleus.
The figure illustrates what happens around the sodium ion. The negative oxygen atoms in water are attracted to the positive sodium. The positive hydrogen atoms are repelled by the positive sodium. Something similar happens around the chlorine ion. The positive hydrogen atoms are attracted to the negative chlorine, and the negative oxygen atoms are repelled by the negative chlorine.
This is all very similar to how a particular water molecule interacts the rest of the water molecules. The positive and negative poles of the particular water molecule behave similarly to the explicitly charged sodium and chlorine.
In general, charged or polar molecules behave similarly to water molecules. This allows them to dissolve in water; it allows them to mix with water on a molecular level.
Everyone is aware that oil and water do not mix. It has even made it into the lexicon as a metaphor for incompatibility. No matter how much you stir oil and water, the oil separates from the water and floats to the top. This is called the hydrophobic effect.
The hydrophobic effect is poorly understood. We will present, in this adventure, a primitive picture of the effect. We may go a little deeper in a later adventure.
The effect is a consequence of the fact that the molecules that make up oil, or lipids as they are called in biology, are uncharged and there is very little localized charge anywhere on the molecule. Water molecules, therefore, cannot interact with oil molecules electrostatically, through the attraction and repulsion of charges. The interaction of oil with water is a consequence of oil molecules disrupting the interactions among water molecules.
Lipid molecules repel water molecules because water molecules are able to attract each other better when they are not in the presence of a lipid molecule. I will describe this a little better in the next post.
The display illustrates a single lipid molecule surrounded by four water molecules. The water molecules are in a very fragile state connected to each other by hydrogen bonds. The water molecules encircle the lipid. They do not interact with the lipid electrostatically because lipid molecules are not charged, nor do they have localized charge distributions. This causes the water molecules to lie flat on the lipid molecule with both positive and negative domains of the water molecule against the lipid.
If you click the start button on the lower left, you can see the water molecules find this fragile equilibrium. If the toggle is on, the equilibrium is one in which water molecules encage the lipid molecule. Because this equilibrium is unstable, we used some numerical tricks to direct the system to the state.
If the toggle is turned off, the numerical tricks are turned off and the system reverts to its natural state. In that case you see the water molecules slide off the lipid and move away from the lipid.
The bottom line is that water molecules near the lipid interact with other water molecules near the lipid to cause the molecules to slide away from the lipid. This is the hydrophobic effect.
The hydrophobic (fear of water) effect is sometimes called the lipophilic effect (love of oil) because as water moves away from lipids, lipids are forced together. To an observer, this looks like lipids are attracted to each other. There is no force that causes the lipid molecules to attract each other, however. The attraction is completely due to water molecules avoiding lipid molecules. The clumping of lipid molecules drives the lipid out of solution, which causes the separation of oil and water.
The hydrophilic and hydrophobic effects become biologically interesting when a molecule has both water and lipid characteristics. For instance, there is a class of molecules called proteins that are composed of long chains of smaller molecules called amino acids. Some amino acids are hydrophobic while others are hydrophilic. When proteins are placed in water, the hydrophobic amino acids try to avoid the water, while the hydrophilic amino acids are attracted to the water. This causes the protein to fold into a globular 3D shape with lipid amino acids in the middle of the globule and the hydrophilic amino acids on the surface of the globule.
For those of you who are watching this video as part of the How Water Thinks game, you have some homework. Create a short piece of literature of any kind and submit it through the comments section below. Please keep the length shorter than 200 words. The best submissions will appear on this post. The very best will be included in the game itself.
Read more in Confronting Complexity by Casti, Jones, and Pennock