Hydrophobic and hydrophilic interactions between biological molecules, such as proteins, are shaped by the energy of the bonds formed when the molecules collide as a consequence of thermal movement. Interactions with water dominate in a cell, so formation of hydrogen bonds with water is a dominant characteristic of molecules and determines the structure of cell membranes, as well as meringue bubbles.
Proteins Are Long Polymers of Hydrophobic and Hydrophilic Amino Acids
Typical proteins, such as cellular enzymes or egg white proteins, e.g. ovalbumin and lysozyme, are synthesized in cells by adding one amino acid at a time, like beads on a string. In proteins, some of the amino acids form hydrogen bonds with water (hydrophilic), whereas others do not (hydrophobic). Dissolved in water a protein normally folds up in the lowest energy configuration with the hydrogen bonding amino acids on the outer surface and the hydrophobic amino acids on the inside.
Proteins Dissolved in Water Have Hydrophobic Amino Acids Inside
Proteins are constantly battered by the kinetic energy of their surrounding water. As a result, the proteins continue to shake and change shape. As the proteins reach a stable, minimum energy configuration, it takes more and more energetic collisions to change their shape. That is, as the proteins form the maximum number of bonds with water, they have released the maximum amount of energy during bond formation. Changing the configuration of a molecule that has stabilized typically requires more energy than in a single or a few colliding water molecules.
Proteins Turn Inside Out on Water Surfaces
If a protein comes to the surface of water, half of the protein remains exposed to water, but the other half is no longer able to form hydrogen bonds, since the air contains relatively few molecules. Shaking and shape changing of the protein now results in a new minimum energy configuration with the hydrophobic amino acids facing the air and the hydrophilic amino acids hydrogen bonding with water. In this configuration the protein is similar to a soap molecule that also orients the same way at a water surface, with its long hydrophobic tail in the air and its hydrophilic group, e.g. carboxyl (-COOH), hydrogen bonding in the water. The molecules of protein or soap form a single layer across the surface of the water.
Bubbles Form with Back-to-Back Surface Layers of Molecules
A bubble loop can be dipped into a soap solution and as the loop is withdrawn, the surface layers of soap on each side of the loop come back on each other, so that water is trapped in a thin film between the soap layers. Each surface layer of soap hydrogen bonds in the center with the water and the hydrophobic tails face the air. This is a strong structure for just being a few molecules thick, because the surface tension of the water in the middle holds it together.
Meringue Is Bubbles with Two Layers of Protein Separated by Water
A meringue is similar to a soap bubble, except that the soap is replaced by the proteins of the egg whites. Sugar is whipped into the eggs whites to increase the viscosity of the water and to link the two layers of protein as the water is removed by gentle cooking. When most of the water is removed, the meringue becomes stiff, because the sugar spans and forms hydrogen bonds with proteins on each of the two faces of the egg white bubbles. A cross section of a meringue bubble would be: air outside, hydrophobic amino acids of outside protein, hydrophilic amino acids, sugar, hydrophilic amino acids of inside protein, hydrophobic amino acids, inside air.
Cell Membranes Are In-Side-Out Bubbles Made of Phospholipid Bilayers
Bubbles and cell membranes are held together by the surface layers of water. The water layers in between to soap layers holds a bubble together. Membranes have hydrogen bonding water layers on the inside and outside. These water layers hydrogen bond to hydrophilic portions of oriented layers of phospholipids. The hydrophobic lipid tails of the two layers of phospholipids come together and are concealed in the center of the membrane. Surface tension in the layers of water molecules on the inner and outer surfaces of membranes, hold the membranes together.
By understanding the molecular basis of meringue bubble formation, a biology student can also understand how cellular membranes are structured and function.
Reference
Alberts, B. et al. 2008. Molecular Biology of the Cell, 5th ed., Garland Science.
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