Lesson 1. Water and biological membranes

  1. Role of water in living beings.
  2. Structure and polarity of the water molecule: hydrogen bonds.
  3. Dissolution of ionic and polar substances.
  4. Insolubility of apolar substances: hydrophobic effect.
  5. Influence of water on the structure of macromolecules.
  6. Adequacy of the aqueous environment to living beings
  7. Observation on the role of biological membranes as a solvent

To know more:

Nobel Prize in Chemistry 2003 to Peter Agre

For the discovery of water channels “Boy, this thing is found in red cells, kidney tubes, plant tissues; have you considered it might be the long-sought water channel? " "Although the oocytes expressing the 28 kDa protein and the control oocytes looked the same, when transferred from isotonic solution to distilled water all six control oocytes were unaffected whereas all six 28 kDa oocytes immediately exploded like popcorn."

  1. Role of water in living beings. Life originated in water. It is the most abundant molecule in living beings. It is the essential solvent for biological molecules. It is a substrate for many reactions. Many biological molecules acquire their shape by interaction with water molecules.
  2. Structure and polarity of the water molecule: hydrogen bonds. In the water molecule, the electrons in the oxygen valence shell show sp3 hybridization. That is why the two hydrogen atoms are located at the vertices of a oxygen centered tetrahedron and the two pairs of unshared electrons at the two remaining vertices. The molecule is almost spherical. Since oxygen is more electronegative than hydrogen, the bonds in water are polarized. When a hydrogen atom of a polarized bond approaches the electronegative atom of another molecule, it forms a hydrogen bridge. Water molecules form with each other Hydrogen bonds. On average, each water molecule forms 4 bridges in the ice and 3.6 in the water. Water is a polar molecule due to the spatial arrangement of its polarized links. This polarity determines that the molecules interact strongly, which is reflected in many of its physical properties.
  3. Dissolution of ionic and polar substances. Water dissolves substances well polar and ionic. Biological molecules abound polar and ionic groups (OH, SH, COO, NH3+, PO4-) that facilitate its dissolution.
  4. Insolubility of apolar substances: hydrophobic effect. Water hardly dissolves nonpolar substances. Molecules of this type, immersed in water, spontaneously associate, which is known as the hydrophobic effect. The hydrophobic effect is usually explained as follows. Apolar molecules disturb the structure of the water in its proximity decreasing the number of hydrogen bonds. As a reaction, the water molecules arrange themselves into an ice-like structure, thus increasing the number of hydrogen bonds. This arrangement decreases the entropy of the water. When apolar molecules approach each other and come into contact, the total surface area they expose to water decreases and thus the number of hyper-ordered water molecules also decreases, thereby increasing the entropy of the water. This increase in entropy determines that the association of apolar molecules in water is a spontaneous phenomenon. Some molecules are amphipathic (they contain polar and apolar groups). When dissolved in water, they associate to form micelles or membranes in which their apolar groups associate 'hiding' from the water.
  5. Influence of water on the structure of macromolecules. Determines the shape of proteins and nucleic acids by the hydrophobic effect (apolar interior and polar exterior). Determines the formation of biological membranes.
  6. Other beneficial properties of water for living beings. Its high specific heat makes it a thermal buffer. Its high heat of vaporization originates the cooling action of sweat. The lower density of ice compared to water makes the ice act as an insulator on the surface of ponds, preventing it from freezing.
  7. Observation on the role of biological membranes as a solvent. Just as water determines the shape of flexible molecules that dissolve and interact with it, biological membranes determine the shape of the molecules that reside in them (by dissolving in them better than in water). The most characteristic membrane proteins are described in lesson 6. It is more appropriate to present the structure, properties and functions of biological membranes in the framework of Cell Biology courses for at least two reasons: membranes are not macromolecules and their functions go far beyond their character of solvent for certain proteins (although many of these functions are possible by virtue of this property).