5: Water and biomolecules

Section 1: Core Principles - 5: Water and Biomolecules

Water is the fundamental solvent of life, constituting 70-90% of living cells. Its unique properties, arising from its polar nature and ability to form hydrogen bonds, are critical for biological structure and function. Each water molecule ($H_2O$) has a bent shape, with oxygen being electronegative, creating a partial negative charge ($\delta^{-}$) and hydrogen atoms having partial positive charges ($\delta^{+}$). This polarity allows water molecules to form hydrogen bonds with each other and with other polar molecules.

Key Properties of Water:

• Hydrogen Bonding: The attraction between the $\delta^{+}$ H of one molecule and the $\delta^{-}$ O of another creates a cohesive network. This gives water high surface tension, cohesion (water molecules sticking together), and adhesion (water sticking to other surfaces).
• High Specific Heat Capacity: Water absorbs and releases large amounts of heat with minimal temperature change, stabilizing cellular and environmental temperatures.
• High Heat of Vaporization: Significant energy is needed to convert liquid water to vapor, enabling effective evaporative cooling (e.g., sweating).
• Solvent Properties: Water is an excellent solvent for polar and ionic substances ("hydrophilic" molecules) due to its polarity. It forms hydration shells around ions and competes effectively with hydrogen bonds in polar solutes. Nonpolar substances ("hydrophobic" molecules) do not dissolve well in water and tend to aggregate together.
• Density of Ice: Ice is less dense than liquid water because hydrogen bonds form a stable, open lattice. This allows ice to float, insulating aquatic life below.

Major Classes of Biomolecules: Biomolecules, the organic compounds essential for life, are primarily built from carbon skeletons. Their structure and function are deeply influenced by interactions with water.

1. Carbohydrates: Sugars (mono-, di-, polysaccharides) like glucose, sucrose, starch, and cellulose. They serve as energy sources (e.g., glucose) and structural components (e.g., cellulose). Polar hydroxyl (-OH) groups make most carbohydrates hydrophilic.
2. Lipids: Hydrophobic molecules including fats (triglycerides for energy storage), phospholipids (primary components of cell membranes with hydrophilic heads and hydrophobic tails), and steroids (e.g., cholesterol). Their insolubility in water drives membrane formation and fat droplet organization.
3. Proteins: Polymers of amino acids, folded into complex 3D structures crucial for catalysis (enzymes), structure, transport, signaling, and defense. Folding is driven by interactions with water: hydrophilic amino acids face the aqueous environment, while hydrophobic ones cluster inside, minimizing water contact.
4. Nucleic Acids (DNA & RNA): Polymers of nucleotides storing and transmitting genetic information. Their sugar-phosphate backbones are hydrophilic, facing the aqueous cellular environment, while the nitrogenous bases are somewhat hydrophobic, facilitating base stacking within the double helix.