Biochemistry Chapter 2: Water

Water Properties and Biological Relevance

• Water's polarity and hydrogen-bonding capability are essential for life, influencing the structure and function of biological molecules.
• Organisms contain 70-90% water, making it crucial for biochemical reactions, which predominantly occur in an aqueous environment.

Structure of the Water Molecule

• Water follows the octet rule, with oxygen atom covalently bonded to two hydrogen atoms and possessing two nonbonding lone pairs.
• The water molecule exhibits a distorted tetrahedral geometry, resulting in a net dipole moment due to the electronegativity of oxygen, facilitating hydrogen bonding.

Hydrogen Bonds

• Hydrogen bonds are strong interactions between a hydrogen atom and electronegative atoms like oxygen or nitrogen, significant for the unique properties of water.
• These bonds contribute to the structural and functional properties of proteins, DNA, polysaccharides, enzyme-substrate binding, and mRNA-tRNA matching.

Solvent Properties of Water

• Water effectively dissolves charged and polar solutes such as amino acids, small alcohols, and carbohydrates.
• Nonpolar substances like nonpolar gases and aromatic compounds show poor solubility in water due to lower entropy, as water molecules form a structured shell around them.

Fluid Mosaic Model and Membranes

• The lipid bilayer forms the basis of cell membranes, incorporating proteins like CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) which is linked to cystic fibrosis, a condition affecting multiple organs.

Ionization of Water

• Water ionizes to H⁺ and OH⁻, with the equilibrium constant (Keq) represented by [H+][OH-]/[H2O].
• At 25°C, Keq is 1.8 x 10⁻¹⁶ M, and the ionic product of water (Kw) is 1 x 10⁻¹⁴ M².

Important Concepts

• The matching of mRNA and tRNA is essential for protein synthesis.
• Understanding the different types of molecular interactions (hydrogen bonds, ionic, hydrophobic, van der Waals) is critical in biochemistry.
• Nonpolar and amphipathic molecules behave distinctly in water, influencing interactions and solubility.
• Micelles, membranes, and lipid bilayer models illustrate important structural and functional aspects of cellular components.
• The CFTR protein mutations can lead to cystic fibrosis, diagnosed typically with sweat tests.

Key Questions for Review

• Differentiate between hydrogen bonds and ionic, hydrophobic, and van der Waals interactions.
• Explain water's polarity and its ability to hydrogen bond with itself and other biological molecules.
• Describe the characteristics of nonpolar and amphipathic molecules in aqueous solutions.
• Analyze solubility trends relative to polarity and water.
• Detail the structure and function of the CFTR molecule and its association with cystic fibrosis.
• Examine the ionization of water and the relationship between Keq and Kw.