Properties of Water
Water is a Polar Molecule: Water is polar because of the difference in electronegativity between oxygen and hydrogen. Oxygen, being more electronegative, attracts electrons more strongly, creating partial negative (δ-) and partial positive (δ+) charges on the molecule. This polarity allows water molecules to form hydrogen bonds with one another and with other polar molecules. These hydrogen bonds are responsible for many of water’s unique properties, such as high surface tension and its ability to dissolve a wide range of substances, making water an ideal biological solvent.
Hydrogen Bonding: Water molecules can form up to four hydrogen bonds. This bonding contributes to its high melting and boiling points, as well as its heat of vaporization. These properties are crucial for life, as they help in maintaining temperature stability in biological systems. The hydrogen bonds give water a structure that influences the physical properties of biomolecules like proteins, nucleic acids, and lipids.
Water as an Ideal Biological Solvent: Water’s ability to dissolve many substances, especially polar biomolecules, is vital for cellular processes. The formation of hydrogen bonds with solutes enables water to dissolve salts, sugars, proteins, and other polar molecules. However, nonpolar molecules do not form favorable interactions with water and are poorly soluble. This is why biological membranes, which contain lipid bilayers, have a hydrophobic (water-fearing) interior, keeping nonpolar substances separated from the aqueous environment.
Ionization of Water: Water can dissociate into hydrogen ions (H⁺) and hydroxide ions (OH⁻). This dissociation is responsible for the acidic or basic nature of water. The concentration of hydrogen ions in pure water is 10⁻⁷ M, which gives it a neutral pH of 7. This ionization plays a crucial role in maintaining cellular pH and regulating biochemical reactions.
pH and Buffer Systems
pH and Its Importance: The pH scale measures the concentration of hydrogen ions (H⁺) in a solution. A neutral pH (pH 7) occurs when the concentration of H⁺ equals that of OH⁻. Solutions with a higher concentration of H⁺ are acidic (pH < 7), while those with a lower concentration are basic (pH > 7). pH is critical for enzyme activity, biochemical reactions, and maintaining cellular functions, as most biological molecules and enzymes are pH-sensitive.
Weak Acids and Conjugate Bases: Weak acids like acetic acid (CH₃COOH) do not dissociate completely in water. Instead, they establish an equilibrium between the acid and its conjugate base (acetate ion, CH₃COO⁻). The equilibrium constant (Ka) and its logarithmic form (pKa) describe this dissociation. The pKa indicates the strength of the acid: the lower the pKa, the stronger the acid. This concept is essential in biochemistry because many biochemical reactions involve weak acids and bases, and understanding their behavior helps in manipulating reactions and processes in vitro and in vivo.
Buffers: Buffers are solutions containing weak acids and their conjugate bases that help maintain a stable pH despite the addition of acidic or basic substances. This is crucial in biological systems, where many processes require precise pH control. For example, blood maintains a stable pH using the bicarbonate buffer system. The Henderson-Hasselbalch equation is often used to calculate the pH of a buffer solution and understand the relative concentrations of acid and conjugate base required to resist pH changes.
- The Henderson-Hasselbalch equation: pH = pKa + log ([A−] / [HA]) where [A⁻] is the concentration of the conjugate base and [HA] is the concentration of the weak acid.
Buffers are vital in physiological processes like enzyme activity, protein folding, and maintaining the integrity of cellular structures. They ensure that the pH of cells and organs stays within the optimal range for biochemical reactions.
