Water and Its Role in Biochemistry

Water, Acids, Bases, and Blood Buffers

• Water is essential for life, covering 70% of the earth's surface and crucial for all living organisms.

• The human body (and single cells) are comprised of over 70% water.

• Water constitutes 45%-75% of total human body weight.

• Water is distributed in both intracellular and extracellular compartments.

• Water is the biological solvent where biochemistry occurs.

• Water plays a crucial role in various metabolic aspects including absorption, transport, digestion, excretion, and maintaining body temperature.

• Direct participation in reactions like hydrolysis and condensation also involve water.

• Water is a polar molecule due to a highly electronegative oxygen atom that attracts electrons from hydrogen atoms.

• The uneven distribution of charge leads to polar H-O bonds and hydrogen bonds between water molecules.

• These bonds contribute to a high specific heat in water, enabling it to resist temperature changes, crucial for maintaining body temperature.

• Water is cohesive, meaning water molecules stick to each other, and adhesive, meaning water molecules stick to other substances. These properties involve capillary action, which is important for many biological processes in cells and organisms.

• Water exists in three states: liquid, solid, and gas.

• Water acts as a polar solvent, easily dissolving charged or polar substances (like salts, ions, sugars).

• Hydrophilic substances dissolve in water, while hydrophobic substances aggregate or cluster together to avoid water.

• The similar forces hold non-polar parts of molecules together; this is called hydrophobic interaction, very important in structures like biological membranes.

• Water ionizes to form H+ and OH- ions.

• The product (H+) + (OH-)—the ion product of water—is constant even when dissolving additional acids/bases in water.

• At 25 °C, pure water contains H+ and OH- ions at concentrations of 1×10⁻⁷ mol L⁻¹ each. This makes it neutral.

• pH is a measure of the concentration of H+ ions in a solution and forms a range from 0 to 14

• Blood pH normally stays within the narrow range of 7.35-7.45.

• The equilibrium constant for water is written as Kw = [H+] [OH-] = 1x10⁻¹⁴, where [10⁻⁷] refers to the concentration of H+ ions and OH- ions in pure water at 25°C.

Major Ions

• Major intracellular cations: K+ and Mg²⁺
• Major intracellular anions: Organic acids, proteins, HCO₃⁻, and Cl⁻
• Major plasma cations: Na⁺, Ca²⁺, Mg²⁺, and K⁺
• Major plasma anions: Cl⁻ and HCO₃⁻

Amphipathic Compounds

• Amphipathic compounds have polar (charged) regions and nonpolar regions.
• In water, their polar hydrophilic regions interact favorably with the solvent (water) leading to dissolution.
• Nonpolar hydrophobic regions tend to aggregate to avoid water.
• These stable structures in water are called micelles.
• Many biological molecules are amphipathic (including proteins, pigments, some vitamins, sterols, and phospholipids).
• Liposomes carry water-soluble and fat-soluble drugs, often used in delivery systems

Body's Acid-Base Balance, and Buffer Systems

• The body naturally produces more acids than bases, from foods, and during metabolism.
• Cellular metabolism produces CO₂. CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻
• Buffers are aqueous systems that prevent drastic changes in pH when small amounts of acids or bases are added.
• Buffer pairs consist of a weak acid and its corresponding conjugate base.
• They absorb or release H⁺ to maintain pH
• Examples of biological buffering systems include bicarbonate, protein, hemoglobin, and phosphate systems.
• The bicarbonate/carbonic acid system is critical for pH balance in blood plasma.
• The ratio of bicarbonate(HCO₃⁻) to carbonic acid(H₂CO₃) in blood is roughly 20/1 under normal conditions.
• Hemoglobin (Hb) is also a significant buffer, particularly within red blood cells (erythrocytes). Acid-base balance involving Hb is essential for gas transport and pH control because it reacts chemically with protons.
• The phosphate buffer system is important within the intracellular fluid and important for buffering phosphate compounds.
• The respiratory mechanism can help adjust pH by eliminating excess CO₂ which would lower pH.
• Kidney function is vital for regulating acid-base balance through H+ ion excretion.
• Disturbances in acid-base balance (acidosis or alkalosis) can have serious health consequences.