BIOCHEMISTRY 1

• Approximately 60% of our body is water
• Water acts as a solvent for substances like K, glucose, ATP, and proteins
• Water is important for the transport of molecules and heat
• Compounds produced in the body and dissolved in water contain chemical groups that act as acids or bases, releasing or accepting hydrogen ions
• The hydrogen ion content and the amount of body water are controlled to maintain homeostasis for cells
• Significant deviations from a constant environment like acidosis or dehydration may be life-threatening
• Water is distributed between intracellular and extracellular compartments
• Extracellular compartments include interstitial fluids, blood, and lymph
• Water is a dipolar molecule with an uneven distribution of electrons between hydrogen and oxygen atoms
• Water forms hydrogen bonds with other polar molecules and acts as a solvent
• Water dissociates to form hydrogen (H+) and hydroxyl (OH-) ions
• The concentration of hydrogen ions determines the acidity of the solution, expressed as pH
• An acid can release hydrogen ions, and a base can accept hydrogen ions
• Almost all molecules of a strong acid dissociate and release hydrogen ions in water
• A weak acid has a characteristic dissociation constant, Ka
• Buffers are mixtures of an undissociated acid and its conjugate base, resisting changes in pH when H+ or OH- is added
• Buffers have the greatest buffering capacity near their pKa
• Normal metabolism generates CO2, metabolic acids (e.g., lactic acid, ketone bodies), and inorganic acids (e.g., sulfuric acid)
• The body uses buffers like bicarbonate, phosphate, and hemoglobin to maintain pH of body fluids in a range compatible with life
• Respiratory mechanisms remove carbonic acid through CO2 expiration, and the kidneys excrete acid as ammonium ion and other ions
• Hydrogen bonds between water molecules are arranged with their hydrogen atoms closest to the anion
• The oxygen atom of water molecules interacts with inorganic cations such as Na⁺ and K⁺ to surround them with a hydration shell
• Hydrogen bonds are strong enough to dissolve polar molecules in water and to separate charges, but weak enough to allow movement of water and solutes
• The strength of the hydrogen bond between two water molecules is approximately 4 kcal, roughly 1/20th of the strength of the covalent O-H bond in the water molecule
• The average hydrogen bond between water molecules lasts only about 10 picoseconds, and each water molecule in the hydration shell of an ion stays for only 2.4 nanoseconds
• Water resists temperature change due to its high heat of fusion, high thermal conductivity, high heat capacity, and high heat of vaporization
• Water responds to heat input by decreasing the extent of hydrogen bonding and to cooling by increasing the bonding between water molecules
• The pH of pure water is 7, considered neutral because [H⁺] and [OH⁻] are equal
• Acids donate a hydrogen ion (H⁺) to a solution, while bases accept hydrogen ions
• Strong acids like sulfuric acid and hydrochloric acid dissociate completely in solution, while weak acids like organic acids dissociate to a limited extent
• Intravenous saline, a solution of 0.9% NaCl, is used for rehydration in cases like osmotic diuresis to balance the high osmolality and water loss
• Water moves between fluid compartments based on osmolality, with water moving from low solute concentration to high solute concentration compartments to achieve equal osmolality
• The semipermeable cellular membrane contains ion channels allowing water movement, and water moves freely through capillaries separating interstitial fluid and plasma
• The blood contains a high content of dissolved negatively charged proteins and electrolytes to balance charges, and water moves to maintain osmolality balance
• The equilibrium constant for dissociation of a weak acid is denoted by its Ka
• The Henderson-Hasselbalch equation converts the formula for the dissociation constant of a weak acid to a logarithmic equation
• The term pKa represents the negative log of Ka
• A weak acid is 50% dissociated at a pH equal to its pKa
• Metabolic carboxylic acids have a pKa between 2 and 5, reflecting the strength of the acid
• Buffers consist of a weak acid and its conjugate base, resisting changes in pH when hydrogen ions or hydroxide ions are added