Water And Acid-Base Balance Lecture PDF

Introduction The water molecule and its ionization products, H1 and OH2, profoundly influence the structure, self-assembly, and properties of all cellular components, including proteins, nucleic acids, and lipids. The noncovalent interactions responsible for the strength and specificity of...

Introduction The water molecule and its ionization products, H1 and OH2, profoundly influence the structure, self-assembly, and properties of all cellular components, including proteins, nucleic acids, and lipids. The noncovalent interactions responsible for the strength and specificity of “recognition” among biomolecules are decisively influenced by water’s properties as a solvent, including its ability to form hydrogen bonds with itself and with solutes. Hydrogen bonding gives water its unusual properties Water has a higher melting point, boiling point, and heat of vaporization than most other common solvents. These unusual properties are a consequence of attractions between adjacent water molecules that give liquid water great internal cohesion. Butanol (CH3(CH2)2CH2OH) has a relatively high boiling point of 117 8C, whereas butane (CH3(CH2)2CH3) has a boiling point of only 20.5 8C. Explain? Water forms hydrogen bonds with polar solutes Hydrogen bonds are not unique to water. They readily form between an electronegative atom (the hydrogen acceptor, usually oxygen or nitrogen) and a hydrogen atom covalently bonded to another electronegative atom (the hydrogen donor) in the same or another molecule. Water forms hydrogen bonds with polar solutes Alcohols, aldehydes, ketones, and compounds containing N—H bonds all form hydrogen bonds with water molecules and tend to be soluble in water. Sugars dissolve readily in water because of the stabilizing effect of hydrogen bonds between the hydroxyl groups or carbonyl oxygen of the sugar and the polar water molecules. Examples of hydrogen bonds other than in water Give examples of amphipathic molecules? What is hydrophobic interactions? Why hydrophobic interactions are crucial for living systems? Water disordering in enzyme-substrate interaction Water is the solvent of life. Explain? Virtually all cells have water in them (cytoplasm) and water in the surrounding environment. Many of water’s properties depend on the structure of water molecules. Water has a number of important properties essential for life. Many of the properties below are due to the hydrogen bonds in water: thermal, cohesive and solvent properties. Thermal properties Specific heat capacity: Water has a specific heat capacity of 4.2 J g-1 °C-1, which means that it takes 4.2 joules of energy to heat 1 g of water by 1°C. This is unusually high and it means that water does not change temperature very easily. This minimises fluctuations in temperature inside cells, and it also means that sea temperature is remarkably constant. Latent heat of vaporization: Water requires a lot of energy to change state from a liquid into a gas, and this is made use of as a cooling mechanism in animals (sweating and panting) and plants (transpiration). As water evaporates it extracts heat from around it, cooling the organism. Latent heat of fusion: Water also requires a lot of heat to change state from a solid to a liquid, and must loose a lot of heat to change state from a liquid to a solid. This means it is difficult to freeze water, so ice crystals are less likely to form inside cells Why do we sweat when temperature increases? Cohesion Water molecules "stick together" due to their hydrogen bonds, so water has high cohesion. This explains why long columns of water can be sucked up tall trees by transpiration without breaking. It also explains surface tension, which allows small animals to walk on water. Solvent Because it is charged, water is a very good solvent. Charged or polar molecules such as salts, sugars, amino acids dissolve readily in water and so are called hydrophilic ("water loving"). Uncharged or non-polar molecules such as lipids do not dissolve so well in water and are called hydrophobic ("water hating"). Department of Biochemistry and Molecular Biology General Biochemistry Chapter 1- Part 2 Acid-Base Balance Dr.Enas Sarahna September 2024 Ionization of water Pure water is slightly ionized. Hydrogen ion does not exist as H+ but as hydronium ion (H3O+) due to hydrogen bonds between water molecules. The ionization of water can be measured by its electrical conductivity; pure water carries electrical current as H3O+ migrates toward the cathode and OH- toward the anode → Proton hopping which is a fast process Explain: Acid-Base reactions in aqueous solutions are exceptionally fast. Equilibrium constant Keq Can be defined in terms of the concentrations of reactants (A and B) and products (C and D) at equilibrium. For a given reaction: The equilibrium constant is fixed and characteristic for any given chemical reaction at a specified temperature. Equilibrium constant Keq For water, at 25◦C : Kw represents the ion product of water at 25◦C. The value for Keq, determined by electrical conductivity measurements of pure water, is 1.8 3 10216 M at 25◦C. At neutral pH: Worked examples The pH Scale Designates the H+ and OH- Concentrations If the concentration of hydrogen ion is 1x10-7 then pH can be calculated as follows: The pH of an aqueous solution can be approximately measured with various indicator dyes, including litmus, phenolphthalein, and phenol red. These dyes undergo color changes as a proton dissociates from the dye molecule. Measurement of pH Accurate determinations of pH in the chemical or clinical laboratory are made with a glass electrode that is selectively sensitive to H+ concentration but insensitive to Na+, K+, and other cations. Explain: Measurement of pH is one of the most important and frequently used procedures in biochemistry. Frequent vomiting in gastroenteritis Diabetes Weak Acids and Bases Have Characteristic Acid Dissociation Constants Hydrochloric, sulfuric, and nitric acids, commonly called strong acids, are completely ionized in dilute aqueous solutions. Strong bases NaOH and KOH are also completely ionized. Weak acids and bases—those not completely ionized when dissolved in water. These are ubiquitous in biological systems and play important roles in metabolism and its regulation. Acids may be defined as proton donors and bases as proton acceptors. When a proton donor such as acetic acid (CH3COOH) loses a proton, it becomes the corresponding proton acceptor, in this case the acetate anion (CH3COO-). A proton donor and its corresponding proton acceptor make up a conjugate acid-base pair Weak Acids and Bases Have Characteristic Acid Dissociation Constants Each acid has a characteristic tendency to lose its proton in an aqueous solution. The stronger the acid, the greater its tendency to lose its proton. The tendency of any acid (HA) to lose a proton and form its conjugate base (A-) is defined by the equilibrium constant (Keq) for the reversible reaction: Equilibrium constants for ionization reactions are usually called ionization constants or acid dissociation constants, often designated Ka. Weak Acids and Bases Have Characteristic Acid Dissociation Constants Stronger acids, such as phosphoric and carbonic acids, have larger ionization constants; weaker acids, such as mono hydrogen phosphate ,have smaller ionization constants. pKa is analogous to pH and defined as the value of pH when half the concentration of a weak acid is ionized. The stronger the tendency to dissociate a proton, the stronger is the acid and the lower its pKa.