Properties of Water

Properties of Water

Overview

• Water is the most abundant biomolecule in the body, accounting for 60-95% of living cells.
• Water distribution in the human body:
  • Intracellular fluids: 55%
  • Plasma: 8%
  • Interstitial and lymph: 22%
  • Connective tissue, bone, and cartilage: 15%

Functions of Water as a Solvent in Biochemical Reactions

• Acts as a transport medium across membranes, carrying substances in and out of cells.
• Helps maintain body temperature.
• Acts as a solvent in the digestive and waste excretion systems.

Water Balance in the Body

• Healthy humans experience daily water intake and loss; a water balance must be maintained.
• Dehydration: Occurs when water loss significantly exceeds intake.
• Edema: Occurs when water loss is significantly less than intake.

Polarity and Hydrogen Bonding of Water

• O-H Bonds in Water
  • Electrons are pulled toward oxygen, creating a partial negative charge (δ-) on the oxygen atom and partial positive charges (δ+) on the hydrogen atoms.
• Polar Bonds
  • Bonds with unequal sharing of electrons due to electronegativity differences are called polar bonds.
  • Molecules with polar bonds are polar molecules (e.g., water).
  • If the electronegativity difference is small (e.g., C-H bonds in hydrocarbons), the sharing of electrons is nearly equal, and the bond is essentially nonpolar.

Hydrogen Bonds (H-Bonds)

• H-bonds are non-covalent interactions; a special case of dipole-dipole interaction.
• Formed between hydrogen atoms covalently bonded to one electronegative atom (H donor) and a lone pair of electrons on another electronegative atom (H acceptor).
• In water, the H atom covalently bonded to O is the H donor, and the lone pair on O of another water molecule is the H acceptor.

Ability of Water to Form H-Bonds

• H bonding enables water to dissolve many organic biomolecules that contain functional groups that can participate in H-bonding.
• Hydrogen Acceptors: O-atoms in carbonyl and carboxyl groups (aldehydes, ketones, amides).
• Both H-acceptors and H-donors: Alcohols, carboxylic acids, and amines.

Solute-Solvent Interactions

• Ionic and Polar Substances Dissolve in Water
  • Hydrophilic substances: Ionic and polar substances that can be dissolved in water.
  • Polar substances are hydrated by water through dipole-dipole interactions.
  • Some polar substances participate in hydrogen bonding, enhancing solubility.
• Non-polar substances do not dissolve in water.
  • Hydrophobic substances: Non-polar substances that cannot be dissolved in water.
  • Incapable of forming dipole-dipole interactions or H bonds with water but can interact with each other through hydrophobic interaction.
  • The hydrophobic effect is critical for folding proteins and the self-assembly of biological membranes.

Micelles

• Amphipathic substances (e.g., detergents) have both hydrophilic and hydrophobic regions (non-polar tail and an ionic/polar end).
• Micelles: Structures formed when amphipathic substances are dispersed in water; non-polar tails associate in the center, minimizing contact with water.

Acids and Bases

• The biochemical behavior of many important compounds depends on their acid-base properties.
• Bronsted-Lowry definition: An acid is a proton donor, and a base is a proton acceptor.

Measures of Acidity

1. pH Scale

• A wide range of possible hydrogen ion and hydroxide ion concentrations exist in aqueous solutions, usually in exponential forms.
• pH is a quantity for expressing these concentrations more conveniently.
• pH = -log[H^+] where [H^+] is the molar concentration of the hydrogen ion.
• A difference of one pH unit implies a tenfold difference in [H^+].

Calculations:

• What is the pH of 1 x 10^{-3} M HCl?
  • pH = -log[H^+]
  • = -log[1 x 10^{-3}]
  • = 3
• What is the hydrogen ion concentration of a solution with pH 4.63?
  • pH=-log[H^+]
  • [H^+] = 10^{-pH}
  • = 10^{-4.63}
  • = 2.34 x 10^{-5}

2. Acid Dissociation Constant (Ka) and pKa

• Acid strength can be expressed by the acid dissociation constant, K_a.
• The value of K_a has a fixed numerical value at a given temperature.
• The higher the K_a, the stronger the acid.
• In biochemistry, most acids encountered are weak acids.
• pKa = -log[Ka] to avoid numbers with very large negative exponents.
• The lower the pK_a value, the stronger the acid.

Buffers

Composition of a Buffer

• A buffer is a composition of a weak acid and its conjugate base (salt).
• Examples:
  • Acetic acid (CH3COOH) and acetate (CH3COO^-
  • Carbonic acid (H2CO3) and carbonate ion (CO_3^{2-})

How Buffers Work

• Based on the nature of weak acids and their conjugate bases.
• If acid is added, it reacts with the conjugate base to form a weak acid.
• If base is added, it reacts with the weak acid to form water and the conjugate base.
• This keeps the pH much more stable than if the same acid or base has been added to an unbuffered system.

How to Choose a Buffer

• Choose a buffer by knowing what pH we are trying to maintain.
• For an experiment requiring a solution to stay at pH 7.5, look for a buffer with a pKa of 7.5 because buffers work best when the pH is close to the buffer pKa.

Bicarbonate Buffer System

• The normal pH range of the blood is between 7.35-7.45.
  • Below this range, there is abundant hydrogen ions (H^+) in the blood (acidic).
  • Above this pH, there is little H^+ (basic).
  • Acidosis: Blood pH is low.
  • Alkalosis: Blood pH is high.
• Acidosis and alkalosis are prevented from occurring through the Bicarbonate Buffer System, which is represented by the equation:
  • CO2 + H2O \leftrightarrow H2CO3 \leftrightarrow H^+ + HCO_3^-
• Two key organs involved:
  • Lungs: Interaction between CO2 and H2O (leftmost side of the equation).
  • Kidneys: Interaction between H^+ and HCO_3^- (rightmost side of the equation).

Types of Acidosis and Alkalosis

• Acidosis: Decrease in blood pH (increase in H^+). Compensation: expel more CO_2.
• Alkalosis: Increase in blood pH (decrease in H^+). Compensation: keep more CO_2 in the lungs.

Respiratory Acidosis/Alkalosis (Lungs)

• Acidosis or alkalosis caused by disturbance of the interaction between CO2 and H2O.
  • Respiratory acidosis: Increase of CO_2, resulting in a shift of the buffer reaction to the right, which produces more H^+ and lowers blood pH.
  • Respiratory alkalosis: Decrease of CO_2, resulting in a shift of the buffer reaction to the left, which lowers H^+ and increases blood pH.

Metabolic Acidosis/Alkalosis (Kidneys)

• Acidosis or alkalosis caused by disturbance of the interaction between H^+ and HCO_3^-.
  • Metabolic acidosis: Decrease of HCO_3^-, resulting in a shift of the buffer reaction to the right, which produces more H^+ and lowers blood pH.
  • Metabolic alkalosis: Increase of HCO_3^-, resulting in a shift of the buffer reaction to the left, which lowers H^+ and increases blood pH.