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 ($K<em>a$) and $pK</em>a$

• 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.

• $pK<em>a = -log[K</em>a]$ 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 ($CH<em>3COOH$) and acetate ($CH</em>3COO^-$

  • Carbonic acid ($H<em>2CO</em>3$) 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 $pK<em>a$ of 7.5 because buffers work best when the pH is close to the buffer $pK</em>a$.

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:

  • $CO<em>2 + H</em>2O \leftrightarrow H<em>2CO</em>3 \leftrightarrow H^+ + HCO_3^-$

• Two key organs involved:

  • Lungs: Interaction between $CO<em>2$ and $H</em>2O$ (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 $CO<em>2$ and $H</em>2O$.

  • 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.