Biochemistry 1: Importance of Water

Course Information

• Course: Biochemistry 1
• Department/Semester: Fourth Semester
• Semester: Autumn 2024 – 2025
• Lecture: First Lecture
• Instructor: Ouida T. Khujli Abbas

Biomedical Importance of Water

• Water is the primary component of living organisms.
• Water's dipolar structure allows it to dissolve various organic and inorganic molecules through hydrogen bonding.
• Water is involved in many metabolic reactions, acting as a reactant or product.
• Water has a slight tendency to dissociate into hydroxide ions and protons.
• Bicarbonate and other buffers maintain the pH of extracellular fluid between 7.35 and 7.45.
• Acidosis (blood pH < 7.35) is caused by conditions like diabetic ketosis and lactic acidosis.
• Alkalosis (blood pH > 7.45) can result from vomiting acidic gastric contents.

Water as a Biological Solvent

• Water molecules form dipoles due to the electronegative oxygen atom pulling electrons away from hydrogen nuclei.
• This creates a partial positive charge on hydrogen and a partial negative charge on oxygen.
• Water's strong dipole and high dielectric constant enable it to dissolve numerous charged compounds (salts).
• Water molecules form hydrogen bonds with each other and with other molecules.
• This self-association of water molecules influences its physical properties (viscosity, surface tension, and boiling point).
• Hydrogen bonding allows water to dissolve many organic substances.

Water Molecules and Hydrogen Bonds

• Water molecules contain both hydrogen bonding donors and acceptors.
• Hydrogen bonding leads to the self-association of water molecules into ordered arrays.
• Hydrogen bonding significantly impacts water's physical properties.

Physiochemical Properties of Water

• Water is colorless, odorless, and tasteless.

Structure of Water Molecules

• Water molecules consist of two hydrogen atoms bonded to an oxygen atom.
• The O–H bond distance is 0.958 Å, and the angle between the three atoms is 104.5°.
• Water molecules form hydrogen bonds.
• Oxygen has partial negative charge and hydrogen has partial positive charge.

Energy of Hydrogen Bonds

• Energy of a hydrogen bond is approximately -20 kJ/mol.
• Energy of covalent bond is approximately 460 kJ/mol.
• Ice is a crystalline structure of hydrogen-bonded water molecules.
• Water molecules in ice are tetrahedrally surrounded by their nearest neighbors.

Liquid Water Density and Ice Structure

• Liquid water density is approximately 1.00 g/mL.
• Ice density is approximately 0.92 g/mL.

Structure of Liquid Water

• Liquid water structure is irregular.
• Networks of water molecules continually break and reform every 2x10^-11 seconds.

Self-Ionization of Water

• Water dissociates into H+ and OH− ions.
• [H+][OH−] = 10⁻¹⁴ mol²/L² at 25°C.

pH

• pH measures the hydrogen ion concentration.
• pH below 7 is acidic, above 7 is alkaline.
• Neutral pH is 7.

Acids and Bases

• Acids release protons (H+) and gain a negative charge (anion).
• Bases accept protons and gain a positive charge (cation).

Ionization of Amino Acids

• Amino acids possess both acidic (-COOH) and basic (-NH2) groups.
• Depending on the pH, amino acids can exist in various charged forms.
• The pH at which an amino acid has no net charge is called the isoelectric point (pI).

Buffers

• Buffer solutions resist changes in pH upon the addition of acids or bases.
• A weak acid and its salt (conjugate base) can act as a buffer.
• Buffers are crucial for maintaining stable pH in biological systems.

Biological Buffers

• Phosphate, bicarbonate are used as buffers in cells.