Biology Chapter on Water and pH

Water pH and Buffers

• Sixty percent of human body weight is water.
• Water is the solvent of life.
• Water dissolves and transports compounds in the blood.
• Water provides a medium for molecule movement in and through cellular components.
• Water separates charged molecules.
• Water dissipates heat.
• Water participates in chemical reactions.

Learning Outcomes

• Describe the properties of water.
• Use structural formulas to represent organic compounds that act as hydrogen bond donors or acceptors.
• Explain the orientation of polar and nonpolar regions of macromolecules in an aqueous environment.
• Explain the relationship between pH, acidity, and alkalinity.
• Describe how buffers work.

Water

• Molecular formula: H₂O
• Structural formula: shown in image
• Molecular models: shown in image
• Angle between hydrogen atoms: 104.5°
• Water structure: an irregular tetrahedron with oxygen at its center

Water Structure

• A water molecule is an irregular tetrahedron with oxygen at its center.
• Water has a slightly positive hydrogen side and a slightly negative oxygen side. This creates a dipole.
• Hydrogen bonds form between the slightly positive hydrogen of one water molecule and the slightly negative oxygen of another.

Hydrogen Bond

• Hydrogen bonds are relatively weak attractions.
• Hydrogen bonding influences water's physical properties.
• Hydrogen bonding accounts for water's relatively high boiling point.
• Examples of biological important hydrogen bonds: between the hydroxyl group of an alcohol and water, between the carbonyl group of a ketone and water, between peptide groups in polypeptides.

Water as a Solvent

• Water is a polar solvent.
• Water dissolves most biomolecules, which are generally charged or polar compounds.
• Water forms hydrogen bonds and electrostatic interactions with these molecules.
• Water dissolves polar compounds due to hydrogen bonding interactions.
• Water dissolves charged compounds due to electrostatic interactions

Classification of Compounds

• Hydrophilic compounds dissolve easily in water.
• Hydrophobic compounds do not dissolve in water.
• Amphipathic compounds contain regions that are polar (or charged) and nonpolar.
• Most biomolecules are amphipathic.

Amphipathic Compounds in Water

• When an amphipathic compound is mixed with water, its polar region interacts with the solvent (water), causing it to dissolve.
• The nonpolar region avoids contact with water, leading to the formation of stable structures called micelles.
• Micelles form when hydrophobic tails cluster together in a spherical formation, with hydrophilic heads facing outward towards the water molecules.

Ionization (Dissociation) of Water

• Water molecules have a slight tendency to undergo reversible ionization to yield a hydrogen ion (H⁺) and a hydroxide ion (OH⁻).
• The reaction is: H₂O ↔ H⁺ + OH⁻

Dissociation Constant of Water

• The dissociation constant (K_w) expresses the relationship between the hydrogen ion concentration [H⁺], the hydroxide ion concentration [OH⁻], and the concentration of water [H₂O] at equilibrium.
• K_w = [H⁺][OH⁻]/[H₂O]

pH

• pH is the negative logarithm (base 10) of the hydrogen ion concentration [H⁺].
• pH= -log [H+]

pH Scale

• pH 7 is neutral, [H+] = [OH−].
• pH < 7 is acidic, [H+] > [OH−].
• pH > 7 is basic, [H+] < [OH−].

Example Calculations

• Calculating pH from [H⁺]
• Calculating pH from [OH⁻]

How to Calculate pH of Weak Acid and Weak Base

• The tendency of a weak acid or weak base to dissociate and donate a hydrogen ion (H+) or hydroxide ion (OH−) is weak.
• The tendency is denoted by K_a (equilibrium constant for weak acid dissociation). The higher the K_a, the greater the dissociation tendency.
• The pH of a solution of a weak acid and its salt is given by the Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA])

Buffer

• Almost every biological process is pH-dependent. Small changes in pH can significantly change the rate of a process.
• Cells and organisms maintain a specific and constant cytosolic pH (around pH 7) using buffers.
• Buffers are aqueous systems containing a weak acid and its conjugate base, or a weak base and its conjugate acid.
• Buffers resist pH changes when small amounts of acid (H⁺) or base (OH⁻) are added.
• The major buffer systems in the body include: bicarbonate-carbonic acid buffer, hemoglobin buffer (in red blood cells), phosphate buffer (in all cells), and protein buffers (in cells and plasma).

Test your understanding of the properties of water and pH in biological systems. This quiz explores the concepts of pH, buffer systems, and the role of water as a solvent. Answer questions related to the characteristics of biological molecules and their interactions in aqueous environments.