Water, pH, and Buffers

The Living Environment and Homeostasis

Homeostasis is the maintenance of a stable internal environment in living organisms. Water, pH, and buffers are essential for sustaining life and ensuring that biological processes occur under optimal conditions.

Properties of Water Supporting Life

Covalent and Polar Nature of Water

Water (H2O) is a polar molecule with covalent bonds between hydrogen and oxygen. The oxygen atom is more electronegative, creating a partial negative charge near the oxygen and a partial positive charge near the hydrogens. This polarity allows water to interact with various molecules and ions, making it an excellent solvent.

Hydrogen Bonding

Hydrogen bonds form between the slightly positive hydrogen atom of one water molecule and the slightly negative oxygen atom of another. These bonds are responsible for many of water's unique properties, such as high cohesion, surface tension, and its role as a universal solvent.

Cohesive and Adhesive Forces

Cohesion refers to the attraction between water molecules, while adhesion is the attraction between water molecules and other substances. These forces enable capillary action, which is crucial for water transport in plants and other biological systems.

Surface Tension

Surface tension is the result of cohesive forces at the surface of water, allowing it to resist external force. This property enables small organisms, such as water striders, to move across the surface of water without sinking.

Dissolution of Substances

Water's polarity allows it to dissolve ionic and polar substances efficiently. When salts dissolve, water molecules surround and separate the ions, facilitating biochemical reactions in cells.

High Heat Capacity

Water can absorb or release large amounts of heat with minimal temperature change, stabilizing environmental and cellular temperatures. This property is vital for maintaining homeostasis in living organisms.

Anomalous Behaviour and Unique Properties of Ice

Unlike most substances, water expands upon freezing due to the formation of a crystalline structure stabilized by hydrogen bonds. This makes ice less dense than liquid water, allowing it to float and insulate aquatic life during cold periods.

Acids, Bases, and pH

Definitions of Acids and Bases

Acids are substances that donate protons (H+), while bases accept protons. The Brønsted-Lowry definition is commonly used in biology. The Lewis definition expands this to electron pair donors and acceptors.

pH and Its Importance

The pH scale, introduced by S.P.L. Sorensen, measures the hydrogen ion concentration in a solution. It is a logarithmic scale ranging from 0 (most acidic) to 14 (most basic), with 7 being neutral. Biological systems require tightly regulated pH for optimal enzyme activity and metabolic processes.

Definition of pH and pOH

pH is defined as the negative logarithm of the hydrogen ion concentration:

pOH is defined as the negative logarithm of the hydroxyl ion concentration:

In pure water at 25°C, M, so pH = 7. The relationship between pH and pOH is:

Interaction of Strong Acids and Bases with Water

Strong acids and bases dissociate completely in water, significantly altering the pH. Weak acids and bases only partially dissociate, which is important for buffer systems in biological fluids.

Buffers and Their Biological Importance

Definition and Function of Buffers

A buffer is a solution that resists changes in pH when small amounts of acid or base are added. Buffers are essential in biological systems to maintain pH within a narrow, optimal range.

Buffer Systems: The Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation describes the relationship between pH, pKa, and the ratio of conjugate base to acid in a buffer system:

When , pH = pKa, which is the point of maximum buffering capacity.

Buffering of Amino Acids

Amino acids can act as buffers due to their ionizable groups. The titration of amino acids like glycine and lysine demonstrates how their charge state changes with pH, affecting their buffering capacity.

Biological Buffer Systems

Several buffer systems operate in the human body to maintain physiological pH, including:

• Bicarbonate buffer system: Maintains blood pH and involves the equilibrium between carbonic acid (H2CO3) and bicarbonate (HCO3-).

• Phosphate buffer system: Important in intracellular fluids.

• Protein buffer system: Involves amino acid side chains and plasma proteins.

• Hemoglobin buffer system: Buffers blood pH via hemoglobin in red blood cells.

Homeostatic Regulation of pH

The body constantly faces acid-base challenges from metabolism and respiration. The lungs regulate CO2 (affecting carbonic acid), while the kidneys excrete or retain H+ and HCO3- to maintain pH balance. Disruptions can lead to metabolic or respiratory acidosis/alkalosis.

Applications and Clinical Relevance

• Antacids (e.g., Gelusil, ENO) neutralize excess stomach acid, providing rapid relief from acidity.

• Medications like ranitidine and pantoprazole reduce acid production, helping manage gastric acidity and related disorders.

Additional info: For further practice, solve pH calculation problems and consider the physiological impact of acid-base disturbances and their treatments.