2.2 Water and its properties in biological systems

Properties of Water

Properties of water for life

Water is a polar molecule, meaning its electrical charge is unevenly distributed. The oxygen atom pulls electrons closer to itself, becoming slightly negative ($\delta^-$), while the two hydrogen atoms become slightly positive ($\delta^+$). This polarity allows water molecules to form hydrogen bonds with each other and with other polar substances.

These hydrogen bonds give water several properties that are critical for life:

• Cohesion is the attraction between water molecules via hydrogen bonding. It allows water to maintain continuous columns (like in xylem vessels of plants) and produces high surface tension at air-water interfaces.
• High specific heat capacity ($4.184 \text{ J/g°C}$) means water requires a lot of energy to change temperature. This lets organisms absorb and release heat without drastic temperature swings, keeping cellular environments stable.

Water as a cellular solvent

Water is often called the "universal solvent" because it dissolves a wide range of polar and ionic substances, including sugars, amino acids, and ions. This dissolving ability is what makes transport and chemical reactions possible inside cells.

How does it work? Water molecules surround dissolved ions and polar molecules, forming hydration shells. These shells prevent the dissolved particles from clumping back together, keeping them in solution and available for cellular use.

Water also directly participates in chemical reactions:

• In hydrolysis, water breaks bonds by adding $H$ and $OH$ across them (e.g., breaking down polymers into monomers).
• In condensation (dehydration synthesis), water is released when monomers join together.
• Water serves as a reactant or product in major metabolic pathways like photosynthesis ($6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$) and cellular respiration.

Hydrophobic Interactions and Temperature Regulation

Hydrophobic effect in biology

Nonpolar substances (lipids, hydrophobic amino acids) cannot form hydrogen bonds with water. When a nonpolar molecule is placed in water, the surrounding water molecules rearrange into ordered, cage-like structures to maximize their own hydrogen bonding. This ordering decreases entropy, which is thermodynamically unfavorable. To minimize this disruption, nonpolar substances are driven to cluster together, reducing the total surface area exposed to water. That clustering tendency is the hydrophobic effect.

This effect has two major consequences in cells:

• Membrane formation: Phospholipids arrange into bilayers with hydrophobic tails facing inward (away from water) and hydrophilic heads facing the aqueous environment. This is the structural basis of all biological membranes, including the plasma membrane and organelle membranes.
• Protein folding: Hydrophobic amino acid side chains get buried in the protein's interior, away from water. This drives the formation of secondary and tertiary protein structures and is essential for proper protein function.

Water's role in temperature regulation

Water has a high heat of vaporization ($40.7 \text{ kJ/mol}$), meaning it takes a lot of energy to convert liquid water into vapor. When water evaporates from a surface, it carries that energy away as heat. This is why sweating cools your skin and why transpiration prevents leaves from overheating.

Water also plays a role in cold tolerance:

• Freezing point depression occurs when dissolved solutes (salts, sugars) lower the temperature at which water freezes. Cells and body fluids naturally contain solutes that provide some protection against ice formation.
• Some organisms take this further with specialized adaptations. Certain fish, for example, produce antifreeze proteins that bind to small ice crystals and prevent them from growing, allowing survival in sub-zero waters.