Revise Water for Cambridge 9700 (2.4)

Water (H₂O) is a polar molecule. This means there's an unequal sharing of electrons between the oxygen and hydrogen atoms. Oxygen is more electronegative, pulling electrons closer to itself, resulting in a slight negative charge (δ-) on the oxygen and slight positive charges (δ+) on the hydrogen atoms. The H-O-H bond angle of approximately 104.5° gives the molecule a bent or V-shape, which contributes to its overall polarity. These partial charges allow water molecules to form weak attractions called hydrogen bonds with other water molecules and other polar substances. These hydrogen bonds are the secret behind all of water's incredible properties.

Water has a remarkably high specific heat capacity, meaning it requires a significant amount of energy to raise its temperature. Water's specific heat capacity is approximately 4.2 kJ kg^{-1} °C^{-1}, one of the highest of any common substance. This is because much of the incoming heat energy must first be used to break the extensive network of hydrogen bonds between water molecules before the kinetic energy of the molecules themselves can increase.

Biological Importance:

• Temperature Regulation: This property acts as a thermal buffer, preventing rapid and extreme temperature fluctuations in organisms. Cells, which are mostly water, maintain a stable internal temperature, crucial for optimal enzyme activity.
• Aquatic Environments: Large bodies of water (oceans, lakes) provide stable thermal environments, protecting aquatic organisms from drastic temperature changes that would otherwise be lethal.

Water also possesses a high latent heat of vaporisation, meaning a large amount of energy is needed to change water from a liquid to a gaseous state (evaporation). The latent heat of vaporisation for water is very high, at about 2260 kJ kg^{-1}. Again, this is due to the energy required to break the numerous hydrogen bonds holding the molecules together in the liquid state.

Biological Importance:

• Cooling Mechanism: This is vital for cooling organisms. As water evaporates from a surface (e.g., sweat from skin, water from leaves during transpiration), it takes a considerable amount of heat energy with it, leading to an effective cooling effect with minimal water loss. This prevents overheating and helps maintain optimal body temperatures.

Water's polarity makes it an excellent solvent for other polar molecules (like glucose, amino acids) and ionic compounds (like mineral salts). The partial charges on water molecules can surround and interact with the charged regions of solutes. The δ- oxygen atoms are attracted to positive ions (cations), and the δ+ hydrogen atoms are attracted to negative ions (anions), forming 'hydration shells' that keep the ions separated and dissolved.

Biological Importance:

• Transport Medium: Water acts as the primary transport medium in organisms, carrying dissolved nutrients, respiratory gases (oxygen, carbon dioxide), hormones, and waste products (e.g., urea) in blood, tissue fluid, xylem, and phloem.
• Medium for Reactions: Most metabolic reactions occur in an aqueous environment within cells, where reactants are dissolved and can readily interact.

Water molecules exhibit strong cohesion - the attraction between water molecules themselves, due to hydrogen bonding. They also show adhesion - the attraction between water molecules and other polar surfaces (like the hydrophilic cellulose and lignin in the walls of xylem vessels). This combination of cohesion and adhesion results in capillary action, the ability of water to move up narrow tubes against the force of gravity.

Biological Importance:

• Transpiration Stream: Cohesion and adhesion are crucial for the transport of water in plants. Cohesion creates a continuous, unbroken column of water up the xylem vessels, while adhesion helps pull water up the narrow tubes against gravity. This forms the transpiration stream.
• Surface Tension: Strong cohesive forces at the surface of water create high surface tension, allowing some small insects (e.g., pond skaters) to walk on water and supporting organisms in the surface film (neuston).

Unlike most substances, solid water (ice) is less dense than liquid water. Water reaches its maximum density at 4°C. Below this temperature, as it freezes, the hydrogen bonds form a rigid, open crystalline lattice, pushing the molecules further apart. This increases the volume for the same mass, thus decreasing the density and causing it to float on liquid water.

Biological Importance:

• Insulation: In cold environments, ice forms a protective, insulating layer on the surface of lakes and ponds. This prevents the entire body of water from freezing solid, allowing aquatic organisms to survive beneath the ice during winter.

Beyond its physical properties, water is also a direct participant in many biochemical reactions, acting as a metabolite.

• Hydrolysis: Water is added to break down large, complex molecules (polymers) into smaller units (monomers). For example, in digestion, water is used to break glycosidic bonds in carbohydrates, peptide bonds in proteins, and ester bonds in lipids.
• Condensation (Dehydration Synthesis): Water is removed to form new bonds, building larger molecules from smaller ones. Conversely, in condensation reactions, a molecule of water is formed when two smaller molecules join together, such as when two amino acids form a peptide bond.