Molecular Structure of Water

Molecular Structure of Water

• Water is biologically important as a medium for metabolic reactions and a major habitat, covering 71% of Earth's surface.
• A water molecule comprises two slightly positive hydrogen atoms covalently bonded to one slightly negative oxygen atom.
• Electrons are shared unevenly, with oxygen attracting electrons more strongly.
• This results in a slightly negative charge on the oxygen atom and slightly positive charge on the hydrogen atoms, creating an asymmetrical shape.
• Water molecules are dipoles, having one negatively charged end and one positively charged end, making them polar.

Covalent and Hydrogen Bonds

• Water molecules easily bond with each other through hydrogen bonds (cohesion), acting as an excellent solvent.
• Hydrogen bonds form between the positive and negative regions of adjacent water molecules due to polarity.
• Individually, hydrogen bonds are weak and constantly break/re-form; collectively, large numbers form a strong structure.
• These bonds contribute to essential properties like being an excellent solvent, having high specific heat capacity and surface tension, and acting as a reagent.

Polarity and its Roles

• The polarity of water allows hydrogen bonds to form between adjacent water molecules.
• Water's properties are vital for living organisms because of the polarity of its molecules and the number of hydrogen bonds.
• Pure water has a neutral pH of 7.

Water as a Solvent

• As a polar molecule, water dissolves ions and covalently bonded polar substances.
• Dissolved solutes are more chemically reactive, enabling reactions to occur within cells.
• Metabolites are efficiently transported (except for hydrophobic, non-polar molecules) due to water's solvent properties.
• Polarity makes water a universal solvent.

Specific Heat Capacity

• Specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1°C, with water at 4200 J/kg°C.
• Water has a high specific heat capacity, meaning it requires lots of energy to change temperature.
• The numerous hydrogen bonds in water cause the high specific heat capacity.
• High capacity is advantageous for habitats, maintaining constant temperatures ideal for enzyme activity, and transferring heat within the body.

Latent Heat of Vaporization

• A large amount of thermal energy is needed to break hydrogen bonds for water to change from liquid to gas.
• The high latent heat of vaporization allows organisms to lose heat with minimal water evaporation.
• This provides an effective cooling effect through processes like transpiration and sweating.

Cohesion and Adhesion

• Hydrogen bonds between water molecules result in strong cohesion.
• Cohesion allows water columns to move through plant xylem and animal blood vessels, and create surface tension.
• Water also hydrogen bonds to other molecules (adhesion).
• Adhesion enables water movement up the xylem during transpiration.

Overall Functions

• Temperature regulation happens due to high specific heat capacity and easy evaporation.
• Universal solvent means tiny positive and negative charges easily attract other molecules or ions.
• H bonds produce cohesion and surface tension, allowing mass flow.
• Reactivity occurs in hydrolysis reactions, e.g., during digestion.
• Water functions as assists buffer and has a neutral pH (pH 7).